[24 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



thermal, electromagnetic (as light) and chemical. 

 With the exception of one important group, it ap- 

 pears that nearly all receptors are especially sensitive 

 to one form of energy and either completely or rela- 

 tively insensitive to all others. The excepted group 

 consists of those receptors that have a low sensitivity 

 to all types of energy, but will respond to any form 

 of energy that reaches a damaging or near damaging 

 level; these receptors are of course those that give 

 rise to defensive reflexes and are associated with the 

 sensation of pain. The specificity of receptors to a 

 particular form of energy was first propounded in 

 modern times by Miiller (76). As a whole this con- 

 cept is not seriously challenged, but recently an 

 attack has been made on its application to receptors 

 situated in the skin (99). The objections are based 

 both on the finding that there are areas of human 

 skin in which morphologically specialized endings 

 are not found but from which all modalities of sen- 

 sation can be elicited (39), and on the results of 

 certain sensation experiments (65, 66). There need 

 be no rigid correlation between morphological and 

 functional specializations; it is indeed interesting to 

 note that the bulk of the direct evidence for the func- 

 tional specificity of sensory units has come from 

 preparations of frog and toad skin which contain 

 few morphologically differentiated nerve endings 

 (24). As regards to sen.sation experiments it must be 

 realized that sensations are the end result of compli- 

 cated processes and that such experiments, while 

 giving information about sensations and their specific- 

 ity, cannot weigh heavily against direct evidence on 

 the properties of sensory units. 



Direct evidence of the specificity of units has been 

 obtained by recording the responses of single ones to 

 different stimuli. In the earlier experiments of this 

 type single fibers were not isolated in an anatomical 

 sense, but small bundles of nerve fibers were used so 

 that the activity of indi\idual units could he identi- 

 fied and analyzed; using this technique it was possible 

 to show that therrnal and near-damaging stimuli 

 only excited activity in small fibers and did not 

 produce activity in the larger fibers which responded 

 only to mechanical stimuli (2, 50, 103, 104). This 

 type of work has now been carried a stage further by 

 isolating and recording from single afferent fibers 

 that have their receptive fields in the skin of toads and 

 of cats (73). Results of such experiments show that 

 mechanically sensitive units are not easily excited by 

 thermal stimuli [though this has been shown to hap- 

 pen in the cat's tongue (45)] or by acid; that ther- 

 mally sensitive units do not normally respond to 



mechanical stimulation [the rattlesnake pit organ is 

 an exception (15)]; and that units responding to 

 acid, prick or burning do not respond to small 

 mechanical or thermal stimuli. 



The specificity of sensory units is not confined, 

 however, to a simple distinction between different 

 types of energy but involves distinctions between 

 other properties of the stimulus. Of these properties, 

 those connected with its time course are perhaps the 

 most obvious; the different rates of adaptation ex- 

 hibited by different units is an example which will 

 be considered again. .Specificity to a particular band 

 of frequencies of a periodic function is another ex- 

 ample; thus there is evidence that different units in 

 the retina respond to different frequencies of light 

 waves (31, 91) and that primary units from the 

 mammalian cochlea have particular characteristics 

 in relation to the frequency of the sound waves (93). 

 In both these instances the sensory units are display- 

 ing a specificity, ijut there is clearly a difference in 

 the way this specificity is brought about. In the 

 retina it seems probable that individual receptors are 

 different, but in the cochlea it is the mechanical 

 properties of the system that are mainly responsible 

 for the results. Such a distinction between the proper- 

 ties of the receptor and the properties of the support- 

 ing tissues is one that ari.ses in other situations but is 

 one that is irrelevant in the context of describing the 

 properties of sensory units. The examples of specificity 

 so far given in this paragraph are concerned with 

 time factors, but there are others. Thus there are 

 two types of thermal unit found in the cat's tongue; 

 in both types the frequency of the impulse discharge 

 depends on the temperature of the receptors, but in 

 one group the maximum frequency is found at a 

 temperature of 30 to 32 °C (46), while in the other it 

 occurs at 37.5 to 4o°C (22). Again, units in the cat's 

 tongue responding to chemical stimuli, and pre- 

 sumably responsible for the sensation of taste, can 

 be grouped in respect to the substances that are able 

 to set up activity in them (82). 



Adaptation 



When a piece of tissue containing a receptor sensi- 

 tive to mechanical stimuli is subjected to an abrupt 

 increase in the forces applied to it and the new situa- 

 tion is then maintained, the sensory unit will dis- 

 charge impulses at a frequency, which starts at a 

 relatively high value and then decreases with time 

 (4, 74, 75) (fig. i). This decline in frequency is known 

 as adaptation and may be slow or rapid. In those 



