Studies in the Osteopathic
The Nerve Centers: Volume
Louisa Burns, M.S., D.O., D.Sc.O.
THE CONTROL OF THE MOTOR NEURONS
The motor neurons of the first order are those whose axons are distributed
to the active structures of the body. The term is not usually applied
to the sympathetic neurons, though these cells are as truly motor neurons
of the first order as are the cerebrospinal neurons. The lack of
exact information in regard to the relationships within the sympathetic
ganglia renders the use of these exact terms inadvisable at the present
time. So the term “motor neuron of the first order” is usually applied
only to the cerebrospinal neurons whose axons are distributed to the skeletal
muscles. Motor neurons of the second order are those neurons whose
axons form synapses with the motor neurons of the first order, and which
transmit the impulses from the higher centers. The sensory neuron
of the first order might send its axon or collateral to the motor cell;
but since it does not transmit impulses from the higher centers, it could
not properly be called “motor” neuron at all. Thus it appears that
the distinction between the sensory and the motor neurons is not always
easy to make. Both the sensory neurons of the first order and the
motor neurons of the first order are distinguishable by their peculiarities
of structure and of relationships, but the neurons of higher orders are
not easily classified in all cases. The motor neurons of the first
order are controlled by the impulses reaching them from other parts of
the nervous system. Except for the direct effect of the constituents
of the blood and lymph upon the neurons, the ultimate control of the motor
neurons is from the streams of sensory impulses.
The Somatic Motor Neurons
The somatic motor neurons of the first order occupy a column placed in
the anterior horn of the spinal cord and extending into the medulla, pons
and midbrain. The part of this column which lies within the cord
is fairly regular in outline and in the number and position and appearance
of the cells. In the medulla the column is crowded toward the median
line by the spreading of the posterior fissure as it widens out into the
fourth ventricle and the consequent displacing of the lateral and posterior
funiculi. The interposition of the masses of gray matter of the medullary
nuclei, the decussation of the pyramids and of the fillet, and the passing
of the tracts associated with the cerebellar peduncles, all help to distort
the column of the somatic motor neurons in the medulla and pons so greatly
that only the study of the phylogenetic and ontogenetic development of
the neuron groups makes their homology with the spinal somatic motor neurons
motor neurons of the first order are of large size and of rugged outline.
They are very closely crowded during embryonic life. During later
development they are crowded apart by the increasing development and growth
of their own dendrites and by the richly interlacing pericellular baskets
which grow around them. Their axons are distributed to the skeletal
muscles. Those of the spinal cord pass outward by way of the anterior
roots. The axons, in the adult neuron, become medullated within a
few microns of the exit from the cell body, and this medullary sheath is
retained throughout the whole length of the axon, except where it is interrupted
by the nodes of Ranvier, and within a few microns of the termination of
the fiber in the motor nerve ending upon the muscle fiber. The axons
of the cranial motor neurons of the first order are arranged in a similar
manner, but display a few variations, due to the requirements of the peculiarities
of the cranial structures and their development.
pericellular network is very close and complex. It is composed in
part of the dendrites and fibrillae of the motor neurons themselves, and
in part of the branching axons and collaterals from other neurons.
Sometimes a collateral from the axon of the motor cell itself enters into
this basket. It does not seem that there is any protoplasmic continuity
of the fibrils, though this matter is at present a matter of doubt.
It is certain that the network is extremely dense, and that the transmission
of the impulses carried by the axons and collaterals have every opportunity
to affect the dendrites and fibrillae of the motor neurons.
motor neurons of the first order receive impulses from the following sources:
Axons and collaterals from the sensory neurons of the first order form
synapses with the motor neurons of the first order. By this means
the simplest possible reflex actions are made possible.
Axons and collaterals from the sensory neurons of the second order, notably
those of the posterior horns of the spinal cord and of homologous cell
groups in the higher centers, form synapses with the motor neurons of the
first order. By this means reflex movements requiring the coordinated
action of simple muscle groups are secured. (Fig. 44.)
Axons and collaterals from adjacent segments of the cord, and from adjacent
centers in the higher motor cell groups, affect motor neurons of the first
order. By this means the more complex reflex actions are controlled.
Descending fibers carry impulses from the cerebellum. These may be
axons directly from the cerebellar cells, or they may be axons from the
cells of the central cerebellar nuclei, the olive, and perhaps of other
similar cell groups very closely connected with the cerebellar centers.
By this means the most complex of reflex actions, such as those concerned
in walking and standing, may be secured.
Descending fibers from the red nucleus and certain other cell groups included
as basal ganglia make up the rubrospinal tract. The impulses carried
by these fibers stimulate the motor neurons in such a way as to cause the
instinctive and emotional reactions.
Descending fibers from the vestibular nuclei, passing in the vestibulo-spinal
tract, carry to the motor neurons certain impulses originating in the semi-circular
canals. These impulses, of much less importance in mammals than in
fishes and amphibia, are probably concerned in the maintenance of the symmetrical
and the erect position.
Descending fibers from the quadrigeminate bodies are carried by the tecto-spinal
tract (anterior longitudinal bundle). These, like the vestibulo-spinal
fibers, are representative of an ancient arrangement for the coordination
of the impulses in the complex actions necessary to the maintenance of
life. Impulses chiefly from the retina and cochlea initiate descending
impulses over this tract to the motor neurons of the first order.
It is probable that in this way the tone of the skeletal muscles is partly
affected, and that a certain amount of complex coordination is secured,
especially in reactions following visual stimuli.
The pyramidal tracts carry impulses from the central or kinesthetic area
of the brain (Rolandic area). These fibers form synapses directly
with the nerve cells of posterior or base of the anterior horn, and the
axons of these in turn enter into the pericellular basket of the motor
neurons of the first order. The impulses carried by this tract are
concerned in the control of the movements called volitional, and which
are also usually well recognized in consciousness.
Descending fibers from certain autonomic centers in the medulla, midbrain
and the pons carry impulses concerned in the performance of the autonomic
functions to those skeletal muscles needful for those duties. This
is exemplified in the action of the respiratory center in the lower part
of the calamus scriptorius. Impulses from the respiratory centers
are carried downward in the cord, probably through the fasciculus prorius,
to the motor cells controlling the different respiratory muscles.
The vomiting center, the deglutition center, and other of similar action,
all act through descending impulses to the skeletal muscles concerned.
means of these varying methods of control of the motor neurons of the first
order, with their varying degrees of structural complexity and the related
variations in the complexity and numbers of the sources of sensory impulses
from which the different stimulations are derived, the motor impulses are
made of such quality that comparatively simple muscular mechanisms are
enabled to serve remarkably complex purposes. The simple reflexes
serve the purposes of simple needs. They are not especially coordinated,
serving the immediate needs of the organism quickly. The more complex
reflexes serve the needs of the body for the complex actions, expressive
of the needs of the many individuals of the race, alike for all individuals,
the result of racial experiences, of many survivals and of infinite deaths.
more delicately coordinated reactions, especially those depending upon
the cerebellum and its related ganglia for their control, are of even greater
complexity, and are the results of the experience of the individual, chiefly
and perhaps exclusively. These reactions are initiated and modified
and controlled, largely, through educational methods. The volitional
impulses, again, while probably controlled by simpler structures than those
coordinated by the cerebellum and the related centers, are of a more complex
nature in that they are the result of present sensory impulses, modified
by the results of the history of each individual, plus the past of his
neighbors with whose experiences the individual is familiar, plus the results
of the stimulation of the association cells, whereby the action and its
results are foreseen, even though it may be, as a whole, totally apart
from the experience either of the individual or his race. This complexity
of coordination is possible only because the cells of the central convolutions,
as well as of other parts of the cerebral cortex, are affected by inhibiting
Control of the Viscero-motor Neurons
visceral muscles are innervated through the sympathetic nervous system,
as it is illogically termed. This part of the nervous system includes
a number of ganglia placed in various parts of the body cavities, and the
fibers which relate them to the spinal cord, pons, medulla and midbrain.
nerve cells in the sympathetic ganglia are controlled by the impulses from
the cells in the lateral horns of the cord (the lateral group of
the anterior horns, according to certain writers) and in homologous centers
in the medulla, pons and midbrain. These autonomic centers are themselves
controlled by the impulses from higher centers and from the sensory neurons.
The sympathetic ganglia do not, under any but experimental conditions,
nerve fibers which transmit impulses from the autonomic centers in the
central system are of finer calaiber than are the fibers from the somato-motor
neurons. The autonomic fibers make up most of the bands called the
white rami communicant es, which leave the spinal cord in the dorsal region—that
is, from the first or second thoracic segment to the second or third, or
sometimes the fourth lumbar segments. These white rami enter the
sympathetic ganglion nearest their origin, usually, but rarely terminate
until they have passed through one or more ganglia. Then they break
up into fibers which form a part of the pericellular net around the bodies
of the nerve cells of the sympathetic ganglia. Each axon of the white
rami may send collaterals to several different sympathetic cells in a single
ganglion, and it seems probable that collaterals from the white rami fibers
may pass from one ganglion to another, thus bringing the cells of two or
more ganglia under the control of a single white rami fiber. On the
other hand, each sympathetic cell may receive fibers from two or more white
rami fibers. Thus the activity of a single sympathetic cell may be
modified by the impulses from several cells within the central nervous
system, and the impulses form a single cell within the central nervous
system may modify the activity of many sympathetic cells. The complexity
of these structural relationships accounts for the well-known complexity
of the functional relationship between the sympathetic and the central
vagus, the third cranial, the seventh and perhaps the ninth cranial nerves
also send fibers, comparable in function, to the sympathetic ganglia.
In the pelvis, the nervus erigens sends fibers to the hypogastric ganglion.
autonomic cells in the lateral horns of the cord and the homologous centers
in the medulla, pons and midbrain are somewhat smaller than are the cells
of the somato-motor centers. They are not quite so rugged in outline;
the dendrites are shorter and less profusely branching. They are
surrounded by a network which is similar to that already described as surrounding
the somatic motor cell bodies. This network includes the fibrillae
derived from the spongioplasm of the cell body, and also the axons and
collaterals from the following cells in other parts of the nervous system:
Collaterals from the axons of the entering posterior roots of the cord
bring to the lateral horn cells impulses from the sensory neurons of the
first order. (Fig. 45.) These include (a) viscero-sensory neurons,
by means of which the visceral conditions initiate or modify the autonomic
impulses to the sympathetic ganglia and thus to the muscles, glands and
blood vessels of the viscera, (b) somatic sensory neurons, by means of
which impulses from skin, muscles and joint surfaces may modify visceral
Axons and collaterals from the cells of the posterior horns of the cord
probably have functions similar to those just mentioned.
Fibers from the cells of the opposite side of the cord bring the two halves
of the body into functional relationship.
Terminals and collaterals from the descending rubro-spinal tract bring
impulses from the red nucleus, the substantia nigra, and probably others
of the basal ganglia, to the autonomic cells. Thus the emotional
reactions include visceral as well as somatic manifestations.
Descending impulses are carried from the various centers in the medulla,
midbrain and pons to the viscero-motor neurons in the cord. The vaso-motor
center, the heart centers, and other viscero-motor centers act in this
Fig. 40. Cilio –spinal center. – Gray
fiber; Sympathetic ganglion; Sympathetic cell; Anterior root; Rubro-spinal;
White ramus; Lateral horn; Sensory fiber; Sensory cell; Posterior root;
to their termination, the viscero-motor neurons have the following functions:
They contract the walls of the blood vessels, especially of the arterioles,
thus decreasing the blood supply of that certain area and raising the general
blood pressure in corresponding degree.
They cause contractions in several manners of the walls of the alimentary
canal, the heart, the urinary and the gall bladders, the uterus, and other
organs and ducts of the body. In this way the contents of these hollow
viscera are variously propelled.
They contract the pupilo-dilator, the pupilo-constrictor, the ciliary muscles,
and the other non-striated muscle fibers of the orbit.
They contract the pilo-motor muscles, by means of which the hair, feathers,
quills, and other varieties of the exo-skeleton are made erect during cold,
fear, anger, etc. In this way the loss of heat from the body is lessened,
the danger of wounds in battle is lessened, and the animal is caused to
assume a more ferocious appearance.
They increase the secretions of glands in all parts of the body.
Certain of these fibers seem to have the power of inhibiting the action
of those just mentioned. The manner in which the inhibiting fibers
act is one of the greatest puzzles of physiology.
seems that the inhibitory function is exercised only by neurons of the
central nervous system upon other neurons, either of the central system
or of the sympathetic ganglia. The existence of inhibitor neurons
of the first order is not demonstrated, and their existence is extremely
stimulation of the sympathetic nerves to the salivary glands increases
the secretion of these glands, but the secretion is very thick and rich
in organic matters. The blood vessels are greatly constricted.
The stimulation of the cerebrospinal nerves to the same gland, as the chorda
tympani, on the other hand, also increases the secretion, but in this case
the fluid formed is very thin and watery, containing a certain quantity
of inorganic salts, but very little organic matter. The blood vessels
are tremendously dilated. Just what the real relation is between
these nerves and their action is not yet known. The circulatory changes
must modify the character of the secretion also. If all nerve impulses
are identical in quality, as seems indicated by many of the phenomena of
nerve activity, then there must be some structural difference in the relations
of the two classes of nerve with the glands they supply. On the other
hand, if nerve impulses are not identical in quality, then we have before
us the more complex, but not more explicable, problem of differentiating
between the almost infinite varieties of classes of impulses needed for
the determination of the almost infinite variety of physiological activities
controlled by the nervous system.
viscero-motor neurons are not directly influenced by the volitional impulses
from the somesthetic area. But they may be influenced indirectly
in either of two ways.
one remembers distinctly the events which are associated with emotional
reactions in his own past, or if he imagine distinctly, in such a way as
to present vividly before himself a series of incidents which bring to
him any emotional state, the muscles concerned in such emotional state
become contracted involuntarily, and the viscera whose activity is usually
associated with such emotions become active. The reactions thus produced
are not ordinarily so strenuous as those produced by the actual presence
of the emotion-producing circumstances, but at times they seem even to
be increased in memory or imagination beyond that characteristic of the
actual occurrence. This is noted with disastrous circumstances in
the effects of certain mental shocks. In these cases the memory of
the fright is often more unendurable than the occurrence itself seemed
to be. The phenomena of hysteria and of certain insanities illustrate
this reaction. Probably no rational use can be made of this relationship
lateral and anterior horns are so closely associated in the gray matter
that the stimulation of one group of cells is practically certain to affect
the action of other groups of the same segments. Thus the stimulation
of the skeletal muscles of any segment, by means of volitional impulses,
affords a certain amount of normal stimulation to the visceral muscles
and glands also. In those cases of cardiac lesions in which hypertrophy
is desired, it is very good to cause the gentle use of the arm muscles
and the intercostals. In cases of visceroptosis, dilated stomach,
intestinal atony, etc., much good can be accomplished by the patient himself
if he will use conscientiously those exercises which increase the tone
of the skeletal muscles innervated from the same spinal segment.
any forms of exercise, if the elements of enjoyment and desire can be added
to the volitional impulses, if the patient can be made to enjoy the exercises,
or to feel some emulation, then his progress in strength is more rapid.
For this reason useful exercises are best, other things being equal.
In all these cases the additional stimulation due to the emotional state
is associated with a stream of impulses from the red nucleus and related
The Motor Cranial Nerves
is not possible to divide the cranial nerves into exactly two classes,
somatic and visceral, because in the case of these nerves the original
distribution and function has been so greatly modified through both ontogenetic
and phylogenetic development. It becomes necessary, then, to consider
their relations separately, and to view them in the light of their present
functions and relations, noting their ontogenetic and phylogenetic relationships
only with such care as will serve to explain in part their irregularities.
hypoglossus or twelfth cranial nerve is, in the adult, purely motor.
In the embryo it has one or two sensory ganglia, with corresponding embryonic
sensory roots. These become lost in development. The hypoglossus
contains fibers corresponding to about five nerve segments. It arises
from a genetic nucleus in the floor of the fourth ventricle, in the trigonum
hypoglossi. Its nucleus is in direct line with the anterior horns
of the cord, and it is a somatic motor nerve in development as in present
function. The fibers pass through and between the inferior and the
accessory olives and emerge from the anterior sulcus of the medulla.
It is distributed to the muscles of the tongue and the depressors of the
hypoglossal nucleus receives association fibers from the other motor cranial
nerve nuclei, notably the fifth and seventh, and from the sensory cranial
nerve nuclei, notably the fifth, seventh, and eighth.
fibers from the pyramidal cells of the lower part of the precentral convolutions
of the cerebral cortex form synapses with the cells of this nucleus, and
by this pathway the volitional movements of the tongue and probably the
movements of the tongue in speech are effected.
from the deglutition center in the medulla affect this nucleus.
from the red nucleus and other basal ganglia reach the hypoglossal nucleus,
and it is in part because of this connection that in emotional disturbances
the tongue becomes stiff—in other words, the emotional effects upon the
tongue are, directly, inhibiting. It is scarcely needful to state
that this inhibition is frequently less potent than the stimulation afforded
by the descending impulses from the volitional centers in times of emotional
stress of certain types.
The Accessory Nerve
accessory, or spinal accessory, nerve is to be considered in two parts.
The cerebral root originates in the nucleus ambiguous. It is morphologically
and physiologically a part of the vagus, and the fibers derived from this
nucleus join the vagus after their exit from the cranium. The fibers
of the spinal root originate in a column of cells at the lateral portion
of the upper five segments of the cord. These fibers pass upward
in the canal, join the cerebral root, and leave the cranium by the jugular
foramen. It is distributed to the trapezius and the sterno-mastoid
muscles. One or two rudimentary sensory ganglia have been found in embryos
upon the spinal accessory nerve.
origin of this nerve from a nucleus in the column of the viscero-motor
nerves appears at first sight an anomaly. In its phylogenetic development
the peculiarity disappears. The trapezius and sterno-mastoid muscles
are derived from the old branchial musculature, and are, therefore, phylogenetically,
visceral muscles. Their innervation from the column in line with
the other visceral motor nuclei is thus appropriate, though the muscles
are at this time skeletal, striated, and as thoroughly under the control
of the volitional impulses as any other voluntary muscles.
spinal nucleus of the spinal accessory receives impulses from the sources
already mentioned as influencing the activities of the spinal motor neurons.
The Vagus Nerve
motor fibers of the vagus arise chiefly from the nucleus ambiguous, but
some fibers arise also from the neighboring gray masses in the medulla.
This nucleus belongs to the column of the viscero-motor neurons.
The cells are, like those of the spinal lateral horn, rather small, multipolar
cells, with rather fine axons. The vagus is, phylogenetically and
physiologically, a viscero-motor nerve. Its motor fibers are homologous
with the white rami fibers. They terminate by forming synapses with
motor nucleus of the vagus receives impulses from the following sources:
The sensory fibers of the vagus send collaterals and probably terminals
to the nucleus ambiguous.
Axons and collaterals from the sensory nuclei of the fifth, seventh and
eighth cranial nerves, the nucleus gracilis and nucleus cuneatus, form
synapses with the cells of the nucleus ambiguous.
The various centers of the medulla, the cardiac, the vaso-motor, the respiratory,
etc., either are identical with the nucleus ambiguous and the neighboring
gray matter from which the vagus motor fibers arise, or they are very intimately
related to these by association neurons and by collaterals from the afferent
and efferent axons.
The rubro-spinal and tecto-spinal tracts, the descending root of the fifth,
the vestibular nuclei, the olives, and the pontine nuclei, send axons and
collaterals to the nucleus ambiguous. By this means the various activities
of the vagus are modified by sensory impulses from practically every part
of the body.
fibers do not seem to affect the action of the vagus, and are not described
as entering the nucleus ambiguous.
The Glosso-Pharyngeal Nerve
motor fibers of the glosso-pharyngeal nerve arise chiefly from the nucleus
ambiguous, and also from the nucleus of the alae cinerae and certain small
cell groups in the immediate neighborhood. This is phylogenetically
a viscero-motor nerve, and its nucleus is a part of the viscero-motor column.
Its function is essentially visceral, since it is concerned with deglutition.
The constrictors of the pharynx are remnants of the branchial musculature.
The nucleus ambiguous receives associational fibers from the other cranial
nerve nuclei, as has been given in the case of the vagus. The reflexes
with which the glosso-pharyngeal nerve are concerned are chiefly those
glosso-pharyngeal motor neurons receive fibers from the pyramidal cells
of the inferior part of the precentral convolutions. By means of
this relation it becomes possible for the act of swallowing to be voluntarily
performed or inhibited, to a certain extent.
carrying inhibitory impulses seem to reach the glosso-pharyngeal nuclei
from certain autonomic centers in the medulla, since the act of swallowing
is urgently inhibited during coughing, mastication, etc. The descending
impulses from the red nucleus and related centers appear to be chiefly
inhibitory, since emotional disturbances inhibit swallowing. The
dryness of the mouth associated with certain emotions, however, initiates
the swallowing reflex in many emotional conditions.
fibers emerge from the upper end of the posterior sulcus of the medulla,
join the sensory roots, and pass through the jugular foramen to reach their
area of distribution, the pharyngeal constrictors and the stylo-pharyngeus
The Facial Nerve
motor fibers of the facial nerve are the axons of the nerve cells of a
single nucleus in the pons, under the superior fovea. This nucleus
is a part of the continuation of the lateral horn of the cord, and the
nerve is phylogenetically a viscero-motor nerve. Originally its motor
fibers innervated the gill muscles. It is specifically the nerve
of the hyoid arch, and only secondarily is it a nerve of expression.
Its nucleus does not give fibers to the sixth, as is sometimes stated,
but some fibers of the sixth run in the same path for a certain distance
in the neighborhood of the genu of the facial.
fibers leave the groove between the pons and medulla and pass with the
sensory root of the facial to the muscles of expression—that is, to the
muscles of the face, but not those of mastication.
motor nucleus of the seventh has certain peculiarities which affect its
liability to disease. In the first place, its almost constant
use renders its constituent neurons irritable beyond the common habit of
neurons. The liminal values thus being lower than is usual, excessive
stimulation, the presence of the fatigue products in the blood stream,
reflex irritations originating almost anywhere in the body, and, indeed,
the abnormal conditions associated with very many diseases, affect the
neurons of the seventh with especial severity. For this reason the
expression of the face is of considerable value in diagnosis; the facial
muscles are especially liable to the various tics and spasms, and the habitual
uses of certain muscle groups gives rise to the permanent and characteristic
position of the facial tissues which results in the sum of what is called
“expression” or character in appearance. The nucleus of the facial
nerve receives association fibers from many sources. Its most conspicuous
control is that by the red nucleus and related basal ganglia. It
is by this relationship that the various emotional states so greatly affect
the facial expressions.
pyramidal fibers pass to the nucleus directly, though the volitional control
of the facial is less absolute than is the volitional control of certain
other nerves. The facial muscles may be brought under almost absolute
control through constant education. This control is manifest more
as a repression of emotional expression than as increased or modified expressions.
This is shown most clearly in the case of those who wish to imitate an
expression, as in acting. It is commonly recognized that the volitional
imitation of an expression is practically impossible, and that the only
way to imitate the expression of an emotion is to imitate the sensation—that
is, to actually feel the emotion whose expression is desired. In
this way the red nucleus and associated ganglia are brought into control
in such a way as to initiate the very stimulation of the structures needed
as to bring about the real expression of the emotion sought. This
condition must be remembered when one is dealing with those patients in
whom the existence of habitual expressions of ill feelings are manifestly
a cause of depression and ill health.
Fig. 41. Control of the brachial muscles.
Direct pyramidal; Anterior root; Anterior horn; Rubro-spinal; Mixed nerve;
Sensory; Posterior root; Gra;y fiber; Sympathetic; Cell; White fiber.
motor facial nucleus receives fibers from the sensory nuclei of the facial,
the trigeminus, the auditory and the vagus nerves.
and collaterals from the fillet carry impulses from the nucleus gracilis
and nucleus cuneatus. Axons and collaterals from the posterior longitudinal
fasciculus carry impulses from the midbrain and probably from the pontine
nuclei. Through these very complex relations the facial nerve is
constantly under varying degrees of stimulation and inhibition.
Motor Root of the Trigeminal Nerve
The nucleus of the motor part of the fifth nerve lies in the upper part
of the pons and under the floor of the cerebral aqueduct (aqueduct of Sylvius).
Its fibers leave the anterior face of the pons, join the larger sensory
root of the fifth, and pass with its mandibular division to the muscles
nucleus is also of the viscero-motor series, and its function is now of
this order, though the muscles which it innervates are skeletal, striated,
and largely under volutional control.
nucleus of the trigeminus receives fibers from the sensory nuclei of the
fifth, and it is through this relationship that the masticatory reflexes
nucleus of the trigeminal receives fibers of association from the other
sensory nuclei of cranial nerves, from the nucleus gracilis and nucleus
cuneatus, and from the pyramidal tracts. The red nucleus and other
basal ganglia send fibers to this nucleus also. Though this control
is not so conspicuous as is the similar control of the facial, yet the
forcible tension of the temporal and masseter muscles in certain emotional
conditions is indicative of the intensity of the stimulation sent from
the basal ganglia centers to the motor nucleus of the fifth.
constant use of these neurons, as in the case of the facial, renders them
susceptible to the ill effects of abnormal reflexes, as in case of trismus,
and to the action of certain poisons, as in the case of tetanus and eclampsia.
The Ocular Motor Nerves
nuclei of the third, fourth and sixth cranial nerves occupy a column extending
from the floor of the aqueduct under the superior colliculus to the abducent
nucleus in the pons. This column is in the line of the anterior horns
of the cord and the hypoglossal nucleus. It is composed of the somatic
motor neurons, and it is chiefly somatic motor both in its phylogenetic
development and its present function. The different nuclei have
certain peculiarities in common. Each of these nuclei exchanges association
fiber with every other; each receives fibers from the pyramidal tracts;
each receives many fibers from the superior colliculus and the cerebellum;
each receives a few fibers from the red nucleus. Each receives also
a few fibers from the sensory cranial nerve nuclei and from the nucleus
gracilis and the nucleus cuneatus. The different nuclei have certain
nucleus of the abducens lies within the bend of the genu of the facial,
and a group of cells near the facial nucleus is included as part of the
abducens nucleus. It was originally supposed that these fibers, really
from the accessory nucleus of the abducens nucleus, were from the facial
nucleus. The very close morphological relationship renders the existence
of close associational neurons very probable.
trochlear nerve lies beneath the inferior colliculus. Its fibers
decussate, almost or quite completely, so that the fibers from each nucleus
innervate the trochlear muscles of the opposite side. No explanation
has been offered for this phenomenon.
oculo-motor nerve includes several groups of nerve cells, each of which
performs its own function. Besides the somatic motor nuclei, of which
a variable number have been recognized, there is a viscero-motor nucleus
which requires some attention. The viscero-motor nucleus of the third
nerve includes a number of small nerve cells which are homologous with
the lateral columns of the cord and the cranial viscero-motor nerve nuclei.
Its fibers leave the interpeduncular fossa of the midbrain with the somatic
motor fibers of the third, and pass with these to the ciliary ganglion.
Here they terminate by forming synapses with the sympathetic cells of this
ganglion, and it is the non-medullated fibers of these, passing in the
short ciliary nerves, which innervate the ciliary muscle and the constrictor
of the pupil.
nucleus, like the somatic-motor nucleus, receives fibers from the superior colliculus,
the other sensory cranial nerve nuclei, the red nucleus, and perhaps the other
nuclei of the ocular muscles. It does not receive fibers from the pyramidal
tract. This nucleus is of considerable interest in being involved in certain
paralyses. It is rather infrequent to find any other viscero-motor nucleus
the seat of paralysis. This may be due to the fact that a paralysis of
other viscero-motor nuclei would either result in a death so speedy as to preclude
the possibility of diagnosis, or the symptoms would be too vague for the correct
diagnosis to be made, or it may be that the position of the third nerve
viscero-motor nucleus renders it more susceptible to the action of disease conditions
than are other nuclei of the visceral column.