Although a cycle of simple repetitive movements can
be organized without sensory feedback, more sophisticated movements require
feedback as well as what is called feed-forward control. This is provided by
the cerebellum. Many parts of the brain have to be kept informed of
movements being carried out in order to detect error and continually correct
the movement. The cerebellum continuously receives inputs from the trunk,
limbs, eyes, ears, and vestibular apparatus, maintaining in turn a
continuous transfer of information to the motor parts of the thalamus and to
the cerebral cortex.
As a movement is being prepared, a replica of the instructions is sent to
the cerebellum; the cerebellum sends back its own information to the cortex.
The cortex, meanwhile, sends information about the movement to various
afferent neurons that are about to receive information from receptors in the
body parts where the movement is about to begin. This comparison between
instructions sent and movement performed is a fundamental requirement of all
complicated movement. The discharge of impulses from motor to sensory
regions is called the corollary discharge. The mechanisms involving the
cerebellum do not come to consciousness. There are no sensory consequences
of damage to the cerebellum, for the cerebellum is a motor structure.
As series of movements are learned and improved with practice, a replica of
the movement is probably retained in the cerebral hemispheres. (The
mechanisms of this postulated “replica” are as yet unknown.) Whenever the
learned movements are repeated, they are formed and guided by the replica.
This hypothesis of controlling movement by previously practiced patterns was
developed by von Holst. He gave the name efference to the totality of motor
impulses necessary for a movement, and he proposed that whenever the
efference is produced it leaves an image of itself somewhere in the central
nervous system. This image he called the efference copy. According to von
Holst's theory, as the movement is repeated, afferent impulses return to the
brain from receptors activated by muscular activity. These impulses are
called the re-afference. There is then a comparison between the efference
copy and the re-afference. When they are identical, the movement is
“correct” in relation to its previous performance. When the re-afference
differs from the efference copy, corrections have to be made so as to bring
the present pattern of movement back to the original image left in the
brain.
If the cerebellum is damaged or degenerates, any error between the movement
being performed and the efference copy will no longer be corrected, and the
postural adjustments sent from the cerebral hemispheres will no longer be
implemented. The force and extent of movements also will be abnormal, the
movement going too far or not far enough. The various muscles may not come
into play at the right time, and there will be a disturbance in the
relationship of antagonist muscles, so that the accurate arrival on target
will be replaced by oscillation.
Most of what is known about the contribution of the
basal ganglia has been obtained from studying abnormal conditions that occur
when these nuclei are affected by disease. In
Parkinson's disease there is
a loss of the pigmented neurons of the substantia nigra, which release the
neurotransmitter dopamine at synapses in the basal ganglia. Patients
with this disease have a certain type of muscle stiffness called rigidity, a
typical tremor, flexed posture, and difficulty in maintaining equilibrium.
They have difficulty in starting movements, including walking, and they
cannot put adequate force into fast movements. They have particular
difficulty in changing from one movement to its opposite, in carrying out
two movements simultaneously, and in stopping one movement while starting
another.
The organization of posture, which is based on vestibular, proprioceptive,
and visual input to the globus pallidus, is severely damaged when this
region of the basal ganglia degenerates. As a changing posture of the
various parts of the body is a prerequisite of every movement, this factor
alone upsets all movement. Visual reflexes contributing to motion also act
through the globus pallidus. When this structure is degenerate, what the
patient sees may affect his ability to move. One patient may be unable to go
forward if he has to pass through a narrow door, another may not be able to
do so if he has to go into a wide expanse like a field.
Animals have evolved sophisticated sensory receptors
to detect features of the environment in which they live. In addition to the
special senses such as hearing and sight, there are unobtrusive sensory
systems such as the vestibular system, which is sensitive to acceleration.
Acceleration can be considered as occurring in two forms—linear and angular.
One familiar type of linear acceleration is gravity. Because this
environmental feature, unlike any other encountered by an organism, is
always present, highly sophisticated systems have developed to detect
gravity and enable an animal to maintain its position relative to the earth.
A common form of angular acceleration is that induced by rotation, such as a
turning of the head. Through the vestibular apparatus these forces are
detected and appropriate motor activities are organized to counter the
postural perturbations that they induce.
The vestibular
sensory organ is a paired structure located symmetrically on either side
of the head within the inner ear. Inside each end organ are the hair cells,
the detection units for both linear and angular acceleration. Extending from
one surface of each hair cell are fine, hairlike cilia, displacement of
which alters the electrical potential of the cell. Bending the cilia in one
direction causes the cell membrane to depolarize while hyperpolarization is
induced by movement in the opposite direction. Changes in membrane potential
induce alteration in the firing of nerve impulses by the afferent neurons
supplying each hair cell.
The two types of acceleration are detected by two types of vestibular end
organ. Linear acceleration is sensed by a pair of organs—the saccule and
utricle—while there are three receptor organs—called semicircular canals—in
each vestibular apparatus for the detection of angular acceleration. For
more information see the ear.
Each saccule and utricle has a single cluster, or
macula, of hair cells located in the vertical and horizontal planes,
respectively. Resting upon the hair cells is a gelatinous membrane in which
are embedded calcareous granules known as otoliths. Changes in linear
acceleration alter the pressure of the otoliths, causing changes in the
distortion of the cilia and providing an adequate stimulus for membrane
depolarization. Within each macula the hair cells are arranged in two groups
oriented in opposite directions, so that the receptor functions in a
push-pull fashion within each organ. Since many of the nerve fibers passing
from the hair cells to the brain are constantly active, this push-pull
arrangement makes the receptors a highly sensitive detection system for
linear acceleration in the vertical and horizontal planes.
The angular acceleration detectors within the
semicircular canals function in a different way. The three canals—which in
fact are considerably more than a semicircle in circumference—are oriented
at approximately right angles to one another. Two are vertically placed, and
one is at about 30 degrees to the horizontal. In this arrangement the
anterior canal of one side of the head is in the plane of the posterior
canal of the other side. A ridge, or crista, covered by sensory hair cells
is located at the end of each canal within an expanded chamber called the
ampulla. The canals are filled with an endolymphatic fluid. Rotation of the
canals about an axis passing through the centre of each semicircle causes
the fluid to flow toward or away from the crista, generating a force that
bends the cilia by displacement of a gelatinous plate resting upon the
hairs. The cells of the vertical canals are oriented in such a way that
centrifugal movement away from the cristae depolarizes the hair cell
membranes, while the converse applies to the horizontal canal.
As in the case of the utricle and saccule, some of
the nerve fibers conveying information from the cells are constantly active.
The hair cells receive nerve impulses from the brain (via efferent fibers)
and send them to the central nervous system (via afferent fibers).
Excitatory efferent fibers increase the sensitivity of the hair cells, while
inhibitory fibers decrease sensitivity. This system gives the semicircular
canals a plasticity that is essential to maintaining optimal activity under
different environmental conditions—including such extraordinary states as
the microgravity encountered in space travel.
The vestibular apparatus is supplied by neurons that make up the vestibular
portion of the vestibulocochlear, or eighth cranial, nerve. The somata, or
cell bodies, of the afferent fibers lie in the vestibular ganglion near the
end organ. Most of the nerve fibers pass from there to vestibular nuclei in
the pons, while others pass directly to the cerebellum. The efferent fibers
of the vestibular nerve arise from nuclei in the pons.
For vision to be effective, the retinal image must
be stationary. This can be achieved only by maintaining the position of the
eyes relative to the earth and using this stable platform for pursuing a
moving target. The vestibular system plays a critical part in this, mainly
through complex and incompletely understood connections between the
vestibular apparatus and the musculature. Rotation of the head in any
direction is detected by the semicircular canals, and a velocity signal is
then passed via the vestibular nuclei to the somatic and extraocular
muscles. In the case of the eye muscles, the velocity signal reaching the
brain stem is in some way integrated with impulses signaling the eyes'
position, thus ensuring that the eyes maintain their position relative to
space and the object of regard. This integration partly occurs in the
vestibular nuclei, the source of secondary neurons destined for the
extraocular muscle nuclei of both sides.
When the head is oscillated, the eyes maintain their
position in space but move in relation to the head. This so-called
vestibulo-ocular reflex operates in both horizontal and vertical planes,
owing to the arrangement of the three semicircular canals, and it maintains
such stability that the object of vision does not oscillate until quite high
velocities are attained. The other component of the vestibular system, the
saccule and utricle, also contributes to the vestibulo-ocular reflex. Under
normal circumstances the otolith receptors cause torsional movement of the
eyes. For example, tilting the head toward one shoulder results in
counter-rolling of the eyes, thereby stabilizing the image upon the retina.
The two components of the vestibulo-ocular reflex also interact, enabling
appropriate eye movements to be generated when both linear and angular
accelerations are changing.
While the vestibulo-ocular reflex is the best understood of the
vestibulo-motor connections, information from the vestibular receptors is
also known to be passed via vestibular and other brain-stem nuclei to the
somatic musculature of the trunk and limbs. Through these pathways body
posture is adjusted to counter acceleration forces applied to the vestibule.
These reflexes are so important in maintaining vertical posture that
devastating short-term consequences on posture are seen if the
vestibulocochlear nerve is cut.
Besides maintaining input for the generation of
motor reflexes, vestibular impulses reach consciousness and create a
powerful sensation. A person being rotated knows when he is accelerating
even in the absence of an object upon which he can fix his eyes. This occurs
because acceleration is the adequate stimulus for the semicircular canals.
Similarly, information detected by the otoliths is readily brought to
consciousness—as occurs, for example, when a darkened elevator accelerates
up or down. The pathways to the cerebral cortex, which mediate conscious
sensation, are not fully known, but there is considerable evidence that
areas of the parietal and temporal lobes receive connections via the
thalamus.
One important aspect of vestibular physiology is the interaction of
vestibular impulses, which signal changes of position, and impulses from
other sensory receptors that signal changes in bodily movement. For example,
when the head turns to one side about a vertical axis, not only is the
horizontal canal of that side stimulated and that of the other side
inhibited, but receptors in the neck joints and muscles are also stimulated,
and the retina indicates movement if fixation is not maintained perfectly.
This information is fed to the brain via sensory pathways in the spinal cord
and various visual sensory systems. Therefore, within the vestibular nuclei
of the pons, neurons that respond to acceleration signals from the
semicircular canals receive impulses from other sources as well. Other
information from visual and spinal sensory systems pass to the
cerebellum,
which also receives direct impulses from the vestibular apparatus that
bypass the vestibular nuclei. In this way the cerebellum has the opportunity
to compare signals and assess the degree of mismatch between them. (Such
mismatch is an unpleasant experience and is important in the phenomenon of
motion sickness. Motion sickness is often generated by a mismatch between
the various inputs signaling orientation within space. People will
frequently be seasick if they are below decks in rough weather and the
visual system signals no movement while the vestibular system indicates
motion.) In another example, the vestibulo-ocular reflex may be underactive,
so that for a given head movement the eyes do not deviate sufficiently
within the orbit and the object of regard does not remain stationary upon
the retina. Thus, the image slips and cannot be seen clearly during
movement. The cerebellum has the opportunity to detect this mismatch between
the required position of the eyes with respect to the environment and the
movement actually achieved. Through inhibitory connections to the vestibular
nuclei, the cerebellum can then adjust the vestibulo-ocular reflex so that a
more appropriate movement of the eyes is achieved with the next acceleration
signal. In other words, there is a continual updating of the
vestibulo-ocular reflex via the cerebellum or structures associated with it.
A similar situation also obtains for somatosensory input from the spinal
cord. A dramatic demonstration of short-term adaptation via the visual
system occurs when someone dons prisms that reverse the perception of the
environment in the horizontal plane, making everything look upside down. The
person is at first unable to move about because any rotation of the head
results in apparent movement of the environment in the wrong direction.
However, over a few days normal mobility gradually returns. Incredibly,
during this time the vestibulo-ocular reflex is at first diminished in
amplitude and then is reversed. Removal of the prisms results in a rapid
return to the normal state. These experiments are a powerful demonstration
of the plasticity of the vestibulo-ocular reflex, which can continue
functioning throughout life in spite of the various insults that befall it.
In order for the eye to function properly, specific
autonomic functions must maintain adjustment of four types of
smooth muscle: (1) smooth
muscle of the iris, which controls
the amount of light that passes through the pupil to the retina; (2) ciliary
muscle on the inner aspect of the eye, which controls the ability to focus
vision on nearby objects; (3) smooth muscle of arteries providing oxygen to
the eye; and (4) the smooth muscle of veins that drain blood from the eye
and affect intraocular pressure. In addition, the cornea must be kept moist
by adequate secretion from the lacrimal gland.
When bright light is shined into an eye, the pupils
of both eyes constrict. This response, termed the light reflex, is regulated
by three structures: the
retina,
pretectum, and
midbrain. In the retina is a three-neuron circuit consisting of
light-sensitive photoreceptors (rods), bipolar cells, and retinal ganglion
cells. The latter transmit luminosity information to the pretectum, where
particular types of neurons relay the information to parasympathetic
preganglionic neurons located in the Edinger-Westphal nucleus of the
midbrain. These neurons send axons out of the ventral surface of the
midbrain to synapse in the ciliary ganglion. From there, parasympathetic
postganglionic neurons innervate the pupillary sphincter muscle, causing
constriction.
In order to bring a nearby object into focus,
several changes must occur in both the external and internal muscles of the
eyes. The initial stimulus for accommodation is a blurred visual image that
first reaches the visual cortex. From there, through a series of cortical
connections, it reaches two specialized motor centres. One of these, located
in the frontal cortex, sends motor commands to neurons in the oculomotor
nucleus controlling the medial rectus muscles; this causes the eyes to
converge. The other motor centre, located in the temporal lobe, functions as
the accommodation area. Via multineuronal pathways, it activates specific
parasympathetic pathways arising from the ciliary ganglion. This pathway
causes the ciliary muscle to contract, thereby reducing tension on the lens
and allowing it to become more rounded so the visual image of the near
object can be focused on the central part of the retina. Concomitant with
this change is a constriction of the iris, also under control of the
oculomotor parasympathetic system, that acts to enhance the resolution of
the lens.
The kidney is involved in maintenance of water and
electrolyte balance. It also produces vasoactive hormones such as renin, a
proteolytic enzyme that initiates the formation of angiotensin II (a potent
hormone involved in the regulation of blood pressure), and erythropoietin, a
glycoprotein hormone important in formation of red blood cells. While the
autonomic nervous system is not crucial to these functions, the fine-tuning
of some of these processes is regulated by sympathetic fibers. There is no
known parasympathetic innervation of the kidney.
In contrast to renal functions, the normal physiology of the
urinary bladder
depends entirely on the autonomic nervous system. For example, retention of
urine is made possible by activation of sympathetic pathways originating
from lateral horns in spinal segments T11–L2; these cause contraction of
smooth muscle that forms the internal urinary sphincter. Working in concert
with this sphincter is the external urinary sphincter, which is made up of
skeletal muscle controlled by motor fibers of the pudendal nerve. These
fibers, arising from ventral horns of segments S2–S4, provide tonic
excitation of the external sphincter. Because they are under voluntary
control, micturition is initiated by higher brain centres. The first command
causes voluntary inhibition of the sacral motor outflow, resulting in
relaxation of the external urinary sphincter. Simultaneously, an increase in
abdominal pressure, caused by contraction of muscles of the abdominal wall,
starts an initial flow of urine. This is followed by a reflex inhibition of
sympathetic outflow, resulting in relaxation of the internal urinary
sphincter, and by activation of parasympathetic outflow to smooth muscle
that causes the bladder to contract and expel the urine.
The sexual response in both males and females can be
defined by three physiological events. The first stage begins with
psychogenic events in higher neural centres, which travel through
multineuronal pathways and cause excitation of sacral parasympathetic
outflow innervating vascular tissues of the
penis and
clitoris. This results in dilation of these arteries and erection of the
penis and clitoris. The second stage involves secretion of glandular fluids;
this is mediated by sympathetic neurons arising in the T12–L2 levels of the
lateral horns. In the male, it involves contraction of the epididymis, vas
deferens, seminal vesicles, and prostate gland, with the overall effect of
moving fluids into the urethra; at the same time, sympathetic activation
causes a closure of the internal urinary sphincter to prevent retrograde
ejaculation of semen into the bladder. In the female, the response involves
mucous secretions of the greater vestibular glands, resulting in lubrication
of the vaginal orifice.
The third phase is a muscular response. Somatic efferent fibers in the
pudendal nerve produce rhythmic contractions of the bulbocavernous and
ischiocavernosus muscles in the male; this produces ejaculation. In the
female, homologous muscles of the pelvic floor undergo rhythmic contractions
controlled by somatic efferent neurons from the S2–S4 ventral horns.
The adrenal glands lie above the kidney and are
therefore sometimes referred to by the older term suprarenal glands. They
have a cortex and a medulla. The former synthesizes and secretes steroid
hormones that are essential for life, but it is not under autonomic control.
The adrenal
medulla, on the other hand, is innervated by sympathetic preganglionic
neurons. Within the adrenal medulla are
chromaffin
cells, which are homologous to sympathetic neurons and, like sympathetic
neurons, are developed from embryonic neural crest cells. Chromaffin cells
produce
epinephrine (adrenalin) and, to a much lesser extent,
norepinephrine as well as other chemicals such as chromogranins,
enkephalins, and neuropeptide Y—all of which are released into the
bloodstream and act as hormones. Epinephrine, in particular, affects many
different types of tissues throughout the body and has a particularly potent
effect on cells that possess b -adrenergic receptors.
The release of epinephrine prevents
hypoglycemia (low blood
sugar), through the following mechanism. By binding to a 2-adrenergic
receptors embedded in the hormone-releasing cells of the
pancreas, epinephrine
inhibits the release of
insulin. Since insulin promotes the absorption of
glucose from the bloodstream into liver, skeletal muscle, and fat cells,
inhibition of its release results in a greater amount of glucose that is
available for entry into the brain. In addition, by binding to certain b
-adrenergic receptors, epinephrine stimulates the release of glucagon, a
pancreatic peptide hormone that acts in the liver to convert glycogen to
glucose. Under emergency conditions, epinephrine causes even more widespread
effects on glucose metabolism. Glycogen in the liver and skeletal muscle is
broken down to glucose, fat held in adipose cells is converted to fatty
acids and glycerol, and production of glucose and ketone bodies ( b
-hydroxybutyric acid, acetoacetic acid) is increased in the liver. All these
substances can be used as energy sources for the body.
The function of the cardiovascular system is to
maintain an adequate supply of oxygen to all tissues of the body. In order
to maintain this function, the autonomic system must process visceral
information and coordinate neural elements that innervate the heart, blood
vessels, and respiration. In addition, certain
hormones such as angiotensin II and
vasopressin are released and act in concert with the autonomic nervous
system.
Reflex pathways
The cardiovascular system is regulated by sets of
neurons that form two major types of reflex circuit. One type is triggered
by mechanoreceptors found in the major arteries near the heart and in the
heart itself. Lying in the wall of the aortic arch and the carotid sinuses
are receptors sensitive to high pressure. These are innervated by the aortic
branch of the vagus nerve and by a branch of the glossopharyngeal nerve.
Both branches send information regarding increases in arterial blood
pressure into the medulla oblongata and synapse in the nucleus of the
solitary tract. Another group of mechanoreceptors provides information about
venous pressure and volume; these are low-pressure receptors located in the
walls of the major veins as they enter the heart and within the walls of the
atria. Low-pressure afferents also relay sensory information to the solitary
tract.
The mechanoreceptors described above trigger what is called the baroreceptor
reflex, which causes a decrease in the discharge of sympathetic vasomotor
and cardiac outflows whenever an increase in blood pressure occurs. In
addition, the baroreceptor reflex causes stimulation of vagal
cardioinhibitory neurons, which produces a decrease in heart rate, a
decrease in cardiac contractility, and dilation of peripheral blood vessels.
Overall, the net effect is to lower blood pressure.
The second major class of afferents that trigger reflex responses are
chemoreceptors found in the major arteries near the heart in loci close to
the high-pressure mechanoreceptors. Functioning as oxygen sensors, these
receptors are innervated by separate sets of fibers that travel parallel
with the baroreceptor nerves, and they also project to the nucleus of the
solitary tract. Overall, the chemoreceptor reflex regulates respiration,
cardiac output, and regional blood flow, ensuring that proper amounts of
oxygen are delivered to the brain and heart.
Vasopressin is a peptide hormone that is synthesized
in magnocellular neurons of the supraoptic and paraventricular nuclei of the
hypothalamus. These neurons send their axons into the posterior lobe of the
pituitary gland, from which vasopressin is released into nearby capillaries
and distributed throughout the body.
Vasopressin has two main functions: volume regulation and vasomotor tone. It
acts to increase water retention by increasing the permeability of kidney
tubules to water as the kidney filters blood plasma. As more water is
reabsorbed, extracellular fluid volume is increased, and this in turn
increases venous volume and, ultimately, blood pressure. Under emergency
conditions, vasopressin also selectively constricts certain vascular beds
that are nonessential for life (e.g., gastrointestinal, muscle); this shunts
blood to critical tissues such as the heart or brain.
Two major stimuli trigger the release of this hormone: increases in
extracellular fluid osmolality and decreases in blood volume (as in
hemorrhage). Osmotic stimuli cause vasopressin to be released by acting on
specialized brain centres called circumventricular organs. Surrounding the
third and fourth ventricles of the brain and containing neurons with central
projections that alter autonomic and neuroendocrine functions, these
“osmosensitive” areas possess a unique vascular system that permits
diffusion of large molecules such as peptides and ions to cross readily from
the plasma to the brain. Normally, such chemical agents do not have free
passage because the capillaries form a blood-brain barrier, but at these
special sites they have direct access to central neurons. One of the areas,
called the organum vasculosum of the lamina terminalis, lies in the third
ventricle and is involved in osmo- and sodium regulation. Another
circumventricular organ, called the subfornical organ, lies in the dorsal
part of the third ventricle; it is particularly sensitive to hormones such
as angiotensin II and signals changes needed for the regulation of salt and
water balance. Both regions project directly to vasopressin-producing
hypothalamic neurons. The area postrema, which lies on the floor of the
fourth ventricle in the medulla, is the third site involved as a special
chemical sensor of the plasma.
When blood is lost through hemorrhage, atrial receptors and baroreceptors
relay volume and pressure information, via the vagus nerve, into the nucleus
of the solitary tract. Neurons in this nucleus send commands to other relay
neurons that project directly to the magnocellular hypothalamic neurons and
cause the release of vasopressin.
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