Antennal proprioception in the rock lobsterPalinurus vulgaris: Anatomy and physiology of a bi-articular chordotonal organ

1. In the antennae of the rock lobster,Palinurus vulgaris, proprioception of the two distal joints (J2-J3) (Fig. 1) is ensured by a single mechanoreceptor, the J2-J3 chordotonal organ (CO J2-J3). 2. The CO J2-J3 is formed by a large connective tissue sheet spanning both joints. On the sheet, three groups of sensory cells can be distinguished: a proximal group g1, a medial group g2, a distal group g3 (Fig. 2). 3. Because of the triple cuticular insertion of the sheet, CO J2-J3 length is affected differently by J2 and J3 movements. Micrometric observations have shown that g1 region length changes result mainly from J2 movements, g3 region from J3 movements and g2 region length changes result from movements of both J2 and J3 joints (Fig. 3). 4. Electrophysiological study revealed two kinds of sensory afferent: sensory neurones exclusively sensitive to joint positions (static sensitivity) and sensory neurones exclusively sensitive to joint movements (dynamic sensitivity), the latter neurones responding to only one movement direction (extension or flexion) of the joints. 5. Each kind of sensory neurone can be subdivided into different categories according to its sensitivity to J2 and J3 position and movement: the monoarticular neurones encoding the position or movement of only one joint, J2 or J3; the biarticular neurones sensitive either to the positions or to the movements of both joints. Some respond to position changes of J2 and J3 in the same direction (homo-directional neurones); some respond to position changes of J2 and J3 in opposite directions; i.e. J2 flexion-J3 extension (hetero-directional neurones). 6. The origin of the different modalities of sensory responses has been interpreted according to the localization of the sensory neurones on the CO J2-J3 sheet. 7. CO J2-J3 sensory information is compared with that observed in other chordotonal organs and its role in controlling joint position and movement is discussed.