A&P EXAM 2 Study Guide on the Skeletal System PDF

**[CHAPTER 7]** **Introduction to the Skeletal System** - Skeletal system includes the bones of the skeleton, cartilage, ligaments and other connective tissues that stabilize or connect the bones - Bones are the primary organs of the skeletal system - Form the rigid framework of th...

**[CHAPTER 7]** **Introduction to the Skeletal System** - Skeletal system includes the bones of the skeleton, cartilage, ligaments and other connective tissues that stabilize or connect the bones - Bones are the primary organs of the skeletal system - Form the rigid framework of the body and perform other functions - 2 types: compact and spongy - **Compact bone** - relatively rigid, appears white, smooth and solid. About 80% of total bone mass - **Spongy bone** - located internal to compact bone, appears porous. About 20% of total bone mass - Cartilage - semi-grid connective tissue, more flexible that bone - Mature cartilage is avascular - 2 types of cartilage associated with bone - **Hyaline cartilage** - attaches ribs to sternum (costal cartilage) - covers the ends of some bones (articular cartilage), - Is the cartilage within growth plates (epiphyseal plates) - Provides a model during development for the formation of the fetal skeleton - **Fibrocartilage** - weight-bearing cartilage that can withstand compression. - Forms the intervertebral discs, pubic symphysis (cartilage between bones of the pelvis) and the cartilage pads of the knee joints (menisci) - **Ligaments** - dense regular connective tissue that anchors bone to bone - **Tendons** - dense regular connective tissue that connects muscle to bone **BONE** - Living bones are organs and appear yellowish **GENERAL FUNCTIONS OF BONE** - Support and protection - Levers for movement - Hematopoiesis - Storage of mineral and energy reserves [Support and Protection] - Bones provides structural support and serve as framework for the body - Protect delicate tissues and organs from injury/trauma - Rib cage shields heart and lungs - Cranial bones protect brain - Vertebrae protects spinal cord - Pelvis protects urinary and reproductive organs [Lever for Movement] - Bones serve as attachment sites for skeletal muscles, soft tissues and some organs - Muscles attached to bones contract and pull the skeleton - Bones can alter the direction and magnitude of forces generated by skeletal muscles - Movements include powerful contractions for running to delicate movements like removing a splinter [Hematopoiesis ] - The process of blood cell production - Occurs in red bone marrow - Contains stem cells that form blood cells and platelets [Storage of Mineral and Energy Reserves] - Most of the body's reserves of calcium and phosphate are stored in and released from the bone - **Calcium** - essential mineral for muscle contraction, blood clotting, and release of neurotransmitter from nerve cells - **Phosphate** - structural component of ATP, nucleotides, and phospholipids. Important component of plasma membrane - When the body needs calcium or phosphate, some bone connective tissue is broken down and the minerals are released into the blood - Potential energy (lipids) is stored in yellow bone in the shafts of some adult bones **Classification of Bones** 4. classes of bone determined by shape and function - Long bones - Short bones - Flat bones - Irregular bones [Long bones ] - greater in length than width - Elongated, cylindrical shaft (**diaphysis**) - Most common bone shape - Found in upper limbs (arm, forearm, palm and fingers) - Lower limbs (thighs, legs, soles of foot and toes) [Short Bones] - Length nearly equal to width - Carpals (wrist bones) and tarsals (bones of foot) - **Sesamoid bones** (small sesame seed shaped bones along the tendons of some muscles) - Includes patella (kneecap), the larges sesamoid bone [Flat Bones ] - Flat, thin surfaces, may be slightly curved - Provide extensive surface areas for muscle attachment - Protect underlying soft tissues - Roof of the skull, scapulae, sternum, ribs [Irregular bones] - Elaborate, complex shapes - Vertebrae, ossa cosse (hip bones) - Several bones of the skull - Ethmoid, sphenoid, maxilla  **GROSS ANATOMY OF BONES** [Gross Anatomy of a Long Bone] - Long bones are the most common bone shape in the body [Regions of a Long Bone] - **Diaphysis** - the shaft, elongated and usually cylindrical - Provides leverage and major weight support - Composed of compact bone - Contains Spongy bone internally in the form of spicules (needles) - Contains medullary cavity - hollow, cylindrical space - In children, it contains redbone marrow - In adults, it contains yellow bone marrow - **Epiphysis** - expanded, knobby region at each end of long bone - **Proximal epiphysis** - end of bone closest to body trunk - **Distal epiphysis** - end farthest from the trunk - Composed of outer, thin layer of compact bone - Inner region of spongy bone (resists stress in many directions) - **Articular cartilage** on the surface (type of hyaline cartilage that helps reduce frictions and absorb shock in moveable joints) - **Metaphysis** - region of long bone where the bone wides and transfers forces between the diaphysis and the epiphysis - In growing bone, it contains the **epiphyseal plate** (growth plate) - A thin layer of hyaline cartilage that provides for the continued lengthwise growth of the bone - The remnant of the epiphyseal plate in adults is the **epiphyseal line** [Coverings and Linings of Bone] - **Periosteum** - tough sheath that covers the outer surface of the bone except for areas covered by articular cartilage - Consists of 2 layers
 - Outer, fibrous layer of dense irregular connective tissue. - Protects the bone from surrounding structures, - anchors blood vessels and nerves to the surface of the bone. - Serves as an attachment site for ligaments and tendons - Inner cellular layer - includes **osteoprogenitor** cells, **osteoblasts**, and **osteoclasts** - The **osteoprogenitor** cells and **osteblasts** produce circumferential layers of bone matrix, and thus the periosteum is responsible for growth in bone width. - **Perforating fibers** anchor the periosteum to the bone - They are collagen fibers that run perpendicular to the diaphysis - **Endosteum -** a very thin layer of connective tissue - Contains osteoprogentior cells, osteoblasts and osteoclasts - Covers all internal surfaces of bone with medullary cavity - Active during bone growth, repair and remodeling [Gross Anatomy of Other Bone Classes] - Short, flat and irregular bones generally have external surfaces composed of compact bone (covered by a periosteum) - Interior composed entirely of spongy bone - No medullary cavity - In a flat bone of the skull, spongy bone is also called diploë  [Blood Supply and Innervation of Bone] - Bone is highly vascularized, especially in spongy bone - Blood vessels enter bones from periosteum - **Nutrient foramen** - area where one nutrient artery enters and one nutrient vein exits via a small opening or hole in the bone - Blood vessels supply nutrients and oxygen and remove waste products - Nerves also use the nutrient foramen to innervate the bone and the periosteum, endosteum and marrow cavity. - Mainly sensory nerves hat relay nerve impulses from the skeleton when it is injured [BONE MARROW] - Soft connective tissue of bone - Includes red bone marrow and yellow bone marrow - Red bone marrow - Contains reticular connective tissue - Developing blood cells - Adipocytes - Primary function is forming blood cells - In children, red bone marrow is more widely distributed, and is located in the spongy bone of most bones of the body as wells the medullary cavity of long bones - In adults, there is a decrease in developing blood cells and an increase in adipocytes within the medullary cavities of long bones and the inner core of most epiphyses - Called yellow bone marrow
 - Adults have red bone marrow only in selected portions of the axial skeleton , such as - flat bones of the skull - the vertebrae - ribs - sternum - ossa coxae (hip bones) - proximal emphases of each humerus and femur. - Severe anemia (lower blood cells) results in insufficient oxygen reaching the cells and may trigger conversion of yellow bone marrow back to red bone marrow which facilitates the production of additional red blood cells **Microscopic Anatomy: Bone Connective Tissue** - The primary component of bone is bone connective tissue (osseous connective tissue) - Bone connective tissue is composed of cells and extracellular matrix [Cells of Bone] - Four types of cells: osteoprogenitor cells, osteoblasts, osteocytes, and osteoclasts - Osteoprogenitor cells - stem cells derived from mesenchyme - Some become osteoblasts. - cells are located in both the periosteum and the endosteum. - Osteoblasts - formed from osteoprogenitor stem cells. - Often, osteoblasts are positioned side by side on bone surfaces. - Active osteoblasts exhibit a somewhat cuboidal shape and have abundant rough endoplasmic reticulum and Golgi apparatus - Osteoblasts synthesize and secrete the initial semisolid organic form of bone matrix called osteoid - Osteoid later calcifies as a result of salt crystal deposition. As a consequence of this mineral deposition on osteoid, osteoblasts become entrapped within the matrix they produce and secrete, and thereafter they differentiate into osteocytes. - Osteocytes - mature bone cells derived from osteoblasts that have lost their bone-forming ability when enveloped by calcified osteoid. - Connections between some of the original neighboring osteoblasts are maintained by the cytoplasmic projections of the osteocytes. - Osteocytes maintain the bone matrix and detect mechanical stress on a bone. If stress is detected, osteoblasts are signaled, and it may result in the deposition of new bone matrix at the surface. - Osteoclasts - are large, multinuclear, phagocytic cells. - derived from fused bone marrow cells - These cells exhibit a ruffled border where they contact the bone, which increases their surface area exposure to the bone. - An osteoclast is often located within or adjacent to a depression or pit on the bone surface called a resorption lacuna - Osteoclasts are involved in breaking down bone in an important process called **bone resorption

** A diagram of a cell Description automatically generated [Composition of Bone Matrix] - The matrix of bone connective tissue has both organic and inorganic components. 
 - organic component is osteoid - produced by osteoblasts. composed of collagen and a semisolid ground substance of proteoglycans and glycoproteins that suspends and supports the collagen fibers. - The organic components give bone tensile strength by resisting stretching and twisting, and contribute to its overall flexibility. 
 - The inorganic portion of the bone matrix is made up of salt crystals that are primarily calcium phosphate - Calcium phosphate and calcium hydroxide interact to form crystals of hydroxyapatite - The crystals also incorporate other salts (e.g., calcium carbonate) and ions (e.g., sodium, magnesium, sulfate, fluoride) during the process of calcification. - These crystals deposit around the long axis of collagen fibers in the extracellular matrix. - The crystals harden the matrix and account for the rigidity or relative inflexibility of bone that provides its compressional strength. - The correct proportion of organic and inorganic substances in the matrix of bone allows it to function optimally.
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 - INORGANIC PORTION - CALCIUM PHOSPHATE AND OTHER SALTS - FORMS CRYSTALS - ACCOUNTS FOR THE RIGIDITY OF BONE


 - ORGANIC COMPONENT -- OSTEOID - MADE OF COLLAGEN, PROTEOGLYCANS AND GLYCOPROTEINS - TENSILE STRENGTH
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** - **A loss of protein, or the presence of abnormal protein, results in brittle bones; insufficient calcium results in soft bones.** [Bone Matrix]: - Bone formation begins when osteoblasts secrete osteoid. - Calcification/ mineralization occurs to the osteoid when hydroxyapatite crystals deposit in the bone matrix. - Calcification - initiated when the concentration of calcium ions and phosphate ions reaches critical levels and precipitates out of solution, thus forming the hydroxyapatite crystals that deposit in and around the collagen fibers. - The entire process of **bone formation requires** a number of substances, including **vitamin D** (enhances calcium absorption from the gastrointestinal tract) and **vitamin C** (which is required for collagen formation), as well as **calcium** and **phosphate** for calcification. - Bone resorption is a process whereby bone matrix is destroyed by substances released from osteoclasts into the extracellular space adjacent to the bone. - **Proteolytic enzymes** released from **lysosomes** within the **osteoclasts** chemically digest the organic components (collagen fibers and proteoglycans) of the matrix, while **hydrochloric acid** (HCl) dissolves the **mineral parts (calcium and phosphate crystals)** of the bone matrix. - The liberated calcium and phosphate ions enter the blood. - Bone resorption may occur when blood calcium levels are low  [Compact Bone Microscopic Anatomy] - Compact bone is composed of small, cylindrical structures called **osteons** - An **osteon** is the basic functional and structural unit of mature compact bone - Osteons are oriented parallel to the diaphysis of the long bone. An osteon has several components:
 - The central canal - a cylindrical channel that lies in the center of the osteon and runs parallel to it. - Extending through the central canal are the blood vessels and nerves that supply the bone. 
 - Concentric lamellae - rings of bone connective tissue that surround the central canal and form the bulk of the osteon - Lamellae are sorted with alternating patterns, and the alternating pattern of collagen fiber direction gives bone part of its strength and resilience. 
 - Osteocytes - mature bone cells found in small spaces between adjacent concentric lamellae. - These cells maintain the bone matrix. 
 - Lacunae - the small spaces that each house an osteocyte. 
 - Canaliculi - tiny, interconnecting channels within the bone connective tissue that extend from each lacuna, through the lamellae, and connect to other lacunae and the central canal. - Canaliculi house osteocyte cytoplasmic projections that permit intercellular contact and communication. - Nutrients, minerals, gases, and wastes are transported through the cytoplasmic extensions within these passageways, allowing their exchange between the osteocytes and the blood vessels within the central canal. - Several other structures occur in compact bone but are not part of the osteon proper, including the following - Perforating canals - resemble central canals in that they also contain blood vessels and nerves. - However, perforating canals run perpendicular to the central canals and help connect multiple central canals within different osteons, - Forms a channel for a vascular and innervation connection among the multiple osteons. - Circumferential lamellae - rings of bone immediately internal to the periosteum of the bone (external circumferential lamellae) or immediately external to the endosteum (internal circumferential lamellae) - Both external and internal circumferential lamellae extend the entire circumference of the bone (hence their name). - Interstitial lamellae - are either the components of compact bone that are between osteons or the leftover parts of osteons that have been partially resorbed - they often look like a "bite" has been taken out of them. - The interstitial lamellae are incomplete and typically have no central canal. [Spongy Bone Microscopic Anatomy ] - contains no osteons - structure is an open lattice of narrow rods and plates of bone, called **trabeculae** - Bone marrow (when present) fills in between the trabeculae. - When a segment of spongy bone is examined microscopically, you can see parallel lamellae composed of bone matrix. - Between adjacent lamellae are osteocytes resting in lacunae, with numerous canaliculi radiating from the lacunae. - Nutrients reach the osteocytes by diffusion through cytoplasmic processes of the osteocytes, which extend within the canaliculi that open onto the surfaces of the trabeculae. - trabeculae often form a meshwork of crisscrossing bars and plates of small bone pieces. This structure provides great resistance to stresses applied in many directions by distributing the stress throughout the entire framework. - This is accomplished because stresses and forces are distributed throughout the structure. [Microscopic Anatomy: Hyaline cartilage Connective Tissue] - contains a population of cells scattered throughout a glassy-appearing matrix of protein fibers (primarily collagen) embedded within a gel-like ground substance - This ground substance is similar to that of bone in that it includes proteoglycans, such as chondroitin sulfate, but it differs from bone because its inorganic salts do not include calcium. - This makes hyaline cartilage both resilient and flexible. - Additionally, cartilage also contains a high percentage of water (60% to 70% by weight). - The high water content makes it highly compressible, allowing hyaline cartilage to function as a good shock absorber. - Chondroblasts are derived from mesenchymal cells and they produce the cartilage matrix. - Once chondroblasts become encased within the matrix they have produced and secreted, the cells are called chondrocytes and occupy small spaces called lacunae. - These mature cartilage cells maintain the matrix. - Hyaline cartilage---except the articular cartilage---is covered by a dense irregular connective tissue sheet called the perichondrium, which helps maintain its shape. - Mature cartilage is avascular and contains no nerves. - Nutrients and oxygen are supplied to the cartilage by diffusion from blood vessels in the perichondrium.  **CARTILAGE GROWTH** - Certain types of bone formation and bone growth are dependent upon the growth of hyaline cartilage - Cartilage development and growth begin during embryologic development. - Cartilage can grow both in length through the process of interstitial growth and in width by appositional growth [Interstitial growth ] - an increase in length - occurs within the internal regions of cartilage - four steps - Chondrocytes housed within lacunae are stimulated to undergo mitotic cell division. - Following cell division, two cells occupy a single lacuna; they are now called chondroblasts. - As chondroblasts begin to synthesize and secrete new cartilage matrix, they are pushed apart. Each cell now resides in its own lacuna and is called a chondrocyte. - The cartilage continues to grow in the internal regions as chondrocytes continue to produce more matrix. [Appositional growth ] - an increase in width along the cartilage's outside edge, or periphery. - 3 steps - Undifferentiated stem cells at the internal edge of the perichondrium begin to divide. - New undifferentiated stem cells and committed cells that differentiate into chondroblasts are formed. These chondroblasts are located at the periphery of the old cartilage, where they begin to produce and secrete new cartilage matrix. - The chondroblasts, as a result of matrix formation, push apart and become chondrocytes, with each occupying its own lacuna. The cartilage continues to grow at the periphery as chondrocytes continue to produce more matrix. - During early embryonic development, both interstitial and appositional cartilage growth occur simultaneously. - interstitial growth declines rapidly as the cartilage matures because the cartilage becomes semirigid, and it is no longer able to expand. - Further growth can occur only at the periphery of the tissue, so later growth is primarily appositional. - Once the cartilage is fully mature, new cartilage growth typically stops. - cartilage growth usually occurs only after injury to the cartilage, yet this growth is limited due to the lack of blood vessels in the tissue **Bone Formation** - Ossification/osteogenesis refers to the formation and development of bone connective tissue. - begins in embryo and continues as the skeleton grows during childhood and adolescence. - By 8-12 weeks of development, skeleton begins forming from either thickened condensations of mesenchyme (intramembranous ossification) or a hyaline cartilage model of bone (endochondral ossification). [Intramembranous Ossification] - Intramembranous ossification means "bone growth within a membrane." 
 - also called dermal ossification because the mesenchyme that is the source of these bones is in the area of the future dermis of the skin. 
 - produces the flat bones of the skull. some of the facial bones, the mandible, and the central part of the clavicle. 
 - begins when mesenchyme becomes thickened and condensed with a dense supply of blood capillaries. - 4 steps
 - **Ossification centers form within thickened regions of mesenchyme at the week 8 of development**.
- Some cells in the mesenchyme divide, and the committed cells then differentiate into osteoprogenitor cells. 
- Some osteoprogenitor cells become osteoblasts and begin to secrete osteoid. 
- Multiple ossification centers develop within the mesenchyme as the number of osteoblasts increases. 
 - **Osteoid undergoes calcification**. 
- Osteoid formation is quickly followed by calcification, as calcium salts are deposited onto the osteoid and then they crystallize (solidify). 
- When calcification entraps osteoblasts within lacunae in the matrix, the entrapped cells become osteocytes. 
 - **Woven bone and its surrounding periosteum form**. 
- Initially, the newly formed bone connective tissue is immature and not well organized, a type called woven bone, or primary bone. 
- Eventually, woven bone is replaced by lamellar bone, or secondary bone. 
- The mesenchyme that still surrounds the woven bone begins to thicken and eventually organizes to form the periosteum. 
- Mesenchymal cells grow and develop to produce additional osteoblasts. 
- Newly formed blood vessels also branch throughout this region. The calcified trabeculae and intertrabecular spaces are composed of spongy bone.
 - **Lamellar bone replaces woven bone, as compact bone and spongy bone form**. 
- Lamellar bone replaces the trabeculae of woven bone. 
- On the internal and external surfaces of the bone, spaces between the trabeculae are filled and the bone becomes compact bone. 
- Between these external and internal surfaces, the trabeculae are modified slightly and produce spongy bone. 
- The typical structure of a flat cranial bone is composed of two external layers of compact bone with a layer of spongy bone in between  [Endochondral Ossification] - begins with a hyaline cartilage model and produces most bones of the skeleton - Bones of the upper and lower limbs, the pelvis, the vertebrae, and the ends of the clavicle.
 - Includes Long bone development, 6 steps - **The fetal hyaline cartilage model develops**. - During week 8 -12 of development
 - chondroblasts secrete cartilage matrix, and a hyaline cartilage model forms. 
 - Chondrocytes are trapped within lacunae, and a perichondrium surrounds the cartilage.
 - **Cartilage calcifies, and a periosteal bone collar forms**. - Within the center of the cartilage model (future diaphysis), chondrocytes start to hypertrophy (enlarge) and resorb (eat away) some of the surrounding cartilage matrix, producing larger holes in the matrix. - As these chondrocytes enlarge, the cartilage matrix begins to calcify. - Chondrocytes in this region die and disintegrate because nutrients cannot diffuse to them through this calcified matrix. 
 - The result is a calcified cartilage shaft with large holes where living chondrocytes had been. 
 - As the cartilage in the shaft is calcifying, blood vessels grow toward the cartilage and start to penetrate the perichondrium around the shaft. Stem cells within the perichondrium divide to form osteoblasts. 
 - The osteoblasts develop as this supporting connective tissue becomes highly vascularized, and the perichondrium becomes a periosteum. 
 - The osteoblasts within the internal layer of the periosteum start secreting a layer of osteoid around the calcified cartilage shaft. 
 - The osteoid hardens and forms a periosteal bone collar around this shaft. 
 - **The primary ossification center forms in the diaphysis**. 
 - A growth of capillaries and osteoblasts, called a periosteal bud, extends from the periosteum into the core of the cartilage shaft, invading the spaces where the living chondrocytes had been. - The remains of the calcified cartilage serve as a template on which osteoblasts begin to produce osteoid. 
 - This region is called the primary ossification center because it is the first major center of bone formation. 
 - Bone development extends in both directions toward the epiphyses from the primary ossification center. 
 - Healthy bone connective tissue quickly displaces the calcified, degenerating cartilage in the shaft. Most, but not all, primary ossification centers have formed by the twelfth week of development. 
 - **Secondary ossification centers form in the epiphyses**. 
 - The same basic process that formed the primary ossification center occurs later in the epiphyses. 
 - Beginning around the time of birth, the hyaline cartilage in the center of each epiphysis calcifies and begins to degenerate. 
 - Epiphyseal blood vessels and osteoprogenitor cells enter each epiphysis. - Secondary ossification centers form as bone displaces calcified cartilage. - not all secondary ossification centers form before birth; some form later in childhood. - As the secondary ossification centers form, osteoclasts resorb some bone matrix within the diaphysis, creating a hollow medullary cavity.
 - **Bone replaces almost all cartilage, except the articular cartilage and epiphyseal cartilage.** 
 - By late bone development, almost all of the hyaline cartilage has been displaced by bone. 
 - Hyaline cartilage remains as articular cartilage only on the articular surface of each epiphysis and at the epiphyseal plates. 
 - Lengthwise growth continues until the epiphyseal plates ossify and form epiphyseal lines. 
 - **Lengthwise bone growth continues into puberty until the epiphyseal plate is converted to the epiphyseal line, indicating that the bone has reached its adult length**. 
 - Depending upon the bone, most epiphyseal plates ossify to become epiphyseal lines between the ages of 10 and 25. 
 - The last epiphyseal plates to ossify are those of the clavicle in the late 20s. **BONE GROWTH AND BONE REMODELING** - Bone growth and bone remodeling begin during embryologic development [Interstitial Growth] - dependent upon growth of cartilage within the epiphyseal plate. - The epiphyseal plate has 5 zones that are continuous from the first zone nearest the epiphysis to the last zone nearest the diaphysis 
 - **Zone of resting cartilage**. 
 - farthest from the medullary cavity of the diaphysis and nearest the epiphysis. 
 - composed of small chondrocytes distributed throughout the cartilage matrix. 
 - resembles mature and healthy hyaline cartilage. 
 - This region secures the epiphysis to the epiphyseal plate. 
 - **Zone of proliferating cartilage**. 
 - Chondrocytes undergo rapid mitotic cell division, enlarge slightly, and become aligned within their lacunae into longitudinal columns - columns are parallel to the diaphysis. - **Zone of hypertrophic cartilage**. 
 - Chondrocytes cease dividing and greatly enlarge in size (hypertrophy) in this zone. 
 - Walls of the lacunae become thin because the chondrocytes resorb matrix as they hypertrophy. 
 - **Zone of calcified cartilage**. 
 - usually composed of two or three layers of chondrocytes. 
 - Minerals are deposited in the matrix between the columns of lacunae. 
 - This calcification limits diffusion of nutrients, which results in destruction (loss) of the chondrocytes and makes the matrix appear opaque. 
 - **Zone of ossification**. 
 - walls break down between lacunae in the columns and longitudinal channels. 
 - These spaces are invaded by capillaries and osteoprogenitor cells from the medullary cavity. 
 - The osteoprogenitor cells develop into osteoblasts, which deposit new bone matrix on the remaining calcified cartilage matrix.  - Growth in bone length occurs in zone 2 as chondrocytes undergo mitotic cell division and zone 3 as chondrocytes hypertrophy. - These activities combine to push the zone of resting cartilage toward the epiphysis. - the flexible matrix of hyaline cartilage of bone permits this growth. - once growth in length has occurred by chondrocytes in zones 2 and 3, new bone connective tissue is then produced by osteoblasts at the same rate in zone 5. - Thus, growth in length is due to growth in hyaline cartilage connective tissue, which is later replaced with bone. This process is similar to the endochondral ossification process that occurs during bone development - The epiphyseal plate maintains its thickness during childhood as it is pushed away from the center of the shaft. - At maturity, the rate of hyaline cartilage production slows in zones 2 and 3, and the rate of osteoblast activity accelerates in zone 5. - As a result, the epiphyseal plate continues to narrow until it ultimately disappears, and interstitial growth completely stops. - Eventually, the only remnant of each epiphyseal plate is an internal thin line of compact bone called an epiphyseal line. - The loss of the hyaline cartilage and the appearance of the remnant epiphyseal line signal the end of interstitial growth. [Appositional Growth ] - Appositional growth occurs within the periosteum. - osteoblasts in the inner cellular layer of the periosteum produce and deposit bone matrix within layers parallel to the surface, called external circumferential lamellae. - These lamellae are analogous to tree rings: As they increase in number, the structure increases in diameter. - Thus, the bone becomes wider as new bone is laid down at its periphery. - As this new bone is being laid down, osteoclasts along the medullary cavity resorb bone matrix, creating an expanding medullary cavity. - The combined effects of bone growth at the periphery and bone resorption within the medullary cavity transform an infant bone into a larger version called an adult bone. - Appositional growth continues throughout an individual's lifetime. [BONE REMODELING] - The bone continues to renew and reshape itself throughout a person's lifetime. - The constant, dynamic process of continual addition of new bone tissue (**bone deposition**) and removal of old bone tissue (**bone resorption**) is called bone remodeling. - occurs at both the periosteal and endosteal surfaces of a bone. - about 20% of the adult human skeleton is replaced yearly. - bone remodeling does not occur at the same rate everywhere in the skeleton. - compact bone in our skeleton is replaced at a slower rate than the spongy bone, and different components of a bone are replaced at varying rates. - For example, distal part of the femur (thigh bone) is replaced every 4 to 6 months, whereas the diaphysis of this bone may not be completely replaced during an individual's lifetime.**bone remodeling is dependent upon the coordinated activities of osteoblasts, osteocytes, and osteoclasts.** - The relative activities of these cells are influenced by two primary factors: - hormones - mechanical stress to the bone. - Mechanical stress occurs in the form of weight-bearing movement and weight-bearing exercise. - the amount of mechanical stress or load applied to living bone will affect the structure of the bone. - This mechanical stress is detected by osteocytes and communicated to osteoblasts. - Osteoblasts increase the synthesis of osteoid, and this is followed by deposition of mineral salts. - Bone strength increases over a period of time in response to mechanical stress. - Mechanical stresses that significantly affect bone result from skeletal muscle contraction and gravitational forces. - As this mechanical stress occurs over time, the portions of the bone most affected by the stress will develop more bone mass. - As muscles hypertrophy with weight-bearing exercise, the muscle applies added stress to the bone where it is attached. As a result, the bony projections (where the muscles attach) will enlarge and become more robust. - Weight-bearing activities, such as weight lifting, walking, or running, help build and retain bone mass. - aerobic but non-weight-bearing activities, such as cycling or swimming, do not greatly increase bone mass, because there is limited weight-bearing movement.) - In general, weight training and high-impact aerobic activities (such as running or jumping) tend to have the greatest positive effect on bone density, whereas low-impact exercise (e.g., using an elliptical machine) has less of an effect. Research has shown that consistent weight-bearing exercise can increase total bone mass in adolescents and young adults prior to its inevitable reduction later in life, and even 70- and 80-year-olds who perform moderate weight training can increase their bone mass. Please note, however, that excessive or overly intense exercise can actually be detrimental to bone---if mechanical stresses exceed the bone's ability to remodel, the bone may be injured as a result. For example, a runner who increases their workout time or miles run too quickly may develop a stress fracture (see section 7.8) in a leg bone, such as the tibia or fibula. In contrast, removal or significant decrease of mechanical stress weakens bone through both reduction of collagen formation and demineralization. For example, when a person has a fractured bone and wears a cast or is bedridden, the strength of the unstressed bone decreases in the immobilized limb. Thus, while in space, astronauts must exercise to reduce the effects of loss of bone mass due to lack of gravity. [HORMONES THAT INFLUENCE BONE GROWTH AND BONE REMODELING] - Hormones are molecules that are released from one cell into the blood and are transported throughout the body to affect other cells. - Certain hormones influence bone composition and growth patterns by altering the rates of chondrocyte, osteoblast, and osteoclast activity  [Growth hormone, also called somatotropin ] - produced by the anterior pituitary gland - affects bone growth by stimulating the liver to release another hormone called insulin-like growth factor (IGF) - Both growth hormone and IGF directly stimulate growth of cartilage in the epiphyseal plate. 
 [Thyroid hormone ] - secreted by the thyroid gland - stimulates bone growth by influencing the basal metabolic rate of bone cells - If maintained in proper balance, growth hormone and thyroid hormone regulate and maintain normal activity at the epiphyseal plates until puberty. 
 [Sex hormones (estrogen and testosterone)] - begin to be secreted in relatively large amounts at puberty - dramatically accelerate bone growth. - Sex hormones increase the rate of both cartilage growth and bone formation within the epiphyseal plate. - the appearance of high levels of sex hormones at puberty also signals the beginning of the end for growth at the epiphyseal plate. - This happens because bone formation occurs at a faster rate than cartilage growth. - Bone growth eventually overcomes the region of cartilage, replacing all cartilage with bone at the epiphyseal plates. [Glucocorticoids] - a group of steroid hormones that are released from the adrenal cortex and regulate blood glucose levels - High amounts increase bone loss and, in children, impair growth at the epiphyseal plate. - It is because of this relationship that a child\'s growth is monitored if receiving high doses of glucocorticoids as an anti-inflammatory, such as a treatment for severe asthma. 
 [Serotonin ] - most bone cells have serotonin receptors - when levels of circulating serotonin are too high, osteoprogenitor cells are prevented from differentiating into osteoblasts - serotonin plays a role in the rate and regulation of normal bone remodeling because it affects osteoblast differentiation. 
 [Three additional hormones] - parathyroid hormone, - calcitriol, - calcitonin - participate in both regulating bone remodeling and regulating blood calcium levels. **REGULATING BLOOD CALCIUM LEVELS** - Regulating calcium concentration in blood (between 8.9 and 10.1 milligrams per deciliter \[mg/dL\]) is essential because calcium is required for numerous physiologic processes such as - initiation of muscle contraction - exocytosis of molecules from cells including nerve cells (neurons) - stimulation of the heart by pacemaker cells - blood clotting 
 - 2 primary hormones that regulate blood calcium - calcitriol (an active form of vitamin D) - parathyroid hormone. [ACTIVATION OF VITAMIN D TO CALCITROL] - enzymatic pathway of activating vitamin D to calcitriol - 3 STEPS
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 - **Ultraviolet light converts the precursor molecule in keratinocytes of the skin** (7-dehydrocholesterol, a modified cholesterol molecule) **to vitamin D3** (cholecalciferol), **which is released into the blood.** (Vitamin D3 from a meal also is absorbed from the small intestine into the blood.)

 - **Vitamin D3 circulates within the blood throughout the body. As blood passes through the liver, vitamin D3 is converted by liver enzymes to calcidiol** by the addition of a hydroxyl group (---OH). Both steps 1 and 2 occur continuously with limited regulation. 

 - **Calcidiol circulates in the blood. As blood passes through the kidney, calcidiol is converted by kidney enzymes to calcitriol** by the addition of another ---OH group. **Calcitriol is the active form of vitamin D3.** Note that **the presence of parathyroid hormone increases the rate of this final enzymatic step in the kidney. Thus, greater amounts of calcitriol are formed when parathyroid hormone is present.
** - **Vitamin D in its active form of calcitriol hormone - stimulates absorption of calcium ions (Ca2+) from the small intestine into the blood.** [Parathyroid Hormone and Calcitriol] - Parathyroid hormone (PTH) is secreted and released by the parathyroid glands in response to reduced blood calcium levels. The final enzymatic step converting calcidiol to calcitriol in the kidney occurs more readily in the presence of PTH. PTH and calcitriol interact with selected major organs as follows:\ 
 - Bone - PTH and calcitriol act synergistically to increase the release of calcium from the bone into the blood, by increasing osteoclast activity.
 - - Kidneys - PTH and calcitriol act synergistically to stimulate the kidneys to excrete less calcium in the urine and retain more calcium in the blood. - occurs by increasing calcium reabsorption in the tubules in the kidneys
 - Small intestine - A function unique to calcitriol is to increase absorption of calcium from the small intestine into the blood. - The removal of calcium from bone, the decrease in loss of calcium from the kidney, and the increase in calcium absorption from the small intestine result in elevating blood calcium and returning it to within the normal homeostatic range. Subsequently, the release of additional PTH is inhibited by negative feedback. 
 - calcitriol causes both increased absorption of calcium from the small intestine into the blood and activation of osteoclasts, which causes release of calcium from the bone. - The net effect on bone is dependent upon blood calcium levels. - Sufficient calcium in the diet can offset the loss of calcium from bone caused by calcitriol-activating osteoclasts. [CALCITONIN] - Calcitonin aids in regulating blood calcium levels - less significant role than either PTH or calcitriol. - Calcitonin is released from the thyroid gland from its parafollicular cells in response to high blood calcium levels - also secreted in response to stress from exercise. - calcitonin primarily inhibits osteoclast activity. - calcitonin stimulates the kidneys to increase the loss of calcium in the urine. - result is a reduction in blood calcium levels. 
 - limitations of calcitonin: - Calcitonin seems to have the greatest effect under conditions where there is the greatest turnover of bone, such as in growing children. **EFFECTS OF AGING** - Aging affects bone connective tissue in two ways. 
 - \(1) the tensile strength of bone decreases - reduced rate of protein synthesis by osteoblasts. - relative amount of inorganic minerals in the bone matrix increases (due to decreased matrix protein) - bones of the skeleton become brittle and susceptible to fracture. 
 - \(2) bone loses calcium and other minerals (demineralization). - bones of the skeleton become thinner and weaker, resulting in insufficient ossification, a condition called osteopenia - Aging causes all people to become slightly osteopenic. - reduction in bone mass may begin as early as 35--40 years of age, when osteoblast activity declines, while osteoclast activity continues at previous levels. - Different parts of the skeleton are affected unequally. Vertebrae, jaw bones, and epiphyses lose relatively large amounts of mass, resulting in reduced height, loss of teeth, and fragile limbs. 
 - Every decade, women lose roughly more of their skeletal mass than do men. - A significant percentage of older individuals suffer from osteoporosis - a condition characterized by reduction in bone mass sufficient to compromise normal function 
 - vitamin D and numerous hormones, including growth hormone, estrogen, and testosterone, decrease with age. - This decrease in hormone levels contributes to reduction in bone mass. **BONE FRACTURE AND REPAIR** - Bone has great mineral strength, but it may break as a result of unusual stress or a sudden impact. 
 - Breaks in bones are called fractures and are classified in several ways. - stress fracture - a thin break caused by increased physical activity in which the bone experiences repetitive loads (e.g., as seen in some runners). - A pathologic fracture - usually occurs in bone that has been weakened by disease. - simple fracture - the broken bone does not penetrate the skin - compound fracture - one or both ends of the broken bone pierce the overlying skin. 
 - The healing of a simple fracture takes about 2 to 3 months - A compound fracture takes longer to heal. - Fractures heal much more quickly in young children (average healing time, 3 weeks) and become slower to heal as we age. - In the elderly, the normal thinning and weakening of bone increase the incidence of fractures, - some complicated fractures require surgical intervention to heal properly. - **Bone fracture repair can be described as a series of four steps** - **A fracture hematoma forms.** A bone fracture tears blood vessels inside the bone and within the periosteum, causing bleeding. This bleeding results in a fracture hematoma that forms from the clotted blood. 
 - **A fibrocartilaginous (soft) callus forms**. Regenerated blood capillaries infiltrate the fracture hematoma. First, the fracture hematoma is reorganized into an actively growing connective tissue called a procallus. Fibroblasts within the procallus produce collagen fibers that help connect the broken ends of the bones. Chondroblasts in the newly growing connective tissue form a dense regular connective tissue associated with the cartilage. Eventually, the procallus becomes a fibrocartilaginous (soft) callus (kal′ŭs; hard skin). The fibrocartilaginous callus stage lasts at least 3 weeks. 
 - **A hard (bony) callus forms.** Within a week after the injury, osteoprogenitor cells in areas adjacent to the fibrocartilaginous callus become osteoblasts and produce trabeculae of primary bone. The fibrocartilaginous callus is then replaced by this bone, which forms a hard (bony) callus. The trabeculae of the hard callus continue to grow and thicken for several months. 
 - **The bone is remodeled.** Remodeling is the final phase of fracture repair. The hard callus persists for at least 3 to 4 months as osteoclasts remove excess bony material from both exterior and interior surfaces. Compact bone replaces primary bone. The fracture usually leaves a slight thickening of the bone (as detected by x-ray); however, in some instances, healing occurs with no persistent obvious thickening.  **[CHAPTER 9]** **[CHAPTER 9]** [Classification of Joints] - Joint/articulation -- place of contact between bones, between bone/cartilage or between bones and teeth - Bones articulate with each other at a joint. - arthrology - the scientific study of joints - Joints are classified by both their structural characteristics and their functional characteristics (the movements they allow) - Joints are categorized structurally on the basis of whether a space occurs between the articulating bones and the type of connective tissue that binds the articulating surfaces of the bones: - A fibrous joint - has no joint cavity - occurs where bones are held together by dense regular connective tissue. - Cartilaginous - joint has no joint cavity - occurs where bones are joined by cartilage. - A synovial joint - has a joint cavity (filled with a lubricating fluid) that separates the articulating surfaces of the bones. - The articulating surfaces are enclosed within a connective tissue capsule, - the bones are attached to each other by various ligaments.  - Joints are classified functionally based on the extent of movement they permit. - A synarthrosis - an immobile joint. - Two types of fibrous joints and one type of cartilaginous joint are synarthroses. - An amphiarthrosis - a slightly mobile joint. - One type of fibrous joint and one type of cartilaginous joint are amphiarthroses. - A diarthrosis - is a freely mobile joint. All synovial joints are diarthroses. - The motion permitted at a joint ranges - from no movement - such as where some skull bones interlock at a suture, - to extensive movement, - such as that seen at the shoulder, where the humerus articulates with the scapula. - The structure of each joint determines both its mobility and its stability. - There is an inverse relationship between mobility and stability in articulations. - The more mobile a joint, the less stable the joint. - In contrast, the less mobile the joint, the more stable the joint is. - If a joint is immobile, it has maximum stability. A person climbing a rock Description automatically generated **[Fibrous Joints ]** - Articulating bones in fibrous joints are connected by dense regular connective tissue - Fibrous joints have no joint cavity - they lack a space between the articulating bones. - Most fibrous joints are immobile or at most only slightly mobile; - their primary function is to hold together two bones. - Examples - articulations of the teeth in their sockets, - sutures between skull bones, - articulations between either the radius and ulna or the tibia and fibula. - three types of fibrous joints: gomphoses, sutures, and syndesmoses  [Gomphoses] - A gomphosis resembles a "peg in a socket." - The only gomphoses in the human body are the articulations of the roots of individual teeth with the alveolar processes (sockets) of the mandible and the maxillae. - A tooth is held firmly in place by fibrous periodontal membranes. - This joint is immobile is functionally classified as a synarthrosis. - This is why braces can be painful and take so long to correctly position the teeth - Orthodontists reposition these normally immobile joints through the use of clamps, bands, rings, and braces. - In response to the mechanical stressors, osteoblasts and osteoclasts work together to modify the alveolar process - This results in the remodeling of the joints and the slow repositioning of the teeth. [Sutures ] - fibrous joints found only between certain bones of the skull. - functionally classified as synarthroses since they are immobile joints. - have distinct, interlocking, usually irregular edges - increases their stability - decrease the number of fractures at these articulations. - Joins bones, - Permits the skull to grow (by new bone being deposited at these sutures) as the brain increases in size during childhood. - In older adults, the dense regular connective tissue in the suture becomes ossified, fusing the skull bones together. - When the bones completely fuse across the suture line, the obliterated sutures are now called synostoses [Syndesmoses] - fibrous joints in which articulating bones are joined by long strands of dense regular connective tissue only. - allow for slight mobility - they are classified functionally as amphiarthroses. - found between the radius and ulna - and between the tibia and fibula. - The shafts of the two articulating bones are bound by a broad, ligamentous sheet called an interosseous membrane (or interosseous ligament). - provides a pivot where the radius and ulna (or the tibia and fibula) can move relative to one another. **[Cartilaginous Joints ]** - have cartilage between the articulating bones. - Like fibrous joints, cartilaginous joints also lack a joint cavity. - They may be either immobile or slightly mobile. - The cartilage found between the articulating bones is either hyaline cartilage or fibrocartilage - two types of cartilaginous joints - synchondroses - symphyses A diagram of the bones of the human body Description automatically generated [Synchondroses] - An articulation in which bones are joined by hyaline cartilage is called a synchondrosis - Functionally, all synchondroses are immobile - classified functionally as synarthroses. - The hyaline cartilage of epiphyseal plates in children forms synchondroses that bind the epiphyses and diaphysis of long bones - When the hyaline cartilage stops growing, bone replaces the cartilage and a synchondrosis no longer exists - The spheno-occipital synchondrosis is found between the body of the sphenoid and the basilar part of the occipital bone. - This synchondrosis typically fuses between 18 and 25 years of age, making it a useful tool for assessing the age of the skull - Other examples of synchondroses involve costal cartilage. - The costochondral joint, the joint between each bony rib and its respective costal cartilage, is a synchondrosis. - the attachment of the first rib to the sternum by costal cartilage (called the first sternocostal joint) is another synchondrosis. - Here, the first rib and its costal cartilage are united firmly to the manubrium of the sternum to provide stability to the rib cage. - (Note that the sternocostal joints between the sternum and the costal cartilage of ribs 2--7 are synovial joints, and thus are not synchondroses.) [Symphyses] - A symphysis has a pad of fibrocartilage between the articulating bones - The fibrocartilage resists both compression and tension stresses and acts as a resilient shock absorber. - All symphyses are amphiarthroses - they allow slight mobility. - Example - pubic symphysis - located between the right and left pubic bones. - In pregnant females, the pubic symphysis becomes more mobile to allow the pelvis to change shape slightly as the fetus passes through the birth canal. - intervertebral joints, - bodies of adjacent vertebrae are both separated and united by intervertebral discs. - Individual intervertebral discs allow only slight movements between the adjacent vertebrae; however, the collective movements of all the intervertebral discs afford the spine considerable flexibility. **[Synovial Joints]** - freely mobile articulations (i.e., diarthroses). - Most of the commonly known joints in the body are synovial joints, - glenohumeral (shoulder) joint, - temporomandibular joint, - elbow joint, and - knee joint. [Distinguishing Features and Anatomy of Synovial Joints] - bones in a synovial joint are separated by a space called a joint cavity. - Functionally, all synovial joints are classified as diarthroses because all are freely mobile. - Often, the terms synovial joint and diarthrosis are equated. - All synovial joints include several basic features: - an articular capsule, - a joint cavity, - synovial fluid, - articular cartilage, - ligaments, - nerves, and blood vessels (figure 9.4).  - Each synovial joint is composed of a double-layered capsule called the articular capsule, or joint capsule. - Its outer layer is called the fibrous layer, and the inner layer is a synovial membrane (or synovium) - The fibrous layer is formed from dense connective tissue. - It strengthens the joint to prevent the bones from being pulled apart. - The synovial membrane is a specialized type of connective tissue, the cells of which help produce and secrete synovial fluid. - This membrane covers all the internal joint surfaces not covered by cartilage and lines the articular capsule. - All articulating bone surfaces in a synovial joint are covered by a thin layer of hyaline cartilage called articular cartilage. - This cartilage has numerous functions: - reduces friction in the joint during movement, acts as a cushion to absorb compression placed on the joint, and prevents damage to the articulating ends of the bones. - This special hyaline cartilage lacks a perichondrium - mature cartilage is avascular, so it does not have blood vessels to bring nutrients to and remove waste products from the cartilage. - Thus, if cartilage is damaged, its avascularity is correlated with lack of or delayed healing of the tissue. - The repetitious compression and expansion that occurs during exercise is vital to maintaining healthy articular cartilage because this action enhances its obtaining nutrition and its waste removal. - Only synovial joints house a joint cavity (or articular cavity), - a space that permits separation of the articulating bones. - The articular cartilage and synovial fluid (described next) within the joint cavity together reduce friction as bones move at a synovial joint. - Synovial fluid is a viscous, oily substance located within a synovial joint. - It is a product of both the synovial membrane cells and the filtrate formed from blood plasma. - Synovial fluid has three functions: - Lubricates. - Synovial fluid lubricates the articular cartilage on the surface of articulating bones (in the same way that oil in a car engine lubricates the moving engine parts). - Nourishes the chondrocytes. - The relatively small volume of synovial fluid must be circulated continually to provide nutrients to and remove wastes from articular cartilage's chondrocytes. - Whenever movement occurs at a synovial joint, the combined compression and re-expansion of the articular cartilage circulates the synovial fluid into and out of the cartilage matrix. - Acts as a shock absorber - Synovial fluid distributes stresses and force evenly across the articular surfaces when the pressure in the joint suddenly increases. - Ligaments - are composed of dense regular connective tissue, - connect one bone to another bone. - function to stabilize, strengthen, and reinforce most synovial joints. - Intrinsic ligaments represent thickenings of the articular capsule itself. - include extracapsular ligaments outside the articular capsule and intracapsular ligaments within the articular capsule. - Extrinsic ligaments are outside of, and physically separate from, the articular capsule. The specific intrinsic and extrinsic ligaments are specific to each type of joint (e.g., knee, shoulder). - All synovial joints have numerous blood vessels to transport oxygen and nutrients to the tissue, and to remove wastes. - They also have many sensory receptors that innervate the articular capsule and associated ligaments. - these sensory receptors include proprioceptors that detect the movement, stretch, and positioning of the joint - By monitoring stretching at a joint, the nervous system can detect changes in our posture and adjust body movements. - Joints also contain nociceptors that detect painful stimuli in the joint, which provides us with sensory input regarding possible injury to the joint - Tendons - composed of dense regular connective tissue - not part of the synovial joint itself. - attaches a muscle to a bone. - When a muscle contracts, the tendon from that muscle moves the bone to which it is attached, thus causing movement at the joint. - Tendons help stabilize joints because they pass across or around a joint to provide mechanical support, and sometimes they limit the range or amount of movement permitted at a joint. - Synovial joints usually have bursae and fat pads as accessory structures in addition to the main components just described. - A bursa is a fibrous, saclike structure that contains synovial fluid and is lined internally by a synovial membrane - There are numerous bursae in the body, and they are associated with most synovial joints and are where bones, ligaments, muscles, skin, or tendons overlie each other and rub together. - Bursae may be either connected to the joint cavity or completely separate from it. - They alleviate the friction resulting from the various body movements, such as where a tendon or ligament rubs against bone. - An elongated bursa called a tendon sheath wraps around a tendon where there may be excessive friction. - Tendon sheaths are especially common in the confined spaces of the wrist and ankle A diagram of the hand Description automatically generated  - Fat pads are often distributed along the periphery of a synovial joint. - They act as packing material and provide some protection for the joint. - Often, fat pads fill the spaces that form when bones move and the joint cavity changes shape [Classification of Synovial Joints] - there are three major anatomic planes (coronal, sagittal, and transverse). - Synovial joints are classified by the shapes of their articulating surfaces and the types of movement they allow along these planes. - Movement of a bone at a synovial joint may be described in one of three ways: - A joint is said to be uniaxial if the bone moves in just one plane or axis. - A joint is biaxial if the bone moves in two planes or axes. - A joint is multiaxial or triaxial if the bone moves in multiple planes or axes. - All synovial joints are diarthroses, as mentioned, but some are more mobile than others. - From least mobile to most freely mobile, the six specific types of synovial joints are - plane joints - hinge joints - pivot joints - condylar joints - saddle joints - ball-and-socket joints. A diagram of a person\'s body Description automatically generated - plane joint - also called a planar or gliding joint - the simplest synovial articulation and the least mobile type of diarthrosis. - a uniaxial joint because it usually allows only limited side-to-side movements in a single plane, and because there is no rotational or angular movement with this joint. - The articular surfaces of the bones are flat, or planar. - Examples of plane joints include the intercarpal and intertarsal joints (the joints between the carpal bones and tarsal bones, respectively). - hinge joint - formed by the convex surface of one articulating bone fitting into a concave depression on the other bone in the joint. - Movement is confined to a single axis, like the movement seen at the hinge of a door - so a hinge joint is considered a uniaxial joint. - An example is the elbow joint. - The trochlear notch of the ulna fits directly into the trochlea of the humerus, so the forearm can be moved only anteriorly toward the arm or posteriorly away from the arm. - Other hinge joints occur in the knee and the finger (interphalangeal \[IP\]) joints. - pivot joint - uniaxial joint in which one articulating bone with a rounded surface fits into a ring formed by a ligament and another bone. - The first bone rotates on its longitudinal axis relative to the second bone. - An example is the proximal radioulnar joint, where the rounded head of the radius pivots along the ulna and permits the radius to rotate. - Another example is the atlantoaxial joint between the first two cervical vertebrae (i.e., the atlas and axis). - The rounded dens of the axis fits snugly against an articular facet on the anterior arch of the atlas. - This joint pivots when you shake your head "no." - Condylar joints - also called condyloid or ellipsoid joints - are biaxial joints with an oval, convex surface on one bone that articulates with a concave articular surface on the second bone of the joint. - Biaxial joints can move in two axes, such as back-and forth and side to side. - Examples - metacarpophalangeal (MP) joints of fingers 2 through 5. - The MP joints are commonly referred to as knuckles. - Examine your hand and look at the movements along the MP joints; you can flex and extend the fingers at this joint, which is one axis of movement. - You also can move your fingers apart from one another and move them closer together, which is the second axis of movement. - A saddle joint - named because the articular surfaces of the bones have convex and concave regions that resemble the shape of a saddle. - This biaxial joint allows a greater range of movement than either a condylar or hinge joint. - The carpometacarpal joint of the thumb (between the trapezium, which is a carpal bone, and the first metacarpal) is an example of a saddle joint. - This joint permits the thumb to move toward the other fingers so that we can grasp objects. - Ball-and-socket joints - multiaxial joints in which the spherical articulating head of one bone fits into the rounded, cuplike socket of a second bone. - Examples - coxal (hip) - glenohumeral (shoulder) joints. - The multiaxial nature of these joints permits movement in three planes. - The ball-and-socket joint is considered the most freely mobile type of synovial joint. **[The Movement of Synovial Joints]** Four types of motion occur at synovial joints: gliding motion, angular motion, rotational motion, and special movements (motions that occur only at specific joints)  [Gliding Motion] - Gliding - a simple movement in which two opposing surfaces slide slightly back-and-forth or side-to-side with respect to one another. - the angle between the bones does not change, and only limited movement is possible in any direction. - typically occurs along plane joints, such as between the carpals or the tarsals. [Angular Motion] - either decreases or increases the angle between two bones. - These movements may occur at many of the synovial joints. - They include the following specific types: - flexion and extension, - lateral flexion, - abduction and adduction, - circumduction A hand pointing at a whiteboard Description automatically generated - Flexion - movement in an anteriorposterior (AP) plane of the body that decreases the angle between the bones. - Bones are brought closer together as the angle between them decreases. - Examples - the bending of the fingers toward the palm to make a fist - the bending of the forearm toward the arm at the elbow - flexion at the shoulder when the arm is raised anteriorly - flexion of the neck when the head is bent anteriorly and you look down at your feet. - Extension - The opposite of flexion - movement in an anterior-posterior (AP) plane that increases the angle between the articulating bones. - Extension is a straightening action that occurs in an AP plane. - Straightening the fingers after making a clenched fist - straightening the forearm until it projects directly away from the anterior side of your body - Hyperextension - the extension of a joint beyond its normal range of motion. - may occur if someone has extensively mobile joints or an injury at the joint. - Lateral flexion - occurs when the trunk of the body moves in a coronal plane laterally away from the body. - occurs primarily between the vertebrae in the cervical and lumbar regions of the vertebral column - Abduction - means to move away, - the lateral movement of a body part away from the body midline. - occurs when either the arm or the thigh is moved laterally away from the body midline. - Abduction of either the fingers or the toes means that you spread them apart, away from the longest digit that acts as the midline. - Abducting the wrist (also known as radial deviation) involves pointing the hand and fingers laterally, away from the body. - Adduction - The opposite of abduction - means to move toward. - This is the medial movement of a body part toward the body midline. - Adduction occurs when the raised arm or thigh is brought back toward the body midline, or in the case of the digits, toward the midline of the hand. - Adducting the wrist (also known as ulnar deviation) involves pointing the hand and fingers medially, toward the body.  - Circumduction - a sequence of movements in which the proximal end of an appendage remains relatively stationary while the distal end makes a circular motion - The resulting movement makes an imaginary cone shape. - This is demonstrated when you draw a circle on the blackboard. - The shoulder remains stationary while your hand moves. - The tip of the imaginary cone is the stationary shoulder, while the rounded "base" of the cone is the circle made by the hand. - Circumduction is a complex movement that occurs as a result of a continuous sequence of flexion, abduction, extension, and adduction A person in a swimsuit and a black swimsuit Description automatically generated [Rotational Motion] - Rotation - A pivoting motion in which a bone turns on its own longitudinal axis - Rotational movement occurs at the atlantoaxial joint, which pivots when you rotate your head to gesture "no." - Some limb rotations are described as either away from the median plane or toward it. - For example, lateral rotation (or external rotation) turns the anterior surface of the femur or humerus laterally, - medial rotation (or internal rotation) turns the anterior surface of the femur or humerus medially.  - Pronation - the medial rotation of the forearm so that the palm of the hand is directed posteriorly or inferiorly. - The radius and ulna are crossed to form an X - Supination occurs when the forearm rotates laterally so that the palm faces anteriorly or superiorly. - In the anatomic position, the forearm is supinated. [9.5d Special Movements] - Some movements occur only at specific joints and do not readily fit into any of the functional categories previously discussed. - These special movements include - depression and elevation, - dorsiflexion and plantar flexion, - eversion and inversion, - protraction and retraction, - opposition and reposition. - Depression is the inferior movement of a part of the body. - Examples include - opening your mouth (by depressing your mandible) to chew food and the movement of your shoulders in an inferior direction. - Elevation is the superior movement of a body part. - Examples of elevation include - the superior movement of the mandible while closing the mouth and the movement of the shoulders in a superior direction (shrugging your shoulders). A close-up of a foot Description automatically generated **Dorsiflexion and plantar flexion are limited to the ankle joint** - Dorsiflexion - occurs when the talocrural (ankle) joint is bent such that the dorsum (superior surface) of the foot and the toes moves toward the leg. - This movement occurs when you dig in your heels, and it prevents your toes from scraping the ground when you take a step. - Plantar flexion - a movement of the foot at the talocrural joint so that the toes point inferiorly. - When a ballerina is standing on tiptoes, the ankle joint is in full plantar flexion. **Eversion and inversion are movements that occur at the intertarsal joints of the foot only** - Eversion - the sole of the foot turns to face laterally or outward, - inversion - the sole of the foot turns medially or inward during inversion - (eversion is foot pronation, whereas inversion is foot supination.) - Protraction - the anterior movement of a body part from anatomic position, as when jutting your jaw anteriorly at the temporomandibular joint or hunching the shoulders anteriorly by crossing the arms. - In the latter case, the clavicles move anteriorly due to movement at both the acromioclavicular and sternoclavicular joints. - Retraction - the posteriorly directed movement of a body part from the anatomic position. - opposition - At the carpometacarpal joint, the thumb moves toward the palmar tips of the fingers as it crosses the palm of the hand. - It enables the hand to grasp objects and is the most distinctive digital movement in humans. - The opposite movement is called reposition. **[Synovial Joints and Levers]** - When analyzing synovial joint movement and muscle contraction, anatomists often compare the movement to the mechanics of a lever; - this practice of applying mechanical principles to biology is known as biomechanics. [Terminology of Levers] - Lever - an elongated, rigid object that rotates around a fixed point called the fulcrum - A seesaw is a familiar example of a lever. - Levers have the ability to alter the speed and distance of movement produced by a force, the direction of an applied force, and the force strength. - Movement occurs when an effort applied to one point on the lever exceeds a resistance located at some other point. - The part of a lever from the fulcrum to the point of effort is called the effort arm, and the lever part from the fulcrum to the point of resistance is the resistance arm. In the body, a long bone acts as a lever, a joint serves as the fulcrum, and the effort is generated by a muscle attached to the bone. [Types of Levers] - Three classes of levers are found in the human body: - first-class - second-class - and third-class  A diagram of a wheelbarrow and a wheelbarrow Description automatically generated  - First-Class Levers - has a fulcrum in the middle, between the effort (force) and the resistance. - An example of a first-class lever is a pair of scissors. - The effort is applied to the handle of the scissors while the resistance is at the cutting end of the scissors. - The fulcrum (pivot for movement) is along the middle of the scissors, between the handle and the cutting ends. - In the body, an example of a first-class lever is the atlanto-occipital joint of the neck - the muscles on the posterior side of the neck (effort) pull inferiorly on the nuchal lines of the skull and oppose the tendency of the head (resistance) to tip anteriorly. - Second-Class Levers - The resistance in a second-class lever is between the fulcrum and the applied effort. - A common example of this type of lever is lifting the handles of a wheelbarrow, allowing it to pivot on its wheel at the opposite end and lift a load in the middle. - The load weight is the resistance, and the upward lift on the handle is the effort. - A small force can balance a larger weight in this type of lever, because the effort is always farther from the fulcrum than the resistance. - Second-class levers are rare in the body, but one example occurs when the foot is plantar flexed so that a person can stand on tiptoe. - The contraction of the calf muscle causes a pull superiorly by the calcaneal tendon attached to the heel (calcaneus). - Third-Class Levers - A third-class lever is noted when the effort is applied between the resistance and the fulcrum, as when picking up a small object with a pair of forceps. - Third-class levers are the most common levers in the body. - A third-class lever is found at the elbow where the fulcrum is the joint between the humerus and ulna, the effort is applied by the biceps brachii muscle at its attachment to the radius, and the resistance is provided by any weight in the hand or by the weight of the forearm itself. - The mandible acts as a third-class lever when you bite with your incisors on a piece of food. - The temporomandibular joint is the fulcrum, and the temporalis muscle exerts the effort, whereas the resistance is the item of food being bitten **[Features and Anatomy of Selected Joints]** - Both the axial skeleton and appendicular skeleton exhibit many more joints than are individually discussed here. A chart with text and images Description automatically generated with medium confidence [Temporomandibular Joint] - The temporomandibular joint (TMJ) is the articulation formed at the point where the head of the mandible articulates with the temporal bone - specifically, the articular tubercle of the temporal bone anteriorly and the mandibular fossa posteriorly. - This small, complex articulation is the only mobile joint between bones in the skull  - The temporomandibular joint has several unique anatomic features. - A loose articular capsule surrounds the joint and promotes an extensive range of motion. - It contains an articular disc, which is a thick pad of fibrocartilage separating the articulating bones and extending horizontally to divide the synovial cavity into two separate chambers. - As a result, the TMJ is really two synovial joints---one between the temporal bone and the articular disc, and a second between the articular disc and the mandible. - Several ligaments support the TMJ. - The sphenomandibular ligament (an extracapsular ligament) is a thin band that extends anteriorly and inferiorly from the sphenoid to the medial surface of the mandibular ramus. - the temporomandibular ligament (or lateral ligament) is composed of two short bands that extend inferiorly and posteriorly from the articular tubercle of the temporal bone to the mandible. - The temporomandibular joint functions as a hinge during mandibular depression and elevation while chewing. - It also glides slightly forward during protraction of the mandible for biting, and glides slightly from side to side to grind food between the teeth during chewing. [Shoulder Joint] - The joints associated with movement at the shoulder include the - sternoclavicular joint, - acromioclavicular joint, - glenohumeral joint. A screenshot of a medical chart Description automatically generated - Sternoclavicular Joint - a saddle joint formed by the articulation between the manubrium of the sternum and the sternal end of the clavicle - A fibrocartilaginous articular disc partitions the sternoclavicular joint into two parts and forms two separate synovial cavities. - As a result, a wide range of movement is possible, including depression, elevation, and circumduction of the clavicle at this joint.  - Support and stability are provided to this articulation by the fibers of the articular capsule and by multiple extracapsular ligaments, such as the sternoclavicular and costoclavicular ligaments. - This anatomic arrangement makes the sternoclavicular joint very stable. - If you fall on an outstretched hand so that force is applied to the joint, the clavicle will fracture before this joint dislocates. - Acromioclavicular Joint - The acromioclavicular joint is a plane joint between the acromion of the scapula and the lateral end of the clavicle - A fibrocartilaginous articular disc lies within the joint cavity between these two bones. - This joint works with both the sternoclavicular joint and the glenohumeral joint to give the upper limb a full range of movement. A diagram of the human body Description automatically generated - Several ligaments provide great stability to this joint. - The articular capsule is strengthened superiorly by an acromioclavicular ligament. - In addition, a very strong coracoclavicular ligament binds the clavicle to the coracoid process of the scapula. - If this ligament is torn, the acromion and clavicle no longer align properly - Glenohumeral (Shoulder) Joint - commonly referred to as the shoulder joint. - It is a ball-and-socket joint formed by the articulation of the head of the humerus and the glenoid cavity of the scapula - It permits the greatest range of motion of any joint in the body, and so it is both the most unstable joint in the body and the one most frequently dislocated. - The fibrocartilaginous glenoid labrum encircles and covers the surface of the glenoid cavity. - A relatively loose articular capsule attaches to the surgical neck of the humerus. The glenohumeral joint has several major ligaments. - The coracoacromial ligament extends across the space between the coracoid process and the acromion. - The large coracohumeral ligament is a thickening of the superior part of the articular capsule. - It extends from the coracoid process to the humeral head. - The glenohumeral ligaments are three thickenings of the anterior portion of the articular capsule. - These ligaments are often indistinct or absent and provide only minimal support. - In addition, the tendon of the long head of biceps brachii is within the articular capsule and helps stabilize the humeral head in the joint. - Unlike other joints in the body (where ligaments provide most of the support to a joint), the ligaments of the glenohumeral joint provide little support. - Instead, most of the glenohumeral joint's strength is due to the rotator cuff muscles surrounding it - The rotator cuff muscles (i.e., subscapularis, supraspinatus, infraspinatus, and teres minor) work as a group to hold the head of the humerus in the glenoid cavity. - The tendons of these muscles encircle the joint (except for its inferior portion) and fuse with the articular capsule. - Because the inferior portion of the joint lacks support from rotator cuff muscles, this area is weak and is the most likely site of injury. - Bursae help decrease friction at the specific places on the shoulder where both tendons and large muscles extend across the articular capsule. - The shoulder has a relatively large number of bursae. [Elbow Joint ] - The elbow joint is a hinge joint composed of two articulations - \(1) the humeroulnar joint, - where the trochlea of the humerus articulates with the trochlear notch of the ulna, and - (2) the humeroradial joint - where the capitulum of the humerus articulates with the head of the radius - Both joints are enclosed within a single articular capsule  - The elbow is an extremely stable joint for several reasons. - First, the articular capsule is relatively thick, and thus effectively protects the articulations. - Second, the bony surfaces of the humerus and ulna interlock very well, and thus provide a solid bony support. - Finally, multiple strong supporting ligaments help reinforce the articular capsule. - Because of the trade-off between stability and mobility, the elbow joint is very stable but is not as mobile as some other joints, such as the glenohumeral joint. - The elbow joint has two main supporting ligaments. - The radial collateral ligament (or lateral collateral ligament) is responsible for stabilizing the joint at its lateral surface; - it extends from the lateral epicondyle of the humerus to the head of the radius. - The ulnar collateral ligament (or medial collateral ligament) stabilizes the medial side of the joint and extends from the medial epicondyle of the humerus to both the coronoid process and the olecranon of the ulna. - This ligament may be torn with repetitive use of the joint (e.g., as seen with some baseball pitchers). - The torn ligament may be replaced with another body tendon to restore elbow function, in a procedure known as Tommy John surgery. - In addition, the elbow joint has an anular ligament that surrounds the neck of the radius and binds the proximal head of the radius to the ulna. - The anular ligament helps hold the head of the radius in place. - Despite the support from the capsule and ligaments, the elbow joint is subject to damage from severe impacts or unusual stresses. - For example, if you fall on an outstretched hand and the elbow joint is partially flexed, the posterior stress on the ulna combined with contractions of muscles that extend the elbow may break the ulna at the center of the trochlear notch. - Sometimes dislocations result from stresses to the elbow. - This is particularly true when growth is still occurring at the epiphyseal plate, so children and teenagers may be prone to humeral epicondyle dislocations or fractures. [Hip Joint] - The hip joint, also known as the coxal joint, is the articulation between the head of the femur and the relatively deep, concave acetabulum of the os coxae - A fibrocartilaginous acetabular labrum further deepens this socket. - The hip joint's more extensive bony architecture is therefore much more substantial and more stable than that of the glenohumeral joint. - Conversely, the hip joint's increased stability means that it is less mobile than the glenohumeral joint. - The hip joint must be more stable (and thus less mobile) because it supports the body weight. A collage of bones Description automatically generated  - The hip joint is secured by a strong articular capsule, several ligaments, and a number of powerful muscles. - The articular capsule extends from the acetabulum to the trochanters of the femur, enclosing both the femoral head and neck. - This arrangement prevents the head from moving away from the acetabulum. - The ligamentous fibers of the articular capsule reflect (i.e., fold over) around the neck of the femur - These reflected fibers, called retinacular fibers, provide additional stability to the capsule. - Located within the retinacular fibers are retinacular arteries (branches of the deep femoral artery), which supply almost all of the blood to the head and neck of the femur. - The articular capsule is reinforced by three spiraling intracapsular ligaments: - The iliofemoral ligament is a Y-shaped ligament that provides strong reinforcement for the anterior region of the articular capsule. - - The ischiofemoral ligament is spiral-shaped and posteriorly located. - The pubofemoral (pyū′bō-fem′ŏ-răl) ligament is a triangular thickening of the capsule's inferior region. - All of these spiraling ligaments become taut when the femur is extended at the hip joint, so the hip joint is most stable in the extended position. - Another tiny ligament, the ligament of head of femur, also called the ligamentum teres, originates along the acetabulum. - Its attachment point is the fovea of the head of the femur. - This ligament does not provide stability to the joint; rather, it typically contains a small artery that supplies the head of the femur. - The combination of a deep bony socket, a strong articular capsule, supporting ligaments, and muscular padding gives the hip joint its stability. - Movements possible at the hip joint include - Flexion - Extension - Abduction - Adduction - Circumduction - medial and lateral rotation of the femur. [Knee Joint] - The knee joint is the largest and most complex diarthrosis of the body - It is primarily a hinge joint, but when the knee is flexed, it is also capable of slight rotation and lateral gliding. - Structurally, the knee is composed of two separate articulations: - \(1) The tibiofemoral joint is between the condyles of the femur and the condyles of the tibia, - \(2) the patellofemoral joint is between the patella and the patellar surface of the femur. A diagram of the human body Description automatically generated  - - A diagram of the knee joint Description automatically generated  - The knee joint has an articular capsule that encloses only the medial, lateral, and posterior regions of the knee joint. - The articular capsule does not cover the anterior surface of the knee joint; - rather, the quadriceps femoris muscle tendon passes over the knee joint's anterior surface. - The patella is embedded within this tendon, and the patellar ligament extends beyond the patella and continues to where it attaches on the tibial tuberosity of the tibia. - Thus, there is no single unified capsule in the knee, nor is there a common joint cavity. Posteriorly, the capsule is strengthened by several popliteal ligaments - On either side of the knee joint are two collateral ligaments that become taut on extension and provide additional stability to the joint. - The fibular collateral ligament (lateral collateral ligament) reinforces the lateral surface of the joint. - This ligament extends from the femur to the fibula and prevents hyperadduction of the leg at the knee. - (In other words, it prevents the leg from moving too far medially relative to the thigh transpose punctuation). - The tibial collateral ligament (medial collateral ligament) reinforces the medial surface of the knee joint. - This ligament runs from the femur to the tibia and prevents hyperabduction of the leg at the knee. - (In other words, it prevents the leg from moving too far laterally relative to the thigh.) - This ligament also attaches to the medial meniscus of the knee joint, so an injury to the tibial collateral ligament usually affects the medial meniscus as well. - Deep to the articular capsule and within the knee joint itself are a pair of C-shaped fibrocartilage pads positioned on the condyles of the tibia. - These pads are called the medial meniscus and the lateral meniscus. - They partially stabilize the joint medially and laterally, - act as cushions between articular surfaces, - continuously change shape to conform to the articulating surfaces as the femur moves. - Two cruciate ligaments are deep to the articular capsule of the knee joint. - They limit the anterior and posterior movement of the femur on the tibia. - These ligaments cross each other in the form of an X, hence the name cruciate (which means cross). - The anterior cruciate ligament (ACL) extends from the posterior femur to the anterior side of the tibia. - When the knee is extended, the ACL is pulled tight and prevents hyperextension of the leg at the knee joint. - The ACL prevents the tibia from moving too far anteriorly relative to the femur. - The posterior cruciate ligament (PCL) attaches from the anteroinferior femur to the posterior side of the tibia. - The PCL becomes taut on flexion, and so it prevents hyperflexion of the leg at the knee joint. - The PCL also prevents posterior displacement of the tibia relative to the femur. - Humans are bipedal, meaning that we walk on two feet. - An important aspect of bipedal locomotion is the ability to "lock" the knees in the extended position and stand erect without tiring the leg muscles. - At full extension, the tibia rotates laterally so as to tighten the anterior cruciate ligament and squeeze the menisci between the tibia and femur. - Muscular contraction by the popliteus muscle unlocks and flexes the knee joint. [Talocrural (Ankle) Joint] - The talocrural (ankle) joint is a highly modified hinge joint