Comparisons
Part 2

A comparison of HD to Alzheimer's and Parkinson's diseases

Neurobiology: Neuromotor Comparisons

As addressed in the HD Neurobiology chapter, the part of the brain most affected by HD is a group of nerve cells (neurons) at the base of the brain known collectively as the basal ganglia. The basal ganglia is responsible for the muscle-driven, motor movements of the body. As the cells in this area die, a person with HD experiences uncontrollable muscular movements likened to fidgetiness or nervous restlessness.

Similarly, Parkinson's disease patients experience uncontrollable movements due to the disease's effects on a specific area of the basal ganglia called the substantia nigra. The substantia nigra produces a chemical called dopamine, which is a neurotransmitter. Neurotransmitters are special chemicals that help neurons communicatewith each other. Dopamine and another neurotransmitter, acetycholine help to control our movements. In Parkinson's disease, the neurons inthe substantia nigra gradually die off, which causes less dopamine to be made. With less dopamine than normal, there is an imbalance between dopamine and acetylcholine (see figure). This imbalance causes the nerve cells to fire out of control, leaving patients unable to direct their movement in a normal manner.

In Alzheimer's disease, neurons in the brain and the spaces between them become clogged with protein deposits calledbeta amyloid plaques and neurofibrillary tangles. Even in people who don't have AD, plaques and tanglesdevelop as part of the normal aging process. However, in people with Alzheimer's disease, there are many more plaques and tangles. Plaques are dense, mostly insoluble (cannot be dissolved) deposits of protein and cellular material outside and around the neurons. Tangles are insoluble twisted fibers that build up inside the nerve cell. When neurons are clogged with tangles, and the spaces between neurons are clogged with plaques, the transmission of nerve impulses from one neuron to the next does not happen properly. As a result, the brain cannot perform mental functions such as remembering and thinking. There are other senile dementias (like Multi-infarct Dementia) that present like Alzheimer's disease but have very different causes and are not comparable to HD. Though they many share certain neurobiological properties, these are distinctive conditions.

Though Huntington's, Parkinson's, and Alzheimer's disease are caused by unique cellular processes, all three diseases are facilitated by the inability of neurons to communicate with each other. Gradual neuron death in people with HD hinders the ability of neurons to communicate. For people with PD, neurons die, causing an imbalance of dopamine and acetylcholine and the uncontrolled firing of nerve cells. Finally, in people with AD, plaques and tangles inhibit neurons' ability to communicate.

Last Modified: 5-20-04

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