Diencephalon and third ventricle


Diencephalon and Third Ventricle


The diencephalon is the part of brain between the cerebrum and the brainstem. The cavity within it is termed third ventricle.

The diencephalon comprises two major subdivisions: pars dorsalis and pars ventralis. These subdivisions are seen on midsagittal view of the brain, and are separated from each other by a shallow groove, the hypothalamic sulcus which extends from interventricular foramen to the rostral end of the cerebral aqueduct of midbrain (Fig. 11.10).

The main divisions and subdivisions of diencephalon are listed in Table 11.1.


Anatomically the thalamus is a large ovoid mass of grey matter laying above the midbrain, from which it is separated by a small amount of neural tissue, the subthalamus. There are two thalami situated one on each side of a slitlike cavity, the third ventricle (Fig. 11.1).

Each thalamus is 3.5  cm in length and 1.5  cm in breadth.

The long axes of the thalami are set obliquely running backwards and laterally. The pointed anterior ends are nearer to the median plane whereas the wider posterior ends are separated from each other by pineal body, superior colliculi and habenular triangles. The thalami are usually attached across the median plane by a narrow interthalamic connexus of grey matter (also called interthalamic adhesion). Each thalamus forms most of the lateral wall of the third ventricle and floor of the central part of the lateral ventricle.

Functionally, the thalamus is generally considered as the greatsensory gateway to the cerebral cortex. It receives impulses from the opposite half of the body and transmits most of them to the sensory area of the cerebral cortex (Brodmann areas 3, 2, and 1).

External features

Each thalamus has two ends and four surfaces.

Surfaces (Fig. 11.2)

A line of reflection of ependyma of third ventricle from its medial wall to its roof is termed taenia thalami.

Internal structure (Fig. 11.3)

The thalamus consists mainly of grey matter and only a small amount of white matter.

Thalamic nuclei (Fig. 11.3)

Other thalamic nuclei

In addition to the above mentioned nuclei, the thalamus consists of following other nuclei:

The thalamic nuclei are summarized in Table 11.2.

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.

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.

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.


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.

Jan 2, 2017 | Posted by in NEUROLOGY | Comments Off on Diencephalon and third ventricle
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