Tonic and phasic systems in parallel in the eyecup responses of the crab Carcinus

Abstract

Summary 1. Static positions of the eyecup are maintained by control of the frequency of impulses to tonic muscle fibres.

2. In addition, phasic muscle fibres are excited when an optokinetic stimulus containing frequency components faster than about 0.1 Hz. exceeds about 0.1° in amplitude.

3. Within the above limit, components of the stimulus of greater frequency or amplitude excite the phasic system, which adapts in a few seconds. Normally the tonic and phasic systems act in parallel.

4. With oscillatory movements of a striped drum the tonic and phasic units do not have the same phase relation to the stimulus.

5. The numerous components in parallel are so varied and interact at all stages, so that they cannot be related quantitatively to an analysis of the overall response.

6. Although changes in frequency at each movement are large in one muscle and small in another, there is no question of coarse and fine control because none are separately controlled or dependent on the eyecup position. Show more