Sensory and central nervous control of gill ventilation in Limulus

The gills of Limulus are ventilated by a metachronal rhythm of movements of five pairs of gill plates. A gill plate is promoted and remoted by action of alternating nerve impulse bursts to antagonist promotor and remotor muscles. The motor output pattern is centrally generated, requiring no sensory feedback. Intracellularly recorded rhythmic activity of respiratory motoneurons consists of cyclic depolarization and spiking, and repolarization. The repolarizations have reversal potentials that indicate that motoneuron burst terminations result from synaptic inhibition. Intracellular and antidromic stimulation of motoneurons has little effect on other motoneurons. This apparent lack of interaction between motoneurons indicates that the central respiratory pattern is generated at interneuronal levels. Proprioceptive reflexes are present; they play little role in modulating the centrally generated motor pattern, but they are capable of partially entraining the rhythm when all gill plates are cycled at frequencies near the respiratory rate. Respiratory rate in intact animals is proportional to the ambient oxygen content, respiration ceasing in an anoxic environment. This oxygen dependence may result from sensory input from external oxygen receptors located in the cuticle between the coxae of the walking legs and within the lamellas of the book gills. The intercoxal units are inhibited by anoxia. Three classes of units are recorded from the gills: units excited by oxygen, units inhibited by oxygen, and units whose mechanosensitivity is oxygen dependent. These external oxygen receptors may modulate ventilation via command fibers present in the ventral nerve cord.