The thalamus is in the middle of the brain: two egg-shaped masses of grey matter, one on each side of the narrow third ventricle, sitting above the midbrain and under the cerebral hemispheres. [1]
The 3D brain needs JavaScript. The text below still says where the thalamus is and what it does.
Slice
Opens cut by a front (coronal) slice through the thalamus. Drag to turn, pinch or scroll to zoom, tap any part to name it; .
It forms the upper (dorsal) part of the diencephalon, the division of the forebrain between the cerebrum and the midbrain. [1][2]
Deep beneath the cerebrum: together with the hypothalamus, the diencephalon makes up the walls of the third ventricle. [3]
Paired: the inner surface of each thalamus forms the upper part of the side wall of the third ventricle, and the two are usually joined across it by a flat grey band, the interthalamic adhesion, which is sometimes absent or split. [1][4]
The hypothalamus lies below and slightly in front of it. [3]
What it does
Relays sensory and motor signals to the cerebral cortex. [1]
Every sense except smell passes through the thalamus on its way to the cortex: the retina feeds the lateral geniculate nucleus (then the visual cortex), the inferior colliculus feeds the medial geniculate nucleus (then the auditory cortex), and touch and body-position signals go through the ventral posterior nucleus to the primary somatosensory cortex. [1][3]
It processes as well as relays: the part that receives visual information, for example, influences which visual stimuli are treated as important. [3]
Closes the loop for the basal nuclei: their output goes to the thalamus, which relays it to the cerebral cortex. [3]
Connections
Thalamocortical radiations: fibres fan out to the cerebral cortex in all directions. Almost all thalamic neurons (except those of the reticular nucleus) project to the cortex, and every cortical region studied sends fibres back. [1]
Receives the output of the basal nuclei, an inhibitory projection using the transmitter GABA. [3]
Receives the cerebellum’s main output through the superior cerebellar peduncle and passes it on to the cerebral cortex. [5]
Its anterior nuclei receive the mammillothalamic tract from the mammillary bodies, a link in the circuit from the hippocampus through the fornix. [6]
Inside it
Each thalamus has at least 30 nuclei (at least 60 for both). The ventral group is split into ventral anterior, ventral lateral and ventral posterior nuclei; the lateral and medial geniculate nuclei form the metathalamus; the reticular nucleus wraps around the outer side. [1]
Other names
Plural: thalami. From Greek thalamos, “chamber”. [1]
Size
Each thalamus is about four centimetres long and ovoid. [1]
On this model the left thalamus measures about 37 mm front to back, 24 mm top to bottom and 24 mm side to side (the box it fits in, not its volume); the right one is its mirror image. [7]
Centre of the left thalamus compared with the centre of the hypothalamus: 22 mm further back, 12 mm higher, 13 mm further from the midline; 28 mm apart in a straight line. [7]
Centre of the left thalamus compared with the centre of the midbrain: 16 mm higher, 13 mm further from the midline; 20 mm apart in a straight line. [7]
Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); real brains vary in size and shape, so its measurements describe one model, not anyone’s brain.