Acetylcholine is a chemical messenger, a neurotransmitter, released by nerve cells in many parts of the peripheral nervous system. It controls the contraction of all skeletal or voluntary muscles, for instance. It also affects the contraction of smooth and cardiac muscle. Acetylcholine is held in synaptic vesicles in nerve terminals until an electrical signal causes its release onto a specialized portion of a muscle cell membrane equipped with receptors that recognize the neurotransmitter.
Where Nerve and Muscle Meet
Skeletal, or striated, muscle cells contract in response to input 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. These receptor molecules, made of protein, are concentrated where acetylcholine is released. 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.
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Acetylcholine’s Effect On The Muscle Membrane
The influx of sodium depolarizes the muscle cell in the vicinity of the motor endplate. Depolarization means the difference in charge between the inside and outside of the muscle is reduced. A different type of sodium channel, which is activated in response to depolarization, lets more sodium in and the wave of excitation spreads throughout the muscle cell. This leads to the release of calcium ions from storage sites inside the muscle cell. The calcium ions initiate a series of biochemical events involving troponin, tropomyosin and myosin that cause the muscle to contract.
Acetylcholine’s Effect On Smooth Muscle
Acetylcholine activates a different type of receptor present in smooth muscle: the muscarinic receptor. When this receptor binds acetylcholine, one result is the release of calcium ions from internal stores. Acetylcholine's interaction with muscarinic receptors, as with nicotinic receptors, causes channels to open resulting in ion flow that depolarizes the muscle cell. As in skeletal muscle, the depolarization leads to muscle contraction.
Acetylcholine’s Effect On Cardiac Muscle
Like smooth muscle, cardiac muscle has muscarinic receptors. The effect of acetylcholine on cardiac muscle, however, is very different from its effects on skeletal or smooth muscle. In the heart, acetylcholine activation of muscarinic receptors causes channels in the muscle membrane to let potassium pass. This has the effect of slowing contraction of the heart muscle and making it beat with less force.