Cerebellar neurones in the rainbow trout, Salmo gairdneri (Richardson): structural and electrophysiological observations
thesis
posted on 2017-02-08, 03:50authored byWaks, 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.