Thalamus


The thalamus is a large mass of gray matter on the lateral wall of the third ventricle forming the dorsal part of the diencephalon. 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, 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 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. In an MRI study of 12 healthy males with average age 17 years, scans showed mean 'whole thalamus' volume 8.68cm.
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. 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 and the peri-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 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.
Image:Thalmus.png|thumb|Nuclei of right thalamus

Thalamic nuclei

The principal subdivision of the thalamus into nucleus groups is the trisection of each thalamus 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, the stratum zonale, and the interthalamic adhesion.
Combining these division principles yields the following hierarchy, which is subject to many further subdivisions.
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, paramedian thalamic-subthalamic arteries, inferolateral arteries, and posterior 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 spinothalamic tract, which transmits crude touch and pressure.

Function

The thalamus has multiple functions, and is generally believed to act as a relay station, or hub, relaying information between different subcortical areas and the cerebral cortex. In particular, every sensory system includes a thalamic nucleus that receives sensory signals and sends them to the associated primary cortical area.
For the visual system, for example, inputs from the retina are sent to the lateral geniculate nucleus of the thalamus, which in turn projects to the visual cortex in the occipital lobe. Similarly the medial geniculate nucleus acts as a key auditory relay between the inferior colliculus of the midbrain and the primary auditory cortex. The ventral posterior nucleus is a key somatosensory relay, which sends touch and proprioceptive information to the primary somatosensory cortex. In rodents, proprioceptive information of head and whisker movements is integrated already at the thalamic level.
The thalamus is believed to both process sensory information as well as relay it—each of the primary sensory relay areas receives strong feedback connections from the cerebral cortex.
The thalamus also plays an important role in regulating states of sleep, and wakefulness. Thalamic nuclei have strong reciprocal connections with the cerebral cortex, forming thalamo-cortico-thalamic circuits that are believed to be involved with consciousness. The thalamus plays a major role in regulating arousal, the level of awareness, and activity. Damage to the thalamus can lead to permanent coma.
The role of the thalamus in the more anterior pallidal and nigral territories in the basal ganglia system disturbances is recognized but still poorly understood. The contribution of the thalamus to vestibular or to tectal functions is almost ignored. The thalamus has been thought of as a "relay" that simply forwards signals to the cerebral cortex. Newer research suggests that thalamic function is more selective. Many different functions are linked to various regions of the thalamus. This is the case for many of the sensory systems, such as the auditory, somatic, visceral, gustatory and visual systems where localized lesions provoke specific sensory deficits. A major role of the thalamus is support of motor and language systems, and much of the circuitry implicated for these systems is shared.
The thalamus is functionally connected to the hippocampus as part of the extended hippocampal system at the thalamic anterior nuclei. With respect to spatial memory and spatial sensory datum they are crucial for human episodic event memory. The thalamic region's connection to the medial temporal lobe provides differentiation of the functioning of recollective and familiarity memory.
The neuronal information processes necessary for motor control were proposed as a network involving the thalamus as a subcortical motor center. Through investigations of the anatomy of the brains of primates the nature of the interconnected tissues of the cerebellum to the multiple motor cortices suggested that the thalamus fulfills a key function in providing the specific channels from the basal ganglia and cerebellum to the cortical motor areas. In an investigation of the saccade and antisaccade motor response in three monkeys, the thalamic regions were found to be involved in the generation of antisaccade eye-movement.
Recent research suggests that the mediodorsal thalamus may play a broader role in cognition. Specifically, the mediodorsal thalamus may "amplify the connectivity of just the circuits in the cortex appropriate for the current context and thereby contribute to the flexibility to make complex decisions by wiring the many associations on which decisions depend into weakly connected cortical circuits." Researchers found that "enhancing MD activity magnified the ability of mice to "think," driving down by more than 25 percent their error rate in deciding which conflicting sensory stimuli to follow to find the reward."

Development

The thalamic complex is composed of the perithalamus, the mid-diencephalic organiser and the thalamus. The development of the thalamus can be subdivided into three steps.
The thalamus is the largest structure deriving from the embryonic diencephalon, the posterior part of the forebrain situated between the midbrain and the cerebrum.