The basal ganglia are a group of neurons (also called nuclei) located deep within the cerebral hemispheres of the brain. The basal ganglia consist of the corpus striatum (a major group of basal ganglia nuclei) and related nuclei. The basal ganglia are involved primarily in processing movement-related information. They also process information related to emotions, motivations, and cognitive functions. Basal ganglia dysfunction is associated with a number of disorders that influence movement including Parkinson's disease, Huntington disease, and uncontrolled or slow movement (dystonia).
Basal Nuclei Function
The basal ganglia and related nuclei are characterized as one of three types of nuclei. Input nuclei receive signals from various sources in the brain. Output nuclei send signals from the basal ganglia to the thalamus. Intrinsic nuclei relay nerve signals and information between the input nuclei and output nuclei. The basal ganglia receive information from the cerebral cortex and thalamus through input nuclei. After the information has been processed, it is passed along to intrinsic nuclei and sent to output nuclei. From the output nuclei, the information is sent to the thalamus. The thalamus passes the information on to the cerebral cortex.
Basal Ganglia Function: Corpus Striatum
The corpus striatum is the largest group of basal ganglia nuclei. It consists of the caudate nucleus, putamen, nucleus accumbens, and the globus pallidus. The caudate nucleus, putamen, and nucleus accumbens are input nuclei, while the globus pallidus is considered output nuclei. The corpus striatum uses and stores the neurotransmitter dopamine and is involved in the reward circuit of the brain.
- Caudate Nucleus: These C-shaped paired nuclei (one in each hemisphere) are located primarily in the frontal lobe region of the brain. The caudate has a head region that curves and extends forming an elongated body that continues to taper at its tail. The tail of the caudate ends in the temporal lobe at a limbic system structure known as the amygdala. The caudate nucleus is involved in motor processing and planning. It is also involved in memory storage (unconscious and long-term), associative and procedural learning, inhibitory control, decision making, and planning.
- Putamen: These large rounded nuclei (one in each hemisphere) are located in the forebrain and along with the caudate nucleus form the dorsal striatum. The putamen is connected to the caudate nucleus at the head region of the caudate. The putamen is involved in voluntary and involuntary motor control.
- Nucleus Accumbens: These paired nuclei (one in each hemisphere) are located between the caudate nucleus and putamen. Along with the olfactory tubercle (sensory processing center in the olfactory cortex), the nucleus accumbens forms the ventral region of the striatum. The nucleus accumbens is involved in the brain's reward circuit and behavior mediation.
- Globus Pallidus: These paired nuclei (one in each hemisphere) are located near the caudate nucleus and putamen. The globus pallidus is divided into internal and external segments and acts as one of the major output nuclei of the basal ganglia. It sends information from basal ganglia nuclei to the thalamus. The internal segments of the pallidus send the majority of output to the thalamus via the neurotransmitter gamma-aminobutyric acid (GABA). GABA has an inhibitory effect on motor function. The external segments of the pallidus are intrinsic nuclei, relaying information between other basal ganglia nuclei and internal segments of the pallidus. The globus pallidus is involved in the regulation of voluntary movement.
Basal Ganglia Function: Related Nuclei
- Subthalamic Nucleus: These small paired nuclei are a component of the diencephalon, located just below the thalamus. Subthalamic nuclei receive excitatory inputs from the cerebral cortex and have excitatory connections to the globus pallidus and substantia nigra. Subthalamic nuclei have both input and output connections to the caudate nucleus, putamen, and substantia nigra. The subthalamic nucleus plays a major role in voluntary and involuntary movement. It is also involved in associative learning and limbic functions. Subthalamic nuclei have connections with the limbic system through connections with the cingulate gyrus and nucleus accumbens.
- Substantia Nigra: This large mass of nuclei is located in the midbrain and is also a component of the brainstem. The substantia nigra is composed of the pars compacta and the pars reticulata. The pars reticulata segment forms one of the major inhibitory outputs of the basal ganglia and assists in the regulation of eye movements. The pars compacta segment is composed of intrinsic nuclei that relay information between input and output sources. It is involved mainly in motor control and coordination. Pars compacta cells contain pigmented nerve cells that produce dopamine. These neurons of the substantia nigra have connections with the dorsal striatum (caudate nucleus and putamen) supplying the striatum with dopamine. The substantia nigra serves numerous functions including controlling voluntary movement, regulating mood, learning, and activity related to the brain's reward circuit.
Basal Ganglia Disorders
Dysfunction of basal ganglia structures results in several movement disorders. Examples of these disorders include Parkinson's disease, Huntington disease, dystonia (involuntary muscle contractions), Tourette syndrome, and multiple system atrophy (neurodegenerative disorder). Basal ganglia disorders are commonly the result of damage to the deep brain structures of the basal ganglia. This damage may be caused by factors such as head injury, drug overdose, carbon monoxide poisoning, tumors, heavy metal poisoning, stroke, or liver disease.
Individuals with basal ganglia dysfunction may exhibit difficulty in walking with uncontrolled or slow movement. They may also exhibit tremors, problems controlling speech, muscle spasms, and increased muscle tone. Treatment is specific to the causation of the disorder. Deep brain stimulation, electrical stimulation of targeted brain areas, has been used in the treatment of Parkinson's disease, dystonia, and Tourette syndrome.
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- Parr-Brownlie, Louise C., and John N.J. Reynolds. “Basal Ganglia.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 19 June 2016.
- Wichmann, Thomas, and Mahlon R. DeLong. “Deep-Brain Stimulation for Basal Ganglia Disorders.” Basal Ganglia, U.S. National Library of Medicine, 1 July 2011.