Cholinergic means "having to do with acetylcholine". Acetylcholine (not shown) is the neurotransmitter released from the terminals of all preganglionic neurons in both the sympathetic (orange ) and the parasympathetic (blue ) divisions of the ANS.
Acetylcholine binds with receptor molecules embedded in the membrane of postganglionic cell bodies (light blue and gold circles). There are two categories of receptors -- nicotinic and muscarinic.
When nicotine was injected into laboratory animals it caused some organs to respond in the same way they normally respond to acetycholine. The membrane-embedded receptor molecules that bound with both acetylcholine and nicotine were named "nicotinic." There are several subtypes of nicotinic receptors but only the N1 variety is associated with the autonomic nervous system.
N1 receptors are so abundant that they are only shown in one location on the entire model; that location is the medulla of the adrenal gland. Had they been illustrated in all their locations every ganglion (light blue and gold circles) would be labeled.
There are only two places where these receptors are located: (1) the cells of the adrenal medulla and (2) the cell bodies of all ANS postganglionic neurons. Nicotinic 1 receptors are color-coded green in the model because, when they bind with acetylcholine, the response of the target is stimulatory (green means GO); the adrenal medulla secretes its hormones and impulses are initiated in postganglionic neurons.
The first illustration in the inset to the right shows an N1 receptor on the sympathetic postganglionic cell body (gold circle) of a neuron located in a chain ganglion. The next illustration shows an N1 receptor in the adrenal medulla. The third illustration shows an N1 receptor on the sympathetic postganglionic cell body (gold circle) of a neuron located in a collateral ganglion. The last one shows an N1 receptor on the parasympathetic postganglionic cell body (light blue circle) of a neuron located in the terminal/intramural ganglion of the GI tract.
In laboratory animals the compound muscarine, derived from the mushroom Amanita muscaria, caused some organs to respond in the same way that they respond to acetylcholine. Because of this these receptors were named "muscarinic." Some of these responses were stimulatory and others were inhibitory. The stimulatory receptors are color-coded green and the inhibitory receptors are color-coded red.
Of the five known subtypes, M4 and M5 are restricted to the brain. Of the subtypes associated with the ANS, M1 and M3 are stimulatory while M2 is inhibitory. Inspection of the entire model shows inhibitory M2 receptors located in the heart, stimulatory M1 receptors in gastric glands and stimulatory M3 receptors located at numerous other sites. Be advised that the subtypes muscarinic reseptors is a major area of current research thus you will find many discrepancies in the literature. Information presented here is supported by most investigators and major conflicts are not included.
This type of receptor is found on the gastric glands (small circles in stomach) -- see entire model. Acetylcholine binding causes the secretion of acidic 'gastric juice' into the lumen of the stomach. M1 is color-coded green (green = go) because it stimulates secretion.
The inset to the right shows M2 receptors located in two regions of the heart. The circle drawn in the upper left region of the heart represents the cardiac pacemaker that sets the heart rate. Binding of acetylcholine to M2 receptors on pacemaker cells decreases (red = stop) the rate at which these cells normally send impulses to the myocardium of the heart. The more receptors that bind to the neurotransmitter, the slower the heart rate becomes.
The parasympathetic nerves that inhibit the pacemaker also inhibit the myocardium directly (blue dashed line to right). Binding of acetylcholine with these M2 receptors reduces the amount of calcium available within these cells thus reducing their force of contraction. This is why parasympathetic stimulation of the heart simultaneously reduces both the heart rate and the force of its contraction.
M3 receptors are found on smooth muscle in numerous locations including most glands. In the inset to the left, this subtype receptor is shown in the four different locations:
Inspect the entire model to locate the organs that have M3 receptors.
Last update: 9/25/2005