Interlimb re¯exes and synaptic plasticity become
evident months after human spinal cord injury
Blair Calancie, Maria R. Molano and James G. Broton
The Miami Project to Cure Paralysis and Department of
Neurological Surgery, University of Miami School of
Medicine, Miami, FL 33136, USA
Correspondence to: Blair Calancie, PhD, Department of
Neurosurgery, SUNY's Upstate Medical University, 750
East Adams Street, IHP Room 1213, Syracuse, NY 13210,
USA
E-mail: calancib@upstate.edu
Summary
Persons with long-standing injury to the cervical spinal
cord resulting in complete or partial paralysis typically
develop a wide spectrum of involuntary movements in
muscles receiving innervation caudal to the level of
injury. We have previously shown that these movements
include brief and discrete contraction of muscles in the
hand and forearm in response to innocuous sensory
stimulation to the feet and legs, but we have been
unable to replicate these interlimb re¯exes in able-
bodied subjects. Properties of these muscle responses
indicate that the synaptic contacts between ascending
sensory ®bres and motor neurones of the cervical
enlargement are more ef®cacious than normal. If these
connections are present at all times, and require the
more rostrally-placed spinal cord injury to allow their
emergence, one might expect their appearance relatively
soon following injury, as has been shown for studies of
`latent' synapses. Conversely, delayed appearance of
these interlimb re¯exes would suggest either the devel-
opment of new synaptic connections or a profound
strengthening of existing circuits in the cervical spinal
cord due to a combination of afferent target loss and
motor neurone denervation from motor tracts originat-
ing rostral to the injury site. In this study, we used
repeated examinations of persons with acute injury to
the cervical spinal cord to examine the time post-injury
at which interlimb re¯exes are ®rst seen. Using tibial
nerve stimulation at the knee as a screening test, a total
of 24 subjects were found to develop interlimb re¯exes
following spinal cord injury. Latencies between stimula-
tion and EMG were as brief as 32 ms for muscles of
the forearm and 44 ms for muscles in the hand. These
minimal delays all but rule out a supraspinal route for
these interlimb re¯exes. Interlimb re¯exes ®rst became
evident no sooner than ~6 months following injury, and
in some individuals were not seen until well over 1 year
post-injury. Enhanced lower limb segmental excitability
had emerged in nearly all of these subjects weeks or
months prior to the ®rst appearance of interlimb
re¯exes, arguing against a manifestation of traditional
post-traumatic spasticity as a basis for this activity.
This prolonged delay between time of injury and emer-
gence of interlimb re¯ex activity lends support to the
hypothesis that this activity represents an example of
plasticityÐand perhaps `regenerative sprouting'Ðin
the human spinal cord following traumatic injury.
Keywords: spinal cord injury; plasticity; regenerative sprouting; re¯ex; human; cervical
Abbreviations: ADM = hypothenar group of the hand; APB = thenar group of the hand; ASIA = American Spinal Injury
Association; ECR = wrist extensors; FCR = wrist ¯exors; ILR = interlimb re¯exes; Psoas = hip ¯exors; SCI = spinal cord
injury
Introduction
There are numerous examples of axonal regeneration and
synaptic reorganization of neurones (collectively referred to
as `plasticity') following traumatic lesions to the mammalian
spinal cord (for a review, see Guth, 1974; Steward, 1989;
Goldberger et al., 1993; Schwab and Bartholdi, 1996;
Mendell et al., 2001; Siddall and Loeser, 2001; Wolpaw
and Tennissen, 2001). In the absence of speci®c interven-
tions, such plasticity may contribute to the development of
abnormal movement states (McCouch et al., 1958; Nelson
and Mendell, 1979; Hiersemenzel et al., 2000) and/or sensory
disturbances (Christensen and Hulsebosch, 1997; Romero
et al., 2000). Alternatively, certain post-injury alterations of
behaviour may re¯ect the collective action of synaptic
connections that were present at the time of injury, but
ã Guarantors of Brain 2002
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were functionally inactive (or `silent'). These latent connec-
tions have been reported to become `unmasked' within
minutes to hours of a CNS lesion (Goshgarian and Guth,
1977; Nelson et al., 1979; Wall, 1988; Goshgarian et al.,
1989; but compare with Brown et al., 1984). Thus the
timecourse with which certain spinal cord input/output
properties emerge following CNS trauma may provide clues
as to the mechanism(s) underlying those behaviours.
Many of the same mechanisms of plasticity reported from
animal models of spinal cord injury (SCI) can likely be
demonstrated in human subjects, but quantitative histologic
data supporting the possibility of sprouting or synaptogenesis
are indirect (Krassioukov et al., 1999). Behavioural studies
abound though and raise the possibility for both spontaneous
(Calancie et al., 1994) and task-speci®c (Bach and Rita, 1981;
Wernig et al., 1995; Harkema et al., 1997; Barbeau et al.,
1999) plasticity in the human spinal cord caudal to an injury.
Previous reports from this laboratory (Calancie, 1991;
Calancie et al., 1996) have described novel `interlimb
re¯exes' (ILR) in persons who have sustained SCI >1 year
prior to study (i.e. in the chronic phase). These involuntary
movements are characterized by short-latency (i.e. 40±50 ms)
contractions of hand and forearm muscles following a wide
variety of innocuous sensory stimuli delivered to the lower
limb or limbs (including skin stroking, hair pull, tendon taps
and electrical stimulation of peripheral nerves). We have
suggested that such interlimb re¯exes may re¯ect the
consequences of novel synaptic connections formed between
ascending ®rst- and second-order afferent ®bres and motor
neurones of the cervical enlargement partially denervated due
to a more rostrally-placed lesion to the spinal cord (Calancie
et al., 1996). If correct, one would expect to see a signi®cant
delay between the time of injury and the time at which such
ILRs become evident.
In this paper, we report ®ndings from a group of subjects
who ultimately developed interlimb re¯exes after sustaining
traumatic spinal cord injury. We conducted repeated meas-
ures on these subjects over a period of many months to
determine the time after injury when these re¯exes became
evident. Our data are consistent with the hypothesis that these
interlimb re¯exes represent the establishment of new synaptic
connections between nerve populations that do not normally
interact. We suggest that ILR emergence serves as an
example of CNS plasticity (or `regenerative sprouting';
Steward, 1989) in the adult human nervous system following
traumatic injury. This conclusion is included within a much
broader examination of plasticity after human SCI that was
presented previously (Calancie et al., 2000).
Methods
Subjects
Experiments were performed on persons with traumatic
injury to the cervical spine resulting in neurologic de®cit (i.e.
spinal cord injury). In the majority of cases, the initial
examination of a given subject took place within the ®rst
week after injury, with follow-up studies continuing over the
next weeks and months post-injury. All subjects gave their
informed consent to participate in this protocol, which was
approved by the University of Miami's Institutional Review
Board.
Procedures
Self-adhesive surface EMG electrodes (S'Offset; Graphic
Controls Corp., Buffalo NY, USA) were positioned over the
biceps brachii, triceps brachii, wrist extensors (ECR), wrist
¯exors (FCR), thenar group of the hand (APB), hypothenar
group of the hand (ADM), hip ¯exors (Psoas), quadriceps,
hamstring, tibialis anterior (TA), soleus, and foot intrinsics of
the subject's left side. Examinations were made of voluntary
individual muscle contractions, tendon re¯exes and central
motor conduction in response to transcranial magnetic
stimulation, and interlimb re¯exes through surface-applied
electrical stimulation of the tibial nerve at the popliteal fossa.
Tibial stimulation was accomplished using either a Grass
S88 stimulator (initial four subjects; Grass Instrument Co,
Quincy, MA, USA) or a Digitimer D185 stimulator (remain-
ing 20 subjects; Digitimer Inc, Welwyn Garden City, UK).
Stimuli were delivered through pairs of self-adhesive surface
electrodes (Cleartrace; ConMed Corp., Utica, NY, USA)
positioned over the tibial nerve (cathode) and a site medial
and distal to this site (anode). Single pulses of >100 V were
used to de®ne the optimal stimulus site, using soleus direct
muscle response (M-wave) and plantar-going ankle move-
ment as the criteria to guide stimulation. In most cases,
pressure was applied to the cathode while stimuli were
delivered, pushing the electrode closer to the underlying tibial
nerve in order to minimize subject discomfort (in those
subjects who could feel the stimulus) while still eliciting a
strong plantar-¯exion. The stimulus intensity delivered via
the D185 stimulator [based on readings from the stimulator's
liquid crystal display (LCD)] was not less than 125 mA in any
subject tested, while trials using the Grass stimulator
routinely used pulses of 150 V (the maximum capable with
this device). The duration of individual stimuli within a
3-pulse train was 50 ms and 1000 ms for the D185 and S88
stimulators, respectively. (Note that the D185 pulse width
cannot be adjusted from this 50 ms value, but that its
maximum stimulus intensity far exceeds that of the S88,
enabling the D185 output to produce strong plantar-going
twitches when desired.)
Once an acceptable stimulation site was established, a
series of 3-pulse stimulus trains was delivered, each pulse in
the train separated from the next by 2 ms (i.e. three pulses at
2 ms each; `3 @ 2'). A 4-pulse train was used on occasion.
Pulse trains were separated from one another by a minimum
of 1 s. In most cases, the rate of pulse train stimulation was
approximately 0.2 Hz, and was controlled manually (i.e. a
deliberate button push was needed to trigger a stimulus). No
fewer than 10 stimulus trains were delivered, and the EMG
Interlimb re¯exes after spinal cord injury 1151