The task of controlling the extensive motor systems of the human body is a prodigal one. The human brain, however, has developed systems which handle the enormous demands of the task with great ease. The basal ganglia are just one of the three main systems involved in the control of motor function.
The frontal cortex plans and executes movements in several functionally distinct areas, including the primary motor, premotor, and supplementary motor cortices. Activity can be measured in the supplementary motor areas for even considering movement. The cerebellum (Module 10) decides how the brain can switch current "program" which the organism is executing to its intended replacement or successor. The results of its calculations project back to cortex for the actual implementation. The basal ganglia oversee the current state of the organism, making sure that there are no errors in the execution of movements.
The basal ganglia are not a single, coherent anatomical unit, but rather a collection of five nuclei at the base of the forebrain. The caudate nucleus and putamen, together called the neostriatum (or striatum), develop from the telencephalon. The other three nuclei, the globus pallidus, subthalamic nucleus, and substantia nigra, have their origins in the diencephalon. The globus pallidus is further divided into an internal and an external segment. The substantia nigra is subdivided into pars reticulata and pars compacta.
The connections of the basal ganglia are somewhat complex. Note that all input is to the neostriatum, which passes information on to other nuclei. Input to the basal ganglia can take either the direct or the indirect pathway to the thalamus. The direct pathway passes from the neostriatum to the globus pallidus and thence to the thalamus. Input which follows the indirect pathway, however, passes from the neostriatum to the globus pallidus and then through a feedback loop with the subthalamic nucleus before being passed to the thalamus. The direct pathway produces an excitatory effect on the thalamus; the effect of the indirect pathway is inhibitory.
The neostriatum contains about 110 million neurons, the globus pallidus one-half million, and the substantia nigra about 1/10th of a million. These cells in the neostriatum are of two main types: spiny neurons, and giant aspiny interneurons. Spiny neurons account for about 95% of the neurons in the basal ganglia . Their name comes from the thousands of dendritic spines which are found all over their dendrites. The spines increase the surface area of a dendrite, allowing it to receive many inputs. Each spine can be "personalized" to weight its input in a specific way by modifying its size or shape .
Giant Aspiny Interneurons are much rarer, making up about 5% of the total neurons in the basal ganglia. As their name suggests, their dendrites are devoid of spines. They utilize acetylcholine as a neurotransmitter; damage to this capacity produces the lack of ACh seen in Huntington's Chorea.
The cerebral cortex sends its information to the distal dendrites of spiny neurons in a topographically organized manner. These inputs are not haphazardly arranged, but rather structured carefully.
The neostriatum is not homogeneous in composition. Its main portions are called the matrix, which receives sensory and motor inputs; it shows a higher concentration of Ach than other areas. The matrix is studded throughout with patches, called striosomes, which receive limbic input and dopamine feedback from the substantia nigra .
The matrix can be further broken down into regions called matriosomes. The pattern of inputs to the matriosomes is governed by three principles of organization. First, input from various cortical areas is not combined but instead is segregated and remains so throughout the basal ganglia. Second, this brings about specific "streams of influence" over the thalamus and frontal cortex. Four of the major streams are: motor, limbic, oculomotor, and prefrontal. Third and last, within the basal ganglia exist both direct and indirect pathways. As mentioned earlier, the direct pathway leads to a thalamic depolarization, whereas the indirect pathway leads to a thalamic hyperpolarization.
Damage to the various nuclei and pathways of the basal ganglia can have a number of different consequences, the most common being tremors, athetosis, chorea, ballism, and dystonia. Tremors are small movements which occur in a rhythmic, oscillatory manner. Athetosis (from Greek athetos, set aside) is characterized by involuntary twitchings of the hands and feet. In chorea (from Greek choreia, dance), the limbs and facial muscles exhibit abrupt movement. Ballism (from Greek ballein, to throw) consists of violent, flailing movements. Dystonia (from Greek dustonos, ill-tightening) leads to grotesque, distorted body postures.
Parkinson's disease is caused by damage to the substantia nigra. Since the pars compacta generates DA feedback, the neostriatum is deprived of DA inputs. Symptoms of Parkinson's, namely tremors, can be alleviated by the implantation of fetal tissue to replace the damaged region or the administration of L-DOPA, a precursor of dopamine. After extensive treatment with DA drugs, however, a patient may develop a hypersensitivity to DA, which results in a disorder called tardine dyskinesia, which resembles chorea and ballism. Tardine dyskinesia can be relieved by cessation of the DA drug. Parkinson's may be induced by a drug called MPTP.
Another disorder is known as Huntington's Chorea. This has been identified as genetically based, and the defective locus has not been precisely identified. It manifests itself as damage to the neostriatal cells, producing a deficit in both ACh and GABA. Huntington's produces not only chorea but also eventual dementia and death.