{This webpage pertains to lecture. However, I put it up early because it should help with the related lab work. This is an area where there is significant overlap between lecture and lab. Concentrate on the boldface terms, which are required lecture terms and concepts. Use the remainder of the descriptions to help develop a tangible image of autonomic system}
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. The axons of these neurons
then project to the sympathetic ganglion chain. (You can observe the ganglia in the model in lab. In this case, the ganglion is the inferior cervical sympathetic chain ganglion.)
In the sympathetic ganglia, 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 in the vagus nerve. This nerve passes through the jugular foramen and then follows the carotid artery down to the heart.
The cell bodies of the postganglionic neurons are right next to the heart, on the superior surface. 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 following table shows the neurotransmitters and receptors (viewed simply) associated with the sympathetic and parasympathetic neurons:
| Neurotransmitter Released | Type of Receptor Activated | |
|---|---|---|
| Parasympathetic Preganglionic Neuron | Acetylcholine | Nicotinic |
| Parasympathetic Postganglionic Neuron | Acetylcholine | Muscarinic |
| Sympathetic Preganglionic Neuron | Acetylcholine | Nicotinic |
| Sympathetic Postganglionic Neuron | Norepinephrine | Adrenergic |
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 and project to the sympathetic chain ganglia. (but in this case
mainly to the superior cervical ganglion).
In the ganglion are the cell bodies of the postganglionic neurons, and their axons then project up to the head and 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 fissure is the ciliary ganglion, which contains the cell bodies of the postganglionic neurons. Axons from these neurons then proceed to the iris.
The neurotransmitters and receptors for the sympathetic and parasympathetic neurons are the same as in the cardiac example above.