STRUCTURE OF THE THALAMUS; SOMATOSENSORY and MOTOR NUCLEI
The Thalamus is about the size of a walnut, situated in the centre of the brain, and divided into two halves, one on either size of the third ventricle. The internal capsule, which contains axons connecting the cortex with other parts of the CNS, including the motoneurones, separates the thalamus from the basal ganglia.
The Thalamic nuclei include relay stations in all the major sensory pathways; somatosensory modalities use separate nuclei within the ventrobasal and posterior thalamus.
Other nuclei, concerned with motor control, provide feedback to the cortex from the cerebellum and basal ganglia.
The thalamus consists of many nuclei with different specific functions, separated from each other by bands of axons called laminae. Each side of the thalamus is concerned with sensory information from the contralateral side of the body, and objects in the contralateral visual and auditory fields.
The thalamus is surrounded by a thin layer of non-specific nuclei, known as the Reticular Nucleus of the thalamus, which is concerned with regulating the overall activity of the cerebral cortex, and plays a role in the sleep/waking cycle.
Key Words: Thalamic nuclei and connections; sensory and motor functions of the thalamus; Reticular Nucleus of the Thalamus.
The diagram shows the main divisions of the thalamus, and their nomenclature. The laminae separate the anterior, medial and lateral nuclei.
Subdivisions of the Thalamus
The thalamus is divided into anterior, medial, and lateral regions by bands of white matter (axons) called laminae.
The lateral region is divided into anterior and posterior nuclei (pulvinar), and is also a split into dorsal and ventro-basal regions.
The lateral geniculate body (or nucleus, LGN), and the medial geniculate body (or nucleus, MGN) are relay stations on the VISUAL and AUDITORY pathways.
The ventro-basal complex of the thalamus is concerned with SOMATIC SENSATION and consists of two main nuclei: the nucleus ventralis posteromedialis (VPM) and nucleus ventralis posterolateralis (VPL).
The ventral thalamus also has a role to play in relaying signals concerned with MOTOR function from the basal ganglia and the cerebellum to the cortex (ventro-anterior, VA, and ventro-lateral, VL).
Anterior and medial thalamic nuclei are concerned with emotion and behaviour; they process information from the LIMBIC system.
Image source: www.glittra.com
The diagram shows the inputs to different groups of thalamic nuclei., which helps define their functional roles
Connections of the Thalamus
The diagram opposite shows that the connections of different parts of the thalamus and their functions
The ventral thalamus is concerned with MOTOR functions, processing information from the cerebellum, basal ganglia and substantia nigra.
Somatosensory modalities (touch, pain, temperature, proprioception)are relayed by different nuclei within the ventro-basal complex.
Image source: student consult
The image shows subdivisions of the Ventroposterior nuclei - S = superior, I =inferior, M = medial,L = lateral; and their role in signalling specific somatic sensations. In addition there are the non-specific intralaminar nuclei (within the laminae) and the nucleus reticularis of the thalamus, which forms a shell around the thalamus.
Modality-specific somatosensory nuclei in the Thalamus
The ventrobasal complex of nuclei, including the ventroposterior nuclei, handle much of the sensory traffic from the the body, head and neck.
The medial and lateral axis of the ventro-basal complex handles inputs from the dorsal column - medial lemniscal system, concerned with discriminative touch sensation and proprioception.
There is a somatotopic map of the body within the dorsal column nuclei, the ventro-basal complex of the thalamus and the primary somatosensory cortex.
The inferior nuclei within the ventroposterior complex deals with nociception, and receives signals from the anterolateral system, including the spino-thalamic tract.
The inferior nuclei project to the second somatosensory area of cortex and to the Insula, which is now recognised as dealing with some of the emotional/empathic aspects of nociceptive sensation.
The superior nuclei, dealing with proprioception, project to cortical areas (areas 2, 3a) adjacent to those concerned with discriminative touch (areas 1, 3b).
The thalamus also has some sensory nuclei that are not specific for any single modality, and are therefore considered non-specific, in that they have inputs from a number of sensory systems.
These are the non-specific intralaminar nuclei (within the laminae of the thalamus) and the reticular nucleus of the thalamus (RNT), which forms a shell around the thalamus.
These nuclei are concerned with the sleep-arousal system, and project to all areas of the cortex; the terminals of these neurones synapse in the superficial layers of the cortex, and alter the excitability of cortical neurones.