Nerve Signal Transmission

Nerve signals are transmitted along the axon as a wave of electrochemical charges caused by movement of potassium and sodium ions across its membrane. A membrane would be synaptic knobs synaptic knobs

Figure 9.2 Typical motor neuron. (From Fried, 1990. # The McGraw-Hill Companies, Inc. Used with permission.)

fairly impermeable to ions except for the presence of special pores called membrane channels that provide a path for the ions. There are special membrane channels for potassium and others for sodium. One type of membrane channel allows a continuous but slow leak of ions across the membrane. Another type opens and closes in response to voltage across the membrane. In addition, there is a sodium-potassium exchange pump that uses ATP to exchange three intracellular sodium ions with two extracellular potassium ions. In other words, it pumps two potassium ions into the cell for every three sodium ions pumped out. When the nerve cell is resting, the exchange pump maintains an electrostatic potential, or voltage, of —70 mV across the membrane. That is, the inside of the cell is negatively charged, or has a deficiency of positive anions. This voltage is called the resting potential of the cell. The pump also ensures that under resting conditions, the inside of the cell has far more potassium than sodium, and the reverse is true outside the cell.

Nerve signal propogation along the axon begins when a sodium channel is opened at some point, such as by a chemical signal from a synapse. This causes the voltage to increase toward zero, a process called depolarization. Depolarization triggers a sequence of sodium and potassium pore openings and closings. First one, then the other ion floods across the membrane, causing the voltage to increase and then decrease back to the resting potential. This sequence moves along the axon in a wave, transmitting the signal. The entire sequence at one point may occur in 1 ms.

The largest axon fibers, from 4 to 20 mm in diameter, are well insulated by myelin and can propagate signals at speed up to 140 m/s. Others are unmyelinated and are less than 2 mm in diameter; their transmission speed is only about 1 m/s. The faster fibers are used to transmit the senses of balance, body position, and delicate touch. Slower neurons take less space and are used to transmit information on temperature and pain as well as information for organs and glands. About one-third of an adult's neurons are myelinated. Children lack myelination until early adolescence, which partly accounts for their reduced coordination ability. Multiple sclerosis is a disease involving the loss of myelination of axons, which results in muscle paralysis and loss of sensations.

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