Brain Research, 84 (1975) 351-356
351
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Electrophysiology of spinal motoneurons in the pigeon
AARON RABIN* The Rockefeller University, New York, N.Y. 10021 (U.S.A.)
(Accepted October 28th, 1974)
In contrast to the wealth of information about the physiological properties of spinal motoneurons in mammals no equivalent data have been reported in birds. In the present study intracellular recording techniques have been applied to pigeons to investigate the properties of spinal motoneurons and to characterize the synaptic activity that may be recorded from them. The results will show that the synaptic activity of pigeon motoneurons is similar in many respects to that of mammalian motoneurons. Experiments were performed on carneaux pigeons that were either anesthetized with methoxyflurane or decerebrated. The animals were paralyzed with gallamine triethiodide, unidirectionally respirated and the temperature was maintained at 41-42 °C. The condition of each animal and the depth of anesthesia were monitored by recording the electrocardiogram. Tachycardia in excess of 300 beats/min was counteracted successfully by intramuscular administration of methoxamine hydrochloride. The enlargements in the brachial or lumbosacral regions of the spinal cord were exposed by a dorsal laminectomy. At lumbosacral levels, the glycogen body was sometimes removed from the rhomboid sinus by suction to facilitate exposure of the spinal cord. Intracellular records were obtained with glass micropipettes filled with 2 M potassium acetate, saturated with Fast Green FCF and having resistances of 5-15 M f L Motoneurons were identified by antidromic stimulation of peripheral nerves with bipolar platinum hook electrodes. In the brachial region, nerves were isolated from the brachial plexus 1. In the hindlimb, divisions of the sciatic nerve 14 were used for antidromic stimulation. In some experiments stimulating electrodes were inserted into the brain stem in order to also stimulate descending pathways which affect motoneurons. When neurons in the ventral horn were impaled, resting potentials appeared abruptly; in a sample of neurons, the maximum resting membrane potential ranged from 42 to 84 mV (mean = 59 :k 11 mV S.D., n = 31). The intracellular potentials recorded in motoneurons of the spinal cord of the pigeon consisted of action potentials and depolarizing and hyperpolarizing synaptic potentials.
* Present address: The Albert Einstein College of Medicine, Eastchester Road and Morris Park Avenue, Bronx, N.Y. 10461, U.S.A.
352
,42
-+! 50mv L~J 2 mse¢
2/sec
I
200,/see
-lC~ II ~o
J
C2
C3
--+
L__3 2 msec
Fig. 1. Intracellular action potentials in pigeon spinal motoneurons. Time and voltage calibrations for A and B as shown in A: for C as shown in C8. A: antidromic, A1, and orthodromic, As, action potentials evoked in a lumbosacral motoneuron by stimulation of the sciatic nerve. In As, the stimulus strength was adjusted to be just about threshold to reveal both the spike potential and the underlying depolarizing potential from which it was generated. B: an antidromic action potential evoked in another lumbosacral motoneuron at different frequencies of stimulation. C: antidromic spike potential evoked during the passage of depolarizing (Dep) or hyperpolarizing (Hyp) current through the micropipettel Amount of current passed indicated above each trace. Note failure of spike in C8.
Action potentials. O r t h o d r o m i c or antidromic activation o f a m o t o n e u r o n evoked an all-or-nothing action potential (Fig. 1A). Action potentials had amplitudes o f 40-98 mV (mean = 57 ± 13 mV S.D., n = 41) and durations o f 1.1-3.0 msec (mean -- !.6 ~z 0.4 msec S.D., n -~ 33). The amplitudes were similar for both antidromic and o r t h o d r o m i c activation, and both types o f action potentials showed the property of 'overshoot'. Regardless o f the m o d e o f activation, action potentials often exhibited an inflection on their rising phase. The inflection was more p r o n o u n c e d in some nero ons than in others and generally occurred at a level o f 20-30 mV. The break in the ascending portion o f the action potential was more clearly evident when the frequency o f stimulation was increased (Fig. 1B). D u r i n g high frequency stimulation the full action potential sometimes failed, and during that failure an underlying spike potential whose peak corresponded to the point o f inflection was seen in isolation (Fig. 1B3). These spikes have been termed the A spike and B spike respectively 7 and have been attributed to different sites o f spike generation. When b o t h A and B spikes fail, an underlying potential o f a few millivolts was observed (Fig. 1B3); this potential resembled the M spike described by Eccles 4 in the cat. In some m o t o n e u r o n s the spike of the action potential terminated in a prolonged after-hyperpolarization which slowly declined
353 ,#1
A3
A2
ii .I m s e c
B2
B2
÷
B3 ;~4,~ ,/
~ I n o
CJ
C2 1.3T
Dep6no
C3 1.4T
3 1.8T i
D] "I"Y--
ae
D3
I
2reset 5
m "
3
~
+
- -
Fig. 2. Postsynaptic potentials in pigeon spinal motoneurons. Time calibrations for A and B in A3. Time calibrations for C and D in Ca. Voltage calibrations are 500 #V for all records. A : EPSP evoked in a lumbosacral motoneuron by stimulation applied to the tibialis lateralis nerve at different strengths of stimulation. Stimulus intensity indicated in multiples of threshold (T). B : EPSP evoked in a brachial motoneuron by stimulation applied to the brachialis longus superior nerve: B1, in the absence of polarizing current; B~, Ba, during passage of depolarizing (Dep) current through the micropipette. Amount of current passed indicated above each trace. During the passage of depolarizing current of 6 nA the EPSP gave rise to an action potential. C: IPSP evoked in a pectoralis posterior motoneuron by stimulation of the brachialis longus inferior nerve at different intensities. The occurrence of the stimulus is indicated by a dot. The stimulus strength is indicated above each trace in multiples of the threshold (T) for the response. D: IPSP evoked in another pectoralis posterior motoneuron by stimulation of the medial longitudinal fasciculus: D1, in the absence of polarizing current; Dz, D3, during the passage of depolarizing (Dep) or hyperpolarizing (Hyp) current of 20 nA. (Fig. 1A, IB). In other instances, the after-hyperpolarization was preceded by a small after-depolarization (Fig. 1C). Passage of depolarizing or hyperpolarizing currents into a m o t o n e u r o n affected the amplitude o f the action potential that occurred during the period o f current injection (Fig. 1C). The relationship between the values for the amplitude o f the spike and the a m o u n t o f current passed gives an estimate o f the cell m e m b r a n e input resistance 6. In the pigeon this relationship was significantly linear over a wide range, with the m e t h o d o f least squares, and the resistance over this range (calculated for 4 neurons) was between 5-8 M ~ . Excitatory postsynaptic potentials. O r t h o d r o m i c synaptic activation produced depolarizing potentials in m a n y motoneurons. As the intensity o f the stimulus was progressively increased, there was usually a corresponding graded increase in the amplitude o f the m e m b r a n e depolarization with virtually no change in the time course o f the response; this suggests that the observed depolarizations were p r o d u c e d by the
354
30.
°
20
E
I0
l
05
I'-IEPSPs lIE] I PSPs
1.5
2.5
Lolency (msec) Fig. 3. Distribution of segmental central delay for EPSPs and IPSPs evoked in brachial motoneurons
by stimulation of peripheral nerves in the brachial plexus. Central delay was determined by measuring the time between the first positivepeak of the segmentalafferentvolley,recorded on the dorsum of the spinal cord, and the onset of the PSP. An example of this is shown by the inset; upper trace (negativity upward), dorsum potential; lower trace, segmentallyevoked EPSP. spatial summation of many elemental depolarizing potentials 4. In other motoneurons increasing the strength of the stimulus also evoked later depolarizing potentials (Fig. 2Aa). The passage of depolarizing current through the micropipette did not significantly affect the depolarizing potential (Fig. 2B). However, if the neuron was depolarized to a greater extent, bringing the depolarizing potential closer to its threshold value, it was able togive rise to an action potential (Figs. 1Az, 2B3). Since these potentials exhibited the properties of excitatory postsynaptic potentials (EPSPs) observed in other species, they were assumed to be EPSPs. Inhibitory postsynaptic potentials. Stimulation of peripheral nerves or descending pathways also gave rise to hyperpolarizing potentials. Progressive increments in the strength of the stimulus usually evoked graded increments in the amplitude of the response without producing any change in its time course (Fig. 2C). Just as with the EPSP, the observed hyperpolarizing potential appears to be produced by the summation of smaller elemental hyperpolarizing potentials. In some neurons, stronger shocks also produced later hyperpolarizing potentials. Changes in membrane potential produced by passing current through the micropipette also affected the magnitude and polarity of the hyperpolarizing potential (Fig. 2D). Action potentials evoked during the hyperpolarizing potential were depressed in amplitude. Thus these potentials resembled inhibitory postsynaptic potentials and were therefore assumed to be IPSPs. Segmental monosynaptic delay. The interval between the arrival of the segmental presynaptic afferent volley and the onset of a PSP was used as a measure of the central delay for the response (Fig. 3). The arrival of the presynaptic volley was measured in some experiments by a ball-tipped silver-silver chloride electrode placed on the dorsal surface of the spinal cord adjacent to the entry zone of the dorsal root fibers. The afferent volley consisted of a sequence of fast positive-negative-positive potentials followed by a large slow negative wave (inset, Fig. 3). Since the degree of
355 arborization and tapering of the presynaptic afferent nerve terminals is not known, the latency recorded by an electrode on the dorsal white matter does not give the exact arrival time for the afferent volley. Nevertheless, the dorsum potential does reflect the arrival of a synchronous volley of impulses and could therefore be used as a measure of the average arrival time for the afferent volley. Theoretical considerations suggest that the arrival of the afferent volley is signaled by the first point of reversal of the dorsum potential15. However, impulses conducted by fibers having conduction velocities faster than the average conduction velocity would arrive earlier. Therefore the peak of the early positive wave is often taken as a measure of time of arrival of the fastest conducting fibersl°,11,16,tL When calculated from the positive peak the mean segmental delay for the EPSPs was 0.5 4- 0.1 msec S.D. (n -- 57) and for the IPSPs was 1.6 -t- 0.4 msec S.D. (n = 52). When calculated from the first reversal point the values obtained for the segmental delay were shorter on the average by 0.1 msec. The segmental delays for the EPSPs were always shorter than those for the IPSPs. Undoubtedly the EPSPs were monosynaptically evoked. In addition, since the values obtained for the monosynaptic segmental delay include the conduction time along the presynaptic afferent terminals, the average synaptic delay is on the order of 0.3-0.4 msec and is comparable to the value observed in mammals4. The monosynaptic EPSPs were often evoked by stimulation of peripheral fibers with the lowest threshold. This suggests that, as in mammals, the largest muscle afferents may be producing the monosynaptic excitatory response. The foregoing results demonstrate that the potentials recorded from spinal motoneurons in the pigeon are similar in many respects to those observed in motoneurons of other vertebrates. Moreover, the properties of the potentials observed in this study - - e.g. the existence of 'overshoot' for the action potential or the ability of polarizing currents to influence the magnitude and/or polarity of synaptic potentials - are consistent with the models for the ionic mechanisms postulated for the generation of action and synaptic potentials in other species. The average segmental or synaptic delay for EPSPs in the pigeon is comparable to those values observed in mammals. However, the shortest values of 0.25 msec for the segmental delay (Fig. 3) or 0,1-0.15 msec for the synaptic delay are shorter than the values observed in species of other orders. Accordingly, these findings suggest that synaptic transmission may indeed be faster in birds than in mammals. When measured at the frog neuromuscular junction, the Q10 for synaptic transmission is on the order of 3 for temperatures ranging between 2 and 20 °C (see refs. 13, 18), perhaps the normally higher temperature in birds might increase the speed of synaptic transmission. Alternatively, investigators have previously shown that the magnitudes of the delays at electrotonic synapses are on the order of 50 #sec or less and may be as great as a few tenths of a msec in neurons with moderately long time constants z,3. The brevity of the synaptic delay in the pigeon might therefore reflect the existence of an electrotonic component to the synaptic transmission between spinal afferents and spinal motoneurons. In the cat 5 and the toad 17 investigators have reported that the speed of the components of the action potential of motoneurons often varies according to the speed of
356 c o n t r a c t i o n o f c o r r e s p o n d i n g muscles. Thus, the short d u r a t i o n of the action potential in the pigeon is consistent with the observations that, in the main, the muscles innervated by the nerves used in this study are rapidly c o n t r a c t i n g muscles 8. However, pigeons also possess muscles consisting entirely of t o n u s fibers 9, e.g. the latissimus dorsi anterior. Recent a n a t o m i c a l studies have identified the nerves i n n e r v a t i n g this m u s c l O L Accordingly it would be interesting to see if the time course of the action potentials of the m o t o n e u r o n s giving rise to these fibers also parallel the characteristics of t o n u s muscles. This work was supported in part by N I H G r a n t NS 02619.
1 BAUMEL,J. J., Variation in the brachial plexus of Progne sub&, Acta anat. (Basel), 34 (1958) 1-34. 2 BENNETT,M. V. L., Physiology of electrotonic synapses, Ann. N. Y. Acad. Sei., 137 (1966) 509-539. 3 BENNETT,M. V. L., Comparison of electrically and chemically mediated synaptic transmission. In G. D. PAPPASAND D. P. PURPURA(Eds.), Structure and Function of Synapses, Raven Press. New York, 1972, pp. 221-256. 4 ECCLES,J. C., The Physiology of Nerve Cells, Johns Hopkins Univ. Press, Baltimore, Md., 1957. 5 ECCLES,J. C., ECCLES, R. M., AND LUNDaERC,A., The action potentials of alpha-motoneurons supplying fast and slow muscles, J. Physiol. (Lond.), 142 (1958) 275-291. 6 FRANK,K., AND FUORTES,M. G. F., Stimulation of spinal motoneurons with intraceUular electrodes, J. Physiol. (Lond.), 134 (1956) 451-470. 7 FUORTES, M. G. F., FRANK,K., ANDBECKER, M. C., Steps in the production of motoneuron spikes, J. gen. Physiol., 40 (1957) 735-752. 8 GEORGE,J. C., Avian Myology, Academic Press, New York, 1966. 9 GINSBORG, B. L., Some properties of avain skeletal muscle fibres with multiple neuromuscular junctions, J. Physiol. (Lond.), 154 (1960) 581-598. l0 GRILLNER,S., HONGO,T., AND LUND,S., The vestibulospinal tract. Effects on alpha motoneurons in the tumbosacral spinal cord in the cat, Exp. Brain Res., 10 (1970) 94-120. I I GRILLNER,S., HONGO, T., AND LUND, S., Convergent effects on alpha motoneurons from the vestibulospinal tract and a pathway descending in the medial longitudinal fasciculus, Exp. Brain Res., 12 (1971) 457-479. 12 HIKIDA, R. S., AND ]3OCK, W. J., lnnervation of the avain tonus latissimus dorsi anterior muscle, Amer. J. Anat., 130 (1971) 269-280. 13 KATZ,B., AND MILEDI, R., The effect of temperature on the synaptic delay at the neuromuscular junction, J. Physiol. (Lond.), 181 (1965)656-670. 14 KOCH,T., Anatomy of the Chicken and Domestic Birds, Iowa State Univ. Press, Ames, Iowa, 1973. 15 LORENTEDE N6, R., Analysis of the distribution of the action currents of nerve in volume conductors. In A Study of Nerve Physiology. Studies from the Rockefeller Institute for Medical Research, Vol. 132, The Rockefeller Institute for Medical Research, New York, 1947, pp. 384-477. 16 LUND, S., AND POMPEIANO, O., Monosynaptic excitation of alpha motoneurons from supraspinal structures in the cat, Acta physiol, scand., 73 (1968) 1-21. 17 ROSENBERG,M. E., Excitation and inhibition of motoneurons in the tortoise, J. Physiol. (Loml.), 221 (1972) 715-730. 18 TAKEUCHI,N., The effect of temperature on the neuromuscular junction of the frog, Jap. J. Physiol., 8 (1958) 391-404. 19 WILSON,V. J., AND YOSHIDA, M., Comparison of effects of stimulation of Deiters' nucleus and medial longitudinal fasciculus on neck, forelimb, and hindlimb motoneurons, J. Neurophysiol., 32 (1969) 743-758.
351
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Electrophysiology of spinal motoneurons in the pigeon
AARON RABIN* The Rockefeller University, New York, N.Y. 10021 (U.S.A.)
(Accepted October 28th, 1974)
In contrast to the wealth of information about the physiological properties of spinal motoneurons in mammals no equivalent data have been reported in birds. In the present study intracellular recording techniques have been applied to pigeons to investigate the properties of spinal motoneurons and to characterize the synaptic activity that may be recorded from them. The results will show that the synaptic activity of pigeon motoneurons is similar in many respects to that of mammalian motoneurons. Experiments were performed on carneaux pigeons that were either anesthetized with methoxyflurane or decerebrated. The animals were paralyzed with gallamine triethiodide, unidirectionally respirated and the temperature was maintained at 41-42 °C. The condition of each animal and the depth of anesthesia were monitored by recording the electrocardiogram. Tachycardia in excess of 300 beats/min was counteracted successfully by intramuscular administration of methoxamine hydrochloride. The enlargements in the brachial or lumbosacral regions of the spinal cord were exposed by a dorsal laminectomy. At lumbosacral levels, the glycogen body was sometimes removed from the rhomboid sinus by suction to facilitate exposure of the spinal cord. Intracellular records were obtained with glass micropipettes filled with 2 M potassium acetate, saturated with Fast Green FCF and having resistances of 5-15 M f L Motoneurons were identified by antidromic stimulation of peripheral nerves with bipolar platinum hook electrodes. In the brachial region, nerves were isolated from the brachial plexus 1. In the hindlimb, divisions of the sciatic nerve 14 were used for antidromic stimulation. In some experiments stimulating electrodes were inserted into the brain stem in order to also stimulate descending pathways which affect motoneurons. When neurons in the ventral horn were impaled, resting potentials appeared abruptly; in a sample of neurons, the maximum resting membrane potential ranged from 42 to 84 mV (mean = 59 :k 11 mV S.D., n = 31). The intracellular potentials recorded in motoneurons of the spinal cord of the pigeon consisted of action potentials and depolarizing and hyperpolarizing synaptic potentials.
* Present address: The Albert Einstein College of Medicine, Eastchester Road and Morris Park Avenue, Bronx, N.Y. 10461, U.S.A.
352
,42
-+! 50mv L~J 2 mse¢
2/sec
I
200,/see
-lC~ II ~o
J
C2
C3
--+
L__3 2 msec
Fig. 1. Intracellular action potentials in pigeon spinal motoneurons. Time and voltage calibrations for A and B as shown in A: for C as shown in C8. A: antidromic, A1, and orthodromic, As, action potentials evoked in a lumbosacral motoneuron by stimulation of the sciatic nerve. In As, the stimulus strength was adjusted to be just about threshold to reveal both the spike potential and the underlying depolarizing potential from which it was generated. B: an antidromic action potential evoked in another lumbosacral motoneuron at different frequencies of stimulation. C: antidromic spike potential evoked during the passage of depolarizing (Dep) or hyperpolarizing (Hyp) current through the micropipettel Amount of current passed indicated above each trace. Note failure of spike in C8.
Action potentials. O r t h o d r o m i c or antidromic activation o f a m o t o n e u r o n evoked an all-or-nothing action potential (Fig. 1A). Action potentials had amplitudes o f 40-98 mV (mean = 57 ± 13 mV S.D., n = 41) and durations o f 1.1-3.0 msec (mean -- !.6 ~z 0.4 msec S.D., n -~ 33). The amplitudes were similar for both antidromic and o r t h o d r o m i c activation, and both types o f action potentials showed the property of 'overshoot'. Regardless o f the m o d e o f activation, action potentials often exhibited an inflection on their rising phase. The inflection was more p r o n o u n c e d in some nero ons than in others and generally occurred at a level o f 20-30 mV. The break in the ascending portion o f the action potential was more clearly evident when the frequency o f stimulation was increased (Fig. 1B). D u r i n g high frequency stimulation the full action potential sometimes failed, and during that failure an underlying spike potential whose peak corresponded to the point o f inflection was seen in isolation (Fig. 1B3). These spikes have been termed the A spike and B spike respectively 7 and have been attributed to different sites o f spike generation. When b o t h A and B spikes fail, an underlying potential o f a few millivolts was observed (Fig. 1B3); this potential resembled the M spike described by Eccles 4 in the cat. In some m o t o n e u r o n s the spike of the action potential terminated in a prolonged after-hyperpolarization which slowly declined
353 ,#1
A3
A2
ii .I m s e c
B2
B2
÷
B3 ;~4,~ ,/
~ I n o
CJ
C2 1.3T
Dep6no
C3 1.4T
3 1.8T i
D] "I"Y--
ae
D3
I
2reset 5
m "
3
~
+
- -
Fig. 2. Postsynaptic potentials in pigeon spinal motoneurons. Time calibrations for A and B in A3. Time calibrations for C and D in Ca. Voltage calibrations are 500 #V for all records. A : EPSP evoked in a lumbosacral motoneuron by stimulation applied to the tibialis lateralis nerve at different strengths of stimulation. Stimulus intensity indicated in multiples of threshold (T). B : EPSP evoked in a brachial motoneuron by stimulation applied to the brachialis longus superior nerve: B1, in the absence of polarizing current; B~, Ba, during passage of depolarizing (Dep) current through the micropipette. Amount of current passed indicated above each trace. During the passage of depolarizing current of 6 nA the EPSP gave rise to an action potential. C: IPSP evoked in a pectoralis posterior motoneuron by stimulation of the brachialis longus inferior nerve at different intensities. The occurrence of the stimulus is indicated by a dot. The stimulus strength is indicated above each trace in multiples of the threshold (T) for the response. D: IPSP evoked in another pectoralis posterior motoneuron by stimulation of the medial longitudinal fasciculus: D1, in the absence of polarizing current; Dz, D3, during the passage of depolarizing (Dep) or hyperpolarizing (Hyp) current of 20 nA. (Fig. 1A, IB). In other instances, the after-hyperpolarization was preceded by a small after-depolarization (Fig. 1C). Passage of depolarizing or hyperpolarizing currents into a m o t o n e u r o n affected the amplitude o f the action potential that occurred during the period o f current injection (Fig. 1C). The relationship between the values for the amplitude o f the spike and the a m o u n t o f current passed gives an estimate o f the cell m e m b r a n e input resistance 6. In the pigeon this relationship was significantly linear over a wide range, with the m e t h o d o f least squares, and the resistance over this range (calculated for 4 neurons) was between 5-8 M ~ . Excitatory postsynaptic potentials. O r t h o d r o m i c synaptic activation produced depolarizing potentials in m a n y motoneurons. As the intensity o f the stimulus was progressively increased, there was usually a corresponding graded increase in the amplitude o f the m e m b r a n e depolarization with virtually no change in the time course o f the response; this suggests that the observed depolarizations were p r o d u c e d by the
354
30.
°
20
E
I0
l
05
I'-IEPSPs lIE] I PSPs
1.5
2.5
Lolency (msec) Fig. 3. Distribution of segmental central delay for EPSPs and IPSPs evoked in brachial motoneurons
by stimulation of peripheral nerves in the brachial plexus. Central delay was determined by measuring the time between the first positivepeak of the segmentalafferentvolley,recorded on the dorsum of the spinal cord, and the onset of the PSP. An example of this is shown by the inset; upper trace (negativity upward), dorsum potential; lower trace, segmentallyevoked EPSP. spatial summation of many elemental depolarizing potentials 4. In other motoneurons increasing the strength of the stimulus also evoked later depolarizing potentials (Fig. 2Aa). The passage of depolarizing current through the micropipette did not significantly affect the depolarizing potential (Fig. 2B). However, if the neuron was depolarized to a greater extent, bringing the depolarizing potential closer to its threshold value, it was able togive rise to an action potential (Figs. 1Az, 2B3). Since these potentials exhibited the properties of excitatory postsynaptic potentials (EPSPs) observed in other species, they were assumed to be EPSPs. Inhibitory postsynaptic potentials. Stimulation of peripheral nerves or descending pathways also gave rise to hyperpolarizing potentials. Progressive increments in the strength of the stimulus usually evoked graded increments in the amplitude of the response without producing any change in its time course (Fig. 2C). Just as with the EPSP, the observed hyperpolarizing potential appears to be produced by the summation of smaller elemental hyperpolarizing potentials. In some neurons, stronger shocks also produced later hyperpolarizing potentials. Changes in membrane potential produced by passing current through the micropipette also affected the magnitude and polarity of the hyperpolarizing potential (Fig. 2D). Action potentials evoked during the hyperpolarizing potential were depressed in amplitude. Thus these potentials resembled inhibitory postsynaptic potentials and were therefore assumed to be IPSPs. Segmental monosynaptic delay. The interval between the arrival of the segmental presynaptic afferent volley and the onset of a PSP was used as a measure of the central delay for the response (Fig. 3). The arrival of the presynaptic volley was measured in some experiments by a ball-tipped silver-silver chloride electrode placed on the dorsal surface of the spinal cord adjacent to the entry zone of the dorsal root fibers. The afferent volley consisted of a sequence of fast positive-negative-positive potentials followed by a large slow negative wave (inset, Fig. 3). Since the degree of
355 arborization and tapering of the presynaptic afferent nerve terminals is not known, the latency recorded by an electrode on the dorsal white matter does not give the exact arrival time for the afferent volley. Nevertheless, the dorsum potential does reflect the arrival of a synchronous volley of impulses and could therefore be used as a measure of the average arrival time for the afferent volley. Theoretical considerations suggest that the arrival of the afferent volley is signaled by the first point of reversal of the dorsum potential15. However, impulses conducted by fibers having conduction velocities faster than the average conduction velocity would arrive earlier. Therefore the peak of the early positive wave is often taken as a measure of time of arrival of the fastest conducting fibersl°,11,16,tL When calculated from the positive peak the mean segmental delay for the EPSPs was 0.5 4- 0.1 msec S.D. (n -- 57) and for the IPSPs was 1.6 -t- 0.4 msec S.D. (n = 52). When calculated from the first reversal point the values obtained for the segmental delay were shorter on the average by 0.1 msec. The segmental delays for the EPSPs were always shorter than those for the IPSPs. Undoubtedly the EPSPs were monosynaptically evoked. In addition, since the values obtained for the monosynaptic segmental delay include the conduction time along the presynaptic afferent terminals, the average synaptic delay is on the order of 0.3-0.4 msec and is comparable to the value observed in mammals4. The monosynaptic EPSPs were often evoked by stimulation of peripheral fibers with the lowest threshold. This suggests that, as in mammals, the largest muscle afferents may be producing the monosynaptic excitatory response. The foregoing results demonstrate that the potentials recorded from spinal motoneurons in the pigeon are similar in many respects to those observed in motoneurons of other vertebrates. Moreover, the properties of the potentials observed in this study - - e.g. the existence of 'overshoot' for the action potential or the ability of polarizing currents to influence the magnitude and/or polarity of synaptic potentials - are consistent with the models for the ionic mechanisms postulated for the generation of action and synaptic potentials in other species. The average segmental or synaptic delay for EPSPs in the pigeon is comparable to those values observed in mammals. However, the shortest values of 0.25 msec for the segmental delay (Fig. 3) or 0,1-0.15 msec for the synaptic delay are shorter than the values observed in species of other orders. Accordingly, these findings suggest that synaptic transmission may indeed be faster in birds than in mammals. When measured at the frog neuromuscular junction, the Q10 for synaptic transmission is on the order of 3 for temperatures ranging between 2 and 20 °C (see refs. 13, 18), perhaps the normally higher temperature in birds might increase the speed of synaptic transmission. Alternatively, investigators have previously shown that the magnitudes of the delays at electrotonic synapses are on the order of 50 #sec or less and may be as great as a few tenths of a msec in neurons with moderately long time constants z,3. The brevity of the synaptic delay in the pigeon might therefore reflect the existence of an electrotonic component to the synaptic transmission between spinal afferents and spinal motoneurons. In the cat 5 and the toad 17 investigators have reported that the speed of the components of the action potential of motoneurons often varies according to the speed of
356 c o n t r a c t i o n o f c o r r e s p o n d i n g muscles. Thus, the short d u r a t i o n of the action potential in the pigeon is consistent with the observations that, in the main, the muscles innervated by the nerves used in this study are rapidly c o n t r a c t i n g muscles 8. However, pigeons also possess muscles consisting entirely of t o n u s fibers 9, e.g. the latissimus dorsi anterior. Recent a n a t o m i c a l studies have identified the nerves i n n e r v a t i n g this m u s c l O L Accordingly it would be interesting to see if the time course of the action potentials of the m o t o n e u r o n s giving rise to these fibers also parallel the characteristics of t o n u s muscles. This work was supported in part by N I H G r a n t NS 02619.
1 BAUMEL,J. J., Variation in the brachial plexus of Progne sub&, Acta anat. (Basel), 34 (1958) 1-34. 2 BENNETT,M. V. L., Physiology of electrotonic synapses, Ann. N. Y. Acad. Sei., 137 (1966) 509-539. 3 BENNETT,M. V. L., Comparison of electrically and chemically mediated synaptic transmission. In G. D. PAPPASAND D. P. PURPURA(Eds.), Structure and Function of Synapses, Raven Press. New York, 1972, pp. 221-256. 4 ECCLES,J. C., The Physiology of Nerve Cells, Johns Hopkins Univ. Press, Baltimore, Md., 1957. 5 ECCLES,J. C., ECCLES, R. M., AND LUNDaERC,A., The action potentials of alpha-motoneurons supplying fast and slow muscles, J. Physiol. (Lond.), 142 (1958) 275-291. 6 FRANK,K., AND FUORTES,M. G. F., Stimulation of spinal motoneurons with intraceUular electrodes, J. Physiol. (Lond.), 134 (1956) 451-470. 7 FUORTES, M. G. F., FRANK,K., ANDBECKER, M. C., Steps in the production of motoneuron spikes, J. gen. Physiol., 40 (1957) 735-752. 8 GEORGE,J. C., Avian Myology, Academic Press, New York, 1966. 9 GINSBORG, B. L., Some properties of avain skeletal muscle fibres with multiple neuromuscular junctions, J. Physiol. (Lond.), 154 (1960) 581-598. l0 GRILLNER,S., HONGO,T., AND LUND,S., The vestibulospinal tract. Effects on alpha motoneurons in the tumbosacral spinal cord in the cat, Exp. Brain Res., 10 (1970) 94-120. I I GRILLNER,S., HONGO, T., AND LUND, S., Convergent effects on alpha motoneurons from the vestibulospinal tract and a pathway descending in the medial longitudinal fasciculus, Exp. Brain Res., 12 (1971) 457-479. 12 HIKIDA, R. S., AND ]3OCK, W. J., lnnervation of the avain tonus latissimus dorsi anterior muscle, Amer. J. Anat., 130 (1971) 269-280. 13 KATZ,B., AND MILEDI, R., The effect of temperature on the synaptic delay at the neuromuscular junction, J. Physiol. (Lond.), 181 (1965)656-670. 14 KOCH,T., Anatomy of the Chicken and Domestic Birds, Iowa State Univ. Press, Ames, Iowa, 1973. 15 LORENTEDE N6, R., Analysis of the distribution of the action currents of nerve in volume conductors. In A Study of Nerve Physiology. Studies from the Rockefeller Institute for Medical Research, Vol. 132, The Rockefeller Institute for Medical Research, New York, 1947, pp. 384-477. 16 LUND, S., AND POMPEIANO, O., Monosynaptic excitation of alpha motoneurons from supraspinal structures in the cat, Acta physiol, scand., 73 (1968) 1-21. 17 ROSENBERG,M. E., Excitation and inhibition of motoneurons in the tortoise, J. Physiol. (Loml.), 221 (1972) 715-730. 18 TAKEUCHI,N., The effect of temperature on the neuromuscular junction of the frog, Jap. J. Physiol., 8 (1958) 391-404. 19 WILSON,V. J., AND YOSHIDA, M., Comparison of effects of stimulation of Deiters' nucleus and medial longitudinal fasciculus on neck, forelimb, and hindlimb motoneurons, J. Neurophysiol., 32 (1969) 743-758.
