CONECTORES

A number of authors have calculated the conduction velocity of afferents in the human posterior tibial nerve. Magladery and McDougal (1950), using two different stimulation sites in the popliteal fossa and examining the changes in the latencies of the M and H responses in four individuals, described the highest conduction velocity of the afferents producing the H- reflex as 60 metres per second (m/s), but they acknowledged limitations in their method and considerable variability in their results.

In 1983 Burke, Gandevia and McKeon used microelectrodes to record directly from the posterior tibial nerve and they too produced H-reflexes using two different stimulation sites in four individuals and used the differences in distance and latency to calculate conduction velocity. The fastest group one afferents were found to conduct at between 62 and 67 metres per second and the slowest at 36 to 45 m/s, although the latter figures were perceived as an underestimate, because they were calculated from measurements of the end rather than the onset of the potentials concerned.

Hultborn et al. (1987) used post-stimulus time histograms of the firing o f individual soleus motor units to identify the latencies of facilitaion following stimulation of soleus la fibres at two different sites. They calculated that the fastest fibres conducted at 64 m/s. Hultbom and his colleagues also made similar calculations for the femoral nerve during the same series of experiments. The values above have been used as guidelines in the present study.

Far less information is available concerning human group II afferent conduction times, but in a recent study by Nardone and Schieppati (1998)

the latencies of soleus M waves and H-reflexes were used along with the latencies of the short and medium latency responses (SLR and MLR) to perturbation in soleus and flexor digitorum brevis (FDB) to calculate estimated group II conduction velocities. They estimated the mean conduction velocity of fibres mediating the FDB MLR was 2L4m /s. They were also able to estimate the central delay involved in the ipsilateral group II pathway from FDB, which they calculated to be 6.7ms compared to the estimated central delay of the group I pathway which was 1.4ms.

These figures were compared to the estimates of fastest and slowest rates made by comparing relative rates of conducton between the groups using the detailed study of afferents in the cat by Jack (1978). Jack classified muscle afferents in hindlimb cat nerves according to their relationship with the fastest fibres in that nerve, because there was considerable variation between individual cats and between individual nerves. Group II afferents were those with a conduction velocity of less than 65% of that of the fastest fibres. Such a comparison would yield fastest conduction velocity of group II afferents to be around 40m/s and the slowest around 12m/s. The direct estimates of Nardone and Shieppati (1998) fit comfortably within this range, although the 21.4 m/s value is lower than the comparison with Jack’s might have led one to expect for the fastest group II fibres. However the values are for afferents from a small foot muscle and may not represent the very fastest group II fibres, but this is the most direct estimate available and could point to previous overestimates.

Simonetta-Moreau, Marque, Marchand-Pauvert and Pierrot-Deseilligny (1999) also made estimates of maximum group II conduction velocities, from perceived ipsilateral group II excitatory effects in the human low er limb.Their estimates were close to Table II which was constructed simply to consider minimum possible arrival times using the peripheral nerve

pathway distances described above. The ranges of conduction velocities of the group I afferents in the human posterior tibia! nerve was taken to be from 64 m/s to 40 m/s The range of conduction velocities of group II afferents was taken to be between 40 m/s and 12 m/s. Because it is the arrival of conditioning volleys at the motoneurone pool that can cause changes in excitability, it is the differences in the conduction distance and conduction times of the afferent pathways which are important. An extra 2 ms was added to the travelling time for all contralateral pathways, as the estimated minimum time required to traverse the spinal cord and to include one extra synapse in the pathway. This delay time was extended for group II fibres in the discussions in the light of the findings of Nardone and Schieppati (1998).

5.4c ELECTRICAL STIMULATION OF HUMAN AFFERENTS

In animal experiments the relationships between the thresholds to electrical stimulation of the different populations of afferent fibres in peripheral nerves have been extensively studied. The similarities in the range of conduction velocities and thresholds of the la and Ib afferent populations in cats are well-documented (e.g. Eccles & Lundberg 1959, Matthews 1972, Jack 1978) and led to the development of methods designed to block o r change the threshold of one population in order to allow selective activation (e.g. Coppin, Jack & MacLennan 1970).

It has been suggested that in some cat peripheral nerves it is possible to separate group I stimulation from that of group II fibres (see Jack 1978). In humans the la and Ib fibre populations have thresholds and distributions of conduction velocities which cannot be separated by selective electrical stimulation (Pierrot-Deseilligny, Morin, Bergego & Tankov 1981).

The threshold to activation which can be most easily and most reliably recorded in humans is the threshold of the largest motor axons (MT). This is probably most accurately identified by recording the intensity at which the first visible component appears above or below the baseline surface EMG at the latency of the M wave. Hultbom et al. (1987) have shown that the threshold of human la afferents of soleus is approximately 0.6 times that of the motor axons (which explains why it is routinely possible to obtain an H-reflex response without producing an M wave).

In the cat group II fibres are activated at intensities of between 2.5 and 5 times the la threshold intensity (Eccles and Lundberg 1959, Edgley and Jankowska 1987). If the relative relationship of the thresholds between the fibres is the same in humans as it is in cats, then for the group II fibres to be activated by intensities of between 2.5 and 5 times the la afferents threshold intensity, intensities of between 1.5 and 3 times the threshold of the motor response would be needed.

Because of the large circumference of human peripheral nerve trunks, fibres will respond to low intensity percutaneous stimulation not only because they have certain thresholds to excitation, but also in relation to where they lie in the nerve trunk. Fibres which do not lie in that section of the trunk which is adjacent to the surface electrode may require higher stimulus intensities than would be predicted in order to be activated. Gracies et al.(1994) found that la afferents were still being activated at 4 times MT. A major implication of such findings is that there may be greater overlap of effects from different fibres. Maximal activation of each group is likely to occur when large numbers of higher threshold fibres have already been recruited.