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Introduction to the Nervous System 3
Consciousness is the awareness of sensation. It is a phenomenon
associated with the excitation of cerebral cortical neurons*. Therefore,
stimuli that are consciously appreciated must give rise to an excitatory
state that reaches the cerebral cortex. These stimuli are relatively few in
number: Olfactory, gustatory, pain, touch, pressure, temperature
(electromagnetic radiation of the infrared spectrum), light (electromagnetic
radiation of the visible spectrum), auditory, stimuli that act on the vestibular
and ampullary receptors of the internal ear, and proprioceptive (joint and
muscle sense) stimuli. It turns out that, with the possible exception of
stimuli that excite vestibular and ampullary receptors, each of these
modalities results in excitation of neurons in a specific area of the cerebral
cortex, which is designated accordingly: Visual cortex, auditory cortex,
somatosensory cortex, etc.
*Some texts state that low levels of consciousness are present in the thalamus.
Fig. 1. Canine brain, lateral
view. Inset shows the brain
with left hemisphere removed.
cruciate sulcus
cruciate sulcus
somatosensory cortex
motor cortex
Fig. 2. Canine, left cerebral
hemisphere, lateral view,
showing sensory and motor
areas of the cerebral cortex.
From Lehrbuch der Anatomie
der Haustiere, Band IV;
Nickel, Schummer, Seiferle,
G. Böhme, Ed., 1992; Verlag
Paul Parey.
cruciate sulcus
Fig. 3. Canine, left cerebral
hemisphere, medal view,
showing sensory and motor
areas of the cerebral cortex.
From Lehrbuch der Anatomie
der Haustiere, Band IV; Nickel,
Stimuli act
on receptors
Schummer,
Seiferle,
G. Böhme,of the body. Afferent and interneurons provide the
which
theParey.
excitatory state extends from receptors to the cerebral
Ed.,path
1992;by
Verlag
Paul
cortex. Interruption of any part of this pathway or loss of receptor function
or of the specific cortical function results in loss of the sensation mediated.
Consciousness and the reticular activating system. What happens
when receptor input is lost owing to injury, disease, or other cause? Since
all efferent output is due to input, if all input were erased, presumably all
output would be lost. The animal would probably not die; for the heart
would continue to beat and other functions of smooth muscle and gland
would not be lost. These autonomic effectors would lose their neural
regulation but would continue to function.
The reticular formation. In contrast to the large aggregations of cell
bodies within the cns, designated nuclei and grey matter, and the
aggregation of axonal fibers, designated tracts, a dispersed network of
neurons, observed under the light microscope as a meshwork of cell
bodies and processes, is designated reticular formation. The reticular
formation extends from the thalamus of the diencephalon to the caudal
spinal cord. The length of the cord it is a lateral, weblike extension of the
grey matter (dorsal to the lateral horn of the grey matter of the thoracic
cord) in relation to the lateral funiculus. Within the brainstem, it appears as
an irregular and diffuse feltwork, filling the spaces between the larger
nuclei and tracts. In the thalamus of the diencephalon, it consists of nuclei
in relation to the external and internal medullary laminae. The laminae are
thin layers of medullated fibers that separate major thalamic nuclei. In the
brainstem, smaller, ill-defined, areas constituting the respiratory, cardiac,
micturition, and vomiting centers are present within the reticular formation.
reticular formation
Fig. 4. Spinal cord x-section.
From Handbuch der
Vergleichenden Anatomie der
Haustiere by Wilhelm
Ellenberger and Hermann
Baum; revised 1943, by Otto
Zietzschmann, Ernst
reticular formation
Fig. 5. Cross-section of the rostral pons at its junction with the mesencephalon.
The right-half is myelin-stained, making the fiber tracts black; nuclear areas remain
pale with this stain, providing contrast. The large grey area dorsally is made up of a
felt-work of fibers and neuron cell bodies, the reticular formation. From Gray’s
Anatomy, 18th US edition; 1920. The figure was taken from Wikipedia.
The reticular activating system is an interneuronal system that
maintains consciousness. I believe (key word; other than its description as
an “arousal” system, there is little to describe its precise manner of
function) that it functions in this way: Probably no neuron is excited by a
single other neuron. Spatial and temporal summation of interneurons and
efferent neurons generally results from the input provided by neighboring
synaptic boutons of several or many neurons. This is undoubtedly true of
cortical neurons (Note: As a neuron that begins and ends within the cns,
cortical neurons satisfy our definition of inteneurons.) that, excited, yield
the phenomenon of awareness, the consciousness of sensation. All
afferent neurons are probably excitatory and, arriving at the CNS, give off
collaterals that feed into the reticular formation. From the formation, there
are generated excitatory pathways that excite and inhibitory pathways that
inhibit succeeding neurons. A major excitatory pathway is designated the
reticular activating system (RAS), which maintains sub-threshold excitation
of cortical neurons and is responsible for consciousness. With the arrival of
excitation provided by interneuronal pathways, which, as well-defined
tracts, lead in more direct fashion from specific receptors, threshold is
reached, the cortical neuron is excited, and the particular sensation is
realized. A prominent part of the RAS is in the midbrain and midbrain
lesions of animals may result in a comatose or semi-comatose state.
Consider also how one normally prepares for sleep, which is a state of
diminished consciousness. We do this by lowering receptor input: We turn
off the light; we turn off the radio; we retire to the bed and thus diminish
tactile stimulation. In this way, the input to the reticular formation and the
activity of the RAS are substantially diminished. Consciousness is not
erased, however, and a sufficiently strong stimulus (for example, the ring of
the alarm) is consciously perceived.
Note: From the pontomedullary reticular formation, important
excitatory and inhibitory reticulospinal tracts pass to efferent neurons innervating striated muscle.
Fig. 6. Diagram of proposed origin of the
reticular activating system. Collateras of
afferent neurons feed into the system,
creating sub-threshold excitation of
cerebral cortical neurons.
reticular activating system (it
extends from the thalamus
through the brainstem and the
length of the cord)
Fig. 7. Diagram of a cerebral cortical
neuron covered (this neuron is not
“covered” but the cell bodies and
dendrites of most interneurons would be
entirely covered by boutons of excitatory
and inhibitory neurons). The proposed
mechanism is that with sub-threshold
excitation maintained by the RAS, the
arrival of excitation generated by specific
receptors results in threshold excitation of
specific cortical neurons and awareness of
the stimulus.
Memory, learning, and thinking are cortical functions. Awareness of
what is seen, heard, touched, and otherwise experienced is a necessary
prerequisite of memory, learning, and thinking, which of course determine
much of behavior. Memory, learning, and thinking are all cortical,
interneuronal, functions and have their effect mediated by interneuronal
pathways that are ultimately expressed in excitation and inhibition of
efferent neurons. Mechanisms of attention by which the effect of some
receptors is accentuated and that of others suppressed probably (key
word) are accomplished in part by efferent innervation to (and, presumably,
inhibition of) receptors. The hair cells of the internal ear have been shown
to receive efferent innervation, and such efferent innervation of receptors is
likely present elsewhere. To the writer’s knowledge, the function of these
efferents to receptors is not established.
The motor cortex is the area of the cerebral cortex at which initiation
of voluntary motor activity takes place. In all the species that we study,
and in humans, the motor cortex is located immediately anterior to the
somatosensory cortex. Voluntary, deliberate, motor activity is the result of
processes of memory, learning, and thought. It is distinguished from
involuntary, reflex, activity that, with the exception of olfactory reflexes,
does not traverse the cerebral cortex.
All stimuli of which the animal is aware give rise to impulses, the
excitatory state, that pass to specific areas of the cerebral cortex.
Conditioned on the stimuli, interneuronal processes of memory and
learning integrate this input within the cortex, resulting in the initiation of
deliberate motor activity in the motor cortex and the initiation of a response
by skeletal muscle and autonomic effectors.
Voluntary motor activity is modulated by subcortical nuclei of the cerebral
hemisphere and the subthalamic nuclei of the diencephalon. Skeletal motor
activity is coordinated by the cerebellum. Motor fibers descending from the
cortex end on efferent neurons in the brainstem and spinal cord that supply
skeletal muscle. These same motor fibers give off collaterals that synapse
in pontine nuclei, aggregations of cell bodies between the fibers of the
basis ponti. Axons of the cells of the pontine nuclei as pontocerebellar
fibers collectively form the middle cerebellar peduncles and end in the
cerebellar cortex. Descending motor fibers of the red nucleus of the
midbrain give off collaterals to the inferior olivary nucleus of the medulla,
which projects with olivocerebellar fibers to the cerebellar cortex. In this
way, prospective voluntary movements are fed into the cerebellum.
The cerebellum coordinates skeletal motor activity and determines
the resting tone of muscle. It functions in the maintainance of normal
posture and equilibrium (equi libra = equal balance). Every particle of the
body mass is subject to the force of gravity. The force acting on each
particle could be represented by a vector, the length of which is
proportional to the force acting on the particle and the direction of which
extends toward the force acting. These vectors can be added to yield a
single vector, whose length is proportional to the total force acting on the
body and which is directed toward the center of the earth from a point on
the body, the body’s center of gravity. Owing to the displacement of its
mass, every movement of the body necessarily alters the center of gravity
and in the moving animal the center of gravity is continually changing. If
there were no accommodating change in the tension of its muscles, the
animal would be unable to maintain its posture and its equilibrium with
respect to the supporting surface, the ground or floor. Every movement
results in changes in the output to its muscles that determine its posture.
This output, and the output that at the same time provides a smooth,
coordinated, movement, is integrated in, and effected by, the cerebellum.
The cerebellum receives input, impulses, from the neuromuscular and
neurotendinous spindle receptors that give it information on the length of
muscles and the rate of change in length of muscles and the tension of
tendons; it receives input from all joint receptors. It also receives input from
the vestibular and ampullary receptors of the middle ear, which are
sensitive to the position of the head in space and to linear and angular
acceleration. It has also been shown that there is input to the cerebellum
from visual and auditory receptors. And it receives input by way of
descending corticopontine, corticomedullary, corticospinal, and rubrospinal
fibers.
From this input, there follows an integration that establishes the necessary
adjustments in output to efferent neurons supplying skeletal muscle to
assure the maintenance of posture and the coordination of movement. All
output of the cerebellar cortex is effected by the Purkinje cells of the
cerebellar cortex, which act chiefly on deep cerebellar nuclei to bring about
these adjustments.
Consciousness is not a function of the cerebellum. Removal or
destruction of the cerebellum leaves consciousness unaffected.
Maintenance of posture, of equilibrium and coordination of muscle activity
are functions of the cerebellum.
In Nervous System 2 was a list of those actions and activities that are
relatively easily examined. In boldface below are those most useful in
examining cerebellar, muscle and joint receptors, and vestibular and
ampullary internal ear function.
Consciousness and behavior;
Standing attitude (posture) and ambulation (how the animal
moves);
Animal’s action or lack of it in response to pain, tactile, visual and
auditory stimuli;
Facial expression, eye movement and resting position of the eyes;
Appearance of pupil;
Appearance of tongue, ability to swallow;
Postural, limb, and anal reflexes;
Atrophy of muscle, integrity of muscle reflexes.