Excitability of human muscle afferents studied using threshold tracking of the H reflex

Abstract

In human peripheral nerves, physiological evidence has been presented for a number of biophysical differences between cutaneous afferents and alpha motor axons. The differences in strength-duration properties for cutaneous afferents and motor axons in the median nerve have been attributed to greater expression of a persistent Na(+) conductance (I(Na,P)) on cutaneous afferents. However, it is unclear whether the biophysical properties of human group Ia afferents differ from those of cutaneous afferents. The present studies were undertaken to determine whether the properties of human group Ia afferents can be studied indirectly using 'threshold tracking' to measure the excitability changes in the H reflex, and to determine whether the excitability of group Ia afferents differs from that of cutaneous afferents. The strength-duration properties of the soleus H reflex and soleus motor axons were measured at rest and during sustained voluntary contractions. Similar experiments were performed on the median nerve at the wrist to study the strength-duration properties of cutaneous afferents, alpha motor axons and H reflex of the thenar muscles. In addition, the technique of 'latent addition' was used to determine whether there was a difference in a low-threshold conductance on soleus Ia afferent and motor axons. The present findings indicate that the strength-duration time constant (tau(SD)) for the H reflex is longer than that for alpha motor axons, but similar to that for cutaneous afferents. There were no differences in tau(SD) for the soleus H reflex at rest and during contractions, suggesting that tau(SD) for the H reflex is largely unaffected by changes in synaptic or motoneurone properties. Finally, the difference in latent addition suggests that the longer tau(SD) of the soleus H reflex may indeed be due to greater activity of a persistent Na(+) conductance on Ia afferents than on soleus alpha motor axons.

Figure 1. Strength-duration data for soleus H reflex and M wave

A and B: left, strength-duration curves for 8 subjects (means ± s.e.m.) for soleus H reflex (10 % of maximum CMAP) with stimulus intervals of 1 s (•) and 2 s (○) (A) and for motor potentials (50 % of maximum CMAP) (B). A and B: right, corresponding charge-duration plots. Threshold charge (= stimulus strength ×stimulus duration) is linearly related to stimulus duration. The time constant (τ_SD) is given by the (negative) intercept of the linear regression line on the duration axis. Rheobase is given by the slope of the regression line. In A and B, the regression lines are based on the equations given in the panels on the right.

Figure 2. Effects of voluntary contraction on τ_SD

A, strength-duration data for the soleus H reflex for 6 subjects (means ± s.e.m.) normalized to the threshold for the 1 ms stimulus and plotted against stimulus duration, at rest (•) and during contractions using weaker stimuli (○) and comparable stimuli (×), as described in Methods. B, charge-duration plots for the data in A. The regression lines in A and B are derived from the equations in B.

Figure 3. Comparison of the strength-duration properties of cutaneous afferents, H reflex and motor axons of the median nerve at the wrist

A, strength-duration curves for cutaneous afferents (○), H reflex (▴) and motor axons (•) of the median nerve at the wrist for 6 subjects (means ± s.e.m.). B, charge-duration plots for the data in A. In A and B, the regression lines are derived from the equations in B.

Figure 4. Relationship between τ_SD and threshold increase at the 0.2 ms interval

Histograms of threshold increase at the 0.2 ms conditioning-test interval in the latent addition paradigm (A) and τ_SD (B) for 6 subjects (means ± s.e.m.) for soleus H reflex and motor axons. In C, there is a significant relationship between τ_SD and the threshold increase at the 0.2 ms conditioning-test interval. The arrows indicate the data points for the subject with the most marked values (for the H reflex). The relationship was significant even when these two data points were eliminated (P + 0.046).