What is the composition and function of the basal ganglia?

Anatomy and Function of the Basal Ganglia

The basal ganglia consists of the striatum (caudate nucleus and putamen), globus pallidus (internal and external segments), substantia nigra, and subthalamic nucleus—not just the striatum and lenticular nucleus. 1, 2, 3, 4

Anatomical Structure of the Basal Ganglia

The basal ganglia is a complex of subcortical nuclei comprising:

1. Striatum

   • Caudate nucleus
   • Putamen
   • Nucleus accumbens (ventral striatum)

2. Globus Pallidus

   • External segment (GPe)
   • Internal segment (GPi)

3. Substantia Nigra

   • Pars compacta (SNc)
   • Pars reticulata (SNr)

4. Subthalamic Nucleus

The lenticular nucleus is actually a subcomponent consisting of the putamen and globus pallidus together 2, 4.

Functional Organization

Input and Output Pathways

• Input: The striatum serves as the primary input station, receiving glutamatergic projections from nearly all areas of the neocortex and thalamic nuclei 2.

• Output: The GPi and SNr function as the output stations, projecting primarily to the thalamus, superior colliculus, and pedunculopontine nucleus 2, 3.

• Modulation: Dopaminergic input from the SNc modulates striatal neuronal activity 4.

Parallel Circuits

The basal ganglia contains two major pathways:

1. Direct Pathway: Striatal neurons project directly to the output nuclei (GPi/SNr) 2, 4.

2. Indirect Pathway: Striatal neurons project to the GPe, which connects to the subthalamic nucleus, which then projects to the output nuclei (GPi/SNr) 2, 4.

These pathways work together to regulate the net activity of the basal ganglia and provide processed information to output targets like the thalamus 4.

Functional Roles

The basal ganglia are involved in:

1. Motor Control: Regulation of voluntary and involuntary movements 5, 3

   • Movement initiation and execution
   • Motor learning
   • Suppression of unwanted movements

2. Cognitive Functions 5, 3

   • Executive functions
   • Behavioral control
   • Cognitive planning of purposive motor acts
   • Shifting attentional sets
   • Spatial working memory

3. Emotional Processing 5, 3

   • Motivation
   • Reinforcement learning
   • Predicting future events
   • Suppressing unwanted behavior

Clinical Significance

Dysfunction of the basal ganglia can manifest as:

1. Movement Disorders 1, 3

   • Parkinson's disease (resting tremor, bradykinesia, rigidity)
   • Huntington's disease
   • Wilson's disease
   • Dystonia

2. Behavioral Disorders 3

   • Abulia (apathy with loss of initiative)
   • Alterations in emotional responses

3. Imaging Findings in Pathological States 6, 7

   • T2 hyperintensity and diffusion restriction in conditions like CJD
   • Signal abnormalities in the basal ganglia, particularly the caudate and putamen, in various disorders
   • The "face of the giant panda" sign in Wilson's disease

Diagnostic Considerations

MRI is the optimal imaging modality for evaluating basal ganglia abnormalities, with diffusion-weighted and T2-FLAIR sequences being most sensitive 6, 7.

Abnormal findings in the basal ganglia may include:

• Hyperintensity on T2-weighted images
• Diffusion restriction
• Volume changes
• Calcifications or iron deposition (hypointense on GRE/T2*/SWI)

These findings can help diagnose various conditions affecting the basal ganglia, including neurodegenerative disorders, metabolic disorders, and infectious diseases 7.