Extracellular recording of the nervous impulse

There are many experimental situations where it is impracticable to use intra-cellular electrodes, so that the passage of impulses can only be studied with the aid of external electrodes. It is therefore necessary to consider how the picture obtained with such electrodes is related to the potential changes at membrane level.

Since during the impulse the potential across the active membrane is reversed, making the outside negative with respect to the inside, the active region of the nerve becomes electrically negative relative to the resting region. With two electrodes placed far apart on an intact nerve as in Fig. 2.5a, an impulse set up by stimulation at the left-hand end first reaches Rx and makes it

Membrane Potential

Fig. 2.4. Intracellular records of resting and action potentials. The horizontal lines (dashed in a and b) indicate zero potential; positive potential upwards. Marks on the voltage scales are 50 mV apart. The number against each time scale is its length (ms). In some cases the action potential is preceded by a stimulus artifact: a, squid axon in situ at 8.5 °C, recorded with 0.5 ^m microelectrode; b, squid axon isolated by dissection, at 12.5°C, recorded with 100 ^m longitudinal microelectrode; c, myelinated fibre from dorsal root of cat; d, cell body of motoneuron in spinal cord of cat; e, muscle fibre in frog's heart; f, Purkinje fibre in sheep's heart; g, electroplate in electric organ of Electrophorus electricus; h, isolated fibre from frog's sartorious muscle. a and b recorded by A. L. Hodgkin and R. D. Keynes, from Hodgkin (1958); c, recorded by K. Krnjevic; d, from Brock, Coombs and Eccles (1952); e, recorded by B. F. Hoffman; f, recorded by S. Weidmann, from Weidmann (1956); g, from Keynes and Martins-Ferreira (1953); h, from Hodgkin and Horowicz (1957).

Fig. 2.4. Intracellular records of resting and action potentials. The horizontal lines (dashed in a and b) indicate zero potential; positive potential upwards. Marks on the voltage scales are 50 mV apart. The number against each time scale is its length (ms). In some cases the action potential is preceded by a stimulus artifact: a, squid axon in situ at 8.5 °C, recorded with 0.5 ^m microelectrode; b, squid axon isolated by dissection, at 12.5°C, recorded with 100 ^m longitudinal microelectrode; c, myelinated fibre from dorsal root of cat; d, cell body of motoneuron in spinal cord of cat; e, muscle fibre in frog's heart; f, Purkinje fibre in sheep's heart; g, electroplate in electric organ of Electrophorus electricus; h, isolated fibre from frog's sartorious muscle. a and b recorded by A. L. Hodgkin and R. D. Keynes, from Hodgkin (1958); c, recorded by K. Krnjevic; d, from Brock, Coombs and Eccles (1952); e, recorded by B. F. Hoffman; f, recorded by S. Weidmann, from Weidmann (1956); g, from Keynes and Martins-Ferreira (1953); h, from Hodgkin and Horowicz (1957).

Time

Time

Time

Fig. 2.5. The electrical changes accompanying the passge of a nerve impulse as seen on an oscilloscope connected to external recording electrodes R1 and R2. S, stimulating electrodes. An upward deflection is obtained when R1 is negative relative to R2. a, diphasic recording seen when R1 and R2 are both on the intact portion of the nerve and are separated by an appreciable distance; b, monophasic recording seen when the nerve is cut or crushed under R2; c, diphasic recording seen with R2 moved back on to intact nerve, much closer to R1.

temporarily negative, then traverses the stretch between R1 and R2, and finally arrives under R2 where it gives rise to a mirror-image deflection on the oscilloscope. The resulting record is a diphasic one. If the nerve is cut or crushed under R2, the impulse is extinguished when it reaches this point, and the record becomes monophasic (Fig. 2.5b). However, it is sometimes difficult to obtain the classical diphasic action potential of Fig. 2.5a because the electrodes cannot be separated by a great enough distance. In a frog nerve at room temperature, the duration of the action potential is of the order of 1.5 ms, and the conduction velocity is about 20 m/s. The active region therefore occupies 30 mm, and altogether some 50 mm of nerve must be dissected, requiring a rather large frog, to give room for complete separation of the upward and downward deflections. When the electrodes are closer together than the length of the active region, there is a partial overlap between the phases, and the diphasic recording has a reduced amplitude and no central flat portion (Fig. 2.5c).

A whole nerve trunk contains a mixture of fibres having widely different diameters, spike durations and conduction velocities, so that even a monophasic spike recording may have a complicated appearance. When a frog's sciatic nerve is stimulated strongly enough to excite all the fibres, an electrode placed

Diagram Monophasic Recording

Fig. 2.5. The electrical changes accompanying the passge of a nerve impulse as seen on an oscilloscope connected to external recording electrodes R1 and R2. S, stimulating electrodes. An upward deflection is obtained when R1 is negative relative to R2. a, diphasic recording seen when R1 and R2 are both on the intact portion of the nerve and are separated by an appreciable distance; b, monophasic recording seen when the nerve is cut or crushed under R2; c, diphasic recording seen with R2 moved back on to intact nerve, much closer to R1.

Compound Nerve Action Potential

Fig. 2.6. A monophasic recording of the compound action potential of a bullfrog's peroneal nerve at a conduction distance of 13.1 cm. Time shown in milliseconds on a logarithmic scale. Amplification for b is ten times that for a. S, stimulus artifact at zero time. Redrawn after Erlanger and Gasser (1937).

Fig. 2.6. A monophasic recording of the compound action potential of a bullfrog's peroneal nerve at a conduction distance of 13.1 cm. Time shown in milliseconds on a logarithmic scale. Amplification for b is ten times that for a. S, stimulus artifact at zero time. Redrawn after Erlanger and Gasser (1937).

near the point of stimulation will give a monophasic action potential that appears as a single wave, but a recording made at a greater distance will reveal several waves because of dispersion of the conducted spikes with distance. The three main groups of spikes are conventionally labelled A, B and C, and A may be subdivided into a, and y. In the experiment shown in Fig. 2.6, for which a large American bullfrog was used at room temperature, the distance from the stimulating to the recording electrode was 131 mm. If the time for the foot of the wave to reach the recording electrode is read off the logarithmic scale of Fig. 2.6«, it can be calculated that the rate of conduction was 41 mm/ms for a, 22 for ^,14 for y, 4 for B and 0.7 for C. The conduction velocities in mammalian nerves are somewhat greater (100 for a, 60 for 40 for y, 10 for B and 2 for C), partly because of the higher body temperature and partly because the fibres are larger.

This wide distribution of conduction velocities results from an equally wide variation in fibre diameter. A large nerve fibre conducts impulses faster than a small one. Several other characteristics of nerve fibres depend markedly on their size. Thus the smaller fibres need stronger shocks to excite them, so that the form of the volley recorded from a mixed nerve trunk is affected by the strength of the stimulus. With a weak shock, only the a wave appears; if the shock is stronger, then both a and 3 waves are seen, and so on. The amplitude of the voltage change picked up by an external recording electrode also varies with fibre diameter. On theoretical grounds it might be expected to vary with the square of diameter, but Gasser's reconstructions provide some support for the view that in practice the relationship is more nearly a linear one. In either case, the consequence is that when the electrical activity in a sensory nerve is recorded in situ, the picture is dominated by what is happening in the largest fibres, and it is difficult to see anything at all of the action potentials in the small non-myelinated fibres.

While there is a wide range of fibre diameters in most nerve trunks, it is in most cases difficult to attribute particular functions to particular sizes of fibres. The sensory root of the spinal cord contains fibres giving A (that is a, 3 and y) and C waves; the motor root yields a, y and B waves, the latter going into the white ramus. It is generally believed that B fibres occur only in the preganglionic autonomic nerves, so that what is labelled B in Fig. 2.6 might be better classified as subdivision S of group A. The grey ramus, containing fibres belonging to the sympathetic system, shows mainly C waves. The fastest fibres (a) are either motor fibres activating voluntary muscles or afferent fibres conveying impulses from sensory receptors in these muscles. The y motor fibres in mammals are connected to intrafusal muscle fibres in the muscle spindles, but in amphibia they innervate 'slow' as opposed to 'twitch' muscles (see p. 123). At least some of the fibres of the non-myelinated Cgroup convey pain impulses, but they mainly belong to postganglionic autonomic nerves. The myelinated sensory fibres in peripheral nerves have also been classified according to their diameter into group I (20 to 12 fxm), group II (12 to 4 fxm) and group III (less than 4 fxm). Functionally, the group I fibres are found only in nerves from muscles, subdivision IA being connected with annulo-spiral endings of muscle spindles, and the more slowly conducting IB fibres carrying impulses from Golgi tendon organs. The still slower fibres of groups II and III transmit other modes of sensation in both muscle and skin nerves.

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