The Diencephalon

The thalamus

This complex structure figure 10serves to process all sensory input (except olfactory) to the cortex, but it also has profound influence on motor (via input from basal ganglia and cerebellum) and cognitive function. Although critically situated, the functions of the thalamus are not well understood. We will mention only the most well-defined clinico-pathologic correlations:

Sensory function:

• Visual input from the optic tract relays in the lateral geniculate nucleus (LGN): lesions result in hemianopia (see figure 11).
• Auditory input from the lateral lemniscus relays in the medial geniculate nucleus. Unilateral lesions have little effect on hearing, because auditory information from each ear ascends bilaterally.
• Somatosensory input from both the posterior column/medial lemniscus system for position and vibration and the spinothalamic system for pain and temperature relay in the thalamus (VPL and VPM nuclei). Lesions affecting this part of the thalamus can therefore cause loss of all sensation on one side of the body. Paradoxically, some patients experience abnormally painful sensations (Athalamic pain@) on the anesthetic side.

Motor function

Thalamic strokes are not known for their motor manifestations, but interruption of the cerebellar input to VA and VL may result in ataxia, and interruption of basal ganglia input (to these same thalamic nuclei, VA and VL) may result in akinesia.

Cognitive function:

• Arousal: bilateral lesions affecting the intralaminar thalamic nuclei, which can be considered extensions of the brainstem reticular formation, can cause unresponsiveness, but the eyes remain open. This has been called coma vigil or akinetic mutism.
• Memory: Lesions affecting medial thalamic structures (the confluence of mammillothalamic and amygdalofugal tracts, dorsomedial and possibly anterior nuclei) can cause profound amnesia.
• Other cognitive functions: aphasia, neglect and visuospatial dysfunction have been described with thalamic lesions, and presumably relate to interruption of reciprocal thalamic connections with the cerebral cortex.

The hypothalamus

The hypothalamus exerts control over the pituitary gland and thus over endocrine function in general, and it has extensive connections with brainstem autonomic nuclei. Lesions of the hypothalamus affect appetite, emotional behavior, temperature control, and numerous other autonomic and endocrine-influenced behaviors.