Sensory Cell Function and Architecture 81 



impulses. These latter impulses, however, are initiated first in 

 the membrane of the axon or axon hillock region, not in the soma- 

 dendrite complex, ^^ 



Even if the axonal membrane is not more responsive to genera- 

 tor currents than other parts of the cell, the above results may be 

 predicted from neuron geometry. Depolarizing currents (flowing 

 within the cell from a source region on the dendritic tree) will 

 cross the membrane in an outward direction, both in the soma and 

 the initial reaches of the axon. The small diameter of the axon, 

 compared with that of the cell body, means that the density of 

 outward current may be greater in the former region, even though 

 most of the total available current will be drawn by the relatively 

 great capacitance of the soma membrane. In addition, there is 

 now some evidence^^ that the excitability of the axon membrane 

 may, in fact, be inherently greater than that of the soma. Thus, 

 the initial activation of the impulse at an axonal locus appears 

 to be certain. 



The most ubiquitous type of primary sensory neuron is of the 

 simple bipolar configuration. The somata of these cells are 

 usually located at the periphery of an animal and are often quite 

 close to the region of stimulus transduction. None the less, the 

 thin distal processes of such cells may be several hundred microns 

 in length. Considering the expected decrement in amplitude of a 

 receptor potential generated at the tip, it is not surprising to find 

 that impulse-initiation can occur at a locus which is distal to the 

 cell body. Evidence for this statement comes from a preparation 

 in which the sensory neurons in question are large enough to be 

 examined with intracellular electrodes. The cells in question 

 occur in pairs within the hypodermis of the crayfish carapace. 

 These cells possess a directional sensitivity, and they are activated 

 to different extents depending upon the direction in which the 

 structural parts of the receptor organs are moved by the stimulus." 

 The external structural part of each receptor is a thin hair, or 

 sensillum; it can be moved by the flow of water past the parent 

 animal, and depending upon whether the direction of flow is 

 anteriorly or posteriorly directed, one or the other neuron from 

 each pair will respond. In the region of the thoracic carapace the 

 hypodermis exists as a thin sheet of tissue. The somata of the 

 sensory neurons are some 50 - 80 microns long, and under 



