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The Thalamus

 

INTRODUCTION

 The thalamus is a large, ovoid mass of grey matter that forms the major part of the diencephalon. There are two thalami and one is situated on each side of the third ventricle. The anterior end of the thalamus is narrow and rounded and forms the posterior boundary of the interventricular foramen. The posterior end is expanded to form the pulvinar. The inferior surface 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 grey matter the interthalamic connection (interthalamic adhesion).

 

 

SUBDIVISIONS OF THE THALAMUS

 

The thalamus is covered on its superior surface by a thin layer of white matter, called the stratum zonale, and on its lateral surface by another layer, the external medullary lamina. The grey matter of the thalamus is divided by a vertical sheet of white matter, the internal medullary lamina, into medial and lateral halves. The internal medullary lamina consists of nerve fibres that pass from one thalamic nucleus to another. Anteriosuperiorly the internal medullary lamina splits so that it is Y-shaped. The thalamus thus 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. Smaller nuclear groups are situated within the internal medullary lamina, and some are located on the medial and lateral surfaces of the thalamus.

 

Anterior Part

 

This part of the thalamus contains the anterior thalamic nuclei. They receive the mammillothalamic tract from the mammillary nuclei. These anterior thalamic nuclei also receive reciprocal connections with the cingulate gyrus and hypothalamus. The function of the anterior thalamic nuclei is closely associated with that of the limbic system and is concerned with emotional tone and the mechanisms of recent memory.

 

Medial Part

 

 This part of the thalamus contains the large dorsomedial nucleus and several 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. It is inter connected with all other groups of thalamic nuclei. The medial part of the thalamus is responsible for the integration with all other groups of thalamic nuclei including somatic visceral and olfactory information, and the relation of this information to one’s emotional feelings and subjective states

 

Lateral Part

 

The nuclei are subdivided into a dorsal tier and a ventral tier.

 DORSAL TIER OF THE NUCLEI

 This tier includes the lateral dorsal nucleus, the lateral posterior nucleus, and the pulvinar. The details o connections of these nuclei are not clear. However, they are known to have interconnections with other thalamic nuclei and with the parietal lobe, cingulate gyrus, and occipital and temporal lobes.

 VENTRAL TIER OF THE NUCLEI

 This tier consists of the following in a craniocaudal sequence

 1. Ventral anterior nucleus. This nucleus connected to the reticular formation, the substia nigra, the corpus striatum, and the premotor cortex as well as to many of the other thalamic nuclei. Since the nucleus lies on the pathway between the corpus striatum and the motor areas of the frontal cortex, it probably influences the activities of the motor cortex.

 2. Ventral lateral nucleus. This nucleus has connections similar to those of the ventral anterior nucleus but, in addition, has a major input from the cerebellum and a minor input from the red nucleus. Its projections pass to the motor and premotor regions of the cerebral cortex. Here again this thalamic nucleus probably influences motor activity.

 3. Ventral posterior nucleus. This nucleus is subdivided into the ventral posteromedial nucleus and the ventral posterolateral nucleus. The ventral postereomedial (VPM) nucleus receives the ascending trigeminal and gustatory pathways, while the ventral postereolateral (VPL) nucleus receives the important ascending sensory tracts, the medial and spinal lemnisci.

 

The thalamocortical projections from these important nuclei pass through the posterior limb of the internal capsule and corona radiata to the primary somatic sensory areas of the cerebral cortex in the postcentral gyrus (areas 3,1 and 2)

   

Other Nuclei of the Thalamus

 

These nuclei include the intralaminar nuclei, the m nuclei, the reticular nucleus, and the medial and lateral geniculate bodies.

 The intralaminar nuclei are small collections of cells within the internal medullary lamina. They receive afferent fibres from the reticular formation and fibres from the spinothalamic and trigeminothalamic they send efferent fibres to other thalamic nuclei, which turn project to the cerebral cortex, and fibres to the corpus striatum. The nuclei are believed to influence the levels of consciousness and alertness in an individual.  

The midline nuclei consist of groups of nerve adjacent to the third ventricle and in the interthalamic connection. They receive afferent fibres from the reticular formation. Their precise functions are unknown.  

The reticular nucleus is a thin layer of nerve cells sandwiched between the external medullary lamina and the posterior limb of the internal capsule. Afferent fibres converge on this nucleus from the cerebral cortex and the reticular formation, and its output is mainly to other thalamic nuclei. The function of this nucleus is not fully understood, but it may be concerned with a mechanism by which the cerebral cortex regulates thalamic activity.

The medial geniculate body forms part of the auditory pathway and is a swelling on the posterior surface of the thalamus beneath the pulvinar. Afferent fibres to the medial geniculate body form the inferior brachium and come from the inferior colliculus. It will be remembered that the inferior colliculus receives the termination of the fibres of the lateral lemniscus. The medial geniculate body receives auditory information from both ears but predominantly from the opposite ear.  

The efferent fibres leave the medial geniculate body to form the auditory radiation, which passes to the auditory cortex of the superior temporal gyrus.

The lateral geniculate body forms part of the visual pathway and is a swelling on the undersurface of the pulvinar of the thalamus. The nucleus consists of six layers of nerve cells and is the terminus of all but a few fibres of the optic tract (except the fibres passing to the pretectal nucleus). The fibres are the axons of the ganglion cell layer of the retina and come from the temporal half of the ipsilateral eye and from the nasal half of the contralateral eye, the latter fibres crossing the midline in the optic chiasma. Each lateral geniculate body therefore, receives visual information from the opposite field of vision.

 The efferent fibres leave the lateral geniculate body to form the visual radiation, which passes to the visual cortex of the occipital lobe.

 

Connections of the Thalamic Nuclei

See notes given by Terry Parker

 

FUNCTIONS OF THE THALAMUS

1.The thalamus is made up of collections of nerve cells are centrally placed in the brain and are interconnected

 

2. A vast amount of sensory information (except smell converges on the thalamus and is integrated through interconnections between the nuclei. The resulting information pattern is distributed to other parts of the central nervous system. It is probable that olfactory information is first integrated at a lower level with taste and other sensations and is relayed to the thalamus from the amygadaloid complex and hippocampus through the mammiliothalamic tract.

 

3.The thalamus and the cerebral cortex are closely link The fibre connections have been established, and it is known that following removal of the cortex the thalamus can appreciate crude sensations. However, the cerebral cortex is required for the interpretation of sensations based on past experiences.

 

4. The thalamus possesses certain very important nuclei that include the ventral posteromedial nucleus, the ventral posterolateral nucleus, the medial geniculate body and the lateral geniculate body.

 

5. The ventroanterior and ventrolateral nuclei of the thalamus form part of the basal nuclei circuit and thus are involved in the performance of voluntary movements. These nuclei receive input from the globus pallidus and send fibres to the prefrontal, supplemental, and premotor areas of the cerebral cortex.

 

6. The large dorsomedial nucleus has extensive connections with the frontal lobe cortex and hypothalamus. There is considerable evidence that this nucleus lies on the pathway that is concerned with subjective feeling states and the personality of the individual.

 

7. The intralaminar nuclei are closely connected with the activities of the reticular formation and they receive much of their information from this source. Their strategic position enables them to control the level of overall activity of the cerebral cortex. They are thus able to influence the levels of consciousness and alertness in an individual.