The structure and function of a connective chordotonal organ in the cockroach leg

Abstract

The micromorphology of the tibio-tarsal joint of Periplaneta is described, based on the mesothoracic limb. There are two muscles acting on the joint, numbered 144 and 145. There is a connective chordotonal organ, which branches off from the trunk of N5 proximally and inserts on the intersegmental membrane distally. In addition, a previously undescribed group of campaniform sensilla is reported from the tibiotarsal joint. The tibio-tarsal chordotonal organ subdivides into a main branch and two side branches. It contains 26 bipolar sensory neurons, whose dendrites are associated distally with 14 scolopales. These scolopidia are arranged in three groups which differ in position, fine structure and number of cells per scolopale. There is one group 1 scolopidium in each of the side branches and two proximally in the main branch. Group 2 scolopidia are spread out along the main branch and group 3 scolopidia occur distally in the main branch. Group 1 scolopidia consist of two bipolar neurons, 15 to 20 µm in diameter, whose dendrites, heavily sheathed, insert in a single scolopale. Group 2 scolopidia consist of two bipolar neurons, 8 to 15 µm in diameter, whose dendrites are less well sheathed and insert in a single scolopale. Group 3 scolopidia consist of a single bipolar neuron, about 10 µm in diameter, with a short, poorly sheathed dendrite inserting in a single scolopale. The scolopales of groups 1 and 2 are identical, consisting of the usual pattern of a ring of scolopale rods inserting into a distal cap and enclosing the cilia on the dendrite terminations. In this case, the cap is particularly long and pointed and both the cilia and the scolopale rods penetrate a long way into it. The cilia of the two members of a pair of dendrites are identical but the ciliary roots differ between the two members of a pair. Electrophysiological recordings show that the organ responds to downward and backward deflexion of the tarsus. The response comprises at least two classes of sensory fibre distinguishable both by their size and by their behaviour. The larger fibres show a unidirectional phasic and tonic response to extreme deflexion of the tarsus and are identified with the group 1 scolopidia. The smaller fibres show a unidirectional tonic response to the full range of deflexion of the tarsus and are identified with the group 2 scolopidia. On structural evidence, it is suggested that the differences in adaptation between these two groups of scolopidia is not likely to be caused by differences in mechanical attachment. The combination of electrophysiological and fine structural evidence indicates that in this chordotonal organ the adequate stimulus of the scolopidia is an increase in their longitudinal tension but it is not possible to say which fine structural component responds to strain.