thalamus The
cerebral hemispheres of the
brain consist of an outer layer of
grey matter called
cerebral cortex, with a core of
white matter, surrounding masses of
grey matter. The thalamus (of which there is one on each side) is a large and important mass of nerve cells or
neurons. All sensory pathways (from eyes, ears, skin, etc.), except some of those concerned with the sense of smell, pass through the thalamus on their way to the cerebral cortex. The incoming fibres from each sensory system terminate on a dense clump of nerve cells called a
nucleus, or on a set of such nuclei. In turn, the fibres of the cells in each thalamic nucleus run up to a particular area (or areas) of the
cerebral cortex. Important pathways concerned with the control of movement, from the cerebellum and from motor nuclei called the
basal ganglia, also pass through the thalamus. But the thalamus is more than merely a relay station.
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 nuclei
The 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 nuclei
In 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.
