Neural Control of Radular Retractor Muscles of the Pulmonate Snail, Planorbarius Corneus (l.): Functional Anatomy and Properties of Neuromuscular Units

The paired supralateral radular retractor muscles of Planorbarius are innervated by eight (four pairs) of buccal motoneurones. Each motoneurone supplies both muscles, the ipsiand contralateral motor responses being similar, and identical to those produced by its contralateral partner. This arrangement apparently secures equivalence of motor output to the two muscles, irrespective of discrepancies in spiking between homologous motoneurones across the midline, and relates to the strong mechanical linkage between the muscles exerted by the radula. The neuromuscular units are essentially non-overlapping, have properties which range from tonic to phasic, and may act independently providing the flexibility in control required of bifunctional muscles participating in both tensing and retraction of the radula during each feeding cycle.