Responses of the purkyně cells of a selachian cerebellum ( Mustelus canis)

Electrophysiological investigations have been performed on the cortex of the dogfish cerebellum paralleling those carried out on the mammalian cerebellum. The principal objective has been to see how far the neuronal structures of the dogfish cerebellum exhibit behaviors comparable with their mammalian counterparts. In their responses to antidromic invasion the dogfish and mammalian Purkyně cells are similar, there being blockage of propagation along the dendrites. However, after this invasion, a much longer time (about 40 msec) is required for full recovery of the dogfish Purkyně cell. Presumably, by their synapses on the dendritic spines, a parallel fiber volley is very effective in generating the discharge of impulses from Purkyně cells, there being often a double or a triple discharge at a frequency as high as 200/sec. By their depth profile, these synaptically evoked impulses reveal that they have approximately the same limitations of dendritic involvement as do the antidromically propagating impulses, and subsequently the Purkyně cell exhibits the same slow recovery of about 40 msec. In addition to this synaptic excitatory action, a parallel fiber volley may have a prolonged inhibitory action on Purkyně cells, as tested by depression of their antidromic invasion. It is postulated that this inhibition of about 500 msec duration is mediated by the stellate cells of the molecular layer, the inhibitory pathway being parallel fibers to stellate cells to Purkyně cell dendrites. Stimulation of the afferent fiber pathway to the cerebellum often evokes a powerful unitary excitation of Purkyně cells that is characterized by a repetitive spike discharge of 3 or 4 impulses superimposed on a depolarization of 20–30 msec duration. This is a typical climbing fiber response. Extracellular recording often reveals that the rhythmic spike responses of adjacent Purkyně cells are so well synchronized that they give a rhythmic wave of about 200/sec and decrementing over 3 or 4 cycles. Sometimes responses typical of a climbing fiber are evoked with a latency as long as 10 msec. It is postulated that the climbing fiber to the Purkyně cell under observation is a branch from a parent stem deep in the cerebellum, and that the stimulus excited another branch of this fiber, so that there is a long conduction delay via an axon reflex pathway. There has been a discussion of these results in relation to investigation on the cerebellar cortices of other vertebrates.