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, membrane 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 does not contribute to the generation of action potentials in vivo where spikes are solely generated from synaptic potentials. Synaptic potentials occur in phase with either the cardiac or the respiratory cycle or at irregular intervals. Postsynaptic interaction of these various inputs at the level of SPNs as well as presynaptic gating mechanisms in relation to the respiratory cycle have been observed. The discharge pattern is also modified by specific membrane properties which function to limit their discharge rate in the absence of axon collaterals. Finally the discharge of SPNs is affected by local factors: Since asphyxia causes a strong sympathetic activation when synaptic inputs to other neurones are already non-functioning synapses on SPNs are resistant to hypoxia or changes in the extracellular fluid somehow influence the activity of these neurones.