Nucleus gracilis responses to knee joint motion: a frequency response study.

(1) A systems analysis approach was used to study the dynamic responses of sensory cells in the nucleus gracilis of anesthetized cats. Sinusoidal changes in knee angle were used as inputs and the average firing rate of single gracile cells was recorded as the output. Frequency response functions were derived from data displayed as Bode plots. (2) Fifty-nine cells were studied. The majority of these cells exhibited an acceleration sensitivity but 5 cells exhibited a velocity sensitivity. No position sensitivity was evident and no slowly adapting or tonic responses were recorded. (3) The acceleration sensitive cells demonstrated significant non-linear responses. The gain associated with their frequency response function depended upon both static knee angle and input excursion amplitude. These cells also exhibited a bi-directional response to sinusoidal and square wave inputs. (4) These data are taken as evidence that the dorsal columns may carry acceleration and velocity information from receptors in the knee joint, but that positional information may travel by other pathways to cells located in other areas. I f such is the case, lesion experiments involving the dorsal columns should reveal loss of velocity and acceleration sense but no decrement in position sense.