Did You Know?

The word thalamus is derived from the Greek language, and means a room or a chamber.

The upper portion of the brain is split into two hemispheres, and its lower potion connects to the spinal cord via the brain stem. If the brain is split through the center, the thalamus would be seen held atop the brain stem. The thalamus is thus located close to the central region, buried underneath the cerebral cortex between it and the mid-brain below with nerve fibers projecting out to the cerebral cortex in all directions. It lies above the hypothalamus. In humans, the thalamus can be divided into two halves, which are prominent bulb-shaped masses located obliquely and symmetrically on either sides of the third ventricle.

The thalamus can be called the relay station of the body, that takes in sensory and motion-related information from different parts and passes it on to the cerebral cortex. Information is also passed in the reverse direction―from the cerebral cortex to the thalamus―which is then in-turn sent out to the other parts of the body. The thalamus is composed of many nuclei having sensory, motor, and limbic connections. Some of the nuclei are nonspecific in nature. These nuclei help it in receiving inputs from the many cortical and sub-cortical structures. Mentioned below are a few of the important bodily systems and the thalamic nuclei that are associated with them. The input from the retina of the eyes which is relayed to the brain through the optic nerve is received by the lateral geniculate nucleus of the thalamus. It then relays it to the primary visual cortex in the occipital lobe. The medial geniculate nucleus of the thalamus relays information between the inferior colliculus of the mid-brain and the primary auditory cortex. The ventral posterior nucleus in the thalamus is an important somatosensory relay that is responsible for transmitting information relating to touch and proprioception to the primary somatosensory cortex. A major role of the thalamus is related to the motor control system of the body. It performs the positive reinforcement of the non-reflex motor behavior initiated by the cortex. The following are the thalamic nuclei associated with motor control, along with their specific functions:

• Ventrolateral: It is responsible for the coordination and modulation of movement.
• Ventroanterior: It is involved in the planning and initiation of movement.
• Ventrointermedial: It helps coordinate different movements.

The ventral posteromedial nucleus in the thalamus conveys facial sensory information. It also receives taste-related information from various parts of the gustatory system. The ventral posterolateral nucleus receives sensory inputs from the body, including the joints, limbs, etc. The thalamus is part of the limbic system which is responsible for combining higher mental functions and primitive emotions into one system. The following are the thalamic nuclei involved in this.

• Anterior: It is involved in memory storage and emotion.
• Mediodorsal: It is responsible for motivation, drive, and other related emotions.
• Centromedian: It deals with the emotional aspect of pain.

The following are some other functions that the thalamus performs. The thalamus has strong reciprocal connections with the cerebral cortex, forming the thalamo-cortico-thamlamic circuits, which take care of consciousness. It plays a major role in regulating arousal, wakefulness, level of awareness, and activity. Thus, any damage to the thalamus carries with it the risk of comatose. Together with the brainstem and the higher cortical areas, the thalamus plays an important role in a human's ability of filtering out insignificant information, and directing attention towards the stimuli of interest by allowing the related information to pass. This allows parents, for instance, to sleep soundly through the loud noises of rainfall and wind, but be instantly aware of their child's slightest whimpering. Thalamus is also capable of a crude awareness of many other types of sensations in the body, without being able to identify their specific location or intensity.