Thalamus, Epithalamus

Thalamic Nuclei

All thalamic nuclei except one (reticular nucleus) have reciprocal excitatory connections with the cerebral cortex. The Y-shaped internal medullary lamina of white matter divides the thalamus into three large cell groups: anterior, medial dorsal (MD), and lateral nuclear groups ( Fig. 25.1 ). The lateral group consists of dorsal and ventral nuclear tiers. At the back of the thalamus are the medial and lateral geniculate bodies . The external medullary lamina separates the thalamus from the shell-like reticular nucleus . The thalamic nuclei are categorised into three functional groups: relay (specific) nuclei, association nuclei , and ‘non-specific’ nuclei.

Within each thalamic nucleus, most neurons are glutamatergic excitatory projection neurons (e.g. thalamocortical, thalamostriatal) and the remainder are inhibitory GABAergic interneurons. The projection neurons are further organised into core (driver) and matrix (modulatory) cells, but the number of each cell type again differs among the nuclei. Core cells receive modality-specific inputs and project in a topographically organised manner; those projecting to the cerebral cortex predominantly transmit to layer 4. Matrix cells receive less precise inputs, project more diffusely, and those projecting to the cerebral cortex predominantly transmit to layer 1 and have the potential to synchronise activity across broad areas of the cortex.

Input to the thalamic nuclei is predominantly glutaminergic and excitatory and can be divided into two main types. Some are specific (driver) and represented by the medial lemniscus input to the ventral posterior lateral (VPL) or cortical (layer 5) for the medial dorsal (MD). However, the majority is regulatory (modulatory) and serves as a feedback system.

The result of this excitatory and inhibitory input on thalamic projection neurons causes them to assume two different states and firing patterns. Tonic firing is the ‘typical’ state of neurons where firing frequency reflects input and the magnitude of that input. The other state is burst firing where activation of a thalamic neuron produces a burst of action potentials followed by a period of inactivation. Discreteness of response does not occur, but a burst firing pattern functions as an ‘alerting’ response to a novel or unexpected stimulus or event.

Relay Nuclei

Table 25.1 and Fig. 25.1 provide details for each of these nuclei and define the reciprocal connection to their corresponding area of cerebral cortex.

The anterior nucleus is involved in cognition (memory), emotion, and executive (behavioural) control and is a component of the Papez circuit (see Chapter 33 ).

The ventral posterior nucleus (VP) ( Fig. 25.2 ) receives fibres of the medial, spinal, and trigeminal lemnisci. It projects to the somatosensory cortex (SI), and a smaller projection is sent to the second somatic sensory area (SII) at the foot of the postcentral gyrus. The VP is somatotopically organised ( Fig. 25.3 ). The portion of the nucleus devoted to the face and head is called the ventral posteromedial nucleus (VPM) and that for the trunk and limbs is called the ventral posterolateral nucleus (VPL) .

Modality segregation is a feature of both VP nuclei, with proprioceptive neurons most anterior and tactile neurons posterior. Most spinothalamic nociceptive input does not project to the VP nucleus, but further posteriorly to other nuclei ( ventromedial posterior , ventral posterior inferior nucleus , and anterior nucleus of the thalamus ) . In addition, there is no evidence in the VP of an antinociceptive mechanism comparable to that found in the substantia gelatinosa region of the spinal cord and spinal trigeminal nucleus. However, an unexplained disorder, the thalamic pain syndrome , may follow a vascular lesion that interferes with the spinothalamic system and these other posterior thalamic nuclei. In this condition, a period of complete sensory loss may occur on the contralateral side of the body and later bouts of severe pain occurring either spontaneously or in response to tactile stimuli. (See also Chapter 35 , central poststroke pain .)

The medial geniculate nucleus is the thalamic nucleus of the auditory pathway. It receives the inferior brachium from the inferior colliculus (which carries auditory signals from both ears, see Chapter 20 ), and it projects to the primary auditory cortex in the superior temporal gyrus.

The lateral geniculate nucleus is the principal thalamic nucleus for vision. It receives retinal inputs from both eyes via the optic tract, and it projects to the primary visual cortex in the occipital lobe. The visual pathways are described in Chapter 31 .