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Thermal Sensibility

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Abstract

The sections in this article are:

1 Cutaneous Thermal Sensibility in Man and Other Primates
1.1 Thermal Sensations Elicited by Localized Changes in Skin Temperature
1.2 Detection of Incremental Changes in Skin Temperature
1.3 Sensing and Differentiating Sudden Suprathreshold Changes in Skin Temperature
2 Peripheral Neural Mechanisms of Thermal Sensibility
2.1 Fiber Populations Responding to Changes in Skin Temperature in Primate and in Other Mammals
2.2 Specific Thermoreceptive Fibers
3 Central Neural Mechanisms of Thermal Sensibility
3.1 Effect on Thermal Sensibility of Discrete Lesions Within Central Nervous System
3.2 Transmission of Thermal Sensory Information Within Spinal Cord
3.3 Thermoreceptive Neurons Within Spinal Dorsal Horn
3.4 Thermoreceptive Neurons in Brain Stem Trigeminal Complex
3.5 Thermoreceptive Neurons in Thalamus and Cerebral Cortex
4 Final Comment
Figure 1. Figure 1.

Left: time course for adaptation in human subjects to gradual changes in temperature of skin of forearm. Subject could adjust temperature of thermode in contact with skin (area of contact 14.4 cm2) and was instructed to keep stimulus just noticeably warm or cool. Thermode temperature is plotted at increasing intervals after either cooling or warming of the skin was begun; data points are based on observations made on four subjects. Right: effect of rate of change of skin temperature on a subject's capacity to detect change. Ordinate is threshold change in skin temperature, based on observations on three subjects. With changes in skin temperature at rates greater than 0.1°C/s, threshold did not change significantly.

Left: data from Kenshalo and from Kenshalo and Scott . Right: data from Kenshalo and from Kenshalo et al.
Figure 2. Figure 2.

Thresholds in man for detecting warming or cooling of skin of forearm. Prior to estimation of threshold, skin had been adapted at a steady temperature in the range 28°C‐40°C. Beyond the neutral zone of adapting temperatures (31°C‐36°C) there was a persisting sensation of warmth or coolness. When this was so, subject first reported a decrease in persisting background sensation of, say, coolness and then, with more intense stimulation, a reversal of thermal sensation to one of warming.

Data from Kenshalo
Figure 3. Figure 3.

Relationship of threshold for detecting a sudden rise in skin temperature to area of warming. Data for skin of back (triangles), forearm (squares), and forehead (circles); both radiant (open symbols) and conducted (solid symbols) heat used for warming skin. Dashed line, radiant heating of forehead.

Data for back, forearm, forehead from Kenshalo et al. ; data for radiant heating of forehead from Hardy and Oppel
Figure 4. Figure 4.

Estimated rise in temperature of skin at 0.3 mm and 0.7 mm below its surface during a period of steady radiant heating lasting 12 s. Observations made at 3 levels of thermal energy absorption (ϕ, absorbed irradiance of 3.36, 9.80, and 22.1 mcal/s × cm2). Skin warmed was that of front of trunk of a man. There was a near‐linear rise in temperature at each depth in skin. Compare this change in skin temperature elicited by radiant heat with response pattern of a warm fiber responding to a conducted “ramp” increase in skin temperature, illustrated in Fig. .

Adapted from Stevens et al.
Figure 5. Figure 5.

Relationship between subjective estimate of amplitude of a stepwise cooling pulse applied to thenar eminence and its actual amplitude. Skin had been previously adapted at 34°C. Stimulus duration 4 s. Subject periodically presented with a 5.3°C cooling pulse, which was arbitrarily scaled as 10.

Data from Darian‐Smith and Dykes
Figure 6. Figure 6.

Subjective estimate of magnitude of cutaneous warming stimulus. Closed circles, subjective estimate of intensity obtained when stimuli were delivered in rotating sequence to nine separate loci on thenar eminence. Under these conditions, mean interstimulus interval at each spot was 225 s. Open circles, subjective estimate obtained using a mean interstimulus interval of 25 s, with stimuli delivered to a single locus on thenar eminence. Dashed lines, scale values for boundaries between each category. Each stimulus scale value represents mean of normal distribution of category responses obtained for a given stimulus temperature and interstimulus interval. Distribution of responses to 45°C stimulus delivered with longer interstimulus interval is illustrated and is representative of the other distributions.

From LaMotte and Campbell
Figure 7. Figure 7.

Subjective estimate of intensity of a radiant warming stimulus, illustrating spatial summation of stimulus input. Each stimulus lasted 3.0 s and was of fixed irradiance (mW/cm2) and areal extent. Two regions of skin used for the experiment were forehead (left) and skin of back (right). Symbols indicate areal extent of each stimulus. Spatial summation was greatest with low‐intensity stimuli and absent with the most‐intense, noxious stimuli.

From Stevens et al.
Figure 8. Figure 8.

Discrimination of incremental changes in amplitude of stepwise shift in skin temperature. Stimulus design is shown on right. Skin of thenar eminence was first adapted at 34°, 29°, or 39°C. A train of nearly identical temperature pulses was then presented with stimulus repetition rate of 1/10 s, each stimulus lasting 4.0 s. Subject was required to judge larger of each successive pair of stimuli presented. Graphs at left, relationship between subject's discriminative capacity (difference limen) and mean intensity of stimulus pairs being differentiated (Weber functions). Weber functions for cooling stimuli were similar at adaptation temperatures of 29°, 34°, and 39°C. With warming pulses discrimination did depend on adaptation temperature being greater at 34°C than at 29°C. As indicated at right, same stimulus sequences could be used in studies of intensity discrimination based on responses of single warm fibers innervating palmar skin in macaque (i.e., estimates of discriminable stimulus increment).

Based on data from Johnson et al.
Figure 9. Figure 9.

Receptive‐field organization of a warm fiber. Responses were elicited by a warming pulse with stimulus thermode located at different positions in fiber's receptive field. Edge of thermode is plotted relative to central most responsive zone in fiber's receptive field. Warming pulses were presented once every 30 s. Family of curves illustrate response profiles for warming pulses with intensities of 2°C, 4°C, 6°C, and 8°C. Response patterns observed with cold fibers were similar to those of warm fibers.

Data from Darian‐Smith et al.
Figure 10. Figure 10.

Distribution of conduction velocities of thermoreceptive fibers innervating digital and palmar skin of macaque's hand.

From Darian‐Smith et al.
Figure 11. Figure 11.

Mean discharge rates of cold (two left‐hand curves) and warm (two right‐hand curves) fibers innervating glabrous skin of macaque's hand or foot at steady skin temperatures in range 20°C‐50°C. Open circles from data from Kenshalo's laboratory ; solid circles from data from Darian‐Smith's laboratory .

Figure 12. Figure 12.

Mean discharge rates of 6 warm fibers innervating hairy skin in man at steady skin temperatures in range 35°C‐46°C .

From Konietzny and Hensel
Figure 13. Figure 13.

“Paradoxical” discharge in cold fibers. Mean discharge rates in a sample of cold fibers with skin adapted at temperatures in range 29°C‐53°C. All 10 fibers in sample innervated glabrous skin of macaque's palmar skin. Three curves illustrate relationship of deep‐body temperature to occurrence of paradoxical discharge in cold fibers at steady skin temperatures higher than 45°C

From Long
Figure 14. Figure 14.

Responses in a warm fiber elicited by warming pulses. Stimulus profile is shown in upper left of figure. Fiber's receptive field was on thenar eminence. Adaptation temperature was 34°C. Ensemble of impulses (vertical strokes) in individual responses to stimuli with intensities of 8.0°C‐0.0°C is shown on left side and poststimulus time histograms (PSTHs) on right side. Ensemble displays 2 typical responses for each temperature step. The PSTHs based on mean 5 responses at each intensity; bin width, 200 ms. Stimuli presented once every 60 s.

From Darian‐Smith et al.
Figure 15. Figure 15.

Response of warm fiber to linear temperature ramp. Temporal form of stimulus is shown in upper left of figure. Stimuli presented as trains of stimuli at 22‐s intervals. Poststimulus time histograms (PSTHs) from these data are shown on right. Intensity of stimuli in train was set at 2°C, 4°C, 6°C, and 8°C to generate family of curves.

From Darian‐Smith et al.
Figure 16. Figure 16.

Responses of a cold fiber innervating palm of macaque's hand to stepwise cooling pulses. The skin had been previously adapting at 34°C. Lowest record is of steady discharge of fiber at 34°C. Successive records above this are responses to cooling pulses graded in intensity up to a maximum of 10°C (i.e., a plateau temperature of 24°C). Top record indicates stepwise shift in skin temperature.

From Darian‐Smith and Dykes
Figure 17. Figure 17.

Poststimulus time histograms (PSTHs) of a warm fiber (receptive field on index finger) responding to warming pulses of intensities 2°C, 4°C, 6°C, and 8°C. Skin was previously adapted at 34°C. Each histogram constructed from mean of five consecutive responses selected from a train of stimuli at 60‐s intervals; bin width, 200 ms. Profile of warming pulse shown in black beneath histograms. In B and C, PSTHs of responses to stimuli of intensities 2°C and 8°C are matched against PSTH of a response to a warming pulse of 4°C, shaded in both figures. Data normalized in each instance by equating total number of impulses occurring within first 4 s of stimulation.

From Darian‐Smith et al.
Figure 18. Figure 18.

Intensity functions of single warm fibers at different skin adaptation levels (29°C, 34°C, and 39°C). Warming pulses lasted 4 s and were presented in random order every 60 s. Lower right, plot of average intensity function for each of these fiber samples; there were 17 fibers common to each sample. With an additional 5 warm fibers, intensity function was estimated at skin adaptation level = 34°C and either 29°C or 39°C.

From Darian‐Smith et al.
Figure 19. Figure 19.

Families of intensity functions relating mean response of a thermoreceptive fiber (cumulative impulse count during first 4 s of stimulation) to intensity of temperature pulse when test stimulus is one of a train of stimuli and stimuli preceding it are all specified. Stimulus train had the following characteristics: duration of temperature pulses, 4 s; stimulus repetition rate, 1/10 s; I0, intensity of stimulus; I1, intensity of stimulus preceding I0; I2, intensity of stimulus preceding I1. Left: responses of a cold fiber (averaged data from 10 cold fibers), with I2 = 0°C for whole sequence; family of curves illustrates suppressive action of I1, as its intensity was increased from 0°C to 8°C. Middle: responses for a warm fiber (averaged data from 14 warm fibers) to a similar sequence of warming pulses; similar family of intensity curves. Right: family of intensity functions illustrate responses of same sample of warm fibers examined in middle graph. However, now I2 = 8°C; even I2 has a substantial suppressive action on warm fiber response. Finally, intensity function of a slowly adapting (SA) mechanoreceptive fiber responding to sequence of cooling pulses described above is shown in left graph; although responsive to cooling pulses, SA mechanoreceptive fibers are insensitive when compared with cold fibers.

Data from Darian‐Smith et al.
Figure 20. Figure 20.

Responses of a single warm fiber innervating hairy skin on dorsum of hand in man. Skin temperature was increased rapidly (1.5°C/s) from adaptation temperature to a new level 5°C above this. Fiber became more responsive to test stimulus as adaptation temperature was increased from 27°C to 32°C to 37°C.

From Konietzny and Hensel
Figure 21. Figure 21.

Series of intensity functions for a single cold fiber, relating cumulative impulse count over 4 s to amplitude of cooling pulse, at different skin adaptation values of 25°C, 29°C, 34°C, 39°C. Interval between stimuli was 25 s to reduce temporal suppressive action. Slopes of these intensity functions did not change with changes in skin adaptation temperature over this range. When adaptation temperature of skin was 41°C, this fiber was unresponsive to cooling pulses less than 2°C in intensity; about one‐third of fibers examined at this adaptation level were similarly unresponsive to comparable small cooling pulses.

Figure 22. Figure 22.

Subjective estimates of intensity of identical warming pulses when presented in different stimulus trains. Base temperature 38°C. When interstimulus interval between successive stimuli was 225 s (abscissa), subject's estimate of its intensity was greater than when interstimulus interval was short (ordinate). This observation illustrates the reflection of the suppressive interaction observed in thermoreceptive fibers (Fig. ) in the subject's performance.

Adapted from LaMotte and Campbell
Figure 23. Figure 23.

Graphical representation of steps in analysis of a warm fiber's capacity to resolve incremental changes in intensity of a warming pulse, as measured by discriminable stimulus increment (DSI). In this example skin was adapted at 34°C; warming pulses used in experimental sequences had amplitudes of 4°C and 4.5°C and were presented alternately in a continuing train, as shown in response sequence in lower part of figure. Mean cumulative impulse count at 2 intensity levels of 4°C and 4.5°C, sensitivity (dr/dI, change in mean cumulative impulse count/1°C change in stimulus intensity), standard deviation of difference between individual response pairs (Δr), and finally DSI (0.67 × σΔr)/(dr/dI), are plotted in 4 graphs for successively longer segments of stimulus period.

From Darian‐Smith et al.
Figure 24. Figure 24.

Distribution of discrimination stimulus increment (DSI) for 13 warm fibers. Adaptation temperature, 34°C; amplitude of warming pulse, 4°C. DSI estimated for successively longer integration intervals, increasing by 200‐ms increments. Lines plot changing DSI with stimulus duration for 2 typical fibers of sample. Shaded zone is bounded by difference limen for human subjects differentiating identical sequences of warming pulses (0.04°C) and longest decision time used by these subjects (3 s).

From Darian‐Smith et al.
Figure 25. Figure 25.

Families of curves illustrating dependence of the discriminable stimulus increment (DSI; see text) for a population of warm fibers on stimulus period over which response was measured. In left and middle graphs each curve describes relationship for population of n fibers, where n = 1, 2, 5, 10, 25, 50, and 100. Adaptation temperature for skin was 34°C; mean amplitude of warming pulses was 6°C. In left graph each fiber was considered to respond independently of its neighbors; equal weighting was given to each fiber's contribution to overall population response. In middle graph each fiber also responded independently, but now its contribution to overall population response was weighted to permit maximum possible resolution of changes in intensity of stimulus. In right graph each fiber's contribution to population response was given equal weighting, but now the fibers were not responding entirely independently of their neighbors. Correlation coefficient, ρ, defines dependent variability between pairs of fibers in responding population; when ρ = 0, fibers were responding independently, but when ρ = 1, all fibers varied entirely in accord. Shaded zone defines behavior of a trained observer differentiating pairs of warming pulses similar to those used in fiber study, zone being bounded by difference limen and decision time for this task. Only when function for a particular fiber population overlaps the shaded zone does that population signal sufficient information to brain to account for human discriminative behavior.

Adapted from Johnson et al.
Figure 26. Figure 26.

Families of Weber functions relating discriminable stimulus increment (DSI) for a population of responding warm fibers (in macaque) to mean intensity of warming pulses being differentiated. Each curve describes Weber function for a population of n fibers, where n = 1, 2, 5, 10, 25, 50, and 100. Skin was adapted at 34°C for all observations. Experimental conditions pertaining for each of the three graphs matched with those described in legend of Fig. . Response measure for all observations was cumulative impulse count during first 2 s of stimulation. Shaded zone defines best observed intensity resolution (difference limen) determined in human subjects for stimulus sequences similar to those used in fiber study.

Adapted from Johnson et al.
Figure 27. Figure 27.

Response of a warm fiber to graded warming pulses when adaptation temperature of skin was 39°C. Discharge by warming pulses with intensities of 0°C, 2°C, 4°C, 6°C, and 8°C is shown on the left, along with profile of warming pulse. Plateau temperature of warming pulses are also indicated on extreme left. This fiber did not respond at all when pulse plateau temperature was raised to 49°C. Right: mean response profiles; each profile constructed by averaging 5 successive responses.

From Darian‐Smith et al.
Figure 28. Figure 28.

Family of intensity functions relating mean discharge (cumulative impulse count during first 3 s of a heating pulse) of a nociceptive fiber to amplitude of stimulus. Adaptation temperature was 38°C; intensity of heating pulse is indicated in terms of plateau temperature (45°C‐53°C). There was a monotonic increase in response over this range, which contrasts with responses of warm fibers (Fig. ). Suppressive interaction such as occurs in warm fibers was also observed in nociceptive fibers; if stimulus preceding test stimulus (Sn‐1) was high, response to test stimulus was reduced, as seen in family of intensity functions.

From LaMotte and Campbell
Figure 29. Figure 29.

Responses of a cold fiber to near‐rectangular cooling and warming pulses (lasting 10 s) before and after exposure to noxious heating of skin. Steady skin temperature between successive stimuli was 35°C (arrow). Solid curve labeled 1–2 indicates fiber response prior to noxious heating of skin. Following this, noxious heating responses (dashed curve 3–4) to skin cooling were unchanged, but terminal was now sensitized to warming. Fiber now responded when skin temperature was raised to 43°C‐44°C, whereas with initial heating of skin no response was elicited until temperature of skin was raised to about 50°C.

From Dubner et al.
Figure 30. Figure 30.

Diagram summarizing available data concerning terminations of thermoreceptor fibers and origin of spinothalamic projections within cervical segments of primate spinal cord. Most warm and cold fibers terminate in marginal lamina I of Rexed. Also, the few specifically thermoreceptive dorsal horn cells that have been identified were mostly within lamina I. Many of these marginal cells are spinothalamic; the majority pass into contralateral anterolateral column, but some enter ipsilateral column. Branches of large‐diameter mechanoreceptor fibers ascending in dorsal columns and the spinocervical projection (not thermoreceptive) are also shown. Few if any spinothalamic neurons have their cell bodies located within the substantia gelatinosa.

Figure 31. Figure 31.

Responses of low‐threshold thermoreceptive neurons within marginal lamina I of coccygeal spinal cord of rhesus monkey. In A and C, upper record is of average rate of discharge in 2 different thermoreceptive cells during cooling of skin, caused by evaporating ethyl chloride; changing skin temperature is indicated in lower trace. Neuron responding to cooling in A was also examined when skin was warmed by infrared radiation. This response is shown in B, in which upper trace is of intervals between successive impulses. Neither of the thermoreceptive cells responded to innocuous mechanical stimulation of skin.

From Kumazawa and Perl
Figure 32. Figure 32.

Camera lucida drawing of a neuron within marginal lamina I of the sacral/coccygeal spinal cord of the cat. Neuron was stained by intracellularly injected horseradish peroxidase following recording of its responses. It was excited to discharge by innocuous cooling of skin and inhibited by low‐intensity mechanical stimulation of same area of skin. Cell also discharged in response to electrically evoked dorsal root volleys when these included discharging unmyelinated fibers. Hatching marks boundary between white matter and marginal lamina. Open arrow indicates axon.

From Light et al.
Figure 33. Figure 33.

Distribution of cell bodies of spinothalamic neurons in lumbosacral enlargement of spinal cord in macaque. Cells were identified by retrograde uptake of horseradish peroxidase following its injection either into ventral posterolateral nucleus (upper half of figure), or more medially into central lateral nucleus (lower figures). Histograms show relative numbers of cell bodies within different laminae of Rexed; spinothalamic neurons with contralateral and ipsilateral thalamic projections are plotted separately. Spinal cord sections illustrate locations of each cell body observed in a succession of serial sections.

Data from Willis et al.
Figure 34. Figure 34.

Projections to diencephalon from contralateral dorsal column nuclei, lateral cervical nucleus, and anterolateral column of spinal cord of squirrel monkey (Saimiri sciureus). Coronal section in stereotaxic anterior planes +5, +6, and +7 mm, as defined in stereotaxic atlas by Emmers and Ackert . CL, central lateral nucleus; CM, centre médian n.; LP, lateral posterior n.; MD, mediodorsal n.; MGN, medial geniculate n.; Pf, parafascicular n.; PO, posterior group; Pul, pulvinar n.; R, reticular n.; St, subthalamic n.; Vim, ventral intermedial n.; VL, ventrolateral n.; VPI, ventral posteroinferior n.; VPL, ventral posterolateral n.; VPMpc, paracentral decision of VPM.

Adapted from Berkeley
Figure 35. Figure 35.

Responses of thermoreceptive neurons within nucleus caudalis of brain stem trigeminal complex of cat. Upper record: thermoreceptive neuron specifically excited by cooling skin of lower lip (inset). Upper tracing, skin temperature; lower tracing, discharge rate of cell. Lower left record: similar sequence, but this neuron was specifically excited by warming skin at outer canthus of eye (inset). Lower right record: discharge rates in the two cells at steady skin temperatures; responses of cold unit were maximal at about 15°C.

From Dostrovsky and Hellon
Figure 36. Figure 36.

Discharge patterns in 2 neurons located within ventrobasal complex of thalamus of squirrel monkey (Saimiri sciureus). Lower graph: cell was specifically excited by cooling its receptivefield zone on tongue; cell was unresponsive to innocuous mechanical stimulation. Upper graph: cell, although excited by cooling the tongue, also responded to innocuous mechanical stimulation of same zone. Same sequence of thermal stimulation was used to examine responses of both neurons; 30‐s stimulus periods were alternated with a 90‐s period during which skin was at 35°C. Numbers immediately below the lower graph indicate pattern of alternating skin temperatures

From Poulos and Benjamin


Figure 1.

Left: time course for adaptation in human subjects to gradual changes in temperature of skin of forearm. Subject could adjust temperature of thermode in contact with skin (area of contact 14.4 cm2) and was instructed to keep stimulus just noticeably warm or cool. Thermode temperature is plotted at increasing intervals after either cooling or warming of the skin was begun; data points are based on observations made on four subjects. Right: effect of rate of change of skin temperature on a subject's capacity to detect change. Ordinate is threshold change in skin temperature, based on observations on three subjects. With changes in skin temperature at rates greater than 0.1°C/s, threshold did not change significantly.

Left: data from Kenshalo and from Kenshalo and Scott . Right: data from Kenshalo and from Kenshalo et al.


Figure 2.

Thresholds in man for detecting warming or cooling of skin of forearm. Prior to estimation of threshold, skin had been adapted at a steady temperature in the range 28°C‐40°C. Beyond the neutral zone of adapting temperatures (31°C‐36°C) there was a persisting sensation of warmth or coolness. When this was so, subject first reported a decrease in persisting background sensation of, say, coolness and then, with more intense stimulation, a reversal of thermal sensation to one of warming.

Data from Kenshalo


Figure 3.

Relationship of threshold for detecting a sudden rise in skin temperature to area of warming. Data for skin of back (triangles), forearm (squares), and forehead (circles); both radiant (open symbols) and conducted (solid symbols) heat used for warming skin. Dashed line, radiant heating of forehead.

Data for back, forearm, forehead from Kenshalo et al. ; data for radiant heating of forehead from Hardy and Oppel


Figure 4.

Estimated rise in temperature of skin at 0.3 mm and 0.7 mm below its surface during a period of steady radiant heating lasting 12 s. Observations made at 3 levels of thermal energy absorption (ϕ, absorbed irradiance of 3.36, 9.80, and 22.1 mcal/s × cm2). Skin warmed was that of front of trunk of a man. There was a near‐linear rise in temperature at each depth in skin. Compare this change in skin temperature elicited by radiant heat with response pattern of a warm fiber responding to a conducted “ramp” increase in skin temperature, illustrated in Fig. .

Adapted from Stevens et al.


Figure 5.

Relationship between subjective estimate of amplitude of a stepwise cooling pulse applied to thenar eminence and its actual amplitude. Skin had been previously adapted at 34°C. Stimulus duration 4 s. Subject periodically presented with a 5.3°C cooling pulse, which was arbitrarily scaled as 10.

Data from Darian‐Smith and Dykes


Figure 6.

Subjective estimate of magnitude of cutaneous warming stimulus. Closed circles, subjective estimate of intensity obtained when stimuli were delivered in rotating sequence to nine separate loci on thenar eminence. Under these conditions, mean interstimulus interval at each spot was 225 s. Open circles, subjective estimate obtained using a mean interstimulus interval of 25 s, with stimuli delivered to a single locus on thenar eminence. Dashed lines, scale values for boundaries between each category. Each stimulus scale value represents mean of normal distribution of category responses obtained for a given stimulus temperature and interstimulus interval. Distribution of responses to 45°C stimulus delivered with longer interstimulus interval is illustrated and is representative of the other distributions.

From LaMotte and Campbell


Figure 7.

Subjective estimate of intensity of a radiant warming stimulus, illustrating spatial summation of stimulus input. Each stimulus lasted 3.0 s and was of fixed irradiance (mW/cm2) and areal extent. Two regions of skin used for the experiment were forehead (left) and skin of back (right). Symbols indicate areal extent of each stimulus. Spatial summation was greatest with low‐intensity stimuli and absent with the most‐intense, noxious stimuli.

From Stevens et al.


Figure 8.

Discrimination of incremental changes in amplitude of stepwise shift in skin temperature. Stimulus design is shown on right. Skin of thenar eminence was first adapted at 34°, 29°, or 39°C. A train of nearly identical temperature pulses was then presented with stimulus repetition rate of 1/10 s, each stimulus lasting 4.0 s. Subject was required to judge larger of each successive pair of stimuli presented. Graphs at left, relationship between subject's discriminative capacity (difference limen) and mean intensity of stimulus pairs being differentiated (Weber functions). Weber functions for cooling stimuli were similar at adaptation temperatures of 29°, 34°, and 39°C. With warming pulses discrimination did depend on adaptation temperature being greater at 34°C than at 29°C. As indicated at right, same stimulus sequences could be used in studies of intensity discrimination based on responses of single warm fibers innervating palmar skin in macaque (i.e., estimates of discriminable stimulus increment).

Based on data from Johnson et al.


Figure 9.

Receptive‐field organization of a warm fiber. Responses were elicited by a warming pulse with stimulus thermode located at different positions in fiber's receptive field. Edge of thermode is plotted relative to central most responsive zone in fiber's receptive field. Warming pulses were presented once every 30 s. Family of curves illustrate response profiles for warming pulses with intensities of 2°C, 4°C, 6°C, and 8°C. Response patterns observed with cold fibers were similar to those of warm fibers.

Data from Darian‐Smith et al.


Figure 10.

Distribution of conduction velocities of thermoreceptive fibers innervating digital and palmar skin of macaque's hand.

From Darian‐Smith et al.


Figure 11.

Mean discharge rates of cold (two left‐hand curves) and warm (two right‐hand curves) fibers innervating glabrous skin of macaque's hand or foot at steady skin temperatures in range 20°C‐50°C. Open circles from data from Kenshalo's laboratory ; solid circles from data from Darian‐Smith's laboratory .



Figure 12.

Mean discharge rates of 6 warm fibers innervating hairy skin in man at steady skin temperatures in range 35°C‐46°C .

From Konietzny and Hensel


Figure 13.

“Paradoxical” discharge in cold fibers. Mean discharge rates in a sample of cold fibers with skin adapted at temperatures in range 29°C‐53°C. All 10 fibers in sample innervated glabrous skin of macaque's palmar skin. Three curves illustrate relationship of deep‐body temperature to occurrence of paradoxical discharge in cold fibers at steady skin temperatures higher than 45°C

From Long


Figure 14.

Responses in a warm fiber elicited by warming pulses. Stimulus profile is shown in upper left of figure. Fiber's receptive field was on thenar eminence. Adaptation temperature was 34°C. Ensemble of impulses (vertical strokes) in individual responses to stimuli with intensities of 8.0°C‐0.0°C is shown on left side and poststimulus time histograms (PSTHs) on right side. Ensemble displays 2 typical responses for each temperature step. The PSTHs based on mean 5 responses at each intensity; bin width, 200 ms. Stimuli presented once every 60 s.

From Darian‐Smith et al.


Figure 15.

Response of warm fiber to linear temperature ramp. Temporal form of stimulus is shown in upper left of figure. Stimuli presented as trains of stimuli at 22‐s intervals. Poststimulus time histograms (PSTHs) from these data are shown on right. Intensity of stimuli in train was set at 2°C, 4°C, 6°C, and 8°C to generate family of curves.

From Darian‐Smith et al.


Figure 16.

Responses of a cold fiber innervating palm of macaque's hand to stepwise cooling pulses. The skin had been previously adapting at 34°C. Lowest record is of steady discharge of fiber at 34°C. Successive records above this are responses to cooling pulses graded in intensity up to a maximum of 10°C (i.e., a plateau temperature of 24°C). Top record indicates stepwise shift in skin temperature.

From Darian‐Smith and Dykes


Figure 17.

Poststimulus time histograms (PSTHs) of a warm fiber (receptive field on index finger) responding to warming pulses of intensities 2°C, 4°C, 6°C, and 8°C. Skin was previously adapted at 34°C. Each histogram constructed from mean of five consecutive responses selected from a train of stimuli at 60‐s intervals; bin width, 200 ms. Profile of warming pulse shown in black beneath histograms. In B and C, PSTHs of responses to stimuli of intensities 2°C and 8°C are matched against PSTH of a response to a warming pulse of 4°C, shaded in both figures. Data normalized in each instance by equating total number of impulses occurring within first 4 s of stimulation.

From Darian‐Smith et al.


Figure 18.

Intensity functions of single warm fibers at different skin adaptation levels (29°C, 34°C, and 39°C). Warming pulses lasted 4 s and were presented in random order every 60 s. Lower right, plot of average intensity function for each of these fiber samples; there were 17 fibers common to each sample. With an additional 5 warm fibers, intensity function was estimated at skin adaptation level = 34°C and either 29°C or 39°C.

From Darian‐Smith et al.


Figure 19.

Families of intensity functions relating mean response of a thermoreceptive fiber (cumulative impulse count during first 4 s of stimulation) to intensity of temperature pulse when test stimulus is one of a train of stimuli and stimuli preceding it are all specified. Stimulus train had the following characteristics: duration of temperature pulses, 4 s; stimulus repetition rate, 1/10 s; I0, intensity of stimulus; I1, intensity of stimulus preceding I0; I2, intensity of stimulus preceding I1. Left: responses of a cold fiber (averaged data from 10 cold fibers), with I2 = 0°C for whole sequence; family of curves illustrates suppressive action of I1, as its intensity was increased from 0°C to 8°C. Middle: responses for a warm fiber (averaged data from 14 warm fibers) to a similar sequence of warming pulses; similar family of intensity curves. Right: family of intensity functions illustrate responses of same sample of warm fibers examined in middle graph. However, now I2 = 8°C; even I2 has a substantial suppressive action on warm fiber response. Finally, intensity function of a slowly adapting (SA) mechanoreceptive fiber responding to sequence of cooling pulses described above is shown in left graph; although responsive to cooling pulses, SA mechanoreceptive fibers are insensitive when compared with cold fibers.

Data from Darian‐Smith et al.


Figure 20.

Responses of a single warm fiber innervating hairy skin on dorsum of hand in man. Skin temperature was increased rapidly (1.5°C/s) from adaptation temperature to a new level 5°C above this. Fiber became more responsive to test stimulus as adaptation temperature was increased from 27°C to 32°C to 37°C.

From Konietzny and Hensel


Figure 21.

Series of intensity functions for a single cold fiber, relating cumulative impulse count over 4 s to amplitude of cooling pulse, at different skin adaptation values of 25°C, 29°C, 34°C, 39°C. Interval between stimuli was 25 s to reduce temporal suppressive action. Slopes of these intensity functions did not change with changes in skin adaptation temperature over this range. When adaptation temperature of skin was 41°C, this fiber was unresponsive to cooling pulses less than 2°C in intensity; about one‐third of fibers examined at this adaptation level were similarly unresponsive to comparable small cooling pulses.



Figure 22.

Subjective estimates of intensity of identical warming pulses when presented in different stimulus trains. Base temperature 38°C. When interstimulus interval between successive stimuli was 225 s (abscissa), subject's estimate of its intensity was greater than when interstimulus interval was short (ordinate). This observation illustrates the reflection of the suppressive interaction observed in thermoreceptive fibers (Fig. ) in the subject's performance.

Adapted from LaMotte and Campbell


Figure 23.

Graphical representation of steps in analysis of a warm fiber's capacity to resolve incremental changes in intensity of a warming pulse, as measured by discriminable stimulus increment (DSI). In this example skin was adapted at 34°C; warming pulses used in experimental sequences had amplitudes of 4°C and 4.5°C and were presented alternately in a continuing train, as shown in response sequence in lower part of figure. Mean cumulative impulse count at 2 intensity levels of 4°C and 4.5°C, sensitivity (dr/dI, change in mean cumulative impulse count/1°C change in stimulus intensity), standard deviation of difference between individual response pairs (Δr), and finally DSI (0.67 × σΔr)/(dr/dI), are plotted in 4 graphs for successively longer segments of stimulus period.

From Darian‐Smith et al.


Figure 24.

Distribution of discrimination stimulus increment (DSI) for 13 warm fibers. Adaptation temperature, 34°C; amplitude of warming pulse, 4°C. DSI estimated for successively longer integration intervals, increasing by 200‐ms increments. Lines plot changing DSI with stimulus duration for 2 typical fibers of sample. Shaded zone is bounded by difference limen for human subjects differentiating identical sequences of warming pulses (0.04°C) and longest decision time used by these subjects (3 s).

From Darian‐Smith et al.


Figure 25.

Families of curves illustrating dependence of the discriminable stimulus increment (DSI; see text) for a population of warm fibers on stimulus period over which response was measured. In left and middle graphs each curve describes relationship for population of n fibers, where n = 1, 2, 5, 10, 25, 50, and 100. Adaptation temperature for skin was 34°C; mean amplitude of warming pulses was 6°C. In left graph each fiber was considered to respond independently of its neighbors; equal weighting was given to each fiber's contribution to overall population response. In middle graph each fiber also responded independently, but now its contribution to overall population response was weighted to permit maximum possible resolution of changes in intensity of stimulus. In right graph each fiber's contribution to population response was given equal weighting, but now the fibers were not responding entirely independently of their neighbors. Correlation coefficient, ρ, defines dependent variability between pairs of fibers in responding population; when ρ = 0, fibers were responding independently, but when ρ = 1, all fibers varied entirely in accord. Shaded zone defines behavior of a trained observer differentiating pairs of warming pulses similar to those used in fiber study, zone being bounded by difference limen and decision time for this task. Only when function for a particular fiber population overlaps the shaded zone does that population signal sufficient information to brain to account for human discriminative behavior.

Adapted from Johnson et al.


Figure 26.

Families of Weber functions relating discriminable stimulus increment (DSI) for a population of responding warm fibers (in macaque) to mean intensity of warming pulses being differentiated. Each curve describes Weber function for a population of n fibers, where n = 1, 2, 5, 10, 25, 50, and 100. Skin was adapted at 34°C for all observations. Experimental conditions pertaining for each of the three graphs matched with those described in legend of Fig. . Response measure for all observations was cumulative impulse count during first 2 s of stimulation. Shaded zone defines best observed intensity resolution (difference limen) determined in human subjects for stimulus sequences similar to those used in fiber study.

Adapted from Johnson et al.


Figure 27.

Response of a warm fiber to graded warming pulses when adaptation temperature of skin was 39°C. Discharge by warming pulses with intensities of 0°C, 2°C, 4°C, 6°C, and 8°C is shown on the left, along with profile of warming pulse. Plateau temperature of warming pulses are also indicated on extreme left. This fiber did not respond at all when pulse plateau temperature was raised to 49°C. Right: mean response profiles; each profile constructed by averaging 5 successive responses.

From Darian‐Smith et al.


Figure 28.

Family of intensity functions relating mean discharge (cumulative impulse count during first 3 s of a heating pulse) of a nociceptive fiber to amplitude of stimulus. Adaptation temperature was 38°C; intensity of heating pulse is indicated in terms of plateau temperature (45°C‐53°C). There was a monotonic increase in response over this range, which contrasts with responses of warm fibers (Fig. ). Suppressive interaction such as occurs in warm fibers was also observed in nociceptive fibers; if stimulus preceding test stimulus (Sn‐1) was high, response to test stimulus was reduced, as seen in family of intensity functions.

From LaMotte and Campbell


Figure 29.

Responses of a cold fiber to near‐rectangular cooling and warming pulses (lasting 10 s) before and after exposure to noxious heating of skin. Steady skin temperature between successive stimuli was 35°C (arrow). Solid curve labeled 1–2 indicates fiber response prior to noxious heating of skin. Following this, noxious heating responses (dashed curve 3–4) to skin cooling were unchanged, but terminal was now sensitized to warming. Fiber now responded when skin temperature was raised to 43°C‐44°C, whereas with initial heating of skin no response was elicited until temperature of skin was raised to about 50°C.

From Dubner et al.


Figure 30.

Diagram summarizing available data concerning terminations of thermoreceptor fibers and origin of spinothalamic projections within cervical segments of primate spinal cord. Most warm and cold fibers terminate in marginal lamina I of Rexed. Also, the few specifically thermoreceptive dorsal horn cells that have been identified were mostly within lamina I. Many of these marginal cells are spinothalamic; the majority pass into contralateral anterolateral column, but some enter ipsilateral column. Branches of large‐diameter mechanoreceptor fibers ascending in dorsal columns and the spinocervical projection (not thermoreceptive) are also shown. Few if any spinothalamic neurons have their cell bodies located within the substantia gelatinosa.



Figure 31.

Responses of low‐threshold thermoreceptive neurons within marginal lamina I of coccygeal spinal cord of rhesus monkey. In A and C, upper record is of average rate of discharge in 2 different thermoreceptive cells during cooling of skin, caused by evaporating ethyl chloride; changing skin temperature is indicated in lower trace. Neuron responding to cooling in A was also examined when skin was warmed by infrared radiation. This response is shown in B, in which upper trace is of intervals between successive impulses. Neither of the thermoreceptive cells responded to innocuous mechanical stimulation of skin.

From Kumazawa and Perl


Figure 32.

Camera lucida drawing of a neuron within marginal lamina I of the sacral/coccygeal spinal cord of the cat. Neuron was stained by intracellularly injected horseradish peroxidase following recording of its responses. It was excited to discharge by innocuous cooling of skin and inhibited by low‐intensity mechanical stimulation of same area of skin. Cell also discharged in response to electrically evoked dorsal root volleys when these included discharging unmyelinated fibers. Hatching marks boundary between white matter and marginal lamina. Open arrow indicates axon.

From Light et al.


Figure 33.

Distribution of cell bodies of spinothalamic neurons in lumbosacral enlargement of spinal cord in macaque. Cells were identified by retrograde uptake of horseradish peroxidase following its injection either into ventral posterolateral nucleus (upper half of figure), or more medially into central lateral nucleus (lower figures). Histograms show relative numbers of cell bodies within different laminae of Rexed; spinothalamic neurons with contralateral and ipsilateral thalamic projections are plotted separately. Spinal cord sections illustrate locations of each cell body observed in a succession of serial sections.

Data from Willis et al.


Figure 34.

Projections to diencephalon from contralateral dorsal column nuclei, lateral cervical nucleus, and anterolateral column of spinal cord of squirrel monkey (Saimiri sciureus). Coronal section in stereotaxic anterior planes +5, +6, and +7 mm, as defined in stereotaxic atlas by Emmers and Ackert . CL, central lateral nucleus; CM, centre médian n.; LP, lateral posterior n.; MD, mediodorsal n.; MGN, medial geniculate n.; Pf, parafascicular n.; PO, posterior group; Pul, pulvinar n.; R, reticular n.; St, subthalamic n.; Vim, ventral intermedial n.; VL, ventrolateral n.; VPI, ventral posteroinferior n.; VPL, ventral posterolateral n.; VPMpc, paracentral decision of VPM.

Adapted from Berkeley


Figure 35.

Responses of thermoreceptive neurons within nucleus caudalis of brain stem trigeminal complex of cat. Upper record: thermoreceptive neuron specifically excited by cooling skin of lower lip (inset). Upper tracing, skin temperature; lower tracing, discharge rate of cell. Lower left record: similar sequence, but this neuron was specifically excited by warming skin at outer canthus of eye (inset). Lower right record: discharge rates in the two cells at steady skin temperatures; responses of cold unit were maximal at about 15°C.

From Dostrovsky and Hellon


Figure 36.

Discharge patterns in 2 neurons located within ventrobasal complex of thalamus of squirrel monkey (Saimiri sciureus). Lower graph: cell was specifically excited by cooling its receptivefield zone on tongue; cell was unresponsive to innocuous mechanical stimulation. Upper graph: cell, although excited by cooling the tongue, also responded to innocuous mechanical stimulation of same zone. Same sequence of thermal stimulation was used to examine responses of both neurons; 30‐s stimulus periods were alternated with a 90‐s period during which skin was at 35°C. Numbers immediately below the lower graph indicate pattern of alternating skin temperatures

From Poulos and Benjamin
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Ian Darian‐Smith. Thermal Sensibility. Compr Physiol 2011, Supplement 3: Handbook of Physiology, The Nervous System, Sensory Processes: 879-913. First published in print 1984. doi: 10.1002/cphy.cp010319