Basal Ganglia

Deep within the cerebral hemispheres are five nuclei (masses of grey matter) collectively known as the basal ganglia. They are:

• Globus pallidus and putamen (otherwise known as the lentiform nucleus).
• Caudate nucleus (the putamen and caudate nucleus are collectively known as the striatum)
• Substantia nigra (although located in the midbrain technically)
• Subthalmic nuclei (although located in the midbrain technically)
• Receives input from the cerebral cortex and provides output back to motor areas via the thalamus
• Major function is to help initiate and terminate movements of the body
• Also suppresses unwanted movements and regulates muscle tone.

Role of the Basal Ganglia in Motor Control

In a resting person the internal segment of the globus pallidus (GPi) and the pars reticulate of the substantia nigra (in the midbrain) sends spontaneous inhibitory signals to the thalamus, through the inhibitory neurotransmitter GABA. Inhibition of the excitatory neurones in the thalamus leads to a reduction of activity in the motor cortex’s and therefore a lack of muscular activity.

When the pre frontal region of the cerebral cortex (decision making and planning) initiates movement it sends signals to the motor cortex’s. These then send signals through the basal ganglia in order to decide which muscles will participate in the movement.

Direct loop ALLOWS MOVEMENT

• The motor cortex’s send signals to inhibitory cells of the striatum (caudate and putamen)
• They then send inhibitory signals to the internal segment of the globus pallidus (GPi) and the substantia nigra (with the inhibitory neurotransmitter GABA)
• These inhibitory signals then stop the original spontaneous inhibitory signals (that usually happen in a resting person) meaning that the thalamus is now free to excite the motor cortex’s
• This accelerates motor cortex activity and will eventually result in muscle contractions
• Therefore two minus make a plus!!

Indirect loop INHIBITS UNWANTED MOVEMENT

• Operates in conjunction with the direct loop to help prevent unwanted muscle contractions from competing with voluntary movements.
• At the same time that signals are being sent through the direct loop, the motor cortex’s send signals to inhibitory cells of the indirect loop in the striatum (caudate and putamen)
• These cells then send inhibitory signals to the external segment of the globus pallidus (GPe) reducing activity in that area.
• The GPe normally send inhibitory signals to the Subthalmic nucleus
• When the indirect loop is activated these inhibitory signals are reduced which leaves the Subthalmic nucleus free to excite the GPi and the substantia nigra
• Which in turn are then free to send more inhibitory signals to the thalamus, which prevents the development of activity in the motor cortex
• This loop prevents the activation of motor cortical areas that would compete with the voluntary movement
• Therefore preventing unwanted muscle contractions (BOOM!!!)

Parkinson’s – neurons in the substantia nigra release dopamine. The loss of these neurones and therefore the loss of dopamine results in overactive inhibition of the thalamus, causing bradykinesia (because the substantia nigra sends to many spontaneous signals to the thalamus which reduce muscular activity). Dopamine is also a modulatory neurotransmitter and the loss of it results in a change in tone, resulting in rigidity.

Huntington’s – inhibitory signals (GABA) from the striatum to the GPe (indirect) and the GPi and substantia nigra (direct) are damaged. Damage to the indirect loop leads to chorea (random purposeless movements) and damage to the direct loop results in increased inhibition of the thalamus which results in bradykinesia and rigidity.