Investigation of the Vertical Vestibulo-Ocular Reflexes in the Dogfish Scyliorhinus canicula (L.)

Chouhdary, Rukhsana (1988) Investigation of the Vertical Vestibulo-Ocular Reflexes in the Dogfish Scyliorhinus canicula (L.). PhD thesis, University of Glasgow.

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In the dogfish Scyliorhinus canicula (L.) an investigation of the vertical vestibulo-ocular reflexes (VOR) has been made by recording the vertical eye reflexes and the myographic activity of the extraocular muscles (EOM) which induce these reflexes. Control of this reflex has been determined by selective ablation of the different components of the vestibular apparatus. The physiological profile of the EOM has been determined by investigating their histochemical, immunohistochemical, mechanical properties, their innervation, and the size of their motor units. In the dogfish all six EOM contain TYPE I and TYPE II fibres. Type I fibres are small, lie mostly in the orbital region, stain negatively with the Mg2+ activated +m -ATPase, give a positive reaction for the succinate dehydrogenase and stain positive with antisera specific for slow fibre myosin (a ALD and a SHC). TYPE II fibres are large, lie mostly in the global region, stain positively with Mg2+ activated myosin-ATPase, give a negative reaction for SDH and do not stain with a ALD and a SHC. Both Type I and TYPE II fibres are innervated by the enplaque and engrappe endplates. Mechanically two levels of response are shown by the EOM fibres and individual twitches and tetanic contraction is elicited in all the EOM. The size of motor units in the + SO, IO and SR is 9 muscle fibres per one motor axon, and in the EXT-R is 13 muscle fibres per one motor axon. In their physiological and mechanical properties, and in their innervation the TYPE I ans TYPE II fibres resemble the slow and fast fibres of other vertebrates. Vertical eye reflexes have been studied by fixing the animal in a frame and recording eye movements by video and movement transducer techniques. Vertical eye reflexes are compensatory and vestibular effects are strongly shown. Vision is generally weaker and is only seen when the animal is provided with a strong visual stimulus. The gain of the eye movements is generally smaller during tilts in the pitch plane. There is no nystagmus during the vertical eye reflexes, but it occurs in the horizontal VOR. The gain of the eye reflexes is significantly reduced after ablation of the vertical semicircular canals and the utriculus. Myographic recordings in all six EOM have been made during tilts in the roll and pitch planes and at intermediate angles. Co-activation of the SO and SR combined with inactivity of 10 and + IR induces counter-rolling of the down ward eye in the roll plane. Reciprocal activation occurs in the muscles of the upward eye. In the pitch plane SO and SR of the two eyes remain co-activated, and it is the difference in the relative strength of firing of the two EOM which induces the observed torsional movement in the two eyes. The action of the horizontal EOM stabilizes the activity of vertical EOM in all planes of the tilt. Tilts in vertical intermediate planes have demonstrated that transitions occur in the relationship between the cycle of tilt and the phase of EOM myogram burst. For right leading rotations between tilt planes, beginning with roll, the phase shifts occur in the left EOM around 45 and 225, and in the right eye around 135 and 315. Nystagmus is totally absent during tilts in the vertical plane, although when tilted in yaws, not only the horizontal EOM, but also the vertical EOM show a nystagmus response. A strong tonic firing is shown by the EOM during static tilts. The control of the vertical VOR by the vertical semicircular canals and the utriculus have been determined by selective ablation of these two components at a range of frequencies. The effects of utriculus ablation are best seen at 0.2Hz, while the semicircular canals ablation affects the reflexes at 0. 8Hz. The sequence of ablation experiments has ranged from the ablation of a single vertical canal to the ablation of all four vertical canals and the utriculus, and in each has produced a particular pattern of EOM firing for tilts between 0-360, which is different from the algebric model of the input strength of the vertical semicircular canals. The ablation of a single vertical canal reduces the strength of both the ipsilateral and contralteral EOM significantly. The ablation of posterior vertical canal suggests an inhibitory interaction between the vertical canal outputs. In experiments where only single vertical canal is left intact the myographic response of the EOM in two eyes is controlled by contralateral effects. These patterns of EOM firing under various ablated conditions differ from those predicted by a simple algebric model of the input strengths of the 4 vertical semicircular canals, and suggests that a complex central wiring exists between different components of the vestibular system.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Zoology, Neurosciences
Date of Award: 1988
Depositing User: Enlighten Team
Unique ID: glathesis:1988-77718
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jan 2020 11:53
Last Modified: 14 Jan 2020 11:53

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