Preganglionic and sensory axons in developing bullfrog sympathetic ganglia express three neuropeptides during early tadpole stages

The sacral autonomic outflow is parasympathetic: Langley got it right

A recent developmental study of gene expression by Espinosa-Medina, Brunet and colleagues sparked controversy by asserting a revised nomenclature for divisions of the autonomic motor system. Should we re-classify the sacral autonomic outflow as sympathetic, as now suggested, or does it rightly belong to the parasympathetic system, as defined by Langley nearly 100 years ago? Arguments for rejecting Espinosa-Medina, Brunet et al.’s scheme subsequently appeared in e-letters and brief reviews. A more recent commentary in this journal by Brunet and colleagues responded to these criticisms by labeling Langley’s scheme as a historical myth perpetuated by ignorance. In reaction to this heated exchange, I now examine both sides to the controversy, together with purported errors by the pioneers in the field. I then explain, once more, why the sacral outflow should remain known as parasympathetic, and outline suggestions for future experimentation to advance the understanding of cellular identity in the autonomic motor system.

An aortic projection of lumbar paravertebral sympathetic neurons in the bullfrog

A population of sympathetic neurons which projects from paravertebral ganglia to the dorsal aorta was identified in the bullfrog based on its electrophysiology and anatomy. The aorta is bilaterally supplied by about a dozen connective nerves arising from sympathetic ganglia 7-10. From compound action potentials, the majority of fibers can be identified as postganglionic axons belonging to both the B- and C-cell groups. Tracing experiments indicated that 95 +/- 21 (mean +/- SD) neurons project into each connective and that axons in connectives penetrate the wall of the aorta where they break into small bundles. Staining wholemounts of the aorta for neuropeptide Y revealed a plexus of varicose axons which presumably arise from sympathetic C neurons. These observations imply that the aorta is sympathetically innervated by paravertebral C neurons.

Development of fast synaptic transmission in bullfrog sympathetic ganglia

Extracellular recordings were made of postsynaptic responses originating in sympathetic ganglia 9 and 10 of bullfrog tadpoles and adults. At stage III, when the length and diameter of the developing hindlimb bud are equal, preganglionic stimulation elicits postganglionic action potentials in spinal nerves 9 and 10 near the sciatic plexus. Although they fluctuate in amplitude, these responses follow short trains of repetitive stimuli at 20 Hz. Their mediation by nicotinic synapses was demonstrated by reversible blockade in low Ca2+, high Mg2+ Ringer and in nicotine. Parallel sympathetic B and C systems are clearly defined by stage III. They can be selectively activated by appropriate segmental stimulation of the sympathetic chain and are characterized by distinct conduction velocities which both lie in the C fiber range (less than 1 m/s). Throughout subsequent tadpole stages, the conduction velocities of the developing B and C systems gradually double while the magnitudes of their compound action potentials grow exponentially by 100-fold. Conduction velocities reach adult values after completion of metamorphosis. These results provide physiological evidence that synapse formation in sympathetic ganglia supplying the hindlimbs begins by the earliest stages of limb bud development, is selective, and progresses over a protracted period of months, prior to myelination.