Question

Describe the neurotransmitters and neurotransmitter receptors of the parasympathetic nervous system.

Answer 1

The parasympathetic nervous system uses chiefly acetylcholine (ACh) as its neurotransmitter, although peptides (such as cholecystokinin) can be used.

Acteycholine

Acetylcholine, an ester of choline and acetic acid that serves as a transmitter substance of nerve impulses within the central and peripheral nervous systems.

the parasympathetic nervous system, the part of the autonomic nervous system (a branch of the peripheral nervous system) that contracts smooth muscles, dilates blood vessels, increases bodily secretions, and slows heart rate. Acetylcholine can stimulate a response or block a response and thus can have excitatory or inhibitory effects.

Within the autonomic nervous system, acetylcholine behaves in a similar manner, being discharged from the terminal of one neuron and binding to receptors on the postsynaptic membrane of other cells. Its activities within the autonomic nervous system affect a number of body systems, including the cardiovascular system, where it acts as a vasodilator, decreases heart rate, and decreases heart muscle contraction. In the gastrointestinal system, it acts to increase peristalsis in the stomach and the amplitude of digestive contractions. In the urinary tract, its activity decreases the capacity of the bladder and increases voluntary voiding pressure. It also affects the respiratory system and stimulates secretion by all glands that receive parasympathetic nerve impulses. In the central nervous system, acetylcholine appears to have multiple roles. It is known to play an important role in memory and learning and is in abnormally short supply in the brains of persons with Alzheimer disease.

The ACh acts on two types of receptors, the muscarinic and nicotinic cholinergic receptors

When stimulated, the preganglionic neuron releases ACh at the ganglion, which acts on nicotinic receptors of postganglionic neurons. The postganglionic neuron then releases ACh to stimulate the muscarinic receptors of the target organ.

Types of muscarinic receptors

The five main types of muscarinic receptors:

The M1 muscarinic receptors are located in the neural system.

The M2 muscarinic receptors are located in the heart, and act to bring the heart back to normal after the actions of the sympathetic nervous system: slowing down the heart rate, reducing contractile forces of the atrial cardiac muscle, and reducing conduction velocity of the sinoatrial node and atrioventricular node

The M3 muscarinic receptors are located at many places in the body, such as the endothelial cells of blood vessels, as well as the lungs causing bronchoconstriction. The net effect of innervated M3 receptors on blood vessels is vasodilation, as acetylcholine causes endothelial cells to produce nitric oxide, which diffuses to smooth muscle and results in vasodilation. They are also in the smooth muscles of the gastrointestinal tract, which help in increasing intestinal motility and dilating sphincters. The M3 receptors are also located in many glands that help to stimulate secretion in salivary glands and other glands of the body. They are also located on the detrusor muscle and urothelium of the bladder, causing contraction.

The M4 muscarinic receptors: Postganglionic cholinergic nerves, possible CNS effects

The M5 muscarinic receptors: Possible effects on the CNS

Types of nicotinic receptors

In vertebrates, nicotinic receptors are broadly classified into two subtypes based on their primary sites of expression:

muscle-type nicotinic receptors (N1) primarily for somatic motor neurons;

neuronal-type nicotinic receptors (N2) primarily for autonomic nervous system