Thalamus structure and function

Thalamus is a large ovoid mass of gray matter, situated bilaterally in diencephalon. Both thalami form 80% of diencephalon. Thalami on both sides are connected in their rostral portions by means of an intermediate mass. Caudal portions are more widely separated by corpora quadrigemina.

THALAMIC NUCLEI

Thalamic nuclei are classified by two methods:

A. Anatomical clasasification

B. Physiological classification.

ANATOMICAL CLASSIFICATION

Thalamus on each side is divided into five main nuclear groups by ‘Y’-shaped internal medullary lamina.

1. Midline nuclei

2. Intralaminar nuclei

3. Medial mass of nuclei

4. Lateral mass of nuclei

5. Posterior group of nuclei.

1. Midline Nuclei

Midline nuclei are a group of small nuclei, situated on the medial surface of thalamus near the midline.

2. Intralaminar Nuclei

Intralaminar nuclei are smaller nuclei present in the medullary septum of thalamus.

3. Medial Mass of Nuclei

Medial mass of nuclei are situated medial to septum and it comprises two nuclei:

i. Anterior nucleus

ii. Dorsomedial nucleus.

4. Lateral Mass of Nuclei

This group of nuclei are situated lateral to septum. Lateral mass of nuclei are again divided into two

subgroups:

i. Dorsal group of lateral mass with two nuclei:

a. Dorsolateral nucleus

b. Posterolateral nucleus

ii. Ventral group of lateral mass with three nuclei:

a. Ventral anterior nucleus

b. Ventral lateral nucleus

c. Ventral posterior nucleus. It consists of two parts:

Ventral posterolateral nucleus

Ventral posteromedial nucleus.

5. Posterior Group of Nuclei

Posterior group of nuclei are the continuation of lateral mass of nuclei. It has two subgroups:

i. Pulvinar

ii. Metathalamus which consists of two structures:

a. Medial geniculate body

b. Lateral geniculate body.

Thalamic reticular nucleus

Thalamus also includes thalamic reticular nucleus, which is a thin layer of neurons covering the lateral

aspect of thalamus. It is separated from thalamus by external medullary lamina. It receives information from reticular formation, cerebral cortex and other thalamic and sends inhibitory signals to other thalamic nuclei.

PHYSIOLOGICAL CLASSIFICATION

On the basis of functions and their projections, thalamic nuclei are classified into five groups. This type of

classification is also called Bondok classification. Five groups of thalamic nuclei are:

1. Specific sensory relay nuclei

2. Specific motor nuclei

3. Association or less specific nuclei

4. Non-specific nuclei

5. Limbic system nuclei.

CONNECTIONS OF THALAMIC NUCLEI

THALAMIC RADIATIONS

Thalamic radiation is the collection of nerve fibers connecting thalamus and cerebral cortex. It contains

both thalamocortical and corticothalamic fibers. All these fibers between thalamus and cerebral cortex pass through internal capsule. Fibers of thalamic radiation are divided into four groups, which are called thalamic peduncles or thalamic stalks. Thalamic peduncles are:

1. Anterior (frontal) thalamic peduncle or radiation

2. Superior (centroparietal) thalamic peduncle or radiation

3. Posterior (occipital) thalamic peduncle or radiation

4. Inferior (temporal) thalamic peduncle or radiation

ANTERIOR (FRONTAL) THALAMIC PEDUNCLE OR RADIATION

Anterior thalamic peduncle connects the frontal lobe of cerebral cortex with medial and lateral thalamic nuclei. It contains mostly motor nerve fibers.

SUPERIOR (CENTROPARIETAL) THALAMIC PEDUNCLE OR RADIATION

Fibers of this peduncle connect postcentral gyrus (somesthetic area) of parietal lobe and adjacent area

in frontal cortex with lateral mass of thalamic nuclei. It contains mainly the sensory fibers.

POSTERIOR (OCCIPITAL) THALAMIC PEDUNCLE OR RADIATION

Posterior thalamic peduncle connects occipital lobe of cerebral cortex with pulvinar and lateral geniculate body. It contains the nerve fibers concerned with vision.

INFERIOR (TEMPORAL) THALAMIC PEDUNCLE OR RADIATION

Fibers of this peduncle connect temporal lobe and insula with pulvinar and medial geniculate body. This peduncle contains the nerve fibers concerned with hearing

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