142 



HANDBOOK OF l'H%SIOLOGY 



NEUROPHYSIOLOGY I 



inner sh 



FIG. 14A. Diagram of a cross section of a portion of a frog's 

 muscle spindle, at resting length, in the region of the sensory 

 innervation. Inner sh., intrafusal muscle fiber inner sheath; 

 m.nuc, muscle nuclei; mf., myofilaments; sarc, sarcoplasm; 

 m., mitochondria; peri, subst., perimuscular substance; ax., 

 axons; Sch., Schwann cells. [By courtesy of J. D. Robertson.] 



FIG. 14B. Diagram of a transverse section of the central core 

 of a Pacinian corpuscle based on electronmicrographs. [By 

 courtesy of A. Quilliam.] 



HYPOTHESES CONCERNING THE MECHANISMS 

 OF RECEPTORS 



Many of our present ideas on the mechanisms in- 

 volved in the initiation of impulses by receptors stem 

 from the idea of nerve as a model sense organ (9). 

 This concept invokes two parts; first that a constant 

 current would excite repetitive discharges in a nerve 

 fiiaer, and secondly that such currents are produced 

 in nerves under physiological conditions by the devel- 

 opment of generator potentials in the receptors. 



It is now known that many receptors produce recep- 

 tor potentials and it is probably safe to assume that 

 this is a generalization that applies widely. There is 

 good evidence, which has already been considered, 

 that these receptor potentials are the immediate cause 

 of the impulse discharges. At present there is no evi- 

 dence or need to suppose that the part of the afferent 

 fiber in which the impulses are .set up differs from 

 other parts of nerve fibers in its response to a flow of 

 current, whether this be a flow of current due to a 

 receptor potential, to an external source or to the 

 summated effects of both. There is evidence that has 

 already been considered which indicates that in the 

 Pacinian corpuscle impulses are set up at the first 

 node of Ransier and that the terminal nonmyelinated 

 portion of the nerve fiber does not appear capable of 

 conducting impulses. Similar conclusions can be 

 drawn for the stretch receptor of the crayfish, though 

 in this instance it is not possible to put such clear ana- 

 tomical limits to impulse conduction. It may well be 

 a general property of receptors that impulses are set up 

 at a point central to the sensitive terminals by currents 

 which are generated elsewhere. The summation noted 

 between natural stimuli and externally applied cur- 

 rents would result from a passive summation of the 

 discharging process in this region of the membrane. 



The results just considered further suggest that the 

 part of the nerve fiber that is unable to conduct a nerve 

 impulse is the site at which the receptor potential is 

 generated. This view is supported by the fact that the 

 conditions under w hich receptor potentials have been 

 recorded from the three mechanical receptors indi- 

 cate that the current must have crossed the nerse fiber 

 membrane peripheral to the point of recording. Esti- 

 mates of the absolute value of the maximum receptor 

 potentials suggest that it is unlikely that those currents 

 that traverse the membrane of the nerve terminal are 

 secondary to acti\ity in an external source. Further- 

 more the fine structure of these terminals shows certain 

 distinctive features. Thus, to take the specific example 

 of the Pacinian corpuscle, the nonmyelinated terminal 



