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Phys Exam 1: Ch7

Excitation of Skeletal Muscle

Describe initiation of a muscle contraction 1. AP reaches end of axon; 2. ACh is released @ NMJ; 3. ACh initiates an AP in the sarcolemma; 4. AP causes release of Ca from the SR; 5. Intracellular Ca initiates muscle contraction
Describe the Neuromuscular Junction (NMJ) 1. A motor neuron axon ends at an axon terminal on a muscle fiber; 2. Each axon terminal is embedded into the surface of the muscle fiber; 3. This location is called the Motor End Plate.
What is the synaptic cleft? b/n the axon terminal and the muscle fiber; ~20-30nm wide
What is a subneural cleft? folds in the muscle fiber surface below the axon terminal
What are the components of an axon terminal? Voltage-gated Ca channels, Vesicles of NT (ACh)
Describe the process of events that occur in the axon terminal in regards to Ca and ACh 1. An AP opens Ca channels; 2. Ca influx causes vesicles to fuse to pre-synaptic membrane (~100+ vesicles released per nerve impulse); 3. Exocytosis occurs - ACh is expelled and the vesicle becomes part of the membrane
Where are the Ca channels located? Voltage-gated Na channels? ACh receptors? Ca = axon terminal; Na = bottom of subneural cleft on post-synaptic membrane; ACh = opening of subneural cleft on post-synaptic membrane
What is an achetylcholine(ACh) receptor? How does it work? 1. A trans-membrane protein w/a binding site and an ion channel; 2. Two ACh molecules attach, opens the channel (it is an ACh-gated ion channel)
What are nicotinic ACh receptors? Ligand-gated ion channels in the NMJ
What molecule inhibits the nicotinic ACh receptors? Curare
What are muscarinic ACh receptors? G-protein coupled receptors found on target organs of the sympathetic NS
What molecule inhibits muscarinic ACh receptors? What excites them? Inhibit: atropine; Stimulate: pilocarpine
Describe the ion movement through ACh-gated ion channels 1. Na is in high concentration in synaptic cleft; 2. K is held inside by negatively charged cytoplasm; 3. Some Ca also enters
What is the primary ion that depolarizes the post-synaptic muscle membrane? Na crosses the post-synaptic membrane, enters muscle, depolarizes interior
What is depolarization of the post-synaptic membrane called? What is it called when this depolarization spreads? End Plate Potential; Muscle Action Potential
What is the location, action, and function of acetylcholinesterase? Location: on fibers in synaptic cleft; Action: cuts ACh into acetate ion and choline; Fxn: clears receptor for next signal
What is reuptake? most of the choline is reabsorbed into the pre-synaptic membrane
What do reuptake-inhibitors do? Increase the amount of NT available in the synapse
Describe how vesicles and ACh are made 1. Vesicles are produced by the Golgi in neuron cell body and transported into the axon terminal; 2. ACh is synthesized in the axon terminal, then moved into the vesicles
Describe Clathrin 1. Contractile protein in axon terminal; 2. After each nerve impulse, 100+ vesicles fuse w/axon terminal membrame; 3. Clathrin molecules bind to original vesicle membranes; 4. Bound clathrin contracts, forms pits --> vesicles, all inside axon terminal
What is Excitation-Contraction Coupling? 1. An AP spreads across & along the sarcolemma; 2. Transverse tubules allow surface signal to access center of muscle fiber; 3. Depolarization causes Ca release from SR
Describe the relationship of the T-tubule (transverse tubule) and SR (see image) Branches of T-tubules lie directly over the actin-myosin overlap, allowing Ca to be applied to the region of the myofibril that can most quickly and efficiently use it.
Discuss the action of DHP and Ryanodine receptors on the SR in response to Ca (see image) 1.In skeletal muscle, DHP on sarcolemma are linked to ryanodine receptors on SR; 2.AP depolarizes interior, channels on SR open, Ca is released into cell; 3.Cell interior repolarizes, Ca channel closes, Ca pump returns Ca to the SR
Describe the Ca flow in contraction of cardiac muscle (see image) Contraction: (CICR) AP opens surface Ca channels. Ca influx opens ryanodine channels on SR, which dump Ca onto the myofibrils; Relaxation: Ca pump returns Ca to SR and extracellular fluid
What is the action of curare? Blocks ACh RECEPTOR. No transmission --> no contraction --> FLACCID paralysis
What is the action of botulism toxin? What is an example? Blocks ACh RELEASE. No transmission --> no contraction --> FLACCID paralysis; Botox - low dose, blocks contraction
What is the action of neostigmine? Inactivates ACh-esterase. ACh builds up --> spasms--> RIGID paralysis
What is the action of nicotine? Strongly binds to ACh RECEPTOR. Continuous depolarization --> stimulant --> spasms
What is the disease process of the tetanus toxin? Released by anaerobic Clostridium tetani. Blocks ACh release from inhibitory motor neurons --> RIGID paralysis
What are the Congenital Mysethenic Syndromes? What are the sx? Mutations in ACh receptor; sx: weakness, breathing difficulty
What is myasthenia gravis? An autoimmune disease - the body makes ABs to the ACh-gated channel protein
What are the sx of Myasthenia gravis? No signal to muscle --> flaccid paralysis. First sxs ptosis, diplopia, dysphagia. There are cycles of flareup and remission. Involvement of respiratory muscles can be life-threatening, otherwise life expectancy is normal
Created by: hclark86



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