Basal Ganglia Overview

Definition

• Basal ganglia (BG) or basal nuclei are subcortical nuclei in vertebrate brains.
• Located at the base of the forebrain and the top of the midbrain.
• Strong connections with the cerebral cortex, thalamus, brainstem, and other areas.

Functions

• Regulate voluntary motor movements.
• Involved in procedural learning and habit formation.
• Play a role in conditional learning, eye movements, cognition, and emotion.

Main Components

1. Striatum
   • Dorsal striatum: includes caudate nucleus and putamen.
   • Ventral striatum: composed of nucleus accumbens and olfactory tubercle.
   • Receives input from various brain areas, outputs to the basal ganglia.
2. Globus Pallidus
   • External (GPe) and internal (GPi) regions.
   • Sends inhibitory output to motor-related areas.
3. Substantia Nigra
   • Source of striatal dopamine input.
   • Consists of pars compacta (SNc) and pars reticulata (SNr).
4. Subthalamic Nucleus
   • Projects to the globus pallidus.
   • Only basal ganglia part that produces excitatory neurotransmitter, glutamate.

Role in Action Selection

• Inhibitory influence on motor systems; release of inhibition permits motor system activation.
• Involvement in both motor and cognitive action selection.

Disorders

• Dysfunction linked to neurological disorders such as Parkinson's disease, Huntington's disease, Tourette syndrome, OCD, and addiction.
• Dopamine-related disorders due to substantia nigra degeneration.

Development

• Central nervous system classified based on three brain vesicles: prosencephalon, mesencephalon, rhombencephalon.
• Basal ganglia development involves lateral and medial ganglionic eminences.

Connectivity

• Receives glutamatergic inputs and sends GABAergic outputs.
• Dopaminergic pathways from ventral tegmental area and substantia nigra.
• Multiple pathways: direct, indirect, and hyperdirect.

Clinical Significance

• Excessive or insufficient output from basal ganglia can cause hypokinetic or hyperkinetic disorders.
• Associated with several mental health conditions such as ADHD, schizophrenia, and major depressive disorder.

Anatomical and Historical Context

• Basal ganglia considered a major cerebral system.
• Historical evolution of the term and understanding of structures within the basal ganglia.
• Comparative anatomy shows presence across vertebrate species.