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Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics
in 2 minutes or less. In this installment I will discuss the basal ganglia.
The basal ganglia are a group of structures found deep within the cerebral hemispheres
and the brainstem that include the caudate, putamen, globus pallidus, substantia nigra,
and subthalamic nucleus. The caudate and putamen are often referred to collectively as the
striatum, and the globus pallidus and substantia nigra are each made up of multiple nuclei.
Although there are a variety of nonmotor functions associated with the basal ganglia, they are
best known for their role in facilitating movement.
Much of the information the basal ganglia receives comes from the cerebral cortex and
travels first to the caudate or putamen, the main input nuclei of the basal ganglia. The
globus pallidus and substantia nigra are the main output nuclei, and they send projections
out from the basal ganglia to the cerebral cortex, mostly by way of the thalamus, as
well as to nuclei in the brainstem.
Activity in the nuclei of the basal ganglia doesn’t cause movement independently, but
instead the basal ganglia influence activity in other areas of the brain like the motor
cortex to affect movement. The ways in which the basal ganglia do this are not fully understood,
but one hypothesis is that there are different circuits in the basal ganglia that promote
and inhibit movement, respectively. According to this model, the main output of the basal
ganglia is inhibitory and the neurons in the globus pallidus are constantly inhibiting
the thalamus to prevent unwanted movements.
When a signal to initiate movement is sent from the cortex to the basal ganglia it follows
a circuit in the basal ganglia known as the direct pathway, which leads to the silencing
of neurons in the globus pallidus. This frees the thalamus from the inhibitory effects of
the globus pallidus and allows movement to occur. There is also a circuit within the
basal ganglia called the indirect pathway, which involves the subthalamic nucleus and
leads to increased suppression of unwanted movements. It is thought that a balance between
activity in these two pathways may allow for smooth movement.