A Critical Analysis of the Contribution of the Collision Method to “Pyramidal” Identification of Pericruciate Neurones in the Chloralosed Cat

Abstract

1. In cats anaesthetized with chloralose, the identification of motor cortex neurones with axons distributed within the pyramidal tract (as assessed by their antidromic response to a bulbar pyramid shock) is critically analysed in view of the known complexity of the neuronal circuitry in this cortical area. 2. The conclusions drawn from the two classical criteria of antidromic invasion – invariable latency and ability to follow high rates of repetitive stimulation — are compared with the results obtained by initiating collision between orthodromic and antidromic impulses travelling along the same axonal pathway: the principle of this method is summarized and results from 39 peri-cruciate neurones, recorded by extracellular micropipettes, are presented. 3. It is shown that the collision test helps in the accurate identification of pericruciate pyramidal neurones, particularly for some neurones which do not follow the frequency criterion owing, probably, to inhibitory mechanisms in the motor cortex. 4. A possible pitfall of the collision method is however stressed in that collisions can be observed when an axon is only partially common to the orthodromic and the antidromic impulses; hence an isolated collision cannot be used to identify true antidromic invasion. 5. Each of the three criteria used in identifying a neurone as antidromically invaded is critically evaluated.