EXTERNAL LOOPS OF HUMAN PREMOTOR SPINAL OSCILLATORS IDENTIFIED BY SIMULTANEOUS MEASUREMENTS OF INTERSPIKE INTERVALS AND PHASE-RELATIONS

Single nerve-fibre action potentials (APs) were recorded extracellular ly from alpha and gamma-motoneurons and secondary muscle spindle affer ents from a ventral S4 nerve root (some afferents are contained in low er sacral motor roots) in an individual with traumatic spinal cord les ion sub TH1. Simultaneous interspike intervals (IIs) of, and phases be tween, the APs of 5 nerve fibres were measured, and distributions were constructed. The II distributions were of a broad peak type. Phase di stributions showed 1 to 3 peaks interpreted as phase relations between the firings of the nerve fibres. Under certain phase relations, the r hythmic firing of alpha and gamma-motoneurons is further interpreted a s an interaction of oscillatory firing neuronal subnetworks driving al pha and gamma-motoneurons. Following repetitive touch and pin-prick st imulation in- and outside the anal reflex area, the II distributions o f alpha and gamma-motoneurons and of secondary spindle afferents assim ilated partly or fully, while preserving their phase relations. This c oordinated firing is interpreted as the oscillatory firing of alpha ne uronal networks building up an external loop to the periphery via the gamma-loop. Upon touch, pin-prick, and anal reflex stimulation, and an al and bladder catheter pulling, the values and the number of the phas e relations changed. Mostly two phase relations per oscillation cycle were observed. Two phase relations probably represent the physiologic case for the somatic nervous system. Only one phase relation was found when full synchronization of all units occurred. Three phase relation s were found when the parasympathetic nervous system division interact ed with the somatic one. Based on data obtained from brain-dead indivi duals it is discussed that the increased synchronization and instabili ty in the number and the values of phase relations suggested pathologi c functioning of the caudal functionally disconnected spinal cord in p atients with spinal cord lesions: Oscillatory firing neuronal networks , which lost their specific properties, interacted more easily and uns pecifically with other oscillatory firing networks. Further, it is dis cussed that physiologic tremor is caused by chance synchronization of oscillatory firing neuronal networks and therefore originates in the c entral nervous system (CNS). Since spinal oscillators build up externa l loops to the periphery, it is suggested that in patients with incomp lete spinal cord lesions it should be possible to re-preformate oscill atory firing neuronal networks by a rhythm training, to reduce spastic ity and to re-train useful movements, especially locomotion.