PROPRIOCEPTIVE FIELDS OF CRAYFISH CLAW MOTOR NEURONS

Action potentials of crayfish claw motor neurons were recorded during both imposed constant-velocity displacements and imposed alternating sequences of opening and closing step movements of the dactyl. Peristimulus time (PST) histograms show that the firing probabilities of two neurons, the opener inhibitor (OI) and the slow closer excitor (CE) consistently increased during opening ramp movements and declined during closing ramp movements. Hyperpolarizing synaptic potentials were observed in both cells during closing movements. The proprioceptive field organizations of OI and CE were analyzed with response planes and countour planes. Each PST histogram in a plane displays the firing probability of the neuron as a function of time following step displacements at a given position. A relatively uniform early primary response followed each successive opening step. The probability of occurrence of later activity, when present, usually became more pronounced as the joint angle increased. Often both cells were silent during closing steps; when the cell were active, their firing probabilities were highest at the more open joint angles. When both OI and CE were active, their spike trains were usually temporally correlated. The other claw efferents did not respond to imposed movements in a consistent manner. When CE was active it was most likely to respond to closing movements near the closed position. It is concluded that OI and CE are strongly and similarly influenced by proprioceptive reflexes. The responses of the two cells to imposed dactyl movements change as a function of joint angle, time after movement, and direction of movement.