Acta physiol. scand. 1975. 95. 166-178 From the Department of Anatomy, Karolinska Institutet, Stockholm, Sweden
The Postnatal Development of Some Twitch and Fatigue Properties of the Ankle Flexor and Extensor Muscles of the Cat BY and J - 0 . KELLERTH C. HAMMARBERG Received 27 March 1975
Abstract HAMMARBERG, C. and J - 0 . KELLERTH. The postnatal development of some twitch and fatigue properties of the ankle flexor and extensor muscles of the cat. Acta physiol. scand. 1975. 95. 166-178. The isometric responses of the medial gastrocnemius (MG), soleus (SOL) and anterior tibia1 (TA) muscles to single shocks and different modes of repetitive stimulation were studied in kittens of varying postnatal ages and i n adult cats. The postnatal decrease in time-to-peak and half-relaxation time of the twitch contractions was similar for the M G and TA muscles and adult values were attained at around 6-7 weeks of age. The SOL muscle displayed a transient decrease in contraction time during the first postnatal weeks, followed later by a slowing towards adult values. The susceptibility to fatigue during iterative stimulation was smallest in the SOL at all ages studied, and usually largest in TA. It changed only little for the M G and SOL postnatally while increasing markedly for the T A up until 6-7 weeks of age. Tetanic contraction resulted in similar depressions i n contractile tension of all three muscles in the youngest kittens, but the SOL displayed a greater ability to recover from this depression than the MG and, in particular, the T A muscles. Tetanus resistance increased postnatally and adult responses were attained at 6-7 weeks of age.
It has long been recognized (see Ranvier 1874) that the limb muscles of adult mammals may be classified into so-called “fast” and “slow” twitch muscles which differ in their speeds of contraction. As was first described by Banu (1922), however, a t birth all the limb muscles of the kitten contract slowly, the twitch response having a time course similar to that of adult slow twitch muscles. The ensuing process of limb muscle differentiation during the postnatal stages of cat ontogeny has been the subject of several investigations dealing with for example isometric twitch characteristics (Denny-Brown 1929, Buller et a/. 1960, Buller and Lewis 1965), twitch/tetanus ratios (Buller and Lewis 1965, Close and H o h 1967), force/velocity properties (Close and Hoh 1967) and post-tetanic potentiation of peak twitch tension (Buller and Lewis 1965, Nystrom 1968 a, f ) . An additional important aspect of muscle performance which has received less attention is the sensitivity t o fatigue during prolonged activity. In the adult cat Burke et al. (1971, 1973, 1974) found this parameter t o be of major significance in the classification of different types of motor units. The present investigation was undertaken t o study the changes in some twitch and fatigue 166
TWITCH AND FATIGUE IN MUSCLES. I
167
properties which occur in the anterior tibial, medial gastrocnemius and soleus muscles of the cat during postnatal development. The results obtained also serve as a basis for following physiological studies on the postnatal differentiation of various motor unit types in the cat ankle muscles (Hammarberg and Kellerth 1975 b). The histochemical staining properties of these muscles during postnatal and adult stages have been described previously (Hammarberg 1974 a, b).
Methods The experiments were performed on 7 adult cats and 16 kittens varying in ages between 6 and 65 days. The animals were anaesthetized with 40 mg/kg of pentobarbitone sodium (Nembutal) given intraperitoneally. Subsequent doses, when necessary, were given through a catheter inserted in the left femoral vein. The animals were usually allowed to breathe spontaneously through a tracheal cannula, and expired CO, was monitored continuously. In one adult animal artificial respiration was employed in order to study the effects of hyper- and hypoventilation upon muscle performance (see Fig. 1 and 2). The anterior tibial (TA), medial gastrocnemius (MG) and soleus (SOL) muscles of the right hind limb were exposed and dissected free except for their proximal attachments, care being taken to preserve their blood and nerve supplies. The corresponding motor nerves were cut in the popliteal fossa and mounted on stimulating electrodes. In some animals muscles from both legs were used for the experiment. The animals were mounted in a steel frame with the long axis of the hind limb almost horizontal. A steel drill transfixed the distal end of the femur while the distal ends of the tibia and fibula were rigidly clamped. The cut distal muscle tendons were arranged for separate isometric connection to a strain gauge myograph through short lengths of brided suture (compliance 0.05 mm/kg.cm). Care was taken to align the strain gauge with the natural direction of muscle pull. The strain gauge was attached to a sliding metal rod provided with a catch which made it possible to extend the muscles millimetre by millimetre. In order to provide a standard by which to compare the contractions of the different muscles at various postnatal ages, the length of each muscle was always adjusted to give maximal twitch responses (Buller et al. 1960). By elevating the surrounding skin flaps it was possible to immerse the exposed tissues in a pool of paraffin oil. Muscle temperature was maintained by means of infrared light. With the youngest kittens, however, the small volumes of the paraffin oil pools made control difficult and variations in intramuscular temperature between 34-38°C had to be accepted. Body temperature was kept at 36-38°C. Muscle contractions were evoked by stimulating the appropriate muscle nerves with supramaximal square wave pulses of 0.3 ms duration. Because of the varying ages of the animals strain gauges of different maximal strains had to be used to record the mechanical responses. The unloaded natural resonant frequency of the strain gauges exceeded 550 Hz in all cases. The output of the strain gauge was amplified and displayed on an oscilloscope and on a penwriter (Devices M2). The oscilloscope traces were photographed o n paper film. Testing procedures (a) Twitch contractions were elicited by single stimulating pulses. The “twitch rise time” was measured from
the start of the contraction to its peak, while the “twitch half-relaxation time” was measured from the peak to the point representing 50% passive decay in twitch tension. Because the speed of muscle contraction has a fairly high temperature coefficient and because muscle temperature was often difficult to control in the smallest kittens, the recorded values for contraction time have been adjusted in Fig. 3 to give the corresponding values at 38.0”C by using the correction factor TI”: 1.55 (Gordon and Phillips 1953, Buller et a/. 1968). The maximal adjustment made in Fig. 3 amounted to a 21 ”/D reduction of the recorded value. (b) Fatigue properties of the muscles were examined by the following series of tests: 1. “Fatigue test”. Short trains of stimuli with a stimulus frequency of 40 Hz and a duration of 330 ms were repeated once a second (see Fig. 1 and 2). This test, which will be referred to below as the “fatigue test”, was originally introduced by Burke et al. (1971) to assess the sensitivity to fatigue of single motor units. The amount of fatigue exhibited by the various muscles after two minutes of stimulation will be expressed as the “fatigue index”, i.e., the ratio of the tension output during the 120th tetanus to the tension produced by the first tetanus of the sequence. 2. “Pause test”. After about 2 min of the “fatigue test” stimulation was suddenly interrupted for ten
168
C. HAMMARBERG AND J - 0 . KELLERTH
Fig. 2
Fig. 1
Fig. 1 and Fig. 2. The contractile responses of the M G (Fig. 1) and SOL (Fig. 2) muscles of an adult cat during the “fatigue”, “tetanus” and “pause tests”. In each record the concentration of expired CO, has been indicated. A. Normal breathing. B. Hyperventilation. C. Hypoventilation. It is seen that deviations in either direction from normal ventilation will markedly affect the ability of both fast and slow muscles to maintain their contractile tensions during sustained activity. B and C were recorded after 5 min of hyperand hypoventilation, respectively, when the new concentration of expired CO, had reached its steady state. Between B and C the animal was allowed to breathe spontaneously for 45 min which was sufficient for the complete restitution of normal contractile responses seen in A. seconds. This is referred to as the “pause test”. When stimulation was resumed the amount of recovery in contractile tension following the short rest was studied (see Fig. 1 and 2). 3. “Tetanus test”. Approximately 30 s after the “pause test”, when tension output had returned to a more or less steady level, the muscle was exposed for 30 s to sustained tetanic stimulation at a frequency of 40 Hz (see Fig. I and 2). The iterative stimulation was then resumed and the changes in tension output occurring both during and after this so-called “tetanus test” were studied. 4. About 30-45 s after the end of the maintained tetanic activation a second “pause test” was performed and the ensuing recovery in contractile tension measured.
Results Twitch rise times (time-to-peak) of the MG, SOL and TA muscles at various postnatal ages are shown in Fig. 3. The results are essentially in agreement with the observations of Buller et al. (1960), Buller and Lewis (1965), Mann and Salafsky (1970) and Westerman et al. (1973). In the smallest kittens all three muscles displayed similarly slow contractions, although there was a tendency for the SOL muscle to have a somewhat higher mean value in all the age groups studied here. There was a parallel decrease in rise time for the 3 muscles during the first 3 4 weeks, after which the SOL rise time increased, while the rise times of the other 2 muscles continued to decrease. It appears, therefore, that the soleus muscle contracts faster at around 3 weeks of age than in the adult stage (p .10.002). The most striking difference between the adult and kitten twitches is seen in the fast MG and TA muscles, the adult muscles being markedly faster. The adult values for twitch rise time were fairly well acquired by the fast muscles at around 6-7 weeks of age, while those of the SOL muscle were acquired somewhat later (cf. also Buller et al. 1960, and Mann and Salafsky 1970). In all age groups studied here there was a consistent tendency for the TA to contract slightly faster than the MG.
169
TWITCH AND FATIGUE IN MUSCLES. I 0 SOLEUS
TWITCH RISE TIME
0 GASTROCNEMIUS V ANTERIOR TIWAL
MSEC
7
1
12-15 DAYS 17~24DAYS
i
i
0
00
;;
43-40 DAYS
ii
63-65 DAYS
1 20
30
40
50
70 DAYS ’
60
.
’ ADULT ’AGE
r r
10
Fig. 3. The postnatal changes in twitch rise time (time-to-peak) for the MG, SOL and TA muscles. Each bar shows the range of observed values, the number of which is indicated above each bar. The round and triangular symbols show the mean values.
Twitch half-relaxation time ( H R T ) : Fig. 4 illustrates the postnatally occurring changes in HRT of the three muscles. Here also the fast MG and TA muscles acquired their adult values at around 6-7 weeks of age, while the SOL possibly took somewhat longer. The mean values for HRT at different ages are generally consistent with the findings of Buller et al. (1960) and Mann and Salafsky (1970) except in the case of kittens around 1 week of age, where the present study found the value for SOL in particular to be considerably smaller than in the earlier studies (HRT mean values 48, 0 It: 15.5 ms and 35.5 k9.9 ms for
7
loo--
T
80 --
i 17-24 DAYS
P
43-48 DAYS
63-65 DAYS
Fig. 4. The postnatal changes in twitch half-relaxation time for the MG, SOL and T A muscles. Each b a r indicates the range of observed values, while the round and triangular symbols show the mean values. The number of observations is indicated above each bar.
170
C. HAMMARBERG AND J . - 0 . KELLERTH
7 DAYS
18 DAYS
48 DAYS
ADULT
B
R
C
P
- -
low
80sec
Fig. 5 . Typical examples of the contractile responres of the MG ( A ) , SOL ( B ) and TA (C) muscles during the “fatigue”, “tetanus” and “pause tests” at different postnatal ages.
SOL and MG, respectively, compared to approximately 70-75 ms and 40-45 ms, respectively, according to Fig. 4 of Buller eta/. 1960). With respect to the general pattern of maturation the contraction times (Fig. 3) and half-relaxation times (Fig. 4) for the different muscles were found to display similar time courses (see also Buller et al. 1960). “Fatigue test”. Fig. 5 shows some typical examples of the behaviour of the MG, SOL and T A muscles (A, B and C, respectively) at different postnatal ages during prolonged activity. The M G and T A muscles (A and C, respectively) of the 7-day-old kitten of Fig. 5 showed a moderate reduction in contractile tension after 2 min of intermittent stimulation, the reduction being somewhat more pronounced for the latter muscle. The SOL muscle (B), o n the other hand, produced approximately the same amount of contractile tension after
171
TWITCH AND FATIGUE IN MUSCLES. I
FAT IGU E INDEX 0 SOLEUS 0 GASTROCNEMIUS V ANTERIOR TlElAL
6
’if -9 DAY5
11-24 DAYS
P
63-48 DAYS
L
61-65 DAYS
1
Fig. 6. The postnatal changes in “fatigue index” for the MG, SOL and TA muscles. Each bar illustrates the range of observed values, while the round and triangular symbols show the mean values. The number of observations is indicated above each bar.
2 niin of stimulation, although earlier in the sequence an initial increase reminiscent of posttetanic potentiation had gradually disappeared. In the 18-day-old kitten a qualitatively similar “fatigue test” picture was obtained: both the MG and, in particular, the TA showed a progressive reduction in tension output during the 2 min of stimulation, while the SOL remained virtually unaffected. The same general tendency was present in the 48-day-old kitten, as well as in the adult animal. Fig. 6 illustrates the fatigue indices (see Methods) for each of the tested muscles at various postnatal ages. The change in fatigue index occurring during postnatal development was quite conspicuous in the case of the TA muscle which became less resistant to fatigue with increasing age (p
The Postnatal Development of Some Twitch and Fatigue Properties of the Ankle Flexor and Extensor Muscles of the Cat BY and J - 0 . KELLERTH C. HAMMARBERG Received 27 March 1975
Abstract HAMMARBERG, C. and J - 0 . KELLERTH. The postnatal development of some twitch and fatigue properties of the ankle flexor and extensor muscles of the cat. Acta physiol. scand. 1975. 95. 166-178. The isometric responses of the medial gastrocnemius (MG), soleus (SOL) and anterior tibia1 (TA) muscles to single shocks and different modes of repetitive stimulation were studied in kittens of varying postnatal ages and i n adult cats. The postnatal decrease in time-to-peak and half-relaxation time of the twitch contractions was similar for the M G and TA muscles and adult values were attained at around 6-7 weeks of age. The SOL muscle displayed a transient decrease in contraction time during the first postnatal weeks, followed later by a slowing towards adult values. The susceptibility to fatigue during iterative stimulation was smallest in the SOL at all ages studied, and usually largest in TA. It changed only little for the M G and SOL postnatally while increasing markedly for the T A up until 6-7 weeks of age. Tetanic contraction resulted in similar depressions i n contractile tension of all three muscles in the youngest kittens, but the SOL displayed a greater ability to recover from this depression than the MG and, in particular, the T A muscles. Tetanus resistance increased postnatally and adult responses were attained at 6-7 weeks of age.
It has long been recognized (see Ranvier 1874) that the limb muscles of adult mammals may be classified into so-called “fast” and “slow” twitch muscles which differ in their speeds of contraction. As was first described by Banu (1922), however, a t birth all the limb muscles of the kitten contract slowly, the twitch response having a time course similar to that of adult slow twitch muscles. The ensuing process of limb muscle differentiation during the postnatal stages of cat ontogeny has been the subject of several investigations dealing with for example isometric twitch characteristics (Denny-Brown 1929, Buller et a/. 1960, Buller and Lewis 1965), twitch/tetanus ratios (Buller and Lewis 1965, Close and H o h 1967), force/velocity properties (Close and Hoh 1967) and post-tetanic potentiation of peak twitch tension (Buller and Lewis 1965, Nystrom 1968 a, f ) . An additional important aspect of muscle performance which has received less attention is the sensitivity t o fatigue during prolonged activity. In the adult cat Burke et al. (1971, 1973, 1974) found this parameter t o be of major significance in the classification of different types of motor units. The present investigation was undertaken t o study the changes in some twitch and fatigue 166
TWITCH AND FATIGUE IN MUSCLES. I
167
properties which occur in the anterior tibial, medial gastrocnemius and soleus muscles of the cat during postnatal development. The results obtained also serve as a basis for following physiological studies on the postnatal differentiation of various motor unit types in the cat ankle muscles (Hammarberg and Kellerth 1975 b). The histochemical staining properties of these muscles during postnatal and adult stages have been described previously (Hammarberg 1974 a, b).
Methods The experiments were performed on 7 adult cats and 16 kittens varying in ages between 6 and 65 days. The animals were anaesthetized with 40 mg/kg of pentobarbitone sodium (Nembutal) given intraperitoneally. Subsequent doses, when necessary, were given through a catheter inserted in the left femoral vein. The animals were usually allowed to breathe spontaneously through a tracheal cannula, and expired CO, was monitored continuously. In one adult animal artificial respiration was employed in order to study the effects of hyper- and hypoventilation upon muscle performance (see Fig. 1 and 2). The anterior tibial (TA), medial gastrocnemius (MG) and soleus (SOL) muscles of the right hind limb were exposed and dissected free except for their proximal attachments, care being taken to preserve their blood and nerve supplies. The corresponding motor nerves were cut in the popliteal fossa and mounted on stimulating electrodes. In some animals muscles from both legs were used for the experiment. The animals were mounted in a steel frame with the long axis of the hind limb almost horizontal. A steel drill transfixed the distal end of the femur while the distal ends of the tibia and fibula were rigidly clamped. The cut distal muscle tendons were arranged for separate isometric connection to a strain gauge myograph through short lengths of brided suture (compliance 0.05 mm/kg.cm). Care was taken to align the strain gauge with the natural direction of muscle pull. The strain gauge was attached to a sliding metal rod provided with a catch which made it possible to extend the muscles millimetre by millimetre. In order to provide a standard by which to compare the contractions of the different muscles at various postnatal ages, the length of each muscle was always adjusted to give maximal twitch responses (Buller et al. 1960). By elevating the surrounding skin flaps it was possible to immerse the exposed tissues in a pool of paraffin oil. Muscle temperature was maintained by means of infrared light. With the youngest kittens, however, the small volumes of the paraffin oil pools made control difficult and variations in intramuscular temperature between 34-38°C had to be accepted. Body temperature was kept at 36-38°C. Muscle contractions were evoked by stimulating the appropriate muscle nerves with supramaximal square wave pulses of 0.3 ms duration. Because of the varying ages of the animals strain gauges of different maximal strains had to be used to record the mechanical responses. The unloaded natural resonant frequency of the strain gauges exceeded 550 Hz in all cases. The output of the strain gauge was amplified and displayed on an oscilloscope and on a penwriter (Devices M2). The oscilloscope traces were photographed o n paper film. Testing procedures (a) Twitch contractions were elicited by single stimulating pulses. The “twitch rise time” was measured from
the start of the contraction to its peak, while the “twitch half-relaxation time” was measured from the peak to the point representing 50% passive decay in twitch tension. Because the speed of muscle contraction has a fairly high temperature coefficient and because muscle temperature was often difficult to control in the smallest kittens, the recorded values for contraction time have been adjusted in Fig. 3 to give the corresponding values at 38.0”C by using the correction factor TI”: 1.55 (Gordon and Phillips 1953, Buller et a/. 1968). The maximal adjustment made in Fig. 3 amounted to a 21 ”/D reduction of the recorded value. (b) Fatigue properties of the muscles were examined by the following series of tests: 1. “Fatigue test”. Short trains of stimuli with a stimulus frequency of 40 Hz and a duration of 330 ms were repeated once a second (see Fig. 1 and 2). This test, which will be referred to below as the “fatigue test”, was originally introduced by Burke et al. (1971) to assess the sensitivity to fatigue of single motor units. The amount of fatigue exhibited by the various muscles after two minutes of stimulation will be expressed as the “fatigue index”, i.e., the ratio of the tension output during the 120th tetanus to the tension produced by the first tetanus of the sequence. 2. “Pause test”. After about 2 min of the “fatigue test” stimulation was suddenly interrupted for ten
168
C. HAMMARBERG AND J - 0 . KELLERTH
Fig. 2
Fig. 1
Fig. 1 and Fig. 2. The contractile responses of the M G (Fig. 1) and SOL (Fig. 2) muscles of an adult cat during the “fatigue”, “tetanus” and “pause tests”. In each record the concentration of expired CO, has been indicated. A. Normal breathing. B. Hyperventilation. C. Hypoventilation. It is seen that deviations in either direction from normal ventilation will markedly affect the ability of both fast and slow muscles to maintain their contractile tensions during sustained activity. B and C were recorded after 5 min of hyperand hypoventilation, respectively, when the new concentration of expired CO, had reached its steady state. Between B and C the animal was allowed to breathe spontaneously for 45 min which was sufficient for the complete restitution of normal contractile responses seen in A. seconds. This is referred to as the “pause test”. When stimulation was resumed the amount of recovery in contractile tension following the short rest was studied (see Fig. 1 and 2). 3. “Tetanus test”. Approximately 30 s after the “pause test”, when tension output had returned to a more or less steady level, the muscle was exposed for 30 s to sustained tetanic stimulation at a frequency of 40 Hz (see Fig. I and 2). The iterative stimulation was then resumed and the changes in tension output occurring both during and after this so-called “tetanus test” were studied. 4. About 30-45 s after the end of the maintained tetanic activation a second “pause test” was performed and the ensuing recovery in contractile tension measured.
Results Twitch rise times (time-to-peak) of the MG, SOL and TA muscles at various postnatal ages are shown in Fig. 3. The results are essentially in agreement with the observations of Buller et al. (1960), Buller and Lewis (1965), Mann and Salafsky (1970) and Westerman et al. (1973). In the smallest kittens all three muscles displayed similarly slow contractions, although there was a tendency for the SOL muscle to have a somewhat higher mean value in all the age groups studied here. There was a parallel decrease in rise time for the 3 muscles during the first 3 4 weeks, after which the SOL rise time increased, while the rise times of the other 2 muscles continued to decrease. It appears, therefore, that the soleus muscle contracts faster at around 3 weeks of age than in the adult stage (p .10.002). The most striking difference between the adult and kitten twitches is seen in the fast MG and TA muscles, the adult muscles being markedly faster. The adult values for twitch rise time were fairly well acquired by the fast muscles at around 6-7 weeks of age, while those of the SOL muscle were acquired somewhat later (cf. also Buller et al. 1960, and Mann and Salafsky 1970). In all age groups studied here there was a consistent tendency for the TA to contract slightly faster than the MG.
169
TWITCH AND FATIGUE IN MUSCLES. I 0 SOLEUS
TWITCH RISE TIME
0 GASTROCNEMIUS V ANTERIOR TIWAL
MSEC
7
1
12-15 DAYS 17~24DAYS
i
i
0
00
;;
43-40 DAYS
ii
63-65 DAYS
1 20
30
40
50
70 DAYS ’
60
.
’ ADULT ’AGE
r r
10
Fig. 3. The postnatal changes in twitch rise time (time-to-peak) for the MG, SOL and TA muscles. Each bar shows the range of observed values, the number of which is indicated above each bar. The round and triangular symbols show the mean values.
Twitch half-relaxation time ( H R T ) : Fig. 4 illustrates the postnatally occurring changes in HRT of the three muscles. Here also the fast MG and TA muscles acquired their adult values at around 6-7 weeks of age, while the SOL possibly took somewhat longer. The mean values for HRT at different ages are generally consistent with the findings of Buller et al. (1960) and Mann and Salafsky (1970) except in the case of kittens around 1 week of age, where the present study found the value for SOL in particular to be considerably smaller than in the earlier studies (HRT mean values 48, 0 It: 15.5 ms and 35.5 k9.9 ms for
7
loo--
T
80 --
i 17-24 DAYS
P
43-48 DAYS
63-65 DAYS
Fig. 4. The postnatal changes in twitch half-relaxation time for the MG, SOL and T A muscles. Each b a r indicates the range of observed values, while the round and triangular symbols show the mean values. The number of observations is indicated above each bar.
170
C. HAMMARBERG AND J . - 0 . KELLERTH
7 DAYS
18 DAYS
48 DAYS
ADULT
B
R
C
P
- -
low
80sec
Fig. 5 . Typical examples of the contractile responres of the MG ( A ) , SOL ( B ) and TA (C) muscles during the “fatigue”, “tetanus” and “pause tests” at different postnatal ages.
SOL and MG, respectively, compared to approximately 70-75 ms and 40-45 ms, respectively, according to Fig. 4 of Buller eta/. 1960). With respect to the general pattern of maturation the contraction times (Fig. 3) and half-relaxation times (Fig. 4) for the different muscles were found to display similar time courses (see also Buller et al. 1960). “Fatigue test”. Fig. 5 shows some typical examples of the behaviour of the MG, SOL and T A muscles (A, B and C, respectively) at different postnatal ages during prolonged activity. The M G and T A muscles (A and C, respectively) of the 7-day-old kitten of Fig. 5 showed a moderate reduction in contractile tension after 2 min of intermittent stimulation, the reduction being somewhat more pronounced for the latter muscle. The SOL muscle (B), o n the other hand, produced approximately the same amount of contractile tension after
171
TWITCH AND FATIGUE IN MUSCLES. I
FAT IGU E INDEX 0 SOLEUS 0 GASTROCNEMIUS V ANTERIOR TlElAL
6
’if -9 DAY5
11-24 DAYS
P
63-48 DAYS
L
61-65 DAYS
1
Fig. 6. The postnatal changes in “fatigue index” for the MG, SOL and TA muscles. Each bar illustrates the range of observed values, while the round and triangular symbols show the mean values. The number of observations is indicated above each bar.
2 niin of stimulation, although earlier in the sequence an initial increase reminiscent of posttetanic potentiation had gradually disappeared. In the 18-day-old kitten a qualitatively similar “fatigue test” picture was obtained: both the MG and, in particular, the TA showed a progressive reduction in tension output during the 2 min of stimulation, while the SOL remained virtually unaffected. The same general tendency was present in the 48-day-old kitten, as well as in the adult animal. Fig. 6 illustrates the fatigue indices (see Methods) for each of the tested muscles at various postnatal ages. The change in fatigue index occurring during postnatal development was quite conspicuous in the case of the TA muscle which became less resistant to fatigue with increasing age (p
