Neuro: 2:00 - 3:00 Scribe: Dipesh Patel

Neuro: 2:00 - 3:00 Scribe: Dipesh Patel

Monday, February 15, 2010 Proof:

Dr. Keyser The Autonomic Nervous System: An Overview Page 1 of 6

I.  [S1]: The Autonomic Nervous System: An Overview

a.  Last Wednesday had lecture on cerebellum. If I say put everything away and take out paper and said pop quiz for 75%, some of you would have urge to go to bathroom, heart rate would go up, stress hormones released into blood stream. Everything is mediated by autonomic nervous system. On Wednesday, we will talk about neuro and endocrine system and how it coordinates all these activities.

II.  [S2] The Autonomic Nervous System Comprises…

a.  This is part of NS that comprises neural circuit for any aspects of physiology and it works in concert with endocrine system to maintain homeostasis. Homeostasis is a fancy word for maintaining constancy of certain internal environments; concentrations, things like that. The autonomic is distinct from somatic nervous system. It controls all the skeletal muscles. One of main roles of the autonomic nervous system is to provide motor outflow to visceral organs. There is also motor output to internal organs and too smooth muscle and vasculature. There are two divisions of the autonomic nervous system. The sympathetic and parasympathetic. And if that was not bad enough, each of these has two levels of organization: the preganglionic neurons and postganglionic neurons.

III.  [S3] The spinal cord extends from…

a.  The spinal cord, where lots of the preganglionic neurons live, extends from base of skull to lumbar vertebrae. The neurons of the cord communicate with the periphery by 31 pairs of spinal nerves, one at each segment. This shows the basic organization that you are already familiar with.

IV.  [S4] Figure 15.4

a.  Here are the two divisions, the sympathetic and parasympathetic that exist. One thing to notice from his very busy slide is that both systems innervate pretty much the same organs. Not every time, but pretty much the same organ systems. In fact, they do that b/c they tend to work in opposition. The sympathetic does one thing to an organ and the parasympathetic does the other thing. So, I’ll give you examples of that as we go along.

b.  Most of these organs are innervated by both systems. We will start with the sympathetic system. The cell bodies of sympathetic system originate in this part of spinal cord.

V.  [S5] Somatic and autonomic styles of motor innervations

a.  Its different and distinct from the somatic system and this is how it is different. In somatic system you have the motor neuron in spinal cord that sends axon out to skeletal muscles and makes a synapse and release of neurotransmitter from that motor neuron terminal.

b.  This causes depolarization of muscle, action potentials, and contraction. In contrast the autonomic system interposes another neuron in that circuit. In this case, there is the preganglionic neuron, cell body (which is in the brain stem or spinal cord), sends an axon out to autonomic ganglion. This is a collection of second order neurons. These guys send axon to innervate between tissues.

c.  There are two different ways to do this. B shows what happens with the sympathetic system. And this (C) is what happens in the parasympathetic system.

VI.  [S6] The preganglionic neurons of the sympathetic system…

a.  B/c the cell bodies, the preganglionic cell bodies, live in the thoracic and lumbar spinal cord, the sympathetic system is sometimes called the thoracolumbar system. The cell bodies live in a distinct region of the spinal cord gray matter called the intermedial lateral cell column. That’s this little bulging guy right here. That contains the preganglionic cell bodies. They give rise to axons that exit the spinal cord thru the ventral root. Just as you know, the motor neurons that innervate skeletal muscles live in the ventral horn and extend axons out the ventral root. The axons that leave the preganglionics use this same pathway.

b.  These axons do not project directly to the target tissue. They project to the sympathetic ganglion that contains the postganglionic neurons. So here is the preganglionic and intermedial lateral cell column. The axons go out ventral root and enter, sometimes, chain ganglia. So these chain ganglia run on either side of the spinal cord and these ganglia contain the ganglionic neurons. The axons for the ganglionic neurons, project out to the target tissue.

c.  There are two kinds of sympathetic ganglia. The ones in the sympathetic chain are very close to the spinal cord and called the paravertebral ganglia. But there are other ganglia further away from spinal column called the prevertebral ganglia. In each case the cell bodies of the ganglia, receive input from preganglionic neurons and they enter and project to the target tissue.

d.  The route these axons take can be actually pretty complicated. It turns out that all these organs that are innervated by the autonomic system also send sensory input back to the spinal cord. Just as you have somatic sensory (input from skin, Golgi tendon organs, interfusal muscle fibers), maybe the organ systems innervated also have sensory cells. That information enters thru the dorsal root of the spinal cord. The cell bodies lie in the dorsal root ganglion along with the cell bodies of the sensory neurons.

e.  We have motor outflow and sensory input. So we have substrates for local reflexes that control organs in the systems we will see.

VII.  [S7] Details of the organization of the SNS and its sensory inputs

a.  Just to review there is the intermedial lateral cell column. The axons leave by the ventral root. They can synapse on paravertebral ganglia or prevertebral ganglia and those cells in turn project to the target. Sensory information from organ system, such as the gut, come in thru dorsal root and synapses usually on an interneuron. Then conveys sensory information back to the preganglionics on the intermedial cell column. This is the route for the reflex control of organ systems.

VIII. [S8] The SNS is organized segmentally

a.  The exact route that these axons take as it leaves spinal cord can be complicated. This is the cartoon of the sympathetic system again. We are looking at probably thoracic, 3 or 4. Here is the intermedial lateral cell column. The axons leave by the ventral root. Then they can do lots of different things. Sometimes these axons are myelinated but not always and sometimes they are unmyelinated. They can project to the paravertebral ganglion of that segment or they can travel up or down these chain ganglia and synapse on one of the ganglionic neurons in an adjacent segment. Alternatively they can project and leave the sympathetic chain ganglia all together and project one of the prevertebral ganglia. There are lots of routes to get to their targets.

IX.  [S9] Sympathetic Division

a.  The thoracolumbar system, aka the sympathetic system is arranged visceral topically and by that, I mean the preganglionic cell bodies in the rostral most part of sympathetic region (that is the rostral most part of thoracic spinal cord) project to regions in the head. That would include lacrimal gland, salivary gland, and the iris of eye.

b.  When you come the thoracic cord, the preganglionics project to the stomach, pancreas, gallbladder, and liver.

c.  When get into the lumbar region, they project to regions of gut, bladder, and other regions down there.

d.  This visceral topic organization means that these axons project to target tissue in various stereotype bundles. So typically you can wear they are. For example, if you are a neurosurgeon, you want to avoid some of these nerve bundles so you do not cut off control of bladder or some other body part.

e.  Important specialization of sympathetic system is the adrenal medulla which sits on kidney. By many criteria It’s a specialized sympathetic ganglion and functions as endocrine organ. It releases epinephrine into blood stream and other important molecules. It’s a modified sympathetic ganglion.

X.  [S10] Figure 15.3

a.  – This shows the anatomy of the prevertebral ganglia. These are named after the arteries with which they are associated. The paravertebral ganglia are in chains on either side of spinal cord. These ganglia are not necessarily paired and they are associated with different parts of the vascular system.

XI.  [S11] Parasympathetic division

a.  The parasympathetic system is organized fairly differently. The preganglionic neurons are found in the nuclei in the brainstem and sacral spinal cord. It’s called the cranioscaral part of autonomic system. Some of the cell bodies that live in the brainstem send axons out thru the cranial nerves to reach their targets. The targets are things such as tear glands, salivary glands, the cilliary ganglion at the back of the eye (which helps it control pupillary diameter). It’s important clinically for diagnosis. Others project into the thorax to gallbladder, pancreas, stomach, heart, and the bronchial system.

b.  In this case these cells project out all the way to the organ system and the ganglionic neurons are embedded in the target tissue. For example, the cilliary ganglion is a separate ganglion but its located on optic nerve just behind the globe of the eye. There are nerves plexuses in the stomach where the ganglionic neurons are actually embedded in the wall of the target tissue. This means that the preganglionic axons are very long and postganglionic axons are short.

c.  In case where neurons are embedded in target tissue, frequently, they are not consolidated into identifiable ganglion. It’s dispersed into a plexus, which will be a small group neurons and a bunch of fibers. These cells embedded in target tissue also receive sensory input from sensory neurons that are also in the target tissue.

XII.  [S12] The dorsal surface of the brainstem…

a.  – Here is a diagram of the dorsal surface of the brainstem. It shows the location of cranial nerve nuclei that are the source of cranial nerve. The color-coding shows whether the clusters of cells are motor, sensory input from periphery, or mixed.

b.  The cranial nerves can be strictly sensory (as in vestibulochoclear), strictly motor or mixed.

c.  “Don’t need to remember all this stuff but I thought I would be a good reference in case you need it”

XIII. [S13] Figure 7

a.  – One of the major parasympathetic circuits is the vagal motor pathways. Its parasympathetic and it’s involved with control of heart, gut and provides input to muscles of the pharynx, esophagus and larynx. Here’s the dorsal vagal nucleus and it provides input to the gut. There is sensory inflow to the solitary tract nucleus. A nucleus ambiguus also provides inputs to these structures here. All of these things work together provide some of these reflex actions, as I will show in just a minute.

XIV.  [S14] Visceral and somatic afferents…

a.  The visceral info and somatic sensory info have cell bodies that reside in dorsal root ganglia. The sensory inflow in both cases is thru the dorsal root. In both cases the info project to lamina one and five of the dorsal horn of the spinal cord. These relay cells provide the substrate for the reflex actions. For a long time it was thought that projections of the somatic sensory axons and visceral sensory axons were completely overlapped and interwoven. It was hard to figure out how any system could work that way since ending in same lamina of spinal cord.

b.  A few years a very sensitive tract tracing method was used and the somatic afferent has a very different distribution pattern than the visceral afferents. So the visceral afferent distribution is centered on the intermedial lateral cell column while for the somatic afferents it’s further dorsally.

XV. [S15] The enteric nervous system….

a.  In addition to the parts we talked about there is a whole other nervous system you may not have heard of. It’s the enteric nervous system. There are plexuses of neurons embedded in the walls of the gut. If you peel away walls of the gut and get down to the longitudinal and circular muscles of the gut, you find a plexus called Auerbach’s plexus. It’s a plexus of interconnected neurons that do a variety of things. It controls smooth muscle of the gut, it can control secretions into the lumen of the gut and variety of functions.

b.  If you keep peeling away layers of the gut going towards the lumen there is another plexus called the submucosal or Meissner’s plexus which is between the circular muscle and intermucosal. In addition of these there are deep muscular plexuses, periglandular plexus, and villus plexus. Its been estimated that the enteric nervous system contains as many neurons as the entire spinal cord. It’s a nervous system of which we are mostly blissful unaware of. There are times we become aware of it and that is usually not a good thing.

c.  This image shows what a plexus looks like in a gut. These fibers are the preganglionic parasympathetic projections into the wall of the gut. The cells stained in blue are neurons that make up some of these plexuses. You can see you have a small cluster of neurons that receive input thru preganglionics and right up here there would be another plexus or group of neurons. All in all it makes a network that completely surrounds the gut and extends up and down its length.