Encoding of the angular position of the two distal joints of the spiny lobster antennae by a single mechanoreceptor

In the spiny lobster antennae, the two distal joints (J2, J3) rotate in approximately the same plane, their combined movements extending and flexing the flagellum forwards and backwards from the cephalothorax. A single mechanoreceptor (the chordotonal organ CO J2.J3) provides information about movement and angular position of these two joints. Four categories of static sensory neurones were identified. The specific neurones are selectively sensitive to either the J2 position or the J3 position. Homo-directional neurones code J2- and J3-flexed positions or J2- and J3-extended positions. Hetero-directional neurones code the position of each of the two joints in opposite ways, increasing and then decreasing their activity when J3 and J2 are successively flexed. The origin of these different kinds of static response is discussed with regard to the morphology and mechanical properties of the chordotonal organ and to the distribution of the bipolar sensory neurones on the elastic sheet which forms this proprioceptor. Its functional properties are briefly discussed with regard to the contribution of the CO J2.J3 to reflex actions. The sensitivity of this receptor is compared to that of joint receptors in other species.