HD Blog

Excitotoxicity. Overstimulation of cells by natural chemicals found in the brain.

Defective energy metabolism. A defect in the power plant of the cell, called mitochondria, where energy is produced.

Oxidative stress. Normal metabolic activity in the brain that produces toxic compounds called free radicals.

Trophic factors. Natural chemical substances found in the human body that may protect against cell death.

Several HD studies are aimed at understanding losses of nerve cells and receptors in HD. Neurons in the striatum are classified both by their size (large, medium, or small) and appearance (spiny or aspiny). Each type of neuron contains combinations of neurotransmitters. Scientists know that the destructive process of HD affects different subsets of neurons to varying degrees. The hallmark of HD, they are learning, is selective degeneration of medium-sized spiny neurons in the striatum. NINDS-supported studies also suggest that losses of certain types of neurons and receptors are responsible for different symptoms and stages of HD.

What do these changes look like? In spiny neurons, investigators have observed two types of changes, each affecting the nerve cells' dendrites. Dendrites, found on every nerve cell, extend out from the cell body and are responsible for receiving messages from other nerve cells. In the intermediate stages of HD, dendrites grow out of control. New, incomplete branches form and other branches become contorted. In advanced, severe stages of HD, degenerative changes cause sections of dendrites to swell, break off, or disappear altogether. Investigators believe that these alterations may be an attempt by the cell to rebuild nerve cell contacts lost early in the disease. As the new dendrites establish connections, however, they may in fact contribute to nerve cell death. Such studies give compelling, visible evidence of the progressive nature of HD and suggest that new experimental therapies must consider the state of cellular degeneration. Scientists do not yet know exactly how these changes affect subsets of nerve cells outside the striatum.

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National Institute of Neurological Disorders and Stroke
National Institutes of Health
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Reviewed July 1, 2001