Thalamic nuclei are classified by two methods:

1. Anatomical classification.
2. Physiological classification.

Anatomical Classification

Thalamus on each side is divided into five main nuclear groups by means of a Y-shaped internal medullary lamina.

I. Midline Nuclei

It is a group of small nuclei, situated on the medial surface of thalamus near midline.

II. Intralaminar Nuclei

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

III. Medial Mass of Nuclei

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

1. Anterior nucleus.
2. Dorsomedial nucleus.

IV. Lateral Mass of Nuclei

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

V. Posterior Group of Nuclei

It is the continuation of lateral mass of nuclei.

Posterior group has two subgroups:

1. Pulvinar.
2. Metathalamus. It consists of two structures:
   1. Medial geniculate body.
   2. 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.

Based on functions and their projections, thalamic nuclei are classified into five groups. This type of classification is also called Bondok classification.

Connections of different groups of nuclei are given below.

Afferents from

1. Occipital lobe
2. Parietal lobe
3. Precuneus cortex
4. Hypothalamus
5. Globus pallidus
6. Midbrain nuclei
7. Red nucleus
8. Reticular formation
9. Dentate nucleus
10. Trigeminal lemniscus
11. Lateral lemniscus
12. Medial lemniscus
13. Spinal lemniscus
14. Auditory tract
15. Optic tract

Efferents to

1. Prefrontal cortex
2. Parietal lobe
3. Occipital lobe
4. Auditory cortex
5. Visual cortex
6. Limbic cortex
7. Hypothalamus
8. Putamen
9. Caudate nucleus

All the fibers between thalamus and cerebral cortex pass through internal capsule.

Fibers of thalamic radiation are divided into four groups.

Thalamus is primarily concerned with somatic functions.

1. Relay Center for Sensations

Thalamus forms a relay center for sensations.

Impulses of almost all the sensations reach thalamic nuclei, particularly in ventral posterolateral nucleus.

After being processed in thalamus, the impulses are carried to cerebral cortex.

2. Center for Processing of Sensory Information

Thalamus forms major center for processing 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.

3. Center for Determining Quality of Sensations

Thalamus is also a center for determining quality of sensations, that is, to determine the affective nature of sensations.

Sensations have two qualities:

1. Discriminative nature.
2. Affective nature.

i. Discriminative Nature of Sensation

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

It is the function of cerebral cortex.

ii. Affective Nature of Sensation

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.

4. Center for Sexual Sensations

Thalamus forms the center for perception of sexual sensations.

5. 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.

6. Center for Reflex Activity

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

7. 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 blood vessels supplying thalamus.

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 a neurological disease caused by infarction of posteroventral part of thalamus.

It is a rare disease and it has many names.

In thalamic syndrome, the whole body becomes hypersensitive to pain.

Effects of thalamic lesion occur in the contralateral side.

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 loss of tactile and kinesthetic sensations in thalamic syndrome.

3. Ataxia

Ataxia refers to incoordination of voluntary movements which 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

Persons with amputated limb sometimes feel sensations in the missing limb.

This is called phantom limb.

It is because of response by thalamus to inputs from cut ends of sensory nerves.

5. Amelognosia

It is the illusion felt by the patient that his limb is absent.

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 light touch may be unpleasant.

Sometimes, patient feels pain even in the absence of pain stimulus.

It becomes worse in conditions such as emotional disturbance and exposure to cold or heat.

Pain is due to overactivity 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.

8. Thalamic Hand or Athetoid Hand

Thalamic hand is an abnormal attitude of the hand in thalamic lesion.

It is characterized by moderate flexion at wrist and hyperextension of all fingers.