Motoneurons innervating the dorsal superficial muscles of the hermit crab,Pagurus pollicarus, and their reflex asymmetry
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The morphology of the second ganglionic roots and motoneurons innervating the dorsal superficial muscles of the fourth abdominal segment of the hermit crab,Pagurus pollicarus, is described. No significant differences between left and right sides in axon diameters of fibers in the root above 2 μn nor in soma diameters filled with cobalt centripetally diffused in the second root were observed.
Five excitors and two inhibitors innervate the three subregions of the dorsal superficial muscles previously described (Chappie, 1977). The largest excitor innervates the phasic central medial region, a large inhibitor (next in size) innervates muscles in all three regions, a smaller excitor distributes to all the muscle fibers, and three excitors, similar in extracellular amplitude and conduction velocity innervate different subregions. The smallest fiber is an inhibitor that innervates the tonic muscles.
The three small excitors fire at different tonic frequencies. Pairs of bilateral homologues are physiologically asymmetrical in their tonic frequencies: those on the left fire at higher frequencies than those on the right.
Conduction velocities of motoneurons in left and right second roots do not differ significantly.
It appears that the source of the asymmetry is not located in the motoneurons, but arises centrally in the absence of any major peripheral asymmetry.
Numbers and relative sizes of motoneurons are more conservative in evolution than reflex connections and the details of innervation of the muscle fibers.
KeywordsCobalt Muscle Fiber Relative Size Conduction Velocity Medial Region
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