- Email: [email protected]
Chapter 11 Responses of muscle spindles depend on their history of activation and movement
W . Hamann and A. lggo (Eds.) Progress m Brain Research, Vol. 14
6 1988 Elsevier Science Publishers B.V. (Biomedical Division)
85
CHAPTER 11
Responses of muscle spindles depend on their history of activation and movement J.E. Gregory, D.L. Morgan and U. Proske Departments of Physiology and of Electrical Engineering, Monash University, Clayton, Vic., Australia
Introduction
After-effects, defined here as variations in the responsiveness of muscle spindles resulting from preceding activity and movement of the muscle, have long been known to exist (Hunt and Kuffler, 1951). They are commonly encountered as the ‘stuck’ spindle where the resting discharge in the passive muscle may undergo large ‘spontaneous’ changes in rate (Eldred et al., 1976). However, the resting discharge is only one of several aspects of a spindle’s response that may be altered by the preceding history. Others include the ‘initial burst’ of impulses at the onset of a ramp stretch (Brown et al., 1969), the response to a tendon tap (Gregory et al., 1987) and the response to fusimotor stimulation (Emonet-Denand et al., 1985a,b). Emonet-Denand et al. have shown that the response of a spindle to dynamic fusimotor stimulation during a slow stretch can be large or small depending on whether or not this is preceded by a conditioning period of rapid movements or fusimotor stimulation. The test used in their experiments was a brief fusimotor tetanus and the measure of the after-effect was the amplitude of the resulting burst of afferent impulses. A limitation of this test is its binary nature, signalling the presence of an after-effect simply by whether the burst is large or small. Another quite different problem is that the slow stretch during which the test tetanus is applied in one sequence may act to
condition the test response recorded in the next sequence. In other words, the persistent nature of after-effects makes it necessary to carefully define the unconditioned state. Because of this, in our own experiments we have chosen t o always condition the muscle by a defined procedure, and to avoid any ‘unconditioned state’. Rather than using a brief fusimotor tetanus for the test we have preferred to use tetani of longer duration. This converts a large or small afferent burst into a change in delay of onset of a sustained afferent response (Morgan et al., 1984). The advantage of such a test is that it allows after-effects to be quantified. After trying a variety of different conditioning and test procedures to show up after-effects we have concluded that the underlying cause lies in the mechanical properties of the intrafusal .fibres. Depending on their previous history of activation and length changes they may or may not be able to shorten themselves, when a shortening is imposed on the whole muscle. If not, they may fall slack. The presence of any intrafusal slack is signalled by a delay in onset of the afferent response to an intrafusal contraction, representing the time required for the shortening fibre to take up the slack. (A small afferent burst in response to a brief tetanus simply means that the intrafusal contraction did not last long enough to do more than just take up the slack and produce a small modulation of the afferent discharge, Fig. 1.) Procedures
86
which, we believe, introduce slack in the intrafusal bundle include stretching the muscle and holding it stretched for some seconds before returning it to the length at which the fusimotor test is to be applied. Here it is immediately apparent that if the test procedure itself involves a slow stretch then this will act to condition any subsequent test. Procedures which remove existing intrafusal slack include whole muscle contraction or fusimotor stimulation at the test length, or a series of rapid lengthening and shortening movements ending at the test length. Stimulation at a length longer than the test length but immediately followed by shortening to the test length also prevents development of any slack. We propose that the time required for development of slack represents the slow rate of formation of long-lasting cross-bridges (Hill, 1968) between actin and myosin of intrafusal fibres. It is these cross-bridges which are thought to give muscle its ‘stiction’ or thixotropic property (Hagbarth et al., 1985). Similarly, the short-range elasticity of intrafusal fibres is thought to give rise to the initial burst in the response of the spindle during a ramp stretch (Brown et al., 1969). We report here two experiments designed to test our hypothesis for development of slack in intrafusal fibres. The first considers the effect of altering the amount by which conditioning and test lengths differ. If the muscle is contracted at a length longer than the test length, held there for several seconds and then brought back to the test length, the amount of intrafusal slack developed and hence the size of the delay in onset of the response to a test fusimotor tetanus should increase as the length difference is made bigger. However, when the conditioning length is shorter than the test length, any pre-existing slack in intrafusal fibres should be taken up during the stretch required to return the muscle to the test length. Methods and results
The preparation used in these experiments is essentially the same as that described in our previous
reports on this subject (Gregory et al., 1986, 1987). We have used the cat soleus muscle and recorded from identified primary afferents of spindles in dorsal root filaments. Muscle length was related to the maximum in the body (Lma,). The test length was chosen to correspond approximately to the optimum for a muscle contraction. For the results presented here, the conditioning procedure involved stretching or releasing from the test length at a rate of 10 mm/s, holding the muscle there, and then stimulating the muscle nerve at 30 pulses/s with a stimulus strength sufficient to engage fusimotor fibres. After stimulation the muscle was kept still for a further 5 s and then returned to the test length. The testing procedure consisted of measuring the delay in onset of the afferent response to tetanic stimulation (500 ms tetanus at 50 pulses/s) of an identified dynamic fusimotor fibre supplying the test spindle. The fusimotor tetanus was given 1 s after commencement of a slow stretch (1 mm/s), the stretch being initiated 3 s after return to the test length (see Fig. 1). The delay or latency was measured as the time from delivery of the stimulating pulses to 50% of the peak of the spindle response. Measurement of latency had a resolution of 5 ms and reproducibility of 5 - 10 ms. For all five spindle and dynamic fusimotor pairs studied, the afferent response to stimulation rose sufficiently rapidly for the choice of point of measurement not to be critical. The entire testing sequence is shown in Fig. 1 . When conditioning involved stretching to a length longer than the test length (LONG), the subsequent test response was identified as ‘LONG’. When conditioning was at a shorter length than the test length this was identified by ‘SHORT’. The afferent discharge is shown as an instantaneous frequency display and the test response to the fusimotor tetanus is indicated by an arrow. Here, for the sake of illustration, we have used a brief test tetanus, one which shows the effect of conditioning as a change in size of the afferent burst. In all subsequent figures a longer tetanus was used, long enough to measure a delay in onset of afferent response (see Fig. 2).
87
t
SHORT
5s
Fig. 1. The effect of alternative conditioning procedures on the response of the primary ending of a soleus muscle spindle to a brief dynamic fusimotor tetanus applied during a slow stretch. In the lower part of the figure are shown two superimposed traces of muscle length, while the upper part shows the corresponding traces of spindle discharge as instantaneous frequency (reciprocal of interval between successive impulses). The muscle was initially at a length 11 mm shorter than maximum length in the body and was then either stretched (long) or shortened (short) by 3 mm, following which the muscle nerve was stimulated at a rate of 30 s - ’ for 1 s. The muscle was then held at the long or short conditioning length for a further 5 s before being returned to the mean test length. After a short pause, the test dynamic fusimotor tetanus (100 s - ’ , 60 ms duration) was delivered during a slow stretch. The response (indicated by arrow) is of smaller amplitude following conditioning at the long length than after conditioning at the short length.
In Fig. 2 is shown the lack of any change in latency when the test is preceded by a conditioning contraction at a length 0.5 mm longer o r shorter than the test length (A). However, a large latency shift becomes apparent when the conditioning length is increased to 3 mm longer or shorter than the test length. This result is in accord with our predictions. For conditioning at lengths shorter than or equal to the test length (negative length differences), no intrafusal slack develops and the measured latency is small and constant. For positive length differences where now the condi-
tioning length is longer than the test length a latency difference between the ‘LONG’ and ‘SHORT’ condition becomes apparent, but only if the conditioning length exceeds the test length by a sufficient amount. For example in Fig. 2, 0.5 mm is clearly not sufficient. Making the conditioning length longer results in development of a latency difference which gets bigger with further length increases but which reaches a plateau value depending on the muscle length at which the test measurements are made. When the test length is long (Lmax - 6) the maximum latency difference (130 ms) is measured with a conditioning length 2 mm longer than the test length. When the test A
Too
7 Too
Fig. 2. Effect of size of conditioning length step on latency of response to test dynamic fusimotor stimulation. Both A and B show two superimposed spindle responses above two superimposed length traces in which the length changes from long or short conditioning lengths to the test, mean length can be seen to the left. Interval between the end of conditioning whole muscle tetanus and start of return to test length is 10 s. (A) Conditioning at lengths 0.5 mm longer and shorter than the test length produces no difference in the latency of the responses to test dynamic fusimotor stimulation at 50 s - ’ for 0.5 s. (B) The latency of the response following conditioning at a length 3 mm longer than the test length is 0.28 s longer than after conditioning at the short length. Mean muscle length 11 mm shorter than maximum length in the body (Lmax - 11).
88
length is shorter (Lmax- 11) a maximum latency difference of 250 ms is measured for a 4 mm difference between conditioning and test lengths (Fig. 3). The shorter latency measured at longer muscle lengths suggests that some of the intrafusal slack introduced by conditioning is ‘spontaneously’ taken up as a result of the greater passive tension in the muscle. The fact that latency has already reached a plateau with a length difference of 2 mm suggests that at these longer lengths intrafusal fibres will not develop more than a limited amount of slack. Perhaps slack beyond a certain point is again taken up ‘spontaneously’ as lateral compressive forces from the intrafusal fibre’s immediate extrafusal surroundings and the fibre’s own passive resistance to bending reach sufficient levels to lead to detachment of some of the longlasting cross-bridges, and their reformation after some fibre shortening. Notice too in Fig. 3 that at longer test lengths the minimal length difference for a latency shift is 1 - 2 mm. At the shorter test length, a 1 mm difference already gives a latency shift of nearly 200 ms. This again suggests that in the face of a greater passive tension a larger shortening step must be used to introduce measurable slack. The second experiment described here considers the effect of varying the time for which the muscle is held at the conditioning length before being returned to the test length. Our theory predicts that this hold time must be long enough to allow sufficient numbers of stable cross-bridges to form, following muscle contraction, to stiffen the intrafusal fibres enough for them not to shorten but to fall slack when the whole muscle is shortened. As muscle fibre shortening is brought about by passive forces, these would be expected to be greater at longer mean muscle lengths so that the amount by which an intrafusal fibre must be stiffened to prevent it from shortening would be expected to be greater at longer lengths, implying a longer necessary hold time. The result of an experiment exploring this point is shown in Fig. 4. Here the conditioning length was always made either 3 mm longer or shorter
Q
Lrnax-6
+ Lrnax-I1
-6
-4
-2 0 2 Length Difference (rnm)
4
6
Fig. 3 . For one spindle, plots of difference between conditioning and test lengths against latency of responses to dynamic fusimotor stimulation, at two test lengths. Details of stimulation etc. as in Fig. 2, which shows examples of records from which values plotted were read. Latency is the same for all conditioning lengths shorter than the test length, but increases progressively when the conditioning length is made longer than the test length. When muscle mean length is increased, the latency reaches a lower maximum.
than the test length. When the muscle contraction at the conditioning length was followed by immediate return to the test length (0 s, Fig. 4) no latency difference developed. When, however, the muscle was held at the conditioning length for 20 s after the contraction a large latency difference between the ‘long’ and the ‘short’ condition developed (20 s, Fig. 4). This experiment was repeated at a number of different muscle lengths for a range of different hold times of between 0.1 s and 100 s (Fig. 5). As predicted, the hold time required to achieve the same latency shift was found to be longer at longer mean muscle lengths. Notice that at Lm,,-2 mm the latency shift was small, 40 - 50 ms, and needed a hold time of at least 10 s to become apparent. Once again the conclusion is that some ‘spontaneous’ intrafusal shortening can occur following development of slack and this becomes progressively more pronounced at longer muscle lengths. Here presumably there is slippage in some sarcomeres, or part of the muscle, and sticking in other parts. Partial shortening, uniformly distri-
89
buted along the whole length of the intrafusal fibre, seems unlikely as once the cross-bridges of a sarcomere are broken the sarcomere would be expected to continue shortening until there remained little or no passive tension within it. Another feature of Fig. 5 is that intrafusal fibres continue to become more firmly stuck over hold intervals of 20 s or more. There is a progressive increase in latency over this whole period. Such behaviour is consistent with the known properties of stable cross-bridges (Hill, 1968).
300 -
-m ~rnax-2
+ 4
Lmax-4 Lmax-6
E
v
z1
c
! I1 0 0 -
0
I
0.1
1
Interval(sec)
100
Fig. 5 . For one spindle, plots of duration of periods of holding muscle at conditioning length after whole muscle contraction (Interval) against latency of response to test dynamic fusimotor stimulation, at five different mean muscle lengths. Values read from records similar to those shown in Fig. 4. Latency increases with increased interval. As the muscle length is increased, a larger interval is required to achieve the same latency. Note the logarithmic time scale.
0s
....
.'* *
10
.
. .
Conclusion
/
Is
Fig. 4. Effect of changing the interval during which the muscle is held at the long or short length after the conditioning whole muscle contraction. Two length traces and two records of spindle discharge are superimposed in each panel. Details of muscle and fusimotor stimulation as in Fig. 2. In the upper part of the figure, when the interval is zero (i.e., immediately after the end of contraction, shortening to the test length begins), there is no difference in latency between conditioning at lengths longer and shorter than the test length. With an interval of 20 s, the response to fusimotor stimulation following conditioning at the long length is delayed by 0.25 s. Mean muscle length Lmax- 11.
To conclude, we believe the experiments described here support our explanation of after-effects and are difficult to explain in any other way. The general proposition is that a spindle will develop depression of its responsiveness if it is held passive for a sufficient period and is then shortened. The time necessary for development of this depression is greater at longer muscle lengths, ranging from seconds to tens or even hundreds of seconds. According to our explanation this time is required for 'permanent' or 'slow-turnover' cross-bridges to form in sufficient numbers to resist the passive forces imposed on sacromeres by elastic tissue during muscle shortening. Anything that leads to detachment of these bridges during their period of formation will tend to reduce spindle depression by allowing intrafusal fibres to subsequently shorten along with the whole muscle, thereby reducing the likelihood of development of any slack.
90
References Brown, M.C., Goodwin, G.M. and Matthews, P.B.C. (1969) After effects of fusimotor stimulation on the response of muscle spindle primary afferent endings. J. Physiol. (London), 205: 677-694. Eldred, E., Hutton, R.S. and Smith, J.L. (1976) Nature of the persisting changes in afferent discharge from muscle following its contraction. In: s. Homma (Ed.), ‘Undersfanding The Stretch Reflex, Progress in Brain Research, Vol. 44, Elsevier, Amsterdam, pp. 157- 170. Emonet-Denand, F., Hunt, C.C. and Laporte, Y. (1985a) Fusimotor after-effects on response of primary endings to test dynamic stimuli in cat muscle spindles. J. Physiol. (London), 360: 187-200. Emonet-Denand, F., Hunt, C.C. and Laporte, Y. (1985b) Effects of stretch on dynamic fusimotor after-effects in cat muscle spindles. J. Physiol. (London), 360: 201 - 213. Gregory, J.E., Morgan, D.L. and Proske, U. (1986) After-
effects in the responses of cat muscle spindles. J . Neurophysiol., 56: 451 -461. Gregory, J.E., Morgan, D.L. and Proske, U. (1987) Changes in the size of the stretch reflex of cat and man attributed to after-effects in muscle spindles. J. Neurophysiol., in press. Hagbarth, K.E., Hagglund, J.V., Nordin, M. and Wallin, E.U. (1985) Thixotropic behaviour of human finger flexor muscles with accompanying changes in spindle and reflex responses to stretch. .I. Physiol. (London), 368: 323 - 342. Hill, D.K. (1968) Tension due to interaction between the sliding filaments in resting striated muscle. The effect of stimulation. J . Physiol. (London), 199: 637 - 684. Hunt, C.C. and Kuffler, S.W. (1951) Further study of efferent small-nerve fibres to mammalian muscle spindles. Multiple spindle innervation and activity during contraction. J . Physiol. (London), 113: 283 - 297. Morgan, D.L., Prochazka, A. and Proske, U. (1984) The aftereffects of stretch and fusimotor stimulation on the responses of primary endings of cat muscle spindles. J . Physiol. (London), 356: 465 - 477.
6 1988 Elsevier Science Publishers B.V. (Biomedical Division)
85
CHAPTER 11
Responses of muscle spindles depend on their history of activation and movement J.E. Gregory, D.L. Morgan and U. Proske Departments of Physiology and of Electrical Engineering, Monash University, Clayton, Vic., Australia
Introduction
After-effects, defined here as variations in the responsiveness of muscle spindles resulting from preceding activity and movement of the muscle, have long been known to exist (Hunt and Kuffler, 1951). They are commonly encountered as the ‘stuck’ spindle where the resting discharge in the passive muscle may undergo large ‘spontaneous’ changes in rate (Eldred et al., 1976). However, the resting discharge is only one of several aspects of a spindle’s response that may be altered by the preceding history. Others include the ‘initial burst’ of impulses at the onset of a ramp stretch (Brown et al., 1969), the response to a tendon tap (Gregory et al., 1987) and the response to fusimotor stimulation (Emonet-Denand et al., 1985a,b). Emonet-Denand et al. have shown that the response of a spindle to dynamic fusimotor stimulation during a slow stretch can be large or small depending on whether or not this is preceded by a conditioning period of rapid movements or fusimotor stimulation. The test used in their experiments was a brief fusimotor tetanus and the measure of the after-effect was the amplitude of the resulting burst of afferent impulses. A limitation of this test is its binary nature, signalling the presence of an after-effect simply by whether the burst is large or small. Another quite different problem is that the slow stretch during which the test tetanus is applied in one sequence may act to
condition the test response recorded in the next sequence. In other words, the persistent nature of after-effects makes it necessary to carefully define the unconditioned state. Because of this, in our own experiments we have chosen t o always condition the muscle by a defined procedure, and to avoid any ‘unconditioned state’. Rather than using a brief fusimotor tetanus for the test we have preferred to use tetani of longer duration. This converts a large or small afferent burst into a change in delay of onset of a sustained afferent response (Morgan et al., 1984). The advantage of such a test is that it allows after-effects to be quantified. After trying a variety of different conditioning and test procedures to show up after-effects we have concluded that the underlying cause lies in the mechanical properties of the intrafusal .fibres. Depending on their previous history of activation and length changes they may or may not be able to shorten themselves, when a shortening is imposed on the whole muscle. If not, they may fall slack. The presence of any intrafusal slack is signalled by a delay in onset of the afferent response to an intrafusal contraction, representing the time required for the shortening fibre to take up the slack. (A small afferent burst in response to a brief tetanus simply means that the intrafusal contraction did not last long enough to do more than just take up the slack and produce a small modulation of the afferent discharge, Fig. 1.) Procedures
86
which, we believe, introduce slack in the intrafusal bundle include stretching the muscle and holding it stretched for some seconds before returning it to the length at which the fusimotor test is to be applied. Here it is immediately apparent that if the test procedure itself involves a slow stretch then this will act to condition any subsequent test. Procedures which remove existing intrafusal slack include whole muscle contraction or fusimotor stimulation at the test length, or a series of rapid lengthening and shortening movements ending at the test length. Stimulation at a length longer than the test length but immediately followed by shortening to the test length also prevents development of any slack. We propose that the time required for development of slack represents the slow rate of formation of long-lasting cross-bridges (Hill, 1968) between actin and myosin of intrafusal fibres. It is these cross-bridges which are thought to give muscle its ‘stiction’ or thixotropic property (Hagbarth et al., 1985). Similarly, the short-range elasticity of intrafusal fibres is thought to give rise to the initial burst in the response of the spindle during a ramp stretch (Brown et al., 1969). We report here two experiments designed to test our hypothesis for development of slack in intrafusal fibres. The first considers the effect of altering the amount by which conditioning and test lengths differ. If the muscle is contracted at a length longer than the test length, held there for several seconds and then brought back to the test length, the amount of intrafusal slack developed and hence the size of the delay in onset of the response to a test fusimotor tetanus should increase as the length difference is made bigger. However, when the conditioning length is shorter than the test length, any pre-existing slack in intrafusal fibres should be taken up during the stretch required to return the muscle to the test length. Methods and results
The preparation used in these experiments is essentially the same as that described in our previous
reports on this subject (Gregory et al., 1986, 1987). We have used the cat soleus muscle and recorded from identified primary afferents of spindles in dorsal root filaments. Muscle length was related to the maximum in the body (Lma,). The test length was chosen to correspond approximately to the optimum for a muscle contraction. For the results presented here, the conditioning procedure involved stretching or releasing from the test length at a rate of 10 mm/s, holding the muscle there, and then stimulating the muscle nerve at 30 pulses/s with a stimulus strength sufficient to engage fusimotor fibres. After stimulation the muscle was kept still for a further 5 s and then returned to the test length. The testing procedure consisted of measuring the delay in onset of the afferent response to tetanic stimulation (500 ms tetanus at 50 pulses/s) of an identified dynamic fusimotor fibre supplying the test spindle. The fusimotor tetanus was given 1 s after commencement of a slow stretch (1 mm/s), the stretch being initiated 3 s after return to the test length (see Fig. 1). The delay or latency was measured as the time from delivery of the stimulating pulses to 50% of the peak of the spindle response. Measurement of latency had a resolution of 5 ms and reproducibility of 5 - 10 ms. For all five spindle and dynamic fusimotor pairs studied, the afferent response to stimulation rose sufficiently rapidly for the choice of point of measurement not to be critical. The entire testing sequence is shown in Fig. 1 . When conditioning involved stretching to a length longer than the test length (LONG), the subsequent test response was identified as ‘LONG’. When conditioning was at a shorter length than the test length this was identified by ‘SHORT’. The afferent discharge is shown as an instantaneous frequency display and the test response to the fusimotor tetanus is indicated by an arrow. Here, for the sake of illustration, we have used a brief test tetanus, one which shows the effect of conditioning as a change in size of the afferent burst. In all subsequent figures a longer tetanus was used, long enough to measure a delay in onset of afferent response (see Fig. 2).
87
t
SHORT
5s
Fig. 1. The effect of alternative conditioning procedures on the response of the primary ending of a soleus muscle spindle to a brief dynamic fusimotor tetanus applied during a slow stretch. In the lower part of the figure are shown two superimposed traces of muscle length, while the upper part shows the corresponding traces of spindle discharge as instantaneous frequency (reciprocal of interval between successive impulses). The muscle was initially at a length 11 mm shorter than maximum length in the body and was then either stretched (long) or shortened (short) by 3 mm, following which the muscle nerve was stimulated at a rate of 30 s - ’ for 1 s. The muscle was then held at the long or short conditioning length for a further 5 s before being returned to the mean test length. After a short pause, the test dynamic fusimotor tetanus (100 s - ’ , 60 ms duration) was delivered during a slow stretch. The response (indicated by arrow) is of smaller amplitude following conditioning at the long length than after conditioning at the short length.
In Fig. 2 is shown the lack of any change in latency when the test is preceded by a conditioning contraction at a length 0.5 mm longer o r shorter than the test length (A). However, a large latency shift becomes apparent when the conditioning length is increased to 3 mm longer or shorter than the test length. This result is in accord with our predictions. For conditioning at lengths shorter than or equal to the test length (negative length differences), no intrafusal slack develops and the measured latency is small and constant. For positive length differences where now the condi-
tioning length is longer than the test length a latency difference between the ‘LONG’ and ‘SHORT’ condition becomes apparent, but only if the conditioning length exceeds the test length by a sufficient amount. For example in Fig. 2, 0.5 mm is clearly not sufficient. Making the conditioning length longer results in development of a latency difference which gets bigger with further length increases but which reaches a plateau value depending on the muscle length at which the test measurements are made. When the test length is long (Lmax - 6) the maximum latency difference (130 ms) is measured with a conditioning length 2 mm longer than the test length. When the test A
Too
7 Too
Fig. 2. Effect of size of conditioning length step on latency of response to test dynamic fusimotor stimulation. Both A and B show two superimposed spindle responses above two superimposed length traces in which the length changes from long or short conditioning lengths to the test, mean length can be seen to the left. Interval between the end of conditioning whole muscle tetanus and start of return to test length is 10 s. (A) Conditioning at lengths 0.5 mm longer and shorter than the test length produces no difference in the latency of the responses to test dynamic fusimotor stimulation at 50 s - ’ for 0.5 s. (B) The latency of the response following conditioning at a length 3 mm longer than the test length is 0.28 s longer than after conditioning at the short length. Mean muscle length 11 mm shorter than maximum length in the body (Lmax - 11).
88
length is shorter (Lmax- 11) a maximum latency difference of 250 ms is measured for a 4 mm difference between conditioning and test lengths (Fig. 3). The shorter latency measured at longer muscle lengths suggests that some of the intrafusal slack introduced by conditioning is ‘spontaneously’ taken up as a result of the greater passive tension in the muscle. The fact that latency has already reached a plateau with a length difference of 2 mm suggests that at these longer lengths intrafusal fibres will not develop more than a limited amount of slack. Perhaps slack beyond a certain point is again taken up ‘spontaneously’ as lateral compressive forces from the intrafusal fibre’s immediate extrafusal surroundings and the fibre’s own passive resistance to bending reach sufficient levels to lead to detachment of some of the longlasting cross-bridges, and their reformation after some fibre shortening. Notice too in Fig. 3 that at longer test lengths the minimal length difference for a latency shift is 1 - 2 mm. At the shorter test length, a 1 mm difference already gives a latency shift of nearly 200 ms. This again suggests that in the face of a greater passive tension a larger shortening step must be used to introduce measurable slack. The second experiment described here considers the effect of varying the time for which the muscle is held at the conditioning length before being returned to the test length. Our theory predicts that this hold time must be long enough to allow sufficient numbers of stable cross-bridges to form, following muscle contraction, to stiffen the intrafusal fibres enough for them not to shorten but to fall slack when the whole muscle is shortened. As muscle fibre shortening is brought about by passive forces, these would be expected to be greater at longer mean muscle lengths so that the amount by which an intrafusal fibre must be stiffened to prevent it from shortening would be expected to be greater at longer lengths, implying a longer necessary hold time. The result of an experiment exploring this point is shown in Fig. 4. Here the conditioning length was always made either 3 mm longer or shorter
Q
Lrnax-6
+ Lrnax-I1
-6
-4
-2 0 2 Length Difference (rnm)
4
6
Fig. 3 . For one spindle, plots of difference between conditioning and test lengths against latency of responses to dynamic fusimotor stimulation, at two test lengths. Details of stimulation etc. as in Fig. 2, which shows examples of records from which values plotted were read. Latency is the same for all conditioning lengths shorter than the test length, but increases progressively when the conditioning length is made longer than the test length. When muscle mean length is increased, the latency reaches a lower maximum.
than the test length. When the muscle contraction at the conditioning length was followed by immediate return to the test length (0 s, Fig. 4) no latency difference developed. When, however, the muscle was held at the conditioning length for 20 s after the contraction a large latency difference between the ‘long’ and the ‘short’ condition developed (20 s, Fig. 4). This experiment was repeated at a number of different muscle lengths for a range of different hold times of between 0.1 s and 100 s (Fig. 5). As predicted, the hold time required to achieve the same latency shift was found to be longer at longer mean muscle lengths. Notice that at Lm,,-2 mm the latency shift was small, 40 - 50 ms, and needed a hold time of at least 10 s to become apparent. Once again the conclusion is that some ‘spontaneous’ intrafusal shortening can occur following development of slack and this becomes progressively more pronounced at longer muscle lengths. Here presumably there is slippage in some sarcomeres, or part of the muscle, and sticking in other parts. Partial shortening, uniformly distri-
89
buted along the whole length of the intrafusal fibre, seems unlikely as once the cross-bridges of a sarcomere are broken the sarcomere would be expected to continue shortening until there remained little or no passive tension within it. Another feature of Fig. 5 is that intrafusal fibres continue to become more firmly stuck over hold intervals of 20 s or more. There is a progressive increase in latency over this whole period. Such behaviour is consistent with the known properties of stable cross-bridges (Hill, 1968).
300 -
-m ~rnax-2
+ 4
Lmax-4 Lmax-6
E
v
z1
c
! I1 0 0 -
0
I
0.1
1
Interval(sec)
100
Fig. 5 . For one spindle, plots of duration of periods of holding muscle at conditioning length after whole muscle contraction (Interval) against latency of response to test dynamic fusimotor stimulation, at five different mean muscle lengths. Values read from records similar to those shown in Fig. 4. Latency increases with increased interval. As the muscle length is increased, a larger interval is required to achieve the same latency. Note the logarithmic time scale.
0s
....
.'* *
10
.
. .
Conclusion
/
Is
Fig. 4. Effect of changing the interval during which the muscle is held at the long or short length after the conditioning whole muscle contraction. Two length traces and two records of spindle discharge are superimposed in each panel. Details of muscle and fusimotor stimulation as in Fig. 2. In the upper part of the figure, when the interval is zero (i.e., immediately after the end of contraction, shortening to the test length begins), there is no difference in latency between conditioning at lengths longer and shorter than the test length. With an interval of 20 s, the response to fusimotor stimulation following conditioning at the long length is delayed by 0.25 s. Mean muscle length Lmax- 11.
To conclude, we believe the experiments described here support our explanation of after-effects and are difficult to explain in any other way. The general proposition is that a spindle will develop depression of its responsiveness if it is held passive for a sufficient period and is then shortened. The time necessary for development of this depression is greater at longer muscle lengths, ranging from seconds to tens or even hundreds of seconds. According to our explanation this time is required for 'permanent' or 'slow-turnover' cross-bridges to form in sufficient numbers to resist the passive forces imposed on sacromeres by elastic tissue during muscle shortening. Anything that leads to detachment of these bridges during their period of formation will tend to reduce spindle depression by allowing intrafusal fibres to subsequently shorten along with the whole muscle, thereby reducing the likelihood of development of any slack.
90
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