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img069 - depolarizes and fires an action potential that...

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Unformatted text preview: depolarizes and fires an action potential that then simulates contraction. The sequence of events that links the’agtion potential to the contraction is calleckigcrtatr; on=contmctrorr _ #4“ m‘ . mmmm: [y mm M m “I” m: G0 to the next screen: The connection between a "ohm-Md Lew-“Mbmhhmw motor neuron and a muscle cell IS called the o‘e-m- neuromuscular M. ction is similar to any synapse. The 3%: [1‘ :‘wwxmmw motor neuron (presynaptic cell) transmits an action or:- potential and secretes the neurotransmitter ow'm— acetylcholine upon its arrival at the axon terminal. °°""""'" After release, acetylcholine diffuses to the plasma 3:; membrane of the muscle cell (the postsynaptic cell) where it binds to specific receptors, triggering a change in ion permeability that results in W depolarization. The We r: ,l 6 threshold and triggers an action potential in the muscle cell‘ WW the WW!“ Go to the next screen: Once an action potential is initiated in a muscle cell, it propagates through the entire sarcolemma G0 to the next screen: From the sarcolemma the action potential travels through theTtu\bule_s_(these are essentially “mm. mm muted-um sun-4mm if VWWPMW) Wm ouch-annu- Oat-ungua- own-um 0‘1"" Own-Inn (ecu-Iii. Dec-— Ora-Inh- nun-I- mwmmrmmm mmmhmsnmm. w WM-Wflflhm Wop-n Go to the next screen: As the action potential travels through the T tubules, it triggers the release of calcium from the nearby sirgplasmic reticulum. When a muscle cell is relaxed, the concentration of calcium in the cytosol is very low because the membrane of the sarcoplasmic reticulum (SR) is equipped with pumps that actively pump calcium ions from the cytosol into the SR. The pumps permit the SR to assuage. cflcmfima Mnmient and fimction :t : 'um s W» n addition to calcium pumps the membrane of the SR contain voltage-gated calcium channels that are normally closed. This prevents calcium inside the SR from lealing out. An action potential in the T tubules b 'efl allowin calcium to flow out into the WM 0 {030. .1.— The gating mechanism is a bit unusual because the electrical signal triggers a gate in the membrane of the T tubule — not the membrane of the SR itself. Proteins called ryanodjne mtors bridge the gap between adjacent T tubules and SR membranes. The ryanodine receptors also fimctions as W5. Where the ryanodine receptors come into contact with the T tubule membranes are other proteins called 9%? dihydrophyridine rem that firnction as Wm The potential traveling through the T tubules trigger the HP receptors that then undergo a mnfnnnatinnalchange. They transmit the signal to the ryanodine receptors and this signals the opening of the calcium channels. The initial release of calcium triggers the mleasgofexenmgre calcium from the SR. 7 _. Page 4 of 6 ...
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