In case you have yet to receive your PhD in Neuroscience
Before you can understand Metachromatic Leukodystrophy, you must first have a general understanding of certain terms and concepts. Please click on the links above to learn more about each area.
The nervous system is made up of two
types of cells. There are neurons and there are glia. Neurons are cells that receive
information and transmit "messages" to other neurons via
The human cerebral
cortex contains approximately 12 to 15 billion neurons.
Most neurons have 3 major components. They are made up of the soma, dendrites, and a single axon. The vertebrate soma ranges in diameter from 0.005 mm to 0.1 mm. Dendrites are thin fibers that branch out and narrow as they extend further and further from the soma. They serve as the information-receiving poles of neurons. The axon is much longer then the dendrites and serves as the information-sending pole of the neuron. It conveys impulses toward other neurons or toward glands and muscles. Most axons are covered with a myelin sheath which serves as an insulating material. These myelin sheaths form in segments along the axon, and are separated by gaps known as the nodes of Ranvier (RAHN-vee-ay). Through a process known as saltatory conduction, impulses are sent down the axon -- jumping from one node to the next. When a neuron is suitably stimulated, an electrical impulse swiftly jumps down the axons. This process of "passing the impulse" is much faster then transmission through axons that do not use myelin. Arrival of the electrical disturbance at the neurons presynaptic terminals triggers a series of events that cause the neuron to either inhibit or stimulate adjacent cells. If previously myelinated axons were to lose their sheaths, the axon would no longer be able to function correctly and the necessary messages would fail to reach their targets in any type of coordinated manner -- thereby killing their host!
In the late-nineteenth-century, Gregory Mendel demonstrated that inheritance occurs through genes. Contrary to the prior belief that inheritance was a blending process, it is now known that genes maintain their structural identity throughout generations.
Genes come in pairs -- which makes sense since genes are aligned along chromosomes, which are also paired. When someone has two identical genes on two chromosomes, they are homozygous for that gene. Being heterozygous for a gene would mean having an unmatched pair of genes on two chromosomes.
Genes can either be dominant or they can be recessive. Dominant genes are strong regardless of whether they are homozygous or not. Recessive genes, however, are only potent when homozygous.
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