Each thalamus is a large, bullet-shaped mass, consisting of a number of closely-packed nuclei. The two lie side by side, roughly in the middle of the entire brain, forming the walls of the central fluid-filled cerebral ventricle called the third ventricle. And above each thalamus is the lateral ventricle, within the cerebral hemisphere. In some individuals they are joined by a bridge of grey matter, the massa intermedia, stretching across the third ventricle. Below and in front of each thalamus is the hypothalamus, and to the side of each is the internal capsule – the important band of white matter containing the fibres of thalamic nerve cells, passing to the cerebral cortex above, together with fibres from the cortex running down. The thalamus is split into three major parts, lateral, medial and anterior, by a thin, Y-shaped fibrous sheet (see figure), containing so-called ‘non-specific’ nuclei (see below).
‘Specific’ thalamic nucleiThe lower part of the lateral thalamus contains large and very important nuclei, called specific nuclei, which relay sensory information to primary areas of the cerebral cortex. For instance, the ventral posterior nucleus (VP) is the main relay for information from the skin and deep tissues of the body, passing it up to the somatic sensory area of the cortex. The ventral anterior and ventral lateral nuclei receive fibres from, respectively, the corpus striatum (part of the basal ganglia) and the cerebellum, and they send fibres to parts of the frontal lobe of the cortex that are concerned with motor control. At the rear end of the thalamus are the smaller medial and lateral geniculate nuclei, relaying auditory and visual pathways, respectively, to the auditory and visual areas of the cortex.
In the major sensory nuclei of the thalamus, incoming fibres are generally distributed in strict topographic order and they therefore form ‘maps’ of the array of sensory receptors from which they derive. For instance, the opposite half of the body is represented within each VP. Fibres ascending from the spinal cord reach the lateral part, carrying signals from the main part of body, while the medial part receives input from the face and head via the fifth cranial nerve, the trigeminal nerve. This spatial arrangement is conserved as the fibres of cells in the VP run up, so that the input from the whole body is draped, upside down, over the somatic sensory cortex (which occupies a strip running vertically down the middle of the side of each hemisphere).
The nuclei of the upper part of the lateral thalamus send their fibres to regions of the cortex other than the primary sensory and motor areas. The lateral posterior nucleus and pulvinar are large structures, which send fibres to regions of the parietal and occipital lobes that are concerned with visual understanding and the integration of sensory information. The lateral dorsal nucleus receives fibres from the hippocampus and sends its axons to regions of limbic cortex on the inner surface of the hemisphere.
The medial thalamus consists essentially of the mediodorsal nucleus, which relays signals from hypothalamus and amygdala to the frontal lobe and also to the corpus striatum.
The anterior thalamus contains the anterior nucleus, which receives input from the mamillary body of the hypothalamus and sends fibres to a region called the cingulate cortex, on the midline surface of the hemisphere. All these structures are parts of the limbic system, which is concerned with the regulation of vital rhythms, emotions, appetites, and memory.
The cerebral cortex and the thalamus have evolved in parallel during mammalian evolution, and they constitute an intimately related functional system. Perhaps the most intriguing and least-understood feature of the thalamus is the fact that each specific nucleus receives at least as many fibres from its region of cerebral cortex as it sends fibres to that region. Thus thalamus and cortex are reciprocally interconnected, with some sort of recursive interplay occurring between them. Some argue that the thalamus acts not just to relay signals to the cerebral cortex, but as a dynamic conduit of information between one cortical area and another, regulating the flow of information according to attention and intention.
‘Non-specific’ thalamic nucleiIn between the specific sensory and motor relay nuclei of the thalamus is a system of smaller, more diffuse ‘non-specific’ nuclei, which are organized quite differently. They receive ascending fibres from the reticular formation, which stretches through the core of the brainstem and itself receives and integrates input from the sensory pathways. In turn, the non-specific nuclei of the thalamus send their fibres diffusely over the cerebral hemispheres, rather than to distinct areas. This pathway, from reticular formation to non-specific nuclei to the cortex, which is called the ascending reticular activating system, is thought to be involved in regulating the state of ‘arousal’ of the cortex during the cycle of sleeping and waking.
Laurence J. Garey, and Colin Blakemore
See also brain; cerebral cortex; hearing; somatic sensation; sleep; vision.
A collection of cell body clusters located in the middle of the forebrain.
The thalamus is a relatively large collection of cell body clusters shaped like two small footballs. It is involved in receiving sensory information from the eyes and other sense organs, processing that information, and then transmitting it to primary sensory zones in the cerebral cortex. The thalamus also processes pain signals from the spinal cord as well as information from different parts of the cerebral hemispheres, and relays it to the cerebellum and the medulla. Together with the hypothalamus , the thalamus forms part of the forebrain called the diencephalon.
By registering the sensory properties of food, such as texture and temperature, the thalamus plays a role in appetite. It is also known to be involved in the control of sleep and wakefulness. Cognitive researchers have found that the thalamus activates or integrates language functions, plays a role in memory , and that a portion of the thalamus, called the pulvinar, helps in refocusing attention . Together with the hippocampus and parts of the cortex, it is instrumental in the formation of new memories, which are then thought to be stored in the cerebral cortex.
See also Brain.
thal·a·mus / ˈ[unvoicedth]aləməs/ • n. (pl. -mi / -ˌmī/ ) Anat. either of two masses of gray matter lying between the cerebral hemispheres on either side of the third ventricle, relaying sensory information and acting as a center for pain perception. DERIVATIVES: tha·lam·ic / [unvoicedth]əˈlamik/ adj. .