A dozen or so highly interconnected structures lie within the cerebrum beneath the cerebral cortex and in the brainstem, and they interact with the cortex to control movements. Their influence is transmitted indirectly to the motor neurons both by pathways that go to the cerebral cortex and by descending pathways that arise from some of the brainstem nuclei.
It is not known to what extent, if any, these structures initiate movements, but they definitely play a prominent role in planning and monitoring them, establishing the programs that determine the specific sequence of movements needed to accomplish a desired action. Subcortical and brainstem nuclei are also important in learning skilled movements.
Prominent among the subcortical nuclei are the paired basal ganglia (see Figure 12-2b), which anatomically consist of a closely related group of separate nuclei. (Despite their name, these neuronal clusters are technically nuclei because they are within the central nervous system.) They form a link in some of the looping parallel circuits through which activity in the motor system is transmitted from a specific region of sensorimotor cortex to the basal ganglia, from there to the thalamus, and then back to the cortical area from
PART TWO Biological Control Systems
PART TWO Biological Control Systems which the circuit started. Some of these circuits facilitate movements and others suppress them.
Parkinson's Disease In Parkinson's disease, the input to the basal ganglia is diminished, the interplay of the facilitory and inhibitory circuits is unbalanced, and activation of the motor cortex (via the basal ganglia-thalamus limb of the circuit mentioned above) is reduced. Clinically, Parkinson's disease is characterized by a reduced amount of movement (akinesia), slow movements (bradykinesia), muscular rigidity, and a tremor at rest. Other motor and nonmotor abnormalities may also be present. For example, a common set of symptoms include a change in facial expression resulting in a masklike, unemotional appearance, a shuffling gait with loss of arm swing, and a stooped and unstable posture.
Although the symptoms of Parkinson's disease reflect inadequate functioning of the basal ganglia, a major part of the initial defect arises in neurons of the substantia nigra ("black substance"), a subcortical nucleus that gets its name from the dark pigment in its cells. These neurons normally project to the basal ganglia where they liberate dopamine from their axon terminals. The substantia nigra neurons, however, degenerate in Parkinson's disease, and the amount of dopamine they deliver to the basal ganglia is reduced, which decreases the subsequent activation of the sensorimotor cortex.
The most powerful drugs currently available for Parkinson's disease are those that mimic the action of dopamine or increase its availability. The major drug is L-dopa, a precursor of dopamine. L-dopa enters the bloodstream, crosses the blood-brain barrier, and is converted to dopamine (dopamine itself is not used as medication because it cannot cross the blood-brain barrier). The newly formed dopamine activates receptors in the basal ganglia and improves the symptoms of the disease. Another drug inhibits the brain enzyme that breaks down dopamine so that more neurotransmitter reaches the neurons in the basal ganglia. Other therapies include the electrical destruction ("lesioning") of overactive areas of the basal ganglia or stimulation of the underactive ones. Still highly controversial is the transplantation into the basal ganglia of neurons from either human fetuses or animals such as fetal pigs or cells that have been genetically engineered or taken from dopamine secreting tissues in the patient's own body. Regardless of their source, the implanted cells then synthesize the necessary dopamine as well as important growth factors.
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