Basal Ganglia and Motor Control System

Basal Ganglia

• The basal ganglia are a group of nuclei that act as a unified functional unit, closely associated with the cerebral cortex and corticospinal motor control system.

Structure

• The basal ganglia consist of four principal nuclei:
  • Striatum (input nucleus)
  • Globus pallidus (output nucleus)
  • Subthalamic nucleus
  • Substantia nigra

Striatum

• The striatum is the input nucleus of the basal ganglia, receiving afferent projections from the cerebral cortex.
• Three subnuclei comprise the striatum:
  • Caudate nucleus (participates in eye movement control and cognition)
  • Putamen (participates in control of limb and trunk movements)
  • Nucleus accumbens (participates in emotions)

Globus Pallidus

• The globus pallidus consists of two separate nuclei: the external and internal segments.
• The internal segment is one of the major output structures of the basal ganglia, while the external segment is part of their intrinsic circuitry.

Substantia Nigra

• The substantia nigra includes two separate nuclei: the pars compacta and pars reticulata.
• The pars compacta contains dopaminergic cells that project heavily to the striatum.

Subthalamic Nucleus

• The subthalamic nucleus is a small nucleus situated between the thalamus and the substantia nigra.
• It receives projections from the external segment of the globus pallidus, the cerebral cortex, thalamus, and brain stem, and sends output to both segments of the globus pallidus and to the substantia nigra pars reticulata.

Basal Ganglia Input

• The basal ganglia receive input from:
  • Parietal cortex (primary and secondary somatosensory information, secondary visual information)
  • Temporal cortex (secondary visual and auditory information)
  • Cingulate cortex (limbic and emotional status information)
  • Frontal cortex (primary and secondary motor information)
  • Prefrontal cortex

Cortico-Basal Ganglia-Thalamocortical Circuits

• There are two important pathways through which striatal information reaches the globus pallidus (internal): the direct pathway and the indirect pathway.
• The direct pathway facilitates movement, while the indirect pathway inhibits movement.

Functions of Basal Ganglia

• The basal ganglia are involved in:
  • Voluntary movement
  • Postural control
  • Control of muscle tone
  • Role in arousal mechanism
  • Speech
  • Cognitive control of motor activity
  • Planning and programming of movements
  • Timing and scaling of intensity of movements
  • Subconscious execution of some movements

Lesions of Basal Ganglia

• Lesions of different nuclei can result in:
  • Globus pallidus: athetosis (spontaneous writing movements of the hand, arm, neck, and face)
  • Putamen: chorea (flicking movements of the hands, face, and shoulders)
  • Substantia nigra: Parkinson's disease (rigidity, tremor, and akinesia)
  • Subthalamus: hemiballismus (sudden flailing movements of the entire limb)
  • Caudate nucleus and putamen: Huntington's chorea (loss of GABA-containing neurons to globus pallidus and substantia nigra)