Generally, coherence is observed in a frequency band between 15 and 30 Hz, although coherence peaks may also be seen around 40–60 Hz (Farmer et al., 1993, 1997; McAuley, Rothwell & Marsden, 1997;
Brown et al., 1998). The coherence peaks reflect the frequency content of the last-order input to the spinal motoneurones, and in the case of the coher-ence between 15 and 30 Hz, the evidcoher-ence suggests that it depends on intact transmission in the pyra-midal tract (cf. Farmer et al.,1993,1997).
General conclusions
Methods
A number of methods have been developed to inves-tigate the changes in excitability of human motoneu-rones after a conditioning volley. The simplest one is the modulation of the on-going EMG, which pro-vides rapidly a full-time course of the changes in motoneuronal excitability. This is a distinct advan-tage when investigating patients, but the temporal resolution of the method is weak. Because the H reflex enables a comparison of the results obtained at rest and during movement, it remains the only avail-able method with which it is possible to investigate how transmission in spinal pathways is changed by motor tasks in human subjects. The technique of the H reflex is simple but, besides changes in motoneu-ronal excitability, the size of the reflex depends on mechanisms acting on the afferent limb of the reflex and on ‘pool problems’. The drawbacks related to the complexity of the so-called monosynaptic reflex pathway can usually be controlled by parallel inves-tigations on single motor units (using post-stimulus time histograms for voluntarily activated units, or the unitary H reflex). Such studies should be per-formed systematically when studying motor control physiology in human subjects, keeping in mind the fact that the recordings provide data only for the lowest-threshold motoneurones in the pool, and that the fractionation and focusing of voluntary drives necessary for PSTHs may result in an unnaturally biased input to the pool. The F wave provides a
flawed measure of the excitability of the motoneu-rone pool and has little place as a research tool. Cor-tical stimulation has made it possible to investigate corticospinal excitation of motoneurones and the corticospinal control of spinal pathways. However, a single stimulus can generate multiple corticospinal volleys and this complicates the interpretation of the results. Spatial facilitation is an indirect tech-nique used to demonstrate the convergence of two inputs onto common interneurones projecting to the motoneurone(s) being tested, and is an indispens-able tool for probing interneuronal circuits in human subjects.
Development
There has been considerable development of meth-ods available to explore spinal pathways in human subjects since the first investigations performed with the H reflex in the 1950s: PSTHs of single units, spatial facilitation techniques, cortical stimulation.
These developments have resulted in considerable advances in motor control physiology and in new diagnostic procedures. An important principle is that, in human experiments, conclusions are more firmly based when supported by evidence using more than one technique.
R´esum´e
Monosynaptic reflex
Initial studies
The monosynaptic reflex was introduced in animal studies in the early 1940s as a tool for investiga-ting excitability changes in the motoneurone pool. In human subjects, the first motoneurones discharging in the soleus H reflex elicited by electrical stimula-tion of the posterior tibial nerve have been shown to do so at a latency consistent with a monosynaptic pathway.
Underlying principles
Ia afferents have monosynaptic excitatory projec-tions onto homonymous motoneurones, and this pathway is responsible for the tendon jerk. In the control reflex, a group Ia volley causes some motoneurones to discharge and creates EPSPs in other motoneurones which, though they do not dis-charge, are slightly depolarised. If motoneurones are facilitated by a conditioning volley, the size of the test reflex increases because some of these subliminally excited motoneurones will discharge in response to the summation of conditioning and test EPSPs.
Conversely, if motoneurones receive conditioning IPSPs, the test Ia volley will no longer be able to discharge the motoneurones last recruited into the control reflex, and the size of the test reflex will be decreased.
Basic methodology
(i)H reflexes can be recorded at rest from soleus, quadriceps, semitendinosus, and FCR, and from virtually all limb muscles during weak vol-untary contractions.
(ii) Reflexes are recorded through bipolar surface electrodes placed over the corresponding mus-cle belly. Reflex latency is measured to the first deflection of the H wave from baseline, and its amplitude usually assessed peak-to-peak.
Contamination of the recording by the EMG of another muscle may occur due to spread of the test or conditioning stimuli, volume conduc-tion of the condiconduc-tioning potential, or to the fact that the reflex response occurs in a number of muscles, not just the one being studied. Palpa-tion of muscle tendons may help identify this problem. Another simple way of ensuring that the reflex response originates from the mus-cle over which it is recorded is to check that it increases during voluntary contraction of that muscle.
(iii) H reflexes are obtained by percutaneous elec-trical (or magnetic) stimulation of Ia afferents in the parent nerve. The optimal stimulus
dura-tion for eliciting the H reflex is long (1 ms).
The best method involves placing the cathode over the nerve and the anode on the opposite side of the limb. However, in areas where there are many nerves, bipolar stimulation should be used to avoid encroachment of the stimulus upon other nerves. Reflex attenuation due to post-activation depression is sufficiently small after 3–4 s to allow routine testing at 0.2–0.3 Hz for relaxed muscles.
(iv) H and M recruitment curves: when increas-ing the intensity of the electrical stimulus to the parent nerve, there is initially a progres-sive increase in the reflex amplitude. When the threshold of motor axons is exceeded, a short-latency direct motor response (M wave) appears in the EMG. Further increases in sti-mulus intensity cause the M wave to increase and the H reflex to decrease. The test reflex should never be on this descending part of the recruitment curve. Finally, when the direct motor response is maximal (Mmax), the reflex response is totally suppressed, because the antidromic motor volley set up in motor axons collides with and eliminates the reflex dis-charge. Mmax provides an estimate of the response of the entire motoneurone pool and must always be measured, and the amplitudes of the reflexes should be expressed as a per-centage of Mmax. The constancy of a small M wave may be used to monitor the stability of the stimulation conditions.
(v) Tendon jerks may be convenient for test-ing the excitability of motoneurones of prox-imal muscles, although this introduces two complications: the mechanical delay due to the tendon tap, and the possibility that changes in
drive might alter the sensitivity of muscle spindle primary endings to percussion (how-ever, see Chapter3).
(vi) Control and conditioned reflexes should be randomly alternated, because this prevents the subject from predicting the reflex sequence, and regular alternation produces erroneously large results.
R´esum´e 51
(vii) The H reflex technique underestimates the central delay: in individual motoneurones, the rise time of the EPSP ensures that the spike evoked by the monosynaptic input in the last recruited motoneurones occurs suffi-ciently late to be altered by a disynaptic PSP. An EPSP elicited by a conditioning volley enter-ing the spinal cord after the test volley may summate with the test Ia EPSP and cause the motoneurone to discharge ‘too early’. In the motoneurone pool, the test reflex discharge is desynchronised.
(viii) The recovery cycle of the H reflex is the result of too many phenomena to allow useful phys-iological insights.
(ix) With threshold tracking, test stimuli are var-ied to maintain an H reflex of constant size.
Reflex facilitation produces a decrease in the current required to produce the test reflex.
There are advantages of threshold tracking over the conventional technique of ampli-tude tracking: less variability, constant popu-lation of motoneurones contributing to the test response, avoiding the problem of size-related changes in test reflex sensitivity, and the dynamic range of threshold tracking is wide.
There are also disadvantages: changing stimu-lus intensity changes the intensity of the affer-ent volley, and the reflex size also depends on mechanisms acting on the afferent volley (see below); when excitability changes, there is a delay before a new threshold can be reached.
Reflex size also depends on mechanisms acting on the afferent volley
As a result, many mechanisms other than changes in motoneurone excitability can alter reflex size.
(i) Changes in the excitability of Ia afferent axons will occur when there is a change in the discharge rate of those axons. An increased discharge rate will pro-duce ‘activity-dependent hyperpolarisation’ of the active axons. When axons hyperpolarise, a constant stimulus will produce a smaller afferent volley.
(ii) Changes in presynaptic inhibition of Ia ter-minals must always be considered when there is a change in the amplitude of the monosynaptic reflex.
To that end, several methods have been developed (see Chapter8).
(iii) Post-activation depression of the monosynap-tic reflex is due to reduced transmitter release from previously activated Ia afferents, and is prominent at short intervals of 1–2 s or less (see Chapter 2).
The depressive effects of stimulus rate on the reflex size are generally taken into account in reflex studies, but post-activation depression occurs under any cir-cumstance that activates the Ia afferents responsible for the test reflex, and this is often ignored. Misinter-pretations have arisen when comparing test reflexes recorded at rest and during or after a voluntary con-traction because this phenomenon was neglected.
(iv) Autogenetic inhibition elicited by the test vol-ley helps limit the size of the reflex. The quadriceps H reflex may be suppressed by conditioning volleys that, by themselves, do not depress the on-going EMG or the background firing of single motor units.
The suppression is due to convergence between the conditioning volleys and group I (Ib) afferents in the test volley for the H reflex onto inhibitory interneu-rones mediating disynaptic group I inhibition. This helps limit the size of the H reflex, and creates a prob-lem for H reflex studies, because the reflex cannot be considered exclusively monosynaptic. Only those changes that affect the entire monosynaptic excita-tory peak in the PSTH of single units to the same extent, and specifically those that affect the initial 0.5–1.0 ms of the peak, can be considered to have affected the monosynaptic pathway.
‘Pool problems’
(i) Expressing the changes in the H reflex size as a percentage of control values causes changes to appear much larger with the smaller test H reflexes.
This is a mathematical consequence of normalising to control H reflexes of different size, and can be avoi-ded by expressing the results as a percentage of Mmax. (ii) Size-related sensitivity of the test reflex (non-linearity within the motoneurone pool). Because the
input–output relationship in the motoneurone pool is sigmoid, when the conditioning input is strong, the number of additional motoneurones recruited (or de-recruited) by a constant input increases with increasing size of the control test reflex and then decreases. When the effect of the conditioning input is more modest, the relationship is relatively flat between the two phases of increase and decrease.
The input–output relationship must be taken into account when the size of the control H reflex evoked by a constant test stimulus is different (e.g. when comparing rest and contraction). To set the test sti-mulus intensity so that the reflex remains within the linear range may be a solution when the condition-ing effect is moderate. Otherwise, the intensity of the test stimulus may be adjusted so that the size of the unconditioned reflex is the same in the two situ-ations, but this introduces problems because chan-ging the intensity of the test stimulus alters the afferent volley responsible for the reflex.
(iii) Changes in the recruitment gain of the reflex.
Despite a control reflex of constant size, a greater change in the H reflex could occur (e.g. during movement) if the conditioning input had different effects on low- and high-threshold motoneurones, thus compressing the range of thresholds in the motoneurone pool and increasing the slope of the input–output relationship for the test reflex (i.e.
altering the ‘recruitment gain’ of the reflex). As a result, a constant conditioning volley would then recruit more motoneurones than in the control situation, even though there was no change in the specific pathway explored. The only way to control for this possibility is to investigate whether the conditioning EPSP is changed in single motor units.
(iv) Plateau potentials can develop in motoneu-rones, and may be triggered by peripheral inputs and/or voluntary effort. The triggering of ‘plateaux’
would greatly distort the input–output relationship of the pool.
Normative data
Reflex amplitude varies widely in normal subjects, and amplitude measurements in patients are of little
value except when pathology is asymmetrical. Reflex latency has a strong correlation with height and a weak but significant correlation with age.
Conclusions
The H reflex technique is attractively simple, but strict methodology is required for valid results. The reflex pathway is not as simple as it first seems, but most of the complexities may be controlled by parallel investigations on the discharge of sin-gle motor units. The H reflex enables a compari-son of results obtained at rest and during move-ment, and it therefore remains the standard method for investigating how transmission in spinal path-ways is changed during motor tasks in human subjects.
F wave
Principles of the method and basic methodology
A supramaximal electrical shock delivered to a nerve will elicit, in addition to the Mmaxresponse, a late response, termed the F wave. The pathway involves an antidromic volley in motor axons, ‘backfiring’ of motoneurones and conduction of an orthodromic volley to the muscle. For many muscles, F waves occur in high-threshold motoneurones preferen-tially.
Characteristics
F waves can be recorded from any muscle. They typi-cally vary from trial to trial in amplitude, latency and shape. The persistence is the percentage of stimuli that produce F waves. The latency of the F wave is roughly similar to that of the H reflex, and its ampli-tude is normally below 5% of Mmax. Its sensitivity to changes in motoneurone excitability is low and it has little place as a research tool.
R´esum´e 53
F wave studies
These are useful clinically in detecting acquired demyelinating polyneuropathies, where the latency of the F wave may be quite prolonged, and in sus-pected proximal nerve lesions that are otherwise inaccessible to routine testing.
Modulation of the on-going EMG Principles of the method and basic methodology
The on-going EMG during a steady voluntary con-traction is full-wave rectified, averaged, and plot-ted against the conditioning stimulus. An excitatory input to motoneurones will facilitate the on-going EMG activity, and an inhibitory one will suppress it.
The central delay of a conditioning effect can be cal-culated from the expected time of arrival of the con-ditioning volley at the segmental level of the tested motoneurone pool.
Changes in the on-going EMG do not necessarily parallel those in the H reflex This is because: (i) the on-going EMG is more sensi-tive to inhibition than the monosynaptic reflex, and (ii) mechanisms that can alter the efficacy of the group I test volley can alter the H reflex.
Critique: advantages, limitations, conclusions Advantages
The full time course of the changes in motoneu-ronal excitability can be recorded more easily and more rapidly than when using the monosynaptic reflex; comparing the modulation of the on-going EMG during various motor tasks may give a gen-eral idea of the different patterns elicited by a given stimulus in these tasks; the absence of test stimu-lation avoids problems due to instability of the stimulating electrodes for the test volley during movement.
Limitations
The technique can be used only in an active motoneurone pool; the temporal resolution of the method is limited; when there is an initial facilita-tion, the subsequent post-spike AHP and recurrent inhibition can obscure late synaptic events; the type of motor unit involved in the EMG modulation can-not be specified.
Conclusions
Modulation of on-going EMG activity has the advan-tages of simplicity and speed, and this is an asset par-ticularly in studies on patients. However, the tech-nique can only provide a general idea of the response of the motoneurone pool to a stimulus.
Post-stimulus time histograms (PSTHs) of the discharge of single motor units
Almost by definition, the ‘pool problems’ inherent in the compound H reflex are not an issue when study-ing the responses of sstudy-ingle motor units.
Underlying principles
The effect of a particular input to a single motoneurone can be determined by constructing a histogram of the timing of motoneurone spikes fol-lowing repeated presentation of an appropriate sti-mulus. This procedure extracts from the naturally occurring spike train only those changes in firing probability that are time-locked to the stimulus.
Basic methodology Recording
A prerequisite for PSTH investigations is the isola-tion of a single motor unit during voluntary contrac-tion. This is possible with a window discriminator with variable upper and lower levels, and has been made easier with sophisticated template-matching paradigms that allow automatic recognition of the
shape of motor units. The background discharge of the unit must be stable to avoid false peaks or troughs in the PSTH.
Stimulation
Stimuli may be delivered randomly, or with respect to the discharge of the motor unit, each stimulus then being triggered at a fixed delay after the preced-ing motoneurone discharge. The latter allows one to avoid the AHP or conversely to use the AHP to atten-uate the monosynaptic discharge of the motor unit.
The intensity of the stimulation should be subliminal for the compound response.
Assessment of the timing of the changes in firing probability
The recording window usually begins after a fixed delay following the stimulus, when stimulus arte-fact and the M wave have disappeared. The latency of the first bin of a group of consecutive bins with a significant change in firing probability is taken as the latency of the effect. The time resolution of the method is excellent, dependent only on the bin width. The onset of a period of increased (EPSP) or decreased (IPSP) firing probability may be iden-tified from a cumulative sum (CUSUM) display by the onset of a positive (or negative) slope. CUSUMs commonly allow more confident estimates of latency than can reasonably be achieved using raw his-tograms which inevitably contain irregular bin-to-bin fluctuations. To estimate the central delay of an effect, it is convenient to compare the latency of the peak (or trough) to that of homonymous mono-synaptic Ia excitation in the same unit.
Assessment of the size and significance of the peaks and troughs in the PSTH
When stimulation is delivered randomly with respect to the discharge of the motor unit, the background firing is calculated during the period immediately preceding the stimulation. When stimulation is trig-gered by the previous motor unit discharge, the
probability of firing is affected by the AHP follow-ing this discharge. To take the gradual recovery of membrane potential during the spike inter-val into account, a histogram of firing probability is constructed under control conditions without sti-mulation, control and conditioned situations being randomly alternated in the same sequence. The con-trol histogram is then subtracted from the condi-tioned one. To normalise the results it is convenient to express the number of counts in each bin as a per-centage of the number of triggers. The PSTH method may be used to calculate the conduction velocity of the Ia afferents.
Conclusions
The PSTH is a valuable method, which allows the investigation of single motonerones in human sub-jects with good time resolution. It can be applied in virtually all muscles, and the use of needle electrodes allows studies on high-threshold motor units. It is an indispensable complement of the H reflex in avoid-ing ‘pool problems’. The most important limitation is that it requires a voluntary contraction of the tested muscle.
Unitary H reflex
Methodology
A ‘unitary’ H reflex is the H reflex of a single motor unit and is recorded with a needle electrode. Using a threshold tracking technique, measurements are made of the current required to just discharge the motoneurone at rest and of the current required to produce a liminal homonymous Ia peak in PSTHs for the unit. The difference between these values has been termed the ‘critical firing stimulus’ (CFS) and represents the size of the EPSP needed to produce a discharge of the single motoneurone.
The CFS size
This can be used as a measure of the size of the test Ia EPSP necessary to activate the motoneurone. When conditioning stimuli produce an IPSP or EPSP within