Brain Research, 91 (1975) 315-320 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
315
Relevance of motoneuronal firing adaptation to tension development in the motor unit
F. BALDISSERA ANDF. PARMIGGIANI Laboratorio di Fisiologia dei Centri Nervosi del CNR, Via Mario Bianco, 9-20131 Milan (Italy)
(Accepted March 1lth, 1975)
Spinal motoneurones fire repetitively in response to steps of current injected through the cell membrane 9. Due to 'summation' of the potassium conductance (Gr~) changes responsible for the afterhyperpolarization (AHP) 1,2,4,11,12, the initial firing frequency rapidly decreases (adaptation) to a steady value which is reached after a number of intervals which varies according to the intensity of the injected current a,l°. The high frequency firing which precedes adaptation should be 8 'functionally important for rapid utilization of muscular activity' also in view of the 'catch properties' of the mammalian motor unit v. Thus, experiments were designed to test how muscle tension in a single motor unit is developed after injection of current steps of increasing intensity into the motoneurone. Recordings were made in intact cats, anaesthetized with Nembutal (40 mg/kg, initial dose). The nerves of both hindlimbs were severed except for the left gastrocnemius-soleus (G-S) nerve. The lumbar enlargement was exposed by laminectomy. The Achilles tendon was cut and connected through an inelastic thread to a Grass FTO 3C force transducer (self-resonant frequency 330 Hz) and stretched to the length giving the strongest twitch response for the whole muscle. G-S motoneurones in L7-SI were impaled with glass microelectrodes filled with 3M KCI. Steps of steady current (220 msec duration) of different strengths were injected through the recording electrode to induce repetitive firing of the cell. The same cell was then stimulated with trains (300 msec duration) of short depolarizing pulses at different frequencies. Simultaneously, the isometric tension responses of the motor unit to the two types of stimulation were recorded. Fig. 1 shows the tension response of a motor unit after stimulation of its motoneurone by current steps (B-F) and by trains of single pulses (G-L). This motor unit should be of the F (fast) type, as suggested by the duration of the motoneuronal AHP (around 60 msec), the twitch contraction time (18 msec) and the maximal tetanic tension (56.2 g)6. Increasing the intensity of the current steps from 16 to 36 nA causes a parallel growth of both the tension developed (up to the maximal tetanic tension, E, F) and the velocity of the force transient. Note that, at the lower frequencies (B and C), the peak tension is reached, transiently, at the beginning
316 A J
G
16 Hz
....
B
16 nA
H
27
C
20
I
39
D
24
J
45
K
56
L
98
E 28 L;l~ill~i
F
36
.I,~L,L,LL~ II
40 msec
10gr A,B,C
Fig. 1. Tension development in a G-S motor unit upon stimulation of its motoneurone with current steps and with constant rate intracellular pulses. On each record, upper trace gives the motoneurone's membrane potential, lower trace the isometric tension response of the motor unit. A: single twitch response. B-F: tension developed by repetitive firing produced by injections of current steps of intensities as indicated in nA (10 -9 A) on each record. G-L: tension developed upon constant rate firing produced by trains of single intracellular pulses at the frequencies as indicated on each recoi~d. Spike amplitude 80 inV. Spikes retouched. of the discharge. One to 2 rain after having obtained records A - F , the same cell was stimulated with trains of single pulses. A m o n g the various frequencies tested, those selected for illustration (G-L) were closer to the steady state frequencies obtained with current step injections. By comparing the two rows of records, it is apparent that for a similar frequency of steady firing the tension developed by current step stimulation grows much faster and reaches, at least transiently, higher values than the tension developed by a train of single shocks at constant rate. Similar results, recorded in another motor unit are illustrated in Fig. 2. In this unit the tension of the single twitch was subthreshold for the gauge (F). When double shock stimulation was used, the twitch following the second shock had a contraction time of 44 msec (not illustrated). The A H P duration was about 80 msec (F) and the maximal tetanic tension 21.8 g (E-J). Considered together, these characteristics should indicate 6 that the unit belongs to type S (slow). Note that in this unit the tension develops gradually and continuously after both types of stimulation. However, at low frequency of stimulation, the initial tension, i.e., the maximal tension developed in the first 200 msec, is higher on the left column (A-E current step stimulation) than on the right column ( G - J constant rate stimulation).
317 A
10 nA
F
G
B
i
14 r
:
I
,
3 3 Hz , r
I
C
18
H
45
D
26
I
67
E
30
J
110
4 mV F I 100 m V G - J
'40'msec 20 m s e c F
Fig. 2. Same as in Fig. l, for another G-S motor unit. A-E: responses to injection of current steps. G-J: responses to constant-rate stimulation. F: single pulse stimulation of the motoneurone, twitch response subthreshold for the transducer. Spike amplitude 75 inV. Spikes in A-E outpassed the upper border of the screen. The initial part of their upstroke has been retouched.
The results of Fig. 2 have been quantified in the graphs of Fig. 3. In Fig. 3A is represented the frequency-to-current (f/i) relation for the motoneurone, while in B the curves are shown relating the strength of the injected current to the maximal initial tension (open circles) and to the velocity of the tension development (open squares). The velocity is expressed as the ratio AF/At, where AF is 9 0 ~ of the maximal tension recorded in the first 200 msec and t the time from the onset of the tension to the value AF (see insert D). It seems worth noting, by comparing A and B, that the steep initial portion of both curves in B corresponds to the steep rise of the first interval curve in A (curve range between 10 and 18 nA). Shortening of the second interval (from 22 to 34 nA) is related to the second, less steep portion of the curves in B. Such correlations underline the functional importance, in tension recruitment, of the presence of a plateau in the time course of the A H P conductance 1,~. The slope of the steep portion of the f/i relation for the first firing intervals is, in fact, strictly dependent on the characteristics of the S-shaped plateau region of the G~: time course3, 4. The curves of graph A and B have been utilized to correlate the tension parameters F and AF/At with the firing frequency attained at steady state by injection of current steps. On the steady-state firing curve (s.s.) of graph A the current intensities
318
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B .7
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315
Relevance of motoneuronal firing adaptation to tension development in the motor unit
F. BALDISSERA ANDF. PARMIGGIANI Laboratorio di Fisiologia dei Centri Nervosi del CNR, Via Mario Bianco, 9-20131 Milan (Italy)
(Accepted March 1lth, 1975)
Spinal motoneurones fire repetitively in response to steps of current injected through the cell membrane 9. Due to 'summation' of the potassium conductance (Gr~) changes responsible for the afterhyperpolarization (AHP) 1,2,4,11,12, the initial firing frequency rapidly decreases (adaptation) to a steady value which is reached after a number of intervals which varies according to the intensity of the injected current a,l°. The high frequency firing which precedes adaptation should be 8 'functionally important for rapid utilization of muscular activity' also in view of the 'catch properties' of the mammalian motor unit v. Thus, experiments were designed to test how muscle tension in a single motor unit is developed after injection of current steps of increasing intensity into the motoneurone. Recordings were made in intact cats, anaesthetized with Nembutal (40 mg/kg, initial dose). The nerves of both hindlimbs were severed except for the left gastrocnemius-soleus (G-S) nerve. The lumbar enlargement was exposed by laminectomy. The Achilles tendon was cut and connected through an inelastic thread to a Grass FTO 3C force transducer (self-resonant frequency 330 Hz) and stretched to the length giving the strongest twitch response for the whole muscle. G-S motoneurones in L7-SI were impaled with glass microelectrodes filled with 3M KCI. Steps of steady current (220 msec duration) of different strengths were injected through the recording electrode to induce repetitive firing of the cell. The same cell was then stimulated with trains (300 msec duration) of short depolarizing pulses at different frequencies. Simultaneously, the isometric tension responses of the motor unit to the two types of stimulation were recorded. Fig. 1 shows the tension response of a motor unit after stimulation of its motoneurone by current steps (B-F) and by trains of single pulses (G-L). This motor unit should be of the F (fast) type, as suggested by the duration of the motoneuronal AHP (around 60 msec), the twitch contraction time (18 msec) and the maximal tetanic tension (56.2 g)6. Increasing the intensity of the current steps from 16 to 36 nA causes a parallel growth of both the tension developed (up to the maximal tetanic tension, E, F) and the velocity of the force transient. Note that, at the lower frequencies (B and C), the peak tension is reached, transiently, at the beginning
316 A J
G
16 Hz
....
B
16 nA
H
27
C
20
I
39
D
24
J
45
K
56
L
98
E 28 L;l~ill~i
F
36
.I,~L,L,LL~ II
40 msec
10gr A,B,C
Fig. 1. Tension development in a G-S motor unit upon stimulation of its motoneurone with current steps and with constant rate intracellular pulses. On each record, upper trace gives the motoneurone's membrane potential, lower trace the isometric tension response of the motor unit. A: single twitch response. B-F: tension developed by repetitive firing produced by injections of current steps of intensities as indicated in nA (10 -9 A) on each record. G-L: tension developed upon constant rate firing produced by trains of single intracellular pulses at the frequencies as indicated on each recoi~d. Spike amplitude 80 inV. Spikes retouched. of the discharge. One to 2 rain after having obtained records A - F , the same cell was stimulated with trains of single pulses. A m o n g the various frequencies tested, those selected for illustration (G-L) were closer to the steady state frequencies obtained with current step injections. By comparing the two rows of records, it is apparent that for a similar frequency of steady firing the tension developed by current step stimulation grows much faster and reaches, at least transiently, higher values than the tension developed by a train of single shocks at constant rate. Similar results, recorded in another motor unit are illustrated in Fig. 2. In this unit the tension of the single twitch was subthreshold for the gauge (F). When double shock stimulation was used, the twitch following the second shock had a contraction time of 44 msec (not illustrated). The A H P duration was about 80 msec (F) and the maximal tetanic tension 21.8 g (E-J). Considered together, these characteristics should indicate 6 that the unit belongs to type S (slow). Note that in this unit the tension develops gradually and continuously after both types of stimulation. However, at low frequency of stimulation, the initial tension, i.e., the maximal tension developed in the first 200 msec, is higher on the left column (A-E current step stimulation) than on the right column ( G - J constant rate stimulation).
317 A
10 nA
F
G
B
i
14 r
:
I
,
3 3 Hz , r
I
C
18
H
45
D
26
I
67
E
30
J
110
4 mV F I 100 m V G - J
'40'msec 20 m s e c F
Fig. 2. Same as in Fig. l, for another G-S motor unit. A-E: responses to injection of current steps. G-J: responses to constant-rate stimulation. F: single pulse stimulation of the motoneurone, twitch response subthreshold for the transducer. Spike amplitude 75 inV. Spikes in A-E outpassed the upper border of the screen. The initial part of their upstroke has been retouched.
The results of Fig. 2 have been quantified in the graphs of Fig. 3. In Fig. 3A is represented the frequency-to-current (f/i) relation for the motoneurone, while in B the curves are shown relating the strength of the injected current to the maximal initial tension (open circles) and to the velocity of the tension development (open squares). The velocity is expressed as the ratio AF/At, where AF is 9 0 ~ of the maximal tension recorded in the first 200 msec and t the time from the onset of the tension to the value AF (see insert D). It seems worth noting, by comparing A and B, that the steep initial portion of both curves in B corresponds to the steep rise of the first interval curve in A (curve range between 10 and 18 nA). Shortening of the second interval (from 22 to 34 nA) is related to the second, less steep portion of the curves in B. Such correlations underline the functional importance, in tension recruitment, of the presence of a plateau in the time course of the A H P conductance 1,~. The slope of the steep portion of the f/i relation for the first firing intervals is, in fact, strictly dependent on the characteristics of the S-shaped plateau region of the G~: time course3, 4. The curves of graph A and B have been utilized to correlate the tension parameters F and AF/At with the firing frequency attained at steady state by injection of current steps. On the steady-state firing curve (s.s.) of graph A the current intensities
318
A
B .7
,6
t,.
200
'2*
F max
2O
f
/
N
3:
¢
OD
,e,.
.4AF/~t
o
@
u. / r/
100
3 lO
/P
2
/t,," s.s.
,1
1'0
3"o
2()
4'o
5'o II.AI
10
20
.......
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20.
50 I(nA)
/' /
(
4)
I: max .5
ItI u.
40
O .x
C
J. m
30
