Primary Parkinson’s Disease
Currently primary PD is attributed to a decrease in dopamine production on the part of the substantia nigra. The reason for the decrease in dopamine production, however, is unknown. Supposedly the substantia nigra just goes bad one day and stops producing the way it should. But what causes such a small and highly localized area of the brainstem to suddenly start to go bad while the rest of the brain, for the most part, is working fine?
The picture in the upper left corner above is a sagittal (side view) of the brain stem with a cross section removed and layed out in the main picture. In this picture the substantia nigra (long grey area on either side of the cross section) is actually part of a system of interconnecting nerve centers, called the basal ganglia, and a router called the thalamus located in the core of the brain and brainstem that fine tunes the coordination of the movement of muscles. The basal ganglia take in and process information from different hard drives in the brain, called lobes, which among others include the motor and premotor cortex of the frontal lobe. The motor cortex sends the commands out for movement and the premotor cortex provides stored programs for that particular movement such as riding a bike or throwing a ball.
For another perspective the picture above is a coronal view (looking at the back of the head and removing a slice in the middle of the brain from top to bottom and from side to side. The first three structures listed are part of the basal ganglia and the substantia nigra and thalamus are also shown.
Memories of motor activities, however, reside in all the lobes of the brain, not just the motor cortex. The brain pictures riding a bike in the occipital lobes, at the rear of the brain, and stores long term memory of things about riding a bike in the temporal lobes, located on either side of the brain next to the ears, such as watch out for potholes and roads are slippery when wet. The parietal lobes, above the top of the ears, integrate and associate the information from smell, sight, sound, taste and touch. Some western athletes and all eastern martial arts use Zen visualization techniques, such as watching videos and mental imaging, to stimulate the occipital lobe to train muscles without using them.
Muscle movement requires fast timing and ultrahigh speed components. Consequently, in contrast to most of the brain, the interconnecting system of the basal ganglia uses a high speed neurotransmitter called acetylcholine.
The grey matter of the brain is for processing and integrating information and uses slower speed neurotranmitters such as dopamine. The substantia nigra also uses dopamine. We are now finding out that there is much more to dopamine than simply muscle movements and tremors. Among other things, it appears to play a role in long term memory.
The role of the substantia nigra and it’s neurotransmitter dopamine is to take the rough edges off movement by slowing down and smoothing out muscle activity which prevents overexcitation and tremors.
Again, the picture above shows the substantia nigra in blue on the brainstem and other structures of the basal ganglia.
The thalamus is the top of the brainstem which is located in the core of the brain and is extremely complex. But one of its primary roles is to serve as a router for distributing information about muscle movement to the different hard drives of the brain. The hypothalamus sits below the thalamus and also appears to play a role in muscle movement.
In contrast to the substantia nigra, which when malfunctioning produces resting tremors as seen in Parkinson’s, injuries to the hypothalamus appear to play a role in wild, erratic, exaggerated tremors such as those seen in Huntington’s Disease. Interestingly, Huntington’s disease is similar to PD in many ways and will be discussed separately on this website.