Basal ganglia

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Revision as of 21:55, 21 April 2008

The basal ganglia are a collection of nuclei found on both sides of the thalamus, outside and above the limbic system, below the cingulate gyrus, and within the temporal lobes. The basal ganglia are associated with movement, cognition, emotions, and learning. There are two sets of basal ganglia in the brain, one in the right hemisphere and one in the left. The basal ganglia are comprised of a series of circuits that project to specific nuclei within the basal ganglia. The largest group of these nuclei is called the corpus striatum. The corpus striatum is made up of the caudate nucleus, the putamen, the globus pallidus, and the nucleus accumbens. Basal ganglia video

The caudate nucleus sends messages to the frontal lobe and is responsible for informing one that something is not right and that one should formulate a proper action to fix the problem. Obsessive compulsive disorder is a result of an overactive caudate, whereas ADD, depression, and lethargy are all results of an underactive caudate. The putamen is involved in coordinating automatic behaviors such as riding a bike. The globus pallidus receives inputs from the caudate and putamen and provides outputs to the substantia nigra which acts to facilitate the movement. The globus pallidus consists of two parts, globus pallidus externa and globus pallidus interna. Globus pallidus interna specifically deals with inhibiting all movements one will not do and defining the amplitude of the movement. The nucleus accumbens receives input signals in the form of dopamine from the prefrontal cortex and sends signals back through the globus pallidus.

The basal ganglia are responsible for all ballistic movements. Ballistic movements are open loop movements with no necessarily defined endpoint. These movements also help to modify voluntary movements. The basal ganglia select the movement by considering all possibilities and suppressing all but one, and scale the movement by regulating its amplitude. The motor functions of a human greatly rely upon the corpus striatum because it is the main input zone for other brain areas to connect to the basal ganglia. The circuit begins in the motor cortex which sends input to the basal ganglia via the striatum, then to the motor regions of the thalamus, and back to the motor cortex. What the motor cortex sends to the spinal cord depends on what occurs during the loop. In short, the basal ganglia inhibit the thalamus which in turn excites the cortex, causing movement. Inside the basal ganglia are two circuits. The first, the Direct Pathway, facilitates movement by disinhibition or inhibiting the inhibition. The Indirect Pathway discourages movement by inhibiting the globus pallidus externa, which inhibits the globus pallidus interna, which finally inhibits the thalamus which would have sent neurotransmitters to the motor cortex (inhibiting the disinhibition).

The substantia nigra is a very important part of the basal ganglia circuit because it releases dopamine which is a key toward facilitating the actual movement. It facilitates movement by releasing dopamine that excites the direct pathway and inhibits the indirect pathway. If the substantia nigra is damaged, one loses that ability to excite the direct pathway and inhibit the indirect pathway. In other words, one would lose the ability to amplify any movement one would like to make and inhibit any movement one would not like to make. Thus, both pathways are virtually dependent upon the substantia nigra.

Lesions in specific nuclei of the basal ganglia produce specific deficits. The most famous of which is Parkinson’s Disease which is the slow and pervasive loss of dopamine neurons in the substantia nigra. The symptoms of this disease are tremor, rigidity, and bradykinesia. Huntington’s disease is another example. This disease results from the degeneration of the caudate and putamen and produces continuous movements of the face and limbs.