Printed in Great Britain PHYSIOLOGICAL IDENTIFICATION AND ASYMMETRY OF LOBSTER CLAW CLOSER MOTONEURONES

The somata of the fast closer exciter (FCE) and slow closer exciter (SCE) motoneurones to the dimorphic claw muscles of the lobster Homarus ameri-canus were physiologically identified in an isolated claw-ganglion preparation, by recording from a cell body simultaneously with recording from an identified axon. The FCE, SCE, and opener exciter (OE) somata are the largest cells in the anterior ventral region of the ganglion and appear con-sistently as a triad. A marked asymmetry occurred in the responsiveness of the identified FCE and SCE somata to sensory stimulation via the 2nd root. The SCE soma produced a longer burst with higher spike frequencies than the FCE soma in either cutter or crusher claw. When homologous somata were compared, the crusher FCE and SCE produced higher frequencies and longer bursts of spikes than their cutter counterparts. Since this occurred in response to both sensory stimulation via the 2nd root and depolarization of the soma it suggests that differences in motor output between homologous motoneurones have an extrinsic and intrinsic origin. Furthermore, asymmetry sometimes occurred within an individual motoneurone with a larger number of spikes in the soma than in the axon. Thus the homologous FCE and SCE motoneurones are differentiated into physiologically different types