Organization of the Nervous System: An introduction for students in the Human Anatomy Course

I. Neurons and glia

The basic structural and functional unit of the nervous system is the nerve cell or NEURON. It is important to come to grips with the neuron and the terminology relating to its parts now, otherwise much of the material on organization of the nervous system will not make sense to you. Here is a schematic drawing of a typical nerve cell

All neurons have a cell body (or SOMA) which contains cellular organelles which are typical of most of the cells in the body, including a nucleus, a nucleolus, lots of rough endoplasmic reticulum, and so on. Most nerve cells have processes called DENDRITES, which act like antennae for the cell, in that they receive input to the cell. Most neurons also have a single long process called an AXON, which is capable of transmitting a pulse of electricity (NERVE IMPULSE or ACTION POTENTIAL) from the cell body to some distant target in the brain or the periphery. These axons may be quite long (up to a meter or more in the case of a nerve cell whose some sits in the spinal cord and has an axon which contacts a muscle in the foot). Axons usually break up into smaller branches (terminal branches) near their target. These terminal branches end in swellings which make a specialized contact with the target cell. If the target cell is another neuron, the swelling is called a BOUTON, and the specialized contact is called a SYNAPSE. If the target is a muscle fiber, the bouton is often called a MOTOR ENDPLATE and the synapse is often referred to as a NEUROMUSCULAR JUNCTION. There is usually a gap between the terminal swelling and the target cell (POSTSYNAPTIC CELL). The electrical impulse does not cross this gap, but rather causes a chemical (NEUROTRANSMITTER) to be released from the axon terminals. The neurotransmitter diffuses across the gap and causes electrical changes to occur in the postsynaptic cell.

Neurons come in all sizes and shapes, but the basic functions of all neurons are more or less similar: they receive (and integrate) inputs, and relay their output, in the form of an action potential, to some other target cell. The cell body is mainly responsible for meeting the metabolic needs of the cell and its position with respect to the axon and dendrites is somewhat variable. The drawing below shows you a type of neuron you will encounter in your study of the peripheral nervous system. It is a dorsal root ganglion cell, or primary sensory neuron. Note that the arrangement of dendrites, cell body and axon is somewhat different than for the neuron drawn above.

Note that over part of its length, the sensory axon is actually conducting nerve impulses TOWARD the cell body; over the rest of its length the axon is conducting nerve impulses AWAY from the cell body. The best functional definition of an axon is that it is a nerve process which is capable of transmitting a nerve impulse (action potential) over some distance.

The nervous system also contains cells which are not neurons and which do not DIRECTLY participate in the task of sending and receiving electrical signals. These supporting cells are called GLIA. There are several types of glia, but for our present purposes we will be concerned with only two types: those that form MYELIN SHEATHS around axons in the central and peripheral nervous systems.

Generally, axons are not naked, as we have drawn them above. Rather, they are often wrapped in an insulating material referred to as MYELIN. Myelin is formed by glial cells that wrap themselves around axons (see drawing below). The presence of a myelin sheath around an axon increases the velocity at which an axon will conduct a nerve impulse down its length because the nerve impulse effectively jumps from one space to another between insulating cells. Nerve impulses therefore travel faster in MYELINATED axons than in UNMYELINATED axons.

The myelin sheath is formed by flattened out cells that wrap themselves jelly-roll style around the axon. In the central nervous system the cells that form the myelin sheath are called OLIGODENDROGLIA or OLIGODENDROCYTES; in the peripheral nervous system they are called axon sheath cells. Below is a drawing, which is highly schematic, of a flattened axon sheath cell, and to the right of that is a cross-section of a myelinated axon, so you can see how the axon sheath cell wraps around the axon.

Notice that the sheath itself is essentially flattened cell membrane, with all of the cytoplasm squeezed out except in the outermost layer. Recall that the major component of a cell membrane is the phospholipid bilayer. If you have many layers of membrane stacked on top of one another it is going to have a fatty appearance due to the presence of this phospholipid. Unembalmed lipid (such as fat found on meats) has a glistening white appearance. Myelinated axons therefore will have a glistening white appearance in the central and peripheral nervous systems, and are referred to as WHITE MATTER. Areas containing mainly cell bodies tend to lack myelin and are referred to as GRAY MATTER.
 

TERMINOLOGY RELATING TO AGGREGATIONS OF NEURONS IN THE CENTRAL AND PERIPHERAL NERVOUS SYSTEMS

The schematic drawing below summarizes some terms which are used to describe aggregations of nerve cell bodies or axons in the central nervous system (above the drawing) and the peripheral nervous system (below the drawing). More details regarding particular terms are provided in the text which follows.

GANGLION: In the periphery, cell bodies are not usually found in isolation. Rather, they exist in clusters, which are known as GANGLIA.

NERVE: A bundle of axons traveling together in the periphery. If the nerve contains sensory axons only, it is called a SENSORY NERVE. If it contains motor axons (going to muscles) only, it is called a MOTOR NERVE. Virtually all nerves in the body contain both sensory and motor axons and are therefore called MIXED NERVES.

Many of the axons in any nerve will be myelinated, which is what gives nerves their glistening white appearance (see above). In addition, there are some connective tissue elements associated with nerves that you should be familiar with:

Individual axons are enveloped in a connective tissue wrapping called ENDONEURIUM. Bundles, or FASCICLES, of axons are wrapped in a connective tissue covering called PERINEURIUM. The nerve as a whole is enveloped in a connective tissue sheath called the EPINEURIUM. These connective tissue sheaths help to give peripheral nerves a certain toughness and resistance to tearing which you should notice when dissecting.
 

II. Major subdivisions of the nervous system.

It is best to begin by thinking in very general terms about the function of the nervous system. The nervous system is a network of millions of cells which communicate with one another by means of nerve impulses transmitted along axons of nerve cells. Simplifying quite a bit. the nervous system allows you to SENSE what is going on the internal or external environment, PROCESS that information, and REACT to it in some way, which usually involves MOVEMENT and therefore muscle contraction (this could be speaking, walking, or writing).
 

The nervous system consists of two major subdivisions: the CENTRAL NERVOUS SYSTEM (CNS) and the PERIPHERAL NERVOUS SYSTEM (PNS). The CNS consists of the BRAIN and SPINAL CORD. This is where almost all the important PROCESSING in the nervous system takes place. There appears to be no communication between neurons receiving sensory information and neurons encoding motor output outside of the CNS. Therefore, for even the simplest reflex activity to take place, the sensory information evoking the reflex must be relayed to the CNS, and the motor output must leave the CNS and go to the muscles.

Here we confront an anatomical problem: the CNS is housed entirely within the DORSAL CAVITY of the body, which is made up of the CRANIAL CAVITY, housing the brain, and the VERTEBRAL CANAL, housing the spinal cord. (The dorsal cavity is shown in black on the schematic drawing below.) The receptors which relay sensory input to the CNS, and the muscles which are to be controlled, lie almost entirely OUTSIDE the dorsal cavity. Therefore, there needs to be some way of relaying sensory input and motor outflow between the periphery and the CNS. This is the function of the peripheral nervous system.
 

 

III. The peripheral nervous system
 
The PNS is a collection of neurons and their processes which relay information from the periphery to the CNS, in which case they are AFFERENT or SENSORY; or from the CNS to the periphery, in which case they are EFFERENT or MOTOR. The parts of the PNS that actually connect the CNS and the periphery are the NERVES. The nerves which connect the brain with the periphery (mainly in the head and neck) are called CRANIAL NERVES. There are 12 pairs of cranial nerves which leave the brain on its underside (see Atlas 7-31), then exit the cranial cavity by a series of holes in the base of the skull, called FORAMINA (see Atlas 7-32, 7-34). Some cranial nerves are almost purely sensory, such as those which mediate smell, vision, and hearing. Others are almost purely motor, such as those which move the eyes and tongue. Others are mixed. Cranial nerves and their functions will be discussed in more detail in conjunction with the dissection of the head and neck.
 
The nerves linking the spinal cord and the periphery, which are responsible for sensory and motor innervation of the body outside of the head and neck, are called spinal nerves. The spinal nerves exit the vertebral canal by way of spaces between adjacent vertebrae, known as INTERVERTEBRAL FORAMINA (see figure below).
 

There are 31 pairs of spinal nerves, which are named according to the intervertebral foramina through which they pass. There are 8 pairs of CERVICAL spinal nerves, 12 THORACIC, 5 LUMBAR, 5 SACRAL, and 1 COCCYGEAL. The cervical nerves all exit ABOVE the vertebra for which they are narned, except for C8, which exits between C7 and T1. (Remember, there are only 7 cervical vertebrae.) All other spinal nerves exit BELOW the vertebra for which they are named. All spinal nerves are MIXED (sensory and motor), with the possible exception of the first cervical and coccygeal nerves, which often lack a sensory component. It is very important that you understand the origin and composition of the spinal nerves. These details are explained below,
 

IV. Spinal nerves: somatic components
 
For the somatic portion of the nervous system (the portion dealing with body parts other than viscera, vascular smooth muscle, and glands) there are two major types of nerve cells which connect the spinal cord to the periphery. These are PRIMARY SENSORY NEURONS (or AFFERENT NEURONS), which relay input from the periphery to the spinal cord, and SPINAL CORD MOTOR NEURONS (or EFFERENT NEURONS) which convey motor outflow from the spinal cord to the periphery. Primary sensory neurons have peripheral processes in the skin or muscles which are usually in contact with some kind of a specialized receptor (such as those for touch or muscle stretch). Their central processes enter the spinal cord where they make synapses with other neurons using a variety of neurotransmitters.  Axons of spinal cord motor neurons pass to th eperiphery to innervate striated muscle.  ACETYLCHOLINE is used as the neurotransmitter at the neuromuscular junction.

The simplest circuit you could envision would be a TWO NEURON REFLEX ARC or a circuit for a MONOSYNAPTIC REFLEX.

The sole elements of this circuit would be the primary sensory neuron relaying information from the periphery, the spinal cord motor neuron relaying motor out put to the muscles, and the synapse between them.  This is the kind of curcuit that can mediate SIMPLE reflex activity, such as the stretch reflex, the classic example of which is the knee jerk reflex.  When you tap the patellar tendon it stretches the tendon.  This stretch is relayed to the spinal cord by a primary sensory neuron, which synapses directly on a spinal cord motor neuron which in turn sends a nerve impulse in its axon out to the quadriceps femoris muscle, causing it to contract, and jerking the lower leg forward.  You should realize that aside from the intrinsic interest that the knee jerk reflex holds for the clinician, this reflex has functional importance in everyday life in terms of being an "anti-gravity" reflex.  The most common stimulus that would put a stretch on the patellar tendon would be if the knees were to start to buckle.  In this case, the shortening of the quadriceps femoris muscle would serve to straighten the leg at the knee to help prevent falling.  The fact that there is only one synapse in this circuit makes it an especially fast circuit, which is important to its role as an antigravity reflex.  (Every synapse in a circuit introduces a delay of about 1/2 msec. in response time, so the fewer synapses, the faster the response.

Realize, however, that most circuits are not quite this simple.  Even for most other reflexes, such as the withdrawal reflex, when you burn your finger, there is at least one other neuron (an INTERNEURON) interposed in the circuit. And for more complex behaviors, such as feeling a pen that is put into your hand and deciding how to hold it, the circuit will involve connections that ascend to the brain and descend back to the spinal cord.
 

At this point it is appropriate to consider where these elements are situated in the spinal cord and the bony structures of the vertebral column. The schematic drawing immediately below is a summary of the major points. Particular elements will be discussed in the text that follows.
 

The vertebral column and spinal cord have been cut in cross section. The vertebra has been stippled, and selected parts labeled. Realize that everything on this drawing is bilaterally symmetrical, although, for convenience, many elements have been drawn or labeled on one side only. When the spinal cord is cut in cross section the white matter is on the outside and is made up mainly of axons that are ascending or descending in the spinal cord. The gray matter (cell bodies) is centrally located and has a butterfly shape (shaded gray on the drawing). There is some functional segregation in the spinal cord, so that neurons in the DORSAL HORN are mainly sensory, while those in the VENTRAL HORN are mainly motor. Spinal cord motor neurons sit in the ventral horn of the spinal cord. Their axons pass ventrally and laterally, forming what are known as VENTRAL ROOTS, up to the point where they pass through an intervertebral foremen. The sensory axon, on the other hand, starts in the vicinity of a receptor in the periphery. It probably travels with some motor axons on its way toward the spinal cord, and enters the vertebral canal through an intervertebral foremen. The cell body sits just inside the intervertebral foremen, along with other cell bodies of primary sensory neurons, in what are called DORSAL ROOT GANGLIA. The central process of the sensory axon travels dorsally and medially, forming a DORSAL ROOT to enter the spinal cord in the vicinity of the dorsal horn. Within the gray matter of the spinal cord the axon can then form synapses either with interneurons or with spinal cord motor neurons. 

Review the summary diagram above, and make sure that you can understand and verify all of the following statements. 

  1. Cell bodies of spinal cord motor neurons sit in the VENTRAL HORN of the spinal gray matter.
  2. Cell bodies of primary sensory neurons sit in aggregations called DORSAL ROOT GANGLIA just inside the intervertebral foramina.
  3. The central process of the primary sensory neuron running between the intervertebral foremen and the spinal cord is part of a DORSAL ROOT.
  4. The axon of the spinal cord motor neuron, where it runs between the ventral horn and the intervertebral foremen, is part of a VENTRAL ROOT.
  5. Where the dorsal and ventral roots come together and exit through the intervertebral foremen, they form a SPINAL NERVE.
  6. DORSAL ROOTS ARE SENSORY. VENTRAL ROOTS ARE MOTOR. SPINAL NERVES ARE MIXED.
Let us focus now on the fate of the spinal nerves once they pass through the intervertebral foramina and enter the periphery. The drawing below shows the course of a TYPICAL spinal nerve, in this case a thoracic nerve, in a schematic cross sectional view.

Upon emerging from the intervertebral foremen the spinal nerve gives off two main branches:

  1. A DORSAL RAMUS- this supplies the skin in the region overlying the vertebral column and the intrinsic musculature of the back.
  2. A VENTRAL RAMUS- this continues forward, giving off branches that supply the remainder of the skin and musculature of the trunk or it continues into the limbs. (You should already have traced the distribution of ventral rami in the thoracic wall in the laboratory.

Nerves emerging through a specific intervertebral foremen tend to supply particular parts of the body in a manner that is consistent, by and large, from one body to another. This characteristic is referred to as SEGMENTAL ORGANIZATION of spinal nerves. In some parts of the body, such as the thorax, the segmental organization is relatively simple. Each nerve emerges from an intervertebral foremen, passes along the body wall between two ribs, and innervates the skin and musculature lying between (or adjacent to) those two ribs. In other regions the arrangement is more complex. Upon emerging from the intervertebral foramina, nerves in some regions revert themselves and recombine, forming PLEXUSES. The drawing below shows plexuses in the upper cervical region (cervical plexus, innervating the neck and shoulder region), lower cervical and upper thoracic region (brachial plexus, innervating the upper limb), the lumbar region (lumbar plexus, innervating the lower limb), and sacral plexus (innervating the pelvis, gluteal region, leg and foot). Contrast this with the simpler segmental organization of most of the thoracic nerves.
 

Whether a spinal nerve enters into plexus formation or retains a simple segmental distribution, each spinal nerve innervates a particular area of skin in a predicatable and orderly way. The area of skin innervated by sensory fibers from a particular spinal segment is called th eDERMATOME for that segment. The map showing the location of the dermatome for each spinal segment is called a DERMATOME MAP. Refer to the dermatome map below or in your Atlas (5-41)
 

Realize that typical dermatome maps are somewhat deceiving. There is actually considerable overlap between the territories supplied by nerves arising from adjacent spinal cord segments, as is summarized schematically below,
 

This point has considerable clinical significance. The implication is that in order to achieve complete loss of sensation on an area of skin, a patient would have to do damage to (or have anesthesia applied to) three adjacent spinal nerves.
 

SELF INSTRUCTIONAL EXERCISE FOR REVIEW OF SOMATIC COMPONENTS OF SPINAL NERVES.
 
1. Use the skull provided in your bone box, and several vertebrae or an articulated skeleton.
 
Using Atlas figures 7-32 and 7-34 as a guide, examine the interior of the skull (the cranial cavity). Locate the FORAMEN MAGNUM, the large opening at the base of the skull through which the brain is continuous with the spinal cord. Note also some of the smaller openings (FORAMINA) through which cranial nerves leave the cranial cavity. Do not attempt to memorize names and/or locations of cranial nerves at this time, however. (You will have a lab that deals specifically with this later in the semester.)
 
Now use either an articulated skeleton or line up several of the vertebrae from your bone box in proper anatomical position, using the figure from p. 35 of this handout as a guide. Locate the vertebral canal, which houses the spinal cord. Find an intervertebral foremen and realize that each spinal nerve exits through an intervertebral foremen. Dorsal root ganglia also sit just inside the intervertebral foramina.

2. Sluggo, a fourth-year medical student, is attempting to do a pleural tap on a patient at Grady in order to sample fluid which he suspects has accumulated in the costodiaphragmatic recess of the pleura. Sluggo has located the ideal spot to do this procedure, in the intercostal space between ribs 8 and 9, approximately 2.5 cm posterior to the midaxillary line. In Sluggo's excitement, however, he has forgotten to apply anesthetic to the area. As he inserts the needle to do the pleural tap, the patient yells in pain, and the intercostal muscles contract, causing the needle to jiggle.

a. On the schematic drawing below, use a dotted line to indicate the pathway for pain impulses to get from the skin to the spinal cord. Mark the location of the cell bodies for the sensory neurons with an X. Label the nerves involved. Use a solid line to indicate the pathway for motor impulses to get from the spinal cord to the intercostal muscles. Label the nerves involved. Mark the location of the cell bodies of the motor neurons with a diamond.
 

b. In what dermatome is the pain of the needle piercing the skin felt?

c. Sluggo is now panicked, having caused the patient a certain amount of pain already.  He remembers that he can produce anesthesia of the skin by injecting spinal nerves as they emerge through intervertebral foramina.  (This would NOT be a recommended conservative approach for this procedure, but remember, Sluggo is NERVOUS.)  Which spinal nerves would Sluggo have to anesthetize to produce complete anesthesia in the area in which he is going to do the pleural tap?

d. Fortunately for the patient, Brilliant, a first-year medical student, walks in, sees what is happening, and tactfully stops Sluggo from performing this rather drastic procedure. Brilliant has just completed her dissection (and presentation) of the thoracic wall and knows that there is an easier way to produce anesthesia on the thoracic wall, including both the skin and muscle layers. Using your knowledge of anatomy, describe in detail the procedure Brilliant has suggested to Sluggo.

ANSWERS to Question 2:

b. T8. The T8 dermatome is by definition the area of skin innervated by the T8 spinal nerve, which runs in the groove on the lower border of the 8th rib and supplies the area of skin between the 8th and 9th ribs.

c. Sluggo would have to anesthetize the 7th, 8th, and 9th thoracic nerves. Because of the overlap of skin areas innervated by adjacent spinal nerves, it would not suffice to anesthetize T8 alone.

d. Brilliant has recommended that Sluggo inject a local anesthetic up beneath the lower border of rib 8, Just posterior to the area where he wants to do the pleural tap. This would anesthetize the 8th intercostal nerve, which is mainly responsible for motor and cutaneous innervation to the area between the 8th and 9th ribs. Because there is some overlap of cutaneous innervation from adjacent dermatomes, Sluggomay also want to apply a little bit of anesthetic to the area of skin he intends to puncture, to deaden any branches of the 7th and 9 intercostal nerves which innervate this skin area.

(Please note: none of the above are necessarily real, approved clinical procedures.)
 

V. Spinal nerves: visceral components

The discussion above centered on SOMATIC components of spinal nerves. Somatic components of spinal nerves mediate skin sensation and proprioception (position sense) for the body wall and limbs, as well as motor innervation to skeletal (striated or "voluntary") muscle. Your intuition will tell you, however, that there must also be nerves which are capable of relaying sensory information (particularly pain or stretch) from the internal organs or VISCERA, and of conveying motor outflow to glands, smooth muscle in viscera and blood vessels, or to the (cardiac) muscle of the heart. These are the visceral components of spinal nerves. (There are also visceral components to cranial nerves, but these will be discussed in more detail in the part of the course dealing with the head and neck.)

VISCERAL MOTOR SYSTEM (AUTONOMIC NERVOUS SYSTEM)
 
The portions of the CNS and PNS which are concerned with regulation of visceral MOTOR functions (efferent fibers) have historically been referred to as the AUTONOMIC NERVOUS SYSTEM. However, for a variety of reasons, this designation is less widely accepted now, with some authors preferring to use the terms VISCERAL MOTOR or VISCERAL Et~kRENT to designate the neurons which relay motor outflow to cardiac muscle, smooth muscle, and glands. Historicallv. these neurons have been thought of as regulating body functions that are not under voluntary control ("automatic functions"), such as heart rate, blood pressure, gut motility, etc. However, recent advances in use of "biofeedback" techniques have shown that these functions can. to some extent, be controlled voluntarily.

The visceral motor system is somewhat different in its plan of organization from the somatic motor system. Recall that for the SOMATIC motor system the skeletal (striated) muscles are controlled by a motor neuron sitting in the ventral horn of the spinal cord, which sends its axon into the ventral root, then into a spinal nerve. The peripheral portion of the SOMATIC motor control system is therefore a ONE-NEURON SYSTEM. (Realize, however, that activity in a spinal cord motor neuron does not occur independently. Rather, it is influenced by other spinal cord neurons or bv descending axons of neurons whose cell bodies are in the brain.) Unlike the somatic motor control system, the peripheral component of the autonomic or VISCERAL MOTOR SYSTEM is a TWO-NEURON SYSTEM. The cell body of the first neuron, called a PREGANGLIONIC NEURON is located in the CNS. In the spinal cord, cell bodies of preganglionic visceral motor neurons are located in the INTERMEDIOLATERAL CELL COLUMN, rather than in the ventral horn, where the somatic motor neurons lie. However, as with other motor axons, axons of preganglionic visceral motor neurons leave the spinal cord by way of ventral roots.
 

The cell body of the second neuron, called a POSTGANGLIONIC NEURON, is located outside the dorsal cavity, in what are called AUTONOMIC or VISCERAL GANGLIA.
 

The visceral motor system is commonly divided into 2 parts: the SYMPATHETIC division and the PARASYMPATHETIC division. Most of the viscera receive innervation from both sympathetic and parasympathetic fibers, but the effects produced by activity in the two divisions generally oppose one another. Activity in the SYMPATHETIC NERVOUS SYSTEM is generally associated with an increase in the level of excitation of an organism. It is sometimes called the "fight or flight" system. The parasympathetic nervous system, on the other hand, is generally thought of as "vegetative", being concerned with the body's recovery from exertion, or active when the body is in its resting state. The table below is a brief summary of actions associated with the sympathetic and parasympathetic divisions of the autonomic nervous system.
 
 

SYMATHETIC PARASYMPATHETIC
increase in heart rate decrease in heart rate
decrease in gastric motility increase in gastric motility
decrease in secretion of salivary and digestive glands  increase in secretion of salivary and digestive glands
dilation of pupils constriction of pupils
ejaculation penile erection
vasoconstriction contraction of smooth muscle in walls of bladder
dilation of bronchioles
increased secretion of sweat glands
Aside from these functional differences between the sympathetic and parasympathetic divisions of the visceral motor system, there are significant differences in their anatomical organization. These are summarized on the drawing below. Preganglionic axons are represented by solid lines, postganglionic axons by interrupted lines. The major points are explainedin the text belong.
 

DIFFERENCES IN THE ORGANIZATION OF THE SYMPATHETIC AND PARASYMPATHETIC DIVISIONS OF THE VISCERAL MOTOR SYSTEM
 
1. LOCATION OF PREGANGLIONIC CELL BODES: Sympathetic preganglionic cell bodies are located in the spinal cord from T1 levels to L2 (or 3). The sympathetic division is often referred to as the THORACOLUMBAR DIVISION of the visceral motor system. Parasympathetic preganglionic cell bodies are located in the brain stem and in the spinal cord at S2 - levels. The parasympathetic division is often referred to as the CRANIOSACRAL division of the visceral motor system.
 
2. LOCATION OF POSTGANGLIONIC CELL BODES: Sympathetic postganglionic cell bodies are generally located near the vertebral column in PARAVERTEBRAL GANGLIA (SYMPATHETIC CHAIN GANGLIA) which are located atallvertebral levels, orPREVERTEBRAL(PREAORTIC) GANGLIA, which are located in the abdomen anterior to the vertebral column. near the stems of the major branches of the abdominal aorta. Parasympathetic postganglionic cell bodies are generally located within or very near to the target structure.

3. LENGTH AND TRAJECTORY OF PREGANGLIONIC AXONS: Sympathetic preganglionic axons pass only from thoracolumbar levels to ganglia located near the vertebral column, whereas parasympathetic preganglionic axons pass directly from craniosacral levels to the vicinity of the target organ. Sympathetic preganglionic axons are therefore (relatively) short, and parasympathetic preganglionic axons are (relatively) long. Aside from this general rule, several important points concerning the distribution of sympathetic and parasympathetic preganglionic axons need to be made.
 
SYMPATHETIC PREGANGLIONIC AXONS leave the CNS as part of spinal nerves T1-L2. ALL sympathetic preganglionic axons leave spinal nerves soon after their exit from the intervertebral foramen and enter the sympathetic chain ganglion at their own segmental level (see drawing below) by way of a WHITE RAMUS COMMUNICANS (so called because preganglionic axons are generally myelinated and have, theoretically, a glistening white appearance.) Some preganglionic axons synapse with a postganglionic neuron in the paravertebral ganglion at their own level, as does the one in the drawing immediately above, and the examples shown at levels T2-T4 in the summary drawing.

Other preganglionic sympathetic axons do not synapse in the ganglion at their own segmental level. Some preganglionic axons ascend or descend in the sympathetic chain to synapse in paravertebral ganglia at cervical or lower lumbar and sacral levels. An example of a preganglionic axon that behaves in this way is shown emerging from the T1 segment in the summary diagram above.

Other preganglionic axons pass through the paravertebral ganglion at their own segmental level to synapse in prevertebral (preaortic) ganglia (labeled as celiac. superior mesenteric. and inferior mesenteric ganglia in the summary drawing). Examples of preganglionic axons that behave this way are found at levels T5-L2 in the figure. These preganglionic axons form what are known as SPLANCHNIC NERVES on their way to the preaortic ganglia There are usually three major splanchnic nerves (on each side) arising from thoracic levels: the GREATER SPLANCHNIC NERVE usually arises from T5-9 or 10. The LESSER SPLANCHNIC NERVE usually arises from T10 and 11. The LEAST SPLANCHNIC NERVE usually arises from T12. Splanchnic nerves also arise from lumbar levels. These are called LUMBAR SPLANCHNICS.
 
Some splanchnic nerves do not synapse in prevertebral ganglia at all, but continue directly to the SUPRARENAL or ADRENAL GLANDS, where they innervate cells of the adrenal medulla directly. This would seem to be a violation of the rule that the visceromotor system is a two neuron system. Realize, however, that the cells of the adrenal medulla are modified postganglionic sympathetic neurons. You can see examples of preganglionic axons that pass through the celiac ganglion without synapsing in the figure.

PARASYMPATHETIC PREGANGLIONIC AXONS of neurons with cell bodies located in the brain stem leave the CNS with cranial nerves 3, 7, 9, and 10; then may tag along with other cranial nerves to get to their destinations. For the parasympathetic fibers in cranial nerves 3, 7, and 9, these are ganglia in or near the eye, the glands, and the smooth muscle of the head. Parasympathetic components of cranial nerve 10, called the VAGUS (means wanderer) have a more widespread territory, ultimately synapsing in ganglia in the walls of many viscera, including the heart and the digestive tract from the pharynx to the left colic flexure of the large intestine.

The axons of preganglionic parasympathetic neurons in spinal cord segments S2-4 leave the CNS in the ventral roots of spinal nerves S2-4, then form what are known as PELVIC SPLANCHNIC NERVES (or PELVIC NERVES, as in the figure above) on their way to synapse in ganglia in the walls of the pelvic viscera.
 
4. LENGTH AND TRAJECTORY OF POSTGANGLIONIC AXONS: Sympathetic postganglionic axons must pass from cell bodies in paravertebral or prevertebral ganglia to targets in the viscera. Therefore they are (relatively) long. Sympathetic postganglionic axons may reach their targets by:
 
a. rejoining a spinal nerve and traveling to target structures in the limbs or body wall. These targets would be mainly sweat glands, smooth muscle in the walls of blood vessels and arrector pill muscles.
 
b. passing directly from the paravertebral ganglion to the target organ. This is the case for sympathetic innervation of thoracic viscera.
 
c. tagging along with blood vessels supplying the target organ. Most postganglionic axons innervating abdominal or pelvic viscera arrive at their target organ by this route.
 
Parasympathetic postganglionic axons must pass only from cell bodies in ganglia located in or near the target organ to their specific target cells. Therefore they are (relatively) short. When the target is a thoracic, abdominal, or pelvic viscus, the ganglia are located in the walls of the viscus and are so short they are never seen on gross dissection. When the target is a gland or smooth muscle in the head. however, the ganglia sit a short distance from the target, and the postganglionic axons reach their targets by tagging along with other cranial nerves passing to the target structure.
 
5. NEUROTRANSMITTERS: Preganglionic sympathetic neurons use ACETYLCHOLINE as their neurotransmitter at synapses in prevertebral or paravertebral ganglia, but EPINEPHRINE or NOREPINEPHRINE is the neurotransmitter where the postganglionic axon synapses with the target organ.
 
ACETYLCHOLINE is the neurotransmitter for parasympathetic preganglionic and postganglionic neurons at synapses located in parasympathetic ganglia and on target cells.

These general features of the parasympathetic and sympathetic divisions of the visceral motor (autonomic) part of the peripheral nervous system are summarized in the table that follows.
 
 
Location of  
Preganglionic Neuron  
(Cell body)
Preganglionic  
Axon
Neurotransmitter  
at synapse
Location of  
Postganglionic Neuron  
(cell body)
Neurotransmitter  
at Target organ
 
  Smooth muscle  
Cardia muscle  
Gland
Parasympathetic  
Division
Brainstem;  
Intermediate gray  
matter of S2-S4
Cranial nerves  
3,7,9,10- then may  
tag along with  
other nerves
Acetylcholine Terminal ganglia in  
or near organ
Acetylcholine
Spinal nerves  
S2-S4- then as  
pelvic splanchnics
Sympathetic  
Division
Intermediolateral cell  
column of T1-L2
Spinal nerves T1-L2,  
then some run in  
sympathetic trunk or  
as splanchnic nerves
Acetylcholine Paravertebral ganglia  
OR  
Prevertebral ganglia  
OR  
Adrenal Medulla
Norepinephrine  
(Noradrenaline)  
or  
Epinephrine  
(adrenaline)
 

VISCERAL AFFERENTS

Intuition will tell you that there are such things as visceral afferents. (These should NOT be described as "autonomic" or "sympathetic" or "parasympathetic" afferents, as these descriptors should properly be reserved for MOTOR components of the visceral nervous system.) However, visceral sensation (at least insofar as it is consciously perceived) is largely limited to pain. This may be due to overdistension of a viscus, or spasm of smooth or cardiac muscle. A striking characteristic of visceral pain is that very often it is PERCEIVED in a body part other than where it is being produced. For example, the pain of a heart attack is often perceived as a pain radiating down the left arm. The pain of early appendicitis is often localized in the umbilical region of the abdominal wall. Pain which is produced in a viscus, but localized to the body wall or limbs is called REFERRED PAIN and is a notable characteristic of visceral sensation. The anatomical basis of referred pain is poorly understood. It is thought that, since visceral sensation is not usually consciously perceived, when a spinal cord segment is bombarded with pain input from an injured or inflamed viscus, the information is INTERPRETED as arising from body areas from which that segment generally receives input. That is, the pain is perceived as arising from the DERMATOME innervated by that spinal segment.
 
Aside from the special features described above, visceral afferents are highly similar to somatic afferents. That is, they are a one-neuron system, with a sensory axon arising near a receptor in a and passing all the way to the dorsal horn of the spinal cord. As with somatic sensory neurons, the cell body of a visceral sensory neuron sits in a dorsal root ganglion and the central process of the sensory axon passes to the spinal cord by way of a dorsal root. Afferents arising from a viscus will usually pass back to the CNS by tagging along with an autonomic nerve. This means that visceral sensory fibers may be found passing THROUGH autonomic ganglia, but they DO NOT synapse in those ganglia.
 

VI. The enteric nervous system

A complete discussion of the ENTERIC NERVOUS SYSTEM is beyond the scope of this course or this handout. However, you should be aware that it exists and have some idea what it is. Aside from postganglionic parasympathetic neurons found in the walls of the gut, the walls of the viscera of the gastrointestinal tract have been shown to contain a network of millions and millions of nerve cells which play an important role in controlling gut motility. This network forms what is known as the ENTERIC NERVOUS SYSTEM. Estimates are that the enteric nervous system may contain more nerve cells than the spinal cord! The details of the anatomy of the enteric nervous system are still poorly understood, however, and this is an area of active investigation. It is not yet clear how many neurotransmitters are utilized by enteric neurons, but the evidence suggests that SEROTONIN is a major neurotransmitter in this system.