Information transfer rate of nonspiking afferent neurons in the crab

The thoracic-coxal muscle receptor organ (TCMRO) is the only proprioceptor at the thoracic-coxal joint in the crab leg. The S and T afferent neurons of the TCMRO convey signals to the CNS solely by means of graded changes in membrane potential. The rate of information transfer of these afferents was determined by measuring the signal-to-noise ratio (SNuR) of these cells after repeated stimulation of the receptor with identical sequences of random movement and applying the Shannon formula for the information capacity of a Gaussian channel. Intracellular recordings were made from the S and T afferents adjacent to the transduction site at the origin of the receptor and along the axon 5-7 mm distal to this site. These nonspiking afferents transduce receptor movement and transmit this information with extremely high fidelity. The SNR of both neurons near the transduction site was >1000 over most of the 200 Hz stimulation bandwidth, and the mean information transfer rate was approximately 2,500 bits/s. When calculated over a wider bandwidth of 500 Hz, the information rate was >4,600 bits/s. The effect of axonal cable properties on the information rate was evaluated by determining the SNR from membrane potential recordings made 5-7 mm distal to the transduction region. The major effect of graded transmission along the axon was attenuation and low-pass filtering of the sensory signal. The consequent reduction in signal power and bandwidth decreased the information transfer by approximately 10-15% over 200 Hz and approximately 30% over a 500 Hz bandwidth.