The thalamus (pl.: thalami; from Greek θάλαμος, "chamber") is a large mass of gray matter on the lateral wall of the third ventricle forming the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, known as the thalamocortical radiations, allowing hub-like exchanges of information. It has several functions, such as the relaying of sensory and motor signals to the cerebral cortex and the regulation of consciousness, sleep, and alertness. Anatomically, the thalami are paramedian symmetrical structures (left and right), within the vertebrate brain, situated between the cerebral cortex and the midbrain. It forms during embryonic development as the main product of the diencephalon, as first recognized by the Swiss embryologist and anatomist Wilhelm His Sr. in 1893.
Anatomy The thalami are paired structures of gray matter about four centimetres long and ovoid in appearance, located in the forebrain which is superior to the midbrain, near the center of the brain with nerve fibers projecting out to the cerebral cortex in all directions. In fact, almost all thalamic neurons (with the notable exception of the thalamic reticular nucleus) project to the cerebral cortex, and every region of the cortex so far studied has been found to innervate the thalamus. Each of the thalami may be subdivided into at least 30 nuclei, giving a total of at least 60 for the 'whole thalamus'. Estimates of the volume of the 'whole thalamus' vary. A post-mortem study of 10 people with average age 71 years found average volume 13.68 cm
3 {\displaystyle {}^{3}} . In an MRI study of 12 healthy males with average age 17 years, scans showed mean 'whole thalamus' volume 8.68cm
3 {\displaystyle {}^{3}} . The medial surface of the thalamus constitutes the upper part of the lateral wall of the third ventricle, and is connected to the corresponding surface of the opposite thalamus by a flattened gray band, the interthalamic adhesion. The lateral part of the thalamus is the neothalamus, the phylogenetically newest part of the thalamus, which includes the lateral nuclei, the pulvinar nuclei and the medial and lateral geniculate nuclei. The surface of the thalamus is covered by two layers of white matter, the stratum zonale covers the dorsal surface, and the external medullary lamina covers the lateral surface. (This stratum zonale should not be confused with the stratum zonale of the superior colliculus.) The grey matter of thalamus is partitioned by a Y shaped internal medullary lamina which divides the nuclei into anterior, medial, and lateral groups. Derivatives of the diencephalon include the dorsally located epithalamus (essentially the habenula and annexes) and the peri-thalamus (pre-thalamus) containing the zona incerta and the thalamic reticular nucleus. Due to their different ontogenetic origins, the epithalamus and the peri-thalamus are formally distinguished from the thalamus proper. The metathalamus is made up of the lateral geniculate and medial geniculate nuclei. The thalamus comprises a system of lamellae (made up of myelinated fibers) that separate different thalamic subparts. Other areas are defined by distinct clusters of neurons, such as the periventricular nucleus, the intralaminar elements, the "nucleus limitans", and others. These latter structures, different in structure from the major part of the thalamus, have been grouped together into the allothalamus as opposed to the isothalamus. This distinction simplifies the global description of the thalamus.
Thalamic nuclei
The principal subdivision of the thalamus into nucleus groups is the trisection of each thalamus (left and right) by a Y-shaped internal medullary lamina. This trisection divides each thalamus into anterior, medial and lateral groups of nuclei. The medial group is subdivided into the medial dorsal nucleus and midline group. The lateral group is subdivided into ventral, pulvinar, lateral dorsal, lateral posterior and metathalamus. The ventral group is further subdivided into ventral anterior, ventral lateral and ventral posterior. The interior medullary lamina is subdivided into intralaminar nuclei. Additional structures are the reticular nucleus (which envelops the lateral thalamus), the stratum zonale, and the interthalamic adhesion. Combining these division principles yields the following hierarchy, which is subject to many further subdivisions.
anterior group medial group medial dorsal nucleus midline group lateral group ventral group ventral anterior group ventral lateral group ventral posterior group pulvinar group lateral dorsal nucleus lateral posterior nucleus metathalamus lateral geniculate nucleus medial geniculate nucleus intralaminar group reticular nucleus stratum zonale interthalamic adhesion The term "lateral nuclear group" is used with two meanings. It can mean either the complete set of nuclei in the lateral "third" of the trisection by the lamina, or the subset which excludes the ventral group and the geniculate nuclei.
Blood supply The thalamus derives its blood supply from a number of arteries: the polar artery (posterior communicating artery), paramedian thalamic-subthalamic arteries, inferolateral (thalamogeniculate) arteries, and posterior (medial and lateral) choroidal arteries. These are all branches of the posterior cerebral artery. Some people have the artery of Percheron, which is a rare anatomic variation in which a single arterial trunk arises from the posterior cerebral artery to supply both parts of the thalamus.
Connections
The thalamus has many connections to the hippocampus via the mammillothalamic tract. This tract comprises the mammillary bodies and fornix. The thalamus is connected to the cerebral cortex via the thalamocortical radiations. The spinothalamic tract is a sensory pathway originating in the spinal cord. It transmits information to the thalamus about pain, temperature, itch and crude touch. There are two main parts: the lateral spinothalamic tract, which transmits pain and temperature, and the anterior (or ventral) spinothalamic tract, which transmits crude touch and pressure.
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