proteins and low concentration of Na + ions. In contrast, fluid outside the axon (ECF) contains low concentration of K + and high concentration of Na + , and this forms a concentration gradient. This ionic gradient across the resting membrane is maintained by ATP driven Sodium - Potassium pump, which exchanges 3Na + outwards for 2K + into the cells. In this state, the cell membrane is said to be polarized .
In neuron, the resting membrane potential ranges from - 40mV to - 90mV , and its normal value is - 70mV . The minus sign indicates that the inside of the cell is negative with respect to the outside (Figure . ). Figure .
Ionic channels Extracellular Fluid Na + Cl - K + leak channel Sodium- Potassium exchange pump Na + leak channel ATP Protein Protein Sodium Ion (Na - ) Potassium Ion (K - ) Chloride Ion (Cl - ) CYTOSOL K + KEY ADP + + + + + + + + + + + + + + + + + + + + + – – – – – – – – – – – – – – – – – + + + + + + + + – XI Std Biology-Zoology Chapter- shoots rapidly up to +45mV which is called the Spike potential . Repolarisation [Falling Phase] When the membrane reaches the spike potential, the sodium voltage-gate closes and potassium voltage-gate opens . It checks influx of Na + ions and initiates the efflux of K + ions which lowers the number of positive ions within the cell. Thus, the potential falls back towards the resting potential.
The reversal of membrane potential inside the axolemma to negative occurs due to the efflux of K + ions. This is called Repolarisation . Hyperpolarization If repolarization becomes more negative than the resting potential - mV to about - mV, it is called Hyperpolarization . Action membrane potential An action potential occurs when a neuron sends information down an axon, away from the cell body.
It includes following