Cerebellar neurones in the rainbow trout, Salmo gairdneri (Richardson): structural and electrophysiological observations

2017-02-08T03:50:01Z (GMT) by Waks, Miriam
1 . Aspects of the histology, ultrastructure and electrophysiology of the trout cerebellum have been studied in order to determine if the intracerebellar organization was similar to that in mammals. An attempt has also been made to elucidate the kind of information carried in the visual afferents to the cerebellum. 2. Histological and ultrastructural studies have revealed that the three cellular layers (molecular, granule-cell and Purkinje-cell layers), the five types of neurones {granule, Purkinje, Golgi, basket and stellate cells) and the two afferent fibres (mossy and climbing) found in mammalian cerebellum are also present in the trout. The basic synaptic connections seen in all vertebrates, viz. climbing fibre to Purkinje cell and mossy fibre to granule cell, were identified. Granule cell axons ascend to the molecular layer where they bifurcate to form parallel fibres which synapse with Purkinje cells. Granule cells have been seen to make synaptic contact with Golgi, basket and stellate cells. Evidence has also been obtained for feedback of Golgi onto granule neurones and both basket and stellate onto Purkinje cells, as occurs in the mammalian cerebellum. Structural differences between the trout and mammalian cerebella include fewer molecular layer interneurones, particularly basket cells, and the absence of any true 1 basket 1 formation around Purkinje cell bodies. 3. Electrophysiological studies in the trout cerebellar corpus revealed many units with responses similar to those of mammalian Purkinje cells. These presumed Purkinje neurones responded to electrical stimulation of the spinal cord (mossy and climbing afferents) or of the cerebellar surface {parallel fibres) with one or several spikes, followed by a period of depression. A surface conditioning stimulus reduced the excitation of single Purkinje neurones by subsequent surface test-stimuli, and depressed the amplitude of the Purkinje cell field potential. Units with responses similar to those of mammalian inhibitory interneurones were recorded from the trout molecular layer. The depression of Purkinje cell activity for both stimulus situations described above is therefore considered to be a consequence of active inhibition of the Purkinje cells by the molecular layer interneurones. Periods of depression occurring in the responses of presumed granule cells and interneurones recorded in the granule cell layer are interpreted as indirect evidence for Golgi inhibition of granule cell activity. It has been concluded from the electrophysiological and ultrastructural data in the trout that Purkinje cells and probably also granule cells are under the control of inhibitory interneurones and that the functional organization of trout cerebellar neurones is similar to that of mammalian cerebella. A major electrophysiological difference between trout and mammalian cerebellum concerns the Purkinje cell giant spike, which is recorded from the Purkinje cell main dendrite rather than soma, and is generated by climbing fibre but not parallel fibre activity. Preliminary studies of visual responses of single cerebellar units suggest that the mossy fibre pathway in the trout may be encoding simple properties of the visual environment, while the climbing fibre system may be concerned with more complex aspects.