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Differential properties of orthodromic and antidromic impulse propagation across the Mauthner cell initial segment
Brain Research, 190 (1980) 255-260 © Elsevier/North-Holland Biomedical Press
255
Differential properties of orthodromic and antidromic impulse propagation across the Mauthner cell initial segment
DONALD S. FABER and PAUL G. FUNCH Division of Neurobiology, Department of Physiology, State University of New York, Buffalo, N.Y. 14214 and Research Institute on Alcoholism, Buffalo, N. Y. 14203 (U.S.A.)
(Accepted January 10th, 1980) Key words: Mauthner cell - - goldfish -- initial segment -- axon hillock -- neuron modelling
Many vertebrate and invertebrate neurons have distinct cellular regions characterized by their low safety factor for impulse propagation2-4,12,14. Our electrophysiological investigations 7 of the goldfish Mauthner axon (M-axon) have led to observations relevant to spike initiation in the initial segment-axon hillock region and to the electrotonic couplings between the M-axon, initial segment, axon hillock, and soma. In particular, we report here that: (1) the initial segment-axon hillock region has a low safety factor for antidromic activation; and (2) there is a marked asymmetry in the coupling between this region and the axon, such that voltage signals are transmitted quite faithfully in the orthodromic direction, but are strongly attenuated in the antidromic direction. Thus, although there is a low safety factor for antidromic invasion of the initial segment and axon hillock, there is rather a high safety factor for orthodromic activation of the axon, due in part to a significant spread of synaptic current to that region. The experiments were performed in a manner identical to that described in the preceding paper 7, and unless otherwise noted, simultaneous intracellular recordings from different Mauthner cell (M-cell) regions were employed. Simultaneous recordings from the soma and M-axon demonstrated that with antidromic stimulation there is a delay of 0.06 to 0.10 msec between the axon spike and activation of the axon hillock, which results in distinct inflections on the antidromic action potential recorded in the axon (see Fig. 1B of ref. 7). In contrast, orthodromic action potentials recorded in the axon have a simpler waveform as there is minimal or no delay in propagation in this direction (compare Fig. 1C1 and C4 in ref. 7). Figure 1A illustrates the attenuation of the axon spike across the initial segment-axon hillock region. The soma was hyperpolarized in order to further delay or block antidromic invasion of the initial segment and axon hillock. In this case, the underlying axon spike observed in the soma (Fig. lAx) was less than 10 ~ of its amplitude recorded distally in the M-axon (Fig. 1Aa). Typically, there is a 5-10-fold attenuation of the axon spike recorded in the soma relative to its amplitude 0.4--0.6 mm
256
lmsec Fig. 1. Asymmetric coupling across the initial segment. A1-A3: characteristics of antidromic activation as recorded in the M-cell soma, axon cap, and axon, respectively. AI: hyperpolarizing current through the somatic electrode was used to delay or block invasion of the axon hillock. A2, Aa: during simultaneous recordings from the axon cap (2) and the axon 2022/~m distal to the axon hillock (3), invasion of the axon hillock occasionally failed. This axon hillock spike was rather large in the Maxon while, in contrast, the electrotonically conducted axon spike remaining in A1 after blockage of the axon hillock spike was significantly attenuated over the same distance. B: orthodromic spikes and EPSPs recorded in the soma (1) and in the axon (2). EPSP attenuation from the soma to the M-axon is much less than attenuation in the opposite direction of the axon spike (A). Calibrations in B apply to A as well. Superimposed traces in all records, which are from the same experiment. distally in the M - a x o n (see also ref. 9). On the other hand, both the initial segment-axon hillock spike 7 (compare Fig. 1A1 and A 3 ) a n d the eighth nerve-evoked EPSP (compare Fig. 1B1 and B2), that is, potentials generated proximal to the initial segment, show much less attenuation when recorded in the M-axon. On the basis o f both axon spike and EPSP measurements (8 experiments) the average coupling coefficients between the proximal 0.6 m m of the axon and the soma were 0.18 and 0.80 in the anti- and o r t h o d r o m i c directions, respectively. It should be noted that our analysis is consistent with previous conclusionsT, s that the major source o f action current for the large negative spike recorded in the axon cap (Fig. lAg,), and for the impulse recorded in the soma, is the axon hillock, and that the distal initial segment conducts decrementally, at best. Infrequent intracellular recordings from the initial segment which demonstrated no obvious signs o f significant injury, have revealed that the amplitude o f the isolated axon spike propagating in the rostral direction is greatly reduced. These observations indicate that the axon hillock m a y be a region o f low safety factor for antidromic spike invasion as a result o f the large attenuation o f the axon spike as it traverses the initial segment. In fact, as shown "n Fig. 2A1-A3, we have observed spontaneous failures o f the axon hillock spike during antidromic stimulation with essentially no change in threshold for o r t h o d r o m i c activation. Under these conditions the antidromic action potential recorded in the M - a x o n is composed o f only one major component, the axon spike, and lacks the axon hillock spike component. This is corroborated by the absence o f a large negative field potential in the extracellular
257
A
C
~i~i~. ~.~ "!$i
,~C)N
~
~ '
/
Ril
/
Ri2
u
/
",. Ro¢::
./\
6 J
Fig. 2. A model for the electrical couplings between M-cell regions. A: spontaneous failure of antidromic activation (first stimulus) of the axon hillock with normal orthodromic activation (second stimulus). A1 and Aa: respectively, simultaneously recorded extracellular fields in the axon cap and intracellular potentials from the axon at a distance of 1070 pm distal. The antidromic impulse consists of only the axon spike, which is seen as a small positive deflection in the axon cap. The orthodromic spike is full-sized due to the contribution of the axon hillock; the slight inflection on its rising phase is due to refractoriness of the M-axon following the preceding antidromic stimulation. The axon hillock spike is also signalled by the large negative extracellular field potential. As: the first derivative of As. B" intra-axonal recording of the orthodromic impulse at 2600 /~m distal to the axon hillock. Hyperpolarization of the M-axon during suprathreshold stimulation reveals several sharp spike components superimposed upon the axon hillock spike. The 4 superimposed traces were obtained with the same level of current injection. C: schematic electrical model explaining the asymmetric electrotonic couplings. Switches A, B and C induce current flow associated with the axon spike, the axon hillock spike, and the EPSP, respectively. The corresponding transmembrane potentials at the generator sites are: axon spike, 60 mV; axon hillock spike, 100 mV; EPSP, 20 mV. Abbreviations and resistance values: Rax, axonal membrane resistance (3 Mf~); Ril, initial segment longitudinal resistance (0.70 Mf~); Rls-ah, initial segment-axon hillock transmembrane resistance (2.0 Mr1); Rac, extracellular convergence resistance of the axon cap (0.09 Mf]); Rl 2, longitudinal resistance of axon hillock (0.045 Mf~); Rsa, lumped soma-dendritic transmembrane resistance (0.125 Mf~). Dashed line indicates the border of the axon cap.
r e c o r d (Fig. 2A1). T h e o r t h o d r o m i c a c t i o n p o t e n t i a l , b y c o m p a r i s o n , is essentially n o r m a l in a p p e a r a n c e , with a large negative extracellular field confirming the presence o f the a x o n hillock spike. T h e schematic m o d e l we p r o p o s e to e x p l a i n the a b o v e findings is shown in Fig. 2C. D u e to the very s h o r t time constants ( < 0.4 msec) exhibited b y the v a r i o u s regions o f the M-cell6,S, to, capacitances are n o t included. M o s t resistive values were experim e n t a l l y derived. First, the c o u p l i n g coefficients a n d the t r a n s f e r resistance (taken as 120 k f ] ; range 80-200 kf~) between the axon a n d s o m a were used to calculate a x o n a l resistance (Rax = 3.0 Mf~), the l u m p e d equivalent resistance o f the s o m a a n d dendrites a n d the initial s e g m e n t - a x o n hillock region (156 kf~) a n d the resistance c o u p l i n g the two regions (Ri 1 ---- 700 k ~ ) . A n i n d e p e n d e n t check on a x o n a l resistance was o b t a i n e d with successive transfer resistance m e a s u r e m e n t s between two indepen-
258 dent intra-axonal electrodes. The extrapolated input resistance, which should approach Rax/2 when measured far from the soma, ranged from 1.2 to 2.0 M ~) at about 1.5 space constants from the axon hillock 7. The model was then expanded to separate the passive soma-dendritic membranes from the electrically excitable initial segmentaxon hillock region, on the basis of the following considerations : (i) the axon hillock internal resistance (Ri2), which could be estimated by assuming a truncated right cone (height .... 30/~m, base radii := 6 and 30 #m, resistivity := 100 ~Q/cm), is small; (ii) since the ratio of the axon hillock spike recorded intracellularly at the soma and extracellularly in the axon cap was typically 1.25 (see ref. 8), the resistance of the axon cap (Rae) is close to that of the soma-dendritic membrane (Rs0); and (iii) since the extrinsic hyperpolarizing potential generated in the axon cap 9 is barely detectable with intracellular recordings from the axon hillock, the equivalent resistance of the initial segment-axon hillock membrane (Ris ah) is at least one order of magnitude greater than Rae or Rsd. The salient points of the model are that the M-axon membrane resistance is at least one order of magnitude greater than the equivalent resistance of the soma-dendritic and initial segment-axon hillock membranes and that the axon is coupled to these regions by the relatively high internal resistance of the initial segment. Therefore coupling is asymmetriO and favors orthodromic conduction. The values in Table 1 demonstrate that this simple model predicts all the major observations of antidromic and orthodromic activation of the M-cell. First, the large series resistance of the initial segment causes an 82 ~ attenuation of the antidromically activated axon spike from the M-axon to the initial segment-axon hillock region. Consequently, the latter region is depolarized by approximately 10.5 mV at the peak of the axon spike. This could account for the delay seen during antidromic invasion and for spontaneous failures. Second, the model predicts that a threshold EPSP, typically about 20 mV when recorded in the soma, would depolarize the excitable initial segment-axon hillock region by 18.5 mV. The discreTABLE I Experimental and predicted M-cell potential changes
All potentials are referenced with respect to an extracellular ground, with the exception of the transmembrane potentials calculated for the axon hillock. Source
Vuxon (m V)
V~oma (m V)
Predicted Vls An (m V) **
Experimental
Model
Experimental
Model
Intracellular
Extracellular
Axon spike 40-72 EPSP ('threshold) 12-22
60* 15.6
5-10 17 24
8.1 20*
11.0 19.3
0.5 (~ 1.0) 10.5 0.8 18.5
IS-AH spike
52
37-49
47
64
36 (20-40) 100"
~ 58
Transmembrane
* Indicates the amplitude assigned each source. ** Intracellular recordings from the axon hillock are uncommon; experimentally measured extracellular potentials are in parentheses.
259 pancy between these two apparent 'threshold' values reflects the distributed nature of the spike generating focus in the initial segment and axon hillock. Since the axon hillock seems to be the major source of current for the spike generated in that region 7,s, we have placed this locus near the border between the two by setting R t I ~ ~ R t 2. In fact, the experimentally determined value f o r Ri 1 is very close to that which would be calculated for an initial segment 12 # m in diameter and 75/zm in length s,la. Finally, the model also accounts for faithful transmission of EPSPs into the M-axon (22 ~ attenuation), which may contribute to securing orthodromic spike propagation. The transmembrane EPSP recorded in the axon is comparable to that at the axon hillock. Since the intracellularly recorded axon hillock spike elicited orthodromically is attenuated by only 19 ~ when recorded in the M-axon, the active sites in the axon are presumably brought to threshold by the additive effects of synaptic current and the action current generated by the axon hillock. This postulate is consistent with observations that axonal hyperpolarization either blocks orthodromic activation in an all or none fashion or, with higher stimulus strengths, reveals many small components superimposed on the axon hillock spike, as shown in Fig. 2B. These impulse components can be restored by increasing stimulus strength; therefore they presumably represent activation of distinct axonal sites (e.g. axon collateralsT). Thus, synaptic current can contribute to spike initiation in the axon. In cat spinal motoneurons, the transitions from the motor axon to the initial segment and to the soma-dendritic membranes are characterized by variable and low safety factors 2,4A1. For example, the safety factor is quite sensitive to shifts in temperature 11. Similarly, in cerebellar Purkinje cells the gradation from the dendrites to the initial segment membrane of susceptibility to barbiturate action correlates with differences in safety factor for antidromic invasion 3. We have observed a selective loss of the antidromically evoked axon hillock spike of the M-cell during perfusion with ethanol 5 and with small temperature changes. For example, the failure illustrated in Fig. 2A gradually developed during the experiment. When the water respirating the fish was cooled, antidromic invasion was restored within a few minutes. Thus, such regions of low safety factor can be sensitive indicators of small alterations in passive and/or excitable membrane properties, and the relatively simple model presented here for the M-cell may serve as a basis for analyzing the mechanisms underlying these changes. We wish to thank Dr. M. V. L. Bennett for his critical comments on the manuscript, J. Lakatos for preparation of the figures and J. Jordan for secretarial assistance. This research was supported in part by PHS Grants NS 12132 and NS 15335.
1 Bennett, M. V. L., Physiology of electrotonic junctions, Ann. N. Y. Acad. Sci., 137 (1966) 509-539. 2 Coombs, J. S., Curtis, D. R., and Eccles, J. C., The interpretation of spike potentials of motoneurones, J. Physiol. (Lond.), 139 (1957) 198-231. 3 Eccles, J. C., Faber, D. S. and T~tbofikov~t,H., The action of a parallel fiber volley on the antidromic invasion of Purkyne cells of cat cerebellum, Brain Research, 25 (1971) 335-356.
260 4 Eccles, J. C., Libet, B. and Young, R. R., The behaviour of chromatolysed motoneurones studied by intracellular recording, J. Physiol. (Lond.), 143 (1958) 11-40. 5 Faber, D. S. and Klee, M. R., Ethanol suppresses collateral inhibition of the goldfish Mauthner cell, Brain Research, 104 (1976) 347-353. 6 Fukami, Y., Furukawa, T. and Asada, Y., Excitability changes of the Mauthner cell during collateral inhibition, J. gen. PhysioL, 48 (1965) 581-600. 7 Funch, P. G. and Faber, D. S., Impulse propagation along a myelinated vertebrate axon lacking nodes of Ranvier, Brain Research, 190 (1980) 261-267. 8 Furshpan, E. J. and Furukawa, T., lntracellular and extracellular responses of the several regions of the Mauthner cell of the goldfish, J. NeurophysioL, 25 (1962) 732-771. 9 Furukawa, T. and Furshpan, E. J., Two inhibitory mechanisms in the Mauthner neurons of goldfish, J. Neurophysiol., 26 (1963) 140-176. 10 Greeff, K., Die Kabelkonstanten des Mauthner-axons, Experientia (BaseD, 33 (1977) 780. 11 Klee, M. R., Pierau, F.-K. and Faber, D. S., Temperature effects on resting potential and spike parameters of cat motoneurons, Exp. Brain Res., 19 (1974) 478-492. 12 Mellon, DEF., Jr. and Kaars, C., Role of regional cellular geometry in conduction of excitation along a sensory neuron, J. NeurophysioL, 37 (1974) 1228-1238. 13 Nakajima, Y. and Kohno, K., Fine structure of the Mauthner cell: synaptic topography and comparative study. In: D. S. Faber and H. Korn (Eds.), Neurobiology of the Mauthner Cell, Raven Press, New York, 1978, pp. 133-166. 14 Spira, M. E., Yarom, Y. and Parnas, 1., Modulation of spike frequency by regions of special axonal geometry and by synaptic inputs, J. NeurophysioL, 39 (I 976) 882-899. 15 Tauc, L., Site of origin and propagation of spike in the giant neuron of Aplysia, J. gen. Physiol., 45 (1962) 1077-1097.
255
Differential properties of orthodromic and antidromic impulse propagation across the Mauthner cell initial segment
DONALD S. FABER and PAUL G. FUNCH Division of Neurobiology, Department of Physiology, State University of New York, Buffalo, N.Y. 14214 and Research Institute on Alcoholism, Buffalo, N. Y. 14203 (U.S.A.)
(Accepted January 10th, 1980) Key words: Mauthner cell - - goldfish -- initial segment -- axon hillock -- neuron modelling
Many vertebrate and invertebrate neurons have distinct cellular regions characterized by their low safety factor for impulse propagation2-4,12,14. Our electrophysiological investigations 7 of the goldfish Mauthner axon (M-axon) have led to observations relevant to spike initiation in the initial segment-axon hillock region and to the electrotonic couplings between the M-axon, initial segment, axon hillock, and soma. In particular, we report here that: (1) the initial segment-axon hillock region has a low safety factor for antidromic activation; and (2) there is a marked asymmetry in the coupling between this region and the axon, such that voltage signals are transmitted quite faithfully in the orthodromic direction, but are strongly attenuated in the antidromic direction. Thus, although there is a low safety factor for antidromic invasion of the initial segment and axon hillock, there is rather a high safety factor for orthodromic activation of the axon, due in part to a significant spread of synaptic current to that region. The experiments were performed in a manner identical to that described in the preceding paper 7, and unless otherwise noted, simultaneous intracellular recordings from different Mauthner cell (M-cell) regions were employed. Simultaneous recordings from the soma and M-axon demonstrated that with antidromic stimulation there is a delay of 0.06 to 0.10 msec between the axon spike and activation of the axon hillock, which results in distinct inflections on the antidromic action potential recorded in the axon (see Fig. 1B of ref. 7). In contrast, orthodromic action potentials recorded in the axon have a simpler waveform as there is minimal or no delay in propagation in this direction (compare Fig. 1C1 and C4 in ref. 7). Figure 1A illustrates the attenuation of the axon spike across the initial segment-axon hillock region. The soma was hyperpolarized in order to further delay or block antidromic invasion of the initial segment and axon hillock. In this case, the underlying axon spike observed in the soma (Fig. lAx) was less than 10 ~ of its amplitude recorded distally in the M-axon (Fig. 1Aa). Typically, there is a 5-10-fold attenuation of the axon spike recorded in the soma relative to its amplitude 0.4--0.6 mm
256
lmsec Fig. 1. Asymmetric coupling across the initial segment. A1-A3: characteristics of antidromic activation as recorded in the M-cell soma, axon cap, and axon, respectively. AI: hyperpolarizing current through the somatic electrode was used to delay or block invasion of the axon hillock. A2, Aa: during simultaneous recordings from the axon cap (2) and the axon 2022/~m distal to the axon hillock (3), invasion of the axon hillock occasionally failed. This axon hillock spike was rather large in the Maxon while, in contrast, the electrotonically conducted axon spike remaining in A1 after blockage of the axon hillock spike was significantly attenuated over the same distance. B: orthodromic spikes and EPSPs recorded in the soma (1) and in the axon (2). EPSP attenuation from the soma to the M-axon is much less than attenuation in the opposite direction of the axon spike (A). Calibrations in B apply to A as well. Superimposed traces in all records, which are from the same experiment. distally in the M - a x o n (see also ref. 9). On the other hand, both the initial segment-axon hillock spike 7 (compare Fig. 1A1 and A 3 ) a n d the eighth nerve-evoked EPSP (compare Fig. 1B1 and B2), that is, potentials generated proximal to the initial segment, show much less attenuation when recorded in the M-axon. On the basis o f both axon spike and EPSP measurements (8 experiments) the average coupling coefficients between the proximal 0.6 m m of the axon and the soma were 0.18 and 0.80 in the anti- and o r t h o d r o m i c directions, respectively. It should be noted that our analysis is consistent with previous conclusionsT, s that the major source o f action current for the large negative spike recorded in the axon cap (Fig. lAg,), and for the impulse recorded in the soma, is the axon hillock, and that the distal initial segment conducts decrementally, at best. Infrequent intracellular recordings from the initial segment which demonstrated no obvious signs o f significant injury, have revealed that the amplitude o f the isolated axon spike propagating in the rostral direction is greatly reduced. These observations indicate that the axon hillock m a y be a region o f low safety factor for antidromic spike invasion as a result o f the large attenuation o f the axon spike as it traverses the initial segment. In fact, as shown "n Fig. 2A1-A3, we have observed spontaneous failures o f the axon hillock spike during antidromic stimulation with essentially no change in threshold for o r t h o d r o m i c activation. Under these conditions the antidromic action potential recorded in the M - a x o n is composed o f only one major component, the axon spike, and lacks the axon hillock spike component. This is corroborated by the absence o f a large negative field potential in the extracellular
257
A
C
~i~i~. ~.~ "!$i
,~C)N
~
~ '
/
Ril
/
Ri2
u
/
",. Ro¢::
./\
6 J
Fig. 2. A model for the electrical couplings between M-cell regions. A: spontaneous failure of antidromic activation (first stimulus) of the axon hillock with normal orthodromic activation (second stimulus). A1 and Aa: respectively, simultaneously recorded extracellular fields in the axon cap and intracellular potentials from the axon at a distance of 1070 pm distal. The antidromic impulse consists of only the axon spike, which is seen as a small positive deflection in the axon cap. The orthodromic spike is full-sized due to the contribution of the axon hillock; the slight inflection on its rising phase is due to refractoriness of the M-axon following the preceding antidromic stimulation. The axon hillock spike is also signalled by the large negative extracellular field potential. As: the first derivative of As. B" intra-axonal recording of the orthodromic impulse at 2600 /~m distal to the axon hillock. Hyperpolarization of the M-axon during suprathreshold stimulation reveals several sharp spike components superimposed upon the axon hillock spike. The 4 superimposed traces were obtained with the same level of current injection. C: schematic electrical model explaining the asymmetric electrotonic couplings. Switches A, B and C induce current flow associated with the axon spike, the axon hillock spike, and the EPSP, respectively. The corresponding transmembrane potentials at the generator sites are: axon spike, 60 mV; axon hillock spike, 100 mV; EPSP, 20 mV. Abbreviations and resistance values: Rax, axonal membrane resistance (3 Mf~); Ril, initial segment longitudinal resistance (0.70 Mf~); Rls-ah, initial segment-axon hillock transmembrane resistance (2.0 Mr1); Rac, extracellular convergence resistance of the axon cap (0.09 Mf]); Rl 2, longitudinal resistance of axon hillock (0.045 Mf~); Rsa, lumped soma-dendritic transmembrane resistance (0.125 Mf~). Dashed line indicates the border of the axon cap.
r e c o r d (Fig. 2A1). T h e o r t h o d r o m i c a c t i o n p o t e n t i a l , b y c o m p a r i s o n , is essentially n o r m a l in a p p e a r a n c e , with a large negative extracellular field confirming the presence o f the a x o n hillock spike. T h e schematic m o d e l we p r o p o s e to e x p l a i n the a b o v e findings is shown in Fig. 2C. D u e to the very s h o r t time constants ( < 0.4 msec) exhibited b y the v a r i o u s regions o f the M-cell6,S, to, capacitances are n o t included. M o s t resistive values were experim e n t a l l y derived. First, the c o u p l i n g coefficients a n d the t r a n s f e r resistance (taken as 120 k f ] ; range 80-200 kf~) between the axon a n d s o m a were used to calculate a x o n a l resistance (Rax = 3.0 Mf~), the l u m p e d equivalent resistance o f the s o m a a n d dendrites a n d the initial s e g m e n t - a x o n hillock region (156 kf~) a n d the resistance c o u p l i n g the two regions (Ri 1 ---- 700 k ~ ) . A n i n d e p e n d e n t check on a x o n a l resistance was o b t a i n e d with successive transfer resistance m e a s u r e m e n t s between two indepen-
258 dent intra-axonal electrodes. The extrapolated input resistance, which should approach Rax/2 when measured far from the soma, ranged from 1.2 to 2.0 M ~) at about 1.5 space constants from the axon hillock 7. The model was then expanded to separate the passive soma-dendritic membranes from the electrically excitable initial segmentaxon hillock region, on the basis of the following considerations : (i) the axon hillock internal resistance (Ri2), which could be estimated by assuming a truncated right cone (height .... 30/~m, base radii := 6 and 30 #m, resistivity := 100 ~Q/cm), is small; (ii) since the ratio of the axon hillock spike recorded intracellularly at the soma and extracellularly in the axon cap was typically 1.25 (see ref. 8), the resistance of the axon cap (Rae) is close to that of the soma-dendritic membrane (Rs0); and (iii) since the extrinsic hyperpolarizing potential generated in the axon cap 9 is barely detectable with intracellular recordings from the axon hillock, the equivalent resistance of the initial segment-axon hillock membrane (Ris ah) is at least one order of magnitude greater than Rae or Rsd. The salient points of the model are that the M-axon membrane resistance is at least one order of magnitude greater than the equivalent resistance of the soma-dendritic and initial segment-axon hillock membranes and that the axon is coupled to these regions by the relatively high internal resistance of the initial segment. Therefore coupling is asymmetriO and favors orthodromic conduction. The values in Table 1 demonstrate that this simple model predicts all the major observations of antidromic and orthodromic activation of the M-cell. First, the large series resistance of the initial segment causes an 82 ~ attenuation of the antidromically activated axon spike from the M-axon to the initial segment-axon hillock region. Consequently, the latter region is depolarized by approximately 10.5 mV at the peak of the axon spike. This could account for the delay seen during antidromic invasion and for spontaneous failures. Second, the model predicts that a threshold EPSP, typically about 20 mV when recorded in the soma, would depolarize the excitable initial segment-axon hillock region by 18.5 mV. The discreTABLE I Experimental and predicted M-cell potential changes
All potentials are referenced with respect to an extracellular ground, with the exception of the transmembrane potentials calculated for the axon hillock. Source
Vuxon (m V)
V~oma (m V)
Predicted Vls An (m V) **
Experimental
Model
Experimental
Model
Intracellular
Extracellular
Axon spike 40-72 EPSP ('threshold) 12-22
60* 15.6
5-10 17 24
8.1 20*
11.0 19.3
0.5 (~ 1.0) 10.5 0.8 18.5
IS-AH spike
52
37-49
47
64
36 (20-40) 100"
~ 58
Transmembrane
* Indicates the amplitude assigned each source. ** Intracellular recordings from the axon hillock are uncommon; experimentally measured extracellular potentials are in parentheses.
259 pancy between these two apparent 'threshold' values reflects the distributed nature of the spike generating focus in the initial segment and axon hillock. Since the axon hillock seems to be the major source of current for the spike generated in that region 7,s, we have placed this locus near the border between the two by setting R t I ~ ~ R t 2. In fact, the experimentally determined value f o r Ri 1 is very close to that which would be calculated for an initial segment 12 # m in diameter and 75/zm in length s,la. Finally, the model also accounts for faithful transmission of EPSPs into the M-axon (22 ~ attenuation), which may contribute to securing orthodromic spike propagation. The transmembrane EPSP recorded in the axon is comparable to that at the axon hillock. Since the intracellularly recorded axon hillock spike elicited orthodromically is attenuated by only 19 ~ when recorded in the M-axon, the active sites in the axon are presumably brought to threshold by the additive effects of synaptic current and the action current generated by the axon hillock. This postulate is consistent with observations that axonal hyperpolarization either blocks orthodromic activation in an all or none fashion or, with higher stimulus strengths, reveals many small components superimposed on the axon hillock spike, as shown in Fig. 2B. These impulse components can be restored by increasing stimulus strength; therefore they presumably represent activation of distinct axonal sites (e.g. axon collateralsT). Thus, synaptic current can contribute to spike initiation in the axon. In cat spinal motoneurons, the transitions from the motor axon to the initial segment and to the soma-dendritic membranes are characterized by variable and low safety factors 2,4A1. For example, the safety factor is quite sensitive to shifts in temperature 11. Similarly, in cerebellar Purkinje cells the gradation from the dendrites to the initial segment membrane of susceptibility to barbiturate action correlates with differences in safety factor for antidromic invasion 3. We have observed a selective loss of the antidromically evoked axon hillock spike of the M-cell during perfusion with ethanol 5 and with small temperature changes. For example, the failure illustrated in Fig. 2A gradually developed during the experiment. When the water respirating the fish was cooled, antidromic invasion was restored within a few minutes. Thus, such regions of low safety factor can be sensitive indicators of small alterations in passive and/or excitable membrane properties, and the relatively simple model presented here for the M-cell may serve as a basis for analyzing the mechanisms underlying these changes. We wish to thank Dr. M. V. L. Bennett for his critical comments on the manuscript, J. Lakatos for preparation of the figures and J. Jordan for secretarial assistance. This research was supported in part by PHS Grants NS 12132 and NS 15335.
1 Bennett, M. V. L., Physiology of electrotonic junctions, Ann. N. Y. Acad. Sci., 137 (1966) 509-539. 2 Coombs, J. S., Curtis, D. R., and Eccles, J. C., The interpretation of spike potentials of motoneurones, J. Physiol. (Lond.), 139 (1957) 198-231. 3 Eccles, J. C., Faber, D. S. and T~tbofikov~t,H., The action of a parallel fiber volley on the antidromic invasion of Purkyne cells of cat cerebellum, Brain Research, 25 (1971) 335-356.
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