Functional properties of lumbar preganglionic neurones

Lumbar preganglionic neurones projecting through WRL₂ and L₃ to lumbar ganglia caudal to L₄ were investigated for those functional properties which are typical for postganglionic vasoconstrictor neurones supplying muscle and skin and for postganglionic sudomotor neurones. The properties tested were the cardiac rhythmicity of the activity and the reactions to systemic hypoxia, to noxious stimulation of skin and (in part of the experiments) to vibrational stimuli. Furthermore, resting activity and conduction velocities of the axons were measured.426 neurones were investigated. 311 (73%) of them were silent and could — as far as tested — not be excited by the afferent stimuli used. The conduction velocities of the axons of these neurones ranged from 0.5 to about 16 m/sec.115 neurones had resting activity of 0.1–4.6 impulses/sec. The conduction velocities of their axons ranged from 0.5 to about 12 m/sec. 80 preganglionic neurones with resting activity were classified on the basis of the reflexes in these neurones to afferent stimuli.Preganglionic neurones reacting like postganglionic vasoconstrictor neurones to muscle (excited by systemic hypoxia and/or by noxious stimulation of skin; with cardic rhythmicity) were classified as type 1 neurones (26 from 80 neurones tested). The resting activity of these neurones was1.8 ± 1.3impulses/sec(mean± 1S.D.). Their axons conducted with3.9 ± 2.2m/sec.Preganglionic neurones reacting like the majority of the postganglionic vasoconstrictor neurones to hairy and hairless skin (inhibited by systemic hypoxia and/or noxious cutaneous stimuli) were classified as type 2 neurones (48 from 80 neurones investigated). In 40% of these neurones the activity had cardiac rhythmicity. The resting activity was0.9 ± 0.6impulses/sec. The distribution of the conduction velocities of the axons of these neurones was bimodal. They conducted on the average with1.3 ± 0.6m/sec and6.6 ± 2.2m/sec respectively.A neurones were found (6 fronm 80 neurones) which were activated by vibrational stimuli (activation of Pacinian corpuscles by tapping on the hindfoot). Since this type of activation is typical for postganglionic sudomotor neurobes they were classified as type 3 neurones. The activity of these neurones had no cardiac rhythmicity.Indirect measurements of the conduction velocities of preganglionic axons converging onto postganglionic neurones supplying skeletal muscle and hairy skin yielded values which were statistically not different from the conduction velocities of the axons of type 1 and type 2 neurones respectively. These measurements support the classification into type 1 and type 2 preganglionic neurones. The implications of this study are discussed.