Overview
This lecture covers the anatomical and functional classification of thalamic nuclei, their connections, main functions, and the clinical features of thalamic syndrome.
Anatomical Classification of Thalamic Nuclei
- The Y-shaped internal medullary lamina divides thalamic gray matter into anterior, medial, and lateral groups of nuclei.
- The anterior group contains the anterior nucleus.
- The medial group contains the dorsomedial and midline nuclei.
- The lateral group divides into dorsal (lateral dorsal, lateral posterior, pulvinar) and ventral (ventral anterior, ventral lateral, ventral posterior medial, ventral posterior lateral, medial and lateral geniculate bodies) subgroups.
- Intra-laminar nuclei, including the centromedian nucleus, are within the internal medullary lamina.
Functional Classification of Thalamic Nuclei
- Non-specific nuclei (intra-laminar, centromedian, midline) receive and project diffusely to CNS regions.
- Specific nuclei are divided into:
- Sensory relay nuclei: ventral posterolateral, ventral posteromedial, lateral geniculate (visual), medial geniculate (auditory).
- Motor control nuclei: ventral anterior, ventral lateral.
- Association (integrative) nuclei: pulvinar, lateral dorsal, lateral posterior.
- Limbic system nuclei: anterior and dorsomedial nuclei.
Thalamic Connections
- Non-specific nuclei receive input from the reticular activating system, paleospinothalamic tract, striatum, and hypothalamus, and project diffusely to the neocortex and limbic system.
- Medial geniculate body receives from lateral lemniscus/inferior colliculi and projects to auditory cortex (areas 41, 42).
- Lateral geniculate body receives from optic tract and projects to visual cortex (areas 17, 18, 19).
- Ventral posterior nucleus receives somatosensory inputs and projects to the postcentral gyrus (areas 1, 2).
- Ventral lateral receives from dentate nucleus (cerebellum) and globus pallidus, projects to primary motor cortex.
- Ventral anterior receives from globus pallidus/substantia nigra, projects to premotor cortex.
- Anterior nucleus is part of the Papez circuit, receives from mammillary body via mammillothalamic tract, projects to cingulate gyrus.
- Dorsal medial nucleus has reciprocal connections with prefrontal cortex (thinking, memory, judgment).
- Pulvinar integrates auditory and visual inputs, projects to parietal, occipital, superior temporal cortex.
- Lateral posterior receives from superior colliculi, projects to superior parietal cortex.
- Lateral dorsal receives from superior colliculi, projects to cingulate gyrus and parahippocampal gyrus.
Functions of the Thalamus
- Acts as the final sensory relay station (except olfactory).
- Integrates and modifies peripheral sensory impulses.
- Supports crude perception of sensations if cortical projections are cut.
- Integrates motor as well as sensory information.
- Involved in reflexes, arousal, and alertness due to reticular system connections.
- Supports emotional behavior via limbic system connections.
- Involved in language, EEG synchronization, and somatic-visceral integration.
Thalamic Syndrome
- Caused by blockage of the thalamogeniculate branch of the posterior cerebral artery.
- Results in loss/reduction of sensation, asteriognosis, and thalamic phantom limb (inability to localize limbs).
- Thalamic overreaction: exaggerated responses to pain, temperature, and touch.
- May present with ataxia, involuntary movements, and thalamic hand (wrist flexion, finger hyperextension).
Key Terms & Definitions
- Thalamus — major relay station in the brain for sensory and motor signals.
- Internal medullary lamina — Y-shaped white matter dividing thalamic nuclei.
- Intra-laminar nuclei — nuclei inside internal medullary lamina with diffuse connections.
- Pulvinar — thalamic nucleus integrating auditory and visual information.
- Papez circuit — neural circuit involved in emotion and memory.
- Asteriognosis — inability to recognize objects by touch.
- Thalamic hand — clinical sign with wrist flexion and finger hyperextension.
Action Items / Next Steps
- Review the auditory, visual, and ascending sensory pathways, and their connections with the thalamus.
- Study the Papez circuit and its role in memory and emotion.
- Understand clinical presentations and implications of thalamic syndrome.