Many regions of the brain can influence the input
arriving at the lower levels of the nervous system. This so-called
descending inhibition can be selective, with different regions of the brain
inhibiting certain inputs to the spinal cord. Some regions reduce
mechanoreceptive input, and others reduce noxious and warmth inputs.
Descending inhibition can also reduce input from the skin while increasing
that related to movement.
Prominent regions of influence are those that themselves receive noxious
input. For instance, the lateral reticular nuclei of the medulla oblongata
cause a constant inhibition of input brought to the spinal cord by the
nonmyelinated fibers. In the rat (in which the discovery was first made),
descending inhibition can be so effective that a noxious input does not
enter the spinal cord. In other words, normally painful stimuli cause no
reaction or concern, and there is no change in blood pressure, respiration,
and other reflex activities. In these circumstances it seems that pain
simply is not felt.
Electrical stimulation of the nucleus ceruleus, a small nucleus with widely
ranging axons, and the nucleus raphe magnus, a nucleus in the central
reticular formation of the medulla, inhibits input from noxious stimulation
of the skin, and it also inhibits activities of dorsal-horn neurons
receiving mechanoreceptive input. Since it was discovered that pain could be
obliterated in this manner, attempts have been made to stop the chronic pain
of cancer and other conditions by implanting electrodes in the brain. These
can be placed in relevant parts of the brain so that constant stimulation
can inhibit the input coming from the region of the pain.
This built-in system for obliterating or reducing pain can normally be
brought into play by stress. Furthermore, it has always been known that one
pain can mask another. A volley of nerve impulses reaching the brain via the
spinothalamic and spinoreticular tracts and causing a moderate degree of
pain is stopped when the cells of origin of this tract are inhibited.
Experiments have shown that this suppression can be brought about by a more
severe pain or by a pain in a larger area of the body, which causes
descending inhibition of neurons of the spinal tract. This descending
inhibition is the main mechanism of
{cra_acupuncture}
acupuncture analgesia.
The reticular formation
consists of a vast number of small interconnected neurons occupying the
central area of the brain stem. Parts of the reticular formation,
hypothalamus, and thalamus excite the
cerebral hemispheres and keep the cerebral cortex active and alert—partly in
response to noxious input. In fact, it may be said that pain reaches
consciousness in the thalamus. The thalamus receives noxious input from the
spinal cord in two regions, a lateral part called the ventrobasal complex
and a medial part consisting of several nuclei. The ventrobasal complex is
concerned with the accurate temporal and spatial localization of conscious
sensation, while the medial nuclei are concerned with the emotional,
affective, and autonomic components of pain and other sensations. The
ventrobasal nuclei relay impulses to the sensory areas of the postcentral
gyrus. Noxious stimuli also cause responses in many areas of the cortex and
the deeper islands of gray matter. This is to be expected, for, of all
sensations, pain is the least pure sensation: it startles, it excites, and
it has unpleasant qualities. All these aspects of pain are added by
different parts of the brain.
Pain arising within the central nervous system when
there is no damage to the body is known as central pain. The most common
central pain is due to lesions in or near the thalamus and is called the
thalamic syndrome. This condition is characterized by diminution in
sensibility as well as severe pain when any stimulus exceeds a certain
threshold.
With central pain there is both spontaneous pain and excessive pain on
stimulation of all kinds. The pain may occur in the entire half of the body,
affecting even visual and auditory inputs, or it may occur in a restricted
region, such as the upper limb and side of the face or the lower limb.
The term referred pain is used to describe pain felt
in a region where it does not originate but to which it is “referred.” It is
usually used to mean pain arising in hollow viscera and felt in somatic
tissues, such as the body wall. Referred pain is always referred in one
direction—from deep to superficial tissues. It is pain referred from an
unknown or unfamiliar part of the body to a known or familiar part.
Certain regions of the dorsal horns of the spinal cord receive both nerve
fibers from the viscera and nerve fibers reporting noxious events in the
skin and musculature. For example, afferent nerve fibers from the heart come
to the same regions as those from the muscles and skin of the chest wall and
upper limbs. From these two body tissues, visceral and somatic, there is an
intermingling of inputs, and there is similar convergence in the thalamus.
This anatomical arrangement is likely to form the basis of referred pain,
although the mere convergence of impulses from viscera and soma onto the
same neurons does not alone account for the false localization of visceral
pain. It is probable that in sensory areas of the brain, the skin is served
by a large number of neurons, the muscles with fewer, and the viscera with
least, the visceral representation in the cortex being very small compared
to that of the somatic tissues. It is supposed that, as the input to these
sensory regions of the cortex usually comes from the skin and body wall,
localization of the visceral input will be to these tissues and not to the
viscera, the cortical region of which is small and relatively unused.
Normally, electrical stimulation of the sensory
region of the postcentral gyrus does not cause pain. But in many patients
who have a painful state on the opposite side of the body, such as a painful
amputation stump or damage to the median nerve of the hand, stimulation of
this region reproduces the pain. Pain also arises from stimulation of the
white matter deep in this area.
In these cases the character of the sensory region of the cortex changes in
such a manner that neurons that normally never cause pain when stimulated
now invariably produce the pain from which the patient is suffering. Also,
the cortical area subserving the limb enlarges. For instance, the sensory
area receiving impulses from the opposite lower limb is normally at the
upper end of the postcentral gyrus. If there is a painful amputation of the
limb, then the area of cortex in which electrical stimulation induces the
pain spreads downward from the normal area to include the trunk area and
sometimes the upper limb area.
To the biologist, the life of animals (including
that of humans) consists of seeking stimulation and responding
appropriately. A reflex occurs before an individual knows what struck him,
what made him lift a foot or drop an object. It is biologically correct to
be alarmed before one knows the reason. It is only after the immediate and
automatic response that the cerebral cortex is involved and conscious
perception begins.
Perception comes between simple sensation and complex cognitional behaviour.
By the time people are able to talk about it, perception has become so
automatic that they hardly realize that seeing what they see, hearing what
they hear, is an interpretation. Each act of perception is a hypothesis
based on prior experience: the world is made up of things people expect to
see, hear, or smell, and any new sensory event is perceived in relation to
what they already know. People perceive trees, not brown, upright masses and
blotches of green against a background of blue, gray, and white. Once one
has learned to understand speech, it is all but impossible to hear words as
sibilants and diphthongs, sounds of lower and higher frequencies. In other
words, recognizing a thing entails knowing its total shape or pattern. This
is usually called by its German name, gestalt.
As well as perception of the external environment, there is perception of
oneself. Information about one's position in space, for example, comes from
vision, from vestibular receptors, and from somatic receptors in the skin
and deep tissues. This information is collected in the vestibular nuclei and
passed on to the thalamus. From there it is relayed to the central gyri and
the parietal region of the cortex, where it becomes conscious perception.
(For detailed discussion of the perception of movement, see
the
vestibular system.)
Perception relies on what are called the special
senses—the visual, auditory, gustatory, and olfactory. Each begins with
receptors grouped together in sensory end organs, where the sensory input is
organized before it is sent on to the brain. At every synapse on a sensory
pathway there is an important reorganization of impulses, so that by the
time an input arrives at the thalamus, it
is far from being the original input that stimulated the receptors.
The afferent parts of the thalamus fall into two divisions: a medial part,
which is afferent but not sensory, and a ventral and lateral part, which is
sensory. Nerve impulses reaching the medial part are derived from the
reticular formation. This pathway is for emotional and other rapid reactions
such as surprise, alarm, vigilance, and the readiness to react. The lateral
part of the thalamus is a station on the way to areas of the
cortex that are specific for each kind of
sensation.
There are three somatosensory areas of the cortex. The primary sensory area
occupies the postcentral gyrus immediately behind the motor strip. The
secondary area is above the Sylvian fissure, behind the secondary motor
area. The supplementary area is in the upper part of the parietal lobe on
the medial surface of the hemisphere, just behind the primary area.
The primary area receives its input from the ventrolateral thalamus. The
secondary area receives somatosensory input from the lateral part of the
thalamus and also auditory and visual input from the medial and lateral
geniculate nuclei. The primary and secondary areas are reciprocally
connected.
The cerebral cortex (and the thalamus as well) is divided between
nonspecific and specific sensory areas. Most neurons of the specific regions
have small receptive fields in the periphery, respond to only one kind of
stimulus, and follow the features of stimulation exactly. Most neurons of
the nonspecific regions have large receptive fields and respond to many
kinds of stimuli; many do not exactly reproduce the features of the
stimulus.
Although different regions of the body are normally represented by specific
parts of the somatosensory regions of the cortex, the parts where afferent
impulses arrive are not fixed. For example, the leg area is at the top of
the postcentral gyrus, but when there is a painful state in the
periphery—sciatica, for example—the leg area of the cortex can enlarge and
occupy some of the arm area. Furthermore, injury to the peripheral nerves or
brain may alter the sensory “map” of the cortex. These changes in the cortex
and similar changes in anatomical function are referred to as plasticity.
From the somatosensory area, nerve fibers run to other regions of the
cortex, traditionally called association areas. It is thought that these
areas integrate sensory and motor information and that this integration
allows objects to be recognized and located in space. With these regions
acting upon all their inputs, the brain is carrying out those aspects of
neural activity that are commonly labeled mental. It must be remembered,
though, that researchers have not precisely answered the question of how and
where the brain collects messages from all the sensory receptors and then
sifts and arranges them so as to give a complete representation of the world
and of the individual's place in the world.
Of the special senses, the cerebral organization of
the visual and auditory senses are better understood than that behind the
olfactory and gustatory. For this reason, only the former two are discussed
below.
Most investigations of the visual pathways in the
brain have been carried out in the cat.
The area of the human brain concerned with vision comprises the entire
occipital lobe and the
posterior parts of the temporal
and parietal lobes. The
primary visual area, also called the striate cortex, is on the medial side
of the occipital lobe and is surrounded by the secondary visual area. The
visual cortex is sensitive to the position and orientation of edges, the
direction and speed of movement of objects in the visual field, stereoscopic
depth, brightness, and colour. These parallel functions combine to produce
visual perception.
The ganglion neurons of the retina belong to three functional types, called
Y-, X-, and W-cells. X-cells are neurons needed for high-resolution vision.
Y-cells respond to fast movement, whereas X- and W-cells respond poorly to
fast movement. Y-cells are larger than the others and have large peripheral
fields; X-cells have small fields. In the retina, 50 to 55 percent of
ganglion cells are W-type, 40 percent are X-type, and 5 to 10 percent are
Y-type.
As constituent fibers of the optic nerves and optic tracts, X- and Y-cells
connect to the lateral geniculate nucleus of the thalamus, while W-cells
connect mainly to the superior colliculus of the midbrain. From these
regions input from the X-cells goes mainly to the primary visual area, that
from the Y-cells goes to the secondary visual area, and that from the
W-cells goes to the area surrounding the secondary area. The collicular
pathway serves movement detection and direction of gaze. The tract from the
lateral geniculate nucleus is the pathway for visual acuity.
The primary area sends fibers back to the lateral geniculate nucleus, the
superior colliculus, and the pupillary reflex centre for feedback control of
input to the visual areas. It also sends fibers to the secondary area and to
the visual area of the temporal lobe. The secondary area sends fibers to the
temporal and parietal lobes. Also, fibers cross from visual areas of one
cerebral hemisphere to the other in the corpus callosum. This link allows
neurons of the two hemispheres with similar visual fields to make direct
contact with each other.
Neurons of the striate cortex may form the first step in appreciation of
orientation of objects in the visual field. It is thought, however, that
excitation of cortical neurons is insufficient to account for orientation
and that inhibition of other neurons in the visual cortex is also necessary.
Whatever the mechanism, it has been found in experiments on cats and monkeys
that individual neurons are activated by lines at different angles—for
example, at 90 degrees to the horizontal or at an angle of 45 degrees.
Most neurons of the deeper layers are movement analyzers. Some are direction
analyzers, activated by a line or an edge moving in one direction and
silenced when it changes direction (the changed direction then activating
other neurons). One neuron may be excited by a dark line on a bright
background and another by a light line on a dark background. In other
regions of the striate cortex there are form analyzers; for example, some
are activated by rectangles and others by stars. There are also position
neurons, which show a strong response to a spot located in a certain
position and a weak response to stimulation of a larger area; others respond
only to simultaneous binocular stimulation. The striate cortex provides a
fused, binocular picture of the world. There are also colour-specific
neurons sensitive to red, green, or blue. Each of these neurons is excited
by one colour and inhibited by another.
In the secondary visual area many neurons respond particularly to the
direction of moving objects. Neurons activated by colour are not activated
by white light. In the part of this area where there are many neurons
responding to colour, the periphery of the visual field is not mapped; this
is because there are no cones in the periphery of the retina (cones being
the colour receptors). The peripheral field is mapped in an area with
neurons responding to movement—notably in the region of the superior
temporal gyrus.
It seems that one function of the pathway from the superior colliculus to
the temporal and parietal cortices is as a tracking system, enabling the
eyes and head to follow moving objects and keep them in the visual field.
The pathway from the geniculate nucleus to the primary visual area may be
said to perceive what the object is and also how and in what direction it
moves.
Some neurons in the parietal
cortex become active when a visual stimulus comes in from the edge of
the visual field toward the centre, while others are excited by particular
movements of the eyes. Other neurons react with remarkable specificity—for
example, only when the visual stimulus approaches from the same direction as
a stimulus moving on the skin of the animal, or during the act of reaching
for an object and tracking it with the hand. These parietal neurons greatly
depend on the state of vigilance. In monkeys that are apparently merely
waiting for something to happen or that have nothing to which to turn their
attention, the neurons are inactive or minimally active. But when the animal
is looking at a visual target whose change it has to detect in order to
obtain a reward, the parietal neurons become active.
A great number of neurons of the middle temporal area are sensitive to the
direction of movement of a visual stimulus and to the size of an object. In
the inferior temporal area are neurons concerned with shape and colour. The
neurons of the superior temporal polysensory area respond best to moving
stimuli—in particular to movements away from the centre of the visual field.
Both these areas are concerned with the conjunction of visual stimuli and
movement.
Much of the knowledge of the neurological
organization of hearing has been acquired from studies on the bat, an animal
that relies on acoustic information for its livelihood.
In the cochlea
(the specialized auditory end organ of the inner ear) the frequency of a
pure tone is reported by the location of the reacting neurons in the basilar
membrane, and the loudness of the sound is reported by the rate of discharge
of nerve impulses. From the cochlea the auditory input is sent to many
auditory nuclei. From there, it is sent on to the medial geniculate nucleus
and the inferior colliculus, as with the relay stations of the retina. The
auditory input finally goes to the primary and secondary auditory areas of
the temporal lobes.
The auditory cortex provides the temporal and spatial frames of reference
for the auditory data that it receives. In other words, it is sensitive to
aspects of sound more complex than frequency. For instance, there are
neurons that react only when a sound starts or stops. Other neurons are
sensitive only to particular durations of sound. When a sound is repeated
many times, some neurons respond, while others stop responding. There are
neurons that are sensitive to differences of the intensity and timing of
sounds reaching the two ears. Certain neurons that never respond to a note
of constant frequency respond when the frequency falls or rises. There are
others that respond to the rate of change of frequency, providing
information on whether distance from the source of a sound is increasing or
decreasing. Some neurons respond to the ipsilateral ear, others to the
contralateral, and yet others to both ears.
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