INITIATION OF IMPULSES AT RECEPTORS 



141 



40 



30 - 



X 

 o • 



20 



10 



_ £ 



•o 



X 



60 



-•-i- 



100 



-o-»J 



J_ 



_L 



_L 



220 



260 



140 180 



Pressure, mm Hg 



FIG. 13. Effects of hexamethonium on the pressure-response 

 relationship of cat's carotid sinus receptors. Abscissa/ Pressure 

 in sinus in mm Hg. Ordinate: impulse frequency per sec. O, 

 X normal curves, • perfusion with i per cent hexamethonium. 

 [From Diamond (17).] 



and they appear to be due to a direct effect of epi- 

 nephrine on the receptor. Epinephrine can also in- 

 crease the .size of the receptor potential of the Pacinian 

 corpuscle in response to a given stimulus; this results 

 in a lowering of the threshold (6g). In the carotid 

 sinus of the cat there is also an effect of epinephrine, 

 but in this instance the effect appears to be secondary 

 to its action on the muscle of the sinus (17, 63). 



These results show that the activity of receptors 

 may be modified by centrifugal activity. The idea 

 is not, of course, new because the effects of stimulating 

 the efferent fibers to the muscle spindles are well 

 known (59). Centrifugal influences on the activity of 

 the ear (29) and eye (31) are also under investigation, 

 but whether or not these operate at receptor level is 

 not yet clear. This topic is discussed also by Livingston 

 (Chapter XXXI) on central effects on afferent activ- 

 ity in this work. 



Othn Substances 



Histamine is a substance that has been much inves- 

 tigated in relation to receptors, especially those con- 



cerned with the sensation of pain in man. Discussion 

 of this problem belongs to another chapter. Many 

 other agents have also been investigated (80) and 

 special mention should be made of the sensitization of 

 receptors by anesthetics (88, loi). 



MINUTE STRUCTURE OF RECEPTORS 



Electronmicroscopical studies have begun to throw 

 some light on those structural relationships that mav 

 be of importance in explaining the genesis of the re- 

 ceptor potential in mechanical receptors. Sections of 

 muscle spindles from the frog and of Pacinian cor- 

 puscles from the cat's mesentery have been investi- 

 gated. 



In the muscle spindle the finer branches of the af- 

 ferent filler which are nonmedullated lie in close rela- 

 tion to the intrafusal muscle fiber. These fibers, as they 

 approach their termination, lose their Schwann cell 

 sheath and come into direct contact with the muscle 

 fibers; the continuation of the Schwann cell also runs 

 in contact with the muscle but is separated from the 

 axon. Smaller axons, which may represent the final 

 terminations, are also seen in close relation to, but 

 not in contact with, the muscle surface. The terminal 

 parts of the afferent fibers contain many mito- 

 chondria, though with no apparent orientation 

 (fig. 14.4) (87). 



In the Pacinian corpuscle the axon is nonmyeli- 

 nated from the point at which it enters the central 

 core (86). At this point it has a diameter of 2 /x which 

 it maintains until it ends. Over the whole of this 

 nonmyelinated section there are certain characteristic 

 features (85) (fig. 145). There appears to be no 

 Schwann cell sheath; there are numerous mitochon- 

 dria inside the nerve fiber arranged as a palisade 

 around the fiber just beneath its surface membrane. 

 The axon itself is not round but an ellipse in cross 

 section and is surrounded by a complex cellular 

 structure. This cellular structure is divided into two 

 D-shaped parts separated from each other, in the 

 middle by the axon, and on either side by gaps that 

 continue the plane of the long axis of the elliptical 

 nerve fiber. 



At this stage of such investigations, the most striking 

 feature of the.se results is that both types of mechanical 

 receptors show the terminal axon without a Schwann 

 cell sheath. 



