thalamus
Meaning of thalamus: Less a relay station than a gatekeeper, since it decides which signals reach awareness and which are damped down before arriving.
Definition of thalamus
The thalamus is a pair of egg-shaped masses of gray matter buried deep in the center of the brain, one on each side of the fluid-filled third ventricle. Almost every signal heading for the thinking surface of the brain passes through it first, whether from the senses, the movement circuits or the memory system, and it governs how much of that traffic gets through.
What passes through it?
Sight, hearing, touch, temperature, pain and the sense of where the limbs are all stop at the thalamus on the way to the outer layer of the brain. Smell is the exception, reaching the cortex by a separate route. The traffic is not one way either. Signals from the movement circuits and from the cerebellum pass through on their way to the motor areas, and the cortex sends far more fibers back to the thalamus than it receives, which makes the arrangement a two-way conversation rather than a switchboard.
Why call it a gatekeeper?
Because it settles what gets attention. Each patch of the thalamus can be turned up or down, and that adjustment is what allows a conversation to be followed in a noisy room while the noise itself fades from awareness. The same mechanism shifts with the state of arousal: during deep sleep the gate largely closes and most sensory traffic never reaches the cortex, which is part of why sleep is not simply rest.
What happens when it is damaged?
| Region affected | Usual consequence |
|---|---|
| Sensory nuclei | Numbness on the opposite side, sometimes later a burning pain |
| Motor relay | Tremor and clumsiness |
| Memory circuit | Difficulty forming new memories |
| Arousal pathways | Drowsiness or reduced consciousness |
Its central position and dense small blood supply make it a common site of small strokes. Because it sits just above the brain stem and beside structures with no room to spare, a brain tumor growing there is hard to reach surgically and any swelling has little space to expand into.
Also known as
Used in a sentence
The infarct involved the left thalamus and the adjacent internal capsule.
Good to know
Damage on one side produces effects on the opposite side of the body, and the burning thalamic pain that can develop weeks after a small stroke there is often mistaken for a separate new problem.
Word origin
Where “thalamus” comes from
From the Greek thalamos, meaning an inner chamber or bedroom, a name early anatomists gave to the chamber-like space at the base of the brain.