After-potential of spinal axons in vivo

A method is evolved whereby the after-potential sequence intrinsic to longitudinal dorsal column myelinated fibers may be studied in isolation from those events occurring in intact spinal cord subjected to an afferent volley. The recovery sequence intrinsic to dorsal column fibers, after refractoriness is over, is characterized by supernormality approximately four to five times greater than that seen in peripheral nerve. This supemormality averages 15.7 ± 4 per cent (current calibration) at peak and decays exponentially with a half-time of 7.5 msecs. It is not followed by subnormality unless conditioning is repeated more than three times at frequencies greater than 100/sec. Characterization of the recovery curve of dorsal column fibers permits by exclusion, the allocation of the origin of DRV to structures more centrally located. DCV (the dorsal column counterpart of DRV) is seen to exist equally developed in active and passive dorsal column fibers.