SENSORY ORGANS AND RECEPTION 



339 



about the degree of flexion existing in each segment of the abdomen (92, 

 162). 



By intracellular recording it has been shown that excitation of the sensory 

 cell normally starts in the distal portion of the dendrites, which are depolar- 

 ized by stretch deformation. This potential change, a generator potential, 

 spreads electrotonically over the nearby cell soma, reducing the resting 

 potential of the latter. When the membrane resting potential is lowered 

 by stretch to a certain critical level, conducted impulses are initiated in the 

 sensory axon. The accessory nerve fibres are inhibitory and form a direct 



Fig. 8.24. Proprioceptors in Decapod Crustaceans 



(Left) Muscle receptor organ in thoracic muscle. (Right) N-cell on lateral thoraco- 

 abdominal muscle. Both preparations from Homarus vulgaris. (From Alexandrowicz (2).) 



peripheral inhibitory control mechanism, which can modulate the activity 

 of the stretch receptor. Stimulation of the inhibitory fibre decreases the 

 afferent discharge rate, or stops impulses altogether in the stretched re- 

 ceptor cell. The inhibitory mechanism acts by limiting depolarization of 

 the dendrites above a certain level, thus preventing the generator potential 

 from attaining a level sufficient to fire the cell (45, 46, 93). 



PHONORECEPTION 



Sound waves travel readily through water, with higher velocity and less 

 absorption of energy than in air. Because the air-water interface reflects 

 most of the sound waves reaching it, very little of any sound produced 



