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KMID : 0376219700070030379
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Chonnam Medical Journal
1970 Volume.7 No. 3 p.379 ~ p.390
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Responses of the extraocular muscles to stretch and vestibular canal stimulation in the rabbits
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Abstract
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Since crevatin(1990) discovered the muscle spindles in cow’s extra ocular muscles, intensive studies to find the muscle spindles in the extra ocular muscles of various animals and man have been performed up to the present, the spindles were found in the extra ocular muscles of man, cow and sheep but not in those of monkey, dog, cat and rabbit.
The anatomical demonstration of the extra ocular muscle spindle in man and some animals induced a concept with a reflex regulatory mechanism that is analogous to the stretch reflex of the other skeletal muscles may play a role in the central control of eye movements. However, direct evidences demonstrating the stretch reflex action of the extra ocular muscles have not been obtained.
Cooper and Daniel demonstrated afferent excitation of the muscle spindle from the oculomotor nerve of a stretched extra ocular muscle of sheep. Cooper and Fillenz were also able to record such afferent excitation from the extra ocular muscles of monkeys and cats which are known to be devoid of muscle spindle; they ascribed such afferent excitation to the action of nonspecialized stretch receptors in the extra ocular muscles.
In the present experiment, the extra ocular muscles of the decerebrate rabbits were subjected to ramp stretch by means of a spring dashpot stretcher, and the excitatory semicircular canal nerve was stimulated to elicite afferent impulses through the vestibulo-oculomotor pathway. The responses of extra ocular muscles to the stretch and vestibular stimuli were observed in the length tension curve recorded by a torsion wire myograph that was attached to the stretcher and following results were obtained.
1. During the stretch, the tension of the extra ocular muscle increased graphically and the length tension curve showed a relatively constant slope. When the was continued nearly to the 10% of the natural muscle length, terminal part of the length tension curve revealed a steeper slope indicating sharp increment of the muscle tension.
2. After section of the trochlear nerve, the slope of the length-tension curve of the superior oblique muscle decreased considerably.
3. Stimulation of the excitatory semicircular canal nerve increased the slope of the length tension curve depending upon the increment in intensity and frequency of the square waves applied.
4. When two excitatory canal nerves were stimulated simultaneously, the tension increment which was obtained by the separate stimulation of the two canal nerves.
5. Based on the above experimental observations, it is concluded that the extraocular muscles of rabbits are equipped with some stretch receptors which are able to signal the changes of the muscle length to the central nervous system; the two different afferent impulses elicited by the muscle stretch and vestibular excitation interact in the central nervous system by a multiplicative manner. However, any direct evidence supporting the stretch reflex response of the extra ocular muscles was not obtained.
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