What is the anatomy and function of the basal ganglia in the brain?

Anatomy of the Basal Ganglia

Structural Components

The basal ganglia consist of five interconnected subcortical nuclei in the basal forebrain: the striatum (caudate nucleus, putamen, and nucleus accumbens), the globus pallidus (external and internal segments), the substantia nigra (pars compacta and pars reticulata), and the subthalamic nucleus. 1

Primary Nuclei

• Striatum: The main input structure of the basal ganglia, receiving glutamatergic projections from all cortical areas 2, 1

  • Caudate nucleus and putamen form the dorsal striatum
  • Nucleus accumbens comprises the ventral striatum
  • Contains GABAergic medium spiny neurons whose excitability is controlled by pre- and postsynaptic mechanisms and interneuron activity 3

• Globus Pallidus: Divided into external (GPe) and internal (GPi) segments 3

  • GPe occupies a pivotal position in regulating motor output of the basal ganglia 3
  • GPi serves as the primary output nucleus, projecting to the motor thalamus and then to cortex 3, 1

• Substantia Nigra: Contains two functionally distinct parts 1

  • Pars compacta provides dopaminergic input to the striatum
  • Pars reticulata functions as an output nucleus alongside GPi 1

• Subthalamic Nucleus (STN): Serves as one of two entry points into the basal ganglia motor circuit (along with the striatum) 3

Functional Circuitry

Direct and Indirect Pathways

The basal ganglia operate through two opposing pathways that create a center-surround mechanism to focus motor output on specific muscle groups required for performing specific tasks. 4

• Direct Pathway: Disynaptic projections from cortex through striatum to GPi, resulting in disinhibition of thalamic motor nuclei and net facilitation of motor cortex activity 3, 1

• Indirect Pathway: Trisynaptic projections from cortex through striatum and GPe to GPi, exerting inhibitory effects on motor output 3

• Hyperdirect Pathway: Direct cortico-subthalamic projections provide rapid excitatory input to the STN 3

Parallel Processing Loops

The basal ganglia form 4-5 distinct parallel circuits with the cortex and thalamus, maintaining somatotopic organization throughout, allowing simultaneous processing of motor, associative, limbic, and oculomotor functions. 2, 4

• Motor circuit: Projects primarily to supplementary motor area, premotor cortex, and motor cortex 2
• Associative circuit: Connects with prefrontal cortex for executive functions, working memory, and motor planning 2
• Limbic circuit: Involves cingulate cortex for emotional and motivational processing 2
• Oculomotor circuit: Controls eye movements and visual attention 2

Neurotransmitter Systems

• Glutamate: Excitatory input from cortex to striatum and STN 1
• GABA: Primary inhibitory neurotransmitter in striatal output neurons and GPi/GPe projections 3
• Dopamine: Modulates striatal activity and fine-tunes the balance between direct and indirect pathways; also modulates GPe, GPi, and STN activity 3, 1
• Acetylcholine: Released by striatal interneurons to influence medium spiny neuron excitability 1

Functional Roles

The basal ganglia serve as a sensory gating device for motor control, opening sensory channels for expected sensory feedback during movement while suppressing irrelevant information. 4

Motor Control Functions

• Facilitate motor activity by disinhibiting thalamic motor nuclei, which tonically inhibit motor cortex 1
• Regulate both automatic and voluntary motor responses by providing information to the pyramidal system 2
• Predict future events and reinforce wanted behavior while suppressing unwanted behavior 2
• Control movement initiation and spatial working memory 2

Non-Motor Functions

• Associative learning and executive functions 3, 5
• Attention shifting and behavior-guiding rule potentiation 2
• Emotional and motivational processing 3, 2
• Planning and working memory 3

Clinical Relevance

Disruption of basal ganglia circuitry produces movement disorders characterized by involuntary movements, stereotyped movements, or paucity of movement without affecting voluntary motor pathways. 2

• Lesions produce symptoms based on specific anatomical location: abulia (apathy with loss of initiative) and dystonia are the most common behavioral and motor disturbances respectively 2
• Imaging abnormalities most commonly involve the anterior caudate and putamen (60% of choreiform disorders) 6, 7
• MRI without contrast, including susceptibility-weighted sequences, is the optimal imaging modality for evaluating basal ganglia disorders 6