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Mammalian cold receptor afferents: role of an electrogenic sodium pump in sensory transduction
156
Brain Research, 73 (1974) 156-160 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Mammalian cold receptor afferents: role of an electrogenic sodium pump in sensory transductiort
FRIEDRICH-KARL PIERAU*, PETER TORREY AND DAVID O. CARPENTER Neurobiology Department, Armed Forces Radiobiology Research Institute, Bethesda, Md. 20014 (U.S.A.)
(Accepted February 28th, 1974)
The afferent discharge characteristic of specific peripheral cold receptors responding to temperature changes is well known from experiments in a variety of mammals and man~,6, 8. However, the transduction process which underlies this temperature-dependent discharge is unknown. Since specific thermoreceptors are small in diameter and difficult to study, mechanoreceptors have been studied as models of thermoreceptors. Some mechanoreceptors have a fairly high temperature sensitivity. In Pacinian corpuscles, the generator potential increases with rising temperature and, consequently, there is a greater discharge rate at higher temperatures 11. However, the discharge frequency o f cat muscle spindle afferents increases with cooling 10, supposedly due to a depolarizing effect of the fall in temperature at the terminals. But none of these experiments provides an understanding of the generator potential mechanism in specific or even nonspecific temperature sensitive receptors. Recent studies of neurons which are not primary temperature receptors have demonstrated mechanisms which might explain the temperature sensitivity of specific thermoreceptors. In Aplysia neurons, which have a very high specific membrane resistance, a ouabain-sensitive electrogenic sodium pump contributes to membrane potential and tends to cause an increase in excitability on cooling as a result of the high temperature dependence of this active transport process 2,4. This depolarizing effect of cooling is opposed by a greater temperature coefficient of the permeability of sodium (P~a +) than potassium (PK+), which tends to hyperpolarize and consequently to decrease the excitability at lower temperatures. However, in mammalian spinal motor neurons 9, a greater temperature coefficient of PK* causes a depolarization during cooling which is accompanied by an increase in membrane resistance. As a result, these cells are more excitable at cold temperatures. Thus, these previous studies suggest two possible mechanisms for cold transduction (operation of an electrogenic sodium pump or a greater temperature dependence of PK + than P~a+ ) and one for warm transduction (a greater temperature dependence of Pl~a+ than Pg+).
* Permanent address: Kerckhoff Institut der Max-Planck-Gesellschaft, 635 Bad Nauheim, G.F.R.
157
SHORT COMMUNICATIONS
We have attempted to evaluate whether an electrogenic sodium pump could be the mechanism determining the excitability of mammalian cold sensitive primary afferent fibers by a local application of ouabain into the region of the afferent terminal. Ouabain is an inhibitor of sodium-potassium adenosine triphosphatase, the enzyme thought to be responsible for the operation of the sodium transport process 1, and is a relatively specific blocking drug for electrogenic pumps in other preparations 2,4. The effects of local ouabain infiltration were tested on temperature-dependent activity of afferent fibers isolated from the pudendal nerve of rats. This nerve contains a large number of fibers conducting action potentials from specific cold and warm receptors and from temperature-sensitive mechanoreceptors in the scrotal skinL The temperature of the scrotal skin was controlled using a U-shaped metal thermode containing circulating water at different temperatures and placed under the ventral surface of the scrotum. A small thermocouple at the surface of the thermode measured temperature between the thermode and the scrotal skin. A controlled heating pad kept body temperature constant at 38 °C. Small nerve filaments were isolated from the pudendal nerve of rats anesthetized with Equi-Thesin (initial dose, 3.5 ml/kg). The afferent spike potentials were recorded in a conventional manner with a single platinum wire and using a window discriminator to separate spikes in multifiber preparations. The output of the window discriminator triggered a spike generator to produce a pulse which was recorded on a pen recorder along with temperature. After isolation the afferent fiber was studied for 40-60 rain by alternating the temperature of the fluid bathing the metal thermode every 3-4 min between cold (2224 °C) and warm (37-38 °C) temperatures. In agreement with Witt and Hense112 we observed that the fiber discharge usually achieved a steady frequency within 2 min after a temperature change. In most of the multifiber preparations, all of the mechanoreceptors present had their receptive fields in the same general skin area. We assumed 15
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OUABAIN
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e---e ~
22 ° C 38 ° C
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Fig. 1. Effect of ouabain infiltration on the static discharge of a single cold-sensitive mechanoreceptor of the rat scrotal skin at 22 °C (filled circles) and 38 °C (open circles). Temperature was alternated between the two extremes every 3 rain. Every point represents the average impulse frequency over a 30-sec period beginning 2.5 min after the onset of the temperature change.
158 A CONTROL COOLING
SHORT COMMUNICATIONS
. . . . . . . . . 380CI .
.
.
.
.
1_ _2~[~ilLill~flll
~l~li!!!
l!!! tlll!!!!l!l!!!i!!!l~_lllll[
.
23°C
!L!HILl!~ilL illiLM L~ l ~[llLl!l LLI l!!!i.!i!i!ill!!!!!!! !!1!1!!!!!! llll!!!!!!!!! lillll_l!!!!!!!!l!! ~[~ Hi!!!!!!!!!!!!!!!~L~11IH~
B
15 min AFTER 36°C I OUABAIN
23°C
C CONTROL 38°CI WARMING 23oCL_
D 18 min AFTER38°CI OUABAIN 230CL. .
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__
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Fig. 2. The effect of ouabain infiltration on the temperature responses of a cold afferent fiber from rat scrotal skin. Upper trace: fiber discharge; middle trace: temperature recorded at the thermode-skin interface; lower trace: time marker, 1 sec. Notice the different time scales in A and B as compared to C and D. that a similar general relationship holds for receptive fields of cold- and mechanoreceptors for the purpose of directing the ouabain application to the receptive field of nonmechanically activated afferents. Mechanical sensitivity was not re-tested after placement of the thermode in order to assure reproducibility of the thermal stimulus. For each fiber the effect of local injection was tested by subcutaneous administration of mammalian Ringer's solution into the receptor region. If this did not change the discharge pattern, 0.5 ml of a 10-a to 10-SM solution of ouabain at pH 7.4 was injected at the same site and the temperature sensitivity measured with time. A total of 36 fibers was examined. Because of the uncertainty that the ouabain reached the receptor terminals, this report considers only those fibers which showed a response to ouabain (5 cold fibers and 11 temperature-sensitive mechanoreceptors). Ouabain caused a dramatic increase in the static discharge frequencies of both coldand mechano-sensitive fibers at near body temperature but usually did not change discharge by any significant amount at lower temperatures (22-25 °C). Fig. 1 illustrates the effect of ouabain on the static discharge frequency of a typical temperature-sensitive mechanoreceptor. The fiber was silent at near body temperature, but had a slow discharge at 22 °C. The control injection of saline had no influence on the discharge rate at either temperature. After ouabain, this receptor showed a high static discharge rate at 38 °C with a peak of 11 imp./sec at about 8 min after the ouabain infiltration, whereas the static discharge rate at 22 °C remained relatively unchanged. Fig. 2 shows actual records of a similar experiment on a cold fiber which could
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159
not be excited by mechanical stimuli and is therefore presumed to be a specific thermoreceptor. This fiber, as the one illustrated in Fig. 1, was silent at body temperature, but discharged at a frequency o f 1-2 imp./sec at 24 °C. After ouabain, the fiber discharges at body temperature at a frequency at least as great as in the cold, while discharge at 24 °C is relatively unchanged. While there is no clear dynamic response to the relatively slow temperature change in the control cooling, surprisingly such a response does appear after ouabain. These results suggest that an electrogenic sodium pump is the mechanism responsible for the temperature sensitivity of both specific cold receptors and mechanoreceptors. We propose, as has been shown for other preparations 4, that the sodium pump in these afferent terminals has an unequal coupling of active sodium eiflux and potassium influx and that the excess sodium constitutes a net outward positive current which, at higher temperatures, tends to hyperpolarize and decrease the excitability of the afferent terminals. The change of discharge frequency at warm temperatures after ouabain reflects the diminution of this hyperpolarizing force resulting from inhibition of the electrogenic sodium pump. Since sodium transport is very temperature-dependent (Q10 o f at least 2.3, see ref. 1), the lack of ouabain sensitivity in the cold is best explained by a very low rate of pump activity at low temperatures, leaving terminal excitability determined essentially by the passive membrane mechanisms 2,4. In those fibers which were affected by ouabain, the effects were reversible with time and reproducible on multiple application and were consistent with observations made previously on molluscan neurons showing a depolarization at warm temperatures after ouabain2, 4. However, application ofouabain by a local infiltration into the peripheral receptive field is clearly not a totally satisfactory technique, since one can never be sure whether any or what concentration of ouabain reaches the terminals. In a number o f preparations we observed no effect of ouabain and have presumed this was because of misplacement o f the injection and lack of a sufficient ouabain concentration at the terminals to inhibit the pump. In other preparations, such as in Fig. 2, the inhibition of the pump was probably incomplete, since the discharge in the warm after ouabain did not exceed that in the cold and there is now a dynamic response on cooling. In Aplysia neurons which have temperature-dependent endogenous discharge very similar to these cold receptors, application of ouabain does not affect frequency at low temperatures but there is a progressive increase with increasing temperature and an abolition o f the transient increase in discharge on cooling 3. Because of these uncertainties about the degree of pump inhibition it is not possible to interpret absolute dynamic or static frequencies except to note changes after ouabain and on recovery from its effects. It is apparent that an electrogenic sodium pump is not the only mechanism imparting thermosensitivity to these afferent fibers, since, as is shown in Fig. 1, the receptor after ouabain changes from being cold-sensitive to being warm-sensitive. In Aplysia neurons a similar depolarization after ouabain results from a temperaturedependent increase of the PNa+/Px+ ratio which is normally present but is at least partially obscured by the opposed electrogenic pump 2. It is not possible to study the ionic mechanism o f this warm response in the intact rat. However, our experiments
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support the hypothesis that the primary generator potential mechanism causing a t h e r m o s e n s i t i v e afferent to r e s p o n d to c o l d is t h e o p e r a t i o n o f an e l e c t r o g e n i c s o d i u m p u m p w h i c h causes t h e t e r m i n a l s to be d e p o l a r i z e d o n c o o l i n g a n d h y p e r p o l a r i z e d a n d i n h i b i t e d on w a r m i n g . t ALBERS,R. W., Biochemical aspects of active transport, Ann. Rev. Biochem., 36 (1967) 727-756. 2 CARPENTER, D. O., Membrane potential produced directly by the Na + pump in Aplysia neurons, Comp. Biochem. Physiol., 35 (1970) 371-385. 3 CARPENTER, D. O., Ionic mechanisms and models of endogenous discharge of Aplysia neurons. In J. SAL.~NKI(Ed.), Neurobiology of Invertebrates: Mechanisms of Rhythm Regulation, Proc. Syrup. Tihany (Hungary), Akad6miai Kiad6, Budapest, 1973, pp. 35-58. 4 CARPENTER, O. O., AND ALVING, B. O., A contribution of an electrogenic Na + pump to membrane potential of Aplysia neurons, J. gen. Physiol., 52 (1968) 1-21. 5 HENSEL,H., AND BOMAN,K. K. A., Afferent impulses in cutaneous sensory nerves in human subjects, J. NeurophysioL, 23 (1960) 564-578. 6 HENSEL, H., UND ZOTTERMAN, Y., Quantitative Beziehungen zwischen der Entladung einzelner K~iltefasern und der Temperatur, Acta physiol, scand., 23 (1951) 291-319. 7 IGGO, A., Cutaneous thermoreceptors in primates and sub-primates, J. Physiol. (Lond.), 200 (1969) 403~430. 8 IGGO,A., The mechanism of biological temperature reception. In J. O. HARDY, H. P. GAGGEAND J. A. J. STOLWUK (Eds.), Physiological and Behavioral Thermoregulation, Thomas, Springfield, Ill., 1970, pp. 391~07. 9 KEEL, M. R., PIERAU, FR.-K., AND FABER, D., Temperature eff-cts on resting potential and spike parameters of cat motoneurons, Exp. Brain Res., in press. 10 LIPPOLD, O. C. J., NICHOLS, J. G., AND REDFEARN, J. W. T., A study of the afferent discharge produced by cooling a mammalian muscle spindle, J. Physiol. (Lond.), 153 (1960) 218-231. 11 LOEWENSTEJN,W. R., On the 'specificity' of a sensory receptor, J. Neurophysiol., 24 (1961) 150158. 12 WITT, I., UND HENSEL, H., Afferente Impulse aus der Extremitatenhaut der Katze bei thermischer und mechanischer Reizung, Pfliigers Arch. ges. Physiol., 268 (1959) 582-596.
Brain Research, 73 (1974) 156-160 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Mammalian cold receptor afferents: role of an electrogenic sodium pump in sensory transductiort
FRIEDRICH-KARL PIERAU*, PETER TORREY AND DAVID O. CARPENTER Neurobiology Department, Armed Forces Radiobiology Research Institute, Bethesda, Md. 20014 (U.S.A.)
(Accepted February 28th, 1974)
The afferent discharge characteristic of specific peripheral cold receptors responding to temperature changes is well known from experiments in a variety of mammals and man~,6, 8. However, the transduction process which underlies this temperature-dependent discharge is unknown. Since specific thermoreceptors are small in diameter and difficult to study, mechanoreceptors have been studied as models of thermoreceptors. Some mechanoreceptors have a fairly high temperature sensitivity. In Pacinian corpuscles, the generator potential increases with rising temperature and, consequently, there is a greater discharge rate at higher temperatures 11. However, the discharge frequency o f cat muscle spindle afferents increases with cooling 10, supposedly due to a depolarizing effect of the fall in temperature at the terminals. But none of these experiments provides an understanding of the generator potential mechanism in specific or even nonspecific temperature sensitive receptors. Recent studies of neurons which are not primary temperature receptors have demonstrated mechanisms which might explain the temperature sensitivity of specific thermoreceptors. In Aplysia neurons, which have a very high specific membrane resistance, a ouabain-sensitive electrogenic sodium pump contributes to membrane potential and tends to cause an increase in excitability on cooling as a result of the high temperature dependence of this active transport process 2,4. This depolarizing effect of cooling is opposed by a greater temperature coefficient of the permeability of sodium (P~a +) than potassium (PK+), which tends to hyperpolarize and consequently to decrease the excitability at lower temperatures. However, in mammalian spinal motor neurons 9, a greater temperature coefficient of PK* causes a depolarization during cooling which is accompanied by an increase in membrane resistance. As a result, these cells are more excitable at cold temperatures. Thus, these previous studies suggest two possible mechanisms for cold transduction (operation of an electrogenic sodium pump or a greater temperature dependence of PK + than P~a+ ) and one for warm transduction (a greater temperature dependence of Pl~a+ than Pg+).
* Permanent address: Kerckhoff Institut der Max-Planck-Gesellschaft, 635 Bad Nauheim, G.F.R.
157
SHORT COMMUNICATIONS
We have attempted to evaluate whether an electrogenic sodium pump could be the mechanism determining the excitability of mammalian cold sensitive primary afferent fibers by a local application of ouabain into the region of the afferent terminal. Ouabain is an inhibitor of sodium-potassium adenosine triphosphatase, the enzyme thought to be responsible for the operation of the sodium transport process 1, and is a relatively specific blocking drug for electrogenic pumps in other preparations 2,4. The effects of local ouabain infiltration were tested on temperature-dependent activity of afferent fibers isolated from the pudendal nerve of rats. This nerve contains a large number of fibers conducting action potentials from specific cold and warm receptors and from temperature-sensitive mechanoreceptors in the scrotal skinL The temperature of the scrotal skin was controlled using a U-shaped metal thermode containing circulating water at different temperatures and placed under the ventral surface of the scrotum. A small thermocouple at the surface of the thermode measured temperature between the thermode and the scrotal skin. A controlled heating pad kept body temperature constant at 38 °C. Small nerve filaments were isolated from the pudendal nerve of rats anesthetized with Equi-Thesin (initial dose, 3.5 ml/kg). The afferent spike potentials were recorded in a conventional manner with a single platinum wire and using a window discriminator to separate spikes in multifiber preparations. The output of the window discriminator triggered a spike generator to produce a pulse which was recorded on a pen recorder along with temperature. After isolation the afferent fiber was studied for 40-60 rain by alternating the temperature of the fluid bathing the metal thermode every 3-4 min between cold (2224 °C) and warm (37-38 °C) temperatures. In agreement with Witt and Hense112 we observed that the fiber discharge usually achieved a steady frequency within 2 min after a temperature change. In most of the multifiber preparations, all of the mechanoreceptors present had their receptive fields in the same general skin area. We assumed 15
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e---e ~
22 ° C 38 ° C
,-1
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80 TIME
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(mlnutea)
Fig. 1. Effect of ouabain infiltration on the static discharge of a single cold-sensitive mechanoreceptor of the rat scrotal skin at 22 °C (filled circles) and 38 °C (open circles). Temperature was alternated between the two extremes every 3 rain. Every point represents the average impulse frequency over a 30-sec period beginning 2.5 min after the onset of the temperature change.
158 A CONTROL COOLING
SHORT COMMUNICATIONS
. . . . . . . . . 380CI .
.
.
.
.
1_ _2~[~ilLill~flll
~l~li!!!
l!!! tlll!!!!l!l!!!i!!!l~_lllll[
.
23°C
!L!HILl!~ilL illiLM L~ l ~[llLl!l LLI l!!!i.!i!i!ill!!!!!!! !!1!1!!!!!! llll!!!!!!!!! lillll_l!!!!!!!!l!! ~[~ Hi!!!!!!!!!!!!!!!~L~11IH~
B
15 min AFTER 36°C I OUABAIN
23°C
C CONTROL 38°CI WARMING 23oCL_
D 18 min AFTER38°CI OUABAIN 230CL. .
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__
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Fig. 2. The effect of ouabain infiltration on the temperature responses of a cold afferent fiber from rat scrotal skin. Upper trace: fiber discharge; middle trace: temperature recorded at the thermode-skin interface; lower trace: time marker, 1 sec. Notice the different time scales in A and B as compared to C and D. that a similar general relationship holds for receptive fields of cold- and mechanoreceptors for the purpose of directing the ouabain application to the receptive field of nonmechanically activated afferents. Mechanical sensitivity was not re-tested after placement of the thermode in order to assure reproducibility of the thermal stimulus. For each fiber the effect of local injection was tested by subcutaneous administration of mammalian Ringer's solution into the receptor region. If this did not change the discharge pattern, 0.5 ml of a 10-a to 10-SM solution of ouabain at pH 7.4 was injected at the same site and the temperature sensitivity measured with time. A total of 36 fibers was examined. Because of the uncertainty that the ouabain reached the receptor terminals, this report considers only those fibers which showed a response to ouabain (5 cold fibers and 11 temperature-sensitive mechanoreceptors). Ouabain caused a dramatic increase in the static discharge frequencies of both coldand mechano-sensitive fibers at near body temperature but usually did not change discharge by any significant amount at lower temperatures (22-25 °C). Fig. 1 illustrates the effect of ouabain on the static discharge frequency of a typical temperature-sensitive mechanoreceptor. The fiber was silent at near body temperature, but had a slow discharge at 22 °C. The control injection of saline had no influence on the discharge rate at either temperature. After ouabain, this receptor showed a high static discharge rate at 38 °C with a peak of 11 imp./sec at about 8 min after the ouabain infiltration, whereas the static discharge rate at 22 °C remained relatively unchanged. Fig. 2 shows actual records of a similar experiment on a cold fiber which could
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not be excited by mechanical stimuli and is therefore presumed to be a specific thermoreceptor. This fiber, as the one illustrated in Fig. 1, was silent at body temperature, but discharged at a frequency o f 1-2 imp./sec at 24 °C. After ouabain, the fiber discharges at body temperature at a frequency at least as great as in the cold, while discharge at 24 °C is relatively unchanged. While there is no clear dynamic response to the relatively slow temperature change in the control cooling, surprisingly such a response does appear after ouabain. These results suggest that an electrogenic sodium pump is the mechanism responsible for the temperature sensitivity of both specific cold receptors and mechanoreceptors. We propose, as has been shown for other preparations 4, that the sodium pump in these afferent terminals has an unequal coupling of active sodium eiflux and potassium influx and that the excess sodium constitutes a net outward positive current which, at higher temperatures, tends to hyperpolarize and decrease the excitability of the afferent terminals. The change of discharge frequency at warm temperatures after ouabain reflects the diminution of this hyperpolarizing force resulting from inhibition of the electrogenic sodium pump. Since sodium transport is very temperature-dependent (Q10 o f at least 2.3, see ref. 1), the lack of ouabain sensitivity in the cold is best explained by a very low rate of pump activity at low temperatures, leaving terminal excitability determined essentially by the passive membrane mechanisms 2,4. In those fibers which were affected by ouabain, the effects were reversible with time and reproducible on multiple application and were consistent with observations made previously on molluscan neurons showing a depolarization at warm temperatures after ouabain2, 4. However, application ofouabain by a local infiltration into the peripheral receptive field is clearly not a totally satisfactory technique, since one can never be sure whether any or what concentration of ouabain reaches the terminals. In a number o f preparations we observed no effect of ouabain and have presumed this was because of misplacement o f the injection and lack of a sufficient ouabain concentration at the terminals to inhibit the pump. In other preparations, such as in Fig. 2, the inhibition of the pump was probably incomplete, since the discharge in the warm after ouabain did not exceed that in the cold and there is now a dynamic response on cooling. In Aplysia neurons which have temperature-dependent endogenous discharge very similar to these cold receptors, application of ouabain does not affect frequency at low temperatures but there is a progressive increase with increasing temperature and an abolition o f the transient increase in discharge on cooling 3. Because of these uncertainties about the degree of pump inhibition it is not possible to interpret absolute dynamic or static frequencies except to note changes after ouabain and on recovery from its effects. It is apparent that an electrogenic sodium pump is not the only mechanism imparting thermosensitivity to these afferent fibers, since, as is shown in Fig. 1, the receptor after ouabain changes from being cold-sensitive to being warm-sensitive. In Aplysia neurons a similar depolarization after ouabain results from a temperaturedependent increase of the PNa+/Px+ ratio which is normally present but is at least partially obscured by the opposed electrogenic pump 2. It is not possible to study the ionic mechanism o f this warm response in the intact rat. However, our experiments
160
SHORT COMMUNICATIONS
support the hypothesis that the primary generator potential mechanism causing a t h e r m o s e n s i t i v e afferent to r e s p o n d to c o l d is t h e o p e r a t i o n o f an e l e c t r o g e n i c s o d i u m p u m p w h i c h causes t h e t e r m i n a l s to be d e p o l a r i z e d o n c o o l i n g a n d h y p e r p o l a r i z e d a n d i n h i b i t e d on w a r m i n g . t ALBERS,R. W., Biochemical aspects of active transport, Ann. Rev. Biochem., 36 (1967) 727-756. 2 CARPENTER, D. O., Membrane potential produced directly by the Na + pump in Aplysia neurons, Comp. Biochem. Physiol., 35 (1970) 371-385. 3 CARPENTER, D. O., Ionic mechanisms and models of endogenous discharge of Aplysia neurons. In J. SAL.~NKI(Ed.), Neurobiology of Invertebrates: Mechanisms of Rhythm Regulation, Proc. Syrup. Tihany (Hungary), Akad6miai Kiad6, Budapest, 1973, pp. 35-58. 4 CARPENTER, O. O., AND ALVING, B. O., A contribution of an electrogenic Na + pump to membrane potential of Aplysia neurons, J. gen. Physiol., 52 (1968) 1-21. 5 HENSEL,H., AND BOMAN,K. K. A., Afferent impulses in cutaneous sensory nerves in human subjects, J. NeurophysioL, 23 (1960) 564-578. 6 HENSEL, H., UND ZOTTERMAN, Y., Quantitative Beziehungen zwischen der Entladung einzelner K~iltefasern und der Temperatur, Acta physiol, scand., 23 (1951) 291-319. 7 IGGO, A., Cutaneous thermoreceptors in primates and sub-primates, J. Physiol. (Lond.), 200 (1969) 403~430. 8 IGGO,A., The mechanism of biological temperature reception. In J. O. HARDY, H. P. GAGGEAND J. A. J. STOLWUK (Eds.), Physiological and Behavioral Thermoregulation, Thomas, Springfield, Ill., 1970, pp. 391~07. 9 KEEL, M. R., PIERAU, FR.-K., AND FABER, D., Temperature eff-cts on resting potential and spike parameters of cat motoneurons, Exp. Brain Res., in press. 10 LIPPOLD, O. C. J., NICHOLS, J. G., AND REDFEARN, J. W. T., A study of the afferent discharge produced by cooling a mammalian muscle spindle, J. Physiol. (Lond.), 153 (1960) 218-231. 11 LOEWENSTEJN,W. R., On the 'specificity' of a sensory receptor, J. Neurophysiol., 24 (1961) 150158. 12 WITT, I., UND HENSEL, H., Afferente Impulse aus der Extremitatenhaut der Katze bei thermischer und mechanischer Reizung, Pfliigers Arch. ges. Physiol., 268 (1959) 582-596.