Nervous System - Biology Online Archive Article (original) (raw)

The neurolemma or primitive sheath presents the appearance of a delicate, structureless membrane. Here and there beneath it, and situated in depressions in the white matter of Schwann, are nuclei surrounded by a small amount of protoplasm. The nuclei are oval and somewhat flattened, and bear a definite relation to the nodes of Ranvier, one nucleus generally lying in the center of each internode. The primitive sheath is not present in all medullated nerve fibers, being absent in those fibers which are found in the brain and medulla spinalis.

_**Non-medullated Fibers.**—_Most of the fibers of the sympathetic system, and some of the cerebrospinal, consist of the gray or gelatinous nerve fibers (fibers of Remak). Each of these consists of an axis-cylinder to which nuclei are applied at intervals. These nuclei are believed to be in connection with a delicate sheath corresponding with the neurolemma of the medullated nerve fiber. In external appearance the non-medullated nerve fibers are semitransparent and gray or yellowish gray. The individual fibers vary in size, generally averaging about half the size of the medullated fibers.

_**Structure of the Peripheral Nerves and Ganglia.**—_The cerebrospinal nerves consist of numerous nerve fibers collected together and enclosed in membranous sheaths. A small bundle of fibers, enclosed in a tubular sheath, is called a funiculus; if the nerve is of small size, it may consist only of a single funiculus; but if large, the funiculi are collected together into larger bundles or fasciculi, which are bound together in a common membranous investment. In structure the common membranous investment, or sheath of the whole nerve (epineurium), as well as the septa given off from it to separate the fasciculi, consist of connective tissue, composed of white and yellow elastic fibers, the latter existing in great abundance. The tubular sheath of the funiculi (perineurium) is a fine, smooth, transparent membrane, which may be easily separated, in the form of a tube, from the fibers it encloses; in structure it is made up of connective tissue, which has a distinctly lamellar arrangement. The nerve fibers are held together and supported within the funiculus by delicate connective tissue, called the endoneurium. It is continuous with septa which pass inward from the innermost layer of the perineurium, and shows a ground substance in which are imbedded fine bundles of fibrous connective tissue running for the most part longitudinally. It serves to support capillary vessels, arranged so as to form a net-work with elongated meshes. The cerebrospinal nerves consist almost exclusively of medullated nerve fibers, only a very small proportion of non-medullated being present.

The bloodvessels supplying a nerve end in a minute capillary plexus, the vessels composing which pierce the perineurium, and run, for the most part, parallel with the fibers; they are connected together by short, transverse vessels, forming narrow, oblong meshes, similar to the capillary system of muscle. Fine non-medullated nerve fibers, vasomotor fibers, accompany these capillary vessels, and break up into elementary fibrils, which form a network around the vessels. Horsley has demonstrated certain medullated fibers running in the epineurium and terminating in small spheroidal tactile corpuscles or end bulbs of Krause. These nerve fibers, which Marshall believes to be sensory, and which he has termed nervi nervorum, are considered by him to have an important bearing upon certain neuralgic pains.

The nerve fibers, so far as is at present known, do not coalesce, but pursue an uninterrupted course from the center to the periphery. In separating a nerve, however, into its component funiculi, it may be seen that these do not pursue a perfectly insulated course, but occasionally join at a very acute angle with other funiculi proceeding in the same direction; from this, branches are given off, to joint again in like manner with other funiculi. It must be distinctly understood, however, that in these communications the individual nerve fibers do not coalesce, but merely pass into the sheath of the adjacent nerve, become intermixed with its nerve fibers, and again pass on to intermingle with the nerve fibers in some adjoining funiculus.

Nerves, in their course, subdivide into branches, and these frequently communicate with branches of a neighboring nerve. The communications which thus take place form what is called a plexus. Sometimes a plexus is formed by the primary branches of the trunks of the nerves—as the cervical, brachial, lumbar, and sacral plexuses—and occasionally by the terminal funiculi, as in the plexuses formed at the periphery of the body. In the formation of a plexus, the component nerves divide, then join, and again subdivide in such a complex manner that the individual funiculi become interlaced most intricate’y; so that each branch leaving a plexus may contain filaments from all the primary nervous trunks which form the plexus. In the formation also of smaller plexuses at the periphery of the body there is a free interchange of the funiculi and primitive fibers. In each case, however, the individual fibers remain separate and distinct.

It is probable that through this interchange of fibers, every branch passing off from a plexus has a more extensive connection with the spinal cord than if it had proceeded to its distribution without forming connections with other nerves. Consequently the parts supplied by these nerves have more extended relations with the nervous centers; by this means, also, groups of muscles may be associated for combined action.

The sympathetic nerves are constructed in the same manner as the cerebrospinal nerves, but consist mainly of non-medullated fibers, collected in funiculi and enclosed in sheaths of connective tissue. There is, however, in these nerves a certain admixture of medullated fibers. The number of the latter varies in different nerves, and may be estimated by the color of the nerve. Those branches of the sympathetic, which present a well-marked gray color, are composed chiefly of non-medullated nerve fibers, intermixed with a few medullated fibers; while those of a white color contain many of the latter fibers, and few of the former.

The cerebrospinal and sympathetic nerve fibers convey various impressions. The sensory nerves, called also centripetal or afferent nerves, transmit to the nervous centers impressions made upon the peripheral extremities of the nerves, and in this way the mind, through the medium of the brain, becomes conscious of external objects. The centrifugal or efferent nerves transmit impressions from the nervous centers to the parts to which the nerves are distributed, these impressions either exciting muscular contraction or influencing the processes of nutrition, growth, and secretion.

_**Origins and Terminations of Nerves.**—_By the expression “the terminations of nerve fibers” is signified their connections with the nerve centers and with the parts they supply. The former are sometimes called their origins or central terminations; the latter their peripheral terminations.

_**Origins of Nerves.**—_The origin in some cases is single—that is to say, the whole nerve emerges from the nervous center by a single root; in other instances the nerve arises by two or more roots which come off from different parts of the nerve center, sometimes widely apart from each other, and it often happens, when a nerve arises in this way by two roots, that the functions of these two roots are different; as, for example, in the spinal nerves, each of which arises by two roots, the anterior of which is motor, and the posterior sensory. The point where the nerve root or roots emerge from the surface of the nervous center is named the superficial or apparent origin, but the fibers of the nerve can be traced for a certain distance into the substance of the nervous center to some portion of the gray matter, which constitutes the deep or real origin of the nerve. The centrifugal or efferent nerve fibers originate in the nerve cells of the gray substance, the axis-cylinder processes of these cells being prolonged to form the fibers. In the case of the centripetal or afferent nerves the fibers grow inward either from nerve cells in the organs of special sense, e. g., the retina, or from nerve cells in the ganglia. Having entered the nerve center they branch and send their ultimate twigs among the cells, without, however, uniting with them.

_**Peripheral Terminations of Nerves.**—_Nerve fibers terminate peripherally in various ways, and these may be conveniently studied in the sensory and motor nerves respectively. The terminations of the sensory nerves are dealt with in the section on Sense Organs.

Motor nerves can be traced into either unstriped or striped muscular fibers. In the unstriped or involuntary muscles the nerves are derived from the sympathetic, and are composed mainly of non-medullated fibers. Near their terminations they divide into numerous branches, which communicate and form intimate plexuses. At the junction of the branches small triangular nuclear bodies (ganglion cells) are situated. From these plexuses minute branches are given off which divide and break up into the ultimate fibrillæ of which the nerves are composed. These fibrillæ course between the involuntary muscle cells, and, according to Elischer, terminate on the surfaces of the cells, opposite the nuclei, in minute swellings.

In the striped or voluntary muscle the nerves supplying the muscular fibers are derived from the cerebrospinal nerves, and are composed mainly of medullated fibers. The nerve, after entering the sheath of the muscle, breaks up into fibers or bundles of fibers, which form plexuses, and gradually divide until, as a rule, a single nerve fiber enters a single muscular fiber. Sometimes, however, if the muscular fiber be long, more than one nerve fiber enters it. Within the muscular fiber the nerve terminates in a special expansion, called by Kühne, who first accurately described it, a motor end-plate. The nerve fiber, on approaching the muscular fiber, suddenly loses its medullary sheath, the neurolemma becomes continuous with the sarcolemma of the muscle, and only the axis-cylinder enters the muscular fiber. There it at once spreads out, ramifying like the roots of a tree, immediately beneath the sarcolemma, and becomes imbedded in a layer of granular matter, containing a number of clear, oblong nuclei, the whole constituting an end-plate from which the contractile wave of the muscular fiber is said to start.

Ganglia are small aggregations of nerve cells. They are found on the posterior roots of the spinal nerves; on the sensory roots of the trigeminal, facial, glossopharyngeal, and vagus nerves, and on the acoustic nerves. They are also found in connection with the sympathetic nerves. On section they are seen to consist of a reddish-gray substance, traversed by numerous white nerve fibers; they vary considerably in form and size; the largest are found in the cavity of the abdomen; the smallest, not visible to the naked eye, exist in considerable numbers upon the nerves distributed to the different viscera. Each ganglion is invested by a smooth and firm, closely adhering, membranous envelope, consisting of dense areolar tissue; this sheath is continuous with the perineurium of the nerves, and sends numerous processes into the interior to support the bloodvessels supplying the substance of the ganglion.

In structure all ganglia are essentially similar, consisting of the same structural elements—viz., nerve cells and nerve fibers. Each nerve cell has a nucleated sheath which is continuous with the neurolemma of the nerve fiber with which the cell is connected. The nerve cells in the ganglia of the spinal nerves are pyriform in shape, and have each a single process. A short distance from the cell and while still within the ganglion this process divides in a T-shaped manner, one limb of the cross-bar turning into the medulla spinalis, the other limb passing outward to the periphery. In the sympathetic ganglia the nerve cells are multipolar and each has one axis-cylinder process and several dendrons; the axon emerges from the ganglion as a non-medullated nerve fiber. Similar cells are found in the ganglia connected with the trigeminal nerve, and these ganglia are therefore regarded as the cranial portions of the sympathetic system. The sympathetic nervous system includes those portions of the nervous mechanism in which a medullated nerve fiber from the central system passes to a ganglion, sympathetic or peripheral, from which fibers, usually non-medullated, are distributed to such structures, e. g., bloodvessels, as are not under voluntary control. The spinal and sympathetic ganglia differ somewhat in the size and disposition of the cells and in the number of nerve fibers entering and leaving them. In the spinal ganglia the nerve cells are much larger and for the most part collected in groups near the periphery, while the fibers, which are mostly medullated, traverse the central portion of the ganglion; whereas in the sympathetic ganglia the cells are smaller and distributed in irregular groups throughout the whole ganglion; the fibers also are irregularly scattered; some of the entering ones are medullated, while many of those leaving the ganglion are non-medullated.

_**Neuron Theory.**—_The nerve cell and its processes collectively constitute what is termed a neuron, and Waldeyer formulated the theory that the nervous system is built up of numerous neurons, “anatomically and genetically independent of one another.” According to this theory (neuron theory) the processes of one neuron only come into contact, and are never in direct continuity, with those of other neurons; while impulses are transmitted from one nerve cell to another through these points of contact, the synapses. The synapse or synaptic membrane seems to allow nervous impulses to pass in one direction only, namely, from the terminals of the axis-cylinder to the dendrons. This theory is based on the following facts, viz.: (1) embryonic nerve cells or neuroblasts are entirely distinct from one another; (2) when nervous tissues are stained by the Golgi method no continuity is seen even between neighboring neurons; and (3) when degenerative changes occur in nervous tissue, either as the result of disease or experiment, they never spread from one neuron to another, but are limited to the individual neurons, or groups of neurons, primarily affected. It must, however, be added that within the past few years the validity of the neuron theory has been called in question by certain eminent histologists, who maintain that by the employment of more delicate histological methods, minute fibrils can be followed from one nerve cell into another. Their existence, however, in the living is open to question. Mott and Marinesco made careful examinations of living cells, using even the ultramicroscope and agree that neither Nissl bodies nor neurofibrils are present in the living state.

For the present we may look upon the neurons as the units or structural elements of the nervous system. All the neurons are present at birth which are present in the adult, their division ceases before birth; they are not all functionally active at birth, but gradually assume functional activity. There is no indication of any regeneration after the destruction of the cell-body of any individual neuron.

Fasciculi, tracts or fiber systems are groups of axons having homologous origin and homologous distribution (as regards their collaterals, subdivisions and terminals) and are often named in accordance with their origin and termination, the name of the nucleus or the location of the cell body from which the axon or fiber arises preceding that of the nucleus or location of its termination. A given topographical area seldom represents a pure tract, as in most cases fibers of different systems are mixed.

Contributed by asutoshsahu. Posted on June 2, 2006