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The Journal of Experimental Biology 204, 3411-3423 (2001)
© 2001 The Company of Biologists Limited

Functional organisation of anterior thoracic stretch receptors in the deep-sea isopod Bathynomus doederleini: behavioural, morphological and physiological studies

Masazumi Iwasaki¹, Ayako Ohata¹, Yoshinori Okada¹, Hideo Sekiguchi² and Akiyoshi Niida^1,*

¹ Department of Biology, Faculty of Science, Okayama University, Tsushima, Okayama 700-8530, Japan and
² Faculty of Bioresources, Mie University, Japan

*Author for correspondence (e-mail: niida{at}cc.okayama-u.ac.jp)

Accepted July 27, 2001

The relationship between segmental mobility and the organisation of thoracic stretch receptors was examined in the deep-sea isopod Bathynomus doederleini, which shows a developed adaptive behaviour during digging. The movements of segments during digging were analysed from video recordings, which showed that a large excursion occurred in the anterior thoracic segments. Dye-fills of axons revealed four types of thoracic stretch receptor (TSR): an N-cell type (TSR-1), a differentiated N-cell type (TSR-2), a muscle receptor organ (MRO)-type with a long, single receptor muscle (TSR-3) and an MRO-type with a short, single receptor muscle (TSR-4 to TSR-7).

Physiologically, TSR-1 and TSR-2 are tonic-type stretch receptors. TSR-3 to TSR-7 show two kinds of stretch-activated responses, a tonic response and a phasico-tonic response in which responses are maintained as long as the stretch stimulus is delivered. Both TSR-2, with a long muscle strand, and TSR-3, with a single, long receptor muscle, have a wide dynamic range in their stretch-activated response. In addition, TSR-2 is controlled by an intersegmental inhibitory reflex from TSR-3. These results suggest that, although TSR-1 has no receptor muscle and TSR-2 has a less-differentiated receptor-like muscle, they are fully functional position detectors of segmental movements, as are the MRO-type receptors TSR-3 to TSR-7.

Key words: stretch receptor, muscle receptor organ, N-cell, accessory neurone, crayfish, Crustacea, isopod, Bathynomus doederleini.