Skeletal, Muscular, and Integument Systems PDF

SECTION 2 | BIOLOGY CHAPTER 25 | THE SKELETAL SYSTEM Some animals (such as arthropods) have their skeleton on the external surface of their body (an exoskeleton), while others have it on the inside of their body (an endoskeleton) Humans have an endoskeleton. Th...

SECTION 2 | BIOLOGY CHAPTER 25 | THE SKELETAL SYSTEM Some animals (such as arthropods) have their skeleton on the external surface of their body (an exoskeleton), while others have it on the inside of their body (an endoskeleton) Humans have an endoskeleton. The bones that form our appendages (arms and legs) make up the appendicular skeleton, while all the other bones make up our axial skeleton. Note that the pelvis, scapula, and clavicle are considered part of the appendicular skeleton BONE AND CARTILAGE TISSUE HIGH-YIELD Bone consists of cells embedded in a mineralized matrix, which is made of organic (collagen, proteoglycans) and inorganic (hydroxyapatite, calcium, phosphate) components. It is a dynamic structure that is constantly remodelled by specialized cells called osteoblasts, osteocytes, and osteoclasts: Osteoblasts – Synthesize and deposit uncalci ed bone matrix called osteoid Osteocytes – Osteoblasts that become trapped in bone matrix. They are found in small hollow spaces called lacunae and have cellular extensions that tunnel through channels canaliculi to communicate and transport nutrients/waste Osteoclasts – Resorb bone matrix, increasing blood calcium levels There are two types of bone tissue: Compact/Cortical Bone Compact bone tissue is found in the cortex of all bones. It consists of columns called osteons (also known as a Haversian system). Each osteon contains a central Haversian canal with blood vessels, lymphatic vessels, and nerves. Neighbouring Haversian canals are connected via Volkmann’s canals. Each Haversian canal is surrounded by concentric layers of calci ed bone matrix called lamellae, which contain bone-producing cells called osteocytes Spongy/Cancellous/Trabecular Bone Spongy bone tissue is less dense and more vascular than compact bone. It is found at the epiphyses (ends) of long and short bones, and in the middle layer of at bones. It consists of a porous network of trabecula surrounded by bone marrow. Red bone marrow consists of hematopoietic cells and is more plentiful in children, while yellow bone marrow consists of adipocytes and is more plentiful in adults Unlike bone, cartilage is avascular and is not innervated. This is why it takes a long time for cartilage to heal – any nutrients must gain access to the cartilage through slow di usion. There are three types of cartilage tissue: Hyaline Cartilage – The most common cartilage in the body. Appears glass-like, and functions to reduce friction, provide support, and reduce shock. Hyaline cartilage found on the articular surfaces of bones is called articular cartilage. Found in the respiratory system, joints, ribs, and growth plates Elastic Cartilage – Flexible, contains elastin bres. Found in ears, epiglottis, and larynx Fibrous Cartilage – Found in high-stress regions such as ligaments, tendons, intervertebral disks, and pubic symphysis Compact Bone Spongy Bone Osteon Osteocyte Articular Cartilage Haversian Canal Canaliculi Blood Vessels Bone Marrow Epiphyseal Plate 148 􀋂 fi fi fl ff fi  SECTION 2 | BIOLOGY BONE ANATOMY There are ve major anatomical categories of bones: Long Bone – Characterized by a long shaft (diaphysis) with a rounded head at each end (epiphysis).The epiphyseal plate is where growth occurs. The metaphysis is located between the diaphysis and epiphysis. Examples of long bones include the femur, tibia, bula, radius, and ulna Short Bone – Cube-shaped appearance. Examples include the carpal bones of the wrist Flat Bone – Thin and usually curved. Examples include the sternum, scapula, and bones of the skullcap Sesamoid Bone – Embedded within tendons, functioning to increase muscle leverage. Examples include the patella (kneecap) and pisiform bone Irregular Bone – Bones that do not t into any of the above categories. Examples include the vertebrae, pelvis, ethmoid, and sphenoid bones of the skull INTRAMEMBRANOUS OSSIFICATION Intramembranous ossi cation occurs in the formation of at bones. The following steps are involved: 1. Mesenchymal connective tissue near a blood supply in the fetus di erentiates into osteoblasts 2. Osteoblasts secrete new bone matrix into the connective tissue, forming an ossi cation centre 3. Bone matrix is calci ed and develops into trabeculae, forming spongy bone. Osteoblasts are trapped in the new calci ed matrix, becoming osteocytes 4. Compact bone develops super cial to the spongy bone, and blood vessels coalesce into red bone marrow 149 fi fi fi fi fi fi fi fl ff fi  SECTION 2 | BIOLOGY ENDOCHONDRAL OSSIFICATION HIGH-YIELD Endochondral ossi cation occurs in the formation of long bones and the majority of other bone types In contrast to intramembranous ossi cation, this process contains an intermediate cartilage stage. The following steps are involved: 1. Mesenchymal connective tissue di erentiates into chondroblasts, which form a hyaline cartilage model surrounded by a perichondrium 2. Blood vessels gather to form the primary ossi cation centre at the diaphysis. A bony collar also develops from the diaphysis, replacing the perichondrium with the periosteum. Meanwhile the cartilage model grows taller (interstitial growth) and wider (appositional growth) 3. The cartilage matrix is calci ed by osteoblasts 4. Osteoclasts resorb bone from the primary ossi cation centre to form the medullary cavity 5. Secondary ossi cation centres and articular cartilage develop at the epiphyses 6. After birth, endochondral ossi cation continues at a cartilaginous region between the epiphyses and metaphyses, called the epiphyseal plate (also known as the growth plate) 7. Growth terminates when the individual reaches adulthood, and the epiphyseal plate is replaced with a calci ed epiphyseal line Secondary Ossification Centre Spongy Bone Hyaline Cartilage Model Epiphyseal Plate Primary Compact Bone Ossification Periosteum Centre Blood Supply Bone Marrow Articular Cartilage EPIPHYSEAL PLATE ZONES The epiphyseal plate consists of distinct zones that are visible under a microscope: Zone of Reserve – Located closest to the epiphyses. Consists of attened precursor chondrocytes Zone of Proliferation – Consists of actively dividing chondrocytes secreting cartilage matrix Zone of Maturation/Hypertrophy – Consists of hypertrophic (enlarged) chondrocytes Zone of Calci cation – Consists of apoptotic chondrocytes and calcifying cartilage matrix Zone of Ossi cation – Located closest to the diaphysis. Consists of osteoclasts and osteoblasts, which replace the calci ed cartilage with bone PATHOLOGIES Osteoporosis – Loss of spongy bone due to increased resorption by osteoclasts. This increases the risk of fractures from even minor accidents such as falls Osteogenesis Imperfecta – Abnormal collagen synthesis leading to brittle bones Rickets – Weakened bone in children due to vitamin D de ciency. Patients with this disease usually have legs that are bowed (bent) outwards. The equivalent disease in adults is called osteomalacia Fibrodysplasia – Fibrous tissue (muscle, tendon, and ligament) is spontaneously ossi ed when damaged, causing joints to become permanently locked in place 150 􀋂 fi fi fi fi fi fi fi fi ff fi fi fi fi fl fi  SECTION 2 | BIOLOGY CHAPTER 26 | THE MUSCULAR SYSTEM TYPES OF MUSCLE HIGH-YIELD Striations are a characteristic banding pattern visible under light microscopy due to the parallel organization of myosin and actin proteins Autorhythmic describes a cell that is capable of contracting spontaneously without stimulation There are three types of muscle tissue: Skeletal Muscle – Striated, non-autorhythmic, and multinucleate tissue under voluntary control that functions for posture, locomotion, object manipulation, and venous blood ow. Each muscle bre consists of a syncytium, a single cytoplasmic mass with several nuclei formed by the fusion of multiple cells Cardiac Muscle – Striated, autorhythmic tissue under involuntary control that forms the majority of the heart, pumping blood around the body. Each muscle bre consists of a chain of branching cardiomyocytes joined via intercalated disks Smooth Muscle – Non-striated, autorhythmic tissue under involuntary control that is found in hollow organs (i.e. stomach, intestines, bladder, uterus), ducts, and tubes. It performs various actions of the autonomic nervous system. Each muscle bre consists of a single torpedo-shaped smooth muscle cell. Smooth muscle is the only muscle type in which striations are not visible, due to its highly disordered organization of myosin and actin Type of Muscle Muscle Fiber Nuceli Morphology Control Autorhythmic Syncytium of Multinucleate Striated Voluntary No Cells Skeletal Chain of Cells Mononucleate Striated Involuntary Yes Cardiac Individual Cell Mononucleate Smooth Involuntary Yes Smooth 151 􀋂 fi fi fl fi  SECTION 2 | BIOLOGY MUSCLE ORGANIZATION HIGH-YIELD Skeletal muscles are connected to bones via tendons. Each muscle is composed of many bundles called fascicles, and is surrounded by a connective tissue layer called the epimysium. Each fascicle is composed of many muscle bres and is surrounded by its own connective tissue called the endomysium. Finally, each muscle bre is composed of many myo brils and is surrounded by a cell membrane called the sarcolemma The sarcolemma is penetrated by invaginations called transverse tubules (T-tubules), which allow rapid transmission of an action potential into the cell. The T-tubules associate closely with the terminal cisternae of the sarcoplasmic reticulum, an organelle that stores calcium ions for initiating a muscle contraction Muscle Epimysium Tendon Bone Endomysium Sarcolemma Fascicle Myofibril Muscle Fiber Each myo bril consists of repeating protein structures called sarcomeres, which are the smallest functional units of muscle. A sarcomere is composed of the following components: Actin Filament – Also known as “thin laments”. Consists of two chains of actin protein arranged in a double helix Myosin Filament – Also know as “thick laments”. Consists of two ATP-dependent myosin proteins twisted around each other, with ATP hydrolysis occurring at its two head domains Tropomyosin – Protein that twists around actin and covers the actin active sites to prevent myosin head from binding Troponin – Protein that holds tropomyosin in place. Ca2+ ions can bind to troponin and prevent it from holding tropomyosin in place Z Disk – Serves as an anchor for myosin and actin. Also delineates the borders of a sarcomere Titin – Large elastic protein attached to either end of the myosin lament, anchoring it to the Z disk and aligning it with the M line. Responsible for the elasticity of muscle MNEMONIC Titin holds myosin tight 152 fi 􀉢 􀋂 fi fi fi fi fi fi  SECTION 2 | BIOLOGY There are ve important features of a sarcomere visible under a microscope: I Band – Region containing only thin laments (actin) MNEMONIC The I band appears LIGHT under microscopy A Band – Region containing the entire length of thick laments (myosin) MNEMONIC The A band appears DARK under microscopy H Band – Region containing only thick laments (myosin) MNEMONIC The H band is composed of only THICK laments M Line – String of proteins in the middle of the A band that anchors myosin MNEMONIC The M line is in the MIDDLE Z Line – Region containing Z disks, which anchor actin and myosin. Delineates the sarcomere's borders MNEMONIC The Z line contains Z disks The only two bands that change length during a muscle contraction are the H and I bands MNEMONIC Say “HI” to muscle contraction Relaxed Sarcomere Titin Actin Myosin M Line Z Line Z Line H Band I Band A Band I Band Contracted Sarcomere M Line Z Line Z Line H Band I Band A Band I Band 153 􀉢 􀉢 􀉢 􀉢 􀉢 􀉢 fi fi fi fi fi  SECTION 2 | BIOLOGY NEUROMUSCULAR JUNCTION Skeletal muscles are able to voluntarily contract thanks to synapses between the somatic nervous system and muscles, called neuromuscular junctions. The sum of all muscle bres that are innervated by one motor neuron is called a motor unit Synaptic transmission across a neuromuscular junction occurs via the following mechanism: 1. Action potential reaches the presynaptic terminal of a motor neuron, causing the exocytosis of acetylcholine into the synaptic cleft 2. Acetylcholine binds to nicotinic receptors located at the end plate of a muscle cell. This causes voltage-gated Na+ channels to open, causing an action potential that is propagated down the T-tubules and to the sarcoplasmic reticulum 3. The sarcoplasmic reticulum releases Ca2+, which triggers muscle contraction Axon Muscle End Plate Acetylcholine Nicotinic Receptor T-Tubule MUSCLE CONTRACTION HIGH-YIELD Muscle contraction is believed to occur via the sliding lament model, in which the length of the sarcomere shortens but the length of individual bres remains the same. This occurs via the following mechanism: 1. An action potential is received from a motor neuron and spreads from the neuromuscular junction into the individual muscle cells via the T-tubules. This causes Ca2+ to be released from the sarcoplasmic reticulum 2. When the muscle is at rest, the myosin head is in a high-energy “cocked” conformation. However, a protein called tropomyosin blocks actin’s active sites, preventing myosin from binding to actin. Tropomyosin is held onto actin by another protein called troponin. When exposed to Ca2+, troponin is released, causing tropomyosin to disassociate from actin. This allows myosin to form a cross-bridge with actin 3. ADP and Pi are released from the myosin head, returning myosin into its native conformation. This power stroke pushes the actin and myosin laments in opposite directions 4. When a new molecule of ATP binds to the myosin head, the cross-bridge is broken and myosin detaches from actin. Dephosphorylation of the new ATP returns the myosin head to the high-energy “cocked” conformation. This cycle repeats until the muscle stops receiving action potentials or all ATP is exhausted Note that without a constant supply of fresh ATP, myosin is unable to dissociate from actin. This is what causes rigor mortis, the continued state of muscle contraction and rigidity after death Resting State Cross-Bridge Formation Power Stroke Return to Resting State Actin Tropomyosin Ca2+ Troponin Pi ATP ADP ADP Pi ADP ADP Pi Cross-Bridge Pi Myosin Myosin Head 154 􀋂 fi fi fi fi  SECTION 2 | BIOLOGY PATHOLOGIES Botulism – Muscle weakness and paralysis due to bacterial botulinum toxin, which inhibits the release of acetylcholine at the neuromuscular junction Tetanus – Muscle spasms and sustained contraction due to bacterial tetanus toxin, which also inhibits the release of acetylcholine at the neuromuscular junction Muscular Dystrophy – The weakening and atrophy of skeletal muscles over time 155  SECTION 2 | BIOLOGY CHAPTER 27 | THE INTEGUMENT SYSTEM FUNCTIONS OF THE INTEGUMENT Thermoregulation – Maintains body temperature homeostasis through dilation/contraction of arterioles leading to capillary beds that facilitate evaporative cooling Protection – The physical barriers of skin are part of the innate immune, and dendritic cells in the epidermis are part of the adaptive immune system Sensation – Sends a erent information about temperature, pressure, and pain to the CNS Excretion – Excess water and ions are excreted through skin Vitamin D Synthesis – UV radiation activates 7-dehydrocholesterol in the skin, a precursor to Vitamin D Blood Reservoir – Vessels in the dermis hold around 10% of the blood in a resting adult SKIN ANATOMY OVERVIEW HIGH-YIELD Skin consists of two major layers – the epidermis and dermis. The hypodermis is technically not part of the skin. It consists of adipose (fat) and connective tissue. The epidermis and dermis each have their own distinct layers. The order of all skin layers from super cial to deep is: Stratum Corneum Stratum Lucidum Stratum Granulosum Epidermis Stratum Spinosum Stratum Basale Papillary Region Dermis Reticular Region Hypodermis MNEMONIC “Come Let’s Get Sun Burnt” → Corneum Lucidum Granulosum Spinosum Basale There are two types of skin. Thick skin is found on the palms, soles of the feet, and nger tips. It has a thick epidermal layer and no hair follicles. Thin skin is found everywhere else on the body. It has a thinner epidermal layer and does contain hair. Note that hair is a unique mammalian structure that functions for thermal insulation, physical protection, and sensation EPIDERMIS The epidermis is an avascular tissue that depends on the dermis for oxygen and nutrients Important epidermal cells include: Keratinocytes – Precursors to corneocytes, which are waterproof anucleate cells that are routinely sloughed o and replaced. They contain keratin (a structural protein) and lamellar bodies (organelles that exocytose lipids, waterproo ng the epidermis) Melanocytes – Cells that produce and transport melanin pigment to keratinocytes Langerhans Cells – Resident dendritic cells of the skin. They present antigens to helper T-cells Merkel Cells – Slow-adapting mechanoreceptors with a small receptive eld; abundant in highly sensitive areas such as the skin on ngertips 156 􀉢 􀋂 ff fi ff fi fi fi fi  SECTION 2 | BIOLOGY The epidermis consist of various layers that are visually distinct under the microscope: Stratum Corneum – The outermost layer. Contains dead corneocytes, keratin protein, and lipids Stratum Lucidum – Contains additional dead corneocytes for extra protection. Exists only in thick skin Stratum Granulosum – Contains live keratinocytes migrating up from the stratum spinosum. The keratinocytes contain granules lled with keratohyalin, the precursor to keratin. Lamellar bodies are also secreted in this layer Stratum Spinosum – Contains live keratinocytes securely bound to each other via desmosomes. This layer is responsible for most of the skin’s strength and exibility Stratum Basale/Germinativum – Contains stem cells that di erentiate into keratinocytes, as well as Merkel cells and melanocytes. This layer sits right above the basal membrane, which delineates the epidermis from the dermis DERMIS The dermis is a vascular tissue that contains collagen, elastin, hair follicles, glands, adipocytes, and nerves. It contains folds called dermal papillae, which increase the strength of its attachment with the epidermis. The dermis consists of only two layers: Papillary Region – Super cial 20%. Features dermal papillae, upward projections of the dermis into the epidermis which increase surface area and create our ngerprints Reticular Region – Deep 80%. Contains dense irregular connective tissue MNEMONIC “PR” → Papillary Reticular EXOCRINE GLANDS OF THE SKIN Sebaceous Glands – Found at the base of hair follicles. Sebaceous glands produce oily sebum to lubricate and waterproof the skin and hair Sudoriferous Glands – Produce sweat. There are two types: Eccrine – Most common sweat gland, found throughout the body. Eccrine sweat glands secrete a dilute electrolyte uid over the skin to eliminate urea and regulate body temperature through perspiration Apocrine – Found only in the armpits, nipples, and public region. Apocrine sweat glands secrete viscous solutions with pheromone-like compounds. Unlike eccrine glands, they secrete directly into the hair follicle Ceruminous Glands – Found in the ear canal. Ceruminous glands produce cerumen, a wax-like material that lubricates, waterproofs, kills bacteria, and traps foreign particles Mammary Glands – Produce milk to feed newborns after pregnancy 157 􀉢 fl fi fi fl fi ff