Overview of the Autonomic Nervous System (ANS)

00:01 Welcome to this presentation on the autonomic nervous system. This is the first lecture of several lectures that relate to your study of neuroanatomy. It’s a very fascinating subject.

00:15 It’s my pleasure to lead off with the autonomic nervous system, somewhat a mysterious system and has central and peripheral connections. So, let’s see if we can demystify this a little bit for you to make it easy to understand. The first aspect about the autonomic nervous system that I want you to understand is it’s comprised of two systems. We have the parasympathetic system shown here. You see the structures in purple, various nuclei, cranial nerve nuclei contribute to this system. Then you see the nerve fibers, the nerves that also contribute to the peripheral distribution of this system. Then inferiorly in the spinal cord area, the sacral area, you also have contributions to this parasympathetic system. The other system is the sympathetic system. The components of this system are shown in green. You see the centrally located components of the sympathetic system in the spinal cord. Then you see the peripheral distribution to viscera for example. These systems have functions that conflict with one another or have opposite functions. Sympathetics tend to be very stimulatory and then the parasympathetics tend to decrease activity for nourishment and rest for example.

01:56 The autonomic nervous system has a very simple or basic blueprint. The parasympathetic system as well as the autonomic nervous system is comprised of two nerve cells that communicate with various structures. First is that the preganglionic neuron of the autonomic nervous system.

02:23 It’s cell body will have an origin from the central nervous system, either the brain stem or the spinal cord. The preganglionic nerve fiber then will extend outwards to the periphery and then synapse out within a ganglion. Within the ganglion, you have the nerve cell body for the postganglionic neuron. That postganglionic neuron then will extend its axon or nerve fiber further out to the periphery where it will innervate the target cell.

03:04 In the sympathetic nervous system, preganglionic nerve fibers tend to be short.

03:10 Postganglionic nerve fibers are long. However, in the parasympathetics, that is different.

03:18 Preganglionic fibers are long and in parasympathetics, the postganglionic fibers tend to be very, very short as the ganglia are embedded in many cases in the wall of the structure innervated by the parasympathetics. In order to carry out their function, the autonomic nervous system utilizes an array of neurotransmitters and receptors for its communication.

03:48 So, we have a basic blueprint here of the central nervous system. The preganglionic nerve fiber extending out to the periphery synapsing within the ganglion. Then we see the postganglionic nerve fiber extending out to the target cell. This target cell is going to be stimulated by the parasympathetic system. Then these two target cells that we see here are stimulated by the sympathetic system. So, first consideration for you is the preganglionic neuron.

04:22 This will secrete as its neurotransmitter in both the sympathetic system and the parasympathetic system, this neurotransmitter is acetylcholine. The postganglionic neuron receptor and again, that cell body is in the ganglion, that’s going to be the same for all systems, so the autonomic nervous system, parasympathetics and sympathetics. That’s going to be the nicotinic receptor that binds to acetylcholine. Once the postganglionic nerve fiber is activated, action potentials will travel along its length toward the target cell. Then the postganglionic nerve fiber would have to release its neurotransmitter. In the case of the parasympathetic postganglionic nerve fiber, that neurotransmitter is acetylcholine again. Now, if we look at the sympathetics, this is a bit unusual. The main neurotransmitter for the postganglionic sympathetic nerve fibers will be norepinephrine. However, in some cases, sympathetics will use acetylcholine. Innervation of the sweat glands for example are innervated by postganglionic sympathetic nerve fibers that release acetylcholine. Now, the neurotransmitter is released by the postganglionic nerve fibers will have to interact with target cell receptors so that the target cells can be activated to carry out their function. In the case of the acetylcholine released by the postganglionic parasympathetic nerve fiber, the target cell will have as its receptor a muscarinic receptor that then binds to the acetylcholine. In the case of the postganglionic sympathetic nerve fiber, again this would be most of them with norepinephrine being released, the target cell will have adrenergic receptors, alpha or beta, or may have a combination of both. In the case of the sympathetic postganglionic nerve fiber releasing acetylcholine, that target cell will also have the same target cell receptor, muscarinic receptor as did the parasympathetic target cell.