Question

Following exposure to alpha-bungarotoxin, a toxin that blocks the nicotinic acetylcholine receptor, which of the following statements about synaptic transmission at the vertebrate neuromuscular junction is/are correct?

a. The presynaptic terminal would still be able to generate a graded depolarization.

b. The presynaptic terminal would still be able to release acetylcholine into the synaptic cleft.

c. Acetylcholine in the synaptic cleft would still be broken down by acetylcholinesterase.

d. both a) and b) are correct, and c) is incorrect

e. a), b) and c) are all correct

2)

Regarding the patellar tendon reflex...

	a.	The patellar tendon reflex is initiated by stretch receptors in the quadriceps muscle and results in activation of alpha motor neurons innervating extrafusal muscle fibers of the hamstrings.
	b.	The patellar tendon reflex has one synapse in the spinal cord that activates gamma motor neurons innervating extrafusal muscle fibers of the quadriceps.
	c.	Afferent sensory neurons synapse with motor neurons and interneurons that simultaneously activate a flexor and inhibit an extensor muscle group.
	d.	both a) and b) are correct, and c) is incorrect
	e.	a), b) and c) are all correct

Answer 1

1)e.a),b),c) all are correct.

The alpha-bungarotoxin act on nicotinic acetylcholine receptors on postsynaptic neurons and inhibit it,thereby prevents synaptic transmission. It has no action on presynaptic neurons so it will be able to produce graded depolarization. Since it doesn't interfere with presynaptic neurons they will be able to release acetylcholine and in synaptic cleft it will accumulate since it is not binding to its receptors so it is degraded by acetylcholine esterase.

2)a

In patellar reflex, the stimulus is stretching of muscle spindle of quadriceps femoris muscle.this leads to activation of sensory type 1a fibres which carry impulses to spinal cord. There it synapses with alpha motor neurons innervating extrafusal fibres of quadriceps extensors(activated) and hamstrings flexors(inhibited).it also synapses with gamma motor neurons innervating intrafusal fibers of quadriceps which is excited to regulate sensitivity of reflex.