This webpage will remind you of the autonomic nervous system
anatomy that we studied in lab. Focus now on the difference in
function between the sympathetic and parasympathetic
divisions. As well, pay attention to the neurotransmitters
and receptors of the autonomic neurons and their targets.
The central nervous system consists of the brain and spinal cord. On the other hand, the peripheral nervous system is comprised of all neurons either partly or entirely outside the central nervous system.
The peripheral nervous system controls two types of effector organs: skeletal muscles, which move the bones, and the viscera, which are all other organs. The part of the peripheral nervous system controlling the skeletal muscles is called the somatic, while the part controlling the viscera is called autonomic.
Outside the central nervous system, the organization of the autonomic system is more complex than the somatic. Information must flow through two sequential neurons to reach a visceral organ: preganglionic neurons and postganglionic neurons. The figure below shows this relationship. The cell body of the preganglionic neuron is in the central nervous system, while the cell body of the postganglionic neuron is found outside the central nervous system in an autonomic ganglion.
Another complexity is that autonomic control is carried out by two sets of peripheral neurons. The first set is the parasympathetic. The cell bodies of the preganglionic neurons in this case are in the brainstem or in the sacral spinal cord. The axons tend to be long, projecting to autonomic ganglia close to the visceral organ.
The second set are the sympathetic. Here the cell bodies of the preganglionic neurons are in the thoracic and lumbar spinal cord. Their axons tend to be short, with most autonomic ganglia lying close to the spinal cord. However, there are also several sympathetic ganglia in the abdomen.
The following table shows the neurotransmitters and receptors associated with the sympathetic and parasympathetic neurons:
|Neurotransmitter Released||Receptor Activated||Type of Receptor|
|Parasympathetic Preganglionic Neuron||Acetylcholine||Nicotinic||ligand-gated ion channel|
|Parasympathetic Postganglionic Neuron||Acetylcholine||Muscarinic||G-protein coupled receptor|
|Sympathetic Preganglionic Neuron||Acetylcholine||Nicotinic||ligand-gated ion channel|
|Sympathetic Postganglionic Neuron||Norepinephrine||Adrenergic||G-protein coupled receptor|
The two following examples will make the organization of the two autonomic divisions seem more concrete. As you read through the examples, be sure to be thinking about exactly where the neuron cell bodies and axons are located.
The rate at which is the heart beats is controlled by both sympathetic and parasympathetic nerves, with the sympathetic speeding and the parasympathetic slowing the rate.
In the sympathetic case, the cell bodies of the preganglionic neurons are found in the lateral horns of the first five thoracic segments (approximately). The axons of these neurons then project to the sympathetic ganglion chain (mainly in this case the inferior cervical ganglion) lying just outside the vertebral column.
There in the ganglion the preganglionic neurons form synapses with the postganglionic neurons. The axons of the postganglionic neurons then project out of the ganglion and approach the heart along the large blood vessels connected to the heart. They then spread over the surface of the heart before penetrating into the heart muscle tissue.
In the parasympathetic case, the cell bodies of the preganglionic neurons are in the brainstem (medulla), and their axons project out via the vagus nerve. This nerve passes through the jugular foramen and then follows the carotid artery down to the heart.
Right next to the heart, on the superior surface, are the cell bodies of the postganglionic neurons. The postganglionic axons are short, and project a short distance to the special cardiac muscle fibers near the top of the heart that control the heart rate.
The pupil in the eye is also controlled by both sympathetic and parasympathetic neurons. The sympathetic system dilates the pupil by stimulating the contraction of radial muscle in the iris of the eye. The parasympathetic system constricts the pupil by causing contraction of circular muscle in the eye.
The cell bodies of the sympathetic preganglionic neurons again are in the lateral horns of the upper thoracic segments, but project mainly to the superior cervical ganglion of the sympathetic chain ganglia.
In the ganglion are the cell bodies of the postganglionic neurons, and their axons then project up to the head and then through the superior orbital fissure and into the eye.
The cell bodies of the parasympathetic preganglionic neurons are in the brainstem (midbrain) and their axons project out in the oculomotor nerve, which passes through the superior orbital fissure.
On the other side of the fissure is the ciliary ganglion, which contains the cell bodies of the postganglionic neurons. Axons from these neurons then proceed to the iris.