Question

The neurotransmitter acetylcholine (ACh) causes the rapid contraction of skeletal muscle and the slower relaxation of cardiac muscle. Which one of the following is the most likely explanation for this difference?

	A.	ACh activates a GPCR pathway in cardiac muscle and an ion channel coupled receptor in skeletal muscle.
	B.	ACh activates an ion channel coupled receptor in cardiac muscle and a GPCR pathway in cardiac muscle.
	C.	ACh is permeable through the plasma membrane of skeletal muscle cells but not through the plasma membrane of cardiac muscle cells.
	D.	ACh is permeable through the plasma membrane of cardiac muscle cells but not through the plasma membrane of the skeletal muscle cells.
	E.	ACh acts more slowly at the cardiac muscle cells because it must pass through the gap junctions that couple these cells.

Answer 1

Answer: Option A

Acetylcholine is both a neurotransmitter at the neuromuscular junction as well as in the Autonomic nervous system.

At the neuromuscular junction, ACh transmits signals by activating ligand gated ion channels which are the muscle type Nicotinic receptors of ACh. When ACh binds to these receptors the gates open and become permeable to sodium, pottasium and calcium ions.

On the other hand, cardiac muscle contraction is governed by the ANS. The cardiac muscle have Muscarinic type receptors of ACh which have prolonged effect. ACh transduce signal by activating GPCR pathway (G protein coupled receptors) which utilise second messenger system.