Encoding of muscle movement on two time scales by a sensory neuron that switches between spiking and bursting modes

The gastropyloric receptor (GPR) neurons of the stomatogastric nervous syst em of the crab Cancer borealis are muscle stretch receptors that can fire i n either a spiking or a bursting mode of operation. Our goal is to understa nd what features of muscle stretch are encoded by these two modes of activi ty. To this end, we characterized the responses of the GPR neurons in both states to sustained and rapidly varying imposed stretches. The firing rates of spiking GPR neurons in response to rapidly varying stretches were direc tly related to stretch amplitude. For persistent stretches, spiking-mode fi ring rates showed marked adaptation indicating a more complex relationship. Interspike intervals of action potentials fired by GPR neurons in the spik ing mode were used to construct an accurate estimate of the time-dependent amplitude of stretches in the frequency range of the gastric mill rhythm (0 .05-0.2 Hz). Spike trains arising from faster stretches (similar to those o f the pyloric rhythm) were decoded using a linear filter to construct an es timate of stretch amplitude. GPR neurons firing in the bursting mode were r elatively unaffected by rapidly varying stretches. However, the burst rate was related to the amplitude of long, sustained stretches, and very slowly varying stretches could be reconstructed from burst intervals. In conclusio n, the existence of spiking and bursting modes allows a single neuron to en code both rapidly and slowly varying stimuli and thus to report cycle-by-cy cle muscle movements as well as average levels of muscle tension.