Ischaemic changes in refractoriness of human cutaneous afferents under threshold-clamp conditions

Abstract

1. A technique was developed to counteract the changes in threshold to electrical stimuli of large myelinated cutaneous afferents in the human median nerve induced by ischaemia for 13 min. Intermittent application of polarizing currents was used in five subjects, in whom refractoriness, supernormality and the strength-duration time constant (tauSD) were tracked to determine whether compensating for the ischaemia-induced changes in threshold also controlled the ischaemic changes in these excitability parameters. 2. The threshold compensation prevented the ischaemic changes in tauSD, an excitability parameter dependent on nodal Na+ channels. Threshold compensation did not prevent the changes in refractoriness and supernormality, whether the compensation began 10, 100 or 200 ms prior to the test stimuli. 3. In three subjects, continuous polarizing current was injected for 13 min to compensate for the ischaemic change in threshold, thus clamping threshold at the pre-ischaemic level. Again, tauSD was effectively controlled, but there were still ischaemic changes in refractoriness and supernormality. 4. The effective control of tauSD suggests that both the intermittent threshold compensation and the continuous threshold clamp effectively controlled membrane potential at the node of Ranvier. 5. The ischaemic increase in refractoriness when threshold was kept constant could be due to interference with the processes responsible for refractoriness by a metabolic product of ischaemia. The ischaemic change in supernormality during effective compensation probably results from the intrusion of refractoriness into the conditioning-test intervals normally associated with maximal supernormality. 6. The present results indicate that ischaemia has effects on axonal excitability that cannot be readily explained by changes in membrane potential. Specifically, it is suggested that ischaemic metabolites interfere with the recovery of Na+ channels from inactivation.

Figure 1. The effect of intermittent threshold compensation on the τ_SD

A and B illustrate the threshold changes as measured using test stimuli of 0.1 and 1 ms duration, respectively, produced by ischaemia in the absence of threshold compensation (thin traces) and in its presence (thick traces). The ischaemic change in threshold was effectively controlled except at the onset of the ischaemia and, particularly, at its offset. C shows the profile of the current required to compensate for the ischaemic change in threshold illustrated in B. D illustrates τ_SD, calculated from the data in A and B. There was little change in τ_SD during ischaemia. The traces in A-C are the means for five subjects, and the data in D represent the mean ±s.e.m. The compensating current was applied 10 ms before the test stimuli for the data in this figure and in Figs 2 and 3. In this and subsequent figures, the horizontal filled bars indicate the period of ischaemia.

Figure 2. The effects of intermittent threshold compensation on refractoriness, measured as the threshold increase at a conditioning-test interval of 2 ms

A illustrates mean data ±s.e.m. for five subjects. B and C illustrate, respectively, the mean conditioned thresholds and the mean unconditioned thresholds for the five subjects, i.e. the data used to calculate refractoriness in A. Note that C contains the same data as in Fig. 1A.

Figure 3. The effects of intermittent threshold compensation on supernormality, measured as the threshold decrease at a conditioning-test interval of 7 ms

Despite the effective control of threshold (C, same data as in Fig. 1A), there were still changes in the conditioned threshold (measured using a conditioning-test interval of 7 ms, B) and supernormality (A). A illustrates mean data ±s.e.m. for five subjects.

Figure 4. The effects of intermittent compensating currents (D) of different duration on refractoriness (A), supernormality (B) and τ_SD(C)

The data in the left-hand column for the 10 ms compensating current are those illustrated in Figs 1–3. ^, uncompensated; •, compensated. Data are means ±s.e.m. for five subjects.

Figure 5. The effects of a continuous threshold clamp on refractoriness (A), supernormality (B) and τ_SD (C) for three subjects

In A-C, the minimal scatter of data along the X-axis indicates the efficacy of the continuous threshold clamp. ^, four 1 min measurements of the appropriate parameter immediately prior to ischaemia; •, four 1 min measurements towards the end of the 13 min period of ischaemia; ▵, four 1 min measurements made after release of ischaemia. Refractoriness more than doubled in each subject during ischaemia under effective threshold-clamp conditions. Supernormality was abolished in subjects 2 and 3 but decreased only slightly in subject 1 (in whom the absolute increase in refractoriness was least). There was little ischaemic change in τ_SD.

Figure 6. The effect of a continuous threshold clamp on different parameters of axonal excitability

This figure illustrates the mean data for the three subjects during ischaemia, with the change in each parameter being expressed as a percentage of the pre-ischaemic value. Both threshold (lower panel, ♦) and τ_SD (upper panel, ▪) were effectively controlled, except at the onset and offset of the ischaemic episode. The changes in refractoriness and supernormality mirrored one another during ischaemia but diverged substantially following its release.

Figure 7. The relationship between the ischaemic change in supernormality and the ischaemic change in refractoriness during a continuous threshold clamp

The data from Fig. 6 have been replotted, to demonstrate the close relationship between the changes in supernormality and refractoriness (•). There was little change in threshold during ischaemia under threshold-clamp conditions (^) and there was no relationship between the change in threshold and the change in refractoriness.