Extracellular [K+] essentially determines resting membrane potential (Vm) and ensures that excitation occurs at an appropriate time (Chapter 18). Excitation forms the basis of nervous conduction and also ensures the coupling of physiological responses (e.g. muscle contraction). Na+ ions play a central role in the initiation and propagation of excitation (Figure 19.1).
Neurones typically have a Vm = −70 mV, and following excitation (e.g. activation of receptors, the spread of nervous impulses) there is local depolarisation (i.e. the intracellular potential difference becomes less negative) and the membrane reaches threshold. At threshold, voltage-operated Na+ channels (‘fast’ sodium channels) undergo a conformation change and the channel opens. The increase in Na+ permeability causes an influx of Na+ ions down their electrochemical gradient and, as determined by the Nernst equation, the movement of ions attempts to take the membrane towards the equilibrium potential for Na+ (+65 mV).
Ion channels
Excitability is governed by ion channels. These are proteins that span the membrane and are permeable to specific ions. Their activity may be sensitive to voltage (voltage-operated) and their opening and closure are determined by the electrical environment. Other channel types are receptor-operated, in which events such as mechanical changes (e.g. stretch in muscle spindle fibres) cause the channel to open, leading to depolarisation (the receptor generator potential), which may lead to the initiation of action potentials.
The action potential
From threshold (approx. −50 mV) both the chemical and electrical gradients favour Na+ influx (Figure 19.2). As the membrane depolarises to a potential of 0 mV, the electrical gradient dissipates and is no longer apparent at 0 mV. At this point it is the chemical gradient that continues to drive Na+ influx such that the membrane potential becomes positive. As the membrane depolarises from 0 mV the electrical gradient starts to oppose force generated by the chemical gradient. The interplay of the chemical and electrical driving forces mean that there is depolarisation until (an overshoot) a peak membrane potential of approx. +40 mV is reached.
As the Na+ channels open this leads to the next step, which is channel closure. At the same time, the depolarisation due to the Na+ channels, leads to the activation of voltage-sensitive K+ channels. The closure of Na+ and activation of K+ channels limits the depolarisation. More importantly, the opening of K+ channels leads to K+ efflux and this leads to the membrane to repolarise, i.e. initiates the return of the membrane potential to the resting membrane potential and may overshoot to a hyperpolarised (more negative) and less excitable state.
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