A skin or thin layer of tissue serving as a covering, lining or partition. A neuronal membrane is the boundary of brain cells which controls permeability and serves other functions via surface specializations. PLASMALEMMA is the term given to the membrane covering the SOMA and the dendrites (see DENDRITE) of the cell, while the membrane covering the AXON is called the AXOLEMMA. Neuronal membranes are composed of LIPIDS and PROTEINS. The characteristics of the lipids are responsible for the structure of the mem-brane and its permeability. The proteins are held by the structure of the lipids.
Some of the proteins are intrinsic to the lipid layers and others are extrinsic, located on either the internal or external surface of the membrane. Groups of intrinsic proteins act as carriers of substances (such as sugars and amino acids or NEUROTRANSMITTERS) across the membrane. When the cell is at its RESTING POTENTIAL, some protein units are functioning as pumps, actively moving substances across the membrane against the concentration or electrical gradient to maintain the negative charge of the cell. During an ACTION POTENTIAL, the SODIUM-POTASSIUM PUMP ceases, while other groups of intrinsic proteins act as an ION CHANNEL, opening to allow the passage of sodium and potassium across the membrane. Proteins on the external surface of the membrane may be specialized for the binding of neurotransmitters. Of these, some may have an ion channel which is opened directly when a neurotransmitter chemical binds. In other cases, a receptor protein is coupled to G PROTEIN on the internal surface of the membrane, which may stimulate SECOND MESSENGERS and in turn opens an ion channel.
ERIC M.BOWMAN
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