The location of acetylcholine1 neurons 2 can be mapped with respect to the organization of the nervous system (Figure 6).
Acetylcholine neurons are plentiful in the central nervous system, which includes the brain and the spinal cord. The peripheral nervous system includes neurons that connect the brain and spinal cord to muscles, organs and skin to send sensory and motor information. The peripheral nervous system is sub-divided into 1) the somatic motor system, in which skeletal muscles receive information from the spinal cord via motor nerves to cause movement (mostly voluntary) and 2) the autonomic nervous system, in which smooth muscles and other organs receive information from the brain and spinal cord to control organ function (mostly involuntary). Last, the autonomic nervous system is sub-divided into the parasympathetic nervous system3 (PSNS), which is active all of the time, and the sympathetic nervous system4 (SNS), which is active especially during times of stress, fear and emergencies. Acetylcholine neurons are present in all parts of the peripheral nervous system. In the somatic nervous system5, motor nerves release acetylcholine onto skeletal muscle. In the autonomic nervous system, there are 2 types of neurons that contribute to the PSNS and the SNS. The first type of neuron leaves the spinal cord, en route to a cluster of neurons called a ganglion6. In the ganglia, the acetylcholine neurons release acetylcholine onto the second type of neuron. This second type of neuron travels to its final destination (e.g., organs, glands, smooth muscle) and it either releases acetylcholine in the PSNS or it releases another neurotransmitter, norepinephrine7 in the SNS. These 2 nervous systems usually work in opposition to each other. For example, in the lungs, the PSNS causes bronchiole constriction and the SNS causes bronchiole dilation; the PSNS stimulates salivation and the SNS inhibits salivation. In each place where acetylcholine is released, acetylcholine receptors are present on the corresponding target (Figure 6).
Figure 6 Organization of the nervous system including the central and peripheral parts. The peripheral nervous system includes the parasympathetic and sympathetic branches, along with the somatic branch. The arrows point to where acetylcholine is released; at each place, acetylcholine receptors are on the receiving cells. Ach = acetylcholine; NE = norepinephrine
- intestines: contraction (diarrhea, vomit)
- tear ducts: secretion (lacrimation or tears)
- heart: decreased heart rate
- bladder: contraction (urination)
- lungs: bronchiole constriction
- eyes: pupil constriction (miosis)
- salivary glands: secretion (salivation)
- muscles: contraction
- sweat glands: secretion (sweat)
- brain: stimulate (vomit)
1 a neurotransmitter stored in vesicles of nerve terminals; it is found in neurons within the central nervous system, the somatic nervous system, the parasympathetic nervous system and the sympathetic nervous system.
2 a protein to which hormones, neurotransmitters and drugs bind. They are usually located on cell membranes and elicit a function once bound.
3 part of the autonomic nervous system which controls everyday functions of organs and tissues. It consists of 2 types of neurons, pre-ganglionic and post-ganglionic. Both types release acetylcholine.
4 part of the autonomic nervous system which controls the functions of organs and tissues especially during times of stress, fear and emergencies. It consists of 2 types of neurons, pre-ganglionic and post-ganglionic. The pre-ganglionic neurons release acetylcholine and the post-ganglionic neurons release norepineprine.
5 part of the peripheral nervous system that controls movement. Motor nerves leave the spinal cord and innervate skeletal muscles. The motor nerves release acetylcholine to make muscles contract.
6 a bundle of nerve cell bodies, often referred to as the “post-ganglionic neuron”. In the both the PSNS and SNS, the pre-ganglionic neurons release acetylcholine; the post-ganglionic neurons release either acetylcholine (PSNS) or norepinephrine (SNS).
7 a neurotransmitter (chemical messenger) in the catecholamine family that medicates chemical communication in the sympathetic nervous system. It is responsible for the physiologic response to a stressful challenge (the ‘flight or fight’ response).