Thalamus

Where is the thalamus?

Each thalamus is mostly gray matter, part of the diencephalon near the center of the brain, and it sits above the midbrain. (StatPearls, Thalamus)

The two thalami form the side walls of the third ventricle. In most brains they touch across it as the massa intermedia, or interthalamic adhesion, which StatPearls says is absent in about 30% of brains. (StatPearls, Thalamus and Ventricular System; OpenStax A&P 2e, 13.3)

The hypothalamus and the subthalamus lie below it. The upper surface of the thalamus forms part of the floor of the body of the lateral ventricle. (OpenStax A&P 2e, 13.2; StatPearls, Thalamus)

On its outer side it borders the posterior limb of the internal capsule, and in front and to the side it meets the head of the caudate nucleus. (StatPearls, Thalamus)

What does the thalamus do?

Every sense except smell has a thalamic nucleus that takes in its signals and passes them on to the cerebral cortex. (OpenStax A&P 2e, 13.2; StatPearls, Thalamus)

The thalamus filters and processes what it relays, and it has a part in controlling alertness and attention. (OpenStax A&P 2e, 13.2; StatPearls, Thalamus and Thalamic Nuclei)

It carries motor traffic as well. The main output of the basal nuclei, from the internal segment of the globus pallidus and the substantia nigra pars reticulata, goes to the thalamus, which relays it to the cerebral cortex. (OpenStax A&P 2e, 13.2; StatPearls, Basal Ganglia)

Output from the cerebellum leaves through the superior cerebellar peduncle to the red nucleus and the thalamus, and the thalamus returns it to the motor cortex. (OpenStax A&P 2e, 16.5; StatPearls, Cerebellar Dysfunction)

It also takes part in regulating sleep and wakefulness, and, through limbic connections to its anterior nuclei, in learning and episodic memory. (StatPearls, Thalamus)

How is the thalamus divided?

Sheets of white matter inside it, the internal medullary laminae, split each thalamus into anterior, medial and lateral groups of nuclei. (OpenStax A&P 2e, 14.2; StatPearls, Thalamus)

Limbic structures connect to the anterior nuclei, which brings the thalamus into learning and episodic memory. The lateral group holds the nuclei that pass vision, hearing and body sensation on to their sensory areas of the cortex. (OpenStax A&P 2e, 14.2; StatPearls, Thalamus and Thalamic Nuclei)

The reticular nucleus wraps the outer side of the thalamus like a capsule. It sends nothing to the cortex and instead modulates the other thalamic nuclei. (StatPearls, Thalamus)

What connects to the thalamus?

Hearing reaches it from the inferior colliculi of the midbrain. Its medial geniculate nucleus takes that input on to the primary auditory cortex in the temporal lobe. (OpenStax A&P 2e, 13.2; StatPearls, Thalamus)

Visual fibers reach it in the optic tract, which runs on from the optic chiasm and ends mostly in the lateral geniculate nucleus; this model shows that nucleus as the lateral geniculate body. (OpenStax A&P 2e, 14.2 and 14.1; StatPearls, Visual Pathway and Optic Tract)