FUNCTIONS OF THALAMUS

FUNCTIONS OF THALAMUS

Thalamus is primarily concerned with somatic functions and it plays little role in the visceral functions. Following are the various functions of thalamus:

RELAY CENTER

Thalamus forms the relay center for the sensations. Impulses of almost all the sensations reach the thalamic nuclei, particularly in the ventral posterolateral nucleus. After being processed in the thalamus, the impulses are carried to cerebral cortex through thalamocortical fibers.

CENTER FOR PROCESSING OF SENSORY INFORMATION

Thalamus forms the major center for processing the sensory information. All the peripheral sensory impulses reaching thalamus are integrated and modified before being sent to specific areas of cerebral cortex. This function of thalamus is usually called the processing of sensory information.

Functional Gateway for Cerebral Cortex

Almost all the sensations are proaessed in thalamus before reaching cerebral cortex. Very little information of somatosensory function is sent directly to cerebral cortex without being processed by the thalamic nuclei. Because of this function, thalamus is usually called ‘functional gateway’ for cerebral cortex.

CENTER FOR DETERMINING QUALITY OF SENSATIONS

Thalamus is also the center for determining the quality of sensations, i.e. to determine the affective nature of sensations. Usually the sensations have two qualities:

i. Discriminative nature

ii. Affective nature.

i. Discriminative Nature

Discriminative nature is the ability to recognize the type, location and other details of the sensations and it is the function of cerebral cortex.

ii. Affective Nature

Affective nature is the capacity to determine whether a sensation is pleasant or unpleasant and agreeable or disagreeable. Determining the affective nature of sensations is the function of thalamus.

CENTER FOR SEXUAL SENSATIONS

Thalamus forms the center for perception of sexual sensations.

ROLE IN AROUSAL AND ALERTNESS REACTIONS

Because of its connections with nuclei of reticular formation, thalamus plays an important role in arousal and alertness reactions.

CENTER FOR REFLEX ACTIVITY

Since the sensory fibers relay here, thalamus forms the center for many reflex activities.

CENTER FOR INTEGRATION OF MOTOR ACTIVITY

Through the connections with cerebellum and basal ganglia, thalamus serves as a center for integration of motor functions.

THALAMIC LESION

Thalamic lesion occurs mainly because of blockage (due to thrombosis) in thalamogeniculate branch of

posterior cerebral artery. Mostly, posteroventral nuclei of thalamus are affected because the thalamogeniculate branch of posterior cerebral artery supplies this part of thalamus. Lesion of thalamus leads to a condition called thalamic syndrome.

THALAMIC SYNDROME

Thalamic syndrome is the neurological disease caused by infarction of posteroventral part of thalamus. It is a rare disease and it has many names.

Following are the symptoms of thalamic syndrome:

1. Loss of Sensations

Loss of all sensations (anesthesia) occurs as the sensory relay system in thalamus is affected.

2. Astereognosis

Astereognosis is the loss of ability to recognize a known object by touch with closed eyes. It is due to the

loss of tactile and kinesthetic sensations in thalamic syndrome.

3. Ataxia

Ataxia refers to incoordination of voluntary movements. It occurs due to loss of kinesthetic sensation. This

type of ataxia due to loss of sensation is called sensory ataxia. It is very common in thalamic syndrome.

4. Thalamic Phantom Limb

The patient is unable to locate the position of a limb with closed eyes. The patient may search for the limb

in air or may have the illusion that the limb is lost. This is called thalamic phantom limb.

5. Anosognosia

Anosognosia is the lack of awareness or denial of existance of a neurological defect or general illness or

any disability.

6. Spontaneous Pain and Thalamic Over-reaction

Spontaneous pain occurs often. Pain stimulus is felt more acutely than in normal conditions (hyperalgesia). Pain may be so intense, that it even resists the action of powerful sedatives like morphine. Threshold for pain is very much reduced. Even the light touch may be unpleasant. Sometimes, the patient feels pain even in the absence of pain stimulus. It becomes worst in conditions such as emotional disturbance and exposure to cold or heat. Pain is due to over activity of medial mass of nuclei of thalamus, which escape the lesion. Abnormal reaction to various stimuli is called

thalamic over-reaction.

7. Involuntary Movements

Thalamic syndrome is always associated with some involuntary motor movements.

Athetosis

Athetosis means slow writhing and twisting movements.

Chorea

Chorea means quick, jerky, involuntary movements.

Intention tremor

Tremor is defined as rapid alternate rhythmic and involuntary movement of flexion and extension in the

joints of fingers and wrist or elbow. Intention tremor is the tremor that develops while attempting to do any

voluntary act. Intention tremor is the common feature of thalamic syndrome.

8. Thalamic Hand or Athetoid Hand

Athetoid hand is the abnormal attitude of hand in thalamic lesion. It is characterized by moderate flexion

at wrist and hyperextension of all fingers.

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