A reclassification of B and C neurones in the ninth and tenth paravertebral sympathetic ganglia of the bullfrog.

Abstract

The cellular organization of the 9th and 10th paravertebral sympathetic ganglia in the bullfrog was studied with intracellular and extracellular recording methods. An isolated preparation was used in which anatomical details of individual cells could be resolved while making physiological measurements. This permitted the characterization of neurons in terms of their size, the segmental origin of their cholinergic innervation and their orthodromic and antidromic conduction velocities. With these criteria, 3 classes of sympathetic neurons were identified. C cells were distinguished form B cells by the origin of their innervation. C cells are innervated by slowly conducting axons (0.4 m/s) from spinal nerves 7 and 8 and B cells are innervated by rapidly conducting axons (2.4 m/s) from the sympathetic chain above ganglion 7. The conduction velocity of a preganglionic axon generally matches that of its target neuron. A large group of B cells were characterized for which this is not true. The axons of B cells fall into a rapidly conducting group (2.0 m/s) and a slowly conducting group (0.6 m/s). C neurons, like their preganglionic inputs, have only slowly conducting axons (0.3 m/s). Neurons were classified as C type, fast B type and slow B type. Of the B cells that were studied, 59% were slow B cells. These findings were corroborated by measurements of compound extracellular responses in post-ganglionic nerves. Some neurons can be identified also by the size of their cell bodies. C cells are .apprx. 30 .mu.m in diameter while B cells are .apprx. 50 .mu.m in diameter. Of the cells with radius < 16 .mu.m, 96% were C cells and 94% of the cells with radius > 21 .mu.m were B cells. Fast B cells could not be distinguished from slow B cells by size.