Basal ganglia: structure and pathways

Anatomy explained

A deeper look at the brain’s action-selection system

A single purposeful action depends on many smaller movements happening in the right order. The basal ganglia help the brain select useful patterns while reducing competing or unnecessary activity.

Imagine reaching for a plate. The brain must coordinate the arm, hand, fingers, posture, and timing. At the same time, it must suppress movements that would interfere with the task. The basal ganglia contribute to this selection process, working with the cortex and cerebellum rather than controlling movement alone.

What are the basal ganglia?

The basal ganglia are groups of grey-matter nuclei located deep in the cerebral hemispheres and nearby parts of the diencephalon and midbrain. Although the everyday term is “basal ganglia”, “basal nuclei” is more precise: in central nervous system anatomy, clusters of neuron cell bodies are generally called nuclei.

These nuclei form loops with the cerebral cortex. Signals travel into the system, are processed through several connected stations, and return to cortical areas through the thalamus. Different loops are associated with movement, learning, motivation, and aspects of planning.

The main parts

The group is best understood as a network rather than a list of isolated structures. Its commonly discussed components include:

The corpus striatum

The corpus striatum, or striatum in a shorter form, is a major receiving station for information from the cortex. It is commonly divided into dorsal and ventral regions.

Caudate nucleus

• There is one caudate nucleus in each cerebral hemisphere.
• Its curved shape is often described as having a head, body, and tail.
• It receives information from several cortical regions and participates in action selection, learning, memory, and goal-directed behaviour.

Putamen

• The putamen lies beside the caudate nucleus and, with it, forms the dorsal striatum.
• It is strongly connected with motor and sensory regions of the cortex.
• It contributes to the selection and refinement of learned movement patterns.

Nucleus accumbens

• The nucleus accumbens is part of the ventral striatum.
• It receives information from limbic and cortical regions involved in motivation, emotion, and learning.
• It helps connect the expected value of an outcome with a possible action.

Globus pallidus

• The globus pallidus contains external and internal divisions.
• It is an important output station of the basal ganglia.
• Many of its signals use GABA, an inhibitory neurotransmitter, to regulate activity in connected targets.

Subthalamic nucleus and substantia nigra

The subthalamic nucleus sits below the thalamus and communicates with the globus pallidus, substantia nigra, cortex, and other structures. Its activity can increase inhibitory output from the basal ganglia, helping limit competing actions.

The substantia nigra is located in the midbrain. Its pars compacta contains dopamine-producing neurons that influence the striatum, while the pars reticulata is an output region with functions that overlap with those of the internal globus pallidus.

Direct and indirect pathways

The direct and indirect pathways are simplified teaching models for describing how basal ganglia circuits influence movement. They should be understood as interacting routes, not as two completely separate wires.

Direct pathway: supporting a selected action

1. Cortical signals activate parts of the striatum.
2. The striatum inhibits the internal globus pallidus and related output regions.
3. This reduces inhibitory signalling to the thalamus.
4. The thalamus can then provide stronger excitatory input to motor areas of the cortex.

Indirect pathway: suppressing competing actions

1. Other striatal signals influence the external globus pallidus.
2. The external globus pallidus changes its inhibition of the subthalamic nucleus.
3. The subthalamic nucleus can increase activity in basal ganglia output regions.
4. Greater output inhibition of the thalamus helps reduce competing motor commands.

Dopamine from the substantia nigra changes the responsiveness of striatal pathways. The overall result is a carefully balanced system that helps actions begin at the right time and remain appropriately focused.

Why these pathways matter

Because the basal ganglia participate in movement, learning, and motivation, changes in their circuits can affect more than one function. Parkinson’s disease is associated with altered dopamine signalling and difficulty initiating and scaling movement. Huntington’s disease involves progressive changes in neural circuits and can affect movement, thinking, and behaviour.

These examples are medical conditions, not diagnoses that can be made from a webpage. A qualified healthcare professional should assess persistent or concerning changes in movement, coordination, speech, or cognition.