J .B. Fallon et al. Brain Research 888 2001 348 –355
349
postulated that there is mixing of impulse traffic from these withdrawal reflexes and assessing the level of muscle tone.
two sources, leading to pacemaker switching when one The animal’s body temperature was monitored using a
input is at a higher rate than the other [3,6,11]. These rectal probe, and maintained in the range 38618C using a
considerations have led to models of impulse initiation in thermal blanket.
the spindle [4,12]. When discharge rates from different A laminectomy was performed exposing the lumbo-
pacemakers are similar, it has been proposed that there sacral spinal cord from L6 to S2. Dorsal and ventral roots
may be probabilistic mixing of the two impulse streams were cut where they entered the cord and the peripheral
[7]. Finally, on the basis of the amount of summation of Ia portions dissected into small filaments to obtain func-
responses to combined static and dynamic fusimotor tionally single afferent and motor axons. The left soleus
stimulation, it has been proposed that generator current muscle and its tendon were dissected free and the tendon
from one impulse generator may spread along the peripher- attached via a fragment of calcaneum to an electromag-
al afferent tree to influence rates of firing at the second netic muscle stretcher regulated by feedback. Muscle
generator [1]. lengths were referred to the maximum in situ physiological
But does it actually matter whether there is leakage of length L . Muscle tension was monitored using an in-
m
receptor currents between generators? More generally, series strain gauge placed between the stretcher and the
does it matter what kinds of interactions take place muscle. The left hindlimb was extensively denervated,
between different spindle compartments? In a recent study including the hip, but sparing the nerve to soleus. Skin
of the responses of spindles during treadmill locomotion of flaps of the hindlimb and back were fashioned into pools
the decerebrate cat, patterns of fusimotor activity were filled with mineral paraffin oil and warmed with radiant
deduced from discharge profiles of the Ia afferents [17]. heat.
The aim was to obtain insight into central nervous strate- Action potentials were converted to TTL pulses and
gies making use of the fusimotor system during locomo- recorded digitally using a commercial analog to digital
tion. One complication considered was the electrotonic converter PCI-MIO-16E-4, National Instruments Corp.,
interaction from
receptor currents
between impulse
Austin, Texas, USA and a Power Macintosh computer. generators. It was found that when allowance was made for
The recording and analysis were done using the software this kind of interaction, the response profiles predicted
package Igor Pro WaveMetrics, Lake Oswego, Oregon, from known intrafusal activation patterns could be iden-
USA. tified. In other words, understanding the processes within
Afferent axons, dissected in filaments of dorsal root, the spindle allows interpretation of its afferent responses in
were identified as lying in the group I range by their terms of an underlying motor strategy.
conduction velocity. This was calculated from the latency Here we re-examine the evidence for electrotonic spread
of the recorded action potential in the root filament in of current from one generator to the other. In the main, we
response to stimulation of the muscle nerve and the agree with the earlier conclusions that response summation
conduction distance between stimulating and recording seen with combined static and dynamic fusimotor stimula-
electrodes. Afferents were identified as coming from tion is due to electrotonic effects [1]. However, we view
muscle spindles by their ‘in parallel’ behaviour, interrup- this interaction not just as a simple spillover of current
tion of their discharge during a muscle contraction. from one generator site to another, between spindle
Fusimotor axons were isolated in filaments of ventral root compartments which remain independent of one another in
and were identified by the excitatory effect of their all other respects. Some of the observed behaviour is
stimulation on spindle afferents in the absence of extrafus- consistent with the idea that the amount of current gener-
al tension. They were classified as static or dynamic by the ated at one site is influenced by transmission of forces
effect of their stimulation at 100 pulses per second pps between adjacent contracting intrafusal fibres.
on spindle afferent responses to a ramp-and-hold muscle stretch of 5 mm at 10 mm s [8].
Fusimotor axons were stimulated, alone and in pairs, in
2. Methods a pseudo-random sequence, so that the minimum interval
between successive
stimulation epochs
was 20
s. All experiments were approved by the local Standing
Fusimotor stimulation was at 100 pps unless otherwise Committee for Ethics in Animal Experimentation. The
stated. Typically, summation coefficients were measured experiments used the soleus muscle of the anaesthetised
using 4 s duration tetani. cat. A total of 15 cats was used, with weights ranging from
Spindle responses to fusimotor stimulation were quan- 3.5 to 7.0 kg. Anaesthesia was induced with pentobar-
tified by averaging, using a backfill method. Here, instanta- bitone sodium nembutal, Boehringer Ingelheim 40 mg kg
neous rates in each response were joined to create a i.p. and maintained via a cannula in the cephalic vein, 12
continuous record and average rates were calculated for mg ml to effect. Anaesthetic depth was monitored using
successive 200-ms segments of each record. This yielded end tidal CO
levels Datex Normocap via a tracheal approximately 20 data points for each 4-s period of
2
cannula as well as checking for the presence of blink and stimulation.
350 J
Summation of responses of spindles to combined fusimotor neurones, in all but one experiment, identified
fusimotor stimulation was quantified using the summation static and dynamic neurones. In the remaining experiment
coefficient, K [9], defined as: pairs of static fusimotor responses were compared.
[Combined response 2 Larger individual response] 3.1. Intrafusal isometric contractions
]]]]]]]]]]]]]] K 5
[Smaller individual response] Here we refer to afferent responses to fusimotor stimula-
The value of K therefore represents the fraction of the tion when muscle length was held constant. Our working
smaller individual response that would be needed to be hypothesis was that if impulses were set up at separate and
added to the larger response to produce the observed combined response. A negative summation coefficient
independent sites within the spindle afferent tree, repre- indicates that the response on combined stimulation is less
senting the static and dynamic fusimotor pacemakers, there than either of the two individual responses. A value of K
should always be total occlusion between them. In other between 0 and 1 means that there is some summation, if
words, for a pacemaker-switching model the combined less than algebraic, between pairs of responses. A value of
response should never be bigger than the larger individual K .1 implies some, mutual, potentiation between re-
response, yielding a K value of zero. We viewed a K value sponses.
of more than zero as a degree of response summation. For measurement of length effects, the 4 s of stimulation
Comparisons were made between responses to stimulation was done at a rate sufficient to produce a near-maximum
of 20 pairs of static and dynamic fusimotor fibres belong- fusimotor response, typically 200–250 pps. The order in
ing to 16 different spindles. The findings were in line with which fusimotor fibres were stimulated, singly or in
our earlier observations [3] and those reported by others combination, was pseudo-randomised in each of nine trials,
[1]. There was almost always some response summation, representing three sets of comparisons. A series of such
above the values for the individual responses, but with the measurements was carried out at each muscle length. The
combined response reaching less than the algebraic sum. order of selection of each length was also pseudo-random-
The amount of summation varied between different re- ised.
sponse pairs. On some occasions the combined response Ramp stimulation was carried out at or near L –10 mm,
was little bigger than the larger individual response, giving
m
using a similar protocol to the length–effect measure- a K value close to zero. Other responses showed substan-
ments. One of the constant rate tetani was replaced with a tial summation yielding K values of 0.5 or more Fig. 1.
frequency ramp stimulus that rose from 0 to 140 pps in 4 s. When values obtained from the 20 pairs of units were
Experiments using combined stretch and fusimotor pooled, Fig. 5, the mean K value was 0.15660.005
stimulation were done from an initial length of L –16 mm S.E.M..
m
and consisted of a 3-mm stretch at 0.5 mm s during Another measure of response summation made under
fusimotor stimulation at 100 pps. Again, stimulation order isometric conditions was to keep the stimulation rate to
was pseudo-randomised. one fusimotor fibre, either the static or the dynamic fibre,
The passive ramp stretches were performed at a test constant while a ramp increase in stimulation rate was
length of L –16 mm. The muscle was stretched by 5 mm, applied to the other. An example is shown in Fig. 2. Notice
m
a conditioning whole-muscle contraction carried out and that stimulating the g
fibre alone did not produce any
dynamic
the muscle held at this length for a further 5 s. The muscle response from the spindle over the range of stimulation
was then returned to the test length and a further con- rates between 0 and 20 pulses s. Yet during combined
ditioning contraction 1 s at 100 pps was used to stimulation over this range the response was consistently
selectively remove slack in intrafusal fibres see Fig. 4. A slightly above the g
response alone. It suggests that
static
pseudo-random order was used for fusimotor stimulation. even though the spindle was not generating impulses to the
Then the test stretch of 3 mm at 0.5 mm s was imposed. low rates of g
stimulation, it did produce a sub-
dynamic
threshold response which could make a contribution to the 2.1. Statistical analysis
combined response. In general, response summation under these conditions, studied for four pairs of fusimotor
A comparison of values of summation coefficients for responses, was the same as when both fusimotor fibres
20 pairs of static:dynamic and 13 pairs of static:static were stimulated at constant rates Fig. 5.
responses was carried out using an ANOVA Data Desk; Ithaca, NY, USA, since some pairs were from the same
3.2. Effect of muscle length animal.
Our aim in these experiments was to try to obtain some insight into the mechanism responsible for response sum-
3. Results mation. The predominant effect expected from combined
