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muscle fibers per motor unit
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Muscle Physiology
BYU PdBio 305 Rhees Muscle Physiology
| Question | Answer |
|---|---|
| Motor unit | a single motor neuron and the muscle fibers it innervaes |
| muscle fibers per motor unit | 3-6 in hand and eye; 120-500 in large muscles of the back |
| myoneural junction | junction of motor neuron and muscle; it loses its myelin sheath and divides into a number of terminal buttons or end-feet |
| 6 events at a myoneural junction (1-3) | 1)Action potential travels over the axon terminal and triggers the entry and release of clcium into the terminal 2)Ca++ triggers the exocytosis of vesicles of Ach 3)Ach diffuses accross the space and binds with receptors in the muscle cell |
| Events at a myoneural junction (4-6) | 4)Binding brings about the opening of sodium channels generating an action potential in the muscle cell 5)Current flow throughout the muscle cell via the transvers (T) tubule system 6)Ach is broken down by acetylcholinesterase |
| differences between a synapse and a myoneural junction | one to one transmission of acion potentials at a myoneural junction; myoneural junction is always excitatory |
| three types of muscle tissue | smooth, cardiac, skeletal |
| mesoderm | all 3 muscle cell types are derived from it |
| contractile fibers | all 3 types of muscle cells are composed of them |
| smooth muscle | elongated, lack cross striations, and under involuntary control, and have one nucleus which is located at the center of the cell |
| smooth muscel cells control contraction of what | internal orans, walls of blood vessels, digestive system, ducts of reproductive glands, the ureters, the baldder, and skin |
| myogenic | spontaneous muscle contraction |
| neurogenic | muscle contraction induced through innervation |
| cardiac muscel fiber characteristics | cross-striations, one centrally-lcated nucleus, involuntary control, bifurcated to form a synctium |
| skeletal muscle fiber characteristics | elongated, multinucleate, voluntary control, well developed cross striations |
| function of skeletal muscle | movement of bones, maintenance of body posture, facial expression, and breathing movements |
| sarcolemma | plasma membrane in muscle cell |
| fiber | muscle cell |
| sarcoplasm | cytoplasm in muscle cell |
| sarcoplasmic reticulum | endoplasmic reticulum in muscle cell |
| Myofibril | a muscle fiber contains many small, round parallel bundles (100-1000s) called myofibril |
| Myofilament | each myofibril is composed of still smaller units called myofilaments |
| Actin | thin contractile protein in the muscle cell |
| Myosin | thick contractile protein in the muscle cell |
| Sarcomeres | the contractile proteins are arranged into compartments |
| A bands | dark bands in myofilaments |
| I bands | lighter bands in myofilaments |
| Anisotropic | a bands |
| Isotropic | I bands |
| H zone | lighter central regions of the A bands contain only myosin (no myosin heads) |
| Z line | where the actin filaments of adjacent sarcomeres join |
| Actin filaments are composed of | actin, tropomyosin, and troponin |
| G-actin | globular individual actin molecules |
| F-actin | douple spherical chains (double helix) called fibrous actin |
| Troposmyosin covers how many G-actin subunits | 7 |
| Troponin I | unit that binds to actin |
| Troponin T | portion that binds to tropomyosin |
| Troponin C | component that binds calcium (initiates contraction) |
| Myosin filaments | thick myofilaments |
| Light meromyosin | LMM filaments make up the rodlike backbone of the myosin filaments |
| Heavy meromyosin | HMM filaments form the shorter globular lateral cross bridges (heads) which link |
| Binding sites on the myosin cross bridge | actin binding site and an ATPase binding site |
| Isometric contraction | when a muscle develops tension but does not shorten |
| Isotonic contraction | a contraction in which the muscle shortens |
| Twitch contractions | when an isolated muscle is attached to a device that senses and records changes in muscle length and the muscle then receives a single stimulus, the contraction response is referred to as a twitch |
| Three phases of a twitch | 1)latent period which is the time from actual stimulation until contraction begins 2)contraction period 3)relaxation period |
| Wave summation or temporal summation | first muscle twitch is not completely over when the second one begins and thus the muscle is already in a partially contracted state when the second twitch begins, the degree of muscle shortening in the 2nd contraction is greater than with just a single m |
| Tetanus | occures when the stimuli are applied in a very rapid succession and the contractions fuse together and cannot be distinguished one from the other |
| Slow-twitch fibers | found mostly in the postural muscles such as in the back and legs and have a twitch duration of about 100msec; derive energy from oxidative metabolism; have small diameters, many mitochondria, many capillaries, small amounts of glycogen storage |
| Red fibers | slow-twitch fibers; the red color is due to the high concentration of myoglobin which binds O2 |
| Fast-twicth fibers | found mostly in fine, skilled movements and twitch for about 7 msec; fatigue quickly; derive energy from glycolysis; large fibers, high storage of glycogen and few mitochondria |
| White fibers | fast-twitch fibers; they are white because they contain little myoglobin |
| Why is fast-twitch muscle fast? | has more sarcoplasmic reticulum than slow-twitch muscle; also has fast myosin |
| Causes of muscle fatigue (first 2) | 1)ATP use exceeds ATP production 2)lactic acid (H+) accumulation interferes with ATP production and muscle contraction |
| Causes of muscle fatigue (second 2) | 3)glycogen depletion and therefore hypoglycemia may occur 4)inhibition of excitation-contraction coupling. That means inhibition of release of Ca++ from the lateral sacs |
| Sarcoplasmic reticulum of cardiac muscle | is not as extensive |
| Intercalated disks | manner in which cardiac muscle cells are interconnected |
| 2 functions of intercalated disks | 1)provide gap junctions that allow impulses to travel from one cell to another 2)provide desmosomes that anchor one cell to another |
| Cardiac muscle cell refractory period | long absolute and relative refractory periods making tetanus impossible |
| Automaticity | factors that increase intracellular Ca++ in the cardiac muscle such as catechoamines and digitalis, which slow the heart down, will increase the force of contraction. Cholinergic agents will decrease Ca++ concentrations and decrease the contraction force |
| Contraction of cardiac muscle | the action potential does not release Ca++ from sarcoplasmic reticulum but is released from the extracellular fluid; the increase of Ca++ releases Ca++ in the sarcoplasmic reticulum (calcium induced calcium release) |
| What smooth muscle lacks | sarcomeres, striations, t-tubule systems |
| Contraction of smooth muscle | slower, requires less energy; innervated by neurons (autonomic nerves); other smooth muscle cells are not innervated and contract in response to hormones or local factors |
| Smooth muscle mechanism | myosin-regulated; actin and myosin only interact when the myosin is phosphorylated |
| Ca++ involvement in smooth muscle | intracellular messenger that sets off a seris of events that result in the phosphorylation of myosin. Most Ca++ comes from the extracellular fluid |
| Ca++ to phosphorylation in smooth muscle tract | increased intracellular Ca++ binds to and activates calmodulin which binds to and activates another myosin kinase which phosphorylates myosin which binds with actin so cross-bridge cycling can begin |
| Autonomic innervation of smooth muscle cells | postganglionic neurons travel across smooth muscle cell’s surface and release neurotransmitters from multiple bulges (varicosities) as an action potential passes along the axon. |
| Varicosities | bulges on the postganglionic axon in smooth muscle cells that release neuro transmitters as an action potential passes along the axon |
| Adrenergic receptors (alpha and beta)—effects on smooth muscle cells (norepinephrine) | Alpha(1) receptors--cause smooth muscles to contract or be stimulated; Beta (2) receptors—cause smooth muscles to relax or be inhibited |
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