In the resting stage an axon is polarized, that is, the inside of the plasma membrane is electrically negative to the outside. In this stage the membrane potential is about -70 mV; both Na+ ion channel gates (activation and inactivation gates) are closed, and so are the single type K+ ion channel gates. As a consequence, the plasma membrane is only very slightly to Na+ions , and little more permeable to K+ ions, at this stage. Some Na+ ions leak into the cell down their concentration gradient, and some--but fewer-- K+ ions continue to leak through the plasma membrane down their concentration gradient and out of the cell. However, the electrochemical gradients and the polarized state of the neuron are maintained because the NA+/K+ATPase pump keeps ejecting three Na+ions for every two K+ ions that leaks in. An action potential is generated when an electrical impulse causes the Na+ activation gates of the Na+ion channels to open. This results in a brief increase in membrane permeability to NA+ ions. As the Na+ ions rush in, the resting membrane potential becomes less and less negative, and may go to neutrality and beyond. At a membrane potential of about -53 mV (-50mV to -55mV), threshold is reached and the neuron fires , that is, an action potential is generated. This is a brief event of about millisecond duration (1mS). Soon the Na+ inactivation gates close the Na+ channels, as the single type K+ ion channels slowly open. The results are that the membrane permeability to Na+ ions drops sharply, and the membrane permeability to K+ ions rises. The Na+ influx stops as the K+ ions flow out of the neuron. The loss of many K+ ions without countervailing nNa+ replacement causes the inside of the membrane to return to the resting stage in which the potential is negative (about -70mV), and the cell is repolarized. In some cases the membrane potential may over shoot to a level of -100mV or more . Although the threshold for and action potential is aaboud -53mV , when a neuron depolarizes the membrane potential often reaches about +30mV. Also when the membrane repolarizes, the potential may overshoot to values around -100 mV, if the K+ gates remain open for prolonged periods. In such cases the neuron is said to be hyperpolarized
Work Step by Step
A membrane potential must reach a threshold value if an axon is to be generated. The stimulus must be strong enough to cause enough Na+ ion channels to open per unit time so that the number of Na+ ions entering exceeds the number of K+ ions exiting (per unit time). Stimuli on the neuron may be strong, weak or very weak: strong stimuli push the membrane potential to threshold; weak stimuli may also cause the generation of an AP, but they have to be applied for longer periods. Very weak stimuli disperse before threshold is reached and so cannot generate an AP. But the AP, when generated,always fires at the same threshold: if the threshold is reached it fires ; if the threshold is not reached, no AP is generated. There is no such thing as a weak AP produced by a sub-thresh stimulus. The AP is an all or none response.