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Figure 2.11 M-type K+ currents: from voltage-clamp to 'current-clamp'. Recordings from frog sympathetic neurons. Upper traces in each record show currents (I), lower traces show voltage K). (a) Voltage-clamp records showing membrane currents evoked by 0.5-s voltage steps from holding potentials of — 90 mV (left), where KM channels are shut, and —■46 mV (right), where KM channels are open. (b) Voltage responses to 1-s current injections at the same two potentials observed when the voltage-clamp circuit is switched off. Note that the effect of activating the current is to severely reduce the voltage response to current injection. (Adapted from Fig. 6 in Brown, DA (1988) Ion Channels, Vol. 1 (Ed. Narahashi, T), Published by Plenum Press, New York, pp.55-99)

re-depolarises back to where it started. Hence, because M-channels are voltage-sensitive, changes in voltage affect current through M-channels and changes in current through M-channels in turn affect voltage, in such a manner as to stabilise the membrane potential — a negative feedback effect. This is exactly the opposite effect to current through voltage-gated Na+ channels: current through Na+ channels depolarises the membrane and this increases the number of open Na+ channels, so generating more depolarisation, to give positive feedback and hence generating the 'all-or-nothing' action potential.

Figure 2.12 shows another example of how current is converted into voltage — this time a synaptic current. The bottom trace shows a synaptic current recorded under voltage clamp at a preset voltage of — 60 mV from a ganglion cell on giving a single shock to the preganglionic fibres. The synaptic current is generated by acetylcholine released from the preganglionic fibres, which opens nicotinic cation channels in the ganglion cell membrane to produce an inward cation current. The top trace shows what happens when the voltage-clamp circuit is switched off, to allow the membrane potential to change. The inward synaptic current now generates a depolarisation (the synaptic potential), which in turn initiates an action potential. This is exactly what synaptic potentials should do, of course, but no Na+ current is seen under voltage clamp because the membrane potential is held below the threshold for Na+ channel opening. This threshold is readily exceeded when the clamp circuit is turned off.

EXTRACELLULAR RECORDING

The action potential shown in Fig, 2,12 was recorded from inside the neuron with a micro-electrode, In many instances (particularly in humans!) it is neither convenient nor practicable to use intracellular recording, However, action potentials can still be recorded with extracellular electrodes, by placing the electrode near to the cell (Fig, 2,13), In this case, the electrode tip picks up the local voltage-drop induced by current passing into or out of the cell, Note that (1) the signal is much smaller than the full (intracellularly recorded) action potential and (2) it is essentially a differential of the action potential (because it reflects the underlying current flow, not the voltage change), Nevertheless, since neural discharges are coded in terms of frequency and pattern of

Figure 2.12 From voltage-clamp to 'current-clamp': micro-electrode recordings of synaptic current (I, lower trace) and synaptic potential with superimposed action potential (V, upper trace) from a neuron in an isolated rat superior cervical sympathetic ganglion following a single stimulus (S) applied to the preganglionic nerve trunk, The interval between the stimulus and the postsynaptic response includes the conduction time along the unmyelinated axons of the preganglionic nerve trunk, (SJ Marsh and DA Brown, unpublished)

Figure 2.12 From voltage-clamp to 'current-clamp': micro-electrode recordings of synaptic current (I, lower trace) and synaptic potential with superimposed action potential (V, upper trace) from a neuron in an isolated rat superior cervical sympathetic ganglion following a single stimulus (S) applied to the preganglionic nerve trunk, The interval between the stimulus and the postsynaptic response includes the conduction time along the unmyelinated axons of the preganglionic nerve trunk, (SJ Marsh and DA Brown, unpublished)

Figure 2.13 Relation between the action potential recorded intracellularly from a cat spinal motoneuron following antidromic stimulation (int,) and the local field potential recorded with an extracellular electrode (ext,), (Adapted from Terzuolo, AC and Araki, T (1961) Ann. NY Acad. Sci. 94: 547-558), Published by NYAS

Figure 2.13 Relation between the action potential recorded intracellularly from a cat spinal motoneuron following antidromic stimulation (int,) and the local field potential recorded with an extracellular electrode (ext,), (Adapted from Terzuolo, AC and Araki, T (1961) Ann. NY Acad. Sci. 94: 547-558), Published by NYAS

Figure 2.14 Relation between the EEG recorded from an epileptic focus on the surface of the cerebral cortex (EEG) and the activity of a single cortical neuron recorded extracellularly (e.c.) and intracellularly (i.c.) during an experimental epilepsy induced by topical application of penicillin. Note that the large EEG excursions correspond to the large (synchronised) depolarisations of the neuron, not to action potential discharges. (Adapted from Brain Res. 52: Ayala, GF et al. Genesis of Epileptic Interictal Spikes. New Knowledge of Cortical Feedback systems suggests a Neurophysiological Explanation of Brief Paroxysms, 1-17 (1973) with permission from Elsevier Science)

Figure 2.14 Relation between the EEG recorded from an epileptic focus on the surface of the cerebral cortex (EEG) and the activity of a single cortical neuron recorded extracellularly (e.c.) and intracellularly (i.c.) during an experimental epilepsy induced by topical application of penicillin. Note that the large EEG excursions correspond to the large (synchronised) depolarisations of the neuron, not to action potential discharges. (Adapted from Brain Res. 52: Ayala, GF et al. Genesis of Epileptic Interictal Spikes. New Knowledge of Cortical Feedback systems suggests a Neurophysiological Explanation of Brief Paroxysms, 1-17 (1973) with permission from Elsevier Science)

action potential firing, such 'unit recording' provides the most convenient and useful method of studying neural activity in the intact nervous system.

Problems arise when the electrode is in contact with lots of cells. If these are firing asynchronously, the signals may cancel out so that individual action potentials become lost in the noise. This problem becomes less when the cells are made to discharge synchronously, by (for example) electrical stimulation. This is made use of to record evoked potentials with surface electrodes — for example, to measure conduction velocities along peripheral nerve trunks. Evoked potentials can also be recorded from the brain, via the scalp, along with the EEG (see below). However, the signals are very small (not surprisingly) so have to be averaged by computer. These are used to assess function of sensory systems or in evaluating the progress of demyelinating diseases.

THE EEG

This is a record of fluctuations in activity of large ensembles of neurons in the brain — primarily of the cortical pyramidal cells underneath the recording electrode. Unlike evoked responses, the EEG itself does not represent action potential activity: instead, it originates principally from summed synaptic potentials in pyramidal cell dendrites which (being longer-lasting) summate. However, as with extracellular recording in general, the strongest signal arises when activity of many neurons is synchronised. This happens (for example) in sleep, when large slow-wave activity is recorded: when the subject is woken, the EEG becomes desynchronised. Another instance of synchronised activity occurs in epilepsy (Figure 2.14) in which large numbers of neurons show a simultaneous depolarisation (the paroxysmal depolarising shift), again reflecting large underlying synaptic potentials.

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