Question

Why does acetylcholine excite skeletal muscle fibers but inhibit cardiac muscle fibers?

Answer 1

EFFECT ON SKELETAL MUSCLE

Acetylcholine is a neurotransmitter, released by nerve cells from diferent parts of the peripheral nervous system. It help in the contraction of all skeletal muscle
Skeletal muscle cells contract response to input recieved from the nervous system. A motor neuron contacts a muscle cell at a structure called a motor end plate. The muscle cell membrane contains nicotinic receptors that are sensitive to acetylcholine. The nicotinic receptor is a ligand-gated sodium channel. This means that when acetylcholine, the ligand, binds to a receptor, the receptor changes its shape in a way that lets sodium enter the muscle cell.
The influx of sodium into the cell depolarizes the muscle cell . Depolarization means the difference in charge between the inside and outside of the muscle is decreased. A different type of sodium channel, which is activated in response to depolarization, lncrease in sodium in and the wave of excitation spreads throughout the muscle cell. This cause to the release of calcium ions from storage sites inside the muscle cell. The calcium ions initiate biochemical events involving troponin, tropomyosin and myosin that cause the muscle to contract.

EFFECT ON HEART

the heart receives its electrical impulses via the vagus nerve and sympathetic nervous system fibers. The right vagal nerve primarily innervates the sinoatrial node, that is under parasympathetic nervous system control. The parasympathetic nervous system will reduce the activity of heart In other words, the sympathetic response leads to increase in heart, while the parasympathetic maintains your body at rest.
It is parasympathetic and vagus nerve activation that releases acetylcholine into sinoatrial node.This action decreases pacemaker rate by increasing potassium and decreasing calcium and sodium movement. As the pacemaker slows, so does your heart rate. At rest, the acetylcholine released by the vagus nerve can bring your heart rate down to 65 to 80 beats per minute.