The Thalamus

The brain's central switchboard: a pair of egg-shaped structures that relay and gate almost every sensory signal on its way to the cortex, and help regulate sleep, attention and consciousness.

The thalamus is a pair of egg-shaped masses of grey matter, one in each hemisphere, at the centre of the brain just above the brainstem. The name comes from the Greek for "inner chamber".

More than a relay

The thalamus contains dozens of nuclei, each connected to particular parts of the cortex. Every sense except smell passes through a thalamic relay before reaching the cortex: vision through the lateral geniculate nucleus, hearing through the medial geniculate nucleus, touch and pain through the ventral posterior nuclei. Other nuclei relay signals from the cerebellum and basal ganglia to the motor cortex, or connect with memory circuits.

But the thalamus isn't a passive relay. The cortex sends even more connections back to the thalamus than it receives from it, and the thalamus can amplify or suppress signals, acting as a gate for attention.

Rhythms, sleep and consciousness

The thalamus and cortex together generate brain rhythms. During sleep, the thalamus produces sleep spindles — short bursts of 11–16 Hz activity seen on EEG — and its relay neurons shift into a burst-firing mode that partly closes the gate to the outside world. The intralaminar and other "non-specific" nuclei project widely and help maintain arousal; damage to them on both sides can cause coma or severe disorders of consciousness. Deep brain stimulation of the central thalamus has been tried experimentally to improve responsiveness in some patients.

When it goes wrong

Strokes in the thalamus can cause loss of sensation on the opposite side of the body and sometimes a severe chronic pain called thalamic pain syndrome (Dejerine–Roussy syndrome). Abnormal thalamocortical rhythms underlie absence seizures in children.