EXPERIMENTAL
NEI’ROLOCY
Effect
55,
(1977)
j‘&jj;
of Cooling
Rate of Cat
HELEN Dcpnrtrrrcnt
H. MOLINARI of Psychology, Tullahussec.
on the Cold
Dynamic
Response
Units
.IND DAN R. KENSHALO Florida Stutc Florida 32306
1
Ukvcrsifg,
Cold units innervating the cat’s face were studied by extracellular recording in the trigeminal ganglion. The responses of the units to 25°C cooling at six rates (0.04, 0.06, 0.1, 0.5, 1, and 2X/s) from three adapting temperatures (20, 30, and 40°C) were examined. The dynamic responses of the units were examined by calculating peristimulus time histograms and the average frequency during a period including the temperature change and the first 3 seconds at the lower temperature (referred to as the dynamic frequency). These measures were averaged across the units. Varying the adapting temperature produced a change in the magnitude of the units’ responses to the cool stimuli, with the largest responses occurring at 30°C. At all adapting temperatures, cooling at rates of OS”C/s and above produced a rapid increase in firing frequency to a peak, and a less rapid decrease in frequency to a new steady-state level. The sharp response peak was absent for cooling at rates of O.l”C/s and less. The effect of cooling rate on the peristimulus time histogram peak frequencies and the average dynamic frequencies was compared with human psychophysical data. The average dynamic frequency, rather than the magnitude of the response peak, correlates best with human sensitivity.
INTRODUCTION Cold units were first isolated and identified by Zotterman (21) from the nerves innervating the cat’s tongue. Since then they have been further characterized as having the following properties : (a) a steady-state, temperature-dependent discharge; (b) a pbasic increase in discharge frequency with cooling of the receptive field; (c) a phasic decrease in frequency with warming ; and (d) no change in frequency with non painful 1 This research was suppofted by National Science Foundation Grant GB 30610. The data presented here were collected by H. Molinari in partial fulfillment of the requirements for the Master of Science degree at Florida State University. 546 Copyright All rights
0 1977 by Academic Press. of reproduction in any form
Inc. reserved.
ISSN
0014-4886
CAT
COLD
UNITS
547
mechanical stimulation (6). Other studies have characterized in greater detail the steady-state and dynamic responses of cold units in cats (e.g., 1. 3, 8, 9, 11) and primates (e.g.. 2, 10, 1-F. 18, 19). One aspect of the dynamic response of cold units that is not completely understood is the effect of differing rates of cooling on the discharge frequency. Hensel (5) and Uoman ( 1) each examined the response of a single cold unit in the cat infraorhital nerve to different cooling rates. 130th found that decreasing the cooling rate produced a substantial reduction in the dynamic response.
FIG.
circles
1. Receptive fields indicate the eight
(A) cold
of the cold units and (B) of the warm units used in the data analysis.
units.
The
filled
548
MOLINARI
AND
KENSHALO
Due to the variability in the responsesof individual cold units (2 ; 14, Fig. 5), the responseto cool stimuli of any one unit may be unrepresentative. The present study examines the effect of cooling rate on the responses of a number of cat trigeminal ganglion cold units. The data are averaged to provide a measure of the response at each cooling rate. Comparisons with human thermal sensitivity are made. METHODS Subjects and Surgical Procedure. The data were collected from nine cats weighing 2.3 to 5 kg. Anesthesia was induced with intravenous sodium thiopental or with halothane gas and oxygen, and was maintained with sodium thiopental. The cats were paralyzed with succinylcholine chloride and artificially ventilated. The rectal temperature was maintained at 37.5 * 0.5”C. Facial hair was removed with a chemical depilatory, which was found by Hensel and Kenshalo (7) to have no effect on the responseproperties of thermoreceptors. The left trigeminal ganglion was exposed by left craniotomy and hemispherectomy. Several cuts were made in the dura propria of the ganglion to facilitate electrode penetration while protecting the ganglion from drying. Recording. Isonel-coated tungsten electrodes with tip diameters less than 2 pm were used for extracellular recording of neural signals. Following amplification, the neural signals were monitored by standard audiovisual methods and recorded on a direct channel of a magnetic tape. The electrodes were advanced into the ganglion while the skin of the face was alternately warmed by radiant heat and cooled by a hand-held probe. When a cold unit was isolated, the receptive field was mapped and the stimulation sequencebegun. Stimdation. Thermal stimuli were delivered by a Peltier stimulator (13) with a l.O-cm” surface area. The stimulator maintained any temperature between 5 and 45°C with an accuracy of 1%. Changes in temperature could be presented at rates from 0.04 to 2”CJs. The intensity and rate of the temperature change could be varied independently. A thermistor attached to the stimulator surface registered the temperature at the skinstimulator interface. The voltage analogue of this temperature was recorded on an FM channel of the magnetic tape. When a unit had been isolated, identified, and its receptive field mapped, the thermal stimulator was placed on the receptive field. The steady state and dynamic responsesof the unit were examined at three adapting temperatures, 20, 30, and 40°C. After allowing the receptive field to adapt to the adapting temperature for 5 min, the responsesof the unit to a 2.5”C cool stimulus presented at six different rates were recorded. A stimulus was
CAT
COLD
549
IJNITS
0.30
20°C
AT
0.20
0.10 I 0.00 > r
i? 5
0.30
m
0.10
0.20
I
L
0 aLT 0.00 I I CL 0.30 0.20
I
I
I
L
I
I
30°C AT
I
1
I
40°C AT
0.IO I 0.00 0
200
400
MSEC FIG. 2. Steady-state interval histograms averaged for the eight units at the three adapting temperatures (AT), illustrating the probability of occurrence of interspike intervals of Z- to 400~ms duration. defined as a 2.5”C decrease from the adapting temperature to a new static level. Upon completion of the stimulus series the receptive field was again adapted for 5 min. The final 1 min of both 5-tnin adaptation periods at each adapting temperature was recorded and used as an estimate of the steady-state response. The adapting teqerature was then changed and the sequence repeated. The dynamic responses for cooling rates of 0.04, 0.06, 0.1, 0.5, 1, and 2”C/s were recorded at each adapting temperature. The stimulus sequence
550
MOLINARI
AND
KENSHALO
30.0 27.5 I 572 FIG. 3. The responses of unit 49-2 to a 2S”C cool stimulus presented from the 30°C adapting temperatures at rates of 2”C/s (top spike train), O..S”C/s (middle), and O.O4”C/s (bottom). Only a portion of the response to cooling at O.O4”C/s is shown. The temperature trace is superimposed on the spike train.
for each rate consisted of (a) a 20-s prestimulus period ; (b) a 95-s period for the 0.04, 0.06, and O.l”CJs rates or a 25-s period for the 0.5, 1, and 2”C/s rates that was initiated by the stimulus onset; and (c) an 80-s period initiated by the rapid (less than 0.5 s) return of the stimulator to the adapting temperature. The adapting temperature and rate sequence were counterbalanced across units. Data analysis. The data were analyzed off-line using a PDP-12 computer. The recorded nerve impulses were fed through a window in the PDP-12 that discriminated single spikes from background activity on the basis of spike amplitude and duration. The data were then reduced to a series of interspike intervals and were stored on digital tape. The temperature trace was sampled at 125-ms intervals and the voltage analogue of the temperature at each of those times was recorded on the digital tape. Stimulus onset was determined from the temperature samples by calculating the point at which three consecutive values differed from the mean adapting temperature value by more than 1.5 times its standard deviation. The end of the stimulus was defined by applying the same criintervals corterion to the mean slope of the stimulus. The interspike responding to the onset and the end of the stimulus were marked on the digital tape. The steady-state activity of the units was analyzed with time-interval histograms. The histograms were constructed for consecutive interspike intervals using a 2-ms bin width. The dynamic activity of each unit was analyzed with two measures, the dynamic frequency and the peristimulus time histogram. The dynamic frequency was calculated by averaging the frequency during a period that included the cool stimulus and the first 3 seconds at the lower static temperature. The histograms were computed
CAT
COLD
15-
551
UNITS
2.0%
/ SEC
IO-
0 z \
Ol IS-
I
I 0.5”C / SEC
lo-
: VI 5s 0; ii
I
I
I
1
20
30
0.04”C
/ SEC
IO
r-,‘,-i.i’r’i-,-l
0
IO
40
50
60
70
60
SEC FIG. 4. Peristimulus time histograms for unit 49-2 for a 2.5”C cool stimulus presented from the 30°C adapting temperature at rates of 2, 0.5, and O.O4”C/s. The three temperature traces are shown in the lower part of the figure.
by counting the number of interspike intervals plus one in consecutive 150-ms bins. The peristimulus time histograms were calculated for a period beginning 5 s before stimulus onset and ending 30 or 100 s later, depending on the cooling rate. They were smoothed using a three-bin running average, double weighting the center bin. Population measures of the cold-unit steady-state and dynamic activity were calculated by averaging the data across the units for each adapting temperature and each cooling rate. RESULTS Fifteen cold units and four warm units were identified. Each of the 19 units had a single, spotlike receptive field (Fig. 1). The signals from eight cold units were of sufficient amplitude for recording. For all but one of these units the responsesto the static temperature and to all six cooling rates were examined at all adapting temperatures. Warm units have not previously been found in the trigetninal ganglion of the cat (20). The warm units found in the present study were positively
552
MOLINARI
w 0
I
1 0.5 COOLING
AND
I
KENSHALO
I 1.0
I
I
I
1
RATE
FIG. 5. Average dynamic frequencies of the eight cold units as functions of cooling rate and adapting temperature. The curves are the best-fit power functions at each adapting temperature.
identified by their increased firing frequency to warming of the receptive field, decreased firing frequency to cooling, and insensitivity to mechanical stimulation (6). Figure 1B indicates that the receptive fields of these units were not confined to the area innervated by the branch of the infraorbital nerve from which warm units have previously been isolated (7). The spike amplitudes of the warm units were not large enough to permit further examination.
FIG. 6. Average peristimulus time histograms of the eight cold units for cooling 25°C from the 30°C adapting temperature at the six rates (0.04, 0.06, 0.1, 0.5, 1, and 2”C/s). The histograms were smoothed by hand. The six temperature traces are shown in the lower right portion of the figure.
CAT COLD l*NITS
353
The steady-state responses of the eight cold units, at the three adapting temperatures, were comparable to those observed previously in cats (8) and primates (2, 18). The average steady-state response of the units was greatest at 30°C with a mean frequency of 9 impulses/s, and least at 40°C with a mean frequency of 6 impulses/s. Time-interval histograms of the steady-state activity of the individual units and of the eight units combined (Fig. 2) were not himodal, as determined by the criteria used by Kenshalo and Duclaux (l-l), indicating that the units did not consistently discharge in groups or bursts of impulses ( 17) . Examples of the responses of a cold unit to cooling at three of the rates, 2, 0.5, and O.O-F”C/s, are sho\vn in Figs. 3 and 4. This miit was moderately sensitive to the cool stimuli. The peak response frequency to cooling at 0.5”C/s was slightly reduced and occurred later than the peak response for 2”C/s. The O.O-F”C/s cool stimulus elicited very little increase in frequency. It should be noted that t\vo of the eight units displayed the pronounced changes in response with cooling rate described hy Hensel (5) and Boman ( 1 j . Dynamic frequencies were calculated from stimulus onset to 3 s beyond the end of the stimulus. This ensured that the peak response of each unit, which sometimes occurred after the end of the stimulus (Fig. 4), was included in this measure. The average dynamic frequencies at each adapting temperature (Fig. 5) decreased slightly when the cooling rate was decreased from 2” to O.S”C/s, but decreased significantly when the rate was reduced below 0.5’ C/s. The units were most sensitive to cool stimuli presented from the 30°C adapting temperature and least sensitive tu those presented from 40°C. This variation cannot be accomited for simply by the differences in steadystate activity. When the average steady-state frequency at each adapting temperature was subtracted from the average dynamic frequencies the relative positions of the curves were not altered. The mean peristimulus time histograms for the 30°C adapting temperature are shown in Fig. 6. The response magnitudes differed somewhat at the other adapting temperatures, hut the shapes of the curves did not. For the three fastest cooling rates the average response curves exhibit similar characteristics. The frequency rapidly climbed to a peak, then decreased more slowly to a new steady-state level. The peak response decreased and the latency of the peak increased as the cooling rate decreased from 2 to 0.5”C/s. The nature of the response to cooling at 0.1, 0.06, and O.Oq”C/s was quite different from that at the faster rates. Cooling produced much slower increases in firing frequency. The magnitude of the responses decreased only slightly when the cooling rate was decreased from 0.1 to
5.54
MOLINAKI
AND
KENSHALO
O.OLc”C/s. The same final steady-state level was achieved for cooling at all six rates. DISCUSSION Comparisons between human cod sensations and indices of cold unit responsesas functions of the rate of temperature change may provide information aJ,out how stimulus magnitude is coded in the primary affereuts. Human cool thresholds remain constaut at cooling rates of 0.3 to 0.1 ‘C/s, but markedly increase with further reductions in rate (15). Estimates of the co01 sensation magnitudes produced hy suprathresholtl stimuli remain unchanged across rates of 2 to O.S”C/s (H. H. Molinari, J. D. Greenspan, and D. K. Kenshalo. in preparation), but decrease with rate for cooling rates below O.l”C/s (16). Thus human cool sensitivity is altered only by cooling rates of and below about 0.1 ‘C/s. The response peak frequency and the average dynamic frequency are two measures of the cold unit activity that may he compared to sensation magnitude. Because the average dynamic frequency was calculated for an interval determined by the length of time required to present a stimulus plus 3 s, this measure takes into account the rate dependence of the time of occurrence of the maximum cool sensationsfound by Blolinari ef trl. (in preparation). The response peak frequency does not correlate well with human sensation. Peak frequency is reduced more 1)~decreasing the cooling rate front 2 to O.S”C/s, where human sensation magnitude is unaltered, than by decreasing the rate front 0.06 to O.O3”C/s. where a marked change in sensation inagnitucle occurs. On the other hand, the average dynamic frequency correlates well with human cool sensitivity. The average dynamic frequency, like sensation magnitude, changes little across rates of 2 to 0,5”C/s. Both decreaseconsiderably when rates of O.l”C/s and below are used. Likewise, for primate cold units, the average dynamic frequency provides a better index of sensation than the response peak frequency. Changing the cooling rate from 2 to O.d”C/s produced a much smaller decrease in the average dynamic frequency than in the mean peak frequency of rhesus cold units (1-l). These findings are consistent with the proposal made by Hensel (4), Jarvilehto ( 11). and Kenshalo (12) that the tnagnitucle of sensation depends on the number of impulses that arrive in the central nervous system within a certain period of time. In the present study the response of c&l units was examined for a wide range of cooling rates including those that affect sensitivity and those that do not. The results indicate that the average firing frequency during a period of time serves as a nlore accurate index of human thermal sensitivity than the tmagnitude of the response peak.
(‘AT
(‘01.1)
[‘NITS
555
REFERENCES Thermo-
1. BOMAN, K. K. A. 1958. Elektrophysiologische Untcrsuchungen iiher die A-lcttr Z’/f.vsird Srmrd. 44: Suppl. 149. l-79. receptoren cler Gesichtshaut. 2. DARIAN-SMITH, I., K. 0. JOIINSON, AKD R. DYRES. 1973. “Cold” fiber population to cooling pulses. innervating pattnar and digital skin of the monkey : Responses J. Nr~troph.~sic~/. 36 : 325-346. 3. ~OIIT, E., ANN 1’. ZOTTERUAN. 1952. The discharge of specific cold fibers at high temperatures (the paradosical culd). .dc‘t(r f’llysid. .SC~III~. 26 : 35th365. 4. HEP;SEI., H. 1952. Physiologic tier Thermoreception. Er!/c,ll. Plrysid. 47 : 16&368. 5. HENSEL, H. 1953. The time factor in th~rmorrceptor excitation. .-!rlrr physid. Sccmd. 29: 109-116. 6. HENSEL, H., A. Icco, AKU I. WITT. 1960. X quantitative study of sensitive cutaneous thernioreceptors xvith C afferent fihres. J. Plrysifd. ( L~~rrtlnrt) 153 : 113-126. 7. HENSEL, H., AND D. R. KEIVSFIALO. 1969. Warm receptors in the nasal region of cats. 1. Physid. (Lodon) 204 : 99-112. 8. HEKSEL, H., AND R. WCXSTEK. 1970. Static properties of cold receptors in nasal area of cats, J. iVI,frro~h~siol. 33 : 271-275. 1951. The response of the cold receptors to 9. HENSEL, H., AND Y. ZOTTERXIAK. constant cooling. Act0 I’ltysiol. Sccmd. 22 : 96-l 13. 10. Icco, .A. 1969. Cutaneous thermoreccptors in primates and sub-primates. J. L’hysid. (Lomfo~f) 200: 4os430. 11. JARVILENTO. T. 1973. Neural coding in the temperature sense. :lord. .-II.~. Sri. Fcrw. (Bid.) 184 : 1-71. 12. KENSHALO, D. Ii. 1976. C orrelations of temperature sensitivity in man and monkey. ;\ first approximation. Pages 305-330 irl I-. ZOTTEKRIAS, Ed., Scr~snq Ft~r~ctioxs of Sfiirc in Pvir~rclfc*s. Pergamon, h’ew York, 13. KENSHALO, D. R., ANI) D. C. BEKGEX.. 1975. A device to measure cutaneous temperature sensitivity in humans and subhuman species. J. .App/. Physiol. 39 : 1038-1040. 14. E;ENSIIALO, D. R.. AND R. I>V.CLAL~X. 1977. Response characteristics of cutaneous cold receptors in the monkey. J. Ar~~itrtrp/iysiol. in press. 15. KENSHALO, D. R.. C. B. HOLMES. ANI) 1’. B. Woor). 1968. Warm and cool thresholds as a function of rate of stimulus temperature change. Pt,rc’c’bf. P.r~clfr~~hys. 3: 81-84. 16. MARKS, L. E., AND J. C. STEVENS. 1972. Perceived cold ad skin temperature as functions of stimulation level and duration. A-lr~~. J. Psyrll. 85 : 407-419. 17. PO~NOS, D. i\. 1971. Temperature related changes in discharge patterns of squirrel monkey thermoreceptors. Pages 441-455 irl I;. R. KM. K. KOIZ~~MI, AN,) 11. VASSALLE, Eds.. IZrscw~.h ilt ~hy.xir~lll!/y. Ado Gaggi J’uh., Bologna. 18. POULOS, D. A., AND R. A. I,ENDE. 1970. Reslxmse of trigeminal gallglion neurons to thermal stimulation of oral-facial regions, I. Steady state response. J. Nrzjr,o. physiol. 33: 50~-517. 19. POCLOS, D. A., ANII R. A. LESIJE. 1970. Response of trigeminal ganglion lleurolls to thermal stimulation of oral-facial regions. II. Temperature change respon?e, J. Nclrrophysid. 33 : 518-525. 20. RO\VE, M. J., AiXII B. J. SES5I.E. 1972. Kesponses of trigcminal ganglion ant1 brain stem neurons in the cat to mechanical and thermal stimulation of the face, Rntirc IZcs. 42: 367-384. 21. ZUTTERMAN, Y. 1936. Specific action potentials in the lingual nerve of the eat. Skmf. -irch. I’hysiol. 7.5 : 105-119.
NEI’ROLOCY
Effect
55,
(1977)
j‘&jj;
of Cooling
Rate of Cat
HELEN Dcpnrtrrrcnt
H. MOLINARI of Psychology, Tullahussec.
on the Cold
Dynamic
Response
Units
.IND DAN R. KENSHALO Florida Stutc Florida 32306
1
Ukvcrsifg,
Cold units innervating the cat’s face were studied by extracellular recording in the trigeminal ganglion. The responses of the units to 25°C cooling at six rates (0.04, 0.06, 0.1, 0.5, 1, and 2X/s) from three adapting temperatures (20, 30, and 40°C) were examined. The dynamic responses of the units were examined by calculating peristimulus time histograms and the average frequency during a period including the temperature change and the first 3 seconds at the lower temperature (referred to as the dynamic frequency). These measures were averaged across the units. Varying the adapting temperature produced a change in the magnitude of the units’ responses to the cool stimuli, with the largest responses occurring at 30°C. At all adapting temperatures, cooling at rates of OS”C/s and above produced a rapid increase in firing frequency to a peak, and a less rapid decrease in frequency to a new steady-state level. The sharp response peak was absent for cooling at rates of O.l”C/s and less. The effect of cooling rate on the peristimulus time histogram peak frequencies and the average dynamic frequencies was compared with human psychophysical data. The average dynamic frequency, rather than the magnitude of the response peak, correlates best with human sensitivity.
INTRODUCTION Cold units were first isolated and identified by Zotterman (21) from the nerves innervating the cat’s tongue. Since then they have been further characterized as having the following properties : (a) a steady-state, temperature-dependent discharge; (b) a pbasic increase in discharge frequency with cooling of the receptive field; (c) a phasic decrease in frequency with warming ; and (d) no change in frequency with non painful 1 This research was suppofted by National Science Foundation Grant GB 30610. The data presented here were collected by H. Molinari in partial fulfillment of the requirements for the Master of Science degree at Florida State University. 546 Copyright All rights
0 1977 by Academic Press. of reproduction in any form
Inc. reserved.
ISSN
0014-4886
CAT
COLD
UNITS
547
mechanical stimulation (6). Other studies have characterized in greater detail the steady-state and dynamic responses of cold units in cats (e.g., 1. 3, 8, 9, 11) and primates (e.g.. 2, 10, 1-F. 18, 19). One aspect of the dynamic response of cold units that is not completely understood is the effect of differing rates of cooling on the discharge frequency. Hensel (5) and Uoman ( 1) each examined the response of a single cold unit in the cat infraorhital nerve to different cooling rates. 130th found that decreasing the cooling rate produced a substantial reduction in the dynamic response.
FIG.
circles
1. Receptive fields indicate the eight
(A) cold
of the cold units and (B) of the warm units used in the data analysis.
units.
The
filled
548
MOLINARI
AND
KENSHALO
Due to the variability in the responsesof individual cold units (2 ; 14, Fig. 5), the responseto cool stimuli of any one unit may be unrepresentative. The present study examines the effect of cooling rate on the responses of a number of cat trigeminal ganglion cold units. The data are averaged to provide a measure of the response at each cooling rate. Comparisons with human thermal sensitivity are made. METHODS Subjects and Surgical Procedure. The data were collected from nine cats weighing 2.3 to 5 kg. Anesthesia was induced with intravenous sodium thiopental or with halothane gas and oxygen, and was maintained with sodium thiopental. The cats were paralyzed with succinylcholine chloride and artificially ventilated. The rectal temperature was maintained at 37.5 * 0.5”C. Facial hair was removed with a chemical depilatory, which was found by Hensel and Kenshalo (7) to have no effect on the responseproperties of thermoreceptors. The left trigeminal ganglion was exposed by left craniotomy and hemispherectomy. Several cuts were made in the dura propria of the ganglion to facilitate electrode penetration while protecting the ganglion from drying. Recording. Isonel-coated tungsten electrodes with tip diameters less than 2 pm were used for extracellular recording of neural signals. Following amplification, the neural signals were monitored by standard audiovisual methods and recorded on a direct channel of a magnetic tape. The electrodes were advanced into the ganglion while the skin of the face was alternately warmed by radiant heat and cooled by a hand-held probe. When a cold unit was isolated, the receptive field was mapped and the stimulation sequencebegun. Stimdation. Thermal stimuli were delivered by a Peltier stimulator (13) with a l.O-cm” surface area. The stimulator maintained any temperature between 5 and 45°C with an accuracy of 1%. Changes in temperature could be presented at rates from 0.04 to 2”CJs. The intensity and rate of the temperature change could be varied independently. A thermistor attached to the stimulator surface registered the temperature at the skinstimulator interface. The voltage analogue of this temperature was recorded on an FM channel of the magnetic tape. When a unit had been isolated, identified, and its receptive field mapped, the thermal stimulator was placed on the receptive field. The steady state and dynamic responsesof the unit were examined at three adapting temperatures, 20, 30, and 40°C. After allowing the receptive field to adapt to the adapting temperature for 5 min, the responsesof the unit to a 2.5”C cool stimulus presented at six different rates were recorded. A stimulus was
CAT
COLD
549
IJNITS
0.30
20°C
AT
0.20
0.10 I 0.00 > r
i? 5
0.30
m
0.10
0.20
I
L
0 aLT 0.00 I I CL 0.30 0.20
I
I
I
L
I
I
30°C AT
I
1
I
40°C AT
0.IO I 0.00 0
200
400
MSEC FIG. 2. Steady-state interval histograms averaged for the eight units at the three adapting temperatures (AT), illustrating the probability of occurrence of interspike intervals of Z- to 400~ms duration. defined as a 2.5”C decrease from the adapting temperature to a new static level. Upon completion of the stimulus series the receptive field was again adapted for 5 min. The final 1 min of both 5-tnin adaptation periods at each adapting temperature was recorded and used as an estimate of the steady-state response. The adapting teqerature was then changed and the sequence repeated. The dynamic responses for cooling rates of 0.04, 0.06, 0.1, 0.5, 1, and 2”C/s were recorded at each adapting temperature. The stimulus sequence
550
MOLINARI
AND
KENSHALO
30.0 27.5 I 572 FIG. 3. The responses of unit 49-2 to a 2S”C cool stimulus presented from the 30°C adapting temperatures at rates of 2”C/s (top spike train), O..S”C/s (middle), and O.O4”C/s (bottom). Only a portion of the response to cooling at O.O4”C/s is shown. The temperature trace is superimposed on the spike train.
for each rate consisted of (a) a 20-s prestimulus period ; (b) a 95-s period for the 0.04, 0.06, and O.l”CJs rates or a 25-s period for the 0.5, 1, and 2”C/s rates that was initiated by the stimulus onset; and (c) an 80-s period initiated by the rapid (less than 0.5 s) return of the stimulator to the adapting temperature. The adapting temperature and rate sequence were counterbalanced across units. Data analysis. The data were analyzed off-line using a PDP-12 computer. The recorded nerve impulses were fed through a window in the PDP-12 that discriminated single spikes from background activity on the basis of spike amplitude and duration. The data were then reduced to a series of interspike intervals and were stored on digital tape. The temperature trace was sampled at 125-ms intervals and the voltage analogue of the temperature at each of those times was recorded on the digital tape. Stimulus onset was determined from the temperature samples by calculating the point at which three consecutive values differed from the mean adapting temperature value by more than 1.5 times its standard deviation. The end of the stimulus was defined by applying the same criintervals corterion to the mean slope of the stimulus. The interspike responding to the onset and the end of the stimulus were marked on the digital tape. The steady-state activity of the units was analyzed with time-interval histograms. The histograms were constructed for consecutive interspike intervals using a 2-ms bin width. The dynamic activity of each unit was analyzed with two measures, the dynamic frequency and the peristimulus time histogram. The dynamic frequency was calculated by averaging the frequency during a period that included the cool stimulus and the first 3 seconds at the lower static temperature. The histograms were computed
CAT
COLD
15-
551
UNITS
2.0%
/ SEC
IO-
0 z \
Ol IS-
I
I 0.5”C / SEC
lo-
: VI 5s 0; ii
I
I
I
1
20
30
0.04”C
/ SEC
IO
r-,‘,-i.i’r’i-,-l
0
IO
40
50
60
70
60
SEC FIG. 4. Peristimulus time histograms for unit 49-2 for a 2.5”C cool stimulus presented from the 30°C adapting temperature at rates of 2, 0.5, and O.O4”C/s. The three temperature traces are shown in the lower part of the figure.
by counting the number of interspike intervals plus one in consecutive 150-ms bins. The peristimulus time histograms were calculated for a period beginning 5 s before stimulus onset and ending 30 or 100 s later, depending on the cooling rate. They were smoothed using a three-bin running average, double weighting the center bin. Population measures of the cold-unit steady-state and dynamic activity were calculated by averaging the data across the units for each adapting temperature and each cooling rate. RESULTS Fifteen cold units and four warm units were identified. Each of the 19 units had a single, spotlike receptive field (Fig. 1). The signals from eight cold units were of sufficient amplitude for recording. For all but one of these units the responsesto the static temperature and to all six cooling rates were examined at all adapting temperatures. Warm units have not previously been found in the trigetninal ganglion of the cat (20). The warm units found in the present study were positively
552
MOLINARI
w 0
I
1 0.5 COOLING
AND
I
KENSHALO
I 1.0
I
I
I
1
RATE
FIG. 5. Average dynamic frequencies of the eight cold units as functions of cooling rate and adapting temperature. The curves are the best-fit power functions at each adapting temperature.
identified by their increased firing frequency to warming of the receptive field, decreased firing frequency to cooling, and insensitivity to mechanical stimulation (6). Figure 1B indicates that the receptive fields of these units were not confined to the area innervated by the branch of the infraorbital nerve from which warm units have previously been isolated (7). The spike amplitudes of the warm units were not large enough to permit further examination.
FIG. 6. Average peristimulus time histograms of the eight cold units for cooling 25°C from the 30°C adapting temperature at the six rates (0.04, 0.06, 0.1, 0.5, 1, and 2”C/s). The histograms were smoothed by hand. The six temperature traces are shown in the lower right portion of the figure.
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The steady-state responses of the eight cold units, at the three adapting temperatures, were comparable to those observed previously in cats (8) and primates (2, 18). The average steady-state response of the units was greatest at 30°C with a mean frequency of 9 impulses/s, and least at 40°C with a mean frequency of 6 impulses/s. Time-interval histograms of the steady-state activity of the individual units and of the eight units combined (Fig. 2) were not himodal, as determined by the criteria used by Kenshalo and Duclaux (l-l), indicating that the units did not consistently discharge in groups or bursts of impulses ( 17) . Examples of the responses of a cold unit to cooling at three of the rates, 2, 0.5, and O.O-F”C/s, are sho\vn in Figs. 3 and 4. This miit was moderately sensitive to the cool stimuli. The peak response frequency to cooling at 0.5”C/s was slightly reduced and occurred later than the peak response for 2”C/s. The O.O-F”C/s cool stimulus elicited very little increase in frequency. It should be noted that t\vo of the eight units displayed the pronounced changes in response with cooling rate described hy Hensel (5) and Boman ( 1 j . Dynamic frequencies were calculated from stimulus onset to 3 s beyond the end of the stimulus. This ensured that the peak response of each unit, which sometimes occurred after the end of the stimulus (Fig. 4), was included in this measure. The average dynamic frequencies at each adapting temperature (Fig. 5) decreased slightly when the cooling rate was decreased from 2” to O.S”C/s, but decreased significantly when the rate was reduced below 0.5’ C/s. The units were most sensitive to cool stimuli presented from the 30°C adapting temperature and least sensitive tu those presented from 40°C. This variation cannot be accomited for simply by the differences in steadystate activity. When the average steady-state frequency at each adapting temperature was subtracted from the average dynamic frequencies the relative positions of the curves were not altered. The mean peristimulus time histograms for the 30°C adapting temperature are shown in Fig. 6. The response magnitudes differed somewhat at the other adapting temperatures, hut the shapes of the curves did not. For the three fastest cooling rates the average response curves exhibit similar characteristics. The frequency rapidly climbed to a peak, then decreased more slowly to a new steady-state level. The peak response decreased and the latency of the peak increased as the cooling rate decreased from 2 to 0.5”C/s. The nature of the response to cooling at 0.1, 0.06, and O.Oq”C/s was quite different from that at the faster rates. Cooling produced much slower increases in firing frequency. The magnitude of the responses decreased only slightly when the cooling rate was decreased from 0.1 to
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O.OLc”C/s. The same final steady-state level was achieved for cooling at all six rates. DISCUSSION Comparisons between human cod sensations and indices of cold unit responsesas functions of the rate of temperature change may provide information aJ,out how stimulus magnitude is coded in the primary affereuts. Human cool thresholds remain constaut at cooling rates of 0.3 to 0.1 ‘C/s, but markedly increase with further reductions in rate (15). Estimates of the co01 sensation magnitudes produced hy suprathresholtl stimuli remain unchanged across rates of 2 to O.S”C/s (H. H. Molinari, J. D. Greenspan, and D. K. Kenshalo. in preparation), but decrease with rate for cooling rates below O.l”C/s (16). Thus human cool sensitivity is altered only by cooling rates of and below about 0.1 ‘C/s. The response peak frequency and the average dynamic frequency are two measures of the cold unit activity that may he compared to sensation magnitude. Because the average dynamic frequency was calculated for an interval determined by the length of time required to present a stimulus plus 3 s, this measure takes into account the rate dependence of the time of occurrence of the maximum cool sensationsfound by Blolinari ef trl. (in preparation). The response peak frequency does not correlate well with human sensation. Peak frequency is reduced more 1)~decreasing the cooling rate front 2 to O.S”C/s, where human sensation magnitude is unaltered, than by decreasing the rate front 0.06 to O.O3”C/s. where a marked change in sensation inagnitucle occurs. On the other hand, the average dynamic frequency correlates well with human cool sensitivity. The average dynamic frequency, like sensation magnitude, changes little across rates of 2 to 0,5”C/s. Both decreaseconsiderably when rates of O.l”C/s and below are used. Likewise, for primate cold units, the average dynamic frequency provides a better index of sensation than the response peak frequency. Changing the cooling rate from 2 to O.d”C/s produced a much smaller decrease in the average dynamic frequency than in the mean peak frequency of rhesus cold units (1-l). These findings are consistent with the proposal made by Hensel (4), Jarvilehto ( 11). and Kenshalo (12) that the tnagnitucle of sensation depends on the number of impulses that arrive in the central nervous system within a certain period of time. In the present study the response of c&l units was examined for a wide range of cooling rates including those that affect sensitivity and those that do not. The results indicate that the average firing frequency during a period of time serves as a nlore accurate index of human thermal sensitivity than the tmagnitude of the response peak.
(‘AT
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REFERENCES Thermo-
1. BOMAN, K. K. A. 1958. Elektrophysiologische Untcrsuchungen iiher die A-lcttr Z’/f.vsird Srmrd. 44: Suppl. 149. l-79. receptoren cler Gesichtshaut. 2. DARIAN-SMITH, I., K. 0. JOIINSON, AKD R. DYRES. 1973. “Cold” fiber population to cooling pulses. innervating pattnar and digital skin of the monkey : Responses J. Nr~troph.~sic~/. 36 : 325-346. 3. ~OIIT, E., ANN 1’. ZOTTERUAN. 1952. The discharge of specific cold fibers at high temperatures (the paradosical culd). .dc‘t(r f’llysid. .SC~III~. 26 : 35th365. 4. HEP;SEI., H. 1952. Physiologic tier Thermoreception. Er!/c,ll. Plrysid. 47 : 16&368. 5. HENSEL, H. 1953. The time factor in th~rmorrceptor excitation. .-!rlrr physid. Sccmd. 29: 109-116. 6. HENSEL, H., A. Icco, AKU I. WITT. 1960. X quantitative study of sensitive cutaneous thernioreceptors xvith C afferent fihres. J. Plrysifd. ( L~~rrtlnrt) 153 : 113-126. 7. HENSEL, H., AND D. R. KEIVSFIALO. 1969. Warm receptors in the nasal region of cats. 1. Physid. (Lodon) 204 : 99-112. 8. HEKSEL, H., AND R. WCXSTEK. 1970. Static properties of cold receptors in nasal area of cats, J. iVI,frro~h~siol. 33 : 271-275. 1951. The response of the cold receptors to 9. HENSEL, H., AND Y. ZOTTERXIAK. constant cooling. Act0 I’ltysiol. Sccmd. 22 : 96-l 13. 10. Icco, .A. 1969. Cutaneous thermoreccptors in primates and sub-primates. J. L’hysid. (Lomfo~f) 200: 4os430. 11. JARVILENTO. T. 1973. Neural coding in the temperature sense. :lord. .-II.~. Sri. Fcrw. (Bid.) 184 : 1-71. 12. KENSHALO, D. Ii. 1976. C orrelations of temperature sensitivity in man and monkey. ;\ first approximation. Pages 305-330 irl I-. ZOTTEKRIAS, Ed., Scr~snq Ft~r~ctioxs of Sfiirc in Pvir~rclfc*s. Pergamon, h’ew York, 13. KENSHALO, D. R., ANI) D. C. BEKGEX.. 1975. A device to measure cutaneous temperature sensitivity in humans and subhuman species. J. .App/. Physiol. 39 : 1038-1040. 14. E;ENSIIALO, D. R.. AND R. I>V.CLAL~X. 1977. Response characteristics of cutaneous cold receptors in the monkey. J. Ar~~itrtrp/iysiol. in press. 15. KENSHALO, D. R.. C. B. HOLMES. ANI) 1’. B. Woor). 1968. Warm and cool thresholds as a function of rate of stimulus temperature change. Pt,rc’c’bf. P.r~clfr~~hys. 3: 81-84. 16. MARKS, L. E., AND J. C. STEVENS. 1972. Perceived cold ad skin temperature as functions of stimulation level and duration. A-lr~~. J. Psyrll. 85 : 407-419. 17. PO~NOS, D. i\. 1971. Temperature related changes in discharge patterns of squirrel monkey thermoreceptors. Pages 441-455 irl I;. R. KM. K. KOIZ~~MI, AN,) 11. VASSALLE, Eds.. IZrscw~.h ilt ~hy.xir~lll!/y. Ado Gaggi J’uh., Bologna. 18. POULOS, D. A., AND R. A. I,ENDE. 1970. Reslxmse of trigeminal gallglion neurons to thermal stimulation of oral-facial regions, I. Steady state response. J. Nrzjr,o. physiol. 33: 50~-517. 19. POCLOS, D. A., ANII R. A. LESIJE. 1970. Response of trigeminal ganglion lleurolls to thermal stimulation of oral-facial regions. II. Temperature change respon?e, J. Nclrrophysid. 33 : 518-525. 20. RO\VE, M. J., AiXII B. J. SES5I.E. 1972. Kesponses of trigcminal ganglion ant1 brain stem neurons in the cat to mechanical and thermal stimulation of the face, Rntirc IZcs. 42: 367-384. 21. ZUTTERMAN, Y. 1936. Specific action potentials in the lingual nerve of the eat. Skmf. -irch. I’hysiol. 7.5 : 105-119.
