Flexor reflex afferents reset the step cycle during fictive locomotion in the cat |
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Authors: | E D Schomburg N Petersen I Barajon H Hultborn |
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Institution: | (1) Department of Medical Physiology, Division of Neurophysiology, The Panum Institute, University of Copenhagen, Blegdamsvej 3, DK-2200 Copenhagen, Denmark, DK |
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Abstract: | The generation of locomotor-like spinal rhythms has been proposed to involve two neural centres with mutual reciprocal inhibition
(Graham Brown’s ”half-centre” hypothesis). Much later a particular set of segmental flexor reflex pathways were described
as being organized in accordance with this half-centre hypothesis. As these pathways became operative following injection
of monoaminoxidase inhibitors and l-3,4-dihydroxyphenylalanine (l-dopa), i.e. under the same conditions under which a spontaneous locomotor activity may develop, it was assumed that these
particular pathways and spinal rhythm generators involve the same neuronal networks. In order to give further evidence to
this hypothesis, we investigated whether short trains to ”flexor reflex afferents” (FRA) reset the spinal locomotor rhythm,
i.e. shorten or lengthen the stimulated cycle after which the regular rhythm is resumed with step cycles of the original duration.
The experiments were performed in anaemically decapitated, high-spinal curarized cats. A steady locomotor rhythm was induced
by injection of nialamide and l-dopa and the influence of electrical stimulation (trains of 50–1000 ms) of FRA (joint, cutaneous, and group II and III muscle
afferents) onto this rhythm was tested. Stimulation of FRA induced a clear resetting of the locomotor rhythm, which was mainly
characterized by a flexion reflex pattern: during the extension phase the extensor activity was interrupted and a flexion
phase was initiated; during the late flexion phase mainly a prolongation of that phase with a variable change of the following
extension phase was induced. In addition to this prevailing pattern, stimulation of some nerves (in particular nerves to more
distal extensors and the sural nerve) could often prolong extension, when stimulated during the late extension, or terminate the flexor burst and initiate a new extension phase, when
stimulated during the late flexion phase. This pattern is probably due to the concomitant stimulation of group I afferents
in the case of the muscle nerves and to separate non-FRA pathways in the case of the sural nerve. The results demonstrate
that the interneurones of the FRA pathways, which are operative during l-dopa-induced locomotion in spinal animals, can be considered as neuronal elements of the rhythm-generating network for locomotion.
Received: 25 June 1996 / Accepted: 27 April 1998 |
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Keywords: | Fictive locomotion Proprioception Flexor reflex Spinal cord Cat |
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