Thalamus anatomy: where the thalami are and how they relay signals to the cortex

What it is

The thalamus is a paired diencephalic mass that relays sensory and motor information to the cerebral cortex.

Location

It lies on both sides of the third ventricle, above the hypothalamus and below the lateral ventricle, bordered laterally by the internal capsule.

Function

It relays almost all sensory input (except olfaction) to the cortex and regulates consciousness, sleep and attention. It also relays motor information from the basal ganglia and cerebellum.

Body system

Nervous · paired (left and right)

FMA ID

FMA258716 (Foundational Model of Anatomy)

Key facts

• Includes the ventral posterior nuclei for somatosensation, and the geniculate bodies for vision and hearing.
• Divided by the internal medullary lamina.
• The two thalami are usually joined across the third ventricle by a small interthalamic adhesion.

Clinical relevance

Thalamic stroke can cause contralateral sensory loss or thalamic pain.

The thalamus lies at the core of the diencephalon, deep to the cerebral cortex, on each side of the midline. It is shaped like an egg lying along the front-to-back axis: narrow at the front and wide at the back. (Kenhub: Thalamus.)

The medial surface forms the lateral wall of the third ventricle. The lateral surface touches the posterior limb of the internal capsule. The inferior surface joins the hypothalamus in front and is continuous with the tegmentum of the midbrain behind. The anterior end (anterior pole) forms the back wall of the interventricular foramen, and the posterior end swells into the pulvinar. (Radiopaedia: Thalamus.)

The upper surface is covered by a thin layer of white matter, the stratum zonale, and is separated from the body of the fornix above by the tela choroidea. At its lateral edge the stria terminalis and the thalamostriate vein separate it from the caudate nucleus. (Radiopaedia: Thalamus.)

The pulvinar, at the back of the thalamus, lies beside the pineal gland, behind and to the side of the colliculi, above the geniculate bodies and under the splenium of the corpus callosum. Below the thalamus are the cerebral peduncles and the cerebral aqueduct. (Kenhub: Thalamus.)

Thalami is simply the plural of thalamus: everyone has two, a right and a left. They are the largest structures of the diencephalon, two symmetrical egg-shaped masses that are usually joined in the midline by a band of grey matter, the interthalamic adhesion. (Radiopaedia: Thalamus.)

The interthalamic adhesion (massa intermedia) crosses the third ventricle between the two thalami. It is present in about 80% of people (reported range 75 to 96%) and more often in females. It contains glial tissue but no neurons, so it is not a true commissure, and its function is unknown. (Radiopaedia: Interthalamic adhesion.)

The thalamus develops from the diencephalon, one of the two secondary vesicles of the embryonic forebrain (prosencephalon). The same vesicle gives rise to the hypothalamus, and the cavity inside it becomes the third ventricle. (OpenStax Anatomy and Physiology 2e, 13.1: The Embryologic Perspective.)

A Y-shaped sheet of white matter, the internal medullary lamina, divides each thalamus into anterior, medial and lateral parts, each holding a large group of nuclei. Smaller groups lie within the lamina and on the medial and lateral surfaces. An external medullary lamina covers the lateral surface. (Radiopaedia: Thalamus.)

The lateral part has two tiers. The dorsal tier contains the lateral dorsal nucleus, lateral posterior nucleus and pulvinar. The ventral tier contains the ventral anterior, ventral lateral and ventral posterior nuclei; the ventral posterior nucleus is split into ventral posterolateral (VPL) and ventral posteromedial (VPM) parts. The lateral and medial geniculate bodies are rounded bumps on the posteroventral surface. The medial part holds the dorsomedial nucleus, and the anterior part the anterior nucleus. (Radiopaedia: Thalamus.)

Outside these groups are the reticular nucleus, a thin sheet of neurons between the external medullary lamina and the internal capsule, and the intralaminar and midline nuclei. Kenhub counts sixteen thalamic nuclei in total. (Kenhub: Thalamus.)

The lateral geniculate nucleus sits at the end of the optic tract. It has six layers: four dorsal parvocellular layers for colour and form and two ventral magnocellular layers for movement and depth. Together with the medial geniculate body it is sometimes called the metathalamus. (Radiopaedia: Lateral geniculate nucleus.)

The thalamus relays information between the cerebral cortex and the periphery, spinal cord and brainstem. All sensory information except smell passes through it before reaching the cortex. It does not just pass signals on: it processes them, for example helping decide which visual stimuli get attention. It also carries motor information, working with the cerebellum and the basal nuclei. (OpenStax Anatomy and Physiology 2e, 13.2: The Central Nervous System.)

Sensation from the body reaches the VPL nucleus through the medial lemniscus (fine touch, vibration, position sense) and the spinothalamic tracts (pain, temperature, crude touch). Sensation from the face reaches the VPM nucleus. Both nuclei project to the primary somatosensory cortex (Brodmann areas 3, 1 and 2) through the posterior limb of the internal capsule. (Kenhub: Thalamus.)

The internal capsule carries these thalamocortical fibres. Somatosensory fibres from the ventral posterior nucleus run in the posterior limb, the optic radiation from the lateral geniculate nucleus runs in the retrolentiform part, and the auditory radiation from the medial geniculate nucleus runs in the sublentiform part. (Radiopaedia: Internal capsule.)

The anterior and medial groups are linked with the limbic system. The anterior nucleus receives the mammillothalamic tract and connects with the cingulate gyrus, and the dorsomedial nucleus connects with the hypothalamus and prefrontal cortex. These groups take part in emotion and memory. (Kenhub: Thalamus.)

The thalamus also contributes to consciousness, sleep and memory, as well as sensory and motor function. (Radiopaedia: Thalamus.)

Not all thalamic nuclei relay signals from the senses. Guillery described first-order nuclei, which receive their main input from ascending pathways, and higher-order nuclei, which receive most of their driving input from layer 5 of the cortex and seem to pass information from one cortical area to another. (Guillery RW. Anatomical evidence concerning the role of the thalamus in corticocortical communication: a brief review. J Anat. 1995.)

Both belong to the diencephalon and both border the third ventricle. The thalamus is the larger, upper part and mainly relays and processes information on its way to the cortex. The hypothalamus lies below and slightly in front of it and controls homeostasis, acting as the executive centre of the autonomic and endocrine systems. (OpenStax Anatomy and Physiology 2e, 13.2: The Central Nervous System.)

The hypothalamus is connected to the pituitary gland by the infundibulum (pituitary stalk), which is how it controls hormone release. The thalamus has no such connection. (OpenStax Anatomy and Physiology 2e, 17.3: The Pituitary Gland and Hypothalamus.)

The thalamus is supplied mainly by small perforating end arteries from the posterior cerebral artery (PCA). Four territories are described: anterior (tuberothalamic or polar arteries, from the posterior communicating artery), paramedian (thalamoperforating arteries from the P1 segment of the PCA), inferolateral (thalamogeniculate arteries from P2) and posterior (the posterior choroidal arteries). (Radiopaedia: Thalamus.)

The paramedian arteries vary a lot. In 4 to 12% of people they arise as a single trunk, the artery of Percheron, from one PCA and supply both paramedian thalami and usually the upper midbrain. Blocking it causes infarcts in both thalami; patients may be drowsy, comatose or agitated and may have vertical gaze palsy and memory loss. (Radiopaedia: Artery of Percheron.)

Most thalamic damage is ischaemic or haemorrhagic. Damage to the VPL or VPM nucleus causes loss of sensation on the opposite side of the body. Tumours near the front of the thalamus can block the interventricular foramen, and tumours at the back can block the third ventricle and cerebral aqueduct, causing non-communicating hydrocephalus. (Kenhub: Thalamus.)

Dejerine-Roussy syndrome (thalamic pain syndrome) is a form of central post-stroke pain after a stroke in the ventral posterior part of the thalamus. Patients develop chronic neuropathic pain on the opposite side of the body, which can start straight away or years after the stroke. The area is usually supplied by the thalamogeniculate branches of the PCA. (Radiopaedia: Dejerine-Roussy syndrome.)

Radiopaedia lists other thalamic conditions including bithalamic lesions, thalamic infarcts and thalamic glioma, and describes the thalamic syndrome as loss of sensation on the opposite half of the body, often with mood swings. (Radiopaedia: Thalamus.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Fornix of forebrain · nervous
• Stria terminalis · nervous
• Lateral ventricle · nervous
• Choroid plexus of cerebral hemisphere · nervous
• Cingulate gyrus · nervous
• Third ventricle · nervous
• Commissure of fornix of forebrain · nervous
• Caudate nucleus · nervous
• Internal capsule · nervous
• Hair of head · skin