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Introduction to the Nervous System 2
From Introduction to the Nervous System 1, we want to get these concepts: At
any moment of life, afferent neurons are bringing to the brain and spinal cord a
variable wave of excitation. The wave is variable because different receptors are
being stimulated at any particular moment in time. The receptors have their origin
in stimuli that arise outside the body, e.g., heat, light, sound; and in stimuli that
have their origin inside the body, e.g., pH, proprioception, pressure, pain.
Afferent neurons are excitatory. Every synapse that they make with other
neurons tends to excite those neurons.
With one exception (the stretch reflex), the synapses of afferent neurons are with
interneurons (defined for our purposes as neurons that are entirely within the
CNS). Therefore, a single afferent neuron extends between a receptor and the
CNS. Within the CNS, the interneurons are in a vast and complex array of
connecting pathways. Different from afferent neurons, which are essentially all
excitatory, interneurons may be excitatory or inhibitory to the neurons with which
each synapses and it is in this way that the wave of excitation brought to the
CNS yields a varying response by the effectors of the body.
Note: An excitatory neuron will be excitatory at every one of its synapses. An
inhibitory neuron will be inhibitory at every one of its synapses.
The effectors of the body are muscle and gland. A single efferent neuron extends
from the CNS to striated muscle fibers. A chain of two neurons extends from the
CNS to autonomic effectors: smooth muscle, heart muscle, and gland.
Activities such as consciousness, dreaming, thinking , mechanisms of attention,
etc., take place within the CNS itself. They are mechanisms of interneurons, a
part of, and have their effect on, interneuronal circuitry within the CNS. By having
excitatory and inhibitory neurons, the interneuronal circuitry modulates the wave
of afferent excitation and brings about the variable contraction of muscle and
secretion of glands that constitute the actions of the living animal.
What determines that a neuron reaches the excitatory state? A neuron
effects its synapses with other neurons by terminal and collateral bulblike
structures of its axon called boutons. Depending on its function a single
interneuron may have synapses with a few neurons, with hundreds, or with
thousands of other neurons. The body and dendrites of a neuron within the CNS
are covered with synaptic boutons of both excitatory and inhibitory neurons. In
any small area of the cell body or dendrites of a neuron, the boutons of many
neurons will make synaptic contact. The post-synaptic neuron will reach the
excitatory state only if a sufficient area of its cell membrane is depolarized within
a sufficiently brief period of time. These two features, depolarization of the
neuron’s cell membrane covering a sufficient area and the depolarization’s
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occurring over a sufficiently brief period of time are designated spatial summation
and temporal summation respectively.
When spatial and temporal summation occur, threshold is reached and the
excitatory state passes as a wave of depolarization over the entire cell
membrane. The neuron will “fire”, that is the excitatory state will propagate from
the body or dendrite of the cell to depolarize (excite) the entire cell. Reaching the
axon of the cell, the excitatory state extends peripherally along the axon to its
contact with other neurons or, in the case of efferent neurons that innervate
striated muscle, to myofibers; in the case of the first neuron of the two-neuron
autonomic efferent chain, to neurons in autonomic ganglia. Inhibitory neurons
oppose this depolarization. At synapses of inhibitory neurons, the cell membrane
is hyperpolarized; that is, made more resistant to excitation.
bouton
bouton
dendrite of
postsynaptic
neuron
Electron micrograph of synaptic boutons in contact with a dendrite. Note the
synaptic vesicles. Photo is from W. J. Banks’ Applied Veterinary Histology;
1993 Mosby
Probably no interneuron or efferent neuron can be brought to threshold by a
single excitatory neuron. Each neuron, its cell body and dendrites covered with
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boutons, is subject to excitatory and inhibitory stimulation. When conditions of
time and proximity of excitation result in threshold stimulation, it “fires” and
carries impulses (the excitatory state) to all of its synapses. If it is an excitatory
interneuron, every one of these synapses will be excitatory. If it is an inhibitory
interneuron, every one of these synapses will be inhibitory. If it is an efferent
neuron to striated muscle, each of its neuroeffector synapses will be excitatory at
the motor endplate.
Consciousness is awareness of the stimulus. The total nature of consciousness
is not fully known; but it is unquestioned that it results from excitation of cerebral
cortical neurons (neurons in the outer layer of grey matter, the cortex, of the
cerebral hemisphere). Therefore, stimuli that are consciously perceived have
interneuronal pathways that excite neurons in the cerebral cortex. If the excitation
of these cortical neurons results in action, that action is designated a response or
a conditioned reflex. Such an action’s taking place due to the animal’s perception
of stimuli is a learned response. Such actions are present only after the animal
has learned the appropriate response. They are to be distinguished from
pathways that result in action but have not reached the cerebral cortex. For
example, the “patellar reflex” can be elicited in the unconscious animal. Such an
action is a reflex; or it may be designated an unconditioned reflex. Reflexes, also
designated unconditioned reflexes, are unlearned responses and are present as
soon as the neuronal pathways are functional.
Consider the racing horse. It is running hard when a proximal
sesamoid bone of its right forelimb suddenly fractures. Pain and other stimuli
result in increased inhibition to the motor neurons innervating antigravity
muscles of its injured limb. The horse reduces or avoids entirely using the
extensor muscles that permit the horse’s placing weight on the injured limb.
The horse quickly learns that bearing weight on the limb increases the pain
that it feels. Impulses arising from pain receptors pass by afferent neurons to
the spinal cord (CNS) and synapse with interneurons. Interneurons in this
case provide a number of different pathways but we shall consider only a few
of them. The horse is aware of the pain; therefore, a pathway led to the
cerebral cortex and the horse consciously withdrew the limb to avoid pain.
Pain also gives rise to avoidance reflexes that would result in flexion of the
limb even in the unconscious animal. Neurons innervating extensor muscles of
the limb, antagonists of the flexors, would be subject to interneuronal
inhibition.
Memory, learning, thinking all require consciousness = excitation of cortical
neurons. Dreaming is the excitation of cortical neurons without the external
stimulus being present.
Thinking appears to be (key words) an auditory phenomenon; that is, you “hear”
(excitation of cortical neurons without the external stimulus being present) what
you think.
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Voluntary action is also called voluntary motor activity. It is the conscious
activation of skeletal muscle. It is obviously the result of thinking and therefore
must have its origin in the cerebral cortex. Its origin is by excitation of
interneurons in an area of the cerebral cortex designated the motor cortex.
All stimuli ultimately contribute to effector action. Those that are consciously
appreciated utilize pathways that traverse the cerebral cortex and bring about
action designated a response (conditioned reflex). Stimuli whose interneuronal
pathways to effectors are limited to the lower parts of the brain or the spinal cord
bring about effector action designated a reflex (unconditioned reflex). A stimulus
can give rise to interneuronal pathways that lead to both a response and a reflex.
For example, a bright light shines into the animal’s eye. The pupil constricts, a
reflex. The animal attempts to turn its head, a learned response to the brightness
of the light.
The patellar reflex. This reflex takes place in the anesthetized animal.
Therefore, it does not require consciousness. The anesthetized animal is not
aware of the receptors stimulated to bring about this reflex; the animal is not
aware that the reflex has taken place. The conscious animal is aware that it has
carried out the reflex. This awareness is due to activation of muscle, tendon, and
joint receptors (proprioceptors) and perhaps due to cutaneous receptors
stimulated by bending and stretching of the skin. Of course, the conscious animal
has also felt the tap of the reflex hammer. The effectors are chiefly (not entirely,
as other muscles, chiefly the cranial part of the biceps femoris, are also stretched
by the patellar tap) myofibers of the quadriceps femoris muscle. So, for every
efferent neuron stimulated, probably several hundred myofibers contract.
In spinal cord injury resulting in the interruption of descending motor
pathways originating from the brain, the patellar reflex is exaggerated; it is
stronger. This is due to loss of inhibition from certain descending motor
pathways. In this case, it is easier for the stimulation provided by the patellar tap
to bring about reflex contraction of the quadriceps (and the cranial part of the
biceps femoris). Put another way, it is easier for efferent neurons innervating the
affected muscles to reach threshold.
A question. A dog has been trained to remain calm while a single rabbit crosses
its path. The owner releases three rabbits in front of the dog; the dog fidgets but
remains in place. The receptor by which the dog sees the rabbits is? The
effectors to carry out running movements of the limbs are? The normal reaction
of the untrained dog would be to chase the rabbits. This reaction, were it to take
place, would involve a voluntary act of the dog brought about by circuitry that is
within its cerebral cortex and resulting in descending excitation passing from
neurons in its cerebral cortex to efferent neurons to the appropriate muscles of
the dog’s limbs. The dog’s remaining in place while the rabbits are released must
be due to inhibitory synapses ending on……?
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Relatively few of the stimuli that stimulate receptors of the body are
consciously perceived: visual stimuli; auditory stimuli; PTT = pain, tactile and
temperature stimuli, proprioceptive stimuli (tell the length and rate of change in
length of striated muscles, pressure and tension receptors in joints); taste; smell;
force of gravity. Pathways proceeding from excitation of these receptors pass to
specific areas of the cerebral cortex: visual cortex, auditory cortex,
somatosensory cortex (ptt + proprioception), olfactory cortex. The cortical area to
which interneuronal pathways of receptors for taste and the force of gravity pass
are less clear.
Figure is from Lehrbuch
der Anatomie der
Haustiere, Band IV;
Nickel, Schummer,
Seiferle, G. Böhme, Ed.,
1992; verlag Paul Parey.
Motor c.
Somatosensory (PTT + proprioception)
c.
Visual c.
Auditory c.
Olfactory c.
Canine Brain. Areas of the cerebral cortex to which interneuronal pathways pass from synapses with
afferent neurons of the specified receptors. The motor cortex (Area motorica, Motor c.) is the area of
the cortex from which interneuronal pathways lead to efferent neurons that supply striated muscle.
A dog encounters a bear. The hair on the back of the dog’s neck stands up, its
heart beats faster, its adrenal gland secretes norepinephrine and its blood
glucose level rises… The stimulus is the look and smell of the bear, perhaps the
noise that the bear makes. The reflex result is the contraction of smooth muscles
of the hair follicles and increased rate of contraction of heart muscle, reflex
stimulation of the adrenal medulla… The dog also will adopt a particular
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expression and defensive posture, etc. This dog has never before encountered a
bear. These actions must be a reflex or response?
In examining the animal, only a few actions/activities are relatively easily
determined…
Consciousness and behavior;
Standing attitude (posture) and ambulation (how the animal moves);
Animal’s action or lack of it in response to pain, tactile, proprioceptive,
visual, and auditory stimuli, and temperature;
Facial expression, eye movement and the resting position of the eyes;
Appearance of the pupil;
Appearance of the tongue, ability to swallow;
Postural, limb, and anal reflexes;
Atrophy of muscle, integrity of muscle reflexes.
Less easily: taste, olfaction.
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