Resting membrane potential and Action potential

At rest (not transmitting an action potential) neuron maintains a voltage difference across its plasma membrane. The cytoplasmic fluid next to the membrane is negatively charged whereas the interstitial fluid outside the membrane is positively charged. This voltage difference is called the resting membrane potential. The resting membrane potential is –70 mv. The concentration of Na+ as well as K+ is higher outside the membrane and at the onset of conduction permeability of Na+ increases. This is the first event of action potential. Figure 3.4 illustrates the process of spike generation.

	Figure 3.4: Spike generation

This is followed by a gradient change of voltage from –70 mv to +35 mv, which in turn activates Na+- K+ pump. During the first phase, called as absolute refractory period, Na+ ions cannot enter the membrane and hence no current excites the neuron. It lasts for 1-10 milliseconds and is followed by relative refractory period which is 2-3 times that of the absolute refractory period. Only superthresholds are received during this period. The third phase is known as supernormal phase and its duration is 12 ms. During this phase stimulus at sub-threshold level are received. This is followed by the supernormal phase that lasts for 70 ms. and finally the nerve fiber attains resting membrane potential (-70 mv) and gets ready to receive incoming signals. Figure 3.5 illustrates the mechanism of Na+- K+ pump.

	Figure 3.5: Na+- K+ pump