Thalamus
The thalamus is a part of the brain that is found in the third ventricle. It is egg-shaped and along with the hypothalamus comprises the area known as the diencephalon.
The thalamus acts as a central relay station for all sensory impulses travelling from other parts of the spinal cord and brain to the cerebral cortex (outer portion of the brain). It receives all sensory impulses (except those associated with smell) and directs them to the appropriate regions of the cerebrum for appropriate interpretation and integration. The thalamus acts as the last portion of the brain for sensory input before the cerebrum.
The thalamus connects to various parts of the brain via specialized nerve fibers that originate in the upper part of the reticular activating system (RAS). The RAS (which extends from the medulla oblongata to the thalamus) sorts out stimuli from the sense organs and passes on only those impulses that require immediate attention. We are not aware of most of the sensory impulses received by the central nervous system. Because the thalamus channels only certain sensory impulses to the cerebrum, our brains do not become overloaded with stimuli. For this reason, the thalamus is sometimes referred to as the "gatekeeper of the cerebrum." The thalamus also receives input from the cerebrum and from other parts of the brain that regulate emotion and arousal.
Encephalopathy or brain diseases of the thalamus have been associated with Parkinson's disease, epilepsy, bipolar disorder and chronic solvent abuse.
Removal of the ventral thalamus (thalamotomy) has been reported to significantly reduce or completely stop tremor in 80-90% of Parkinson's patients who undergo the procedure. Neurostimulation of the thalamus also reduces tremors without removal of thalamic tissue. In this procedure, a tiny electrical pulse generator is surgically implanted near the patient's collarbone. The generator is connected to four electrodes that have been implanted in the thalamus and can deliver programmed pulses which the patient can turn on or off using a magnet held over the skin. When the pulse is turned on, the tremor is suppressed. Neurostimulation of the thalamus is also being tested on epileptics to suppress seizures.
It has been found that patients suffering from bipolar disorder (maniac-depression) have elevated levels of VMAT2 (a protein that regulates the transport of important neurotransmitters) in the thalamus and brain stem. Since the thalamus plays a role in the anatomy of mood and emotion, this finding may be related to the extreme emotional swings evident in this disorder.
The thalamus of chronic solvent abusers who inhale volatile fumes in order to attain a high have been found by MRI (magnet resonance imaging) pictures to have a combination of diffuse white matter changes and low signal intensity. This may be related to the partitioning of solvents into the lipid membranes of the thalamus.
It is also estimated that 10-15% of strokes (blood clots in the brain) that involve loss of consciousness are thalamic in origin.
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