Now, with that basic understanding of the circuitry, we can now apply that with our understanding of
of various clinical correlations. Now, I want you to think about Parkinson’s disease and what’s going on
here with this circuitry. What we have in place here is a circuitry that exists between the substantia
nigra and the striatum. The nigrostriatal pathway utilizes dopaminergic neurons. However,
in Parkinson’s disease, we lose, there’s a loss of those neurons. As a result, the direct pathway
is no longer excited. If you can’t excite the direct pathway, you cannot excite the cerebral cortex.
In addition, this loss of dopaminergic neurons on the striatum has an effect on the indirect pathway
in that it is no longer inhibited. So now, it’s going to have a more substantial influence on the cerebral
cortex. As you can recall, when the indirect pathway is activated, it inhibits the cerebral cortex.
So, that is the net effect then that an indirect pathway will inhibit those motor neurons. When those motor
neurons are inhibited, then your ability to execute motor function will be diminished. Here’s a table
that demonstrates the symptoms of Parkinson’s disease on the left. On the right side, we have some
structural changes and causes that attend to this clinical disorder. One of the symptoms of Parkinson’s
is bradykinesia or akinesia. Muscle rigidity is a symptom as well. Individuals will have a resting tremor
of their hands and fingers, kind of in a pill-rolling manner. But when they move their hands and fingers,
that tremor is no longer present. These individuals have difficulty initiating movements because again,
there’s a lack of excitation of the motor cortex. When movements are initiated, they tend to be
abnormally slow in their execution. They have a decreased facial expression, so it’s kind of a masked
face expression; don’t see much emerging from their facial expressions. Now let’s take a look at the
structural changes and causes of Parkinson’s. First, some of the structural changes. We have a couple.
First is that there will be a depigmentation of the substantia nigra due to a loss of the dopaminergic
neurons. You will find the presence of Lewy bodies in this disorder, histologically. This is a pathologic
indicator. Then some of the causes of Parkinson’s disease include genetic factors, environmental factors,
certain medications can result in Parkinson’s, MPTP, as well as vascular insult.