Intracellular recording of the membrane potential

When for the first time Hodgkin and Huxley measured the absolute magnitude of the electrical potential in a living cell by introducing a 50 fxm capillary electrode into a squid giant axon, they found that when the tip of the electrode was far enough from the cut end it became up to 60 mV negative with respect to an electrode in the external solution. The restingpotential across the membrane in the intact axon was thus about — 60 mV, inside relative to outside. On stimulation of the axon by applying a shock at the far end, the amplitude of the action potential (Fig. 2.3) — or spike, as it is often called — was found to be over 100 mV, so that at its peak the membrane potential was reversed by at least 40 mV. Typical values for isolated axons recorded with this

Fig. 2.2. A squid giant axon into which a double spiral electrode has been inserted, photographed under a polarizing microscope. Its diameter was 700 ^m type of electrode (Fig. 2.4a) would be a resting potential of — 60 mV and a spike of 110 mV, that is to say an internal potential of + 50 mV at the peak of the spike. Records made with 0.5 fxm electrodes for undissected axons in situ in the squid's mantle give slightly larger potentials, and the underswing or positive phase at the tail of the spike is no longer seen (Fig. 2.4b). At 20 °C the duration of the spike is about 0.5 ms; the records in Fig. 2.4 were made at a lower temperature.

As may be seen in Fig. 2.4c—h, every kind of excitable tissue, from mammalian motor nerve to muscle and electric organ, gives a similar picture as far as the sizes of the resting and action potentials are concerned. The resting

Peak

Fig. 2.3. Nomenclature of the different parts of the action potential and the afterpotentials that follow it.

Peak

Resting potential

Foot

.Negative after-potential y/ /

Positive phase Positive after" P°tential

Fig. 2.3. Nomenclature of the different parts of the action potential and the afterpotentials that follow it.

potential always lies between —60 and —95 mV, and the potential at the peak of the spike between +20 and +50 mV. However, the shapes and durations of the action potentials show considerable variation, their length ranging from 0.5 ms in a mammalian myelinated fibre to 0.5 s in a cardiac muscle fibre, with its characteristically prolonged plateau. But it is important to note that for a given fibre the shape and size of the action potential remain exactly the same as long as external conditions such as the temperature and the composition of the bathing solution are kept constant. As will be explained later, this is an essential consequence of the all-or-nothing behaviour of the propagated impulse.

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