In this article we will discuss about the subject-matter and functional aspects of thalamus.
Subject-Matter of Thalamus:
i. It forms major part of diencephalon.
ii. It is strategically placed between tracts and cerebral cortex. Thalamus forms medial boundary for internal capsule.
iii. In thalamus, there are many nuclei.
iv. Also included in thalamus are medial and lateral geniculate bodies.
v. It is either in anterior, medial and lateral group, there are some specific nuclei. In Y-shaped structure, the non-specific nuclei.
Nuclei in Thalamus:
a. Specific projection nuclei
b. Non-specific projection nuclei
From specific projection nuclei, impulses are sent to discrete areas of cerebral cortex and from nonspecific projection nuclei, impulses are sent to almost all areas of cerebral cortex.
Some of the important specific projection nuclei are:
a. Ventroposterolateral nuclei (VPL)
b. Ventroposteromedial nuclei VPM)
c. Ventrolateral nuclei (VL)
d. Ventroanterior nuclei (VA)
e. Centromedian nuclei (CM)
f. Medial geniculate body (MGB)
g. Lateral geniculate body (LGB), etc.
Intralaminar nucleus (ILN), pulvinar nucleus and midline nucleus (MLN) are grouped under nonspecific projection nuclei.
Some of the important afferent and efferent connections along with the nueleus involved have been shown in Table 9.4.
Functional Aspects of Thalamus:
1. It acts as a relay station for almost all sensory input except for olfactory impulses (pathway or tract does not relay here). But these days it is also mentioned that even olfactory sensations is relayed in thalamus.
2. It acts as a relay station between cerebral cortex and cerebellum and also for impulses from basal ganglia. It helps in normal co-ordination of movements and also helps to maintain normal muscle tone.
3. Ascending reticular activity system (ARAS) is important for emotional reactions. ARAS fibers during the course to reach cerebral cortex do get relayed inthalamic nuclei.
4. Anterior thalamic nuclei is responsible for recent memory.
5. Seat for subcortical pain sensations. Even if sensory cortex is removed we are able to feel pain sensations. Localization and differentiation of different modalities of pain and intensity discrimination is not possible, if impulses do not reach cerebral cortex area no. 3, 1, 2. So for better localization, intensity assessment, discriminate between different types of pain and also to bring about balanced emotional reactions during painful situation, impulses must reach cerebral cortical areas.
Thalamic Syndrome:
When thalamogeniculate branch of posterior cerebral artery is infarcted (blocked), there will be degeneration of posterior and ventral group of nuclei due to non- supply of oxygen and nutrition.
Characteristic features of this syndrome are:
1. Loss of all sensations mediated by tract of Goll and Burdach as these tracts synapses in VPL nucleus of thalamus.
2. Pain is still felt as pain impulse also reaches ILN and MLN through spinoreticular tract. This pain is poorly localized and threshold for pain is increased. However, when once threshold is crossed, there will be excruciating pain.
3. Light touch, exposure to cold may sometimes bring about perception of pain.
4. Over reaction for pain sensation:
In a normal person, extent of emotional reaction is always proportionate to intensity of pain. In these individuals, there will be exaggerated emotional reactions for pain.
5. Motor disturbances:
Impulses coming from cerebellum and basal ganglion get relayed in ventral group of nuclei. Since these nuclei are damaged, it leads to in-coordination of movements and results in ataxia.
6. Tone of muscle decreases and there can be muscular weakness.
7. Epicratic sensations:
Epicratic sensations are more refined or synthetic sensations, like tactile localization, tactile discrimination, stereognosis, fine touch, etc. All these sensations are lost in thalamic syndrome.
8. Protopathic sensations:
These are more primitive, not refined sensation, e.g. pain temperature, and crude touch. They will be intact in thalamic syndrome.