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Changes in the pause in muscle spindle discharge during a sequence of twitches
EXPERIhfENTAL
NE1’ROI.OC.Y
Changes
60,
in the during
M. Istit&o
DECANDIA,
di Fisiologia
Unzaxa
201-212 (1978)
Pause
in Muscle
a Sequence
AND P. CRENNA 1
II, U~lielersitd dcgli October
Discharge
of Twitches
M. SCHIEPPATI,
Received
Spindle
Studi
di drlilauo,
Milarzo,
Italy
21. 1977
A study was made of the change in duration of the pauses in discharge from spindle afferents occurring during a series of isometric twitches of the cat’s triceps surae muscle. These were induced by stimulation of the muscle nerve at frequencies of 0.1 to 1.0 Hz. At frequencies of 0.2 Hz and higher, the duration of the pause exhibited by group II and tonic and phasic group Ia afferents decreased progressively until stabilizing within 5 to 15 s. The time course of this change was largely independent of the frequency of stimulation, but the eventual duration in the steady state was strongly dependent on the frequency, being shorter at higher rates. At a given frequency, the pause was shorter at longer muscle lengths. The decrease in pause duration was not accompanied by a parallel change in duration or amplitude of the muscle tension responses, indicating that it arose from some alteration intrinsic to the spindle organ. We suggest that the progressive change in sensory discharge could serve the initial reinforcement of repetitive movements.
INTRODUCTION The sensory information supplied by muscle spindles as to the length status of a muscle is coded in terms of static levels of discharge corresponding to the steady length and dynamic changes in rate related to the velocity of length change. The latter can take the form of a decrease in firing, as illustrated by the complete pause in discharge that occurs during a muscle twitch (15). Granit and Van der Meulen (7) distinguished two patterns of such pausesexhibited by Ia afferent fibers: that of “tonic” afferent units, which simply resumed firing at essentially the steady baseline rate, and that 1 The authors are grateful to Dr. Earl Eldred for his helpful ing the manuscript. This work was supported by Grant CT Consiglio Nazionale delle Ricerche. Dr. Decandia’s present degli Studi di Cagliari, Istituto di Fisiologia Umana, Cagliari,
comments in prepar75.00499.04 from the address is Universita Italy.
201 0014-4886/78/0602-0201$02.00/O Copyright 0 1978 by AcademicPress,Inc.
AlI rights
of reproduction
in any
form
reserved.
202
DECANDIA,
SCHIEPPATI,
AND
CKENNA
of ‘il~hasic” units, which showed a fleeting burst of impulses during the fall in muscle tension. The pause in the discharge has significant reflex effect. Bianconi et nl. (2) showed that there is a correlation between pause of the spindles and changes in excitability of the spinal motoneurons on which they impinge. Furthermore, the pause has a role in the peripheral control of movement, because it is considered responsible for the “silent period” of the electrical activity of the voluntary muscle contraction in man (17) and for the silent period of the reflex electrical activity in decerebrate cats (13). The present study is an analysis of a sequential shift in duration of the pause which occurs in the course of a series of twitches. The relation of this shift to frequency of stimulation and muscle length, and its occurrence in group II and tonic and phasic group Ia units are described. Preliminary results were published in a short communication (19). METHOD The experiments were done on cats. Under deep ether anesthesia the carotid and vertebral arteries were clamped, The anesthesia was then discontinued, ventilation maintained by a respirator, and rectal temperature kept at 37 * 0.5”C by a heating pad. A lumbosacral laminectomy was made, ventral roots L5 to S2 to one leg were cut, and the leg was thoroughly denervated except for the gastrocnemius-soleus nerves. After mounting the animal in a frame, the leg was rigidly fixed to the frame by clamps and the severed Achilles tendon was connected by a stiff wire to a strain gauge, which yielded only 50 pm/kg and had a time constant of 1 ms. Warmed oil pools were constructed in the popliteal space and over the spinal cord. The L7 or Sl dorsal root was split with forceps until a filament containing only one afferent unit from the triceps was present, and this was identified by its response to stretch, twitch response, and conduction rate. The latter was calculated from the time interval between responses elicited by stimuli applied to the triceps nerve through two pairs of electrodes separated by 5 cm. Fibers with conduction velocities of 80 to 120 m/s were considered to be group Ia fibers, and those conducting at less than 55 m/s were considered to be group II units (16). After the fiber was identified, stimuli at just-maximal or submaximal strength for elicitation of the twitch response were delivered to the triceps nerve to evoke nearisometric twitches. This strength of stimulus was used to avoid y-fiber excitation ( 14)) although fusimotor excitation through the p-fiber innervation may have occurred (4). No significant differences in the findings were noted when stimulus strengths somewhat below that needed for a maximal twitch were used. In several experiments in which the nerve to the fast-contracting medial gastrocnemius muscle (22) alone was pre-
203
served, tmits displayed the same phenomenon as when both gastrocnemius and soleus nerves were intact. Observations were begun about 5 h after discontinuing the anesthesia. The muscle was stretched to successive steady lengths beyond the resting physiological length, as the ef-fects of a series of twitches elicited at given rate were tested. The interval hetween different series was never less than 20 s. RESULTS The patterns of discharge of groups Ia and II afferents from triceps surae spindles were observed during a series of isometric twitches induced by repetitive stimulation of the muscle nerve at 0.1 to 1.0 Hz, that is, below frequencies leading to fusion of the tension responses. We noted that the pause in the spindle afferent discharge decreased progressively as a function of time when the successive twitches were evoked at frequencies higher than 0.2 Hz. After a transient phase of a
FIG. 1. Pauses in discharge of a group Ia afferent fiber (100 m/s) of the tonic type obtained during twitches evoked at 0.8, 0.2, and 0.1 Hz. Each pair of traces presents tension above and afferent discharge below. The slight irregularities in the tension recordings are related to the heart beat. The muscle was stretched 2 mm beyond the resting length.
204
DECANDIA,
SCHIEPPATI,
AND
CRENNA
2751 c
250
c : 225. i .F200z 2 a 175
b
’
lb
'
2'0
'SIX
3'0
FIG. 2. Duration of the pause in spindle discharge versus time of stimulation, seen at several frequencies of stimulation. Data are from the afferent represented in Fig. 1.
few seconds, the pause tends to a steady value which was found to be frequency and muscle length dependent. A total of 51 afferents were studied: 17 group Ia tonic, 20 group Ia phasic, and 14 group II units. Group Ia Tonic Afferents. Figure 1 illustrates the patterns of response
FIG. 3. Plots on the graph show the total duration (upper curves) of the successive twitches evoked at 1 Hz (dotted lines) and at 0.8 Hz (solid lines). Due to unavoidable inaccuracy in performing the measurements on the baseline, large variations in duration appear among the twitches. However, no trend to a progressive decrease in value is evident. Measurements between the two values of tension corresponding to 25% of the peak give more reliable results, as the lower curves show. The recordings on the right were obtained by superimposition of all the muscle twitches evoked at the indicated frequencies and of their corresponding derivatives. Note the constancy of tension, duration, and rate of change in the successive twitches.
PAUSE
IN
MUSCLE
SPINL)LES
20.5
of a typical tonic afferent fiber within a series of twitches elicited at three different rates of stimulation. The classical response (10) is seen, consisting of a very brief “early discharge,” followed by complete cessation of discharge through the rising phase of the tension response and almost all of its descending phase. This pause, however, is not constant in duration within a series, but decreases progressively during the first five to six twitches. The trend is hardly detectable at 0.1 Hz, but is prominent in the 0.8-Hz series, where a shortening in pause duration of about 3076 occurs. The effects of stimulation at frequencies of stimulation higher than 1.0 Hz were not tested in detail, for fusion of the twitch responses complicated the interpretation ; but we noticed that further reduction in duration occurred at somewhat higher rates. Plots of pause length versus time (Fig. 2) exhibited by the same group Ia unit at different frequencies of nerve stinmlation demonstrate that : (a) The duration decreased progressively as a function of time of stimulation until, after 15 s or less, the duration became stabilized ; and (b) the duration of the pause was decidedly shorter at higher frequencies of stirnulation. These changes in pause duration are not accompanied by parallel changes 275
1
I 0
I
I
1 lengthening
2 of
1
I
3
4
muscle
I 5 mm
FIG. 4. The graph shows the effect of muscle length on duration of the pause at several frequencies of twitch stimulation. Durations were measured after they had reached a constant value. To the right are represented twitches at l-mm increments, from the physiological resting length (LO) to 6 mm (LO). Note the increase in amplitude and duration of the tension responses with increase in muscle length.
206
DECANDIA,
SCHIEPPATI,
AND
CRENNA
in amplitude and duration of successive twitches. During the entire period of testing at any rate at a given muscle length, the plots (Fig. 3) of the total duration of twitches and of duration measured at the point where the tension falls to 25% of the peak value show that the mechanical events have no trend to a progressive change with time. This is confirmed by the recordings (Fig. 3) of superimposed traces of the successive muscle twitches and of the rate of change in tension obtained within a given series. The duration of the pause, when at its stabilized value, decreased with the amount of extension of the muscle at lengths more than 1 mm beyond the resting muscle length (Fig. 4). This is the opposite of wh.at might be expected from the unloading effect of the twitch contractions, which were increased at greater muscle lengths. In fact, the recordings inserted in Fig. 4 show that the muscle twitches, as expected (S), became greater in amplitude and duration for several millimeters of muscle lengthening. Whereas pause duration was related to muscle length, the time course of the progressive decrease in duration at a given frequency of muscle twitches was little affected by the length of the muscle. This is evident in Fig. 5, where the absolute values of the pauses during twitches evoked at 1.0 Hz at the indicated muscle lengths are plotted on the left, and the
275 160
250
1,'SW 225
: ; .c :
200
1
2 a 175
-I
125
1 1 0
I 2
I 4
I 6
1 6
set
, 10
1 0
I 2
1 4
6
, 8
# set
10
FIG. 5. Relation between duration of the afferent pause and time of stimulation, as determined at several muscle lengths. On the left the absolute values are plotted, and on the right the durations are expressed as percentages of the stabilized mean value.
I’AI’SE
IN
1 set
1
FG. tlvitch
A”“‘Y
6. Responses series elicited
RtUSC1.E
207
SI’INDLES
02,‘sec
05’sec
500
I 500
msec
of a group Ia afferent at 1.0, 0.5, and 0.2 Hz.
fiber
(108
m/s)
g
of the
phasic
type
in
durations as a percent of the mean for the pauses at their steady state are plotted on the right. As there is 1x1 systematic shift in the latter curves, the time required for stal)ilizatioii of pause duration is independent of muscle estension. Gvolo la Plmsic L4flcrcl/fs. In Fig. 6 is illustrated the behavior of a “@ask” unit (7)) which during the decline in tension displayed a burst of closely spaced spikes, followed by a secondary gap in firing before the stable discharge was resumed. Such units were found in preparations with ouly the medial gastrocnemius nerve preserved, as well as those with triceps innervation intact. It is seeu that hot11 the early coiiiponent of the pause preceding the burst and the total l)ause to onset of the steady discharge decreased in duration with successive stimuli (top to bottom). Figure 7 sunmarizes the changes in duration of the early pause (solid lines) and total pause (dotted lines) shown by this same group Ia fiber at different frequencies of stimulation. The curves described hy the early pause are similar to those of the total pause. or, expressed differently, the burst tends to fall at ahout the same proportionate point in the total pause. The behavior sho\~n by the burst of inlpulses is puzzling. Generally the
208
DECANDIA,
(Y
SCHIEPPATI,
AND
CRENNA
0
500
0 1/set .
: 0. P
O.Z/sec
-
05Jsec
IJSSC
300
I>=
A
10
2b
30
se=
40
FIG. 7. Duration of the total pause (dotted lines, above) and of the first component (solid lines, below) as a function of time of stimulation. Data are from the afferent fiber of Fig. 6.
burst occurred at a time when the tension trace had not yet completely returned to the baseline. The frequency within the burst was very much greater than the control rate at the constant muscle length, indicating either that the ending was briefly subjected to accelerating length change or that the sensitivity of the ending was decidedly enhanced at that moment. However, the latter possibility seems to be in contrast to the observation that firing within the burst became much slower later along the twitch series (Fig. 6) when shortening of the total pause would indicate that sensitivity of the ending was in fact enhanced. The decrease in frequency within the bursting discharge was observed also during subsequent ramp stretches applied to the muscle (18) or to the single muscle spindle (11) at frequencies lower than 1.0 Hz. This phenomenon seemsto be due to a decrease in stiffness of the intrafusal fibers, caused by breaking of crossbridges between thick and thin filaments which are formed at rest, as Brown et al. (3) suggested. Gr0u.p II Afferents. The group II afferents exhibited a highly tonic pattern of twitch response, a steady rate of discharge being immediately resumed after the pause. The behavior of the pause (Fig. 8) in terms of relation to frequencies of stimulation and the time course in a given series of twitches were similar to those of group Ia tonic units. The duration of
PAUSE
IN
MUSCLE
I 0
200
SPINDLES
20
10
‘SC
io
FIG. 8. Duration of the pause in twitch response of a group II fiber (40 m/s) versus time of stimulation, as influenced by frequency of stimulation.
the pause at its stabilized value was again progressively less at muscle lengths nlore than about 1 mm beyond the resting length (Fig. 9), but
the mean magnitude of the pacse for the population of group II units was lessthan that for the group Ia units.
I 0 lengthening
I 1 of
muscle
I
I
2
3 mm
9. Effect of muscle length on pause duration in discharge of a group II afferent (54 m/s) at several rates of twitch elicitation. The durations were measured after they had reached a constant value. FIG.
210
DECANDIA,
SCHIEPPATI,
AND
CRENNA
DISCUSSION Clearly, the pause in firing characteristic of a passive spindle during a near-isometric muscle twitch is not invariable under a given set of muscle and stimulus conditions, but changes progressively within a sequence of twitches. The mechanism responsible for the effect seems to be time limited, as, with a stimulus rate having intervals as long as 10 s, shortening of the pause was scarcely detectable. Shortening increased as intervals were reduced to 5 s, and increased even more as the interval was shortened to 1 S. Observations at yet higher frequencies were complicated by the beginning of tetanic fusion, but extrapolation would suggest that further shortening would occur until, with the developing tetanus, unloading of spindles became continuous. Following completion of a series of twitches we noted that the steady level of afferent discharge occasionally remained somewhat elevated compared to that in the control period. Such elevation in spindle discharge after whole muscle contraction was observed by other authors (6, 9, 12, 21), and was attributed by Hutton et al. (12) to the stiction phenomenon described by Brown et al. (3) in their study on the effects of isolated stimulation of intrafusal fibers on afferent discharge. The rapidity (5 to 15 s) of the complete course of shortening of the pause and the fact that the postdischarge rate after a single twitch usually was decreased (Fig. 6) compared to the control rate are features that distinguish the present phenomenon from the postcontraction changes analyzed by Hutton et al. (12). Shortening of the pause in successive twitches did not depend on any alteration in duration, amplitude, or speed of relaxation of the muscle contraction, because no such effects were seen in the tension traces. Whether or not contraction of the intrafusal fibers was a requisite is not clear, for although stimuli were held below the strength for y-fiber excitation (14), the p-fiber innervation to spindles was probably excited (4). This latter possibility has little support however, because no differences were found in the pattern of the sequential pause changes among the spindles studied. If a prominent @ effect was present, this would have been limited to only a few of them (1, 4). Whatever explanation the phenomenon may have, it must take into account the fact that shortening of the pause, as seen in both group Ia and group II afferents, was dependent on the duration of the interval between twitches and on the initial length of the muscle; moreover, the time required for stabilization of pause duration is independent of muscle lengthening. Possibly these phenomena may be explained on the basis of changes in the viscoelastic properties of the equatorial region of the spindles as a consequence of their repetitive changes in length. It was found (11) that the stiffness of the intrafusal fibers decreases during
PAUSE
IN
MUSCLE
SPINDLES
211
stretches applied in sequence to the isolated spindles. This is probably due to the breaking of the crossbridges between thick and thin filaments which are formed at rest (20). Under our conditions the passive elongation of the spindle occurs during the falling phase of the muscle twitch. As a consequence, when the successive twitches are evoked at intervals shorter than 10 s, i.e., the time required for the formation of stable crossbridges at resting conditions, as Proske and Gregory would suggest (IS), the enhanced extensibility of the spindles would allow a faster recovery of the initial length of the equatorial region and, therefore, a progressive shortening of the pause in the spindle discharge. According to the length-tension diagram proposed by Gordon et al. (5), the extensibility of the intrafusal fibers is expected to increase with increasing muscle length beyond the physiological resting length, as the number of possible crossbridges decreases. This may explain the shortening in pause duration as a function of the initial length of the muscle (Figs. 4, 9). Regarding the constancy of the time course of the progressive decrease in the pause, which appears in Fig. 5, we propose as an explanation that it results from a disappearance of a constant percentage of crossbridges following each muscle twitch at any muscle length. On the other hand, the increase in extensibility of the intrafusal fibers, which is expected to occur as a function of their increase in length, as stated above, would explain the changes in pause duration observed at different muscle lengths (Fig. 5, left). Any behavioral consequences of the progressive shortening of the pause would most likely appear in repetitive movements. The “silent period” in the electromyograph activity, probably due to reflex effects of the pause in the spindle’s afferent discharge ( 13, 17)) should be reduced in length, and in general the enhanced facilitatory inflow resulting from shortening of the pauses and development of the postcontraction sensory discharge would serve to progressively strengthen the initial contractions during a warming-up period. REFERENCES 1. 2.
3.
4. 5.
ADAL, M. N., AND D. BARKER. 1965. Intramuscular branching of fusimotor fihres. J. Plcysiol. (Lotdm) 117 : 28%299. BIANCONI, R., R. GRANIT, ASD D. J. REIS. 1964. The effects of extensor muscle spindles and tendon organ on homonymous motoneurones in relation to y-bias and curarization. Arfn Plqsiol. Scn~d. 61 : 331-347. BROWN, M. C., G. M. GOODWIN, AND P. B. C. MATTIIEWS. 1969. After-effects of fusimotor stimulation on the response of muscle spindle primary endings. J. Playsiol. (Lomfon) 205 : 677-694. ES~ONET-DANAND, F. L., L. JAM, AND Y. LAPORTE. 1975. Skeleto-fusimotor axons in hindlimb muscles of the cat. J. Pltysiol. (Loduu) 249 : 153-166. GORDON, A. M., A. F. HUXLEY, AND F. J. JULIAN. 1964. The length-tension
212
6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
20.
21. 22.
DECANDIA,
SCHIEPPATI,
AND
CRENNA
diagram of single vertebrate striated muscle fibers. J. Physiol. (Lotadon) 171: ZSP. GRANIT, R., S. HOMMA, AND P. B. C. MATTHEWS. 1959. Prolonged changes in the discharge of mammalian muscle spindles following tendon taps or muscle twitches. Acta Physiol. Scatzd. 46 : 185-193. GRANIT, R., AND J. P. VAN DER MEULEN. 1962. The pause during contraction in the discharge of the spindle afferents from primary end organs in cat extensor muscles. Acta Physiol. Stand. 55: 231-244. HILL, A. V. 1970. Pages 124-125 in First atzd Last Exjwrintettts in Muscle Me&a&s. Cambridge University Press, Cambridge. HNIK, P., A. HUDLICKA, J. KUCERA, AND R. PAYNE. 1969. Activation of muscle afferents by non proprioceptive stimuli. Am. J. Physiol. 217 : 1451-1458. HUNT, C. C., AND S. W. KUFFLER. 1951. Stretch receptor discharges during muscle contraction. J. Physiol. (Londotz) 113: 298-31.5. HUNT, C. C., AND D. OTTOSON. 1976. Initial burst of primary endings of isolated mammalian muscle spindles. J. Neurophysiol. 39 : 324-330. HUTTON, R. S., J. L. SMITH, AND E. ELDRED. 1973. Post-contraction sensory discharge from muscle and its source. J. Newrophysiol. 386: 1090-1103. JANSEN, J. K. S., AND T. RLIDJORD. 1964. On the silent period and Golgi tendon organs of the soleus muscle of the cat. Acta Physiol. Scartd. 62 : 364-379. KIJFFLER, S. W., C. C. HUNT, AND J. P. QUILLIAM. 1951. Function of medullated small nerve fibres in mammalian ventral roots: Afferent muscle spindle innervation. J. Neurophysiol. 14: 29-54. MATTHEWS, B. H. C. 1933. Nerve endings in mammalian muscle. J. Physiol. (London) 78: l-53. MATTHEWS, P. B. C. 1972. Pages 143-146 in Mawmalian Muscle Receptors and their Central Action. Arnold, London. MERTON, P. A. 1951. The silent period in muscle of the human hand. J. Pltysiol. (Lolzdon) 114 : 183-198. PROSKE, U., AND J. E. GREGORY.1977. The time-course of recovery of the initial burst of primary endings of muscle spindles. Braitt Res. 121 : 358-361. SCHIEPPATI, M., C. SEGRE, P. CRENNA, AND M. DECANDIA. 1974. Modal& di scarica delle fibre Ia durante singole scosse muscolari ripetute. Congress0 Sot. Ital. Fisiol., p. 263. SIEGMAN, M. J., T. M. BUTTLER, S. U. MOOERS, AND R. E. DAVIS. 1976. Crossbridge attachment, resistance to stretch, and viscoelasticity in resting mammalian smooth muscle. Sciefzce 191 : 383-385. SMITH, J. L., R. S. HUTTON, AND E. ELDRED. 1974. Postcontraction changes in sensitivity of muscle afferents to static and dynamic stretch. Brain Res. 78: 193-202. WUERKER, R. B., A. M. MCPHEDRAN, AND E. HENNEMAN. 1965. Properties of motor units in a heterogeneous pale muscle (m. gastrocnemius) of cat. J, Neurophysiol.
28 : 85-99.
NE1’ROI.OC.Y
Changes
60,
in the during
M. Istit&o
DECANDIA,
di Fisiologia
Unzaxa
201-212 (1978)
Pause
in Muscle
a Sequence
AND P. CRENNA 1
II, U~lielersitd dcgli October
Discharge
of Twitches
M. SCHIEPPATI,
Received
Spindle
Studi
di drlilauo,
Milarzo,
Italy
21. 1977
A study was made of the change in duration of the pauses in discharge from spindle afferents occurring during a series of isometric twitches of the cat’s triceps surae muscle. These were induced by stimulation of the muscle nerve at frequencies of 0.1 to 1.0 Hz. At frequencies of 0.2 Hz and higher, the duration of the pause exhibited by group II and tonic and phasic group Ia afferents decreased progressively until stabilizing within 5 to 15 s. The time course of this change was largely independent of the frequency of stimulation, but the eventual duration in the steady state was strongly dependent on the frequency, being shorter at higher rates. At a given frequency, the pause was shorter at longer muscle lengths. The decrease in pause duration was not accompanied by a parallel change in duration or amplitude of the muscle tension responses, indicating that it arose from some alteration intrinsic to the spindle organ. We suggest that the progressive change in sensory discharge could serve the initial reinforcement of repetitive movements.
INTRODUCTION The sensory information supplied by muscle spindles as to the length status of a muscle is coded in terms of static levels of discharge corresponding to the steady length and dynamic changes in rate related to the velocity of length change. The latter can take the form of a decrease in firing, as illustrated by the complete pause in discharge that occurs during a muscle twitch (15). Granit and Van der Meulen (7) distinguished two patterns of such pausesexhibited by Ia afferent fibers: that of “tonic” afferent units, which simply resumed firing at essentially the steady baseline rate, and that 1 The authors are grateful to Dr. Earl Eldred for his helpful ing the manuscript. This work was supported by Grant CT Consiglio Nazionale delle Ricerche. Dr. Decandia’s present degli Studi di Cagliari, Istituto di Fisiologia Umana, Cagliari,
comments in prepar75.00499.04 from the address is Universita Italy.
201 0014-4886/78/0602-0201$02.00/O Copyright 0 1978 by AcademicPress,Inc.
AlI rights
of reproduction
in any
form
reserved.
202
DECANDIA,
SCHIEPPATI,
AND
CKENNA
of ‘il~hasic” units, which showed a fleeting burst of impulses during the fall in muscle tension. The pause in the discharge has significant reflex effect. Bianconi et nl. (2) showed that there is a correlation between pause of the spindles and changes in excitability of the spinal motoneurons on which they impinge. Furthermore, the pause has a role in the peripheral control of movement, because it is considered responsible for the “silent period” of the electrical activity of the voluntary muscle contraction in man (17) and for the silent period of the reflex electrical activity in decerebrate cats (13). The present study is an analysis of a sequential shift in duration of the pause which occurs in the course of a series of twitches. The relation of this shift to frequency of stimulation and muscle length, and its occurrence in group II and tonic and phasic group Ia units are described. Preliminary results were published in a short communication (19). METHOD The experiments were done on cats. Under deep ether anesthesia the carotid and vertebral arteries were clamped, The anesthesia was then discontinued, ventilation maintained by a respirator, and rectal temperature kept at 37 * 0.5”C by a heating pad. A lumbosacral laminectomy was made, ventral roots L5 to S2 to one leg were cut, and the leg was thoroughly denervated except for the gastrocnemius-soleus nerves. After mounting the animal in a frame, the leg was rigidly fixed to the frame by clamps and the severed Achilles tendon was connected by a stiff wire to a strain gauge, which yielded only 50 pm/kg and had a time constant of 1 ms. Warmed oil pools were constructed in the popliteal space and over the spinal cord. The L7 or Sl dorsal root was split with forceps until a filament containing only one afferent unit from the triceps was present, and this was identified by its response to stretch, twitch response, and conduction rate. The latter was calculated from the time interval between responses elicited by stimuli applied to the triceps nerve through two pairs of electrodes separated by 5 cm. Fibers with conduction velocities of 80 to 120 m/s were considered to be group Ia fibers, and those conducting at less than 55 m/s were considered to be group II units (16). After the fiber was identified, stimuli at just-maximal or submaximal strength for elicitation of the twitch response were delivered to the triceps nerve to evoke nearisometric twitches. This strength of stimulus was used to avoid y-fiber excitation ( 14)) although fusimotor excitation through the p-fiber innervation may have occurred (4). No significant differences in the findings were noted when stimulus strengths somewhat below that needed for a maximal twitch were used. In several experiments in which the nerve to the fast-contracting medial gastrocnemius muscle (22) alone was pre-
203
served, tmits displayed the same phenomenon as when both gastrocnemius and soleus nerves were intact. Observations were begun about 5 h after discontinuing the anesthesia. The muscle was stretched to successive steady lengths beyond the resting physiological length, as the ef-fects of a series of twitches elicited at given rate were tested. The interval hetween different series was never less than 20 s. RESULTS The patterns of discharge of groups Ia and II afferents from triceps surae spindles were observed during a series of isometric twitches induced by repetitive stimulation of the muscle nerve at 0.1 to 1.0 Hz, that is, below frequencies leading to fusion of the tension responses. We noted that the pause in the spindle afferent discharge decreased progressively as a function of time when the successive twitches were evoked at frequencies higher than 0.2 Hz. After a transient phase of a
FIG. 1. Pauses in discharge of a group Ia afferent fiber (100 m/s) of the tonic type obtained during twitches evoked at 0.8, 0.2, and 0.1 Hz. Each pair of traces presents tension above and afferent discharge below. The slight irregularities in the tension recordings are related to the heart beat. The muscle was stretched 2 mm beyond the resting length.
204
DECANDIA,
SCHIEPPATI,
AND
CRENNA
2751 c
250
c : 225. i .F200z 2 a 175
b
’
lb
'
2'0
'SIX
3'0
FIG. 2. Duration of the pause in spindle discharge versus time of stimulation, seen at several frequencies of stimulation. Data are from the afferent represented in Fig. 1.
few seconds, the pause tends to a steady value which was found to be frequency and muscle length dependent. A total of 51 afferents were studied: 17 group Ia tonic, 20 group Ia phasic, and 14 group II units. Group Ia Tonic Afferents. Figure 1 illustrates the patterns of response
FIG. 3. Plots on the graph show the total duration (upper curves) of the successive twitches evoked at 1 Hz (dotted lines) and at 0.8 Hz (solid lines). Due to unavoidable inaccuracy in performing the measurements on the baseline, large variations in duration appear among the twitches. However, no trend to a progressive decrease in value is evident. Measurements between the two values of tension corresponding to 25% of the peak give more reliable results, as the lower curves show. The recordings on the right were obtained by superimposition of all the muscle twitches evoked at the indicated frequencies and of their corresponding derivatives. Note the constancy of tension, duration, and rate of change in the successive twitches.
PAUSE
IN
MUSCLE
SPINL)LES
20.5
of a typical tonic afferent fiber within a series of twitches elicited at three different rates of stimulation. The classical response (10) is seen, consisting of a very brief “early discharge,” followed by complete cessation of discharge through the rising phase of the tension response and almost all of its descending phase. This pause, however, is not constant in duration within a series, but decreases progressively during the first five to six twitches. The trend is hardly detectable at 0.1 Hz, but is prominent in the 0.8-Hz series, where a shortening in pause duration of about 3076 occurs. The effects of stimulation at frequencies of stimulation higher than 1.0 Hz were not tested in detail, for fusion of the twitch responses complicated the interpretation ; but we noticed that further reduction in duration occurred at somewhat higher rates. Plots of pause length versus time (Fig. 2) exhibited by the same group Ia unit at different frequencies of nerve stinmlation demonstrate that : (a) The duration decreased progressively as a function of time of stimulation until, after 15 s or less, the duration became stabilized ; and (b) the duration of the pause was decidedly shorter at higher frequencies of stirnulation. These changes in pause duration are not accompanied by parallel changes 275
1
I 0
I
I
1 lengthening
2 of
1
I
3
4
muscle
I 5 mm
FIG. 4. The graph shows the effect of muscle length on duration of the pause at several frequencies of twitch stimulation. Durations were measured after they had reached a constant value. To the right are represented twitches at l-mm increments, from the physiological resting length (LO) to 6 mm (LO). Note the increase in amplitude and duration of the tension responses with increase in muscle length.
206
DECANDIA,
SCHIEPPATI,
AND
CRENNA
in amplitude and duration of successive twitches. During the entire period of testing at any rate at a given muscle length, the plots (Fig. 3) of the total duration of twitches and of duration measured at the point where the tension falls to 25% of the peak value show that the mechanical events have no trend to a progressive change with time. This is confirmed by the recordings (Fig. 3) of superimposed traces of the successive muscle twitches and of the rate of change in tension obtained within a given series. The duration of the pause, when at its stabilized value, decreased with the amount of extension of the muscle at lengths more than 1 mm beyond the resting muscle length (Fig. 4). This is the opposite of wh.at might be expected from the unloading effect of the twitch contractions, which were increased at greater muscle lengths. In fact, the recordings inserted in Fig. 4 show that the muscle twitches, as expected (S), became greater in amplitude and duration for several millimeters of muscle lengthening. Whereas pause duration was related to muscle length, the time course of the progressive decrease in duration at a given frequency of muscle twitches was little affected by the length of the muscle. This is evident in Fig. 5, where the absolute values of the pauses during twitches evoked at 1.0 Hz at the indicated muscle lengths are plotted on the left, and the
275 160
250
1,'SW 225
: ; .c :
200
1
2 a 175
-I
125
1 1 0
I 2
I 4
I 6
1 6
set
, 10
1 0
I 2
1 4
6
, 8
# set
10
FIG. 5. Relation between duration of the afferent pause and time of stimulation, as determined at several muscle lengths. On the left the absolute values are plotted, and on the right the durations are expressed as percentages of the stabilized mean value.
I’AI’SE
IN
1 set
1
FG. tlvitch
A”“‘Y
6. Responses series elicited
RtUSC1.E
207
SI’INDLES
02,‘sec
05’sec
500
I 500
msec
of a group Ia afferent at 1.0, 0.5, and 0.2 Hz.
fiber
(108
m/s)
g
of the
phasic
type
in
durations as a percent of the mean for the pauses at their steady state are plotted on the right. As there is 1x1 systematic shift in the latter curves, the time required for stal)ilizatioii of pause duration is independent of muscle estension. Gvolo la Plmsic L4flcrcl/fs. In Fig. 6 is illustrated the behavior of a “@ask” unit (7)) which during the decline in tension displayed a burst of closely spaced spikes, followed by a secondary gap in firing before the stable discharge was resumed. Such units were found in preparations with ouly the medial gastrocnemius nerve preserved, as well as those with triceps innervation intact. It is seeu that hot11 the early coiiiponent of the pause preceding the burst and the total l)ause to onset of the steady discharge decreased in duration with successive stimuli (top to bottom). Figure 7 sunmarizes the changes in duration of the early pause (solid lines) and total pause (dotted lines) shown by this same group Ia fiber at different frequencies of stimulation. The curves described hy the early pause are similar to those of the total pause. or, expressed differently, the burst tends to fall at ahout the same proportionate point in the total pause. The behavior sho\~n by the burst of inlpulses is puzzling. Generally the
208
DECANDIA,
(Y
SCHIEPPATI,
AND
CRENNA
0
500
0 1/set .
: 0. P
O.Z/sec
-
05Jsec
IJSSC
300
I>=
A
10
2b
30
se=
40
FIG. 7. Duration of the total pause (dotted lines, above) and of the first component (solid lines, below) as a function of time of stimulation. Data are from the afferent fiber of Fig. 6.
burst occurred at a time when the tension trace had not yet completely returned to the baseline. The frequency within the burst was very much greater than the control rate at the constant muscle length, indicating either that the ending was briefly subjected to accelerating length change or that the sensitivity of the ending was decidedly enhanced at that moment. However, the latter possibility seems to be in contrast to the observation that firing within the burst became much slower later along the twitch series (Fig. 6) when shortening of the total pause would indicate that sensitivity of the ending was in fact enhanced. The decrease in frequency within the bursting discharge was observed also during subsequent ramp stretches applied to the muscle (18) or to the single muscle spindle (11) at frequencies lower than 1.0 Hz. This phenomenon seemsto be due to a decrease in stiffness of the intrafusal fibers, caused by breaking of crossbridges between thick and thin filaments which are formed at rest, as Brown et al. (3) suggested. Gr0u.p II Afferents. The group II afferents exhibited a highly tonic pattern of twitch response, a steady rate of discharge being immediately resumed after the pause. The behavior of the pause (Fig. 8) in terms of relation to frequencies of stimulation and the time course in a given series of twitches were similar to those of group Ia tonic units. The duration of
PAUSE
IN
MUSCLE
I 0
200
SPINDLES
20
10
‘SC
io
FIG. 8. Duration of the pause in twitch response of a group II fiber (40 m/s) versus time of stimulation, as influenced by frequency of stimulation.
the pause at its stabilized value was again progressively less at muscle lengths nlore than about 1 mm beyond the resting length (Fig. 9), but
the mean magnitude of the pacse for the population of group II units was lessthan that for the group Ia units.
I 0 lengthening
I 1 of
muscle
I
I
2
3 mm
9. Effect of muscle length on pause duration in discharge of a group II afferent (54 m/s) at several rates of twitch elicitation. The durations were measured after they had reached a constant value. FIG.
210
DECANDIA,
SCHIEPPATI,
AND
CRENNA
DISCUSSION Clearly, the pause in firing characteristic of a passive spindle during a near-isometric muscle twitch is not invariable under a given set of muscle and stimulus conditions, but changes progressively within a sequence of twitches. The mechanism responsible for the effect seems to be time limited, as, with a stimulus rate having intervals as long as 10 s, shortening of the pause was scarcely detectable. Shortening increased as intervals were reduced to 5 s, and increased even more as the interval was shortened to 1 S. Observations at yet higher frequencies were complicated by the beginning of tetanic fusion, but extrapolation would suggest that further shortening would occur until, with the developing tetanus, unloading of spindles became continuous. Following completion of a series of twitches we noted that the steady level of afferent discharge occasionally remained somewhat elevated compared to that in the control period. Such elevation in spindle discharge after whole muscle contraction was observed by other authors (6, 9, 12, 21), and was attributed by Hutton et al. (12) to the stiction phenomenon described by Brown et al. (3) in their study on the effects of isolated stimulation of intrafusal fibers on afferent discharge. The rapidity (5 to 15 s) of the complete course of shortening of the pause and the fact that the postdischarge rate after a single twitch usually was decreased (Fig. 6) compared to the control rate are features that distinguish the present phenomenon from the postcontraction changes analyzed by Hutton et al. (12). Shortening of the pause in successive twitches did not depend on any alteration in duration, amplitude, or speed of relaxation of the muscle contraction, because no such effects were seen in the tension traces. Whether or not contraction of the intrafusal fibers was a requisite is not clear, for although stimuli were held below the strength for y-fiber excitation (14), the p-fiber innervation to spindles was probably excited (4). This latter possibility has little support however, because no differences were found in the pattern of the sequential pause changes among the spindles studied. If a prominent @ effect was present, this would have been limited to only a few of them (1, 4). Whatever explanation the phenomenon may have, it must take into account the fact that shortening of the pause, as seen in both group Ia and group II afferents, was dependent on the duration of the interval between twitches and on the initial length of the muscle; moreover, the time required for stabilization of pause duration is independent of muscle lengthening. Possibly these phenomena may be explained on the basis of changes in the viscoelastic properties of the equatorial region of the spindles as a consequence of their repetitive changes in length. It was found (11) that the stiffness of the intrafusal fibers decreases during
PAUSE
IN
MUSCLE
SPINDLES
211
stretches applied in sequence to the isolated spindles. This is probably due to the breaking of the crossbridges between thick and thin filaments which are formed at rest (20). Under our conditions the passive elongation of the spindle occurs during the falling phase of the muscle twitch. As a consequence, when the successive twitches are evoked at intervals shorter than 10 s, i.e., the time required for the formation of stable crossbridges at resting conditions, as Proske and Gregory would suggest (IS), the enhanced extensibility of the spindles would allow a faster recovery of the initial length of the equatorial region and, therefore, a progressive shortening of the pause in the spindle discharge. According to the length-tension diagram proposed by Gordon et al. (5), the extensibility of the intrafusal fibers is expected to increase with increasing muscle length beyond the physiological resting length, as the number of possible crossbridges decreases. This may explain the shortening in pause duration as a function of the initial length of the muscle (Figs. 4, 9). Regarding the constancy of the time course of the progressive decrease in the pause, which appears in Fig. 5, we propose as an explanation that it results from a disappearance of a constant percentage of crossbridges following each muscle twitch at any muscle length. On the other hand, the increase in extensibility of the intrafusal fibers, which is expected to occur as a function of their increase in length, as stated above, would explain the changes in pause duration observed at different muscle lengths (Fig. 5, left). Any behavioral consequences of the progressive shortening of the pause would most likely appear in repetitive movements. The “silent period” in the electromyograph activity, probably due to reflex effects of the pause in the spindle’s afferent discharge ( 13, 17)) should be reduced in length, and in general the enhanced facilitatory inflow resulting from shortening of the pauses and development of the postcontraction sensory discharge would serve to progressively strengthen the initial contractions during a warming-up period. REFERENCES 1. 2.
3.
4. 5.
ADAL, M. N., AND D. BARKER. 1965. Intramuscular branching of fusimotor fihres. J. Plcysiol. (Lotdm) 117 : 28%299. BIANCONI, R., R. GRANIT, ASD D. J. REIS. 1964. The effects of extensor muscle spindles and tendon organ on homonymous motoneurones in relation to y-bias and curarization. Arfn Plqsiol. Scn~d. 61 : 331-347. BROWN, M. C., G. M. GOODWIN, AND P. B. C. MATTIIEWS. 1969. After-effects of fusimotor stimulation on the response of muscle spindle primary endings. J. Playsiol. (Lomfon) 205 : 677-694. ES~ONET-DANAND, F. L., L. JAM, AND Y. LAPORTE. 1975. Skeleto-fusimotor axons in hindlimb muscles of the cat. J. Pltysiol. (Loduu) 249 : 153-166. GORDON, A. M., A. F. HUXLEY, AND F. J. JULIAN. 1964. The length-tension
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