Part	Nuclei
Anterior part	Anterior nucleus
Medial part	Medial dorsal nucleus, medial ventral nucleus
Lateral part
• Dorsal tier nuclei	Lateral dorsal, lateral posterior, pulvinar
• Ventral tier nuclei	Ventral anterior (VA), ventral lateral (VL), ventral posterior—(a) ventral posterolateral (VPL), (b) ventral posteromedial (VPM)
Other parts	Intralaminar nuclei, reticular nucleus, medial and lateral geniculate bodies

The thalamic nuclei are classified into three main functional groups: specific, nonspecific, and reticular.

Connections of thalamic nuclei (Figs 11.4, 11.5)

Connections of the specific nuclei

These nuclei receive input from certain ascending tracts and project it to the specific (primary) cortical areas. Nuclei of this group comprise ventral tier nuclei and medial and lateral geniculate bodies. Their connections are enumerated in Table 11.3.

Table 11.3

Connections of the specific thalamic nuclei

Fig. 11.4 Main connections of the thalamus. The afferent fibres are shown on the left side and the efferent, fibres on the right side. (NG = nucleus gracilis, NC = nucleus cuneatus.)

Fig. 11.5 Scheme to show connections of ventral and lateral groups of thalamic nuclei. The figure inset on left upper corner shows the dorsolateral view of thalamus and its major subdivisions. (LD = lateral dorsal nucleus, LP = lateral posterior nucleus, VA = ventral anterior nucleus, VI = ventral intermediate N, VPL = ventral posterolateral n. VPM = ventral posteromedial n.)

Clinical Correlation

• From a clinical point of view the connections of ventral posterior nucleus are most important because its smaller medial portion, the ventral posteromedial nucleus (VPM) receives general sensory modalities from the head and face through trigeminal lemniscus and taste sensations from taste buds through solitariothalamic tract; and its larger lateral portion, the ventral posterolateral nucleus (VPL) receives exteroceptive sensations (pain, touch and temperature) through spinal lemniscus and proprioceptive sensations (muscle and joint sense, vibration, two point discrimination) through medial lemniscus, from rest of the body except face and head.

All the sensations reaching the ventral posterior nucleus are carried to the primary sensory area of the cerebral cortex by fibres passing through the posterior limb of the internal capsule (superior thalamic radiation). The vascular lesions involving posterior limb of internal capsule, which are not uncommon cause impairment of all forms of sensibility on the opposite side of the body.

• The integrity of anterior nucleus and its connections is necessary for attention and recent memory, therefore a lesion involving them can lead to loss of recent memory.

• Since the medial dorsal nucleus is associated with “moods” (“feeling tone”) and emotional balance, depending on the nature of the present sensory input and past experience, the mood may be that of well-being or malaise, or of euphoria or mild depression.

Connections of the nonspecific nuclei

Nonspecific nuclei do not receive afferents from ascending tracts, but have abundant connections with other diencephalic nuclei. They mostly project to the cortical ‘association areas’ in the frontal and parietal lobes. Nuclei of this group comprise anterior nucleus, dorsal medial nucleus and dorsal tier nuclei of thalamus. Their connections are enumerated in Table 11.4.

Table 11.4

Connections of the nonspecific thalamic nuclei

Connections of the reticular nuclei

The reticular nuclei of thalamus include reticular nucleus, intralaminar nuclei and median nuclei. These nuclei are connected with the reticular formation. There connections are given in Table 11.5.

Table 11.5

Connections of the reticular nuclei of thalamus

Functions of thalamus

• It is a sensory integration and relay station of all the sensory pathways except for the olfactory pathway, which is projected directly to the cerebral cortex without being relayed in the thalamus.

• It is capable of recognition, though poorly of the pain, thermal and some tactile sensations at its own level.

• It influences voluntary movements by receiving impulses from basal ganglia and cerebellum and relaying them to the motor cortex, which in turn influences lower motor neurons through corticonuclear and corticospi-nal pathways.

• Through ascending reticular activating system, the thal-amic reticular component participates in the maintenance of the state of wakefulness and alertness.

• By receiving impulses from hypothalamus and projecting them to the prefrontal and cingulate gyri, it participates in affective reactions, viz. determination of mood.

• It is thought to have role in recent memory and emotions.

• It influences the electrical activity of the cerebral cortex, i.e. it plays a role in synchronization or desynchroniza-tion of EEG waves.

Clinical Correlation

• Thalamic syndrome

It usually occurs subsequent to a vascular lesion of the thalamus (viz. thrombosis of thalamogenicu-late artery). when the patient is recovering from a thalamic infarct.

Characteristic features

The threshold for pain, touch and temperature is decreased on the opposite side of the body (thalamic overreaction) but, when the threshold is reached, the sensations are exaggerated, perverted and disagreeable. For example, the prick of a pin may be felt as a severe burning sensation, and even music that is ordinarily pleasing may be disagreeable. Sometimes even light touch may produce excruciating pain. The spontaneous pain may occur in some instances that may become intractable and fail to respond to powerful analgesics (pain relieving) drugs.

There may be emotional instability, with spontaneous (or forced) laughing and crying.

• Thalamic hand

It is sometimes seen in thalamic lesions. The opposite hand is held in an abnormal posture. The forearm is pronated, wrist flexed, metacarpophalangeal joints flexed and interphalangeal joints extended. The fingers can be moved but slowly, due to altered muscle tone in the different muscle groups.

The metathalamus consists of the medial and lateral geniculate bodies (Fig. 11.6). These are small rounded elevations on the inferior aspect of the posterior part of thalamus, lateral to each side of the midbrain. The medial and lateral geniculate bodies are relay stations for the auditory and visual pathways respectively.