Basic Neuronal Arrangements
CLASSIFICATION OF NEURONS AND OF NERVE FIBRES IN PERIPHERAL NERVES ACCORDING TO FUNCTION
The nerve fibres that make up any peripheral nerve can be divided into two major types as follows.
a. Fibres that carry impulses from the CNS to an effector organ (e.g., muscle or gland) are called efferent or motor fibres.
b. Fibres that carry impulses from peripheral structures (e.g., skin) to the CNS are called afferentfibres. Some afferent fibres carry impulses that make us conscious of sensations like touch or pain:such fibres may, therefore, be called sensory fibres. Other afferent fibres convey information which is not consciously perceived, but is necessary for reflex control of various activities of the body.
Both afferent and efferent fibres can be further classified on the basis of the tissues supplied by them. The tissues and organs of the body can be broadly divided into two major categories − somatic and visceral. Somatic structures are those present in relation to the body wall (or soma). They include the tissues of the limbs (which represent a modified part of the body wall). Thus, the skin, bones, joints and striated muscles of the limbs and body wall are classified as somatic. In contrast, the tissues that make up the internal organs like the heart, lungs or stomach are classified as visceral. These include the lining epithelia of hollow viscera, and smooth muscle.
A distinction between somatic and visceral structures may also be made on embryological considerations.
1. Structures developing from specialised areas of ectoderm e.g., the retina and membranous labyrinth, are classified as somatic while the epithelium of the tongue (and taste buds) which is of endodermal origin is classified as visceral.
2. Striated muscle may be derived, embryologically, from three distinct sources. These are:
a. the somites developing in the paraxial mesoderm;
b. the somatopleuric mesoderm of the body wall; and
c. the mesoderm of the branchial arches.
Keeping in view the distinction between afferent and efferent fibres on one hand, and somatic and visceral structures on the other, we may divide fibres in peripheral nerves into four broad categories.
(a) Somatic efferent, (b) Visceral efferent,
(c) Somatic afferent, and (d) Visceral afferent.
With the exception of somatic efferent fibres each of the categories named above is subdivided into a general and a special group. We thus have a total of seven functional components as follows.
1. Somatic efferent (or somatomotor fibres) fibres supply striated muscle of the limbs and bodywall. They also supply the extrinsic muscles of the eyeballs, and the muscles of the tongue.
2. General visceral efferent fibres (also called visceromotor fibres) supply smooth muscle andglands. The nerves to glands are called secretomotor nerves.
3. Special visceral efferent fibres supply striated muscle developing in branchial arch mesoderm.They are frequently called branchial efferent or branchiomotor fibres. The muscles supplied include those of mastication, and of the face, the pharynx and the larynx.
4. General somatic afferent fibres are those that carry:
a. sensations of touch, pain and temperature from the skin (exteroceptive impulses);
b. proprioceptive impulses arising in muscles, joints and tendons conveying information regardingmovement and position of joints.
5. Special somatic afferent fibres carry impulses of:
b. hearing, and
6. General visceral afferent fibres (also called visceral sensory fibres) carry sensations e.g., painfrom viscera (visceroceptive sensations).
7. Special visceral afferent fibres carry the sensation of taste.
A typical spinal nerve contains fibres of the four general categories. The special categories are present in cranial nerves only.
We will now consider the general disposition of the neurons associated with each functional type of nerve fibre.