THE NERVOUS SYSTEM

The nervous system is the impulse conducting system which regulates all activities of the body. Its fundamental unit is the nerve cell (neuron). The spinal cord and brain are located within the vertebral column and skull, are wrapped by a series of meninges (pia, arachnoid and dura) and are bathed by cerebrospinal fluid. Peripheral nerve fibers originate from specialized sensory corpuscles (afferents) and terminate in motor end plates (efferents).

I. Subdivisions of the Nervous System

A. Central nervous system (CNS)

1. Brain

2. Spinal Cord

B. Peripheral nervous system (PNS)

1. Cranial nerves

2. Spinal nerves

C. Autonomic (visceral) nervous system (ANS)

1. Sympathetic (thoracolumbar) division

2. Parasympathetic (craniosacral) division

II. Functional Components of Peripheral Nerves

A. Afferent (sensory)

1. Somatic (from the body wall)

a. Special (vision, hearing and balance)

b. General (skin-pain, temperature, touch; joints, tendons, ligaments-proprioception)

2. Visceral (from the viscera)

a. Special (taste and smell)

b. General (visceral urges and pain)

B. Efferent (motor)

1. Somatic

a. Special (nonexistent as there are no tissues or organs which fit this description)

b. General (to myomeric musculature)

2. Visceral

a. Special (to branchiomeric musculature)

b. General (to smooth muscle and glands throughout the body - 2 neuron chain)

1. preganglionic fibers

2. postganglionic fibers

III. Peripheral Ganglia (clusters of nerve cell bodies)

A. Dorsal root ganglia -- bodies of GSA + GVA fibers

B. Sympathetic chain ganglia -- postganglionic bodies of sympathetic (GVE) fibers

C. Prevertebral (collateral) ganglia -- postganglionic bodies of sympathetic (GVE) fibers

D. Terminal ganglia -- postganglionic bodies of parasympathetic (GVE) fibers

• A typical spinal nerve is composed of 4 different types of neurons, defined by their function and position. Sensory (AFFERENT) neurons convey impulses from the SOMA and VISCERA to the central nervous system (CNS). Motor (EFFERENT) neurons convey impulses from the CNS to the SOMA and VISCERA. These 4 types of neurons are also referred to as functional components (of a peripheral nerve) and may be defined as follows:

1. Somatic Afferents (SA) - pain, temperature and touch sensations initiated by receptors in the skin, as well as proprioceptive impulses from muscles (tension) and joints (position). Their cell bodies lie in the dorsal root ganglion.

2. Visceral Afferents (VA) - general visceral sensation ranging from the sharp pain of an ulcer to the vague sensation of hunger. Their cell bodies also lie in the dorsal root ganglion.

3. Somatic Efferents (SE) - motor impulses to skeletal muscles. Their cell bodies lie in the ventral horn of gray matter.

4. Visceral Efferents (VE) - motor impulses to smooth muscle, cardiac muscle and glands throughout the body. This component is unique in that it consists of a 2-neuron chain. The cell bodies of the primary (PREGANGLIONIC) neurons lie in the lateral horn of gray matter. The cell bodies of the secondary (POSTGANGLIONIC) neurons lie in paravertebral (SYMPATHIC CHAIN) ganglia or more distant visceral ganglia.

• The visceral components (VA + VE) are distinguished as constituting the AUTONOMIC NERVOUS SYSTEM (ANS) since they are primarily responsible for regulating the involuntary activities of smooth muscles, cardiac muscle and glands. Some neuroanatomist consider the ANS to be purely motor (VE) and thus do not include VA neurons. In this course we will consider the ANS to be equivalent to the "visceral nervous system" (VA + VE).



• While the somatic components (SA + SE) are restricted to the soma, the visceral components are distributed throughout the body (i.e. both viscera and soma!). The reason for this apparent inconsistency is that smooth muscle is a major component of blood vessels and the skin is loaded with glands. Thus, for example, a skeletal muscle is innervated by a SE neuron but the smooth muscle in the blood vessels supplying it is supplied by VE neurons.



• Thus, it is not accurate to refer to somatic nerves as being "sensory" or "motor" since the vast majority of nerves are mixed, i.e. contain both afferent and efferent neurons. For example, nerves to the skin should be referred to as "cutaneous" (not sensory) since they contain both SA (pain, temperature and touch) neurons and VE (smooth muscle and glands) neurons. For the same reason, nerves to skeletal muscle should be referred to as "muscular" (not motor) since they contain both SE (skeletal muscle) neurons and SA (proprioceptive fibers - i.e. joint position and muscle tension) neurons.

• EXERCISE: Study the DISTRIBUTION DIAGRAM of a thoracic spinal nerve and identify the following components:

1. dorsal horn

2. lateral horn

3. ventral horn

4. dorsal root (plus ganglion)

5. ventral root

6. dorsal ramus (=epaxial ramus)

7. ventral ramus (=hypaxial ramus)

8. sympathetic ganglion

9. medial and lateral communicating rami

10. posterior cutaneous branch

11. lateral cutaneous branch

12. anterior cutaneous branch

• EXERCISE: Study the WIRING DIAGRAM of a thoracic spinal nerve and draw in the 4 FUNCTIONAL COMPONENTS.

1. somatic afferent neuron

2. somatic efferent neuron

3. visceral afferent neuron

4. visceral efferent neuron