THE GENERAL ORGANIZATION OF THE VISCERAL NERVES
CONCERNED IN EMOTIONS
The structures of the alimentary canal which are brought into
activity during the satisfactions of appetite or are checked in
their activity during pain and emotional excitement are either the
secreting digestive glands or the smooth muscle which surrounds
the canal. Both the gland cells and the smooth-muscle cells differ
from other cells which are subject to nervous influence--those of
striated, or skeletal, muscle--in not being directly under voluntary
control and in being slower in their response. The muscle connected
with the skeleton responds to stimulation within two or three
thousandths of a second; the delay with gland cells and with smooth
muscle is more likely to be measured in seconds than in fractions
of a second.
The Outlying Neurones
The skeletal muscles receive their nerve supply direct from the
central nervous system, i. e., the nerve fibres distributed to these
muscles are parts of neurones whose cell bodies lie within the brain
or spinal cord. The glands and smooth muscles of the viscera, on the
contrary, are, so far as is now known, never innervated directly
from the central nervous system.[*] The neurones reaching out from
the brain or spinal cord never come into immediate relation with the
gland or smooth-muscle cells; there are always interposed between the
cerebrospinal neurones and the viscera extra neurones whose bodies
and processes lie wholly outside the central nervous system. They
are represented in dotted lines in Fig. 1. I have suggested that
possibly these outlying neurones act as “transformers,” modifying the
impulses received from the central source (impulses suited to call
forth the quick responses of _skeletal_ muscle), and adapting these
impulses to the peculiar, more slowly-acting tissues, the secreting
cells and visceral muscle, to which they are distributed.[21]
*[Footnote: The special case of the adrenal glands will be
considered later.]
The outlying neurones typically have their cell bodies grouped in
ganglia (G’s, Fig. 1) which, in the trunk region, lie along either
side of the spinal cord and in the head region and in the pelvic
part of the abdominal cavity are disposed near the organs which the
neurones supply. In some instances these neurones lie wholly within
the structure which they innervate (see e. g., the heart and the
stomach, Fig. 1). In other instances the fibres passing out from
the ganglia--the so-called “postganglionic fibres”--may traverse
long distances before reaching their destination. The innervation
of blood vessels in the foot by neurones whose cell bodies are in
the lower trunk region is an example of this extensive distribution
of the fibres.
[Illustration: Figure 1.--Diagram of the more important
distributions of the autonomic nervous system. The brain and spinal
cord are represented at the left. The nerves to skeletal muscles
are not represented. The preganglionic fibres of the autonomic
system are in solid lines, the postganglionic in dash-lines. The
nerves of the cranial and sacral divisions are distinguished from
those of the thoracico-lumbar or “sympathetic” division by broader
lines. A + mark indicates an augmenting effect on the activity
of the organ; a - mark, a depressive or inhibitory effect. For
further description see text.]
The Three Divisions of the Outlying Neurones
As suggested above, the outlying neurones are connected with the
brain and spinal cord by neurones whose cell bodies lie within the
central nervous organs. These connecting neurones, represented in
continuous lines in Fig. 1, do not pass out in a continuous series
all along the cerebrospinal axis. Where the nerves pass out from
the spinal cord to the fore and hind limbs, fibres are not given
off to the ganglia. Thus these connecting or “preganglionic” fibres
are separated into three divisions. In front of the nerve roots
for the fore limbs is the head or cranial division, between the
nerve roots for the fore limbs and those for the hind limbs is the
trunk division (or thoracico-lumbar division, or, in the older
terminology, the “sympathetic system”); and after the nerve roots
for the hind limbs the sacral division.
This system of outlying neurones, with postganglionic fibres
innervating the viscera, and with preganglionic fibres reaching
out to them from the cerebrospinal system, has been called by
Langley, to whom we are indebted for most of our knowledge of its
organization, the _autonomic nervous system_.[22] This term indicates
that the structures which the system supplies are not subject to
voluntary control, but operate to a large degree independently. As
we have seen, a highly potent mode of influencing these structures
is through conditions of pain and emotional excitement. The parts
of the autonomic system--the cranial, the sympathetic, and the
sacral--have a number of peculiarities which are of prime importance
in accounting for the bodily manifestations of such affective states.
The Extensive Distribution of Neurones of the “Sympathetic”
Division and Their Arrangement for Diffuse Action
The fibres of the sympathetic division differ from those of the
other two divisions in being distributed through the body very
widely. They go to the eyes, causing dilation of the pupils. They go
to the heart and, when stimulated, they cause it to beat rapidly.
They carry impulses to arteries and arterioles of the skin, the
abdominal viscera, and other parts, keeping the smooth muscles
of the vessel walls in a state of slight contraction or tone,
and thus serving to maintain an arterial pressure sufficiently
high to meet sudden demands in any special region; or, in times
of special discharge of impulses, to increase the tone and thus
also the arterial pressure. They are distributed extensively to
the smooth muscle attached to the hairs; and when they cause this
muscle to contract, the hairs are erected. They go to sweat glands,
causing the outpouring of sweat. These fibres pass also to the
entire length of the gastro-intestinal canal. And the inhibition of
digestive activity which, as we have learned, occurs in pain and
emotional states, is due to impulses which are conducted outward
by the _splanchnic nerves_--the preganglionic fibres that reach to
the great ganglia in the upper abdomen (see Fig. 1)--and thence
are spread by postganglionic fibres all along the gut.[23] They
innervate likewise the genito-urinary tracts, causing contraction
of the smooth muscle of the internal genital organs, and usually
relaxation of the bladder. Finally they affect the liver, releasing
the storage of material there in a manner which may be of great
service to the body in time of need. The _extensiveness_ of the
distribution of the fibres of the sympathetic division is one of
its most prominent characteristics.
Another typical feature of the sympathetic division is an arrangement
of neurones for diffuse discharge of the nerve impulses. As shown
diagrammatically in Fig. 1, the preganglionic fibres from the
central nervous system may extend through several of the sympathetic
ganglia and give off in each of them connections to cell bodies of
the outlying neurones. Although the neurones which transmit sensory
impulses from the skin into spinal cord have similar relations to
nerve cells lying at different levels of the cord, the operation in
the two cases is quite different. In the spinal cord the sensory
impulse produces directed and closely limited effects, as, for
example, when reflexes are being evoked in a “spinal” animal (i. e.,
an animal with the spinal cord isolated from the rest of the central
nervous system), the left hind limb is nicely lifted, in response
to a harmful stimulus applied to the left foot, without widespread
marked involvement of the rest of the body in the response.[24]
In the action of the sympathetic division, on the contrary, the
connection of single preganglionic fibres with numerous outlying
neurones seems to be not at all arranged for specific effects in
this or that particular region. There are, to be sure, in different
circumstances variations in the degree of activity of different
parts; for example, it is probable that dilation of the pupil in the
cat occurs more readily than erection of the hairs. It may be in
this instance, however, that specially direct pathways to the eye
are present for common use in non-emotional states (in dim light,
e. g.), and that only slight general disturbance in the central
nervous system, therefore, would be necessary to send impulses
by these well-worn courses. Thus for local reasons (dust, e. g.)
tears might flow from excitation of the tear glands by sympathetic
impulses, although other parts innervated by this same division might
be but little disturbed. We have no means of voluntarily wearing
these pathways, however, and both from anatomical and physiological
evidence the neurone relations in the sympathetic division of the
autonomic system seem devised for widespread diffusion of nervous
impulses.
The Arrangement of Neurones of the Cranial and Sacral
Divisions for Specific Action
The cranial and sacral autonomic divisions differ from the
sympathetic in having only _restricted_ distribution (see Fig.
1). The third cranial nerves deliver impulses from the brain to
ganglia in which lie the cell bodies of neurones innervating
smooth muscle only in the front of the eyes. The vagus nerves are
distributed to the lungs, heart, stomach, and small intestine.
As shown diagrammatically in Fig. 1, the outlying neurones in
the last three of these organs lie within the organs themselves.
By this arrangement, although the preganglionic fibres of the
vagi are extended in various directions to structures of quite
diverse functions, singleness and separateness of connection of the
peripheral organs with the central nervous system is assured. The
same specific relation between efferent fibres and the viscera is
seen in the sacral autonomic. In this division the preganglionic
fibres pass out from the spinal cord to ganglia lying in close
proximity to the distal colon, the bladder, and the external
genitals. And the postganglionic fibres deliver the nerve impulses
only to the nearby organs. Besides these innervations the cranial
and sacral divisions supply individual arteries with “dilator
nerves”--nerves causing relaxation of the particular vessels. Quite
typically, therefore, the efferent fibres of the two terminal
divisions of the autonomic differ from those of the mid-division
in having few of the distributed connections characteristic of the
mid-division, and in innervating distinctively the organs to which
they are distributed. The cranial and sacral preganglionic fibres
resemble thus the nerves to skeletal muscles, and their arrangement
provides similar possibilities of specific and separate action in
any part, without action in other parts.
The Cranial Division a Conserver of Bodily Resources
The cranial autonomic, represented by the vagus nerves, is the part
of the visceral nervous system concerned in the psychic secretion
of the gastric juice. Pawlow showed that when these nerves are
severed psychic secretion is abolished. The cranial nerves to the
salivary glands are similarly the agents for psychic secretion in
these organs, and are known to cause also dilation of the arteries
supplying the glands, so that during activity the glands receive a
more abundant flow of blood. As previously stated (see p. 13), the
evidence for a psychic tonus of the gastro-intestinal musculature
rests on a failure of the normal contractions if the vagi are
severed before food is taken, in contrast to the continuance of the
contractions if the nerves are severed just afterwards. The vagi
artificially excited are well known as stimulators of increased
tone in the smooth muscle of the alimentary canal. Aside from these
positive effects on the muscles of the digestive tract and its
accessory glands, cranial autonomic fibres cause contraction of
the pupil of the eye, and slowing of the heart rate.
A glance at these various functions of the cranial division reveals
at once that they serve for bodily conservation. By narrowing the
pupil of the eye they shield the retina from excessive light. By
slowing the heart rate, they give the cardiac muscle longer periods
for rest and invigoration. And by providing for the flow of saliva
and gastric juice and by supplying the muscular tone necessary
for contraction of the alimentary canal, they prove fundamentally
essential to the processes of proper digestion and absorption by
which energy-yielding material is taken into the body and stored.
To the cranial division of the visceral nerves, therefore, belongs
the quiet service of building up reserves and fortifying the body
against times of need or stress.
The Sacral Division a Group of Mechanisms for Emptying
Sacral autonomic fibres cause contraction of the rectum and distal
colon and also contraction of the bladder. In both instances the
effects result reflexly from stretching of the tonically contracted
viscera by their accumulating contents. No affective states precede
this normal action of the sacral division and even those which
accompany or follow are only mildly positive; a feeling of relief
rather than of elation usually attends the completion of the act
of defecation or micturition--though there is testimony to the
contrary.
The sacral autonomic fibres also include, however, the nervi
erigentes which bring about engorgement of erectile tissue in the
external genitals. According to Langley and Anderson[25] the sacral
nerves have no effect on the _internal_ generative organs. The vasa
deferentia and the seminal vesicles whose rhythmic contractions
mark the acme of sexual excitement in the male, and the uterus whose
contractions in the female are probably analogous, are supplied
only by lumbar branches--part of the sympathetic division. These
branches also act in opposition to the nervi erigentes and cause
constriction of the blood vessels of the external genitals. The
sexual orgasm involves a high degree of emotional excitement; but
it can be rightly considered as essentially a reflex mechanism;
and, again in this instance, distention of tubules, vesicles, and
blood vessels can be found at the beginning of the incident, and
relief from this distention at the end.
Although distention is the commonest occasion for bringing the sacral
division into activity it is not the only occasion. Great emotion,
such as is accompanied by nervous discharges via the sympathetic
division, may also be accompanied by discharges via the sacral
fibres. The involuntary voiding of the bladder and lower gut at times
of violent mental stress is well known. Veterans of wars testify
that just before the beginning of a battle many of the men have to
retire temporarily from the firing line. And the power of sights and
smells and libidinous thoughts to disturb the regions controlled by
the nervi erigentes proves that this part of the autonomic system
also has its peculiar affective states. The fact that one part of
the sacral division, e. g., the distribution to the bladder, may
be in abeyance, while another part, e. g., the distribution to the
rectum, is active, illustrates again the directive discharge of
impulses which has been previously described as characteristic of
the cranial and sacral portions of the autonomic system.
Like the cranial division, the sacral is engaged in internal service
to the body, in the performance of acts leading immediately to
greater comfort.
The Sympathetic Division Antagonistic To Both
The Cranial and the Sacral
As indicated in the foregoing description many of the viscera are
innervated both by the cranial or sacral part of the autonomic and
by the sympathetic. _When the mid-part meets either end-part in any
viscus their effects are antagonistic._ Thus the cranial supply
to the eye contracts the pupil, the sympathetic dilates it; the
cranial slows the heart, the sympathetic accelerates it; the sacral
contracts the lower part of the large intestine, the sympathetic
relaxes it; the sacral relaxes the exit from the bladder, the
sympathetic contracts it. These opposed effects are indicated in
Fig. 1 by + for contraction, acceleration or increased tone; and
by - for inhibition, relaxation, or decreased tone.[*]
*[Footnote: The vagus nerve, when artificially stimulated, has
a primary, brief inhibitory effect on the stomach and small
intestine; its main function, however, as already stated, is to
produce increased tone and contraction in these organs. This
double action of the vagus is marked thus, ∓, in Fig. 1.]
Sherrington has demonstrated that the setting of skeletal muscles
in opposed groups about a joint or system of joints--as in flexors
and extensors--is associated with an internal organization of the
central nervous system that provides for relaxation of one group of
the opposed muscles when the other group is made to contract. This
“reciprocal innervation of antagonistic muscles,” as Sherrington has
called it,[26] is thus a device for orderly action in the body. As
the above description has shown, there are peripheral oppositions
in the viscera corresponding to the oppositions between flexor and
extensor muscles. In all probability these opposed innervations of
the viscera have counterparts in the organization of neurones in the
central nervous system. Sherrington has noticed, and I can confirm
the observation, that even though the sympathetic supply to the eye
is severed and is therefore incapable of causing dilation of the
pupil, nevertheless the pupil dilates in a paroxysm of anger--due,
no doubt (because the response is too rapid to be mediated by the
blood stream), to central inhibition of the cranial nerve supply to
the constrictor muscles--i. e., an inhibition of the muscles which
naturally oppose the dilator action of the sympathetic. Pain, the
major emotions--fear and rage--and also intense excitement, are
manifested in the activities of the sympathetic division. When in
these states impulses rush out over the neurones of this division
they produce all the changes typical of sympathetic excitation,
such as dilating the pupils, inhibiting digestion, causing pallor,
accelerating the heart, and various other well-known effects. The
impulses of the sympathetic neurones, as indicated by their dominance
over the digestive process, are capable of readily overwhelming
the conditions established by neurones of the cranial division of
the autonomic system.
Neurones of the Sympathetic Division and Adrenal Secretion
Have the Same Action
Lying anterior to each kidney is a small body--the adrenal gland.
It is composed of an external portion or cortex, and a central
portion or medulla. From the medulla can be extracted a substance,
called variously suprarenin, adrenin, epinephrin or “adrenalin,”[*]
which, in extraordinarily minute amounts, affects the structures
innervated by the sympathetic division of the autonomic system
precisely as if they were receiving nervous impulses. For example,
when adrenin is injected into the blood, it will cause pupils to
dilate, hairs to stand erect, blood vessels to be constricted,
the activities of the alimentary canal to be inhibited, and sugar
to be liberated from the liver. These effects are not produced
by action of the substance on the central nervous system, but by
direct action on the organ itself.[27] And the effects occur even
after the structures have been removed from the body and kept alive
artificially.
*[Footnote: The name “adrenalin” is proprietary. “Epinephrin”
and “adrenin” have been suggested as terms free from commercial
suggestions. As _adrenin_ is shorter and more clearly related to
the common adjectival form, _adrenal_, I have followed Schäfer in
using _adrenin_ to designate the substance produced physiologically
by the adrenal glands.]
The adrenals are glands of internal secretion, i. e., like the
thyroid, parathyroid, and pituitary glands, for example; they have
no connection with the surface of the body, and they give out into
the blood the material which they elaborate. The blood is carried
away from each of them by the lumbo-adrenal vein which empties either
into the renal vein or directly into the inferior vena cava just
anterior to the openings of the renal veins. The adrenal glands are
supplied by preganglionic fibres of the autonomic group,[28] shown
in solid line in Fig. 1. This seems an exception to the general
rule that gland cells have an outlying neurone between them and
the neurones of the central nervous system. The medulla of the
adrenal gland, however, is composed of modified nerve cells, and
may therefore be regarded as offering exceptional conditions.
The foregoing brief sketch of the organization of the autonomic
system brings out a number of points that should be of importance
as bearing on the nature of the emotions which manifest themselves
in the operations of this system. Thus it is highly probable that
the sympathetic division, because arranged for diffuse discharge,
is likely to be brought into activity as a whole, whereas the
sacral and cranial divisions, arranged for particular action on
separate organs, may operate in parts. Also, because antagonisms
exist between the middle and either end division of the autonomic,
affective states may be classified according to their expression in
the middle or an end division and these states would be, like the
nerves, antagonistic in character. And finally, since the adrenal
glands are innervated by autonomic fibres of the mid-division,
and since adrenal secretion stimulates the same activities that
are stimulated nervously by this division, it is possible that
disturbances in the realm of the sympathetic, although initiated
by nervous discharge, are automatically augmented and prolonged
through chemical effects of the adrenal secretion.
REFERENCES
[Footnote 21: Cannon: The American Journal of Psychology, 1914,
xxv, p. 257.]
[Footnote 22: For a summary of his studies of the organization of
the autonomic system, see Langley: Ergebnisse der Physiologie,
Wiesbaden, 1903, ii², p. 818.]
[Footnote 23: See Cannon: American Journal of Physiology, 1905,
xiii, p. xxii.]
[Footnote 24: See Sherrington: The Integrative Action of the Nervous
System, New York, 1909, p. 19.]
[Footnote 25: Langley and Anderson: Journal of Physiology, 1895,
xix, see pp. 85, 122.]
[Footnote 26: Sherrington: _Loc. cit._, p. 90.]
[Footnote 27: Elliott: Journal of Physiology, 1905, xxxii, p. 426.]
[Footnote 28: See Elliott: Journal of Physiology, 1913, xlvi, p.
289 ff.]