Peripheral Terminations of Nerves of General Sensations

Human Body X. The Organs of the Senses and the Common Integument Peripheral Terminations of Nerves of General Sensations

1e. Peripheral Terminations of Nerves of General Sensations

The peripheral terminations of the nerves associated with general sensations, i. e., the muscular sense and the senses of heat, cold, pain, and pressure, are widely distributed throughout the body. These nerves may end free among the tissue elements, or in special end-organs where the terminal nerve filaments are enclosed in capsules.

Free nerve-endings occur chiefly in the epidermis and in the epithelium covering certain mucous membranes; they are well seen also in the stratified squamous epithelium of the cornea, and are also found in the root-sheaths and papillæ of the hairs, and around the bodies of the sudoriferous glands. When the nerve fiber approaches its termination, the medullary sheath suddenly disappears, leaving only the axis-cylinder surrounded by the neurolemma. After a time the fiber loses its neurolemma, and consists only of an axis-cylinder, which can be seen, in preparations stained with chloride of gold, to be made up of fine varicose fibrillæ. Finally, the axis-cylinder breaks up into its constituent fibrillæ which often present regular varicosities and anastomose with one another, and end in small knobs or disks between the epithelial cells.

Under this heading may be classed the tactile disks described by Merkel as occurring in the epidermis of the pig’s snout, where the fibrillæ of the axis-cylinder end in cup-shaped disks in apposition with large epithelial cells.

The special end-organs exhibit great variety in size and shape, but have one feature in common, viz., the terminal nerve fibrillæ are enveloped by a capsule. Included in this group are the end-bulbs of Krause, the corpuscles of Grandry, of Pacini, of Golgi and Mazzoni, of Wagner and Meissner, and the neurotendinous and neuromuscular spindles.

The end-bulbs of Krause (Fig. 934) are minute cylindrical or oval bodies, consisting of a capsule formed by the expansion of the connective-tissue sheath of a medullated fiber, and containing a soft semifluid core in which the axis-cylinder terminates either in a bulbous extremity or in a coiled-up plexiform mass. End-bulbs are found in the conjunctiva of the eye (where they are spheroidal in shape in man, but cylindrical in most other animals), in the mucous membrane of the lips and tongue, and in the epineurium of nerve trunks. They are also found in the penis and the clitoris, and have received the name of genital corpuscles; in these situations they have a mulberry-like appearance, being constricted by connective-tissue septa into from two to six knob-like masses. In the synovial membranes of certain joints, e. g., those of the fingers, rounded or oval end-bulbs occur, and are designated articular end-bulbs.

The tactile corpuscles of Grandry occur in the papillæ of the beak and tongue of birds. Each consists of a capsule composed of a very delicate, nucleated membrane, and contains two or more granular, somewhat flattened cells; between these cells the axis-cylinder ends in flattened disks.

The Pacinian corpuscles (Fig. 935) are found in the subcutaneous tissue on the nerves of the palm of the hand and sole of the foot and in the genital organs of both sexes; they also occur in connection with the nerves of the joints, and in some other situations, as in the mesentery and pancreas of the cat and along the tibia of the rabbit. Each of these corpuscles is attached to and encloses the termination of a single nerve fiber. The corpuscle, which is perfectly visible to the naked eye (and which can be most easily demonstrated in the mesentery of a cat), consists of a number of lamellæ or capsules arranged more or less concentrically around a central clear space, in which the nerve fiber is contained. Each lamella is composed of bundles of fine connective-tissue fibers, and is lined on its inner surface by a single layer of flattened epithelioid cells. The central clear space, which is elongated or cylindrical in shape, is filled with a transparent core, in the middle of which the axis-cylinder traverses the space to near its distal extremity, where it ends in one or more small knobs. Todd and Bowman have described minute arteries as entering by the sides of the nerves and forming capillary loops in the intercapsular spaces, and even penetrating into the central space.

FIG. 935 - Pacinian corpuscle, with its system of capsules and central cavity. a. Arterial twig, ending in capillaries, which form loops in some of the intercapsular spaces, and one penetrates to the central capsule. b. The fibrous tissue of the stalk. n. Nerve tube advancing to the central capsule, there losing its white matter, and stretching along the axis to the opposite end, where it ends by a tuberculated enlargement.

FIG. 936 - Papilla of the hand, treated with acetic acid. Magnified 350 times. A. Side view of a papilla of the hand. a. Cortical layer. b. Tactile corpuscle. c. Small nerve of the papilla, with neurolemma. d. Its two nervous fibers running with spiral coils around the tactile corpuscle. e. Apparent termination of one of these fibers. B. A tactile papilla seen from above so as to show its transverse section. a. Cortical layer. b. Nerve fiber. c. Outer layer of the tactile body, with nuclei. d. Clear interior substance.

Herbst has described a nerve-ending somewhat similar to the Pacinian corpuscle, in the mucous membrane of the tongue of the duck, and in some other situations. It differs, however, from the Pacinian corpuscle, in being smaller, in its capsules being more closely approximated, and especially in the act that the axis-cylinder in the central clear space is coated with a continuous row of nuclei. These bodies are known as the corpuscles of Herbst.

The corpuscles of Golgi and Mazzoni are found in the subcutaneous tissue of the pulp of the fingers. They differ from Pacinian corpuscles in that their capsules are thinner, their contained cores thicker, and in the latter the axis-cylinders ramify more extensively and end in flat expansions.

The tactile corpuscles of Wagner and Meissner (Fig. 936) are oval-shaped bodies. Each is enveloped by a connective-tissue capsule, and imperfect membranous septa derived from this penetrate the interior. The axis-cylinder passes through the capsule, and after making several spiral turns around the body of the corpuscle ends in small globular or pyriform enlargements. These tactile corpuscles occur in the papillæ of the corium of the hand and foot, the front of the forearm, the skin of the lips, the mucous membrane of the tip of the tongue, the palpebral conjunctiva, and the skin of the mammary papilla.

FIG. 937 - Nerve ending of Ruffini. (After A. Ruffini.)

Corpuscles of Ruffini.—Ruffini described a special variety of nerve-ending in the subcutaneous tissue of the human finger (Fig. 937); they are principally situated at the junction of the corium with the subcutaneous tissue. They are oval in shape, and consist of strong connective-tissue sheaths, inside which the nerve fibers divide into numerous branches, which show varicosities and end in small free knobs.

The neurotendinous spindles (organs of Golgi) are chiefly found near the junctions of tendons and muscles. Each is enclosed in a capsule which contains a number of enlarged tendon fasciculi (intrafusal fasciculi). One or more nerve fibers perforate the side of the capsule and lose their medullary sheaths; the axis-cylinders subdivide and end between the tendon fibers in irregular disks or varicosities (Fig. 938).

FIG. 938 - Organ of Golgi (neurotendinous spindle) from the human tendo calcaneus. (After Ciaccio.)

The neuromuscular spindles are present in the majority of voluntary muscles, and consist of small bundles of peculiar muscular fibers (intrafusal fibers), embryonic in type, invested by capsules, within which nerve fibers, experimentally shown to be sensory in origin, terminate. These neuromuscular spindles vary in length from 0.8 mm. to 5 mm., and have a distinctly fusiform appearance. The large medullated nerve fibers passing to the end-organ are from one to three or four in number; entering the fibrous capsule, they divide several times, and, losing their medullary sheaths, ultimately end in naked axis-cylinders encircling the intrafusal fibers by flattened expansions, or irregular ovoid or rounded disks (Fig. 939). Neuromuscular spindles have not yet been demonstrated in the tongue muscles, and only a few exist in the ocular muscles.