Corpus: Thalamus

The thalamus is the largest part of the diencephalon. It consists of numerous nuclei that have strong connections with the entire cerebral cortex. To become aware of sensory information, all ascending pathways — except for the olfactory pathway — must first relay through the thalamus on their way to the cortex. Hence, it is often referred to as the "gateway to consciousness."

The thalamus consists of two halves, each about the size of a pigeon’s egg, medially bordering the third ventricle and laterally adjacent to the internal capsule. The left and right thalamus are usually connected by the interthalamic adhesion.

The thalamus contains over 100 nuclei, which are somewhat controversially divided into two groups based on their cortical connections:

2.1. Specific thalamic nuclei

The specific thalamic nuclei have distinct afferent and efferent connections with defined areas of the cerebral cortex. Collectively, they are known as the palliothalamus. These nuclei receive sensitive and sensory impulses from the periphery and relay them to the cerebral cortex after processing.

Important specific nuclei include:

• Anterior and lateral ventral nuclei — processes motor signals
  • Anterior ventral nucleus (VA)
  • Lateral ventral nuclei (VL)
• Posterior ventral nucleus — relay point for sensory signals for proprioception and touch:
  • Posteromedial ventral nucleus (VPM)
  • Posterolateral ventral nucleus (VPL)
• Anterior nuclei of the thalamus — important for limbic system functions
• Medial nuclei of the thalamus — important for higher cognitive abilities
• Lateral geniculate body — important control center of the visual pathway
• Medial geniculate body — important control center of the auditory pathway
• Pulvinar — important for perception, memory, language, and recognition

2.2. Nonspecific thalamic nuclei

The non-specific thalamic nuclei have weak or no direct cortical connections. They receive information from the reticular formation, cerebellum, and basal ganglia and send outputs to the basal ganglia and specific thalamic nuclei. Collectively, they are known as the truncothalamus.

Important non-specific nuclei include:

• Intralaminar nuclei — crucial for controlling the state of consciousness, with the centromedian nucleus
• Midline nuclei — closely connected to the olfactory and limbic systems

2.3. Blood supply

The thalamus is supplied by the anterior and posterior thalamoperforating arteries, the anterolateral central arteries and the thalamogeniculate artery.

Damage to the thalamus primarily affects the contralateral side of the body and can result in:

• Loss of sensation
• Hemiparesis
• Ataxia
• Hemianopsia
• Clouding of consciousness
• Disturbed attention
• Pain syndrome ("thalamic pain")
• Personality changes
• Affective lability due to damage to the anterior thalamic nuclei (VA)

The typical constellation of these symptoms is referred to as thalamic syndrome.