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Activity of muscle spindle efferents during scratching in the cat
192
t~,.in Re,~ear~/z, 1:?9 ~! -)73) / 92 ) .~ Elsevier/North-Holland Bi<,medica~ Prc>;s
ActivRy of muscle spindle afferents during scratching in the cat
A. G. F E L D M A N , G. N, O R L O V S K Y and C. P E R R E T ~'
Institute o f Problems o f ln]brmation Transmission. Academy o/ Sciences, and Moscow .S'mle U~ffversit)'. Moscow (U.S.S.R.) (Accepted March 10th, 1977)
Scratching a,18 and stepping ~.t7 display many similar features: both phenomena are rhythmic, with reciprocal activation of flexor and extensor muscles: both can be generated by the isolated spinal cord. even after complete deafferentation of the limbs. However, during scratching, only one limb performs rhythmic movements, and the frequency of oscillations is higher than during stepping, mainly due lo shorter extension phases. Coactivation of alpha- and gamma-motoneurones was observed during various stereotyped movements (breathing 4,~a. mastication s, locomouon 1° l:~. 1s,16,19) as well as during voluntary movements 2°, and was suggested to promote the range of muscle lengths and forces available 6,7. The present paper tends to demonstrate that the a l p h a - g a m m a coactivation also exists in scratching. We therefore recorded the activity of muscle spindle afferents during scratching in 5 decorticate (thalamic) and 5 decapitate (spinal) cats. In decapitate cats. scratching was evoked by electrical stimulation of the cervical cord al C 1--C2 level 3,5,1s. In decorticatc animals. it was evoked by light tactile stimulation of the inner part of the pinna: this stimulation was effective after placing a small piece of cotton wool soaked in 0.05';~. solution of D-tubocurarine chloride on the dorsal surface of the cervical cord at CI level s (for detailed description of the methods, see ref. 31. The right hindlimb was denervated except for two muscles, tibialis anterior (TA) and gastrocnemius (G) or its medial part. The rachis and the limb were rigidly fixed and the distal tendons of TA and G were cut and attached to strain gauges, permitting either isotonic or isometric contractions. The muscle length at rest was fixed at the value corresponding to the maximal physiological length. Under isotonic conditions, the muscle load was about 100 g. Maximal contractions corresponded to a shortening of 1 cm. which is about the maximal physiological length change. Under isometric conditions, the muscle length varied by less than 1 ram, corresponding to more than 1 kg tension. Electromyograms (EMGs) were recorded by means of thin wires inserted into the muscles. Since scratching cannot be evoked when the limb is deflected backwards unless it is deafferented 3, deafferentation was performed by cutting the dorsal roots from L3 to * Present address: Laboratoire de Neurophysiologm comparee. Universit6 Paris VI. 4 t~lace Jus-~ieu. 75230 Paris Cedex 05, France.
193 $4, and the limb could thus be fixed in a semi-extended position. Primary (Ia) muscle spindle afferents from TA and G were isolated in dorsal root filaments and identified in the usual way (see ref. 12). Their discharge was recorded on a pen inkwriter after conversion of spikes into pulses of standard duration and amplitude. Both signals, spindle afferent activity and strain gauge output of each muscle, could be fed into the same channel to simplify comparison of their temporal course. Since results obtained in decorticate and decapitate preparations were very similar, they will be described together. Activity of TA 3pindle afferents. There are two components in scratching: a postural one and a rhythmic one3, is. There is first a forward movement of the limb, resulting from a tonic activation of flexor muscles. The rhythmic component develops on this postural background, with short successive extensor bursts corresponding to short pauses in the flexor activity. As shown in Fig. 1, a postural TA contraction developed soon after beginning of the pinna stimulation, with a strong increase of activity in the spindle afferent. Rhythmic contractions appeared later on, each relaxation phase in TA being accompanied by a pause in the la afferent discharge. In each cycle the Ia discharge frequency increased at the beginning of the muscle contraction, or even earlier. It decreased at the end of each muscle shortening (Fig. 1) or later, during lengthening. Sometimes, a rhythmic activity of the afferent fibre could be observed even when no overt contraction (nor EMG) existed (Fig. 3A and C). Activity of G spindle afferents. Usually, G muscle was not active during the initial postural phase of scratching 3. Correspondingly, the spindle afferent discharges were either absent (Fig. 1) or, if present, did not change as compared to resting conditions. When rhythmic contractions developed, different patterns of modulation of the afferent activity could be observed. If the G muscle was under isometric conditions (Figs. 2C and 3) or if its contractions were weak (Fig. 2A, right), the spindle afferent was usually active during each contraction and sometimes even before (Fig. 3). Under isotonic conditions (Figs. 1 and 2A), there was only a minor activity during muscle shortening, and a discharge during lengthening (Fig. 2A). In some cases, rhythmic G afferent discharges were seen in the absence of muscle contraction (and EMG), alternating with TA afferent activity (Figs. 2A and 3A and C). The above data demonstrate that a fusimotor control exists during both stages of scratching. It must be specified that the experimental conditions used here only allowed identification of static fusimotor controle, 9, special methods 11 being necessary
la
TA
"
'
G
~
1
,,o I
,
.
I I
.
.
.
tl
I
Fig. 1. Activity of primary spindle afferents during scratching in the decorticate cat. la TA and la G : discharge of la afferent fibre from TA and from G. TA and G : muscle length recorded under isotonic conditions (contraction up). Notice la TA discharge during both sustained and rhythmic TA contractions, and very weak activity in Ia G. •
194
A EMG TA
l~w" .~; ~ --~ -~ -,~
.,.~--~-~
la TA/TA
; [ L L L
I-vT-
I
' l,,, l
la GM/OM ~,..u, l L . , - ~ ~
t!,l
t I
. I
, V
L I
i f
L T
L T
!-ii.-L
1 ! ! li lit!l !H.LL
B la TA/TA
.............................................................. "_
iuui
jill
,,,-
C EMG TA
q
la TA/TA
~
~
i
~
4
~
w- I -'~ 1 ' -
_ o la GMIGM
J- j J L ~ ' - - j ~ I
ls
l
~
_.udu, - - - -
- - , ..,,, ..uu . , . ~
'
-I
t
~~--~--Ju~-J-IL-~L~-L-~
|
Fig. 2. Activity of primary spindle afferents in the decorticate cat during scratching (A, C') and during
passive muscle stretch (B). G contracted isotonically in A and isometrically in C (contraction up). In B, muscle stretch corresponds to downward deflection. Afferent ]a discharges were superimposed upon records of length (A, B) or force (C) of the corresponding muscle. Notice la G discharges during lengthening in A and B, during shortening in C, without visible variation of length in A (arrows, and right part of record).
to investigate a dynamic control. During the initial postural stage, activity of the TA spindle afferents considerably increased despite muscle shortening (see Fig. l), suggesting an a l p h a - g a m m a linkage to TA. During the rhythmic stage, TA afferents were active during contraction of the muscle; also indicating that gamma-motoneurones are activated together with alpha'motoneurones. TA spindle afferent discharge could also display a clear rhythmic modulation even when no contraction occurred, i.e. at constant muscle length. In some cases, there was activity during muscle lengthening (Figs. 2 and 3), This was also due to g a m m a influence rather than to passive stretch of the muscle, since the spindle activity decreased before the end of this lengthening, at the time when the G burst developed. A rhythmic fusimotor control
195
A EMG TA
I a TA/TA
-_!~! I!W! ii!ii! ILimiiI1~1
l
m
i
~
EMG GM la GM/GM
111111111IIIII1 III11111I1111111IIIII llllll Llill 1
~
B
C
iLddll I|INHI IIHII~IilLIIIIHnllIlIIIIIill lllll iu II.IIIIIII IIIII[ I llllllIIilW I a
ls
n
Fig. 3. Activity of primary spindle afferents during scratching in the decapitate cat. Beginning (A), middle (B), and end (C) of a sequence of scratching are shown. Isotonic recording for TA, isometric for G. Notice rhythmic activity in ]a TA and la G without corresponding muscle contraction at initiation and at end of sequence, discharge during muscle shortening in both afferents, weaker in la G than in la TA. upon G spindle afferents was also observed here. However, the latter influence was weaker, since pauses in the spindle discharge during muscle shortening were only suppressed when changes in the muscle length were either small or reduced by isometric conditions. Moreover, peak values of the frequency discharge were lower in
196 G a f f e r e n t s t h a n i n T A o n e s . S i m i l a r d i f f e r e n c e s b e t w e e n f l e x o r a n d e x t e n s o r fusinaolo~" control have been found previously during locomotion
of the decorticatc cat ~:'
F i n a l l y , t h e e x i s t e n c e o f f u s i m o t o r c o n t r o l i n t h e d e c a p i t a t e c a t d u r i n g s c r a t c h i n g , as e a r l i e r f o u n d d u r i n g stepping~0,13,19, d e m o n s t r a t e s t h a t it is a n i n l r i n s i c p r o p e r t y o f t h e spinal generator for scratching
1 Brown, T. G., The intrinsic factors in the act of progression in the mammal, Pr~,,. ~'~y. Soc. B. 84 (19l 1) 308-319. 2 Crowe, A. and Matthews, P. B. C., Further studies of static and dynamic fusimotor fibres. J. Physiol. (Lond.), 174 (1964) 132--151. 3 Deliagina, T. G., Feldman, A. G., Gelfand, I. M. and Orlovsky, G. N., On the role of centrai program and reflexes in the control of scratching movements in the cat, Brain Reseorch, I~X)(1975) 297-313. 4 Eklund, G., yon Euler, C. and Rutkowsky, S., Spontaneous and reflex activity ~)f intercostal gamma motoneurones, J. Physiol. (Lond.), 171 (1964) 139-163. 5 Feldberg, W. and Fleischhauer, K., Scratching movements evoked by drugs applied to the upper cervical cord, J. Physiol. (Lond.), 151 (1960) 502-517. 6 Feldman, A. G., Control of the length of the muscle, Biophysics, 19 (I974) 766-77}. 7 Feldman, A. G., Control of postural muscle length a n d force: advantages of central coactivation of a and static 7' motoneurones, Biofiziea, 21 (1976) 187-199 (in Russian). 8 Goodwin, G. M. andLuschei, E. S.,Discharge ofspindleafferentsfromjaw-closing muscles during chewing in alert monkeys, J. Neurophysiol., 38 (1975) 560~571. 9 Lennerstrand, G. and Thoden, U., Position and velocity sensitivity of muscle spindles in the cat. Ill. Static fusimotor single-fibre activation of primary and secondary endings, Ac'ta physiol, scaml,, 74 (1968) 30-49. 10 Perret, C., Neural control of locomotion in the decorticate cat. In R. M. Herman, S. Grillner, P. Stein and D. Stuart (Eds.), Neural control of Locomotion, Advances in Behavioral Bioloxy, Vol. t8. Plenum Press, New York, 1976, pp. 587-615. 11 Petter. C. and Berthoz, A., Evidence of static and dynamic fusimotor actions on the spindle response to sinusoidal stretch during locomotor activities in the cat, Exp. Brain Re~, 18 11973) 178-188 12 Perret, C. and Buser, P., Static and dynamic fusimotor activity during locomotor movements ill the cat, Brain Research. 40 (1972) 165-169. 13 Perret. C.. Cabelguen. J. M. et Miltanvoye, M.. Caracteristiques d'un rythme de type Iocomoteur chez le chat spinal aigu, J. Physiol. (Paris), 65 (1972) 472A. 14 Sears, T. A., Efferent discharges in alpha and fusimotor fibres of intercostal nerves of the ca~. J. Physiol. (Lond.), 174 (1964) 295-315. 15 Severin, F. V., Orlovsky, G. N. and Shik. M. L.. Work of the muscle receptors durJ~g controlled locomotion. Biophysics, 12 (1967) 575-586. 16 Severin, F. V.. The role of the gamma motor system in the activation of the extenso~ alpha motor neurones during controlled locomotion, Biophysics, 15 (1970) 1138-1145. 17 Sherrington, C. S., Flexion-reflex of the limb. crossed extension reflex, and reflex stepping and standing, J. Physiol. (Lond.). 40 (1910) 28-121 18 Sherrington, C. S.. Notes on the scratch-reflex of the cat, Quart. J. exp. Physiol.. 3 (t910) 213 220. 19 Sj6str6m, A. and Zangger, P., Muscle spindle control during locomotor movements generated by the deafferented spinal cord. Acta physiol, stand,. 97 (1976) 281-291 20 Vallbo, A. B., Muscle spind[e response at the onset of isometric voluntary contracuons in man, Time differences between fusimotor and skeletomotor effects, J. Physiol. (Lvnd.). 218 (1971) 405~43 I.
t~,.in Re,~ear~/z, 1:?9 ~! -)73) / 92 ) .~ Elsevier/North-Holland Bi<,medica~ Prc>;s
ActivRy of muscle spindle afferents during scratching in the cat
A. G. F E L D M A N , G. N, O R L O V S K Y and C. P E R R E T ~'
Institute o f Problems o f ln]brmation Transmission. Academy o/ Sciences, and Moscow .S'mle U~ffversit)'. Moscow (U.S.S.R.) (Accepted March 10th, 1977)
Scratching a,18 and stepping ~.t7 display many similar features: both phenomena are rhythmic, with reciprocal activation of flexor and extensor muscles: both can be generated by the isolated spinal cord. even after complete deafferentation of the limbs. However, during scratching, only one limb performs rhythmic movements, and the frequency of oscillations is higher than during stepping, mainly due lo shorter extension phases. Coactivation of alpha- and gamma-motoneurones was observed during various stereotyped movements (breathing 4,~a. mastication s, locomouon 1° l:~. 1s,16,19) as well as during voluntary movements 2°, and was suggested to promote the range of muscle lengths and forces available 6,7. The present paper tends to demonstrate that the a l p h a - g a m m a coactivation also exists in scratching. We therefore recorded the activity of muscle spindle afferents during scratching in 5 decorticate (thalamic) and 5 decapitate (spinal) cats. In decapitate cats. scratching was evoked by electrical stimulation of the cervical cord al C 1--C2 level 3,5,1s. In decorticatc animals. it was evoked by light tactile stimulation of the inner part of the pinna: this stimulation was effective after placing a small piece of cotton wool soaked in 0.05';~. solution of D-tubocurarine chloride on the dorsal surface of the cervical cord at CI level s (for detailed description of the methods, see ref. 31. The right hindlimb was denervated except for two muscles, tibialis anterior (TA) and gastrocnemius (G) or its medial part. The rachis and the limb were rigidly fixed and the distal tendons of TA and G were cut and attached to strain gauges, permitting either isotonic or isometric contractions. The muscle length at rest was fixed at the value corresponding to the maximal physiological length. Under isotonic conditions, the muscle load was about 100 g. Maximal contractions corresponded to a shortening of 1 cm. which is about the maximal physiological length change. Under isometric conditions, the muscle length varied by less than 1 ram, corresponding to more than 1 kg tension. Electromyograms (EMGs) were recorded by means of thin wires inserted into the muscles. Since scratching cannot be evoked when the limb is deflected backwards unless it is deafferented 3, deafferentation was performed by cutting the dorsal roots from L3 to * Present address: Laboratoire de Neurophysiologm comparee. Universit6 Paris VI. 4 t~lace Jus-~ieu. 75230 Paris Cedex 05, France.
193 $4, and the limb could thus be fixed in a semi-extended position. Primary (Ia) muscle spindle afferents from TA and G were isolated in dorsal root filaments and identified in the usual way (see ref. 12). Their discharge was recorded on a pen inkwriter after conversion of spikes into pulses of standard duration and amplitude. Both signals, spindle afferent activity and strain gauge output of each muscle, could be fed into the same channel to simplify comparison of their temporal course. Since results obtained in decorticate and decapitate preparations were very similar, they will be described together. Activity of TA 3pindle afferents. There are two components in scratching: a postural one and a rhythmic one3, is. There is first a forward movement of the limb, resulting from a tonic activation of flexor muscles. The rhythmic component develops on this postural background, with short successive extensor bursts corresponding to short pauses in the flexor activity. As shown in Fig. 1, a postural TA contraction developed soon after beginning of the pinna stimulation, with a strong increase of activity in the spindle afferent. Rhythmic contractions appeared later on, each relaxation phase in TA being accompanied by a pause in the la afferent discharge. In each cycle the Ia discharge frequency increased at the beginning of the muscle contraction, or even earlier. It decreased at the end of each muscle shortening (Fig. 1) or later, during lengthening. Sometimes, a rhythmic activity of the afferent fibre could be observed even when no overt contraction (nor EMG) existed (Fig. 3A and C). Activity of G spindle afferents. Usually, G muscle was not active during the initial postural phase of scratching 3. Correspondingly, the spindle afferent discharges were either absent (Fig. 1) or, if present, did not change as compared to resting conditions. When rhythmic contractions developed, different patterns of modulation of the afferent activity could be observed. If the G muscle was under isometric conditions (Figs. 2C and 3) or if its contractions were weak (Fig. 2A, right), the spindle afferent was usually active during each contraction and sometimes even before (Fig. 3). Under isotonic conditions (Figs. 1 and 2A), there was only a minor activity during muscle shortening, and a discharge during lengthening (Fig. 2A). In some cases, rhythmic G afferent discharges were seen in the absence of muscle contraction (and EMG), alternating with TA afferent activity (Figs. 2A and 3A and C). The above data demonstrate that a fusimotor control exists during both stages of scratching. It must be specified that the experimental conditions used here only allowed identification of static fusimotor controle, 9, special methods 11 being necessary
la
TA
"
'
G
~
1
,,o I
,
.
I I
.
.
.
tl
I
Fig. 1. Activity of primary spindle afferents during scratching in the decorticate cat. la TA and la G : discharge of la afferent fibre from TA and from G. TA and G : muscle length recorded under isotonic conditions (contraction up). Notice la TA discharge during both sustained and rhythmic TA contractions, and very weak activity in Ia G. •
194
A EMG TA
l~w" .~; ~ --~ -~ -,~
.,.~--~-~
la TA/TA
; [ L L L
I-vT-
I
' l,,, l
la GM/OM ~,..u, l L . , - ~ ~
t!,l
t I
. I
, V
L I
i f
L T
L T
!-ii.-L
1 ! ! li lit!l !H.LL
B la TA/TA
.............................................................. "_
iuui
jill
,,,-
C EMG TA
q
la TA/TA
~
~
i
~
4
~
w- I -'~ 1 ' -
_ o la GMIGM
J- j J L ~ ' - - j ~ I
ls
l
~
_.udu, - - - -
- - , ..,,, ..uu . , . ~
'
-I
t
~~--~--Ju~-J-IL-~L~-L-~
|
Fig. 2. Activity of primary spindle afferents in the decorticate cat during scratching (A, C') and during
passive muscle stretch (B). G contracted isotonically in A and isometrically in C (contraction up). In B, muscle stretch corresponds to downward deflection. Afferent ]a discharges were superimposed upon records of length (A, B) or force (C) of the corresponding muscle. Notice la G discharges during lengthening in A and B, during shortening in C, without visible variation of length in A (arrows, and right part of record).
to investigate a dynamic control. During the initial postural stage, activity of the TA spindle afferents considerably increased despite muscle shortening (see Fig. l), suggesting an a l p h a - g a m m a linkage to TA. During the rhythmic stage, TA afferents were active during contraction of the muscle; also indicating that gamma-motoneurones are activated together with alpha'motoneurones. TA spindle afferent discharge could also display a clear rhythmic modulation even when no contraction occurred, i.e. at constant muscle length. In some cases, there was activity during muscle lengthening (Figs. 2 and 3), This was also due to g a m m a influence rather than to passive stretch of the muscle, since the spindle activity decreased before the end of this lengthening, at the time when the G burst developed. A rhythmic fusimotor control
195
A EMG TA
I a TA/TA
-_!~! I!W! ii!ii! ILimiiI1~1
l
m
i
~
EMG GM la GM/GM
111111111IIIII1 III11111I1111111IIIII llllll Llill 1
~
B
C
iLddll I|INHI IIHII~IilLIIIIHnllIlIIIIIill lllll iu II.IIIIIII IIIII[ I llllllIIilW I a
ls
n
Fig. 3. Activity of primary spindle afferents during scratching in the decapitate cat. Beginning (A), middle (B), and end (C) of a sequence of scratching are shown. Isotonic recording for TA, isometric for G. Notice rhythmic activity in ]a TA and la G without corresponding muscle contraction at initiation and at end of sequence, discharge during muscle shortening in both afferents, weaker in la G than in la TA. upon G spindle afferents was also observed here. However, the latter influence was weaker, since pauses in the spindle discharge during muscle shortening were only suppressed when changes in the muscle length were either small or reduced by isometric conditions. Moreover, peak values of the frequency discharge were lower in
196 G a f f e r e n t s t h a n i n T A o n e s . S i m i l a r d i f f e r e n c e s b e t w e e n f l e x o r a n d e x t e n s o r fusinaolo~" control have been found previously during locomotion
of the decorticatc cat ~:'
F i n a l l y , t h e e x i s t e n c e o f f u s i m o t o r c o n t r o l i n t h e d e c a p i t a t e c a t d u r i n g s c r a t c h i n g , as e a r l i e r f o u n d d u r i n g stepping~0,13,19, d e m o n s t r a t e s t h a t it is a n i n l r i n s i c p r o p e r t y o f t h e spinal generator for scratching
1 Brown, T. G., The intrinsic factors in the act of progression in the mammal, Pr~,,. ~'~y. Soc. B. 84 (19l 1) 308-319. 2 Crowe, A. and Matthews, P. B. C., Further studies of static and dynamic fusimotor fibres. J. Physiol. (Lond.), 174 (1964) 132--151. 3 Deliagina, T. G., Feldman, A. G., Gelfand, I. M. and Orlovsky, G. N., On the role of centrai program and reflexes in the control of scratching movements in the cat, Brain Reseorch, I~X)(1975) 297-313. 4 Eklund, G., yon Euler, C. and Rutkowsky, S., Spontaneous and reflex activity ~)f intercostal gamma motoneurones, J. Physiol. (Lond.), 171 (1964) 139-163. 5 Feldberg, W. and Fleischhauer, K., Scratching movements evoked by drugs applied to the upper cervical cord, J. Physiol. (Lond.), 151 (1960) 502-517. 6 Feldman, A. G., Control of the length of the muscle, Biophysics, 19 (I974) 766-77}. 7 Feldman, A. G., Control of postural muscle length a n d force: advantages of central coactivation of a and static 7' motoneurones, Biofiziea, 21 (1976) 187-199 (in Russian). 8 Goodwin, G. M. andLuschei, E. S.,Discharge ofspindleafferentsfromjaw-closing muscles during chewing in alert monkeys, J. Neurophysiol., 38 (1975) 560~571. 9 Lennerstrand, G. and Thoden, U., Position and velocity sensitivity of muscle spindles in the cat. Ill. Static fusimotor single-fibre activation of primary and secondary endings, Ac'ta physiol, scaml,, 74 (1968) 30-49. 10 Perret, C., Neural control of locomotion in the decorticate cat. In R. M. Herman, S. Grillner, P. Stein and D. Stuart (Eds.), Neural control of Locomotion, Advances in Behavioral Bioloxy, Vol. t8. Plenum Press, New York, 1976, pp. 587-615. 11 Petter. C. and Berthoz, A., Evidence of static and dynamic fusimotor actions on the spindle response to sinusoidal stretch during locomotor activities in the cat, Exp. Brain Re~, 18 11973) 178-188 12 Perret, C. and Buser, P., Static and dynamic fusimotor activity during locomotor movements ill the cat, Brain Research. 40 (1972) 165-169. 13 Perret. C.. Cabelguen. J. M. et Miltanvoye, M.. Caracteristiques d'un rythme de type Iocomoteur chez le chat spinal aigu, J. Physiol. (Paris), 65 (1972) 472A. 14 Sears, T. A., Efferent discharges in alpha and fusimotor fibres of intercostal nerves of the ca~. J. Physiol. (Lond.), 174 (1964) 295-315. 15 Severin, F. V., Orlovsky, G. N. and Shik. M. L.. Work of the muscle receptors durJ~g controlled locomotion. Biophysics, 12 (1967) 575-586. 16 Severin, F. V.. The role of the gamma motor system in the activation of the extenso~ alpha motor neurones during controlled locomotion, Biophysics, 15 (1970) 1138-1145. 17 Sherrington, C. S., Flexion-reflex of the limb. crossed extension reflex, and reflex stepping and standing, J. Physiol. (Lond.). 40 (1910) 28-121 18 Sherrington, C. S.. Notes on the scratch-reflex of the cat, Quart. J. exp. Physiol.. 3 (t910) 213 220. 19 Sj6str6m, A. and Zangger, P., Muscle spindle control during locomotor movements generated by the deafferented spinal cord. Acta physiol, stand,. 97 (1976) 281-291 20 Vallbo, A. B., Muscle spind[e response at the onset of isometric voluntary contracuons in man, Time differences between fusimotor and skeletomotor effects, J. Physiol. (Lvnd.). 218 (1971) 405~43 I.