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How Changes in the Dopaminergic System Affect Cognitive Aging

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  • 0:03 Brain Tracts
  • 2:01 Nigrostriatal
  • 3:15 Tuberoinfundibular
  • 4:23 Mesocorticolimbic
  • 6:22 Lesson Summary
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Lesson Transcript
Instructor: Devin Kowalczyk

Devin has taught psychology and has a master's degree in clinical forensic psychology. He is working on his PhD.

This lesson examines the changes that occur to the four tracts of the dopaminergic system. We will look at how depredations to each tract lead to a particular type of issue.

Brain Tracts

I have a bad habit of thinking of the brain as this mess of neurons all crammed together or broken into distinct and identifiable lobes. It's a bad habit, and I am working on correcting it. The reason this is wrong is that while there are small groupings of neurons and large lobes of the brain, there are tracts, which are bundled axons with a common purpose. Think of a tract like a piece of braided rope; all the individual strands are different axons that send signals.

There are several tracts, each with a different purpose. Some deal with sensory information; others deal with body movements. The one we are going to focus on is the dopaminergic system, which is a group of four tracts that use only dopamine to activate different brain systems. The four tracts are:

  • Nigrostriatal tract, which is a set of axons involved with the refining of motor movements
  • Tuberoinfundibular tract, which is a set of axons involved with regulating hormones
  • Mesolimbic tract, which is a set of axons involved with regulating emotions
  • Mesocortical tract, which is a set of axons involved with regulating thought

These definitions of the tracts are somewhat simplified. If I cut your nigrostriatal tract, you would lose some of the finesse in your motor movements, but they wouldn't be impossible to make. Many people, in fact, have to deal with rebellious tracts every day of their life. For example, there have been strong implications that the dopaminergic system is related to schizophrenia, with its bizarre motor movements, emotional regulation problems, and unusual thought processes. However, we are going to focus on the aging of this system, not schizophrenia.

Let's look at how the dopaminergic system breaks down as we get older. Research has found that certain pathways will degrade in a particular way.

Nigrostriatal

Let's start off looking at the degradation of the nigrostriatal tract. It is responsible for helping control fine movements. This particular tract is plugged into the part of the brain that deals with different types of inhibition.

What does this mean, exactly? When the nigrostriatal is functioning properly, it helps cut down on superfluous movements. Image a long arm of a crane. It is super long and difficult to control, and a small motion can have big effects. The nigrostriatal tract acts like guide wires on the crane. This means that when the crane is moved, it doesn't move as much because the guide wires rein it in. As we get older, the tract becomes less reactive through general neurodegeneration, defined as neuron and helper cell death.

As we get older, the natural inhibition for extra movements is lost. Kind of like the guide wires breaking down and falling off. This translates to an increase of unnecessary movements, like tremors, muscle ticks, and overstepping or reaching. Small behaviors can lead to excessive movement. This is kind of scary since many older people's bones and reflexes aren't what they used to be.

Tuberoinfundibular

The very complex-sounding tuberoinfundibular tract is linked to the pituitary system. The pituitary is a central gland involved in coordinating nearly all other glands in the body. So this complicated sounding tract is connected right into the system that controls all the hormones and other chemicals dropped into your blood.

The tuberoinfundibular tract has a built-in feedback loop that helps control it. What happens is the tuberoinfundibular is stimulated when there is too much or too little of a hormone. The tract triggers the pituitary to produce more of the hormone. This then stops the activation of the tract until the hormone shifts out of balance again.

As you get older, the tract isn't as sensitive as it used to be, and the pituitary isn't as refined as it used to be. Together, this results in a lowering of hormone levels and a difficulty in getting them to how they were when you were younger. This is a downward spiral where it just gets worse and worse, and by the end, you can't tell if someone is a great grandmother or great grandfather because of the mixed up hormone levels.

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