INITIATION OF IMPULSES AT RECEPTORS 



143 



appears unable to conduct impulses and this same 

 region is structurally specialized, in particular in not 

 having a Schwann cell sheath. The estimated poten- 

 tial change that occurs across the membrane of this 

 part of the fiber during a maximum receptor potential 

 is of the same order of magnitude as the resting and 

 action potentials. Many receptors are sensitive to 

 acetylcholine, though it is not known whether or not 

 the Pacinian corpuscle is sensitive. It is tempting to 

 suggest that the inability to conduct impulses, the 

 sensitivity to acetylcholine, the ability to produce re- 

 ceptor potentials and the absence of the Schwann cell 

 sheath are all connected. There is not howe\er enough 

 evidence at present to support such an assertion. 



Olfactory receptors appear to fall in line with much 

 of what has been said in the last few paragraphs. To 

 some extent photoreceptors may as well, but these 

 considerations belong to other chapters. For the rest 

 of this discussion consideration will be given almost 

 entirely to simple mechanical receptors as it is from 

 receptors of this type that the relevant evidence is at 

 present available. 



The next point to be considered is the immediate 

 source of energy utilized in the production of a recep- 

 tor potential. Maintained receptor potentials that last 

 for minutes have been recorded, and if it is assumed 

 that receptor potentials are responsible for the initia- 

 tion of impulses in certain other receptors, for example 

 the mechanical receptors in the carotid sinus of the 

 cat, receptor potentials must remain constant for 

 hours (17). Such potentials cannot be maintained 

 across a biological membrane without the continual 

 utilization of energy; such energy clearly cannot be 

 provided by the work done during the deformation of 

 the receptor. Since this is so, an internal store of 

 energy must be available. It has already been argued 

 that receptor potentials are generated across the mem- 

 brane of the terminal portions of the afferent nerve 

 fiber. Across this membrane is a store of energy in the 

 form of the electrochemical gradients of the principal 

 ions. It seems likely that it is this energy which is 

 utilized during the activity of the receptor. In all 

 slowly adapting receptors some such internal store of 

 energy must be available; this does not necessarily 

 follow for rapidly adapting processes such as the re- 

 ceptor potential of the Pacinian corpuscle and the 

 dynamic phase of the muscle spindle potential. While 

 it seems reasonable that all mechanical receptors of 

 the relatively simple group under consideration should 

 have fundamentally the same mechanism, there is no 

 conclusive evidence that this is so. In fact it has been 

 suggested (58) that the static and dynamic phases of 



the muscle spindle receptor potential may have dif- 

 ferent mechanisms. 



During receptor activity the potential across this 

 membrane must alter. One suggestion discussed as an 

 explanation of the dynamic phase of the muscle 

 spindle receptor potential was that the potential 

 change was a result of a change of membrane capacity, 

 the total charge remaining constant; it was, however, 

 pointed out that there were quantitative difficulties in 

 this explanation (58). Similar calculations for the 

 Pacinian corpuscle demand large increases in surface 

 area which are known not to occur. A inore likely 

 explanation of receptor activity is that ions transfer 

 charge across the membrane by moving down their 

 electrochemical gradients as a result of changes in the 

 permeability of the membrane to one or more ion 

 species (37, 58). If charge is to be transferred in such a 

 direction as to explain the observed potential changes, 

 cations must enter the fiber or anions leave it. The 

 internal anions of nerve fibers are mostly large and 

 less likely to move than the external cations which are 

 almost entirely .sodium. If the mechanism in question 

 were something of the kind suggested, it would then be 

 expected that the receptor potential would be nearly 

 abolished in the absence of sodium. This is in fact 

 what has been observed in the Pacinian corpuscle. 

 Another observation that can be explained on this 

 hypothesis is that the rate of rise of the receptor poten- 

 tial continues to increase with increasing stimulus 

 strength at a level of stimulus strength at which the 

 amplitude of the potential change remains practically 

 constant. This can be explained by assuming that the 

 permeability of the membrane continues to increase, 

 so increasing the rate at which charge is transferred, 

 while the final potential reached is limited by the 

 equiliijrium potentials of the ionic gradients con- 

 cerned. 



If the hypothesis put forward be accepted for the 

 time being, the next problem is to consider how the 

 changes of membrane permeability are brought about. 

 This might be due to a distortion of the membrane or 

 displacements in relation to surrounding structures, 

 it might be due to a change of pressure in and around 

 the axon or there may be chemical intermediaries 

 outside or inside the axon. The last alternative still 

 leaves the problem of how the mechanical energy 

 produces the chemical intermediaries. At present there 

 are no grounds for choosing between these mecha- 

 nisms. However, if there are chemical intermediaries in 

 the Pacinian corpuscle, the time course of their action 

 (the latency often being less than 0.2 msec.) and their 

 ability to function at room temperature (37) show 



