Studies in the Osteopathic
The Nerve Centers: Volume
Louisa Burns, M.S., D.O., D.Sc.O.
RELATIONS OF NEURONS
The individual neurons are functionally related by means of certain arrangements
of dendrites, axons, collaterals and cell bodies, which bring the protoplasm
of one neuron into very intimate structural relations with the protoplasm
of another. This structural relationship which makes the functional
relationship possible is called “synapsis,” from a Greek word which means
“clasping.” Synapses are formed in various manners in different parts
of the nervous system, and are often very complex. In all cases studied
so far the axon or the collaterals from the axon of one neuron transmit
the impulses to the dendrites or to the cell body of another. Thus,
the axons are cellulifugal and the dendrites cellulipetal in function.
synapses of the neurons concerned in the more complex coordinations, especially
in those controlling the autonomic functions, are so intimate, each neuron
receives impulses from so many sources and sends impulses to so many other
neurons, that the older views of Bethe and Golgi and Nissl, that the whole
nervous system should be viewed rather as a syncytium than as a collection
of individual cells, seems at first to be well founded. This appearance
is noted with particular distinctness in the cell groups of the medulla,
where are situated so many of the chief viscero-motor centers.
Here may be well seen the extremely delicate fibrillae, resembling the
extensions of the spongioplasm of the cell body into the pericellular space,
which have aroused so great discussion. These fibrillae may be traced
from one cell to another in so many cases and with such exactness that
the existence of protoplasmic bridges can not longer be denied. Yet
in these same centers may be found other cells, placed near each other,
which seem to have no structural relationships. It seems probable
that in the viscero-motor centers, at least, those neurons which are most
closely related in function are those whose synapses are most intimate
synaptic relations of neurons are so varied that an almost infinite variety
of reactions to any given environmental change becomes possible, as is,
indeed, manifest to even the most superficial observation of the habits
of the higher animals. Any one axon may give off a number of collaterals,
all of which may form synapses with as many other nerve cells, and any
one nerve cell may receive impulses from several different other nerve
cells. When it is remembered that the nervous system contains thousands
of millions of nerve cells, it is evident that the infinite complexity
of even human thought and action does not surpass the complexity of the
Probably the simplest form of synapsis is that found in the gray matter
of the cord, as well as in other parts of the nervous system. The
entering fibers of the posterior roots, the collaterals from the fibers
of the spinal white matter, the axons of the Golgi cells of Type II of
the cord, all break up into a very find feltwork which surrounds the intrinsic
cells of the gray matter, the motor cells of the first orders and the cells
of the viscero-motor centers of the cord. (Fig. 17.) The dendrites
of the cells just mentioned branch freely among this complex feltwork,
and by this means the impulses from several sources become able to affect
the action of the muscles, the viscera, and the coordinating centers of
the same and adjacent spinal segments.
The Purkinje cells of the cerebellum display two very pretty methods of
synapsis. The bodies of these cells are surrounded by a delicate
network of collaterals from the axons of the stellate or basket cells of
the cerebellum. This basket arrangement is similar to the feltwork
around the spinal cells, but it is rather more easily visible and is more
exactly limited to a single cell. (Fig. 18.) The dendrites
of the Purkinje cells branch very freely in a manner resembling some old
oak tree, except that the branches lie all in one plane. These dendrites
are interlaced and wound around by fine vinelike “climbing fibers,” as
they are called, the prolongations of cells whose location is not exactly
known, but which almost certainly are carried into the cerebellum by way
of the peduncles. (Fig. 19.)
The olfactory lobes show another form of synapsis. The olfactory
nerves entering the lobe pass tangentially over the surface for a variable
distance. They then plunge into the mass of the lobe, giving off
no collaterals; each axon branches and twines into a glomerulus.
In the glomerulus also are found dendrites from the mitral cells of the
olfactory lobe, which branch among the fibrillae of the olfactory axons.
By means of this structure each cell of the olfactory region of the nasal
mucous membrane is able to transmit impulses to a mitral cell, which, in
turn, sends impulses brainward. (Fig. 20.)
Fig. 19. Cross section of lobule of cerebellum
of woman of about 30 years. Semidiagrammatic. 72 diameters.
Fig. 20. Olfactory lobe of kitten, 6 days
old. Semidiagrammatic. 60 diamaters.
Fig. 21. Blood vessels of corpora mammillaria,
human, adult. The vessel is filled with blood corpuscles. Around
the vessel is a pericellular lymph space, which is enclosed by a layer
of neuroglia. Three nerve endings lie upon the vessel’s wall.
Fig. 22. Prodasteroids from cortex of
occipital lobe of new-born baby. 175 diameters.
Fig. 23. Prodasteroids from medulla of
adult cat. 60 diameters.
many parts of the nervous system are found cells, evidently nerve cells,
which have freely branching dendrites, but for which no axons have been
described. The function of these cells is not certainly known, but
it seems probable that they are concerned in associating the nerve cells
of their immediate neighborhood. Thus they would be similar to the
Golgi cells of Type II in function. It may be, indeed, that the amacrine
cells will be found to be of the same structure as the Golgi Type II cells.
the sympathetic ganglia each cell occupies a cagelike basket, which is
formed of the interlaced fibers of the axons from the viscero-motor nuclei
of the cord, medulla or midbrain. These small medullated fibers lose
their sheaths near their termination and break up into the fibrillae which
make up the pericellular baskets of the sympathetic ganglia. Any
one basket may be composed of several axons or collaterals, and any given
axon may send collaterals into several baskets. (I have seen five
branches from a single fiber which was medullated before the branches were
given off.) It seems probable, also, that any one fiber from the
viscero-motor centers may pass through two or more sympathetic ganglia,
giving off one or more collaterals in each ganglion. The synaptic
relations of the viscero-motor neurons are thus no less complex in the
ganglia than in the spinal and medullary centers.
means of the various methods of synapsis the neurons are able to affect
the activities of one another and of the other parts of the body through
the motor neurons of the first order. The nature of these effects
are not yet well understood. It seems evident that neurons may affect
other neurons in at least two different ways.
may stimulate other neurons to increased action. This relationship
is the most conspicuous. The stimulation of the sensory nerves may
be followed by the passage of efferent impulses to the muscles innervated
from the same spinal segment, or adjoining segments, or through the intermediation
of the higher centers.
may inhibit the action of other neurons. This relation is not easily
understood, but is evidently one of the physiological facts. The
stimulation of the cortex inhibits the activity of the spinal centers.
This is noted in the excess of reflexes of any area associated with the
injury of the centers or tracts above the spinal segments innervating that
area. The inhibition of one neuron or neuron group by another is
a very important phenomenon in nerve physiology, and it is of especial
importance in the consideration of those centers associated with consciousness.
A temporary inhibition of any nerve center gives time for the receipt of
nerve impulses from other sources to affect the ultimate reaction.
nature of the inhibitions is not well known. There is some reason
to assume a relationship between the phenomena of the refractory period
and those of inhibition.
the case of mankind the simplest possible reaction is that of the simple
spinal reflex action. In this arc we may consider the possible existence
of a single sensory nerve fiber in relationship with the pericellular basket
of a single motor cell. It is evident that the stimulation of the
sensory neuron can affect the activity of that motor cell, and that only.
It is not evident whether there might be at different times differences
of the effect produced upon that cell by the receipt of the sensory impulse—that
is, whether the physiological effects of the sensory stimulation might
be subject to variation, as well as the liminal values of the two neurons
more complex spinal reflexes depend upon the coordinate action of several
neuron groups. It is evident that the interpolation of even a very
few relay stations adds very greatly to the structural possibilities of
the reaction which might follow upon the receipt of the sensory stimulus.
Probably the simplest action of which we are capable, even reflexly, necessitates
the activity of many nerve cells, extending through several spinal segments.
more complex arc includes in its circuit the spinal efferent and afferent
cells, and the cerebellar centers. Reactions which are subject to
the cerebellar coordinations are as complex as the habits which we learn,
and which become as fixed as any reflex action whose foundations were laid
in the nervous system long before birth.
more complex are the arcs which include the cells of the centers lying
about the base of the brain, in which the emotional and instinctive reactions
are coordinated. In these centers are received, correlated, and sent
out again, the impulses concerned in the movement not only of the skeletal
muscles, but also of those visceral activities which have been found associated
with the best and longest life of the individual and his race through all
the phylogenetic developmental steps. Here are related in function
the impulses concerned in the erection of the hair in fright, the grinding
of the teeth in anger, the movements of the nose in disgust, and all the
other phenomena of the instinctive and emotional reactions.
various reflex arcs are complex; their action necessitates many coordinate
reactions on the part of millions of nerve cells; their activities are
the result of ages of inheritance of those whose nervous systems were adapted
to performing their duties most efficiently, and through the working out
of the many other laws which govern racial development. Yet all of
these reactions appear very simple and predictable when the arcs which
include the cerebral cortex are noted.
impulses from all parts of the body, the cortical centers coordinate and
control the impulses arising from the activities of the lower centers.
The impulses are also coordinated in relation to the past effects of previous
experiences. Through the intermediation of consciousness, not only
the actual past may be enabled to affect the ultimate reaction, but the
elements of past experiences may be dissociated and recombined, in
order that the ultimate reaction may be wise beyond experience—or foolish
beyond experience, if the dissociations and the recombinations were ill
advised. All of this multiplicity of reaction capacity and reaction
choice becomes possible only through a multiplicity of synaptic relationships.
any given neuron group, or neuron system, it appears that there must be
an element of choice—not, of course, necessarily conscious. If any
one neuron in synaptic relationship with two other neurons should be stimulated,
it is evident that three different reactions are possible, excluding qualitative
differences. The stimulation of the first may cause the stimulation
of either of the two to which it is related, or it may cause the stimulation
of both. If one neuron is associated with three or more others in
function, the number of the possible reactions becomes increased to seven—the
stimulation of any one of the three, of any combination of two out of the
three, or of all three of the neurons. It is evident, then, that
the element of physiological choice becomes a matter of some complexity.
far as these relationships have been studied, the factors which determine
the choice in any given case are as follows:
The impulses from any nerve cell are apt to be carried over the axon rather
than over the collaterals from the axon. Thus, the impulses from
the motor neurons of the cortex are carried directly to the lower motor
centers, without affecting the basal ganglia cells, unless the stimulation
of the cortical cells be profound. In case of increased stimulation,
the impulses are carried over the collaterals also, and, in most cases,
more active movements result than is the case of the lighter stimulation.
Of two neurons in synaptic relationship with a third, that one which has
the lower liminal value is the more apt to be stimulated. The factors
which modify the liminal value of any neuron have already been discussed.
There is some evidence in favor of the view that the gemmules upon the
dendrites of the nerve cells are capable of amoeboid movement. If
this be true, it is possible that the retraction of the gemmules or of
the dendrites may modify the receptivity of any neuron.
There is, in nerve cells as well as in muscle, gland, and other active
cells, a certain refractory period following stimulation, during which
any stimulus received is followed by no perceptible effect. This
refractory period is very short in the case of the nerve cell, but is demonstrable
in so many instances that it is fair to assume it to be a characteristic
of neuron physiology in general. If, of two neurons receiving impulses
from a third, one has been so recently stimulated that the refractory period
is not yet passed, the other is the more apt to be affected.
Impulses which arise from those parts of the body nearest the spinal cord
are most apt to initiate deep-seated reflexes than impulses arising from
structures more distant from the cord. This is doubtless due to the
fact that the innervation of the body is determined at a time when the
cord occupied a position somewhat more nearly the center of the structures
innervated. The budding of the limbs and the changing form of the
thorax and the abdomen modify the areas of innervation, and while the number
of nerve cells and fibers becomes increased to a certain degree, the central
relationships do not appear to be rendered more complex in correspondingly
great degree. Thus we have the malpositions of the vertebrae, a source
of greater ills than malpositions of the bones of the hands, for
example. Irritation of the skin over the back of the neck may be
a source of considerable reflex irritation, but the irritation of the skin
over the arm or the foot has much less of an evil effect, so far as the
deep reflexes are concerned.
Sensory impulses are more efficient in arousing any given reflex action
the more nearly the functional relation between the area stimulated and
the area affected by the efferent impulses. The sensory impulses
from any part of the naso-pharynx, for example, are more apt to affect
the other parts of the respiratory tract, producing a sneeze or a cough,
than to affect the activity of the muscles of the pharynx, which are nearer
anatomically but less closely related in function. Sensory impulses
form the buccal pharynx, on the other hand, are more apt to initiate the
reflex actions concerned in vomiting.
Those impulses affect consciousness most vividly which arise from parts
of the body most immediately affected by environmental changes. Impulses
from the skin are clearly perceived in consciousness and are located with
more or less accuracy. Impulses from those parts of the skin most
subject to stimulation are those in which the localizing sense is most
acute. Impulses from the skin of the back, for example, while fairly
well adapted to the production of reflex effects, are not well localized
Other things being equal, those impulses arising from parts of the body
whose nerve centers are being left behind in the process of cephalization,
or which are themselves in process of phylogenetic regression, are least
apt to affect consciousness, and are most apt to be efficient stimuli of
the reflex activities. This is noticed in the case of the vestibular
sensations, which are scarcely to be recognized in consciousness even when
the attention is given to the effort, yet which are very efficient in arousing
reflex actions. The phenomena of seasickness probably illustrates
this reaction, and also the phenomena of Meniere’s disease The centers
for the area supplied by the lower sacral nerves seem to be in process
of regression. The impulses from this part of the body are not especially
well adapted to affecting consciousness, though injuries of them may cause
extreme suffering, but even slight injuries of this area are often associated
with reflex effects out of all apparent proportion to the sensory disturbance.
Conscious attention to any sensory impulses increases the power of those
impulses to affect consciousness. This phenomenon may add to the
suffering under certain conditions of disease.
not yet studied are probably responsible for many of the inexplicable effects
of stimulation. In the case of the peripheral irritations, for example,
it is impossible to determine what effect will be produced in any given individual.
The nature of the relations which make possible the inhibition of one neuron
by another needs more study before our knowledge of the interneuronic relations
is at all satisfactory. That these relations are capable of considerable
modification seems probable from facts observed in certain clinic cases.
The effects of cross suturing of nerves, both experimentally upon animals and
in traumatic paralysis in man, seem to indicate that the permeable pathways
of impulses through the central nervous system are capable of change.
In locomotor ataxia, also, the results of careful re-education show that probably
nerve impulses may ultimately employ pathways which, under normal conditions,
would not be used. Many of these things are not to be explained satisfactorily
with our present knowledge of the functional relations of the neurons.