Definition of Basal Ganglia

Definition of Basal Ganglia

Basal Ganglia: An area of the forebrain that is important to smooth muscle movement and actions. This area works in conjunction with the midbrain to help us avoid moving in choppy, fragmented ways.

The basal ganglia are a collection of nuclei found on both sides of the thalamus, outside and above the limbic system, but below the cingulate gyrus and within the temporal lobes. Although glutamate is the most common neurotransmitter here as everywhere in the brain, the inhibitory neurotransmitter GABA plays the most important role in the basal ganglia.

The largest group of these nuclei are called the corpus striatum ("striped body"), made up of the caudate nucleus ("tail"), the putamen ("shell"), the globus pallidus ("pale globe"), and the nucleus accumbens ("leaning"). All of these structures a double ones, one set on each side of the central septum.

The caudate begins just behind the frontal lobe and curves back towards the occipital lobe. It sends its messages to the frontal lobe (especially the orbital cortex, just above the eyes), and appears to be responsible for informing us that something is not right and we should do something about it:

Wash your hands! Lock your door! As these examples are meant to suggest, obsessive compulsive disorder (OCD) is likely to involve an overactive caudate. On the other hand, an underactive caudate may be involved in various disorders, such as ADD, depression, aspects of schizophrenia, and just plain lethargy. It is also involved in PAP syndrome, a dramatic loss of motivation only recently discovered.

The putamen lies just under and behind the front of the caudate. It appears to be involved in coordinating automatic behaviors such as riding a bike, driving a car, or working on an assembly line. Problems with the putamen may account for the symptoms of Tourette's syndrome.

The globus pallidus is located just inside the putamen, with an outer part and an inner part. It receives inputs from the caudate and putamen and provides outputs to the substantia nigra.

The nucleus accumbens is a nucleus just below the previous nuclei. It receives signals from the prefrontal cortex (via the ventral tegmental area) and sends other signals back there via the globus pallidus. The inputs use dopamine, and many drugs are known to greatly increase these messages to the nucleus accumbens.

Another nucleus of the basal ganglia is the substantia nigra ("black substance"). Located in the upper portions of the midbrain, below the thalamus, it gets its color from neuromelanin, a close relative of the skin pigment. One part (the pars compacta) uses dopamine neurons to send signals up to the striatum. The exact function isn't known, but is believed to involve reward circuits. Also, Parkinson's disease is due to the death of dopamine neurons here.

The basal ganglia (or basal nuclei) are a group of nuclei of varied origin (mostly telencephalic embryonal origin, with some diencephalic and mesencephalic elements) 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, eye movements, and cognitive, emotional functions.

Currently popular theories implicate the basal ganglia primarily in action selection, that 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 main components of the basal ganglia are the striatum (also called neostriatum) composed of caudate and putamen, globus pallidus or pallidum composed of globus pallidus externa (GPe) and globus pallidus interna (GPi), substantia nigra composed of both substantia nigra pars compacta (SNc) and substantia nigra pars reticulata (SNr), and the subthalamic nucleus (STN). The largest component, the striatum, receives input from many brain areas but sends output only to other components of the basal ganglia.

The pallidum receives its most important input from the striatum (either directly or indirectly), and sends inhibitory output to a number of motor-related areas, including the part of the thalamus that projects to the motor-related areas of the cortex. One part of substantia nigra, the reticulata (SNr), functions similarly to the pallidum, and another part (compacta or SNc) provides the source of the neurotransmitter dopamine's input to the striatum. The subthalamic nucleus (STN) receives input mainly from the striatum and cortex, and projects to a portion of the pallidum (interna portion or GPi). Each of these areas has a complex internal anatomical and neurochemical organization.

The basal ganglia play a central role in a number of neurological conditions, including several movement disorders. The most notable are Parkinson's disease, which involves degeneration of the melanin-pigmented dopamine-producing cells in the substantia nigra pars compacta (SNc), and Huntington's disease, which primarily involves damage to the striatum.

Basal ganglia dysfunction is also implicated in some other disorders of behavior control such as the Tourette's syndrome, ballismus (particularly hemibalismus), obsessive–compulsive disorder (OCD), and Wilson's disease (Hepatolenticular degeneration); except for Wilson's disease and hemiballismus, the neuropathological mechanisms underlying diseases of ganglia such as Parkinsons' and Huntington's are not very well understood or are at best still developing theories.[citation needed]

The basal ganglia have a limbic sector whose components are assigned distinct names: the nucleus accumbens (NA), ventral pallidum, and ventral tegmental area (VTA). VTA efferents provide dopamine to the nucleus accumbens (ventral striatum) in the same way that the substantia nigra provides dopamine to the dorsal striatum. Because there is much evidence that it plays a central role in reward learning, the VTA→NA dopaminergic projection has attracted a great deal of attention.

For example, a number of highly addictive drugs, including cocaine, amphetamines, and nicotine, are thought to work by increasing the efficacy of the VTA→NA dopamine signal. There is also evidence implicating overactivity of the VTA dopaminergic projection in schizophrenia.