84 The Physiology of Sense Organs 



obtained in different cells — ^the proprioceptor bipolar sensory 

 neurons in the leg of a crab.^"'^^ In that preparation, injured 

 neurons were not used for evidence, but impulses were blocked 

 from the soma by artificially hyperpolarizing the latter. The 

 amounts of polarization required to block orthodromic and anti- 

 dromic spikes were found to be different, suggesting that these 

 impulses enter the soma through different pathways. It was 

 further shown that orthodromic spikes blocked in this manner 

 were not recorded by an electrode placed more centrally on the 

 axon, even though an electrotonic potential from the spike could 

 still be recorded in the soma in response to mechanical stimulation. 



Because of its large capacitance, any area of expanded membrane 

 can be expected to draw current from adjacent regions, thereby 

 creating a zone which has a lowered safety factor for the trans- 

 mission of impulses. Thus, the inclusion of the soma in the 

 conduction pathway of a neuron is likely to limit its frequency 

 capabilities. In fact, it was found that orthodromic impulses in 

 the crayfish thoracic sensory neurons were unable to invade the 

 soma at frequencies higher than 200 per second. While this 

 probably does not represent a physiological encumbrance to a 

 crustacean, vertebrate nervous systems often depend upon 

 afferent input frequencies of 1000 per second. This may be the 

 reason why, in this group, many somatic afferents are monopolar 

 structures having their cell bodies to one side of the major 

 conducting path. 



The initiation of impulses in vertebrate mechanoreceptors, 

 such as the Pacinian corpuscle, appears to be comparable to the 

 situation described above for the bipolar neuron. In the Pacinian 

 corpuscle, the distal tip of the nerve fiber is a mechano-electrical 

 transducer, and a receptor current flows when its membrane is 

 compressed. The axon of this cell is myelinated, and it is inter- 

 esting to note that the myelin sheath extends well out towards 

 the tip of the fiber and may even overlie regions (fig. 37) which 

 are electrically-inexcitable, for impulses are initiated more 

 centrally, at the first node of Ranvier. It would be interesting to 

 confirm this suggestion, for it may represent a true channeling of 

 receptor current by an extracellular structure, the myelin sheath. 

 Another preparation in which this principle may operate is the 

 mechanosensory neuron studied by Wolbarsht.^"^ The region 



