Basal Ganglia and Parkinson’s 101: A Simplification/Clarification

Some visitors felt that the previous post on the Basal Ganglia in Parkinson’s might frustrate the lay person. In the interest of enhancing your understanding, I wrote a clarification.

Several respected “regular” visitors felt that the previous post on the Basal Ganglia in Parkinson’s could possibly be rather confusing to the lay person. In the interest of making this series accessible to all I write this clarification. I am going to simplify the neuroanatomy further and just lay out the essentials.  To make it more interesting and digestible we will tell the story using a specific example: A person moves his/her left bicep muscle.

Details: 

The signal originates in motor  planning regions that then link to the direct bicep control area of the cortex on the opposite side in the right hemisphere of the brain. We will focus on a single nerve fiber even though there could be hundreds involved in actually moving the bicep, and thousands more in the planning region.

Here’s what happens:

1. A signal originates in the motor planning region adjacent to the right bicep direct control region of the motor area on the right side.
2. In our artificially-focused example, a signal from an activated single nerve fiber goes from its cell body in the cortex, through the switching area, the thalamus, crosses to the left side in the brainstem, enters the spinal chord and leaves as part of a nerve going to the bicep muscle on the left.
3. As it passes through the thalamus its activity is sensed by the basal ganglia.
4. The basal ganglia process the signal into a “go” or “slow/halt” signal that goes back to the thalamus.
5. The thalamus processes the “go” signal into a strong feedback signal back to the cortex sustaining the firing of the original bicep nerve fiber, or it processes the “slow” signal into a weak feedback signal back to the cortex causing the original signal cell to slow or stop its firing.
6. The bicep muscle on the left responds accordingly.

How do the basal ganglia decide to send a go or slow signal? This is where things get very complicated. Emotional state, presence of fear, information from the body about bicep position, and the activity of other fibers nearby all factor in to this decision.

We will limit it here to a simple picture. The basal ganglia components get information on the activity of the nerve fiber by monitoring the thalamus. They process that information using a whole host of factors from surrounding structures. When they come to a decision they tell the thalamus which translates their signal into a simpler weak or strong signal back to the cortex. The cortex motor planning region near the bicep control region responds by either enhancing or weakening or halting the outgoing signal to the biceps muscle.

What happens in Parkinson’s? Very simply to make sharp decisions the basal ganglia rely on a nice steady dopamine supply from the substantia nigra below. When this is diminished in Parkinson’s from cell death there, the basal ganglia start acting funny and react in an almost opposite manner than they’re supposed to. This results in stillness when the person tries to initiate movement, slowness when they try to sustain movement, and rigidity and tremor.

I hope this is helpful.

Here’s a more accurate sophisticated diagram (click on it for a full-sized view):

Shown here the right basal gangla involve two major control circuits that feed back to the thalamus and in this case regulate the movement of the left bicep muscle.

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