Physiology of mammalian prevertebral ganglia.

When assessing the physiological role of the prevertebral ganglia, the following should be considered. Ganglion cells in these ganglia receive continuous excitatory synaptic input from peripheral sensory mechanoreceptors and central preganglionic neurons. The latter may result from sensory afferents projecting from the gastrointestinal tract to spinal preganglionic neurons or as a result of endogenous oscillator activity located in the spinal cord (18) or within the ganglion (45). Peripheral and spinal reflex pathways probably do not operate as separate, independent pathways. Rather, they must be considered functionally integrated in the prevertebral ganglia. Because of continuous synaptic input, the prevertebral neuron operates at some level of "neurogenic tone" that provides capabilities for bi-directional responses. An increase in spinal or peripheral synaptic input will be matched by a proportional increase in output firing. The functional effect of spatial summation of peripheral and central input is to prime the prevertebral neuron so that adjustments in its firing frequency can be made without the lag inherent in building up a response in a quiescent system. Not all prevertebral ganglia participate in reflex activity. It may be that reflex activity between prevertebral ganglia and visceral smooth muscle depends upon the nature of the mechanical activity of the muscle. For example, the mechanical activity of smooth muscle of the vas deferens is not characterized by spontaneous, myogenic, regularly occurring contractions. Mechanical activity in this organ occurs in bursts. There is no evidence to suggest that sensory fibers project from the wall of the vas deferens to the hypogastric plexus thereby forming a peripheral reflex arc. Contraction of th smooth muscle of the vas deferens is brought about by bursts of nerve impulses through simple relay connections in the pelvic plexus. Smooth muscles supplied by this ganglion have a direct line to the central nervous system and are secured by a "fail-safe" system. In contrast, the mechanical activity of the gastrointestinal system is characterized by slow spontaneous myogenic activity. Pattern formation of motor activity is achieved by reflex activity within the enteric plexuses. Extrinsic alteration of the nature and pattern of contraction of smooth muscle of the gastrointestinal tract would be best served by prevertebral ganglia that modulate and adjust motor function by continuous integration of convergent weak synaptic input emanating from the peripheral and central nervous systems. When a prevertebral neuron is part of a peripheral reflex loop, its physiological role may be to modulate end organ activity; when it does not participate in a peripheral reflex loop, its role may be to relay CNS input and initiate end organ activity.