10 The Physiology of Sense Organs 



only one sensory neuron at a time. This was first successfully 

 accomplished in 1926 by Adrian and Zotterman* using the 

 sterno-cutaneous muscle of the frog. This muscle, which contains 

 only three or four stretch-sensitive sensory nerve cells, was 

 systematically dissected until the afferent fiber from only a single 

 end-organ remained intact and active. The results of these 

 studies demonstrated conclusively, in records uncomplicated by 

 multifiber discharges, that primary sensory neurons respond to 

 an increase in the intensity of an adequate stimulus by augmenting 

 the frequency of propagated impulses. 



More precise measurements of the relationships between 

 stimulus intensity and impulse frequency were made several years 

 later by Matthews'^ in a systematic series of investigations using 

 a more routinely accessible preparation. Matthews was the first 

 to observe the well-known logarithmic relationship between 

 stimulus strength and steady impulse frequency, to which many 

 sensory cells have been subsequently shown to approximate. 



Now, there are theoretical and practical limits to the maximum 

 frequency capabilities of the neuronal membrane. The finite 

 duration of the action potential and its recovery processes fix this 

 limit at about 1000 per second, although even higher values than 

 this can occur in a few cases among vertebrates. At the same 

 time, the threshold of a receptor must be sufficiently low so that 

 small but significant amounts of stimulus energy — or changes in 

 available energy — can be detected by the organism. While, 

 theoretically, there is really no lower limit to the frequency re- 

 sponse of a neuron (unless it be less than once in the life span!), 

 in practice, the survival of most metazoans depends upon a 

 nervous system having individual units which operate with time- 

 constants of the order of milliseconds. A sensory response of one 

 impulse per 100 seconds would normally be quite useless in terms 

 of information content, since it would be indistinguishable from 

 background noise to cells in the central nervous system. In 

 practical terms, a lower limit of one impulse per second can be 

 chosen as a meaningful threshold frequency. Now, many 

 external sense organs are routinely presented with stimulus 

 energies extending over an enormous range — at least lo^^ in 

 photosensitive cells. Even if the maximum frequency response 

 of a sensory neuron approached 1000 per second, even relatively 



