Thalamus is a mass of gray matter situated at the rostral end of the brainstem. Thalamus functions as an important relay and integrative station for sensory signals and motor information passing to all areas of the cerebral cortex, the basal ganglia, the hypothalamus, and the brainstem. It also regulates consciousness, sleep and alertness.
Anatomy of the Thalamus:
The general appearance of the thalamus is a large, egg-shaped mass of gray matter that forms the major part of the diencephalon. Thalamus has nerve fibers which are projecting out to cerebral cortex in all directions. There are two thalamai which are situated on each side of the third ventricle.
- The anterior end of the thalamus is rounded and narrow which forms the posterior boundary of the interventricular foramen.
- The posterior end of the thalamus is expanded to form the pulvinar, which overhangs the superior colliculus.
- The inferior surface of the thalamus is continuous with the tegmentum of the midbrain.
- The medial surface of the thalamus forms part of the lateral wall of the third ventricle and is usually connected to the opposite thalamus by a band of gray matter called interthalamic connection.
The thalamus derives blood supply from various arteries, including the polar artery (posterior communicating artery) thalamogeniculate arteries, thalamic-sub thalamic arteries and choroidal arteries.
Function of thalamus:
The thalamus is made up of complicated collections of nerve cells that are centrally placed in the brain and are interconnected. Thalamus act as rely station and integrate information between different subcortical areas of brain and the cerebral cortex.
- There are thalamic nuclei for every sensory signal with exception of olfactory system. It receives sensory signals and sends them to associated cortical area.
- Lateral geniculate nucleus of thalamus is associated with visual system. It receive sensory inputs from retina and act rely station and process sensory information to visual cortex.
- Medical geniculate nucleus acts as relay station for auditory system.
- Thalamus play role in regulating sleep, wakefulness and consciousness.
- The ventroanterior and the ventrolateral nuclei of the thalamus form part of the basal nuclei circuit and thus are involved in the performance of voluntary movements.
- The intralaminar nuclei are closely connected with the activities of the reticular formation. Their strategic position enables them to control the level of overall activity of the cerebral cortex. The intralaminar nuclei are thus able to influence the levels of consciousness and alertness in an individual.
Clinical significance of thalamus:
Thalamus is an important relay and integrative center. Disease of this area of the central nervous system will have profound effects. The thalamus may be invaded by neoplasm, undergo degeneration following disease of its arterial supply, or be damaged by hemorrhage.
1. Sensory Loss
Sensory loss occurs when lesions result from thrombosis or hemorrhage of one of the arteries supplying the thalamus. Damage to the ventral posteromedial nucleus and the ventral posterolateral nucleus will result in the loss of all forms of sensation, including light touch, tactile sensation and discrimination, and muscle joint sense from the opposite side of the body.
2. Thalamic Cauterization
The intralaminar nuclei of the thalamus act as relay for pain to the cerebral cortex. Cauterization of these nuclei relieve severe and intractable pain associated with terminal cancer
3. Thalamic Hand
The contralateral hand is held in an abnormal posture in some patients with thalamic lesions. This occurs due to abnormal tone in muscles of hand.
4. Hereditary degeneration of thalamus results in prion disease, cerebrovascular accident lead to thalamic syndrome and damage to mammillary body stems to korsakoff’s syndrome
Subdivision of the thalamus:
The thalamus has two layers.
Stratum zonale: The thalamus is covered on its superior surface by a thin layer of white matter, called the stratum zonale.
External Medullary Lamina: The thalamus is covered on its lateral surface by another layer, the external medullary lamina.
The gray matter of the thalamus is further divided by a vertical sheet of white matter, the internal medullary lamina, into medial and lateral halves.
The internal medullary lamina: It consists of nerve fibers that pass from one thalamic nucleus to another. The internal medullary lamina splits and resembling a Y shape. So the thalamus is subdivided into three main parts. The anterior part lies between the limbs of the Y, and the medial and lateral parts lie on the sides of the stem of the Y. Each of the three parts of the thalamus contains a group of thalamic nuclei. Each part and their nuclei are discussed below.
1. Anterior Part of the Thalamus
The anterior part of the thalamus contains the anterior thalamic nuclei. Anterior thalamic nuclei receive the mammillothalamic tract from the mammillary nuclei. These anterior thalamic nuclei have reciprocal connections with the cingulate gyrus and hypothalamus.
Function of anterior thalamic nuclei: The function of the anterior thalamic nuclei is closely associated with that of the limbic system. The limbic system is concerned with emotional tone and the mechanisms of recent memory.
2. Medial Part of the Thalamus
The medial part of the thalamus contains the large dorsomedial nucleus and several other smaller nuclei. The dorsomedial nucleus has two-way connections with the whole prefrontal cortex of the frontal lobe of the cerebral hemisphere. It also has similar connections with the hypothalamic nuclei. The dorsomedial nucleus is interconnected with all other groups of thalamic nuclei. Function of dorsomeial nucleus: The medial part of the thalamus is responsible for the integration of a large variety of sensory information including somatic, visceral, and olfactory information. Dorsomedial nucleus correlates that information to person emotional feelings and subjective states.
3. Lateral Part of the thalamus
The nuclei are subdivided into a dorsal tier and a ventral tier.
A. Dorsal Tier of the Nuclei: Lateral dorsal nucleus The lateral posterior nucleus The pulvinar
The details of the connections of these nuclei are not clear. They have interconnections with other thalamic nuclei and with the parietal lobe, cingulate gyrus, and occipital and temporal lobes.
Ventral Tier of the Nuclei
Ventral anterior nucleus: This nucleus is connected to the reticular formation, the substantia nigra, the corpus striatum, and the premotor cortex. It influences the activities of the motor cortex.
Ventral lateral nucleus: This nucleus has a major input from the cerebellum. It has similar connections to those of the ventral anterior nucleus. There is minor input from the red nucleus. This thalamic nucleus probably influences motor activity.
Ventral posterior nucleus: This nucleus receives the ascending trigeminal and gustatory pathways. It also receives the important ascending sensory tracts, the medial and spinal lemnisci.
Other Nuclei of the Thalamus
These are small collections of nerve cells within the internal medullary lamina. The nuclei are believed to influence the levels of consciousness and alertness in an individual.
these consist of groups of nerve cells adjacent to the third ventricle. These midline nuclei receive afferent fibers from the reticular formation. Their precise functions are unknown.
Red nucleus fibbers converge on cerebral cortex and the reticular formation, and its output is mainly to other thalamic nuclei. This nucleus may be concerned with a mechanism by which the cerebral cortex regulates thalamic activity.
Medial geniculate body:
It forms part of the auditory pathway. Afferent fibers to the medial geniculate body come from the inferior colliculus. The medial geniculate body receives auditory information from both ears but predominantly from the opposite ear. The efferent fibers leave the medial geniculate body to the auditory cortex.
Lateral geniculate body:
It forms part of the visual pathway. Each lateral geniculate body, therefore, receives visual information from the opposite field of vision through optic chiasm. The efferent fibers leave the lateral geniculate body to form the visual radiation, which passes to the visual cortex of the occipital lobe.
Development of thalamus
Formation of progenitor domains for thalamus occurs then formation of mid-diencephalic organizer. After that maturation and parcellation of thalamus occurs.