ABSTRACT: From measurements of nerve excitability and the changes in
excitability produced by nerve impulses and conditioning currents, it is pos-
sible to infer information about the membrane potential and biophysical
properties of peripheral axons. Such information cannot be obtained from
conventional nerve conduction studies. This article describes a new method
that enables several such measurements to be made on a motor nerve
quickly and reproducibly, with minimal operator intervention. The protocol
measures stimulus–response behavior using two stimulus durations (from
which the distribution of strength–duration time constants can be estimated),
threshold electrotonus to 100-ms polarizing currents, a current–threshold
relationship (indicating inward and outward rectification), and the recovery of
excitability following supramaximal activation. The method was tested on 30
healthy volunteers, stimulating the median nerve at the wrist and recording
from the abductor pollicis brevis. The results were comparable with previ-
ously published normal data, but the recordings took less than 10 min. The
convenience and brevity of the new method make it appropriate for routine
clinical use.
© 2000 John Wiley & Sons, Inc. Muscle Nerve 23: 399–409, 2000
MULTIPLE MEASURES OF AXONAL
EXCITABILITY: A NEW APPROACH IN
CLINICAL TESTING
MATTHEW C. KIERNAN, PhD, FRACP,1 DAVID BURKE, MD, DSc,2
KJELD V. ANDERSEN, MD,3 and HUGH BOSTOCK, PhD1
1 Sobell Department of Neurophysiology, Institute of Neurology, Queen Square,
London WC1N 3 BG, UK
2 Prince of Wales Medical Research Institute and University of New South Wales,
Sydney, NSW, Australia
3 Clinical Neurophysiology Laboratory, University Hospital, Trondheim, Norway
Accepted 7 October 1999
Nerve conduction studies (NCS) remain the
method of choice for the clinician investigating pe-
ripheral nerve function. However, measurements of
action potential amplitude and latency are limited
indices of function, providing information only on
the number of conducting fibers and the conduction
velocity of the fastest. These data do not always cor-
relate well with a patient’s clinical status: on the one
hand, even quite large changes in conduction veloc-
ity may be asymptomatic whereas, on the other hand,
routine NCS may be normal when there is clinical
weakness. Conduction velocity is a nonspe-
cific indicator of pathophysiology; that is, it may be
decreased due to membrane depolarization or hy-
perpolarization, axonal thinning, sodium channel
blockade, and demyelination or remyelination with
short internodes.
Measurements of nerve excitability by threshold
tracking are sensitive to membrane potential at the
site of stimulation, and provide complementary in-
formation to conventional NCS.9 In recent years,
threshold tracking methods have been used increas-
ingly as research tools for investigating physiological
and pathophysiological mechanisms, such as the
mechanisms of postischemic and posttetanic ectopic
discharges, and the biophysical basis for the func-
tional differences between motor and sensory
axons.4–8,10,19,22 To date, however, these techniques
have not found favor for routine diagnosis and as-
sessment of patients. This has been due partly to a
lack of suitable commercially available equipment
and partly to the time-consuming nature of the stud-
ies.
Abbreviations: A/D, analog-to-digital; APB, abductor pollicis brevis;
CMAP, compound muscle action potential; EMG, electromyogram; NCS,
nerve conduction studies; PC, personal computer; RRP, relative refractory
period; SR, stimulus–response; tSD, strength–duration time constant
Key words: nerve excitability; threshold electrotonus; recovery cycle;
strength–duration time constant; supernormality
Correspondence to: H. Bostock; e-mail: H.Bostock@ion.ucl.ac.uk
© 2000 John Wiley & Sons, Inc.
Multiple Excitability Measures MUSCLE & NERVE March 2000 399

In an attempt to broaden the application of such
methods, we have developed a protocol that allows
rapid measurement of multiple parameters of nerve
excitability and an appropriate plotting program.
The aim of the protocol was to enable rapid esti-
mation of a broad range of different membrane pa-
rameters not investigated with conventional NCS,
sufficiently quickly that the protocol could be incor-
porated into routine diagnostic assessments. The
method described provides a convenient assessment
of multiple nerve excitability parameters within a re-
cording session of 10–15 min, and is suitable for
routine screening purposes. It may provide a signifi-
cant expansion of the armamentarium available to
the clinical neurophysiologist for the investigation of
peripheral nerve disorders.
METHODS
The protocol was tested on 30 healthy volunteers (22
men, 8 women, aged 24–58 years). To assess repro-
ducibility, the protocol was repeated on two subjects
on six and ten occasions, and in one subject the
protocol was repeated eight times in rapid succes-
sion in the same experiment without moving the
electrodes. The experiments were performed in
Trondheim, London, and Sydney on healthy volun-
teers with their informed consent, using techniques
approved by the Norwegian Regional Committee for
Medical Research Ethics on Humans (Health Region
IV); the Joint Research Ethics Committee of the Na-
tional Hospital for Neurology and Neurosurgery and
the Institute of Neurology, London; and the Com-
mittee on Experimental Procedures Involving Hu-
man Subjects of the University of New South Wales,
Sydney.
Compound muscle action potentials (CMAPs)
were recorded from thenar muscles using surface
electrodes over the abductor pollicis brevis (APB),
with the active electrode at the motor point and the
reference on the proximal phalanx. The electromyo-
graphic (EMG) signal was amplified (gain 1000,
bandwidth 1.6 Hz to 2 kHz) and digitized by com-
puter (486 or Pentium PC) with an analog-to-digital
(A/D) board (DT2812, Data Translation, Marlboro,
Massachusetts), using a sampling rate of 10 kHz.
Stimulus waveforms generated by the computer were
converted to current with a purpose-built isolated
linear bipolar constant current stimulator (maxi-
mum output ±50 mA) or an equivalent commercial
prototype (DS5, Digitimer, Welwyn Garden City,
Herts, UK). The results with the two stimulators were
indistinguishable. The stimulus currents were ap-
plied via nonpolarizable electrodes (Red Dot, 3M
Health Care, Borken, Germany), with the active elec-
trode over the median nerve at the wrist, and the
reference electrode about 10 cm proximal over
muscle. Stimulation and recording were controlled
by new software, written in BASIC (QTRAC version 4.3,
copyright Institute of Neurology, London, with mul-
tiple excitability protocol TRONDHM).
Test current pulses of 0.2 ms or 1 ms were ap-
plied regularly at 0.8-s intervals, and combined with
suprathreshold conditioning stimuli or subthreshold
polarizing currents as required. The amplitude of
the CMAP was measured from baseline to negative
peak. For all tracking studies, the target CMAP was
set to be 40% of the peak response. Skin tempera-
ture was monitored close to the stimulation site and
was constant in any one study but varied between 30°
and 34°C in different studies. It was not kept within
tight limits so that the effects of temperature varia-
tions on different excitability measures could be
documented for the range of temperatures normally
encountered during diagnostic testing.
The following sequence of recordings was con-
trolled by computer, the only operator intervention
required being to press the “escape” key when each
stimulus–response curve reached its maximum. The
time course of a typical recording is illustrated in
Figure 1.
Stimulus–Response and Strength–Duration Relation-
ships. Stimulus–response curves were recorded
separately for test stimuli of durations 0.2 ms and 1
ms (Fig. 1A). The stimuli were increased in 6% steps,
with two responses averaged for each step, until
three averages were considered maximal. The stimu-
lus–response data were used for several purposes.
First, the 1-ms peak response was used to set the
target submaximal response (40% of peak) for
threshold tracking. Second, the slope of the 1-ms
stimulus–response curve was used in conjunction
with the tracking error (deviation from the target) to
optimize the subsequent threshold tracking. Finally,
when the data were analyzed, the ratio between the
0.2-ms and 1-ms stimuli required to evoke the same
responses was used to estimate the strength–dura-
tion time constants and rheobases of axons of differ-
ent threshold.
Threshold Electrotonus. Prolonged subthreshold
currents were used to alter the potential difference
across the internodal axonal membrane (Fig. 1B), a
process referred to as electrotonus. The changes in
threshold associated with electrotonus normally
have a similar time course to the changes in mem-
brane potential and are known as threshold elec-
trotonus.4 In the present protocol, test stimuli of
1-ms duration were used to produce the target
400 Multiple Excitability Measures MUSCLE & NERVE March 2000