A comparative study of spinocerebellar systems in three classes of poikilothermic vertebrates

Abstract

Spinocerebellar systems have been investigated in three classes of poikilothermic vertebrates. Lesions were made in the spinal cord of a number of dogfish, rudd and frogs which were selected as representatives of their classes. After a postoperative survival period the resulting fibre degeneration was demonstrated by means of a suitable silver impregnation technique. The findings have provided experimental confirmation of a spinocerebellar pathway in each species examined. In the dogfish these fibres form part of a mixed spinal lemniscus, which ascends through the brainstem, and leave it to enter the corpus cerebelli through its peduncles. In the rudd two separate systems were found, one of which corresponds to that seen in the dogfish. The other forms a separate compact fasciculus which lies more dorsally in the brainstem throughout its course. This one can be followed into the cerebellum through the peduncle. Within the corpus cerebelli :it was not possible to distinguish between the two systems. In the frog also two separate systems were seen. These retained their individual identity from the spinal cord to their entrance into the cerebellar peduncle beyond which they appeared to merge. These findings have been compared with the results of other authors, and an attempt has been made to show how and why such systems have evolved.

The activity of cerebellar neurones of the decerebrate dogfish Scyliorhinus during spontaneous swimming movements.

“…had been sectioned, spontaneous movements were performed continually. The significance of this preparation is that since the spinocerebellar fibres lie laterally in the brain stem (Hayle, 1973) and are unaffected by the lesion they can continue to function and influence cerebellar and other neural circuits in a regularly moving animal. Recordings can therefore be made from various neural components in a moving preparation, which is nevertheless stationary with respect to the observer.…”

“…stippled in A. S.c.t., spinocerebellar tract (based on Hayle, 1973). Dotted line in C marks the approximate position of the floor of the IVth ventricle before making the lesion.…”

Ascending spinal systems in the fish,Prionotus carolinus

“…A prominent indirect spinotectal system was observed in Prionotus. Direct and indirect spinotectal connections are present in many other chordates, including hagfish (Ronan and Northcutt, 1990), sharks (Hayle, 1973b), lungfish (Ronan and Northcutt, 1990), and most tetrapods studied to date (summarized in Ronan and Northcutt, 1990;Munoz et al, 1997). Of particular interest is the fact that the spinotectal system arising from the accessory lobes in Prionotus terminates predominantly in the ventral, rostral, and midtectum.…”

The activity of cerebellar nuclear neurones in relation to stimuli which evoke a pectoral fin reflex in dogfish.

“…In the mammal, a substantial input to the cerebellar nuclei via collaterals of afferents relaying to the cortex has been proposed as the major source. In elasmobranch fish, spinal connexions with the nucleus have been described by Hayle (1973) and by Ebbesson & Hodde (1981), presumably as collaterals of the spino-cerebellar projections which reach the cortex.…”