Types of neuronal responses in the rat thalamus to peripheral temperature changes

Recordings were made of spike-trains from 163 neurons of the rostral part of the ventrobasal thalamus complex of the rat while the temperature of the scrotal skin was altered. The following results were obtained: 55 neurons were non-thermosensitive, 7 neurons cold-sensitive and 101 neurons warm-sensitive. In the case of the warm-sensitive cells a definite discrimination was possible: 61.4% of the neurons altered their firing behavior during peripheral cooling, changing from relatively even spike frequency to burst firing. This change could be induced repeatedly by altering the temperature. 38.6% of the neurons, on the other hand, reacted to cooling by slowing down their frequency. By way of statistical methods the firing patterns of the two response-types were analyzed more precisely and the differences in response to temperature stimuli more exactly defined. Likewise, the spatial distribution of the two response-types of warm-sensitive cells exhibited differences; whereas the cells devoid of burst activity occured rather uniformly in the region of the thalamus studied, the cells with bursting activity were confined more to the mediocaudal region. These findings are discussed with regard to the phenomenon of peripheral bursts and to the projection of thermoafferent pathways onto the ventrobasal thalamus complex. The functional interpretation of the various cell reactions and their role in the central processing of thermoafferent signals still remains unexplained and requires comparative studies of peripheral and central parts of the thermoafferent system.

Sorting of signals from thermosensitive areas

Of the several models proposed for the neural regulation of temperature in cold-exposed animals, two have been previously restated in dynamic form using CSMP. Subsequently, computer simulations have led to the design and execution of experiments for selection of the more appropriate model for cold-exposed rats. These experiments, as described in the present paper, have been interpreted as being consistent with the model which sorts signals from thermosensitive areas and channels the selected signals over separate neural pathways to independently control each mode of heat production. Since this model requires multiplication of neural signals, possible neuronal mechanisms which may underline such multiplication are discussed. In addition, parameter variation to account for febril responses and rate sensitivity have been evaluated.

Epidermal thermal conductivity and stratum corneum hydration in cat footpad

Epidermal thermal conductivity (k) was calculated for the cat footpad by measuringtransepidermal heat flux and temperature gradient (¿T) while changes in stratum corneum water content were produced by "internal hydration" (IH; eccrine sweat gland activity by nerve stimulation) or by "external hydration" (HH; exposure to air saturated with water vapor). In some experiments, cutaneous vasoconstriction accompanying IH was prevented by an alpha-adrenergic receptor blocking agent, phenoxybenzamine (POB), 3.52 mg/kg iv. For dry skin k=0.167 plus or minus 0.023 (SE) W.m (-1) degree C (-1). With and without POB, IH produced 49.5% and 17.2% increases in k, respectively; HH after IH did not increase k more. With POB, IH increased k more than did HH alone (49.5% and 15.5%, respectively) and at a higher rate (4.22 times 10 minus 3 and 0.63 times 10 minus 3 W.m minus 1.degree C minus 1. min minus 1, respectively.) As k increased, usually deltaT decreased. Increasing k and decreasing deltaT with skin hydration explain the phenominon that air temperature is felt to be lower the more humid a cold exposure, since the temperature of thermoreceptors at the dermoepidermal junction is closer to that of the skin surface when the corneum is hydrated than when it has a low water content.

[Thermoreceptor function of the stretch receptors of the river crayfish]

Cooling of the crayfish stretch receptor leads to increase and then decrease the spike frequency of slowly adapting (SA) neuron (maximum frequency at 15 degrees). Impulse activity of quickly adapting (QA) neuron arises about 12 degrees and is recoding for a long time (sometimes during 30 min.). Raising the temperature from 20 degrees to 30 degrees results in a gradual increase of the SA neuron spike frequency. Characteristic for cold thermoreceptors dynamic discharge is observed. Infra-red (IR) radiation (lambda=1--3 mum) causes a decrease of SA neuron spike frequency. QA neuron generates a brief discharge after cuting IR radiation. IR radiation acts on somato-dendro-muscle (sensory) region only. Analysis with polarizing microscope shows that the IR radiation effect on the structural organization of neurons is contrary the effect of warming and colding. It is proposed that the IR radiation effect is defined by the rise of temperature gradient in solution (dT/dx). Obtained results indicate that the crayfish stretch receptor belongs to the type of mechanocoldreceptors.

Thermal receptors in the scrotum of the rat

1. The technique of single fibre dissection has been used to study the warm and cold thermoreceptors in the rat scrotum. 2. The warm receptors showed dynamic activity during increases of scrotal temperature and static activity when temperature was constant. The static activity/temperature curve was bell-shaped, with minima at 31 and 45 degrees C and a peak at 42 degrees C. 3. The cold receptors also showed dynamic and static responses to reductions of temperature. At steady temperatures the impulses from some receptors were grouped in bursts. The number of impulses in each burst increased from zero at 30 degrees C to four at 20 degrees C.

Convergence in a thermal afferent pathway in the rat

1. In anaesthetized rats, unit activity was recorded in the afferent somatosensory pathway leading from the scrotum. Recording sites were in the dorsal horn near the entry zone of the scrotal nerve, in the ventrobasal complex of the thalamus and in the somatosensory (SI) cortex. During recording, the temperatures of the left and right sides of the scrotum were varied independently. 2. Almost all (64/67) the units in dorsal horn, thalamus and cortex responding specifically to scrotal temperature were equally affected by temperature changes on either side of the scrotum. The receptive fields of these units were bilateral and large, implying a massive convergence of fibres from thermoreceptors on to each central unit. In contrast, mechanosensitive units responded only to unilateral stimulation. 3. As a consequence of the convergence in the thermal pathway, the firing rate of each central unit was a function of an additive combination, often simply the sum, of the temperatures of the two sides of the scrotum. 4. The relationship between firing rate and the temperature of one side of the scrotum was sigmoid, the position, but not the shape, of the curve depending on the temperature at which the opposite side was maintained. An increase in the maintained temperature shifted the sigmoid response curve towards lower temperatures and vice versa. 5. The convergence which this pathway exhibits would be well suited to integration of the temperature of the scrotal skin, but not to spatial discrimination.

Static and dynamic responses of spinothalamic tract neurons to mechanical stimuli

The activity of primate spinothalamic tract neurons was investigated. Units were identified by antidromic activation from the diencephalon. Most had a spontaneous discharge. The fastest spontaneous discharge rates were from neurons activated by receptors in muscles or joints. However, such activity could be decreased by repositioning the hindlimb. Spontaneous activity was also altered by changes in anesthetic level. Time-interval histograms demonstrated the patterns of spontaneous discharge. Many units could be excited by noxious stimulation. Graded step displacements of the skin often evoked a slowly adapting discharge which was a function of pressure (or displacements above a threshold of some hundreds of microns). Responses continued to increase as pressures became noxious. Rapidly adapting responses of hair-activated and low-threshold spinothalamic tract neurons were tested with ramp stimuli and found to signal acceleration (or a higher derivative of position), velocity, or a combination of these. Responses to stimulus acceleration were prominent when hairs were displaced, while responses to stimulus velocity were seen with displacement of the skin. Possible receptor types which might account for the observations are discussed.

Afferent new fiber activity responding to temperature changes of scrotal skin of the rat

The discharge patterns of single afferent fibers from rat pudendal nerve were studied as a function in temperature of the scrotal skin, an area known to function in temperature regulation. In a number of respects the population of temperature-sensitive afferents here differ from most previously described; 75% of temperature-sensitive afferents were also sensitive to mechanical stimulation. Of the 25% nonmechanosensitive units, half showed dynamic and static responses to cooling, while most of the remaining fibers gave only static discharges to warming. The most frequent thermal reaction of the mechanosensitive units was a dynamic-static cold response or a pure static warm response. However, fibers were also present with only dynamic or only static cold responses. Of the bimodal units, 20% had a dynamic cold response, but showed a minimal static discharge at intermediate temperatures (about 35 degrees C) and an increased discharge on both warming and cooling from that temperature. One unit had static and dynamic warm responses. Whereas a bursting discharge in the cold has previously been considered to be a distinguishing characteristic of specific cold receptors, in the pudendal afferents a bursting discharge on cooling or at low temperatures is common both in mechanosensitive and specific cold fibers. This observation and the identical discharge patterns and mechanisms underlying the thermosensitivity argue for the view that the mechanosensitive afferents participate in thermal sensation and/or regulation along with the specific temperature receptors. Hellon and Misra (7) have concluded that there is processing of thermal information from the scrotal skin at the level of the first synapse in the cord. However, in this study, we have found peripheral afferents which have most of the discharge properties that led Hellon and Misra to conclude that processing had occurred. The unusual characteristics of thermosensitive afferents of the pudendal nerve suggest that there is probably organ specificity of neuronal discharge properties.

Temperature regulation

The general way of looking at short-term temperature regulation has not fundamentaly changed since 1968. Some points nevertheless have been developed and deserve special attention: 1. The influence of water on the skin surface inhibits sweat secretion (55, 106). This fact may be the explanation of sweating fatigue and of discordant conclusions regarding the functioning of the regulator, particularly during exercise in man. 2. Since a large number of studies have shown that appropriate behaviors occur in response to all the stimuli that activate autonomic responses, behavior itself should be considered as an integral part of the thermoregulatory system (1, 2, 16, 18, 19, 21, 23, 25, 31, 32, 34-36, 48, 88, 89, 98, 99, 122, 126, 127, 137). 3. The description of the peripheral input for the control of sweating with regard to mean skin temperature (104) and time dependence (159) has been improved. Among internal temperature sensors those of the spinal cord have been extensively studies (25, 27, 32, 36, 42, 59-63, 71-75, 82, 83, 86, 113-115, 121, 150, 158) and demonstrated to have a sensitivity equal to that of the hypothalamic sensors (73, 75). 4. New hypotheses have been proposed describing the overall mechanism responsible for a constant temperature in the core (58, 96, 97, 135). These stimulating theories have been discussed briefly herein. Mechanisms for the defense against heat and against cold can be dissociated completely from one another. In the same way the control of autonomic responses can be dissociated from the control of behavioral responses. This suggests that temperature regulation is brought about by multiple independent feedback loops. The overall system is well described, in the author's opinion, by the theory of the adjustable set point with proportional control (47).

Static and dynamic activity of warm receptors in Boa constrictor

Afferent impulses from multi- and single-fiber preparations of the trigeminal nerve in Boa constrictor were recorded during exactly controlled thermal stimulation of the receptive field in the labial region. At constant temperatures in the range between 18 and 37 degrees C, multi-fiber preparations showed a continuous discharge with a maximum around 30 degrees C. Dynamic warming caused a high increase of the discharge, whereas dynamic cooling led to a complete inhibition. No cold-sensitive fivers have been found. Mechanical stimulation elicited large spikes from specific mechanoreceptors. Single-fiber preparations from labial warm receptors did not respond to mechanical stimulation. Their discharge was always regular at constant temperatures. The average frequency of a warm receptor population was zero at about 18 degrees C, reached a maximum of 13 sec-1 at 30 degrees C and fell again to zero at 37 degrees C. In addition, a few warm receptors increased their static discharge with temperature up to 36 degrees C, the highest frequency being 38 sec-1. Stepwise warming by delta T = + 5 degrees C caused a marked overshoot in frequency, after which the discharge usually fell to a minimum and then rose again to a new static level. Stepwise cooling by delta T = MINUS 5 DEGREES C led to a transient inhibition of activity followed by an increase until the new static level was reached. In the first group of warm receptors the height of the dynamic overshoot varied with the adapting temperature, the largest average overshoot of 160 sec-1 occurring at an adapting temperature of 30 degrees C. These receptors have their static maximum as well as their highest dynamic sensitivity in the temperature range of the natural tropical habitat of Boidae.

Effect of local cooling on sweating rate and cold sensation

Subjects resting in a 39 degrees C environment were stimulated in different skin regions with a water cooled thermode. This local cooling produced decreases in sweating rate measured at the thigh and increases in magnitude estimates of the cold sensation. The are of cold stimulation varied from 111 cm2 to 384 cm2. Sensitivity coefficients of the changes in sweating rate and magnitude estimate were corrected for differences in size of the area of stimulation and change in skin temperature and were normalized to the responses of the chest. The normalized coefficients showed the following relative sensitivities for changes in sweat rate and magnitude estimate respectively: forehead 3.3, 2.2; back 1.2, 1.4; lower leg 1.1, 0.9; chest 1.0, 1.0; thigh 0.9, 1.0; abdomen 0.8, 0.8. Varying the area stimulated from 122 cm2 to 384 cm2 produced greater changes in the sweating response than in the magnitude estimate. Rate of skin cooling during the period of stimulation had more effect on the sweating response than on the magnitude estimate. We conclude that cooling different body regions produces generally equivalent changes in the sweat rate and sensation, with the forehead showing a much greater sensitivity per unit area and temperature decrease than other areas.

The effect of firing rate on preoptic neuronal thermosensitivity

1. In anaesthetized rabbits, preoptic single units were recorded having positive or negative thermal coefficients (impulses/sec. degrees C) for changes in preoptic temperature.2. A population of forty-two positive coefficient units was divided into four groups based on their level of firing rate at 38 degrees C. In each group, the average thermoresponse curve was determined by averaging the firing rates of the units at 1 degrees C intervals over the 33-43 degrees C range of preoptic temperatures.3. A population of twenty-six negative coefficient units was divided into three groups based on their firing rates at 38 degrees C. Similar average thermoresponse curves were determined for each group.4. As the level of firing rate increased in the positive coefficient units, the preoptic thermosensitivity progressively decreased at temperatures above 39 degrees C, but generally increased at temperatures below 39 degrees C.5. In the negative coefficient units, preoptic thermosensitivity generally increased (especially above 39 degrees C) as the firing rate at 38 degrees C increased.6. The results indicate that in positive coefficient units the level of firing rate determines the temperature range in which units are most thermosensitive. This range reflects whether a neurone is more likely to function in heat-loss or heat-production responses.7. Since peripheral thermal input affects the level of firing rate of positive coefficient units, a neuronal model is suggested to explain the role of peripheral and central thermal signals in temperature regulation.

Temperature sensitivity of the paw of the cat: a behavioural study

1. Cats were trained to respond to differences between the temperatures of the floors of two corridors of a modified T-maze for a food reward.2. The cats were able to respond to differences between warm temperatures or differences between cool temperatures in the range of 1 degrees C.3. Both warm and cool discriminations were mediated by receptors located in or near the footpads.4. The paw of the cat thus appears to be more sensitive to temperature changes than was believed previously, and its temperature sensitivity may be comparable to that of the hand of the primate.

Characteristics, specificity, and efferent control of frog cutaneous cold receptors

1. Thermal stimulation of frog skin produces a discharge in afferents in the dorsocutaneous nerve. The characteristics of this response have been examined with regard to static and dynamic sensitivity to thermal stimuli and to mechanical sensitivity. Frog cutaneous receptors respond only to cooling, with no response to warming through the same thermal range.2. The static temperature at which these receptors are maximally active is about 24 degrees C for Rana pipiens and about 27 degrees C for R. catesbiana.3. The dynamic sensitivity of frog cutaneous receptors is linearly related to both stimulus slope and magnitude. Maximum dynamic sensitivity was between -90 and -120 impulses/ degrees C.sec.4. Antidromic occlusion experiments demonstrate relative insensitivity of these receptors to tonic mechanical stimulation. At high stimulus intensities, however, larger fibres are recruited into the response; this recruitment of action potentials of larger amplitude is a linear function of both stimulus slope and magnitude.5. Spike heights are linearly related to conduction velocities in the dorsocutaneous nerve; tonic mechanoreceptors have a mean spike height of 28.4+/-0.6 muV and conduction velocities about 6-8 m/sec, whereas these temperature sensitive receptors have spike heights 15.8+/-0.4 muV and conduction velocities about 3-4 m/sec.6. Maximum dynamic sensitivity skin is increased following stimulation of the first or second sympathetic ganglion. This increase is both marked and progressive, reaching a maximal enhancement of about 150-160% control at a stimulus rate of 5 stimuli/train, each train delivered once every 5 sec.7. Static sensitivity of the cold receptors is also increased following sympathetic stimulation. This increased sensitivity is shown by both increased discharge rate within the same thermal range and by decreased temperature of maximum static sensitivity.8. Sympathetic modulation of dynamic thermal sensitivity is mimicked by epinephrine and norepinephrine in doses of 10(-6)-10(-7) g/ml. Ephedrine, another adrenergic agonist, also mimics the enhancement of cold receptors by sympathetic stimulation.9. Larger fibres are recruited to account for the increased sensitivity of thermoreceptors following sympathetic stimulation and epinephrine application.10. Propranolol and phentolamine both block the enhancement of the response by sympathetic stimulation, but propranolol blocks the response of the receptor to thermal stimulation as well. Reserpine pre-treatment blocks the effect of sympathetic stimulation on the cold response.