Electric stimulation of the musculocutaneous nerve causes a rapid twitch of the biceps muscle. Electron microscopy shows numerous vesicles within the musculocutaneous nerve terminals. In order for these vesicles to release their contents into the synaptic cleft, which of the following substances is required?

Question

Electric stimulation of the musculocutaneous nerve causes a rapid twitch of the biceps muscle.  Electron microscopy shows numerous vesicles within the musculocutaneous nerve terminals.  In order for these vesicles to release their contents into the synaptic cleft, which of the following substances is required?

Quizplus · Accepted Answer

The synaptic vesicles in motor neuron presynaptic terminals contain acetylcholine (ACh). When an action potential depolarizes the presynaptic terminal membrane, voltage-gated calcium channels open, causing an influx of extracellular calcium into the synaptic terminal. This calcium current stimulates fusion of the presynaptic vesicles with the plasma membrane and exocytosis of ACh. The nicotinic ACh receptors on the postsynaptic skeletal muscle end plate are ligand-gated sodium and potassium ion channels. The increased sodium and potassium conductance that occurs with ACh binding generates a motor end plate potential that signals for depolarization of the muscle cell membrane. ACh in the synaptic cleft is degraded by acetylcholinesterase. (Choice A) Rapid sodium influx is responsible for the upstroke of the action potential in skeletal muscle, smooth muscle, cardiac muscle and Purkinje fibers. In contrast, the initial upstroke of the action potential in the SA node depends on calcium influx. (Choices C- F) Second messengers do not play a role in the release of synaptic vesicle contents (ACh) at the neuromuscular junction. Educational Objective: Acetylcholine release from presynaptic terminal vesicles at the neuromuscular junction depends upon the influx of extracellular calcium into the presynaptic terminal. Calcium influx into the nerve terminal occurs following neuronal depolarization and opening of voltage-gated calcium channels.

Electric Stimulation of the Musculocutaneous Nerve Causes a Rapid Twitch