INTEGRATIVE PROPERTIES OF SYMPATHETIC PREGANGLIONIC NEURONS WITHIN THE THORACIC SPINAL-CORD

The discharge pattern of sympathetic preganglionic neurones (SPNs) in the lateral horn is shaped by the interplay of synaptic inputs, membra ne properties and local factors within the spinal cord. Intracellular recordings in vivo and in vitro have clarified the importance of some of these factors. Pacemaker activity can be recorded in vitro, but doe s not contribute to the generation of action potentials in vivo where spikes are solely generated from synaptic potentials. Synaptic potenti als occur in phase with either the cardiac or the respiratory cycle or at irregular intervals. Postsynaptic interaction of these various inp uts at the level of SPNs as well as presynaptic gating mechanisms in r elation to the respiratory cycle have been observed. The discharge pat tern is also modified by specific membrane properties which function t o limit their discharge rate in the absence of axon collaterals. Final ly the discharge of SPNs is affected by local factors: Since asphyxia causes a strong sympathetic activation when synaptic inputs to other n eurones are already non-functioning synapses on SPNs are resistant to hypoxia or changes in the extracellular fluid somehow influence the ac tivity of these neurones.