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The structural relations of the neurons concerned in this functional relation are as follows: The sensory neurons are those whose cell bodies make up the spinal ganglia. These lie in the intervertebral foramina. The peripheral outgrowths from these cells are distributed to the various tissues of the body and terminate in the remarkable structures called sensory end organs. These peripheral branches from the sensory neuron body Van Gehuchten called dentrites, a name which is very appropriate from the anatomical as well as from the functional standpoint, although nearly all the authors on the subject persist in referring to these cells as being bipolar, and of the peripheral termination as an axon. Every writer agrees that the central prolongations of the sensory cells are axons. These
penetrate the cord as its posterior roots. Each axon divides immediately
upon entering the cord, in a T-shaped manner. The shorter of these
branches passes downward for a distance not exceeding two or three of the
spinal segments. The long branch passes upward to the nucleus gracilis
or the nucleus ceuneatus. Both these branches give off collaterals
very freely near their point of division. These collaterals
are concerned in carrying the sensory impulses from the periphery to the
motor and association neurons which initiate and coordinate efferent impulses.
Collaterals
form synapses with the cells in the anterior horns of the cord. These
anterior horn cells send axons to the striated or skeletal muscles.
By means of this relationship sensory impulses initiate reflex contractions
of these muscles.
Collaterals
form synapses with the cells in the posterior horn. The axons of
these cells enter into relation with cells in the gray matter of the same
and adjoining segments, both of the same and the opposite sides.
By means of the interference of these cells, the complexity of the reflex
pathways is greatly increased.
These structural relations are the same for all, or nearly all, of the cord segments. In the extreme lower portion of the cord there seem to be slight variations in the arrangement. In the cervical and lumbar portions of the cord there are no lateral out-going white rami communicantes, but the axons of the cells of the lateral horn pass through the white matter of the lateral mixed tracts to higher or lower levels, where they terminate in the lateral horn or pass out directly with the anterior roots and help to form the white rami. The lateral horn cells also send out certain other groups of fibers, which make up the splanchnics, the erigens, and others. The centers in the floor of the fourth ventricle and the aqueduct which are viscero-motor in function also send out fibers which pass directly to the anterior ganglia of the sympathetic system. These are the visceral fibers of the third, seventh, tenth, and others of the cranial nerves. These viscero-motor centers in the floor of the fourth ventricle and the aqueduct have not been studied so thoroughly as have the cord centers. The problems offered by these centers are much more complex, partly because of the branchial divisions of the nerve centers, partly because of the changes in the relations and functions of the muscles and other structures innervated by the various nerve centers during the developmental changes, and partly because of other modifications due to the process of cephalization. In
spite of these complications, however, a close study of the structural
relations of the nerve centers, both viscero-motor and somato-motor, in
the medulla, pons, and mid-brain, indicates that in the main the sensory
impulses reaching any segment, or collection of neurons homologous with
a spinal segment, affect all the structures innervated from that segment
or neuron group. The principle as stated at the head of this chapter
is almost certainly true for the sub-cerebral centers, as it is for the
spinal segments.
The
relations of these reflex actions as they are displayed in the spinal centers
are of utmost importance in diagnosis and therapeutics. By means
of these, the body is protected from injury, compensation is secured in
the presence of malfunction or injury to organs, and recovery is facilitated.
On the other hand, by means of the same reflex actions, the injury of any
organ may be a source of embarrassment to others, and malfunction so produced
may, in turn affect adversely the tissues first injured.
Sensory impulses derived from the skin initiate motor impulses to the muscles beneath the point of irritation. Within certain limits, the stronger the initial stimulation, the stronger and the more widespread are the resulting muscular contractions. Sensory impulses from any area initiate also changes in the blood vessels of the same area. Excessive
sensory impulses from the skin may occasionally affect visceral activity.
For example, irritation of the skin of the back of the neck causes dilatation
of the pupils.
Sensory impulses from articular structures cause the contraction of the muscles which move the joint. Thus the joint which is subjected to any irritating influences is held at rest by the forced contraction of all the muscles which move it. This rest is at first a curative measure, but afterwards, if the irritation persists, it renders the joint useless. Sensory
impulses from a joint held in a position of strain initiate motor impulses
to the same muscles. Any tension upon the tissues surrounding a joint
may be a source of the same abnormal contractions. Normally, the
movements of the joints are a source of the impulses which maintain the
tone of the muscles. If any joint is held immovable, either by its
mal-position or by the persistent contraction of the muscles which should
move it, the sensory impulses from it are not normal, and the other muscles
in central relation with the affected joint lose their tone and become
“flabby” and inefficient. In clinic practice, it is not unusual to
find the muscles in the immediate neighborhood of an abnormal joint severely
and painfully contracted, while the more superficial muscles are limp and
flabby.
Sensory
impulses from the viscera initiate the contraction of the skeletal muscles
innervated from the same segment, as well as the changes in their own musculature
and of the size of the blood vessels of their own area. The skeletal
muscles most affected by these viscero-sensory impulses are those which
remained unmodified during the embryonic development. The limb muscles
became so greatly shifted during their growth, and passed through so many
changes, with the concomitant slight rearrangement of the neuron associations,
that the limb muscles are less strongly affected by viscero-sensory impulses
than are the spinal and intercostal muscles. The limb muscles are
somewhat affected by the viscero-sensory impulses, but the effect is usually
slight.
The contraction of the spinal and intercostal muscles in central relation with a disordered viscus is at first of decided benefit. Upon the appearance of a malfunction of any viscus, the sensory impulses initiated by the irritation of its sensory nerve endings affect both its own activity and the activity of other structures innervated from the same segment, for example, the spinal muscles. The contraction of the spinal muscles sends other sensory impulses into the same segment, which in turn affect visceral activities. At the same time these impulses assist in lowering the liminal value of the neurons of the same segment, and so increase their efficiency. The disordered viscus thus becomes stimulated somewhat in excess of the usual amount, and is thereby aided to recovery. It
must be noted that the stimulation thus sent to the viscus is not excessive
in view of its embarrassment, but is in excess of that required under normal
circumstances. After the disorder has disappeared, as it does if
caused by some merely transient abnormality of environment, the stream
of abnormal sensory impulses ceases, and the muscles are permitted to return
to their normal condition of tonicity. This return to the normal
condition is facilitated by the slight fatigue of the muscles and of the
neurons concerned in regulating the whole series of reflex actions.
This is the series of events as they occur under favorable conditions.
When
the disorder is not transient, as in the case of persistent wrong doing
from the dietetic or hygienic standpoint, or of some incurable structural
disorder, or of some continual nerve irritation from mal-adjustments of
articular surfaces, or other more or less permanent causes of malfunction,
then the persistence of these reflexes becomes a source of injury.
The effect of long continued contraction of a muscle is to subject the
sensory nerve ending within it to a functional paralysis. This condition
is a matter of common experience, and is noted whenever any muscle is forced
to remain contracted for too long a time, as in carrying a heavy burden
too far. The muscles of the arm then are kept contracted too long
and too strongly, and they feel numb and dead for some time. The
return of sensation is accompanied by some pain. The same condition
characterizes the sustained contraction of the spinal muscles, except that
the condition is usually so slowly produced, and rest for the spinal muscles
is so easily secured, that consciousness is not very often affected.
The patient is not always conscious that there is any unusual condition
of the spinal muscles at all until a physical examination is made.
The
normal stream of sensory impulses from the alternate contraction and relaxation
of the muscles affected is lacking, in these cases, and all the structures
innervated from the same segment of the cord lack something of their normal
nerve impulses. The contracted muscles themselves are not well nourished,
since their blood vessels are subjected to continual pressure. The
normal flow of the nutrient lymph is also impeded. The vertebrae
are subjected to normal tension, and if the pull of the opposing muscles
be unequal, they are apt to be drawn from their normal alignment.
In any case, the tension exerts a pressure upon the structures surrounding
and penetrating the inter-vertebral foramina, and subjects the articular
surfaces to abnormal strain. The tension upon the articular surfaces
adds to the sum of the abnormal irritation. The stream of abnormal
sensory impulses aroused by these abnormal conditions is self-perpetuating,
as is indicated by the structural and functional relations already discussed,
and is only terminated by the exhaustion of the nerve centers, with their
forced inactivity, or by corrective work upon the structures concerned.
During the period of the existence of such conditions as those just mentioned, the spinal cord itself does not remain unaffected. The circulation through the cord is controlled by vaso-motor impulses derived from its own lateral horns, but reaching its vessels from the sympathetic ganglia. Any abnormal activity of the cord exercises an abnormal effect upon the size of the vessels in that segment. An abnormal increase in the sensory impulses reaching any segment increases the activity of that segment proportionately, and thus increases the dilatation of its vessels to a certain extent. This effect is easily seen in experiments upon animals under anesthesia. The
neurons of the cord are variously injured by the constant congestion, according
to the amount and persistence of the irritation, and other factors.
The arterial supply to the cord and its membranes, the lymphatic and venous drainage, and both outgoing and incoming nerves pass through the intervertebral foramina. Now when the spinal muscles are kept strongly contracted, especially if the tension be greater on one side than the other, the connective tissues around these foramina are subjected to considerable tension. The structures passing through the foramina are pressed upon in a manner that varies according to the direction and force of the contracted muscles and the structural peculiarities of the vertebrae affected. If the pressure is quickly removed the effect is transient and perhaps not at all abnormal. If
the pressure be long continued, the connective tissues, adapting themselves
to the condition by growth changes, as is the habit of connective tissues,
become thickened on the side of greatest pressure, while the ligaments
which are subjected to the constant pull are weakened and lengthened.
The slightly abnormal relations of the vertebrae thus become permanent,
unless corrective work be done. The size of the intervertebral foramina
may be somewhat lessened by the thickening of the connective tissues, but
probably the most serious effects are due to the abnormal tension upon
the intervertebral vessels and nerves, and to the abnormal stream of sensory
impulses from the contracted muscles and the articular surfaces.
Sensory impulses from the viscera initiate motor impulses to the viscera. This reaction is constantly active during life. By this series of reflexes, the varied and complex processes of digestion, circulation, excretion, and all other visceral activities are carried on in a coordinated manner, without conscious effort or sensation. Under slightly abnormal conditions, the increased visceral activity facilitates recovery. Reversed peristalsis follows the ingestion of some toxins, for example; other toxic substances excite merely the increase of the normal peristaltic waves, with increased secretion. In many other instances familiar to every student of physiology the effects of viscero-sensory impulses in securing compensation are very evident. All
viscero-motor impulses are initiated by incoming sensory impulses ultimately,
though in the case of certain cerebral conditions to be discussed in another
connection, the impulses may be a long time retained within the nervous
system before the visceral effects are produced. For the most part,
however, viscero-motor impulses are reflexly initiated, and are therefore
immediate. They are not usually segmental in any very circumscribed
manner, but are more or less diffused.
There is yet great uncertainty as to the exact relation of the sympathetic ganglia in the coordination of the viscero-motor impulses. In a series of experiments performed in the laboratory of physiology of The Pacific College of Osteopathy, the cord was destroyed by thrusting a wire downward from the atlas. After this, no reflexes could be obtained by even the most urgent stimulation. If the cord were merely cut at different places, leaving the nerves uninjured for the most part, a procedure which must have occasioned at least as severe a shock as the destruction of the cord, the reflexes still persisted. These tests, which were repeated a number of times under very various conditions, indicate that the chief, if not the only, pathway of the visceral reflexes includes the spinal cord. The constancy with which the viscero-somatic and the somato-visceral reflexes are noted is additional evidence in favor of the essential function of the cord in coordinating the visceral activities. The sympathetic ganglia are apparently relay stations for the increase or the diffusion of the impulses derived from the lateral horns of the cord and homologous cranial centers by way of the white rami communicantes, the vagi, the erigentes, and others of the same rank. The
lateral horns of the cord and homologous centers in the floor of the fourth
ventricle and the aqueduct, are the immediate centers of origin for the
viscero-motor impulses. These act in accordance with the algebraic
sum of all the impulses reaching them.
Sensory impulses from the skin, muscles, joint surfaces, and other somatic structures initiate viscero-motor impulses. The presence of these impulses is essential to the maintenance of the normal tone of the visceral and vascular walls, since any interference with the pathway of these impulses is followed by a decrease in the tone of the vessels and viscera affected. That is, the injury of the sensory nerves of the skeletal muscles or the skin in any marked area is followed by a loss of tone of the viscera and the blood vessels whose impulses are derived from the segment of the cord controlling the injured somatic structures. This loss of tone is temporary; other related structures compensate in part for the deficiency. Compensation is probably never absolute, however, here or elsewhere. Normal
sensory impulses from skin, muscle, joint surfaces and the like, initiate
normal viscero-motor impulses to the vessels, glands, and visceral walls
innervated from the same spinal segment; abnormal sensory impulses from
skin, muscles, joint surfaces and other somatic structures initiate abnormal
viscero-motor impulses to the same structures.
Now
the effects of the mal-adjustment of the somatic structures vary greatly
in their nature. Slight mal-adjustments of the ribs and vertebrae
and other bones bring tension upon the joint surfaces and upon the surrounding
connective tissues; this tension becomes a source of abnormal impulses
which are at first productive of considerable visceral disturbance.
Under favorable conditions, a more or less perfect compensation occurs,
and the visceral reflexes become fairly normal. If the abnormal structural
conditions be slowly produced, the compensatory reflex actions occur at
the same time, and thus, while the deformity may be excessive, the visceral
mal-function may be very slight. This is the case in Pott’s disease.
On the other hand, if the structural disturbance be suddenly produced,
or if the compensation does not occur, the resulting functional disturbance
may be very great, seeming out of all proportion to the apparently insignificant
structural lesion. The nature of the visceral effects produced by
any given injury depends upon the structural relations of the segment affected
and upon the opportunity given the organs and their nerve centers for adaptation
to the abnormal conditions.
These reflexes serve another very useful purpose in facilitating recovery from disease, and in this relation also, they may increase their evil effects of injudicious over work or over rest. Since the metabolism of any of the tissues of bodies so complex as ours depends in so great measure upon the impulses reaching them from the central nervous system, it follows that the metabolism of all the tissues innervated from the same segment of the cord must be somewhat related. Now this relation makes for health for the most part, else would the relation never have been established or perpetuated. By this means, any organ which is weak, or is being over worked, receives efferent impulses which have been initiated from the sensory impulses from stronger or rested organs, and it is thereby entitled to continue in a fairly normal condition much longer than it would were it affected only by the sensory impulses from its own sensory nerves. On
the other hand, this relationship works an injury to the normal organs
in the presence of an incurable disease. The normal organs are forced
to assist with the burden of the abnormal, and life may be shortened by
the failure of the organs secondarily disordered. In such cases as
these, the exhaustion of the neurons concerned often frees the normal tissues
from the effects of the sensory impulses from the abnormal organ.
This arrangement renders the non-striated muscles indirectly subject to volitional control, and this without permitting the possibility of injudicious interference with visceral activities. The visceral activities are well placed beyond the direct control of psychic influences which are so apt to be poorly planned and destructive. Volitional control of the skeletal muscles is almost absolute and these may therefore be developed at will. Their metabolism is increased by their use, and sensory impulses from them to the central system are thereby increased. The passage of nerve impulses through any nerve center increases the metabolism of that center, lowers the liminal value of its neurons, and in this way increases the nutrition and the efficiency of all the tissues innervated from it. This possibility is recognized in our methods of dealing with many of the disorders characterized by malnutrition or loss of function of certain viscera. For example, when there is lack of tone of the intestinal walls, any corrective measures are increased in their efficiency and recovery is hastened if the patient is instructed in judicious exercises which develop the abdominal muscles. These being strengthened, all other structures innervated from the same segment of the cord are also strengthened. The same principle is of value in facilitating cardiac compensation. The formation of ferments which facilitate muscle metabolism is probably another factor which is concerned in this relation between different groups of muscles. This aspect of the question is discussed in another chapter. Note A.—Segmentation is the primitive arrangement. In the beginning each spinal segment is in relation with a pair of sensory and pair of motor nerves, a double set of muscles laterally placed, and areas of developing bone and skin, all innervated by these same nerves, and all fed by a pair of segmental arteries and drained by a pair of segmental veins. In the process of development some of the veins and arteries became atrophied, some of the muscles were shifted from their original position, some became excessively developed at the expense of others, while yet other muscles grew together or became divided. During all these changes, the muscles retained practically their original nerve supply. Even the muscles which were originally visceral and became skeletal during phylogenetic development retain their innervation from the lateral and viscero-motor nuclei, as in the days when they were visceral muscles. Notice the innervation of the diaphragm, the latissimus dorsi, and the limb muscles, the distribution of the vagus and the spinal accessory nerves as indicative of developmental changes; the spinal muscles and the intercostals are indicative of the persistence of the original segmentation. In the cervical and cranial regions the relationships of the nerves are complicated because of the branchial divisions. The branchial divisions which may be recognized during ontogenetic development are complex enough, but many of the nerve relations of these structures are explicable only in the light of the study of the phylogenetic history of the branchiomeres. Note
B.—The axons of the sympathetic cells are not medullated. They leave
the sympathetic ganglia, pass to the cerebro-spinal nerves, and are distributed,
for the most part, with these. These bundles of gray fibers leaving
the sympathetic ganglia and passing to the cerebro-spinal nerves are called
gray rami communicantes. Some of these gray fibers enter the spinal
cord as the vaso-motor nerves to the vessels of the cord.
COLLATERAL READINGS. The Neurogenic and the Myogenic Theories of the Heart Beat, in Howell’s Text-Book of Physiology, p. 497, Edition of 1905. The Visceral Efferent Division, in Johnston’s Nervous System of Vertebrates. The Sympathetic System, Ibid. |