Giant fibres and directional statocyst fibres : a study of interneurons between the brain and thoracic nervous system in the shore crab, Carcinus maenas (L.)

• Main Author: Fraser, Peter John
• Published: University of Aberdeen 1973
• Subjects: 571.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.592453

A group of five giant fibres. (40 - 6ojx in diameter) occurs in both oesophageal connectives of Caroinus maenas (L.) and similarly in three other species of crab investigated. Four of these fibres react to visual and tactile stimuli in a phasic numner. They seem to be subjected to excitatory and inhibitory control. The fifth fibre has input from the oontralateral statocyst and fires on head up and same side down rotations round the horizontal axes. It alao fires on forced leg movements upwards about the basal joints, summing input from both sides. There are four other directional statocyst fibres in each connective Carcinus responding to rotation round the vertical axis, or to rotation round both horizontal axes. Two of these also have input from the legs. One of these is supplied by the contralateral statocyst and three by the ipselateral one. The detailed structure of the brain of Carcinus is as reported in Bethe (1897) and Bullock and Horridge (1965) except that the antennary and tegumentary neuropiles have been confused by both authors, and the accessory lobe is present (contrary to Bullock and Horridge). The giant fibres all have contralateral cell bodies in the dorsal anterior medial cells. The main branches are in the dorsal deuterocerebrm with a few branches in visual neuropiles of the protocerebrum. All five fibres have an ipsilateral branch with processes in the antennary and Documentary neuropiles which join the main axon where the connective merges into the brain. The detailed anatomy of the connective is similar to the crab leg motor nerve (Horridge and Chapman,1964) only the sheaths round large fibres are thinner withea less well developed inner sheath. There are 2300 fibre in the connective of Carcinus. Approximately 45 of these have diameters greater than 10 jj. Most of the fibres are between 1 and 5 P- diameter. Other interneurons investigated have cell bodies in the posterior cells and ipsilateral dendrites. One other large fibre (40 u iiameter) responded to visual and taotile stimuli and had a oontralateral oell body. This fibre could be pieced with the giant fibres. No additional inputs to any fibres investigated vras revealed in free walking animals with implanted electrodes. No direct effect was apparent on stimulation of these fibresroalthough electrical stimulation of isrtain others both in the connective and optic tract yielded a variety of complicated ordered appendage movements including the full swimming reflex. Swimming is also elicited by head, down rotation of the animal and is activated by sensory information from the statocyst eyes and legs. It is suggested that the visual and tactile giant fibres form a group more similar to giant fibres in the cockroach and locust than in i the crayfish. It is further suggested that the giant statocyst fibre is involved in back leg movements which stabilize the crab during the rearing reflex and also may provide phasio information of integrated leg and body position during walking.