prob_set5_solns - Prob_set5_solns Page 1 of 5 Solution to...

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Solution to PROBLEM SET #4 1. a. Ventral horn motor neuron depolarizes due to flux of cations through protein channels opened by an excitatory neurotransmitter (which is likely to be glutamate). b. Depolarization of the motor neuron post-synaptic membrane is passively conducted to the spike initiating zone. If the change in membrane potential is large enough at the spike initiating zone to open a sufficient number of voltage-sensitive Na + channels there will be a large influx of Na + that will further depolarize the axon. c. This signal is propagated down the axon as an action potential. d. Meanwhile behind the signal, the membrane repolarizes because voltage-sensitive K + channels open, allowing the efflux of K + . For a brief time after the axonal membrane has been excited it is refractory. e. When the action potential reaches the synaptic terminals of the motor neuron, the depolarization of the membrane opens voltage-sensitive Ca 2+ channels, and Ca 2+ enters the terminals, driven by its large electrochemical gradient. f. The increase in intracellular free Ca 2+ in the motor neuron terminal causes synaptic vesicles to fuse with the plasma membrane, liberating ACh into the synaptic cleft. g. ACh diffuses into and across the synaptic cleft, and some of it binds to nicotinic AChRs in the plasma membrane of the muscle. h. The nAChRs to which ACh has bound open their channels, permitting Na + and K + to flow through the channels, which causes Vm to shift toward the equilibrium potential for this current. i. The post-synaptic membrane depolarizes as a result of the mixed Na + and K + current, and voltage- sensitive Na + channels in the sarcolemma (muscle plasma membrane) are activated. j. An action potential sweeps down the cell membrane of the muscle fiber, exactly as it did down the axon of the motor neuron. k. The action potential also invades the membrane of the transverse tubule system (T-tubules), carrying the depolarization into the interior of the muscle fiber. l. Depolarization of the T-tubules activates dihydropyridine receptors, which activate ryanodine receptors, leading to the release of Ca 2+ from the cisternae of the sarcoplasmic reticulum (= excitation- contraction coupling). m. An increase in free Ca 2+ biases the reversible reaction of Ca 2+ binding to troponin toward the state in which Ca 2+ is bound to troponin molecules. n. When Ca 2+ is bound to troponin it causes a change in the structure of tropomyosin molecules, revealing myosin-binding sites on actin sub-units. Page 1 of 5 Prob_set5_solns 5/14/2009
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o. Myosin heads bind to the actin sub-units. p. The angle between the myosin head and the myosin rod becomes smaller, as ADP is released from the head of the myosin, pulling the actin and myosin myofilaments along one another. (See the "sliding filament theory" in your text and/or in the course outline.) q. ATP binds to myosin, breaking the bond between the myosin head and the actin sub-unit (relaxation).
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prob_set5_solns - Prob_set5_solns Page 1 of 5 Solution to...

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