Cardiovascular and electrophysiological responses to sympathetic pathway stimulation

Anesthetized, vagotomized, and paralyzed cats were used to study the relationship between changes in blood pressure and T2 preganglionic nerve activity elicited by stimulation of descending spinal sympathoexcitatory pathways. While low-frequency stimulation (1-10 Hz) resulted in a depressor response, an attenuation of sympathetic nerve evoked discharges was noted after the second pulse of twin-pulse stimulations with 1,000- to 100-ms intervals between stimuli (corresponding to 1-10 Hz). High-frequency stimulation (greater than 10 Hz) resulted in a pressor response, while preganglionic nerve-evoked discharges were enhanced during twin-pulse stimulations with intervals between stimuli less than 50 ms (corresponding to greater than 20 Hz). These data suggest that the depressor response during low-frequency stimulation may be explained by impulses occurring during the silent period of the previous response. During this time the preganglionic neuron is inexcitable and spontaneous activity is abolished; therefore, the blood pressure falls. High-frequency stimulation may result in a pressor response by increasing the number of preganglionic neurons that are activated. This may be due to a process of temporal summation, with initial stimuli bringing cells closer to their threshold for firing and additional stimuli close in time causing these neurons to discharge.