Diencephalon and ventricles

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1. Thalamus (plural thalami)

The thalamus is an egg-shaped relay station in the diencephalon consisting of a number of nuclei. The thalamus is divided into regions or divisions by the internal medullary lamina which separates a medial region, containing only the dorsomedial nucleus, from a lateral region containing many nuclei. Based on function, the lateral region can be further divided into two motor nuclei (ventral lateral nucleus and ventral anterior nucleus) and a far larger number of sensory nuclei. The internal medullary lamina also divides in several places, anteriorly where it encloses the anterior nucleus, and further posteriorly where it encloses intralaminar nuclei consisting of the centromedian nucleus and the parafascicular nucleus. The nuclei within the thalamus are: anterior nucleus, centromedian nucleus, dorsomedial nucleus (medial dorsal nucleus), lateral dorsal nucleus, lateral geniculate nucleus/body (LGN), lateral posterior nucleus, medial geniculate nucleus/body (MGN), midline nuclei, parafascicular nucleus, pulvinar (pulvinar lateral posterior complex), reticular nucleus, ventral anterior nucleus, ventral lateral nucleus, ventral posterolateral nucleus, ventral posteromedial nucleus.

Show list

1.
Thalamus (plural thalami)
2.
Hypothalamus
3.
Lateral ventricle
4.
Fourth ventricle
5.
Midbrain (mesencephalon)
6.
Pons
7.
Medulla (medulla oblongata)
8.
Fornix (plural fornices)
9a.
Rostrum of corpus callosum
9b.
Genu of corpus callosum
9c.
Body of corpus callosum
9d.
Splenium of corpus callosum
10a.
Vermis of cerebellum
10b.
Hemisphere of cerebellum
10c.
Tonsil of cerebellum
11.
Mammillary body
12.
Optic chiasm
13.
Superior colliculi
14.
Medial occipital cortex
15.
Cingulate gyrus
16.
Parietooccipital sulcus
17.
Marginal sulcus (marginal ramus of the cingulate sulcus)
18.
Cingulate sulcus
19.
Paracentral sulcus
20.
Central sulcus (of Rolando)
21.
Paracentral lobule
22.
Calcarine sulcus (calcarine fissure)
23.
Cuneus
24.
Anterior commissure
25.
Lingual gyrus
26.
Precuneus
27.
Subparietal sulcus (splenial sulcus)

In this view we see:

The medial surface of the brain, which has been cut in half lengthwise (a midline or mid-sagittal dissection).
The gyri and sulci are the medial continuation of gyri and sulci covering the lateral surface of the brain (as seen in Image 1).
Three components of the diencephalon (all described below): the thalamus, hypothalamus and part of the epithalamus: the habenula. The other part of the epithalamus – the pineal gland, is absent due to the midline dissection.
A fourth component of the diencephalon, the subthalamus, is not located in the midline and therefore is not visible in this Image (for more on the subthalamus see the subthalamic nucleus entry under Image 6).
Three of the four ventricles (all described below):

the large cavity above the thalamus is the right lateral ventricle.
the third ventricle occupies a slit in the midline surrounding and medial to the thalamus which is difficult to see in this view (see Images 6 and 7 instead).
the fourth ventricle forms a triangle between the cerebellum and the brainstem.

Note that the components of the diencephalon all have thalamus in their name: thalamus, hypothalamus, epithalamus and subthalamus. The thalamus is the largest component of the diencephalon.

Location

As shown in Image 2 (and also Images 5, 6 and 7), the thalamus is located above the brainstem deep within the cerebrum. Here, the thalamus is well positioned to receive sensory information from the brainstem and spinal cord on its way to the cerebral cortex.

Structure

The thalamus contains a number of divisions, all of which contain nuclei. Understanding the structure of the thalamus can be daunting, however the following nuclei in the thalamus are more likely to be encountered than others: medial geniculate nucleus/body (MGN), lateral geniculate nucleus/body (LGN), ventral posterolateral nucleus (VPL), ventral posteromedial nucleus (VPM), ventral lateral nucleus (VL) and the anterior nuclei.

More generally, the thalamus is divided into anterior, medial and lateral divisions by the internal medullary lamina, which consists of myelinated fibres performing a similar function to the fascia separating muscles. For further information on its structure see thalamus in the Glossary.

Function

All incoming sensory information with the exception of smell (olfactory) sensation passes through the thalamus to the cerebral cortex. As a relay station and gatekeeper, the thalamus selectively withholds or transmits sensory information according to our needs. The thalamus also has motor, limbic, multimodal and intralaminar functional divisions.

The motor nuclei convey information from the cerebellum and basal ganglia to the premotor cortex. The majority of thalamic nuclei are specific, such as the sensory and motor nuclei. There are also non-specific nuclei including the intralaminar and reticular nuclei. The intralaminar nuclei are an extension of the reticular formation (an important integrating centre with roles in multiple activities) in the brainstem, and the reticular nuclei deal with saliency – helping isolate novel auditory, visual or tactile information from the normal background noise of cortical activity.

Clinically, the thalamus has a role in consciousness and conveying a specific kind of pain. Damage to the centromedian nucleus can result in loss of consciousness; and a thalamic tumour causes a vague pain which the patient is unable to specifically localise to a particular part of the body.

Notable nuclei with specific functions are:

the lateral (LGN) and medial (MGN) geniculate nuclei: the LGN has a role in vision and the MGN a role in hearing;
the ventral posterolateral (VPL) nucleus is a synaptic region for ascending spinal sensory pathways;
the ventral posteromedial (VPM) nucleus is a synaptic area for the trigeminal lemniscus;
the ventral lateral nucleus (VL), which receives input from the cerebellum;
the anterior nucleus, which together with the hypothalamic mammillary bodies are part of the Papez circuit.