Thalamus, Epithalamus

Chapter Summary
Thalamus
- Thalamic nuclei
- Thalamic peduncles
Epithalamus

Study guidelines

1.

Describe the characteristics of the thalamic nuclei within the specific or relay, association, and nonspecific nuclei groups.
2.

List the afferent and efferent projections for the following relay nuclei: anterior, ventral lateral, ventral posterior, medical geniculate, and lateral geniculate.
3.

List the afferent and efferent projections for the dorsomedial nucleus.
4.

Describe how the thalamic reticular nucleus differs from the other thalamic nuclei.
5.

Discuss what thalamocortical and corticothalamic projections pass through the thalamic peduncles.

Thalamus

The thalamus is the largest nuclear mass in the entire nervous system. It is a prominent feature in magnetic resonance imaging (MRI) scans in each of the three planes in which slices are taken. The afferent and efferent connections of the main nuclear groups are listed in Table 27.1 . The connections are diverse but in general serve to provide sensorimotor integration through conscious perception of sensation (whether external or internal to the body) to guide the motor system.

Table 27.1

Thalamic nuclei and their connections

Type	Nucleus	Afferents	Efferents
Specific (or relay)	Anterior	Mammillary bodies Hippocampus	Cingulate cortex
	Ventral anterior (VA)	Substantia nigra (pars reticulata)	Prefrontal cortex
	Ventral lateral (VL)
	VL, anterior part VL, posterior part	Globus pallidus (internal segment) Cerebellar nuclei	Supplementary motor area Premotor and motor cortex
	Ventral posterior (VP)
	Ventral posterolateral (VPL)	Somatic afferents from trunk and limbs	Somatic sensory cortex
	Ventral posteromedial (VPM)	Somatic afferents from the head region	Somatic sensory cortex
	Medial geniculate body	Brachium of the inferior colliculus	Primary auditory cortex
	Lateral geniculate body	Optic tract	Primary visual cortex
Association	Lateral dorsal (LD)	Hippocampus	Cingulate cortex
	Dorsomedial (DM)	Prefrontal cortex, olfactory and limbic	Prefrontal cortex
	Lateral posterior (LP)/Pulvinar	Superior colliculus, primary visual, auditory, and somatosensory cortex	Posterior parietal and lateral temporal association cortex
Nonspecific	Intralaminar (centromedian, parafascicular, others)	Reticular formation, basal ganglia, limbic system	Cerebral cortex, corpus striatum
	Reticular	Thalamus and cortex	Thalamus

As noted in Chapter 2 the two thalami lie at the centre of the brain. Their medial surfaces are usually linked across the third ventricle and their lateral surfaces are in contact with the posterior limb of the internal capsule. The upper surface of each occupies the floor of a lateral ventricle. The under aspect receives sensory and cerebellar inputs as well as an upward continuum of the reticular formation.

Thalamic nuclei

All thalamic nuclei except one (the 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: medial dorsal, anterior , and lateral ( Figure 27.1A ). 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 .