The response of denervated muscle to long-term electrical stimulation

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The response of denervated muscle to long-term electrical stimulation - 1985
Eur J Trans Myol - Basic Appl Myol 2014; 24 (1): 21-25
- 21 -
The response of denervated muscle to long-term electrical
stimulation
T. Lømo, R.H. Westgaard, R. Hennig, K. Gundersen
Institute of Neurophysiology, University of Oslo, Karl Johansgate 47, 0162 Oslo 1, Norway
Adapted from: Lømo T, Westgaard RH, Hennig R, Gundersen K. The response of denervated
muscle to long-term electrical stimulation, In: Carraro U, Angelini C, eds. Proceedings of the
First Abano Terme Meeting on Rehabilitation, 1985 August 28-30, Abano Terme, Padova,
Italy, An International Symposium, Satellite Meeting of the XIII World Congress of
Neurology, Hamburg 1985. Cleup Padova 1985. pp 81–90.
Eur J Trans Myol - Basic Appl Myol 2014; 24 (1): 21-25
The effects of denervation, cross-reinnervation and
chronic nerve stimulation show that motoneurons
control the properties of skeletal muscles.1-6 Evoked
muscle activity and neurotrophic substances released
from motor nerve terminals may mediate this control,
either independently or in combination. However, the
two mechanisms are difficult to separate in
preparations where the nerve is intact, because changes
in neural activity may influence the content or release
of neurotrophic substances. We have, therefore, as an
extension of earlier work,7,8 denervated rat fast and
slow muscles to remove putative neurotrophic
substances, and then stimulated the muscles electrically
with different stimulation patterns to examine the
effects of evoked activity per se on extrajunctional
membrane properties and contractile properties.
Materials and Methods
Young, adult, male, Wistar rats, weighing 250-350
grams, were used. The operations were done under
barbiturate or ether anesthesia. Soleus and extensor
digitorum longus (edl) muscles were denervated by
cutting and reflecting the sciatic nerve in the thigh. A
pair of Teflon coated multistranded steel wires was
implanted. The distal ends, with the insulation
removed, were placed across the edl or the soleus
muscle, one on each side. And the wires were run
under the skin, through an attachment by screws and
dental cement on the skull to a stimulator above the rat.
The rat was keept in a large bucket, where it could
move freely. Stimulation started 1 day to 9 months
after denervation, lasted from 1 day to 9 months, and
consisted of different trains of stimuli (Table l). Each
stimulus pulse was bipolar. The duration was 0.2 msec
in each direction, and the intensity 5-10 mA.
In one series of experiments the muscles were removed
from the rat and placed in a chamber superfused with
oxygenated Ringer solution at room temperature.
Conventional micropipettes filled with 4 M K-acetate
or 3 M acetylcholine chloride (AChCl) were then used
to record resting membrane potentials (RMP) and
sensitivity to ACh.9 In another series the leg was
inserted into a Perspex chamber containing oxygenated
Ringer solution at 35°C. The distal part of the soleus or
edl was dissected free and connected to a force
transducer, while the main blood supply was kept
intact. Twitch and tetanic contractions, evoked by
direct supra maximal stimuli from large platinum
electrodes, were then measured under isometric
conditions, at optimal length. Lack of reinnervation
was confirmed by stimulating the nerve just outside the
muscle and looking for muscle contractions with the
dissection microscope.
Results
Fig. l A shows first, that denervation causes a striking
increase in extrajunctional Ach sensitivity; second, that
electrical stimulation abolishes this sensitivity; and
third, that stopping the stimulation causes it to
reappear. In Fig. l B the RMP falls after denervation,
and then rises to normal values after the onset of
stimulation. Also in denervated edl muscles stimulation
removes extrajunctional ACh sensitivity and restore
normal RMPs (not shown).
Fig. 2 shows, first, that denervation causes a striking
reduction in force of soleus muscles; second, that
electrical stimulation either maintains or restores the
force output to nearly normal values.
The cross sectional areas of the muscle fibres undergo
parallel changes (not shown). The denervated and
stimulated muscles usually produce less force than
normal muscles, but so do innervated muscles when
stimulated in the same way (Fig. 2). Long-term
stimulation causes similar increases in force output and
muscle fiber cross sectional area in edl muscles (not
shown).
Fig. 3 shows, first, that a fast stimulation pattern
(intermittent 100 Hz) markedly reduces the twitch
contraction time of denervated soleus muscles; second,
that denervation and implantation of sham electrodes

The response of denervated muscle to long-term electrical stimulation - 1985
Eur J Trans Myol - Basic Appl Myol 2014; 24 (1): 21-25
- 22 -
has no comparable speeding up effect; and third, that a
slow stimulation pattern (continuous 10 Hz) maintains
or increases the original contraction time. During fast
pattern stimulation the half relaxation time also
declines markedly, and the properties of "sag" and post
tetanic potentiation, typical of fast muscles,9,10 appear.
In contrast, during tonic, low frequency stimulation (10
Hz, mean frequency 10 Hz), the twitch contraction
time and other contractile properties remain slow.
Fig.4 shows the twitch contraction time of soleus
muscles after about 2 months of denervation and
stimulation with the stimulus patterns indicated in
Table 1. The slowest contraction time (about 45 msec)
is obtained during tonic, low frequency stimulation,
and the fastest (about 12 msec) during intermittent,
high frequency stimulation. When the amount of
stimulation at a given frequency is reduced, the twitch
contraction time is also reduced. For example, at 10 Hz
a 1000 fold reduction in mean stimulus frequency
(from 10 to 0.01 Hz) reduces the contraction time from
~ 45 to 19 msec. However, to make the contraction
time as fast as in the edl (~ 12 msec), high frequency
stimulation is necessary. By changing both the amount
and the frequency of stimulation it is possible to
continuously grade the contraction time within a
certain range (45-12 msec), which we call the
“adaptive range“ for this parameter in the soleus.
Other contractile parameters can also be regulated
continuously within certain adaptive ranges. For
example, during intermittent high frequency
stimulation the intrinsic shortening velocity of the
soleus (whole muscle isotonic shortening velocity
corrected for differences in fiber length) becomes only
half as fast as in the normal edl.11 Furthermore, the
soleus fibres continue to bind anti-slow myosin, while
acquiring the ability to bind anti-fast myosin, i.e. the
stimulated soleus fibres show incomplete trans-
formation and hybrid characteristics.11 In the
denervated edl the adaptive range of time extends from
about 12 to 23 msec. Neither denervation alone, nor
tonic, low frequency stimulation directly after
denervation, or indirectly via the intact nerve
(reference12 and this work), make the contraction time
of the edl slower than 23 msec.

The response of denervated muscle to long-term electrical stimulation - 1985
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Discussion
Long-term electrical stimulation restores normal
extrajunctional membrane properties in denervated
slow and fast muscles in the absence of intramuscular
motor axons and, therefore, in the absence of putative
neurotrophic factors. Stimulation also causes a marked,
but usually incomplete, recovery of muscle size and
force output. The incomplete recovery could easily
have a trivial explanation. For example, musc1e stretch
which is known to affect muscle fiber size,12,13 is
certainly inadequate in these experiments because the
entire rat limb is denervated and the working mode
atypical. The electrodes might also damage or fail to
activate part of the muscle. Furthermore, some atrophy
usually occurs in innervated muscles during long-term
electrical stimulation, although, in this case, putative
neurotrophic substances should be present (Fig. 2, and
references.3,5,6 For these reasons, moderate atrophy in
denervated and stimulated muscles should not be taken
as evidence that essential neurotrophic substances are
lacking. On the contrary, the effects of stimulation on
the extrajunctional membrane properties, and on the
contractile properties of denervated muscles, are so
striking that an essential neurotrophic control
mechanism seems unlikely in the rat.
If this conclusion can be extended to humans, then it
should be possible to maintain and perhaps make some
use of denervated muscles in humans by suitable
electrical stimulation. If, on the other hand,
neurotrophic substances are essential, such prospects
seem less likely.
Contraction speed is determined partly by the amount
of activity. Thus, fast muscles become slower when
stimulated at low or high frequency,3,5,6 while slow
muscles become faster when the amount of activity is
reduced, as after spinal cord section,14,15 or
immobillzation.16,17 Therefore, it has been proposed
that slow contraction speeds are due to tonic activity
per se, while fast speeds may be due to intrinsic muscle
properties6 or some neurotrophic factor,18 which would
make all muscles fast when tonic activity is absent.
Accordingly, fast activity patterns are not considered to
play a significant role in determining fast twitch
properties,19 and the fast speed of muscles cross-
reinnervated with a fast nerve is attributed to the
absence of tonic activity rather than the high frequency
activity of fast motoneurones.20
Our results suggest a different view. Also we find that
the soleus can be made considerably faster merely by
reducing the amount of activity, particularly at low
stimulation frequencies (Fig. 4). However, to obtain
contraction times as short as those found in normal edl
muscles, or in the soleus after cross-reinnervation, high
frequency stimulation is necessary (Fig. 4 and Table
2). Furthermore, when high frequency stimulation is
added to tonic, low frequency activity, evoked either
by electrical stimulation or naturally by an intact soleus
nerve, then the contraction speed increases
considerably despite the increased amount of activity
(Table 2). Therefore, high frequency activity appears
specifically to induce fast contractile properties.
Using stimulation patterns comparable to the firing
patterns of normal soleus and edl motoneurons of
freely moving rats,21 we find that the soleus maintains
a normal slow contraction time during slow pattern
stimulation, and acquires a contraction time as fast as
in the normal edl during fast pattern stimulation (Fig. 4
and Table 2). These effects are virtually the same as
those obtained by self-reinnervation as those obtained
by the slow soleus nerve or cross-innervation by the
fast edl nerve (Table 2). Similar results are obtained in
the edl, where fast stimulation patterns have the same
effects as self-reinnervation by the original fast nerve,
and slow stimulation patterns the same effects as cross-
reinnervation by the slow soleus nerve (Table 2).
However, the two muscles respond strikingly different

The response of denervated muscle to long-term electrical stimulation - 1985
Eur J Trans Myol - Basic Appl Myol 2014; 24 (1): 21-25
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to similar inputs. Thus, the edl acquires a contraction
time of only 23 msec during slow pattern stimulation
or after slow nerve reinnervation, whereas the soleus
acquires a contraction time of 38-40 msec. This
indicates; first, that motoneurons control contraction
speed primarily through the patterns of activity that
they evoke in the muscle fibres; and second, that the
muscle fibres of edl and soleus muscles have
developed different intrinsic properties. Such intrinsic
differences might explain why similar impulse
activities, innervation,2,22,23 or hormones,4 may have
different effects on different types of muscle fibres.
In conclusion, our results indicate; first, that neurally
evoked muscle activity plays an essential role in the
control of extrajunctional membrane and contractile
properties; second, that muscle fibres display adaptive
ranges within which a contractile property, such as
twitch speed, can be continuously graded by different
patterns of impulse activity, and where both the
amount and the frequency of impulses are important;
and third, that different types of muscle fibres have
different adaptive ranges, because of different intrinsic
properties. Thus, the fast speed of a rat edl muscle
apparently results partly from intrinsic properties and
partly from the high frequency activity typical of edl
motoneurons, while the slow speed of rat soleus results
partly from intrinsic properties that are different from
those in edl and partly from the low frequency, tonic
activity typical of soleus motoneurons. As a result
contractile properties may be adapted to the varying
functional demands imposed by the central nervous
system within the characteristic adaptive range of each
muscle fiber.
Corresponding Author
Terje Lømo (MD, PhD), Institute of Basic Medical
Sciences, University of Oslo, Norway
E-mail: terje.lomo@basalmed.uio.no
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