The basal ganglia are responsible for voluntary motor control, procedural learning, and eye movement, as well as cognitive and emotional functions.
Describe the role of the basal ganglia in movement
- The basal ganglia are studied extensively in the context of two disorders of the basal ganglia: Parksinson’s disease and Huntington’s disease.
- Hemiballismus, a movement disorder arising from neuronal damage in the subthalamic nucleus, presents with violent movements of the arms and legs.
- Eye movement, a function of the basal ganglia, is influenced by the superior colliculus, a region of the brain that directs eye movement to specific points in space in response to stimuli.
- Basal ganglia are also thought to play a role in motivation.
- In the basal ganglia,
the majority of the neurons uses gamma-aminobutyric acid (GABA) as the neurotransmitter
and have inhibitory effects on their
- hemiballismus: A rare movement disorder with involuntary flinging motions of the extremities.
- voluntary motor control: The act of directing motion with intent.
- forebrain: The anterior part of the brain, including the cerebrum, thalamus, and hypothalamus.
- nucleus accumbens: A region in the basal forebrain rostral to the preoptic area of the hypothalamus. This region and the olfactory tubercle collectively form the ventral striatum.
Location of the Basal Ganglia
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.
Basal ganglia: Locations of the basal ganglia.
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 targets.
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.
Other neurotransmitters 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.
Main circuits of the basal ganglia: This diagram shows the main circuits of the basal ganglia. Two coronal slices have been superimposed to include the involved basal ganglia structures. The + and – signs at the point of the arrows indicate whether the pathway is excitatory or inhibitory, respectively, in effect. Green arrows refer to excitatory glutamatergic pathways, red arrows refer to inhibitory GABAergic pathways and turquoise arrows refer to dopaminergic pathways that are excitatory on the direct pathway and inhibitory on the indirect pathway.