Thalamus

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A complex nucleus in the DIENCEPHALON, comprising a number of different cell groups. It is subdivided into three component parts—the dorsal thalamus, ventral thalamus and epithalamus—on the basis of differences in connectivity and development. A major function of the thalamus is the relay of incoming information from major sensory, motor or limbic nuclei to specific regions within the CEREBRAL CORTEX; this is carried out by the principal neurons of the dorsal thalamus. The thalamus is also intimately involved in the control of BEHAVIOURAL STATE, as a result of its major input from the CHOLINERGIC, NORADRENERGIC and SEROTONERGIC cell groups in the BRAINSTEM RETICULAR FORMATION.

The dorsal thalamus comprises principal (or relay) nuclei and INTRALAMINAR NUCLEI. The classical organization of principal thalamic nuclei is that they receive a well-defined subcortical input, relay these signals to the middle layers of a restricted cortical region, and receive a reciprocal corticothalamic projection from the same region of cortex. Each principal thalamic nucleus also receives an input from a restricted area of the THALAMIC RETICULAR NUCLEUS, from a specific region of the pallidum as part of the systems of CORTICOSTRIATAL LOOPS, and from the reticular formation. The intralaminar nuclei, found within the fibre LAMINAE of the dorsal thalamus, form an integral system of their own, but also have an organization which implies separable sensory, motor and limbic functions. The ventral thalamus includes the reticular nucleus of the thalamus, a thin sheet of neurons surrounding the lateral and rostral aspects of the dorsal thalamus. It is a ‘non-specific’ thalamic nucleus, which does not project to the cortex, but receives COLLATERALS of the cortical projections to dorsal thalamic nuclei. It also has reciprocal interactions with all nuclei in the dorsal thalamus and receives efferents from the brainstem reticular formation. The EPITHALAMUS contains the medial and lateral HABENULA, and is unique in relation to other thalamic systems because it has strong functional links with the BASAL GANGLIA, ventral STRIATUM and brainstem rather than with the cortex.

Within the dorsal thalamus, the principal sensory nuclei are the LATERAL GENICULATE NUCLEUS (vision), MEDIAL GENICULATE NUCLEUS (hearing) and ventroposterior (somatic sensation), relaying information to the primary visual, auditory and somatosensory cortices respectively.

The ventrolateral nucleus is a comparable motor nucleus relaying activity from the CEREBELLUM to the MOTOR CORTEX. The anterior nuclei and DORSOMEDIAL NUCLEUS OF THE THALAMUS are the principal limbic nuclei (see LIMBIC SYSTEM) and have a particular relationship with the PREFRONTAL CORTEX and CINGULATE CORTEX. Of this diverse group of nuclei, only the dorsolateral geniculate shows a ‘classical’ relationship with the cortex, as described above: it receives a well-defined input from the RETINA and has organised reciprocal connections with the middle layers of the primary VISUAL CORTEX. Other principal nuclei may receive a less clearly defined input pattern, or interact with a number of related cortical areas.

The AFFERENT connections from the reticular nucleus are inhibitory—the only inhibitory outputs from any thalamic nucleus. The nu cleus is so-called as a result of its long and relatively non-divergent output neurons, which are similar to those arising from the brainstem reticular formation. In the thalamic reticular nucleus, the profuse GABAERGIC collaterals form a mesh around passing fibres, and together with electrophysiological evidence suggest a role in gating the activity of thalamocortical relay neurons, in a similar manner to the PURKINJE CELLS in the cerebellum. The interplay between the brainstem reticular formation, thalamus and cortex, has been implicated in the regulation of levels of ATTENTION, VIGILANCE and MOTIVATION during waking, as well as the control of SLOW-WAVE SLEEP and rapid eye movement sleep (REM SLEEP). Efferents to the principal and reticular thalamic neurons from the reticular formation have STATE-DEPENDENT firing patterns (see STATE DEPENDENCY): they operate in distinct modes, which depend on the state of arousal. Their thalamic targets also show differences in activity which are dependent on behavioural state. During alert wakefulness, principal and reticular neurons have similar firing patterns, characterized by spontaneous single-spike tonic discharges which are thought to promote the transfer of information to the cortex. As vigilance moves towards drowsiness, reticular neurons become exceptionally active, firing exceedingly long spike barrages which phasically hyperpolarize (see HYPERPOLARIZATION) principal thalamic neurons; during this stage, principal neurons are characterized by spontaneous oscillations in a 7–14 Hz ‘spindle’ rhythm. Spindle waves in the dorsal thalamus herald the appearance of slow-waves in the cortical ELECTROENCEPHALOGRAM (EEG) during sleep. Although the interactions between thalamic reticular and principal neurons in different states are now relatively well understood in electrophysiological terms, the precise role of other structures such as the brainstem reticular formation—or indeed forebrain structures such as the HYPOTHALAMUS or cholinergic BASAL FOREBRAIN—in modifying thalamic activity, is still not clearly understood.

WENDY L.INGLIS

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