Autonomic Nervous System: Sympathetic Nervous System

 

'Thoraco-Upper Lumbar' Nervous System

 

THIS THORACO-UPPER LUMBAR name describes the central location of cell bodies whose axons project into the periphery to synapse onto their targets. The spinal nuclei are in the upper part of the motor grey column of the spinal cord, located in segments T1 through L2. These are shown as the red and the purple cell bodies in the margin drawing.

   At right is a cross-section of the spinal cord at a typical sympathetic segment ...say... mid-thoracic. Within the red box are the immediate regions of interest for the sympathetic outflow.

   Recall from our study of the spinal cord that all motor axons leave their spinal origins by way of the ventral roots in mammals. All sympathetic axons enter the spinal nerve, go down the ventral ramus of that nerve, and exit shortly into the white ramus (named because these axons are myelinated and glisten white in life). The white ramus in humans is usually a couple of inches long and connects to a lateral chain ganglion located ventrolateral to the bodies of the vertebrae.

   In the illustration at left, the typical pathway includes a preganglionic fiber (= presynaptic) that enters the lateral chain ganglion to synapse there with a ganglion neuron. [These ganglia are derived embryonically from neural crests.] In virtually every case, the postsynaptic cell (blue) returns to the ventral ramus of the spinal nerve and is distributed in the somatic body to innervate such visceral structures as smooth muscle (vessels or hair follicles) and glands, such as sweat glands. These axons may continue with the ventral ramus (commonly) or they may ascend the dorsal ramus to supply dorsal targets. Notice in the drawing there is a tiny unlabeled ramus coming off the spinal nerve proper. This is the "meningeal ramus," made up only of postsynaptic sympathetic fibers to vessels of the meninges.

   In the marginal drawing, white rami are symbolized by the red axons, while the green are the unmyelinated grey rami.

Lateral ganglia are also called "paravertebral" ganglia
because of their location to the side
of the spinal column.

 

 

 

 

 

 

 

 

 

 

Colateral Ganglia

   The alternate sympathetic pathway supplies vessels of the abdominal viscera, plus a few other targets. The preganglionic fiber follows the same route from spinal cord, but when it enters the lateral ganglion, it continues without synapsis into the visceral ramus. Multiple visceral rami combine into several splanchnic nerves that end in colateral ganglia. These colaterals are located along the dorsal aorta, associated with the major (unpaired) visceral arteries. In humans there are three of these ganglia, the coeliac, superior mesenteric, and inferior mesenteric ganglia, named from their adjacent artery. Post ganglionic axons climb these arteries like ivy on a tree and are distributed with their branch arteries to the abdominal viscera.

   The diagram along the left border is over-simplistic. The visceral rami (purple) are not shown joining to form splanchnic nerves, and the colateral ganglia are much too large. If either the lateral or colateral ganglia were drawn to scale in the margin, they would be dots.

Colateral ganglia are also called "prevertebral" ganglia
because of their location in front of the vertebral column
in a standing person.

 

 

 

 

 

The Lateral Chain of Ganglia

LATERAL GANGLIA are joined along the length of the spinal column by connecting rami to form a lateral or sympathetic chain. By way of these connecting rami, it is possible for sympathetic fibers, usually preganglionic, to go to higher or lower levels. Keep in mind that these presynaptic axons may send axon colaterals to several ganglia to magnify their effect.

   This relationship is entirely regular for all of the thoracic and first two lumbar vertebral segments...one pair of white rami; one pair of lateral ganglia, joined to the lateral ganglia above and below by connecting rami; one pair of grey rami returning to the pair of ventral rami. The drawing above is a bit more representational than the preceeding two in terms of gross anatomy.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The Cervical Lateral Chain

CERVICAL spinal nerves do not have white rami, but they do have grey rami and there are cervical ganglia. Recall that there are eight cervical nerves, and, thus, eight sets of grey rami. The cervical ganglia have undergone fusions. The superior cervical ganglion joins to C1-4 and evidently represents merger of the primitive top four ganglia in the sympathetic chain. The middle cervical ganglion connects to C5 and C6. Commonly there is an inferior cervical ganglion that sends grey rami to C7 and C8. Frequently, this inferior cervical may fuse to the ganglion of T1, forming a stellate ganglion, the situation shown at right..

   Since cervical white rami are lacking, all presynaptic input to these cervical ganglia must enter from below, via the uppermost thoracic white rami (principally T1, but T2 and T3 usually contribute some fibers). All cervical synapsis occurs in the three cervical ganglia; these have visceral rami do not contain presynaptic axons -- there are no cervical colateral ganglia. Therefore, all axons exiting via grey or visceral branches are postsynaptic.


The Lumbo-Sacral Lateral Chain

MUCH LIKE the situation in the cervical region, the lower chain has some irregularities. No ventral ramus below L2 gives off a white ramus. Thus, all presynaptic fibers for ganglia L3-5 and S1-4 enter the lateral chain via white rami of the upper two lumbar nerves. Unlike with the cervical ganglia, there has been no fusion between any of the series. However, the coccygeal ganglia (not shown at left) merge into a single midline ganglion.

   This drawing greatly abbreviates reality. All preganglionic fibers in this group of ganglia arise from cells in the L1/L2 grey matter. As with the topmost thoracics, this is actually a heavy outflow of axons. At every lateral ganglion level in this group there are many synapses, and postganglionics exit through each grey ramus for distribution to that body segment. Likewise, there are preganglionics which pass through and exit via each of the visceral rami.


Sympathetic Function

SYMPATHETIC innervation drives the "fight or flight" emergency response system. Highest centers are located in hypothalamic nuclei, although these certainly receive input from cerebral emotional cortex. These nuclei connect to the reticular formation of the brainstem. This latter is a multineuronal network that is in direct or indirect contact with all aspects of brain function, but sympathetic/reticular interactions are very well integrated. The reticulo-spinal tract conducts instructions to all segmental levels of the cord where there are autonomic cells (that includes the sacral parasympathetic output as well).

   You should treat the following text and the figure at right as a summary of details--not to be learned but to be understood. Get a "feel" for what the sympathetic system is doing; you will be able to look up these details, and more, for the rest of your mental life. Keep in mind, what you are about to read is truly bare bones!

…          Pupil -- dilatation (approach someone with a pleasant smile on your face, establish eye contact, and hit them hard in the belly. Notice their pupils flare open. Yours will also when they hit you back 1 or 9 times.)

…          Ciliary muscle -- relax (far focus)

…          Lacrimal gland -- secretion (low level)

…          Parotid gland -- secretion (low level)

…          Submandibular gland -- secretion (low level)

…          Heart -- increased pulse rate and myocardial responsiveness

…          Lung -- dilation of air passages

…          Gut -- reduced motility

…          Liver, pancreas -- increase in blood glucose

…          Kidney -- reduced urine production

…          Urinary bladder -- contraction of exit sphincter and relaxation of bladder wall (having the piss/shit scared out of you is a myth!)

…          Penis -- ejaculation (we'll get to that in more detail later)

…          Arterioles in general -- constriction

…          Arterioles in voluntary muscular system -- response is variable...the entire syndrome of arteriole sympathetic response is to direct blood to those muscles coping with the emergency. Since you have a limited blood volume, this means blood must be directed away from other areas, such as gut and skin (you do turn pale from fright). In general, the brain itself is exempt from this kind of regulation, so that the neural flow rate is maintained. Blood pressure is up and ventricular exit volume are increased; therefore, fainting from fright is more a phenomenon of Hollywood than of neurology. If syncope does occur, the causes is psychological rather than biological.

…          Piloerector muscles (raise hair or goosebumps) -- contraction

…          Sweat glands -- secretion

   Keep in mind that if any of the above happen, all tend to happen (ejaculation being an obvious exception). However, this is a graded, not an absolute, response.

 

 

 

 

 

 

 

 

Interactions with the Parasympathetic System

MOST INTERACTIONS between these two divisions result in suppression of the parasympathetic response. An important exception to this is sexual activity, when both systems are active. This will be discussed shortly as a special case. So, in general you can look at the above list and figure the opposite action is the parasympathetic action.

    There is an important modification of this rule. Thinking back to the anatomic diagrams of the two subsystems, remember that the sympathetic has a major somatic distribution via grey rami to the spinal nerve branches that is entirely lacking in the parasympathetic pathway. Therefore, the sympathetic pathways that actually interact with the parasympathetic are contained within the viscereal ramification from the lateral chain. The parasympathetic innervation does not influence (1) blood sugar release, (2) sweat glands, (3) piloerector muscles, or (4) constriction or dilation of arterioles.

   Also keep in mind that the action of the sympathetic system is global, while parasympathetic activity is local. As an example, the glands of the head which secrete slightly under sympathetic control will produce much larger secretions parasympathetically. However, if you get an object in your left eye and the left lacrimal gland begins to produce tears to wash it out...you do not undertake other parasympathetic functions at the same time, such as emptying the bowel!

 

Sexual Neurobiology

The passage to follow is identical to that in the Parasympathetic Nervous System passage.

SEXUAL ACTIVITY is a complex interaction of sympathetic, parasympathetic, and somatic nerves.

…          Ongoing parasympathetic actions dilate the veins draining erectile tissue. In addition, tonic smooth muscle contraction maintains this drainage. During sexual excitation, these actions are reversed to constrict drainage. At the same time, arterial dilatation increases blood inflow to the spongy erectile tissue.

…          At the beginning of orgasm, sympathetic excitation of smooth muscle in the tube system of the epididymides, ducti deferentia, and accessory gland ducts all move semen into the urethra, within the prostate. At this time, the sphincter of the urinary bladder is tightly closed.

…          Ejaculation results of rhythmic contractions of somatic innervation of the bulbocavernosus, ischiocavernosus, and periurethral striated musculature.

…          Analogous actions to the above three occur in the female genital organs.

   Sensory sex is entirely somatic in its anatomy. The general opinion is that there are no specialized sensory receptors in the genital organs involved with sexual excitation and gratification, but the unique sensory response leaves this question open.

 

Sympathetic Pathology

   BECAUSE the sympathetic and the parasympathetic systems are largely antagonistic, the pathology of the one often involves the other. Pathology of the autonomic nervous system will be reserved until the end of the parasympathetic discussion, when presumably you will have studied both systems in the normal.