Interface unit for on-line measurements of motor unit properties with a small laboratory computer

Some simple circuitry is described which has been successfully used in conjunction with a small laboratory computer for "on-line" measurements of the force of contraction of single motor units in cat gastrocnemius and soleus muscles. Our approach emphasizes use of a sample and hold device to allow high amplification of small forces developed during single motor unit contractions which are often superimposed on relatively large whole muscle passive forces. Two digitally controlled selectable gain amplifiers are used to ensure proper modulation of the computer's ADC and to enable automatic scaling of the measured variables. An analog peak detector is incorporated into the unit such that peak force measurements can be made with the minimum of computer software. The principles governing the present design are directly applicable to the measurement of intracellularly recorded postsynaptic potentials and a variety of other physiological variables which require accurate measurement of a small transient signal superimposed on a large "control" or "resting" signal.