12 The Physiology of Sense Organs 



increase, to be augmented later by discharge of action potentials 

 in the cells with the higher threshold (fig. 3). 



An alternative method of monitoring broad ranges in stimulus 

 energy is to transform the energy in terms of a logarithmic 

 function, a principle which was first observed in single sensory 

 elements thirty-five years ago.'^ At high stimulus intensities, the 

 increase in energy required to bring about a given increase in 

 impulse frequency may be several orders of magnitude greater 

 than it is at threshold. Many sensory neurons display this type 

 of behavior (fig. 4, A-c), which is now regarded as a general 

 condition. The mechanisms responsible for a logarithmic 

 transformation are far from clear, however. Sherrington^^ once 

 stated that sensory end-organs are structures for lowering the 

 threshold of the nerve to a specific form of energy; although the 

 simplicity of this view might now be embellished with the results 

 of many sophisticated electrical investigations which have been 

 performed since its enunciation sixty years ago, we are very little 

 closer to understanding the initial mechanisms involved in the 

 energy absorption process. The difficulty in our way is mainly 

 our present inability to define the energy-utilizing reactions of the 

 membrane to a given stimulus; in other words, we are ignorant 

 of the precise changes which happen to the membrane's molecular 

 architecture when energy is absorbed. Nor is it clear precisely 

 which form of energy is most effective in producing these changes. 

 In studies on mechanoreceptors, for example, stimulation is 

 variously measured in terms of muscle tension, shearing forces 

 applied to a cuticular sensillum, or compression of the receptor 

 structure. One critical question is what exact percentage of the 

 applied energy is actually used in the alteration of membrane 

 structure leading to impulse initiation. An example will illustrate 

 current confusion in this aspect of sensory physiology. In the 

 crustacean muscle stretch-receptor organs, the adequate stimulus 

 is probably the deformation of the sensory nerve-endings which 

 are imbedded in the receptor muscle, and this can be brought 

 about either by increases in muscle length or tension. However, 

 the impulse frequency in the sensory neuron is a linear function 

 of muscle length, but a logarithmic function of muscle tension. 

 It has been pointed out that muscle does not obey Hooke's Law^^; 

 thus, the data obtained may be explained by the possibility that the 



