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Situated deep within the center of the brain, the left and right basal ganglia encircle the thalamus and make up a critical part of the forebrain. These ganglia are believed to be symmetrical in structure and function. Although knowledge of brain disorders affecting the left basal ganglia is abundant, a conclusive, coherent theory on its function has yet to emerge. It is thought that this region of the brain is involved in executive functions, learning and emotion. Experimental studies have indicated that the left basal ganglia is involved in activity selection, activity switching and movement, but just how it makes use of its inhibitory activity to activate motor neurons remains unclear.
The most common disorders affecting left basal ganglia are Huntington's disease and Parkinson's disease. Both of these conditions are characterized by movement abnormalities as well as a decline in cognitive flexibility and executive function. This lends credence to the theory that this region of the brain is responsible for a diverse and complex array of tasks in the brain.
Tourette's syndrome and obsessive-compulsive disorder also have been correlated with the left basal ganglia dysfunction. This is not surprising, because many neuroscientists suspect that activity switching and selection might also be mediated by this part of the brain. Findings that addiction might be mediated in this region lend further support to the theory that the left basal ganglia regulates motivation and behavioral learning.
This might help explain the importance of this region in the vertebrate brain. Every vertebrate species has at least the rudimentary elements of the left basal ganglia. This indicates that it is likely to play an important role in movement, behavior and primitive motivational impulses.
Some experts believe that the left basal ganglia's use of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in most of its neurons instead of the stimulating neurotransmitter glutamate adds to the difficulty of understanding its function. These neurons exert their activity by inhibiting the actions of their targets, so there are countless possibilities for reversals of their signs of influence. This creates the possibility for even more theories about which neural groups work together to accomplish their effects.
Despite the importance of GABA in the left basal ganglia as a whole, one of its sections has a high concentration of cholinergic neurons. The cells within the striatum, although they comprise only a small portion of this ganglia as a whole, use the excitatory neurotransmitter acetylcholine almost exclusively. A significant amount of neurons in the region also use dopamine. This is the neurotransmitter thought to moderate both motivation and addiction as well as play a role in the progression of Parkinson's disease, so its importance to the overall function of the left basal ganglia cannot be overstated.