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What are peripheral nerves?
In general, each individual peripheral nerve cell (also called a neuron) is made up of several parts: the soma (also called the cell body); the dendrites; and an axon. Communication between individual cells occurs at the synapse.
The soma, contains the nucleus of the nerve cell. Most protein synthesis occurs in the cell body. The soma also contains the mitochondria, which manufacture energy for the cell.
Dendrites receive communication from other cells. The dendrites extend from the cell body in branches, and are often referred to as the dendritic tree.
The axon is a long projection from the dendrite. It usually carries electrical information away from the soma (and may also carry information back to the soma). The axon is long, from ten times the length of the soma to up to tens of thousands of times the length of the soma. Most nerve cells only have one axon, but the axon may have many branches. The axon terminal releases neurotransmitters into the synapse. The neurotransmitter transmits information chemically to the dendrites of neighboring nerve cells. Many axons of the peripheral nerves are covered by the myelin sheath, which helps to speed transmission of information.
Motor neurons in the peripheral nervous system carry information from the spine regarding movement. The axon of a motor neuron can be over one meter in length and reaches from the spine to the toes.
Sensory neurons carry sensory information from the body back to the spine. In adults, the sensory neuron carrying information from the toes can be over one-and-a-half meters in length.
When neuromuscular disease affects the nerves in the peripheral nervous system, communication between the nervous system and muscles becomes impaired leading to to muscle weakness and wasting. In some types of neuromuscular disease, sensory nerves are affected causing sensory impairment. Neuromuscular disease may have an effect through directly affecting the muscles, the junction between nerves and muscles, the peripheral nerves in the extremities, or the motor and sensory nerves coming from the spinal cord.
For example, Charcot Marie Tooth (CMT), a genetic disease, actually consists of subtypes and can vary in symptoms. Some subtypes cause breakdown of the myelin sheath, while other subtypes affect the axon of the nerve. Depending on type, CMT may primarily affect motor neurons, sensory neurons, or both types of neurons.
In Myesthenia Gravis (MG), muscle weakness and fatigue occurs. In MG, the problem occurs at the neuromuscular junction. Antibodies block the receptors for the neurotransmitter acetylcholine, which inhibits the post-synaptic cell from firing.
The mitochondrial myopathies are a group of genetic diseases that affect the mitochondria in the cells. These diseases are considered to be a neuromuscular disease, although this disease affects all the cells of the body, causing problems in multiple organ systems. The muscles and neurological system often are particularly affected because these systems require so much energy to function properly.
Having a basic understanding of the nerve cells of the peripheral nervous system can improve understanding of neuromuscular disease. While the various neuromuscular diseases differ in their cause and symptoms, each in some way affects the muscles and the nerves of the peripheral nervous system.
Coon, D., and Mitterer, J., (2007). Introduction to Psychology: Gateways to the Mind and Behavior. Thompson Wadsworth: Belmont, CA.
MDA, (2011). Charcot-Marie-Tooth Disease (CMT). MDA website. http://www.mda.org/disease/cmt.html . Retrieved 1/27/12.
MDA, (2010). Facts About Mitochondrial Myopathies. MDA website. http://www.mda.org/publications/mitochondrial_myopathies.html . Retrieved 1/27/12.
MDA, (2011). Myasthenia Gravis (MG). MDA website. http://www.mda.org/disease/mg.html . Retrieved 1/27/12.
Wikpedia, (2012). Neuron. http://en.wikipedia.org/wiki/Neuron . Retrieved 1/27/2012.
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