Thalamus

Thalamus is a large egg-shaped mass of grey matter present in diencephalon, a part of the forebrain. Thalamus is involved in sensory as well as motor functions of the brain.

It is the part of the brain where the sensory information from all over the body converge and are then sent to various areas of the cortex. It also helps the motor cortex for coordinated voluntary movements of the part. Thus, it has an important role in motor cognition.

In this article, we will discuss the anatomical features such as location, structure, and nuclei as well as input and output fibers of the thalamus. We will also discuss physiological as well as the clinical significance of thalamus.

Anatomy

The anatomical details such as topography or location,
structure and nuclei, input and output fibers as well as blood supply of
thalamus.

Location

Thalamus is a part of the diencephalon. It is located deep in the forebrain, present just above the midbrain. One thalamus is present on each side of the third ventricle. Its anterior part forms the posterior boundary of interventricular foramen.

The posterior end is expanded and forms a structure called pulvinar. The pulvinar of thalamus overhangs the superior colliculus. The inferior surface of the thalamus is continuous with the tegmentum of midbrain.

Structure

Thalamus is an egg-shaped mass of grey matter. It also contains white matter to some extent.

White Matter

The white matter of thalamus consists of three
laminae. These are mentioned below.

On the superior surface, thalamus is covered by a thin layer of white matter. This layer of white matter is called stratum zonale.

The lateral surface of the thalamus is also covered by
a layer of white matter called external medullary lamina.

Another sheet of white matter divides the grey matter
of thalamus into three regions. It is a Y-shaped sheet of white matter called
internal medullary lamina.

Grey Matter

The grey matter is divided by the internal medullary lamina into anterior, medial and lateral parts. Each of these parts along with other associated nuclei are discussed in the subsequent headings.

Anterior part

The anterior part contains anterior thalamic nuclei. These nuclei are a part of the mammillothalamic tract. They have the following connections:

Input fibers are received from the mamillary nuclei,
cingulate gyrus, and hypothalamus.

Output fibers are sent to the cingulate gyrus and
hypothalamus

Medial part

The important nucleus in this part is the dorsomedial
nucleus of the thalamus.

It receives and sends fibers to the prefrontal cortex,
hypothalamus and thalamic. It also sends and receives fibers from all other
nuclei of thalamus.

Lateral part

The nuclei of the lateral part are divided into dorsal tier and ventral tier.

Dorsal tier nuclei

These include lateral dorsal nucleus, lateral posterior nucleus and the pulvinar.

All these nuclei have connections with other nuclei of
thalamus, parietal lobe, cingulate gyrus, occipital, and temporal lobes.

Ventral tier nuclei

These include the following three nuclei:

Ventral anterior nucleus: This nucleus has connections with the reticular formation, substantia nigra, corpus striatum, and premotor cortex. It also receives and sends fibers to other thalamic nuclei.

Ventral lateral nucleus: It has the same connections as the ventral anterior nucleus. As well as, it also has connections with the cerebellum and the red nucleus.

Ventral posterior nucleus: It is further divided into ventral posteromedial and ventral posterolateral nuclei.

The ventral posteromedial nucleus receives tegmental
and gustatory fibers, while the posterolateral nucleus receives the medial and
spinal lemnisci.

Intralaminar Nuclei

These nuclei are present within the internal medullary lamina. They receive fibers from spinothalamic and trigeminal-thalamic tracts. They send efferent fibers to other nuclei of the thalamus.

Reticular Nuclei

These nuclei are present in a thin layer between the
external medullary lamina and the posterior limb of the capsule. They receive
fibers from reticular formation and cerebral cortex and send efferent fibers to
the other nuclei present in thalamus.

Medial Geniculate Body

Medial geniculate body is a swelling on the posterior surface of thalamus just below the pulvinar. It is a part of the auditory pathway. It receives fibers via inferior brachium from the inferior colliculus. The efferent fibers are sent to the auditory cortex.

Lateral Geniculate Body

Lateral geniculate body is another swelling present
just beneath the pulvinar. It is a part of the visual pathway and receives
fibers from optic chiasma. The lateral body sends efferent fibers to the
primary visual cortex.

Blood Supply

The blood supply to the thalamus is derived from the branches of the posterior cerebral artery. These branches include polar artery, paramedian thalamic-subthalamic arteries, inferolateral arteries, and posterior choroidal arteries.

Physiology

In this section, we will discuss the several functions
performed by thalamus.

Relay of Sensory Information

Thalamus occupies the central position between the cortex and spinal cord as well as other areas of the lower brain. Thus, it acts as an important relay center for the signals passing from the lower centers to the higher centers of the brain. Almost all sensory information pass through the thalamus before going to the higher centers of the brain.

Integration of Sensory Information

Thalamus not only acts as a relay or passage to the
sensory information, but it also integrates the sensory information. This
integrated sensory information is then sent to other areas of cortex. It in
this way the sensations of smell and taste are integrated resulting in the
salivatory response.

Emotional Control

Being a part of the mammillothalamic tract, thalamus plays an important role in controlling the emotional tone of a person.

Integration of Sensations with Emotions

The dorsomedial nucleus of thalamus integrates the somatic,
visceral and olfactory sensations of a person. This integrated information is
fed to the mammillothalamic tract resulting in the emotional response to the
sensation.

Hearing and Visual Pathway

The medial and lateral geniculate bodies are part of
auditory and visual pathways, respectively. Thus, it is essential for normal
hearing and visual process of a person. Its disease will affect hearing as well
as vision.

Consciousness

The level of consciousness is controlled by the interlaminar nuclei of thalamus. These nuclei receive information from the reticular formation and control the activity of other thalamic nuclei. Thus, these nuclei control the overall activity of cortex, influencing the level of consciousness and alertness in a person.

Motor control

The ventral anterior and the ventral lateral nuclei of the thalamus are a part of the basal nuclei circuit of voluntary movement control. They receive information from globus pallidus and send information to the motor cortex. They are essential for the cognitive control of motor functions.

Clinical Significance

As thalamus is an important relay and integrative area, the disease of this region of CNS will have profound effects on the body. The thalamus may be damaged by neoplasia, disease in arterial supply or due to hemorrhage.

Following are some important clinical significances of
thalamus.

Sensory Loss

Lesions of thalamus resulting from hemorrhage or thrombosis of arteries can damage ventral posteromedial and ventral posterolateral nuclei of thalamus. This can, in turn, lead to the complete sensory loss.

The sensory loss is complete including light touch, tactile, pain, discrimination, and joint and muscle sensations from the opposite side of the body.

Abnormal Involuntary Movements

The vascular lesions of the thalamus may also lead to choreoathetosis
and ataxia. The ataxia may be a result of loss of appreciation of muscle and
joint movements.

Thalamic Hand

The patients with thalamic lesions have a particular abnormal posture of the contralateral hand. In thalamic hand, the wrist of the person is pronated and flexed, the metacarpophalangeal joints are flexed, and there is an extension at the interphalangeal joints. The movements of the fingers are also slow.

Thalamic Pain

When a patient is recovering from thalamic infarct, he
may experience spontaneous pain. The pain is often excessive and occurs on the
contralateral side of the body.

Conclusion/Summary

Thalamus is a large egg-shaped mass of grey matter, having a small amount of white matter located at the base of the forebrain, just above the midbrain. It is a part of the diencephalon and is located lateral to the third ventricle.

It consists of three lamina of white matter:

• Stratum Zonale, covering the
  superior surface of thalamus
• External Medullary Lamina, present
  on the lateral surface of the thalamus
• Internal Medullary Lamina, divides
  the grey matter into three halves

The grey matter is divided into anterior part, medial
and dorsal part.

The anterior part contains anterior thalamic nuclei, a
part of the mammillothalamic tract.

The medial part contains dorsomedial nuclei having
important connections with the hypothalamus and cingulate gyrus.

The lateral part is further divided into dorsal tier
and ventral tier nuclei.

The other nuclei present are:

• Interlaminar nuclei
• Reticular nuclei
• Medial geniculate body
• Lateral geniculate body

Due to its central location in the CNS, the thalamus acts as an important center for integration and relay center of all types of sensory information. This integrated information is then sent to higher areas of the brain.

It also has a role in integrating emotions with the
sensations of vision, olfaction, and taste.

The interlaminar nuclei control the level of
consciousness and alertness.

Thalamus is also essential for cognitive control of
motor functions.

Thalamic lesions, which may be due to neoplasia, hemorrhage or thrombosis, can result in one of the following conditions:

• Sensory loss
• Thalamic hand
• Movement abnormalities
• Thalamic pain

References

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