Neurones in the dorsal horn of the rat responding to scrotal skin temperature changes

1. Micro-electrode recordings have been made from single neurones in the dorsal horn of male rats anaesthetized with urethane. Scrotal temperature was altered within the range 13-43 degrees C by means of a thermode. The mean firing rate of neurones was correlated with step and ramp changes of temperature.2. In the region where the scrotal nerve enters the cord, 47% of the neurones were responsive to scrotal temperature: half were excited by warming and half by cooling. Most of these thermally responding units were not affected when the scrotal skin was touched and only one-fifth responded to both modalities.3. Both the ;warm' and ;cold' groups of neurones showed responses to step changes of temperature which were classified as dynamic plus static, dynamic only or static only. Comparison of these responses with those published for the scrotal thermoreceptors showed that the incoming thermal information was being processed in the dorsal horn.4. Histological examination of the cord showed that recording sites were in laminae I to V of the dorsal horn.

Neurones in the ventrobasal complex of the rat thalamus responding to scrotal skin temperature changes

1. In rats the scrotal temperature was raised or lowered with a water-perfused thermode while micro-electrode recordings were made of unit activity in the ventrobasal complex of the thalamus. The electrodes were aimed at the region where evoked responses had been found by electrical stimulation of the scrotum. Recording sites were marked by iontophoresis of dye from the micro-electrode.2. Changes in firing rate of thalamic neurones were only found in the scrotal temperature range of 31-40 degrees C. Within this range, 72% of the 123 cells tested were excited or suppressed by skin warming. At temperatures above or below this range, activity was not affected. Most of the cells responded just to temperature and only 7% were also excited by touch.3. Raising temperature in the range 31-40 degrees C caused 82% of the thermally responding cells to increase their firing rate and 18% to decrease their rate. Individual neurones showed a sudden and maintained change in their activity for scrotal temperature increases of only 2, 1 or even 0.5 degrees C. Mean firing rates changed by factors of about 8 or more with these temperature increases and further warming did not change the rate. These step-like changes in firing rate were found at different points over the whole skin temperature range of 31-40 degrees C, but most were between 33 and 38 degrees C.4. For a given neurone the step-like change in activity occurred once its critical temperature was reached, irrespective of whether this was achieved by a step increase of skin temperature over 1-2 sec or by a slow ramp increase lasting several minutes.5. It is not possible to say whether the skin warm receptors, cold receptors or both were responsible for these thalamic responses, but the results do show that incoming thermal information is considerably processed when it reaches the thalamic level.

Neurones in the somatosensory cortex of the rat responding to scrotal skin temperature changes

1. In rats the scrotal temperature was raised or lowered with a water-perfused thermode while micro-electrode recordings were made of unit activity in the somatosensory (SI) cortex. The electrodes were inserted in the area where the largest evoked potentials had been found from electrical stimulation of the scrotum.2. Changes in firing rate of cortical neurones were found only in the scrotal temperature range of 32-41 degrees C. Within this range 40% of all the cells tested were excited or suppressed by skin warming. At temperatures above or below this range, activity was not affected. Most of the cells responded just to temperature and only 14% were also excited by touch.3. Raising temperature in the range 33-41 degrees C caused 83% of the thermally responding cells to decrease their firing rate and 17% to increase their rate. Individual neurones showed a sudden and maintained change in their activity for scrotal temperature increases of only 2, 1 or even 0.5 degrees C. Mean firing rates changed several-fold with these temperature increases and further warming did not change the rate. These step-like changes in firing rate were found at different points over the whole skin temperature range of 33-41 degrees C but most were between 35 and 39 degrees C.4. For a given neurone the step-like change in activity occurred once its critical temperature was reached, irrespective of whether this was achieved by a step increase of skin temperature over 1-2 sec, or by a slow ramp increase lasting several minutes.5. The results are very similar to those found in the thalamus (preceding paper), but the proportions of cortical cells which were excited or suppressed on skin warming were the reverse of the proportions seen in the thalamus.

Central Response to Infra-Red Stimulation of the Pit Receptors in a Crotaline Snake, Trimeresurus Flavoviridis

1. Both action potentials and evoked potentials were recorded from the tectum opticum of a crotaline snake, Trimeresurus flavoviridis, in response to infra-red stimulation of the facial pit organs. Action potentials from single units were recorded throughout the tectum.

2. Most units responded to contralateral stimulation, while some responded to both ipsi- and contralateral stimulation.

3. Firing patterns were tonic, phasic, or phasic-tonic, depending on the position of the stimulus and the type of unit being recorded.

4. Sensitivity to stimulus movement was observed.

5. All potentials differed from peripheral potentials in firing patterns.

6. Firing frequency was directly proportional to stimulus intensity.

7. Measurements were made of the vertical and horizontal response fields of single units.

8. Background discharge was noted in all units and its nature discussed.

9. The integrative function of the tectum in regard to infra-red perception was also discussed, as well as the possibility of stereoscopic perception.

Effects of heating and cooling the spinal cord and medulla oblongata on thermoregulation in monkeys

1. In the unanaesthetized monkey (Macaca cyclopis), heating the spinal cord or the medulla oblongata from 38 to 42-43 degrees C produced subcutaneous vasodilatation, respiratory acceleration (frequently interrupted with a period of apnoea in the medullary experiment), bradycardia and hypotension. The animal became drowsy and had a slight decrease of body temperature. Heating the medulla oblongata often induced retching and/or emesis.2. Cooling the spinal cord or medulla oblongata from 38 to 32-33 degrees C produced subcutaneous vasoconstriction, slower respiration, tachycardia and hypertension. The animal became restless and had a slight increase of body temperature. Cooling the spinal cord induced shivering of the four limbs, while cooling the medulla oblongata induced only shivering of the jaws.3. The effects of heating or cooling of the spinal cord or the medulla oblongata were antagonized by simultaneous application of temperature displacement of the opposite nature in the same areas or vice versa.4. The data suggest that some thermosensitive elements possibly responsible for thermoregulation reside in the spinal cord and the medulla oblongata.

Localization of intra-abdominal thermoreceptors in the ewe

1. In neutral environmental temperatures, shivering was induced in sheep by intra-ruminal cooling. Shivering was then depressed for a period by intra-abdominal heating. Intra-abdominal heating without ruminal cooling induced panting and a reduction of caval temperatures. Posterior caval temperatures were found to be inappropriate to the responses observed.2. Intra-abdominal heating of sheep in cold and warm environments depressed shivering and augmented panting respectively. Unilateral splanchnotomy abolished these responses on the ipsilateral side.3. The results are interpreted to indicate that the thermoreceptors stimulated lie within the walls of the rumen and intestine, and possibly the mesenteric veins.4. The splanchnic nerves are indicated as the afferent pathway for these receptors, and differential splanchnic innervation of the gut is suggested to explain unilateral abolition of the response to warming.

The significance of changes in the temperature of the skin and body core of the chicken in the regulation of heat loss

1. The relationship between the temperatures of the hypothalamus, colon and skin and the control of heat loss mechanisms at ambient temperatures from 20 to 40 degrees C has been studied in unanaesthetized chickens.2. The temperatures of the core and feathered skin varied by not more than 5 degrees C throughout the full range of ambient temperature but the comb, shank and toe varied by up to 20 degrees C and exhibited wide fluctuations in constant environmental conditions.3. At an ambient temperature of 26-27 degrees C, the fluctuations ceased at all of the naked skin sites and there was evidence of a concurrent decrease in the tissue insulation of the extremities. No such change could be detected in the feathered skin.4. Analysis of the various body temperatures suggested that the onset of thermal panting was consistently related to the increment in hypothalamic temperature and that this relationship was influenced both by the peripheral and extra-cranial deep body temperatures.

Evaporative cooling in the rat: differences between salivary glands as thermoregulatory effectors

Rats cool themselves in the heat by evaporating saliva groomed onto their body surfaces. Previous reports indicated that surgical removal of the submaxillary glands severely impaired evaporative water loss and body temperature regulation, whereas parotid desalivation had little effect. In the present experiments saliva was collected directly from the oral cavities of rats exposed to heat stress and was identified by its sodium concentration to determine the roles of individual salivary glands as thermoregulatory effectors. The fundamental difference between the glands was that submaxillary secretion was initiated at a lower level of hyperthermia (38.5 °C) than parotid secretion (40.0 °C), and thus made a greater contribution to thermoregulation. For example, normal rats secreted only submaxillary saliva at an ambient temperature of 36 °C and secreted three times more submaxillary saliva than parotid saliva at 40 °C. These results indicate that the submaxillary glands are primary thermoregulatory effectors of rats during heat stress, and that the parotid glands provide supplementary secretion only during pronounced hyperthermia.