In a neurone’s resting state, when it’s not being stimulated, the outside of the membrane is positively charged relative to the inside. This is due to more positive ions being on the outside. The potential difference of the membrane is -70mV. This difference in voltage is maintained by the sodium-potassium pump and potassium ion channels.
The sodium-potassium pump maintains the potential difference by actively transporting 3 Na+ ions for every 2 K+ ions that are moved in (ATP is required). However, the Na+ ions cannot diffuse back into the neurone as the neuron’s membrane is not permeable to sodium ions. This creates a sodium ion electrochemical gradient.
The steps a neurone takes to generate an action potential (electrical impulse) are:
1) Stimulus. A stimulus excites the neuron’s cell membrane causing the Na+ ion channels to open. Therefore, the membrane becomes more permeable to Na+ ions. As a result, the Na+ ions diffuse into the neurone down their sodium ion electrochemical gradient. This makes the inside of the neurone less negative.
2) Depolarisation. If the potential difference reaches the threshold, voltage-gated Na+ channels open. Therefore, more Na+ ions will diffuse into the neurone leading to mass depolarisation.
3) Repolarisation. When the potential difference increases to around +30mV, the Na+ channels close and the K+ channels open. As a result, K+ will diffuse out of the neurone down its concentration gradient, decreasing the potential difference back down to the resting potential.
4) Hyperpolarisation. The K+ channels are slow to close, so there’s a slight ‘overshoot’ where too many K+ ions diffuse out of the neurone. Therefore, the potential difference becomes more negative than the resting potential.
5) Resting potential. The K+ channels open again, but because there are more K+ outside the neurone than inside, the K+ diffuse down their concentration gradient into the neurone, increase the potential difference from -80mV to -70mv (the resting potential). The resting potential is also maintained by the sodium-potassium pump.
The refractory period is the short period of time after an impulse has been fired when a second action potential cannot be sent. This is due to the hyperpolarisation stage where even if there was another stimulus, the potential difference is too negative to reach the threshold. The refractory period allows discrete (separate) impulses to be sent.
The all or nothing principle is that all action potentials fire at the same change in voltage after the threshold has been reached, or not at all. Therefore, increasing the intensity of the stimulus won’t increase the size of the action potential, only the frequency. This stops the brain getting over stimulated by not responding to very small stimuli.