Structure and Function of Skeletal Muscle
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101 terms
Terms | Definitions |
|---|---|
 Muscle Fiber | Individual muscle cell |
| Sarcolemma | Cytoplasm of muscle fiber |
| T-Tubules | Allow the transport of substances through the muscle fiber (in the sarcoplasm) |
| Perimysium | Separates fascia |
| Epimysium | Separates muscle fibers |
| Alpha Motor Nerve | Diverges and innervates many muscle fibers |
| Motor Unit | Structural and functional unit of a muscle contraction |
| Myofibril | -Subcellular structure of striated muscle proteins, which extend the length of the muscle fiber -Smallest functional unit of a muscle -Contains many sarcomeres |
| Sarcomere | Unit of contractile proteins within a myofibril |
| Tendon | Connects both ends of contractile proteins within a myofibril (bone's outermost covering) |
| Satellite Cells | -Myogenic stem cells locates within the sarcolemma, help regenerative cell growth -Play a role in hypertrophy |
| Chemical Composition of Muscle | -75% water -20% protein -5% salt and other substances |
| Blood Supply to Muscle | Rich vascular network and rhythmic flow (compress during contraction, open during relaxation) |
| Capillarization | -Capillary-to-muscle fiber ratio -Expedites removal of heat and metabolic by products -Increases delivery of O2, nutrients, and hormones |
| Excitability | The ability to respond to a stimulus (neurotransmitter or hormone) by generation and conduction of reversal in membrane potential |
| Contractility | Ability of muscle to contract and generate tension, at the expense of metabolic energy, when an adequate stimulus is received |
| Extensability | Ability of a muscle to be stretched |
| Elasticity | Ability of a muscle to resume resting length after being stretched or contracted |
| Cross Bridges | Projections that bridge the space between adjacent thick (myosin) and thin (actin) filaments |
| M-Line | -Narrow band in the middle of H-zone -Produced by proteins that bind all the thick (myosin) filaments in a sarcomere together -Located in the center of the sarcomere -Keeps thick (myosin) filaments stacked |
| H-Zone | -Located between the ends of the A-bands (actin and myosin interacting) of 2 adjacent sarcomeres -Contain a portion of the thin filaments that do not overlap the thick filaments -It is bisected by the Z-disk |
| I-Band | -Just thin (actin) filament -Center = Z-disk |
| Z-Disk | Keeps actin stacked |
| Thin (Actin) Filament | -Interacts with myosin during E-C coupling -The backbone of the filament |
| Tropomyosin | Transduces conformation change of troponin complex to actin -A protein which blocks attachment sites on actin |
| Troponin C | Binds to Ca2+ to produce conformational change in Troponin I |
| Troponin I | Binds to thin (actin) filament to hold the troponin-tropomyosin complex in place (inhibition) |
| Troponin T | Binds to tropomyosin to form a form a troponin-tropomyosin complex |
| Thick (Myosin) Filament | -Splits ATP and responsible for power stroke of myosin head -2 protein strands twisted together -One folded into a globular head (binding site for actin/enzymatic site [ATPase]) |
| M Protein | Helps hold thick (myosin) filaments in a regular array |
| Myomesin | Provides strong anchor point for protein titin (M-line) |
| Alpha-Actinin (α-Actinin) | Holds thin (actin) filament in place at the Z-line |
| Desminin | -Forms connection between adjacent Z-lines -Helps keep sarcomeres in line to maintain striated look |
| Titin | -Helps keep thick (myosin) filament centered between two Z-lines -Believed to control the number of myosin molecules in the filament -Serves as an adhesion template for the assembly of contractile machinery in muscle cells |
| Nebulin | -Actin-binding protein (I-band) -Acts as a filament "ruler" -Regulates thin filaments length during sarcomere assembly |
| Sarcoplasm | Fluid surrounding myofibrils |
| Skeletal Muscle | Voluntary, maintains posture, striated, could be involuntary if diseased |
| Cardiac Muscle | Controls itself, assisted from nervous and endocrine systems, involuntary |
| Smooth Muscle | Part of blood vessels, involuntary, part of digestive system |
| Fusiform | -Spindle-shaped muscle that has larger belly and fibers are arranged parallel to each other on long axis of muscle -Strap-like order provides greatest degree of shortening and enables the muscle to produce a large range of motion quickly, but it is not a very powerful type of muscle |
| Pennate | -Flet muscle with fibers around one or more central tendons (barbs on a feather) -Shorten only to limited extent, but they can produce very powerful actions |
| Synapse | Axon terminal resting in an invagination of the sarcolemma |
| Neuromuscular Junction | Presynaptic terminal, synaptic cleft, and postsynaptic membrane |
| Neurotransmitter | Substance released from a presynaptic membrane that diffuses across the synaptic cleft and stimulates (or inhibits) the production of an an action potential in the postsynaptic membrane (ex. acetylcholine) |
| Acetylcholinesterase | -A degrading enzyme in synaptic cleft -Prevents accumulation of acetylcholine |
| Sequence of Events Leading to an Action Potential | AP arrives at presynaptic terminal --> voltage-gated Ca2+ channels open --> Ca2+ enters presynaptic terminal --> initiates release of Ach from synaptic vesicles --> Ach is released into synaptic cleft via exocytosis --> Ach diffuses across synaptic cleft --> Ach binds to ligand gated Na+ channel on postsynaptic membrane --> channels open, Na+ enters --> postsynaptic cell depolarizes --> AP surpasses threshold --> unbinds from ligand gated Na+ channels (channel closes) --> Achesterase (attached to post-synaptic membrane) breaks down Ach into acetic acid and choline --> choline is symported with Na+ into presynaptic terminal (recycled), acetic acid diffuses away --> Ach is reformed within presynaptic with recycled choline and acetic acid generated from metabolism |
| Docking | -Vesicle and presynaptic membrane line up in fusion-ready state -Membranes fuse to create a small opening which grows larger until the vesicle membrane collapses into the presynaptic membrane and exocytosis occurs -This is a calcium dependent mechanism |
| G-Proteins | Guide synaptic vesicles to active zones |
| Synaptobrevin and Syntaxin | Proteins that allow the synaptic vesicle and presynaptic membrane to recognize each other in order for docking to occur |
| Following Dockin | A second influx of calcium at active zone occurs and causes vesicle membrane to fuse with the presynaptic membrane, forming a temporary ion channel (fusion pore) |
| Fusion Pore | Connects lumen to the vesicle with space outside neuronal terminal |
| Synaptotagmin | -Acts as a calcium sensor -Pulls the vesicle and presynaptic membrane into closer contact with each other or induce conformational changes in fusion proteins |
| Exocytosis | Process by which the contents of the vesicle are released into the synaptic cleft |
| Excitation-Contraction (EC) Coupling | Mechanism in which an action potential causes muscle fiber contraction |
| Process of EC Coupling | Ach binds to receptors on motor end-plate --> sarcolemma is depolarized --> depolarization travels inside muscle via T-tubules --> Ca2+ is released from terminal cisternae of SR |
| Power Stroke | Ca2+ binds to troponin resulting in conformational change in tropomyosin --> exposes active sites on actin --> Pi on myosin head released --> X-bridge movement begins --> ADP release from myosin --> X-bridge movement stops --> ATP binds to myosin head and is hydrolyzed in ADP and Pi --> myosin head energized and detaches from actin --> Ca2+ pumped back into SR --> Tropomyosin covers active binding sites on actin --> sarcomeres assume resting state |
| Medulla | Serves as a bridge between the spinal cord and pons and cerebellum |
| Midbrain | Connects pons and cerebral hemispheres |
| Cerebellum | -Monitors and coordinates areas of the brain involved in motor control -Fine tunes muscular activity |
| Diencephalon | -Contains thalamus and hypothalamus |
| Hypothalamus | -Regulates many bodily functions -Contains epithalamus and subthalamus |
| 4 Lobes of the Brain | -Frontal lobe -Parietal lobe -Temporal lobe -Occipital lobe |
| Somatic Neurons | Innervate skeletal muscle |
| Autonomic Neurons | Activate smooth muscle, cardiac muscle, sweat and salivary glands, and some endocrine glands |
| Parasympathetic Nervous System (PNS) | Affects thorax, abdomen, and pelvic regions |
| Sympathetic Nervous System (SNS) | Affects heart, smooth muscle, sweat glands, and viscera |
| Reflex Arc | Afferent neurons enter the spinal cord via dorsal root--> transmit sensory input --> synapse with interneurons --> efferent signals return via anterior motor neurons to the effector organ |
| Anterior Motor Neuron | -Consists of a cell body, axon, and dendrites -Allows transmission of an AP/electrochemical impulse from the spinal cord to the muscle -Only conducts impulses down the axon |
| Dendrites | Consist of short neural branches that receive impulses through numerous connections and conduct them toward the cell body |
| Schwann Cell | Covers the bare axon and then spirals around it |
| Neurolemma | -A thinner outermost membrane of a neuron -Covers the myelin sheath |
| Nodes of Ranvier | -Interrupt the Schwann cells and myelin every 1 or mm along the axon's length -Allows impulses to "jump" from node to node as the electrical current travels toward the terminal branches |
| Recruitment | The number of MUs active |
| Rate Coding | The frequency with which each MU fires an AP |
| Size Principle | -Orderly based upon the size of the motor neuron -Occurs in order of smallest to largest, or slowest to fastest; gamma motor neurons are fastest -Small diameter motor neurons are more easily excited by summated excitatory postsynaptic potential (EPSP) than are large diameter motor neurons -Slow oxidative MU are recruited by relatively low levels of activation |
| Isometric Contraction | -No changes in length but tention increases -Postural muscles of body |
| Isotonic Contraction | -Change in length but tension is constant -Either concentric or eccentric contraction |
| Concentric Contraction | Overcomes opposing resistance and muscle shortens |
| Eccentric Contraction | Tension maintained by muscle lengthens |
| Isokinetic | Refers to a special type of contraction where the velocity of muscle shortening remains constant |
| Muscle Twitch | Muscle contraction to a stimulus that causes action potential in one or more muscle fibers |
| Peak Tension (PT) | Highest amount of tension during a muscle twitch |
| Time to Peak Tension (TPT) | Time it takes to reach the highest amount of tension during a muscle twitch |
| Half Relaxation Time (HRT) | Time it takes to reach half of the PT from the PT |
| Contraction Duration (CD) | Time it takes to for the twitch to progress from the beginning of the contraction to the half relaxation time (TPT + HRT) |
| All-or-None Principle | -Either the AP fires or not -Must overcome threshold |
| Sub-Threshold Stimulus | No AP --> no motor units respond |
| Threshold Stimulus | -AP --> one motor unit responds |
| Submaximal Stimulus | -AP --> increasing number of motor units respond |
| Maximal/Supramaximal Stimulus | -AP --> All motor units respond |
| Multiple MU Summation | Strength of contraction depends upon recruitment of motor units |
| Fused/Complete Tetanus | -When APs are close enough together, successive twitch forces add directly to the peak force of the preceding action potential -The rise in force is smooth, the component twitches are not discernible |
| Incomplete Tetanus | Muscle fibers partially relax between contraction |
| Multiple Wave Summation | Muscle tension increases as contraction frequencies increase |
| Active Tension | Force applied to an objet to be lifted when a muscle contracts |
| Stretched Muscle | Not enough cross bridging in a muscle |
| Crumpled Muscle | Myofilaments "crumpled", X-bridges can't contract |
| Passive Tension | Tension applied to load when a muscle is stretched but not stimulated |
| Total Tension | Active tension plus passive tension |
| Hyperplasia | Fiber splitting, more cells |
| Hypertrophy | More myofibrils, actin and myosin filaments, sarcoplasm, and connective tissue |
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