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The Muscular System
- Voluntary movement (innervated by somatic system)
- Appears as striped or striated since the actin and myosin are arranged in repeating units called sarcomeres.
- Multinucleated since individual muscle cells are fused into long rods
- Myoglobin: Oxygen carrier that uses iron in heme group to bind oxygen. Red colour
- Red Fibers or Slow-Twitch Fibers: high myoglobin and get energy aerobically
- Contain lots of mitochondria to carry out oxidative phosphorylation
- White Fibers or Fast-Twitch Fibers: less myoglobin, so there is a lighter colour.
- Both fibers mixed in a specific muscle, but if muscle is meant to contract slowly then it contains more red fibers. Muscles that contract rapidly and fatigue quickly contain mostly white fibers
- Involuntary action and controlled by the autonomic nervous system.
- Found in respiratory tree, digestive tract, bladder, uterus, blood vessel walls, etc.
- Actin and Myosin fibers are not well organized so no striations are seen.
- Capable of more sustained contraction as compared to skeletal muscles.
- Tonus: is a constant state of low muscle contraction.
- Myogenic Activity: smooth muscle contracts without input from nervous system.
- Contract directly in response to stretch or stimuli.
- Both skeletal and smooth muscle characteristics. Has involuntary control, but are striated
- Primarily have one nucleus
- Muscle cells are connected by intercalated discs that contain gap junctions. These are connections between the cytoplasm of adjacent cells and allows for the flow of ions directly between cells.
- Allow for more rapid and coordinated muscle cell depolarization and efficient contraction
- Are able to define and maintain their own rhythm through myogenic activity.
- Sinoatrial Node is where depolarization starts, goes to Atrioventricular nodes. Then spreads to bundle of His and then to the Purkinje Fibers.
- This happens automatically, but nervous and endocrine system can also alter heart rate
- Vagus nerve provides parasympathetic innervation in order to slow down heart rate.
- NE from the sympathetic neuron or epinephrine from the adrenal medulla can bind to the adrenergic receptors of the heart in order to increase its rate.
Microscopic Structure of Skeletal Muscle
- Basic contractile unit of skeletal muscle and is made up of thick and thin filaments
- Thick Filaments: organized bundles of myosin
- Thin Filaments: made up of actin and troponin/tropomyosin (proteins)
- These proteins help regulate the interaction between actin and myosin
- Titin acts as a spring and anchors the myosin and actin filaments together which prevents the excessive stretching of the muscle
- Z-lines: define the boundary of each sarcomere
- M-line: runs down the center of the sarcomere
- I-band: contain exclusively thin filaments
- H-Zone: contains only thick filaments
- A-band: contains the thick filaments and the overlap with the thin filaments
- During contraction, the distance between everything becomes smaller except for the A-Band size, which remains constant.
Gross Structure of Myocytes
- Sarcomeres are attached end-to-end to form myofibrils, which are surrounded by a sarcoplasmic reticulum (SR).
- SR is a modified endoplasmic reticulum that contains a high concentration of Ca2+
- Sarcoplasm is a modified cytoplasm located outside the SR.
- Sarcolemma is the cell membrane of the myocyte
- Capable of propagating an action potential and can distribute the action potential to all sarcomeres in a muscle using a system of transverse tubules
- T-tubules are oriented perpendicular to the myofibrils
- Each myocyte contains many myofibrils arranged in parallel and can be called a muscle fiber. Myocytes in a parallel form a muscle
- The nuclei are found at the peripheral of the cells
- Contraction starts at neuromuscular junction.
- This is where the nervous system communicates with muscles via motor neurons
- Signal travels down the neuron until it reaches the nerve terminal (synaptic bouton), where ACh is released into the synapse
- Nerve terminal also called motor end plate
- Ach causes depolarization. Each nerve terminal controls a group of myocytes, and both the nerve terminal and myocytes makes up the motor unit.
- Depolarization triggers an action potential which spreads down the sarcolemma to the T-tubules.
- T-tubules travel into the SR and Ca2+ is released.
- Calcium ions bind to a regulatory subunit in troponin which triggers a change in the tropomyosin
- Change in tropomyosin exposes the myosin-binding sites on the actin thin filament.
Shortening of Sarcomere
- Globular heads of the myosin bind to the exposed sites on the actin, which forms an actin-myosin cross bridge.
- Cross bridge allows for myosin to pull on actin. This draws the thin filaments towards the M-line, which ultimately shortens the sarcomere.
Sliding Filament Model
- Myosin carries ADP + inorganic phosphate binds to the myosin binding site. The ADP + Pi are released and this provides energy for power stroke (the sliding of actin over myosin).
- ATP binds to myosin head and it is released from the Actin.
- ATP is then hydrolyzed into ADP + Pi which subsequently recocks the myosin head
- The model is based on the repetitive binding and releasing of myosin heads on actin filaments. Which allows the thin filament to slide along the thick filament.
- Acetylcholinesterase is the enzyme that degrades Acetylcholine. This results in the termination of the signal at the neuromuscular junction and allows the sarcolemma to repolarize
- Calcium release ceases as the signal decays and the SR takes up calcium into sarcoplasm.
- SR is responsible for the control of calcium ions in order to only allow contraction when it is necessary.
- ATP binds to myosin head, which allows Sarcomeres to relax. And tropomyosin covers the myosin-binding sites.
Stimulation, Summation and Muscle Fatigue
- Strength of a response depends on the number of motor units that are recruited to respond. The strength of a response form one muscle cell cannot be changed.
- Response of a single muscle fiber to a brief stimulus.
- Latent period: time between reaching threshold and the onset of contraction.
- P spreads along muscle and allows for calcium to be released from the SR.
- Muscle then contracts and relaxes if calcium is present.
Summation and Tetanus
- Frequency summation is the frequent and prolonged stimulation of a muscle fiber.
- Contractions will combine, become stronger and be prolonged since muscle has insufficient time to relax.
- Tetanus: is when the muscle is unable to relax because the contractions are so frequent.
Oxygen Debt and Muscle Fatigue
- Muscles require ATP to function. For aerobic metabolism, large amounts of oxygen are required to generate the large amount of ATP needed.
- Creatine Phosphate: created by transferring phosphate group from creatine during times of rest. Process can then be reversed to quickly generate ATP.
- Myoglobin: reserves are used to keep aerobic metabolism going.
- Fast-twitch muscles rely mainly on glycolysis & fermentation to make ATP
- When muscles overwhelm the body’s ability to deliver oxygen, then even red muscle fibers switch to anaerobic metabolism and produce lactic acid. This is when muscles begin to fatigue.
- Oxygen Debt: difference between amount of oxygen needed by the muscle and the actual amount present.
- Body must metabolize all lactic acid to pyruvate which requires oxygen. Amount of O2 needed is equal to the oxygen debt.
The Skeletal System
- Exoskeletal encase the whole organism and are usually found in arthropods
- Endoskeleton are internal, but cannot protect the soft tissue structure as well as exoskeletons do.
- Axial Structure is the skull, vertebral column, ribcage and hyoid bone and it provides the basic central framework for the body.
- Appendicular Skeleton: bones of the limb, pectoral girdle and pelvis.
- Connective tissue derived from embryonic mesoderm
Macroscopic Bone Structure
- Strength comes from compact bone
- Spongy/Cancellous Bone: lattice structure consists of trabeculae (bony spicules)
- Bone Marrow: cavities between trabeculae
- Red Marrow: filled with hematopoietic stem cells
- Yellow Marrow: composed mainly of fat and is inactive
- Long Bones: found in the appendicular skeleton, and are characterized by cylindrical shafts called Metaphyses swell at the end of each diaphyses. Bone ends are called epiphyses.
- Outermost portions are compact bones and internal is spongy.
- Diaphyses and metaphyses are full of bone marrow.
- Epiphyses use spongy cores to disperse the force and pressure on joints.
- Epiphyseal (growth) Plate: at the internal edge of the epiphysis and is a cartilaginous structure where longitudinal growth occurs.
- Prior to puberty, is filled with mitotic cells that contribute to growth. These plates close at the end of puberty and growth is halted.
- Periosteum: surrounds long bond to protect and acts as a site for muscle attachment.
- Needed for bone growth and repair.
- Tendons attach muscle to bone & Ligaments are bone to bone at joints
- Bone Matrix: includes organic compounds (collagen, glycoproteins, peptides) and inorganic compounds (calcium, phosphate, hydroxide ions form hydroxyapatite).
- Is where the strength of compact bones comes from
- Also stores minerals such as sodium, magnesium, potassium
- Strength comes from uniform distribution of organic and inorganic compounds
- Osteons or Haversian Systems: structural unit of bone matrix
- Lamellae: surround a central microscopic channel. These are concentric circles of bony matrix
- Haversian Canals: longitudinal channels
- Volkmann’s canals: transverse channels
- Canals contain blood vessels, nerve fibers, and lymph vessels that maintain the health of the bone.
- Lacunae: found between the lamellae and are site for mature bone cells (osteocytes).
- Canaliculi: connects the lacunae which allows for the exchange of nutrients and wastes between the osteocytes and canals.
- Osteoblasts build bone, while osteoclasts (polynucleotide macrophages) reabsorb it.
- Bone turnover/remodeling occurs in response to stress and the bone is remodeled in a way to accommodate the repetitive stresses.
- Parathyroid Hormone: peptide hormone that promotes the resorption of bone when calcium levels in the blood are low.
- Vitamin D: activated by parathyroid hormone, promotes the absorption of bone.
- Calcitonin: peptide hormone is released in response to high calcium levels. This promotes bone formation and decreases calcium levels.
- Softer and more flexible than bone. Not innervated with blood/lymph vessels – avascular
- Consists of firm and elastic matrix called the chondrin which is secreted by chondrocyte cells.
- Fetus skeletons are mainly cartilage since they need more flexibility to grow in confined environment.
- In adults, is only located in areas that need extra flexibility or cushioning (nose, ear, walls of larynx/trachea, intervertebral discs and joints)
- Endochondral Ossification: creation of bones through the hardening of cartilage. Is responsible for most of the long bones of the body
- Intramembranous Ossification: undifferentiated embryonic connective tissue (mesenchymal tissue) is transformed into bone. Occurs in the bones of the skull
Joints and Movement
- Immovable Joints: bones that are fused together to form sutures. Found mainly in the head (anchor bones of the skull together)
- Moveable Joints: Permit bones to shift relative to each other and are strengthened by ligaments (fibrous tissue that connects bones to one another).
- Synovial Capsule: encloses joint cavity
- Synovium: soft tissue that secretes synovial fluid.
- Articular Cartilage: coats the articular surfaces of the bones in order to restrict impact to lubricated join cartilage
- Origin: end of a muscle with a larger attachment to bone (usually proximal).
- Insertion: End of muscle with smaller attachment to bone (usually distal)
- Antagonistic Pairs: How the muscles usually work – one muscle relaxes while the other contracts.
- Synergistic: Muscles working together to accomplish the same function
Types of Movement
- Flexor Muscle: decreases the angle across the joint
- Extensor Muscle: Increases the angle across the joint
- Abductor: Moves body away from the midline
- Adductor: Moves body towards midline
- Medial rotator: rotates axis of limb towards the midline
- Lateral rotator: rotates axis away from midline
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