43^ 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



TABLE I Distrihiitinn (if Warm nrul Cold Spots 

 in Human Skin* 



Forehead 



Nose 



Lips 



Other parts of face 



Chest 



Abdomen 



Back 



Upper arm 



Forearm 



Back of hand 



Palm of hand 



Finger dorsal 



Finger volar 



Thigh 



Calf 



Back of foot 



Sole of foot 



* Number per cm-. 



t After Strughold & Porz (85). 



I After Rein (72). 



geminal area which in man is directly exposed to all 

 weathers no doubt has special importance. 



Concerning the thermal sensitivity of animals our 

 knowledge is very scarce and scattered. Until recently 

 cold-blooded animals were not believed to possess any 

 specific thermoceptive organs. Sand (77) using elec- 

 trophysiological methods discovered that the Loren- 

 zinian ampullae of Raja reacted to cooling. The 

 Lorenzinian ampullae of the elasmobranchs are 

 situated laterally in the region of the head and con- 

 sist of a group of small mucous cysts lying subcu- 

 taneously. They are supplied by afferent fibers from 

 the facial nerve. 



The facial pits of the pit viper (Crotalidae), which 

 originally were believed to function as mechano- 

 ceptors specialized for the detection of air vibrations, 

 were clearly shown by Noble & Schmidt (70) through 

 behavioral experiments to detect the body tempera- 

 ture of the snakes' prey. They proved that snakes 

 with the other principal sense organs of the head 

 nonfunctional can still strike correctly at moving ob- 

 jects and can discriminate between warm and cold 

 ones as long as the pits are uncovered. The organ 

 consists of a small pit about 3 mm in diameter covered 

 by a membrane 15 /z thick. This thin membrane is 

 the innervated sensory surface. Leading off from 

 microelectrodes, steel needles with tip diameter of 

 al)out 3 to 7 ;u inserted into the membrane, Bullock 



& Cowles (12), Bullock & Diecke (13) and Bullock & 

 Faulstick (14) pro\ed that the afferent nerve endings 

 serv-e as infrared receptors. They are, so far as we 

 know at pre.sent, the most densely distributed warm 

 receptors and the most effective organ for infrared 

 detection within the animal kingdom. In mammals 

 cold sensiti\ity seems to be located particularily on 

 the bare parts of the nose and on the tip of the 

 tongue. More details are not available as yet. 



Depth of Thermal Nerve Endings 



The fact that the reaction time for warmth is con- 

 sistently longer than that for cold suggested that the 

 warm receptors should be located deeper in the skin 

 than the cold receptors (87, 91). This assumption had 

 many proponents (i, 26, 27, 72). Bazett et al. (5) 

 calculated the depth of the thermal receptors in the 

 prepuce. The skin was stretched out into a flat sheet 

 by means of small ijari^less fish-hooks. Sensitive spots 

 belonging to one layer of skin could thus be stimu- 

 lated from either side of the double fold. The rate of 

 transmission of the temperature wave through the 

 fold was determined by thermoelectrical recording; 

 the value obtained of about i mm per sec. is in agree- 

 ment with more recent measurements of Hensel & 

 Zotterman (55). From this figure and the reaction 

 time of the subject so stimulated it was possible to 

 estimate the depth of the receptors. 



The average depth of the warm receptors was thus 

 found to be 0.3 mm. For the cold receptors the aver- 

 age depth was computed to somewhat less than 0.17 

 mm. The depths of the receptors for cold and warmth 

 computed in this way were in good agreement with 

 the average depth of the Krause and the Ruftini type 

 of end organ respectively as determined histologically. 



This and other previous methods based upon the 

 subjective reaction time to thermal stimuli must be 

 subject to rather large errors because a great number 

 of uncontrollal)le reactions take place between the 

 application of the stimulus and the conscious action 

 of the subject, the time of which is many times longer 

 than that of the actual peripheral events occurring in 

 the thermal receptors themselves. 



By using the method of recording the spike poten- 

 tials in the specific cold fibers Hensel et al. CoO 

 developed a method of physiological depth determina- 

 tion which eliminates the errors of the methods previ- 

 ously used. The method has been used for determining 

 the depth of the cold receptors on the tip of the tongue 

 of the cat and the dog but can of course in principle 

 be applied even to human suijjects. 



