The thalamus has long been regarded as the key to understanding the
organization of the central nervous system. It is involved in the relay and
distribution of most, but not all, sensory and motor signals to specific
regions of the cerebral cortex. Sensory signals generated in all types of
receptors are projected by complex pathways to specific relay nuclei in the
thalamus, where they are segregated and systematically organized. The relay
nuclei in turn supply the primary and secondary sensory areas of the
cerebral cortex. Sensory input to thalamic nuclei is crossed for the
somesthetic and visual systems, bilateral (but mainly crossed) for the
auditory system, and ipsilateral (on the same side) for gustatory (taste)
and olfactory (smell) sense.
The somesthetic relay nuclei of the thalamus, collectively known as the
ventrobasal complex, receive input from the medial lemniscus (originating in
the medulla), from spinothalamic tracts, and from the trigeminal nerve.
Fibers within these ascending tracts that terminate in the central core of
the ventrobasal complex receive input from deep sensory receptors, while
fibers projecting onto the outer shell receive input from cutaneous
receptors. This segregation of deep and superficial sensation is preserved
in projections of the ventrobasal complex to the primary somesthetic (i.e.,
sensory) area of the cerebral cortex.
The medial and lateral
geniculate bodies form what is called the metathalamus. Fibers of the optic
nerve end in the lateral geniculate body, which consists of six cellular
laminae, or layers, folded into a horseshoe configuration. Each lamina
represents a complete map of the contralateral visual hemifield, and all
laminae are in perfect registration. Cells in all layers of the lateral
geniculate body project via the optic radiation to the visual areas of the
cerebral cortex. The medial geniculate body receives auditory impulses from
the inferior colliculus of the midbrain and relays them to the auditory
areas on the temporal lobe. Only the ventral nucleus of the medial
geniculate body is laminated and tonotopically organized; this part projects
to the primary auditory area and is finely tuned. Other subdivisions of the
medial geniculate body project to the belt of secondary auditory cortex
surrounding the primary area.
Major
output from the cerebellum projects to specific thalamic relay nuclei in a
pattern similar to that for somesthetic input. The thalamic relay nuclei in
turn provide a major input to the
primary motor area
of the frontal lobe. This large system appears to provide coordinating and
controlling influences that result in the appropriate force, sequence, and
direction of voluntary motor activities. Output from the corpus striatum, on
the other hand, is relayed by thalamic nuclei that have access to the
supplementary and premotor areas. The
supplementary
motor area, located on the medial aspect of the hemisphere (see figure
to left), exerts modifying influences upon the primary motor area and
appears to be involved in programming skilled motor sequences. The premotor
area, rostral to the primary motor area, plays a role in sensorially guided
movements.
Other major thalamic nuclei, besides those involved in relaying sensory
impulses or controlling influences from the cerebellum and corpus striatum,
include the anterior nuclear group, the mediodorsal nucleus, and the
pulvinar. The anterior nuclear group receives input from the hypothalamus
and projects upon parts of the limbic lobe (i.e., the cingulate gyrus). The
mediodorsal nucleus, part of the medial nuclear group, has reciprocal
connections with large parts of the frontal lobe rostral to the motor areas.
The pulvinar is a huge posterior nuclear complex that, along with the
mediodorsal nucleus, has projections to association areas of the cortex.
Output ascending from the reticular formation of the brain stem is relayed
to the cerebral cortex by intralaminar thalamic nuclei, which lie in laminae
separating the medial and ventrolateral thalamic nuclei. This ascending
system is concerned with arousal mechanisms, maintaining alertness, and
directing attention to sensory events.
The amygdala, (which means
“almond-shaped”), controls our aggression and emotions. Many autistic
individuals are aggressive towards themselves or others, or conversely,
extremely passive. Furthermore, autistic children and adults often appear
emotionless or ‘flat’ (even though they obviously do have emotions).
Experimenters have also shown that when the amygdala is removed or damaged,
animals exhibit behaviors similar to autistic individuals, such as social
withdrawal, compulsive behaviors, failure to learn about dangerous
situations, difficulty retrieving information from memory, and difficulty
adjusting to novel events or situations. In addition, the amygdala is
responsive to a variety of sensory stimuli, such as sounds, sights, and
smells; as well as emotionally or fear-related stimuli. We know that
autistic individuals often have problems with each of these senses.
Interestingly, Georgie, whose childhood was described in her mother’s book,
The Sound of a Miracle, often mentioned being afraid of many sounds prior to
receiving auditory integration training from Dr. Guy Berard.
The
hippocampus, (shaped like a “sea horse”) appears to be primarily responsible
for learning and memory. Damage or removal of the hippocampus will lead to
an inability to store new information into memory. This sounds similar to
Dr. Bernard Rimland's cognitive theory of autism. In his 1964 award-winning
book Infantile Autism, Dr. Rimland theorized that autistic children had
difficulty relating new information to previously stored information. In
addition, when the hippocampus is damaged or removed, animals will display
stereotypic, self-stimulatory behaviors and hyperactivity.
The diencephalon consists of a pair of egg-shaped
nuclear masses that lie on each side of the third ventricle and medial to
the posterior limb of the internal capsule. Four subdivisions are
recognized: (1) the epithalamus, (2) the thalamus, (3) the
hypothalamus, and (4) the ventral thalamus,
or subthalamus.
The epithalamus is represented mainly by the
pineal gland,
which lies in the midline posterior and dorsal to the third ventricle. This
gland synthesizes melatonin and enzymes sensitive to diurnal light. Rhythmic
changes in its activity in response to cyclical photic input suggest that
the gland serves as a biological clock. With age it tends to accumulate
calcium deposits.
The ventral thalamus is represented mainly by the
subthalamic nucleus, a lens-shaped structure lying behind and to the sides
of the hypothalamus and on the dorsal surface of the internal capsule. The
subthalamic region is traversed by fibers related to the globus pallidus.
Discrete lesions in the subthalamic nucleus produce hemiballism, the most
violent form of
dyskinesia known.
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