Neurotransmission

The resting potential of nerve cells and their unique structure provide the basis for the process of neurotransmission. Nerve cells at rest are close to an equilibrium concentration of potassium, but not sodium. Permeabilizing the membrane to ions causes sodium ions to rush in. Nerve cells have gated channels for facilitated transport of Na+ and K+. In the resting state, the gates are closed (Figure 10.32) and the resting potential is about -60 mV. On stimulation at a particular region of an axon, the sodium activation gates open. Sodium rushes in and the membrane potential moves to +40 mV (Figure 10.33). This, in turn, causes the potassium gates to open and potassium leaks out, which reverses the potential gain, but causes it to overshoot to -70 mV. This causes the sodium inactivation gates (different from activation gates) to close and a refractory period ensues during which the sodium channels cannot open until the membrane fully repolarizes and the potassium gate closes.

Depolarization as described above occurs in a single region of a nerve fiber. Transmission of the signal along the fiber occurs because the influx of sodium and efflux of potassium in one section of the fiber causes similar disturbances in adjacent regions of the fiber and the depolarization/repolarization proceeds down the fiber quickly. The traveling pulse is called the action potential. Typical speeds are from 1 to 100 meters per second.

> The action potential does not appreciably decrease with distance transmitted.

> The action potential is like a digital signal - it is either on or off.

> After an impulse has passed, the region behind it in the axon is unable to transmit another impulse during a refractory period of several milliseconds.

See also: Resting Potential, Action Potential, Biochemistry of Neurotransmission (Chapter 21)