Histology of the Nervous System

HISTOLOGY OF THE NERVOUS SYSTEM Three kinds of tissue are found in the nervous system, nerve fibres, nerve cells and a supporting tissue called neuroglia.

Nerve Fibres

may be medullated or non-medullated, but, whichever they are, they consist of the long process or axon of a nerve cell; in a non-medullated nerve this process is either naked or enclosed in a delicate membrane called the primitive sheath or neurilemma, but in a medullated nerve the process or axis cylinder is encased by a white fatty substance called myelin, and so the term "myelinated" is often used instead of "medullated" for these nerves (fig. 1). Outside this white sheath the neurilemma is present in most nerves, but is lost when they are massed to form the white matter of the central nervous system and in the optic nerve. At regular intervals the myelin is interrupted by some substance which stains deeply with silver nitrate, and these breaks are known as nodes of Ranvier. They do not, however, affect the axis cylinder. In a large nerve, e.g., the median, the nerve fibres are collected into small bundles (funiculi), enclosed in a connective tissue sheath (perineurium), and separated from it by a lymph space. From this sheath delicate processes penetrate among the fibres (endaneurium). The funiculi are collected into bundles called faiciculi, and the whole nerve consists of a variable number of fasciculi surrounded by a dense fibrous sheath, the epineurium.

Nerve Cells

are unipolar, bipolar or multipolar. Unipolar cells are found in the ganglia on the posterior roots of the spinal nerves, and only give off an axon or axis cylinder process; this, however, soon divides in a T-shaped manner, and all these cells were originally bipolar, though the cell has grown away from its two axons (or, as they are often regarded, axon and dendrite), leaving a stalk joining it to them at right angles. Bipolar cells are found as an embryonic stage of unipolar, though in fish they persist in the spinal ganglia throughout life. They are also sometimes found in the sympathetic ganglia. Multi polar cells are found in the brain and cord, and are best studied in the anterior horns of the grey matter of the latter, where they are nearly visible to the naked eye (fig. 2).

Neuroglia.

This is the delicate connective tissue which sup ports and binds together the nervous elements of the central nervous system. One part of it, which lines the central canal of the cord and ventricles of the brain, is formed of columnar cells (ependyma), while the rests con sists of small cells with numerous processes which sometimes branch and sometimes do not.

Nerve Endings.—Sensory nerves end by breaking up into fibrillae or by various tactile organs. In the former case the minute fibrils composing the axons or nerve fibres separate and end among epithelial cells of the mucous membrane or skin. In

the latter case the nerve fibres lose their myelin sheath and end in one of the seven following organs :— I. End bulbs of Krause (see fig. 3), oval bulbs composed of elongated cells among which the nerve fibrils end in knobs or coils; each is surrounded by a sheath of neurilemma, and the organs are found in the lips, tongue, conjunctiva, epineurium of nerves, synovial membranes of joints, and in the glans penis et clitoridis, where they have a mulberry-like appearance.

2. Pacinian corpuscles (fig. 3) are large enough to be seen by the naked eye, and are oval bodies made up of a series of concentric capsules of connective tissue rather resembling the structure of an onion; in the centre is a structureless core, at the distal extremity of which the nerve fibre ends in one or more knobs. These bodies are found in the palm and sole, in the mesentery, the genital organs and in joints.

3. Tactile corpuscles of Meissner and Wagner (fig. 3) are oval bodies found in certain of the skin papillae and mucous mem brane, especially of very sensitive parts like the hand and foot, lips, tongue and nipple. They are oval and made of a connective tissue capsule from which septa enter the interior. The nerve fibre generally takes a spiral course through them, loses its myelin sheath, and ends by breaking up into its fibrils, which eventually become bulbous.

4. Tactile corpuscles of Grandry are found in the skin of those parts devoid of hair, and consist of a capsule containing two or more largish cells, between which the nerve fibre ends in the so-called tactile discs.

5. Rujini's endings are flattened oval bodies with a thick connective tissue cap sule, in which the nerve fibre divides into many varicose branches, form a rich plexus, and end in knobs. They are found between the true skin and subcutaneous tissues of the fingers.

6. Organs of Golgi occur in tendons. Nerve fibres penetrate the tendon, bundles and divide in a tree-like manner to end in little disks and varicosities.

7. Neuromuscular spindles are small fusiform bundles of embryonic muscle fibres, among which the nerve fibres end by encircling them and forming flattened disks. These are sensory endings, and must not be confused with the motor end plates.

Motor nerves end in striped muscle by motor end plates. These are formed by a nerve fibre approaching a muscle fibre and suddenly losing its myelin sheath while its neurilemma becomes continuous with the sarcolemma of the muscle fire. The axis cylinder divides, and its ramifications are surrounded by a disk of granular matter containing many clear nuclei.