An implantable device for controllably applying a neural stimulus, the device comprising: a plurality of electrodes including one or more nominal stimulus electrodes and one or more nominal sense electrodes; a stimulus source for providing a stimulus to be delivered from the one or more stimulus electrodes to a neural pathway in order to give rise to an evoked action potential on the neural pathway; measurement circuitry for recording a neural compound action potential signal sensed at the one or more sense electrodes; and a control unit configured to: control application of a neural stimulus as defined by a set of parameter values; measure via the measurement circuitry a neural compound action potential response evoked by the stimulus; determine from the measured evoked response a feedback variable ; compare the feedback variable to a therapy map , the therapy map defining a therapeutic relationship of control variable to feedback variable; alter one or more of the stimulus parameter values to effect the required change in the control variable, and iteratively perform the applying, measuring, comparing and altering, in order to improve alignment of the feedback variable with the therapy map over time, characterised in that the control unit is further configured to provide differential slew rates whereby a rate of change of the control variable in response to a detected overstimulation condition is more rapid than a rate of change of the control variable in response to a detected understimulation condition. a plurality of electrodes including one or more nominal stimulus electrodes and one or more nominal sense electrodes; a stimulus source for providing a stimulus to be delivered from the one or more stimulus electrodes to a neural pathway in order to give rise to an evoked action potential on the neural pathway; measurement circuitry for recording a neural compound action potential signal sensed at the one or more sense electrodes; and a control unit configured to: control application of a neural stimulus as defined by a set of parameter values; measure via the measurement circuitry a neural compound action potential response evoked by the stimulus; determine from the measured evoked response a feedback variable ; compare the feedback variable to a therapy map , the therapy map defining a therapeutic relationship of control variable to feedback variable; alter one or more of the stimulus parameter values to effect the required change in the control variable, and iteratively perform the applying, measuring, comparing and altering, in order to improve alignment of the feedback variable with the therapy map over time, control application of a neural stimulus as defined by a set of parameter values; measure via the measurement circuitry a neural compound action potential response evoked by the stimulus; determine from the measured evoked response a feedback variable ; compare the feedback variable to a therapy map , the therapy map defining a therapeutic relationship of control variable to feedback variable; alter one or more of the stimulus parameter values to effect the required change in the control variable, and iteratively perform the applying, measuring, comparing and altering, in order to improve alignment of the feedback variable with the therapy map over time, characterised in that the control unit is further configured to provide differential slew rates whereby a rate of change of the control variable in response to a detected overstimulation condition is more rapid than a rate of change of the control variable in response to a detected understimulation condition. <EOS>