Bone Tissue Overview

• Bone is dynamic and changes throughout life, containing cells and a calcified extracellular matrix.
• Bone adapts to usage, thickening or thinning in response to external forces.
• Bones undergo remodeling and can change shape to adapt to its use.
• Bones self-repair in response to trauma.

Bone Composition

• Bone tissue, unlike dental enamel, provides a hard, connective tissue that supports organs, maintains body shape, and connects body parts.
• Extracellular matrix is comprised of 33% organic and 67% inorganic compounds.
• The organic component features collagen for flexibility and tension resistance, as well as proteoglycans.
• Collagen absence leads to brittle bones.
• The inorganic part, made of hydroxyapatite and mineral salts, provides hardness and compression resistance.
• Absence of inorganic compounds results in extremely flexible bones.
• Bone cells constitute 2% of bone weight and facilitate adaptation to the environment.

Bone Cells

• There are four types of bone cells which allow bones to adapt
• Osteogenic cells: Stem cells that become other cells, typically osteoblasts
• Osteoblasts: Produce new bone matrix
• Osteocytes: Mature bone cells that communicate, recycle proteins and minerals within the bone matrix
• Osteoclasts: Remove bone matrix
• These cells maintain bone homeostasis, balancing bone destruction and formation.

Bone Structure

• Compact and cancellous bone tissues share the same material ratio but have different structures.
• Compact bones feature an osteon structure (ring of bone).
• Cancellous bones have a trabecular structure (thin bone structures).

Compact Bone

• Compact bone is dense with few gaps, except for foramina allowing blood vessels to supply nutrients to cells.
• Macroscopically, the outer surfaces are dense and impenetrable with periosteum and foramina for blood supply.
• Microscopically, compact bone consists of circumferential lamellae and osteons.

Osteons

• Osteons, lengthwise units in compact bone, provide pathways for nutrients to reach cells in the ECM.
• Osteons consist of a central canal, lamellae, lacunae, and canaliculi.
• The central canal contains blood vessels and nerves.
• Lamellae are cylinders of ECM around the central canal that shape osteons.
• Collagen fibers within lamellae resist forces.
• Lacunae are 'lakes' housing osteocytes, while canaliculi are channels for nutrient transport to osteocytes through the ECM.

Cancellous Bone

• Cancellous bone has a trabeculae structure with criss-crossing lamella bones.
• Marrow fills cavities between trabeculae.
• Nutrients diffuse from marrow, as there is no central canal.
• Osteocytes reside in lacunae between lamellae or on the surface.
• Trabeculae organization resists force from multiple directions.
• Trabeculae can direct body weight force down the shaft, and bone density allows force to spread out distally.
• In the upper and lower body the trabeculae channel weight around the ilia into the femora.

Bone Remodelling and Homeostasis

• Bone remodelling occurs as osteoblasts add bone matrix in lamellae.
• Simultaneously, osteoclasts remove bone from the medullary cavity.
• Long bones widen with age to support increasing weight and stress.
• Bone homeostasis balances osteoblastic (OB) and osteoclastic (OC) activity, with constant bone formation and destruction.
• This process allows the body to mobilize calcium, phosphate, and other minerals from the bone matrix.
• Bones remodel, responding plastically to resist strain.

Bone Loss, Osteoporosis and Key Factors

• Failure to maintain bone homeostasis results in bone mass loss.
• This often occurs with age and greater osteoclastic activity than osteoblastic activity due to inadequate calcium, limited exercise, etc.
• Osteoporosis occurs from a clinically significant imbalance in OB/OC activity where osteoclastic activity exceeds osteoblastic activity.
• Osteoporosis results in thinning in both cancellous and compact bones, causing loss of cortical bone, thinner trabeculae, and developing holes.
• Osteoporosis impairs shock absorption and increases fracture risk.
• Biological females face a higher risk post-menopause due to lower estrogen levels.
• Lifestyle factors, such as lack of exercise, smoking, drinking, poor nutrition, and high salt intake are also contributing factors.
• Peak bone mass is reached in the 20s, and low peak mass at this age increases later risk.

Bone Growth

• Six weeks after fertilization, a cartilage model that serves as a bone blueprint exists.
• Endochondral ossification is the cartilage into bone process.
• Endochondral ossification is ongoing after birth.
• Cartilage gains calcium, which results in bone hardness.
• Blood vessels carry osteoblasts into the cartilage.
• Blood vessels enter the diaphysis of long bones, releasing osteoblasts to convert cartilage into bone.
• The diaphysis hollows out to accommodate blood vessels and marrow.
• The diaphysis is the primary ossification center, while the epiphyses remain cartilage.
• The epiphyses, secondary ossification centers, undergo the same ossification process as the primary centers.
• The epiphyseal plate (growth plate), made of cartilage, separates the epiphyses from the diaphysis.
• Epiphyseal plates facilitate lengthwise bone growth.
• They are 'growth plates' made of cartilage that allow bones to lengthen.
• This begins at puberty, when epiphyses start to fuse with diaphyses.
• Widthwise bone growth is proportional to lengthwise growth (appositional).
• Osteoblast activity produces circumferential lamellae, while osteoblasts mold the bone shape to form a medullary cavity to prevent bones from getting too heavy.

Joint Tissues

• Bones articulate at joints, they connect bones, involve bone ends, soft tissues such as cartilage and DFCT and allow controlled movement
• Key soft tissues in joints consist of cartilage and dense fibrous connective tissue (DFCT).
• The two types of cartilage are hyaline and fibrocartilage.
• Cartilage is formed by chondrocytes in lacuna, with collagen fibers in a jelly-like matrix, there are no blood vessels
• Since there are no blood vessels, nutrients are diffused through the matrix by joint loading.
• Hyaline contains a lot of ground substance making it good at trapping water resisting compression.
• Hyaline doesn’t have many collagen fibers
• Hyaline molds to bone surfaces creating a smooth surface and frictionless movement but it will degrade with age.
• Fibrocartilage has a lot of collagen fibers in bundles, and less ground substance than hyaline.
• Fibrocartilage is good at resisting compression and tension.
• The arrangement of cartilage buffers/absorbs shock by spreading force
• Fibrocartilage is usefull at joints that experience both compression and tension (eg. knee joint experiences compression from body weight and tension from walking)and it will deepen articular surfaces.
• DFCT is made of fibroblasts which then make collagen and elastin fibers
• DFCT is tightly packed, slow to heal and has not many nutrients or water
• DFCT is good at resisting tension.
• DFCT consists of ligaments (bone-bone connections), tendons (muscle-bone-connections), and joint capsules.
• Ligaments have collagen and elastin resist tension while allowing a little stretch and recoil.
• Ligaments pull tight and restrict movement when a bone tries to move incorrectly.
• Tendons have more collagen than elastin, which facilitates and controls movement and transmits the contraction of muscle.
• Bone congruence is the sum of bone surfaces that form articulation. Less congruence means more soft tissue support is needed. More congruence results in a higher surface area contact.

Joint Classifications

• There are three types of joints: fibrous, cartilaginous, and synovial.
• Fibrous joints (eg. skull) have the least movement.
• Cartilaginous joints have some movement.
• Synovial joints (appendicular skeleton) have the most movement.
• Fibrous joints consist of dense fibrous connective tissue ligaments.
• Fibrous joints limits movement and provides stability.
• Cranial sutures are an example of a fibrous joint protecting the brain.
• The distal tibiofibular joint provides stability for the ankle and prevents rotational movements at the ankle joint.
• Cartilaginous joints consist of fibrocartilage tissueand vary in structure
• Cartilaginous joint functions to provide a little bit of movement.
• Bones are connected entirely by fibrocartilage in cartilaginous joints.
• The intervertebral disc needs limited movement to minimise damage, and the pubic symphysis needs movement to shift weight while walking
• Synovial joints consist of many tissues allowing lots of movement.

Joints in the Musculoskeletal System

• There are three joint classifications within the musculoskeletal system which are fibrous, cartilaginous, and synovial, and each have different functions
• Fibrous joints have the least amount of movement and are are made out of ligaments that hold bones together (eg. cranial structures)
• Cartilaginous joints have some movement and are made out of fibrocartilage that holds bones together (eg. pubic symphysis)
• Synovial joints have the most amount of movement.

Structure of Synovial Joints and Movement

• Synovial joints are free-moving, and most limb joints are formed of this.
• The amount of movement, and direction of movement, is determined by the structure of these joints, which is a complex association of tissues and structures.
• Synovial joints facilitate free movement and control of movement.
• Bone ends (shape) determine the range of motion at a joint.
• Every synovial joint connects two bones, has smooth articular cartilage, a joint capsule, a joint cavity and a synovial membrane.
• Articular cartilage covers the bone ends to allow smooth, frictionless movement.
• The joint capsule encapsulates the joint and is made of DFCT which holds the joint together.
• The synovial membrane secretes the synovial fluid for joint lubrication and getting nutrients.
• These capsules help hold bone together, and change structure to change function.

Capsular Ligaments

• Capsular ligaments are part of some joint capsules and intracapsular ligaments are additional bands of DCFT inside joints.
• Medial collateral ligament (MCL) and lateral collateral ligament (LCL) in the knee (capsular ligaments).
• The MCL connects the femur to the tibia and it restricts against abduction
• The LCL connects the femur to the fibula and it restricts against adduction
• The anterior cruciate ligament (ACL), and posterior cruciate ligament (PCL) are intracapsular ligaments of the knee.
• The ACL is connected from the anterior of the tibia to the posterior of the femur, restricting against posterior/backward displacement of the femur.
• The PCL is connected from the posterior of the tibia to the anterior of the femur, restricting against anterior/forward displacement of the femur.
• Fibrocartilaginous pads provide cushioning/shock absorption inside some synovial joints (eg. menisci/meniscus in the knee).

Shapes and Movements of Synovial Joints

• Joints trade off between stability and mobility, with synovial joints having high mobility at the expense of stability.
• Range of motion is determined by bone end shape, ligament location/length, body surface contact, and muscles.
• Synovial joints move in planes/axis, with the axis are opposite direction to a plane.
• The uniaxial joint movement (one axis), biaxial (two axes), and multiaxial (three or more axes).
• The plane joint is multiaxial and is used in flat articular surfaces to promote sliding and gliding intercarpal/intertarsal joints.
• The hinge joint is uniaxial (backward and forward motion), where movements allowed are flexion and extension (ankle, elbow, and, interphalangeal joints).
• The pivot joint is also uniaxial (in the transverse plane), movement allowed is rotational (pronation/supination) radioulnar joints, and the C1,C2 vertebrae.
• The condylar joint is biaxial (in the sagittal plane and transverse plane), where rotation (when flexed) is enabled, both the knee and temporomandibular joint.
• The ellipsoid joint is biaxial, enabling both flexion/extension, and abduction/adduction, the radiocarpal joint (wrist joint).
• The saddle joint is able to do flexion/extension and abduction/adduction.
• The ball and socket joint is multiaxial and enables flexion/extension, abduction/adduction and rotation, the shoulder and hip are both examples of this.

Diffusion and Osmosis

• Diffusion is the movement of molecules from a high to low concentration down the concentration gradient to equalize concentrations.
• Osmosis is the movement of water across a semi-permeable membrane to equalize solute concentration.
• Cell membrane divides our body fluid volumes into intracellular and extracellular spaces.
• Osmosis has to balance the tonicity between the intracellular and extracellular spaces.

Tonicity

• The ECF and ICF are in balance when the cell is isotonic
• When the concentration is higher in the ECF than in the ICF the solution is hypertonic with respect to the ICF
• Osmotic water shift from the ICF into the ECF therefore decreases the volume of ICM
• When the concentration is lower in the ECF than in the ICF the solution is hypertonic with respect to the ICF
• Osmotic water shift from the ECF into the ICF therefore decreases the volume of ECM

Chemical and Electrical Gradients

• Uneven distributions of molecules result in chemical gradients.
• Uneven distributions of charges across the membrane creates an 'electrical gradient'.
• Cationswant stay in the ECM and anions want to be in the ICM
• Ions are driven to move to equilibrate.

Membrane and Ion Channels

• The membrane is semi-permeable to ions due to channels that'll open and rush ions in/out along their gradient through the use of passive transport, with the use of passive ion channels.
• The sodium-potassium exchange pump maintains chemical & electrical gradients using ATP to move molecules, which moves sodium out while bringing potassium in.
• Intracellular space in the cell has a more negative charge which creates an electrical gradient when it is rest to create a specific set point and resting membrane potential.

Depolarization and Repolarization

• Excitable cells use ion movement as signals leading to depolarization and then repolarization
• Chemical stimulus will open sodium channels before sodium ions are transported out of the cytosol again when the stimulus is removed

Three Main Types of Muscle

• All types of muscless are made of fibers and their primary function is to generate force via contraction.
• Smooth muscles which line hollow organs and gut, aren't consciously controlled
• Cardiac muscles which generate force to pump blood, aren't consciously controlled
• Skeletal muscles which apply force to control posture and body movements (mostly)is controlled consciously

Key Elements of Skeletal Muscle Structure

• The primary job of skeletal muscle is to develop force via consciously shortening or contraction
• SKeletal muscles are important for movement and posture
• The secondary jobs of skeletal muscle include provide support/protection, voluntary control of major openings, and conversion and conversion of energy to provide support and protection for soft tissue organs
• Individual muscle cells bundle intop facicles, which bundle into muscles so that they are aligned in the direction they'rea re orientated in, in order to maximise force.
• Fibres, fascicles, and muscles contain many blood vessels and nerves
• Muscle fibers have many more nuclei than other cells in the body
• Skeletal muscles are sheathed in connective tissue, which is gathered to form tendons which lock is locked in connection with the bone.
• Muscle fibers are comprised of bundles of myofibrils, of actin and myosin which gives it striated (striped) appearance.

Sequence of Events that Occur During Skeletal Muscle Excitation

• Muscle fibers possess regulatory structures
• The muscle fibre is lined by a semi-permeable cell membrane called Sarcolemma and has transverse Tubules alongside the sarcoplasmic Reticulum to conduct eletrical signals
• Transverse Tubules are like extensions invaginating that signal to the muscle, while the signal for muscle signals is sent through the sarcolemma
• The sarcoplasmic Recticulum takes and storess calcium to increase concenctration when the muscle contracts

Excitation-Contraction Coupling (EC Coupled)

• Occurs when the signal coming from the nerves is activated by the moving signal which is then intracted nearby causing them to open
• Once the RyR opens calcium poured from the SR into the cell which then activated the myofilaments
• Calcium is then puumped back through SERCA and stops the signalling event with the help of the voltage gated sensor DHPR and RyR.

Cross-Bridge Cycle

• Myofilliaments contain actin(thin) and myosin(thick) which require a signal to act as molecules
• When calcium attaches it allows the actin and myosin to interact so that they can bind together allowing contraction to trigger with it's 5 stage cycle: attached, released, cocked, cross-bridge, power-stroke.
• With Myosin head preparation ATP will bind to preperation to allow energy that causes the release of the actin-myosin complex through dissociation

Determinants of Skeletal Muscle Force Generation

• Muscle tension and force depend on the number of mucle fiberes and the rate at which msucle is stimualted.
• The number of fibers is refulated by how many neurons there are active at one time througha brina "catalog" that reuires to be activated/ reuited in the brain depending on how many are active with different tasks.
• Single Nerve actions results in a pule of of energy and tensio development
• Eventually frequence increases to where there is no no power/ force, and more calcium being used.

Muscle Length/ Strength

• Each muscle is optimal in it's strenhgth,and weaker when over or under the length as that is their changing the overlap between actin and myosin.
• In stretching less connections are made making it harder to reach bridge force
• Muscle slacking is easily contracted with contraction

Fast/Slow Muscle Fibers

• Fast fibers fatigue easily and provide force, while slow fiber provides steady rat and is slow in strength.
• They both consist a large # of fiberd and mitocondrioa of differing tensions that are fast or longer.

Movements at Synovial Joints

• Skeletal muscle is attached to bone that acts as the lever, which affect how many roles it has in the way it
• The bond acts a lever (movemnts) and muscles contract around the fixed points to make to load
• With 2 classes being more effective in overcoming heavy points

Muscle Form and Function

• Muscle form determines its dependance on the length or muscle fibers and it's the the number of and arrangement.
• Fibers of longer lengh can shoten to 50%

• of muscle is proportional number force is called number of fibers

• Fiber has 2 types = parallel vs pennate
• Parallel has smalles CSA, and less strenth
• Pennate has greatCSA and more strenth
• There are thee forms of it (3+ = more/ most strength)

3 Types of Contractions

• Muscle fiber has 3 cotnractions which are Concentric, eccentric or isometric contractions
• Concentric contractions has tension that is greater load that results in changed joint positions.
• Eccetric cotnractions are more stronger than the load with the muscle elongating and pullling the opposite way

Role of Actions and Actions in Actions

• Acions have various controls that oppose and create movements
• Agonist creates with concentric actions that causes constant occurances
• Anthagonis opposed this movement