Multiple gate control of the descending statocyst-motor pathway in the crayfish Procambarus clarkii Girard.

1. Synaptic responses of uropod motoneurons and interneurons to magnetic field stimulation of the statocyst were studied in a whole animal preparation using intracellular recording and staining techniques to characterize the descending statocyst pathways controlling uropod steering behavior. 2. When the animal was engaged in abdominal postural movement, all uropod motoneurons received sustained excitatory input. Motoneurons which were to be activated during steering behavior showed excitatory responses to the stimulus superimposed on the sustained excitation. In the resting state, they showed weaker responses or no visible responses to the same stimulation. 3. Motoneurons to be suppressed during steering showed inhibitory responses to the stimulus only during abdominal movement. These included both active inhibition as well as disfacilitatory suppression of excitatory input to the motoneurons. 4. Premotor nonspiking interneurons, like motoneurons, showed greater responses to the stimulus during abdominal movement than at rest. Unlike motoneurons, however, they did not always receive sustained input during abdominal movement. 5. Descending axons which responded to statocyst stimulation independent of abdominal movement were found in the 4th and 5th abdominal ganglia. Other axons showed greater responses during abdominal movement than at rest. 6. A number of intersegmental descending interneurons with cell bodies and dendrites in the 4th or 5th ganglion were found to receive excitatory inputs from both the statocyst and the motor system controlling abdominal posture. These responses were found to summate with each other to generate spikes. 7. Statocyst signals are thus transmitted to uropod motoneurons by two types of descending pathways: one whose operation is affected by the abdominal system and the other which operates independently. The former pathway functions by recruiting intersegmental abdominal interneurons and makes stronger connections with motoneurons than the latter.