In the walking legs of decapod crustaceans, intersegmental reflex actions originate from various joint proprioceptors. The activity of the ‘accessory flexor’ (AF) muscle, which with the myochordotonal organ (MCO) constitutes a muscle proprioceptor for the mero-carpopodite (M-C) joint, is modulated by the sensory discharge of a joint receptor (CB chordotonal organ) for the more proximal, coxo-basal (C-B) joint. Selective mechanical stimulation of the CB organ also reflexly modifies the motor activities of the main M-C flexor and extensor muscles (recorded as EMGs). 1. Dynamic CB stretch (as would occur during a dorso-ventral C-B movement - i.e. ‘depression’ of the limb) stimulates motor discharge to the M-C extensor muscle, while dynamic release of CB (as during a ventrodorsal C-B movement - or leg ‘elevation’) excites the accessory flexor as well as the main flexor muscle. 2. Successive M-C muscle responses to repetitive sinusoidal changes of CB length differ quantitatively according to the direction (stretch or release) of the first CB movement, in some cases increasing but more commonly ‘adapting’ with repetition. 3. Reflex discharge frequencies of the extensor, flexor and accessory flexor motoneurones increase with velocity of CB movement. 4. Eye illumination, and spontaneous or other sources of increased central excitability, generally increase the CB reflex drive to the flexor and accessory flexor muscles and, in parallel, decrease the reflex action on the extensor muscle. The results are discussed in terms of the role of proprioceptive reflexes in intersegmental co-ordination of the leg joints. In particular the significance of the reflex regulation of the myochordotonal receptors, and thereby the gain of the M-C resistance reflexes, is considered in the light of the observed ‘co-activation’ of main flexor and receptor muscle motoneurones.
In the decapod Crustacea, Palinurus vulgaris and Fasus lalandii, the reflex influences of one particular proprioceptor organ, the coxo-basal chordotonal organ (CB), on all the muscles operating the proximal and distal joints of the same leg, have been analysed. The distal end of CB was clamped in fine forceps mounted on a servo-controlled stretcher, and CB length changes of 2 mm were applied. Motor unit activity of the different muscles was recorded as electromyograms (EMGs). 1. Two types of proprioceptive reflex evoked by CB length changes have been investigated: (a) resistance reflexes of the two levator and two depressor muscles of the same leg segment, the coxopodite, i.e. ‘intrasegmental reflexes’, (b) ‘intersegmental reflexes’ induced in the muscles operating the proximal (T-C) joint of the same leg, and in all eight muscles of the limb segments distat to CB. 2. Both levator muscles respond reflexly to imposed CB stretch (which normally occurs with limb ‘depression’), while both depressors respond during CB shortening (or passive “elevation” of the leg). 3. Intersegmentally CB stretch reflexly activates the M-C extensor muscle, and sometimes facilitates the T-C remotor and C-P bender muscles. Shortening of the single CB organ of a leg excites one or two tonic motor units of the T-C promotor and M-C flexor muscles, and also facilitates the remotor, I-M reductor, and the single stretcher-opener excitatory motoneurone. 4. Some of the muscles, particularly the M-C flexor and extensor muscles, are also influenced intersegmentally by the resting length of CB, usually but not invariably in the same direction as for the corresponding dynamic reflexes. The role of the CB chordotonal organ is discussed, with particular consideration of its intersegmental reflex influence on the posture of the entire leg, and on the more complex motor behaviour of locomotion, where it may be specially significant in coordination of the limb in lateral walking. A complex picture of both tonic and dynamic, inra- and intersegmental reflex regulation of the positions and movements of the limb segments, thus emerges.
1. The reflex activity elicited by movement of the mero-carpopodite (M-C) joint in the cheliped of the crayfish Astacus leptodactylus is investigated and the role of the different proprioceptors (chordotonal and myochordotonal organs) separately studied. 2. The reflex discharge involves mainly the tonic motoneurones of the extensor (E), the flexor (F) and the accessory flexor (AF) muscles. 3. M-C joint posture is also regulated by the cuticular stress detector (CSD2) afferents: they increase mainly the F discharge and secondarily the AF command. 4. The activity of the motor axons supplying the muscles of the meropodite can be also influenced by a variety of natural stimuli applied to other appendages. The effect usually produced is a general flexion reaction which is characterized by a reciprocity between E and F involving both central and peripheral mechanisms. 5. The AF muscle is innervated by two antagonistic motoneurones, an excitatory neurone functionally linked in its discharge with one of the four excitors supplying F and an inhibitory motoneurone, common with E. The resulting competitive effect between these two neurones has been recorded intracellularly in AF muscle fibres. 6. The role of the myochordotonal organ (MCO) in the crayfish is discussed. In particular the modulation of the AF command in relation to the discharges of the motor nerves to the main muscle E and F is studied.