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The basal ganglia (or basal nuclei) are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus, and other brain areas.
The basal ganglia are associated with a variety of functions, including voluntary motor control, procedural learning relating to routine behaviors or habits such as bruxism and eye movements, as well as cognitive and emotional functions.
Currently popular theories hold that the basal ganglia play a primary role in action selection. Action selection is the decision of which of several possible behaviors to execute at a given time.
Experimental studies show that the basal ganglia exert an inhibitory influence on a number of motor systems, and that a release of this inhibition permits a motor system to become active. The behavior switching that takes place within the basal ganglia is influenced by signals from many parts of the brain, including the prefrontal cortex, which plays a key role in executive functions.
The greatest source of insight into the functions of the basal ganglia has come from the study of two neurological disorders, Parkinson's disease and Huntington's disease. For both of these disorders, the nature of the neural damage is well-understood and can be correlated with the resulting symptoms.
Parkinson's disease involves the major loss of dopaminergic cells in the substantia nigra. Huntington's disease involves the massive loss of medium spiny neurons in the striatum.
The symptoms of the two diseases are virtually opposite: Parkinson's disease is characterized by a gradual loss of the ability to initiate movement, whereas Huntington's disease is characterized by an inability to prevent parts of the body from moving unintentionally.
It is noteworthy that, although both diseases have cognitive symptoms, especially in their advanced stages, the most salient symptoms relate to the ability to initiate and control movement. Thus, both are classified primarily as movement disorders.
A different movement disorder, called hemiballismus, may result from damage restricted to the subthalamic nucleus. Hemiballismus is characterized by violent and uncontrollable flinging movements of the arms and legs.
Function in Eye Movement
One of the most intensively studied functions of the basal ganglia is their role in controlling eye movements. Eye movement is influenced by an extensive network of brain regions that converge on a midbrain area called the superior colliculus (SC).
The SC is a layered structure whose layers form two-dimensional retinotopic maps of visual space. A bump of neural activity in the deep layers of the SC drives eye movement toward the corresponding point in space.
Although the role of the basal ganglia in motor control is clear, there are also many indications that it is involved in the control of behavior in a more fundamental way, at the level of motivation. In Parkinson's disease, the ability to execute the components of movement is not greatly affected, but motivational factors such as hunger fail to cause movements to be initiated or switched at the proper times.
The immobility of patients with Parkingson's disease has sometimes been described as a paralysis of the will. These patients have occasionally been observed to show a phenomenon called kinesia paradoxica, in which a person who is otherwise immobile responds to an emergency in a coordinated and energetic way, then lapses back into immobility once the emergency has passed.
The role in motivation of the limbic part of the basal ganglia—the nucleus accumbens (NA), ventral pallidum, and ventral tegmental area (VTA)—is particularly well established. Thousands of experimental studies combine to demonstrate that the dopaminergic projection from the VTA to the NA plays a central role in the brain's reward system.
Numerous things that people find rewarding, including addictive drugs, good-tasting food, and sex, have been shown to elicit activation of the VTA dopamine system. Damage to the NA or VTA can produce a state of profound torpor.
In most regions of the
brain, the predominant classes of neurons use glutamate as the neurotransmitter and have excitatory effects on their targets. In the basal ganglia, however,
the great majority of neurons uses gamma-aminobutyric acid (GABA) as the neurotransmitter
and have inhibitory effects on their
The inputs from the cortex and thalamus to the striatum and subthalamic
nucleus are glutamatergic, but the outputs from the striatum, pallidum, and
substantia nigra pars reticulata all use GABA. Thus, following the
initial excitation of the striatum, the internal dynamics of the basal ganglia
are dominated by inhibition and disinhibition.
have important modulatory effects. Dopamine is used by the
projection from the substantia nigra pars compacta to the dorsal
striatum and also in the analogous projection from the ventral tegmental area
to the ventral striatum (nucleus accumbens).
Acetylcholine also plays an
important role, as it is used both by several external inputs to the striatum and
by a group of striatal interneurons. Although cholinergic cells make up only a
small fraction of the total population, the striatum has one of the highest
acetylcholine concentrations of any brain structure.
Source: Boundless. “The Role of the Basal Ganglia in Movement.” Boundless Anatomy and Physiology. Boundless, 12 Oct. 2016. Retrieved 24 Oct. 2016 from https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/peripheral-nervous-system-13/motor-pathways-135/the-role-of-the-basal-ganglia-in-movement-724-8216/