338 



THE BIOLOGY OF MARINE ANIMALS 



The motor fibres innervate the muscle, and the accessory fibres form 

 synapses with the dendrites of the receptor neurones. It has been observed 

 that the sensory cells of the organs differ in the length and arrangement of 

 their dendrites. The N-cells are somewhat similar sensory neurones, the 

 processes of which are connected with ordinary muscles (those inserted 

 on the epimeral plate of lobsters) (2, 3, 4, 4a, 48). 



Earlier conjectures that the muscle receptor organs are responsive to 

 stretch have been confirmed physiologically. Stretching the isolated organ 

 gives rise to a nervous discharge in its sensory axon (Homarus, Panulirus, 



Fig. 8.23. Responses of a Proprioceptor Organ in the Leg of the Crab 

 Carcinus maenas to Vibration and Passive Movement of the Propodite- 



dactylus Joint 

 Recording from nerves to propodite-dactylus organ. The lower beam indicates 

 movement of the dactylus. (a) Passive extension, (b) (c) Passive flexion at different rates. 

 (d) Two taps on preparation box. (From Burke (24).) 



Cambarus, etc.). The two organs differ considerably in their physiological 

 characteristics (Fig. 8.25). One has high threshold and adapts quickly to 

 strong stretching in less than a minute. The other has low threshold and 

 maintains continuous discharge for several hours under constant stretch. 

 Furthermore, contraction of the receptor-organ muscle itself can initiate 

 discharge in the sensory axon. The receptor muscle linked with the quickly- 

 adapting neurone gives twitch-like contractions and has a high fusion 

 frequency, whereas the receptor muscle connected with the slowly adapting 

 sensory neurone gives slow contractions and has a low fusion frequency. 

 One of the receptor organs thus acts as a phasic receptor signalling sudden 

 flexion of the tail; the other is a tonic receptor, transmitting information 



