Thalamus
thalamus
- Think of it as a "mini-brain" within the brain.
- See Brain Atlas: Thalamus
Nuclei
The internal medullary lamina
- Lies along the anterior–posterior axis of the thalamus and bifurcates, anteriorly.
The anterior thalamic group
- Lies within this bifurcation and communicates with the limbic system (in particular with the mammillary bodies and cingulate gyrus as part of the Papez circuit).
The medial group nuclei
- The dorsomedial nucleus connects with the prefrontal cortex.
The lateral group nuclei
Divides into dorsal and ventral subgroups.
The ventral subgroup
- The ventroanterior nucleus connects with the basal ganglia.
- The ventrolateral nucleus connects with the cerebellum, red nucleus, and, to a lesser extent, the basal ganglia.
- The ventroposterior nucleus further divides into medial and lateral nuclei, both of which receive sensory afferents (medial from the face, lateral from the body) and project them to the somatosensory cortex.
- Sensory information in the thalamus has a very specific somatosensory map in which the fist is adjacent to the mouth.
- Small ventroposterior strokes in the lateral portion of the ventroposteromedial nucleus and medial portion of the ventroposterolateral nucleus result in the characteristic cheiro-oral syndrome in which there is loss of sensation around the mouth and in the fist contralateral to the side of the thalamic infarct.
- The ventroposterior inferior nucleus is a less commonly discussed nucleus.
The dorsal subgroup
- The dorsolateral nucleus: communicates with the limbic system (along with the anterior group nuclei).
- The lateral posterior nucleus, and posteriorly: visual attention.
Clinical Correlation: Dorsal Thalamic Stroke
- The pulvinar (part of the extrageniculate visual pathway).
- Important for visual attention.
The intralaminar group
- Most notably helps form the ascending arousal system for wakefulness.
The midline group nuclei
- Function in limbic-related processes and have important hippocampal connections.
Metathalamus
- Medial geniculate nucleus: underneath the medial aspect of the pulvinar.
- Part of the auditory system; it receives afferents from the inferior colliculus, which it projects to the transverse temporal gyri (of Heschl).
- Lateral geniculate nucleus: underneath the lateral aspect of the pulvinar.
- Involved in the visual system; it receives afferents from the optic tract, which it projects to the primary visual cortex.
Posterior group nuclei
- Span posteriorly from the caudal pole of the ventroposterior nucleus to the medial geniculate nucleus and they also extend medial to the medial geniculate nucleus.
- Posterior nuclear group has broad cortical connections: secondary somatosensory cortical projections of the posterior nucleus, which are involved in nociceptive sensory processing.
Thalamic reticular nucleus
- Forms a shell around the rostral/ventral/lateral thalamus.
- Modulates thalamic output through its GABAergic neurons.
- The posterior limb of the internal capsule lies lateral to the reticular nucleus.
- The external medullary lamina is a white matter layer that surrounds the lateral aspect of the thalamus.
USMLE, STEP 1
- Focus on the ventroposterior lateral and medial nuclei; lateral geniculate nucleus; medial geniculate nucleus; and ventral lateral nucleus.