HARKV GRUNDFEST I 23 



All sensory receptors may be considered as endowed with specialized trans- 

 ducer membrane^ (27, 57, 183). Auditory and labyrinthine cells probably have 

 deformation-sensitive, mechano-transducer membrane. In some mechano- 

 Iransducer sense organs, free nerve terminals themselves act as the receptors. 

 In others, the receptor appears to be a specialized cell, and many anatomically 

 distinguished types of these are known. A graded electrical response is re- 

 corded in the nerve tiber on mechanical stimulation of some (8, 92, 128). It is 

 not known, however, whether the electrogenic response is generated in these 

 cells themselves and then transmitted to their innervating afferent fibers, or 

 whether the cell is only an adjunct to mechano-transducer membrane in the 

 nerve fiber (cf. 183). Responsiveness of mechano-sensory receptors to, or their 

 sensitization by chemical agents has often been reported (cf. 59). For example, 

 the crayfish stretch receptor dendrites are e.xcited by acetylcholine (188). In 

 this case it is clear that the membrane is certainly a mechano-transducer, and 

 its chemical excitation seems to be an adjunct, often exhibited in other cells 

 and differentiated by the classification of 'adequate' and 'inadequate' stimuli. 

 The responsiveness to chemical as well as mechanical stimulation may be 

 indicative of structural similarities between the transducer molecules. In other 

 cases the specialized receptor cells might be the mechano-transducer, releasing 

 a chemical to activate chemo-receptor membrane of the nerve terminals. 



The crayfish stretch receptors also illustrate clearly the differences char- 

 acteristic among sensory receptors of a given class. The slowly adapting 

 receptor is more sensitive to the mechanical stimulus (cf. 127), to applied 

 acetylcholine, and to Ca++ deficiency than is the rapidly adapting receptor cell 

 (188). 



b) Other Types of Sensory Transducers. Visual and temperature receptors 

 may be considered as possessing specialized membrane for transducing photic 

 or thermal energy. The graded potentials of retinal activity are well known 

 (e.g., 91). The actions of thermal transducers have been measured, as yet, 

 only as spikes generated in their nerve fibers (27, 191). Chemo-transducer 

 membrane probably accounts for the receptors of olfaction, taste, and vascular 

 and respiratory control. 



It has been suggested (cf. 45) that the remarkably regular pulsatile dis- 

 charges of various electric fishes subserve orientating functions. These animals 



3 The behavior of insect flight muscles (16) indicates the possibihty that a mechano- 

 transducer with positive feedback is involved, the transducer action being coupled either 

 through elect rogenesis or directly to the mechanical response. The self-exciting rhythmic 

 properties of heart muscle, its peculiar electrogenic activity, responses to neural stimuli and 

 to drugs suggest that mechano-transducers may play a role in this tissue under the special 

 conditions of syncytial structures. Mechano-transducer properties, such as are present in 

 muscle spindles, might also account for the increased potential observed (164) in stretched 

 muscle fibers. 



