Muscarinic acetylcholine receptor: Encyclopedia II - Muscarinic acetylcholine receptor - Physiology

Muscarinic acetylcholine receptor - Physiology

Acetylcholine (ACh) is a neurotransmitter extensively found in brain and autonomic nervous system.

ACh is always used as the transmitter within the autonomic ganglion. Nicotinic receptors on the postganglionic neuron are responsible for the initial fast depolarisation (Fast EPSP) of that neuron. As a consequence of this, nicotinic receptors are often cited as the receptor on the postganglionic neurons at the ganglion. However, the subsequent hyperpolarisation (IPSP) and slow depolarisation (Slow EPSP) which represent the recovery of the postganglionic neuron from stimulation are actually mediated by muscarinic receptors, types M2 and M1 respectively (discussed later).

Another role for these receptors is at the junction of the innervated tissue and the postganglionic neuron in the parasympathetic division of the autonomic nervous system. Here acetylcholine is again used as a neurotransmitter, and muscarinic receptors form the principal receptors on the innervated tissue. In addition, muscinaric acetylcholine receptors pre-synaptically on the post-ganglionic neuron bind to the released acetylcholine and regulate the response of the postganglionic neuron.

By contrast, this junction in the sympathetic division does not tend to use acetylcholine as a neurotransmitter (instead, noradrenaline is used), and therefore neither muscarinic nor nicotinic receptors are involved. A very few parts of the sympathetic system (sweating, for example), do use acetylcholine as a neurotransmitter at this position. In these cases, the receptors are of the muscarinic type. The sympathetic nervous system also has single nerves terminating at the chromaffin cells in the adrenal medulla, which secrete adrenaline and noradrenaline into the bloodstream. Acetylcholine is used as a neurotransmitter, and the receptor is of the nicotinic type. The somatic nervous system uses acetylcholine at the junction between its one peripheral nerve and the innervated tissue, also of the nicotinic type.

Muscarinic acetylcholine receptors are also present and distributed throughout the central nervous system, in post-synaptic and pre-synaptic postions. There is also some evidence for postsynaptic receptors on sympathetic neurons allowing the parasympathetic nervous system to inhibit sympathetic effects.

In summary, then, muscarinic receptors can be found:

• Between the postganglionic neurons and the innervated tissue throughout the parasympathetic nervous system.
• Presynaptically within the postganglionic neurons to regulate acetylcholine release in the parasympathetic nervous system.
• Between the postganglionic neurons and the innervated tissue in a very few parts of the sympathetic nervous system.
• Within the ganglions of the whole autonomic nervous system (sympathetic and parasympathetic), acting as recovery receptors after the primary stimulation of the postganglionic neurons by action on nicotinic receptors by ACh from the preganglionic neuron.
• In various parts of the higher central nervous system, which are currently poorly characterised, but have a role in cognition and memory, and degenerate in Alzheimer's disease.

Adapted from the Wikipedia article "Physiology", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki