Comparison of antidromic and orthodromic action potentials of identified motor axons in the cat's brain stem

Recordings were made from identified central axons at a known distance from their somata, to compare the action potentials resulting from antidromic and synaptic excitation. By taking advantage of the anatomical configuration within the brain stem of the motoneurones innervating the retractor bulbi muscle in the orbit, their axons were penetrated in the VIth nucleus and labelled by electrophoretic injection of horseradish peroxidase. Excitatory post-synaptic potentials recorded in the retractor bulbi axons at about 3 mm from the soma were six times smaller than in the soma. The space constant of the axonal segment between the retractor bulbi and the abducens nucleus was estimated to be 1.7 mm. When the axons propagated action potentials the attenuation was increased to eighteen times due to the nodes of Ranvier intercalated between the soma and the site of recording. Antidromic action potentials displayed stepwise changes in amplitude and shape when stimuli were applied at intervals decreasing from 5 ms to 0.7 ms. The changes were related to the different lengths of refractoriness of the soma, initial segment and axon. Orthodromic action potentials evoked by synaptic excitation displayed similar changes in amplitude and shape. These observations lead to the conclusion that the soma, initial segment and neighbouring nodes of Ranvier contribute significantly to the shape of the action potential. Contrary to the generally accepted view, it appears that the efferent discharge along motor axons can be initiated without a simultaneous activation of the somato-dendritic or even the initial segment membrane, as revealed by the lack of somato-dendritic and/or initial segment contribution to the shape of the synaptically evoked action potentials.