The Leg Depressor and Levator Muscles in the Squat Lobster <i>Munida quadrispina</i> (Galatheidae) and the Crayfish <i>Procambarus clarkii</i> (Astacidae) Have Multiple Heads with Potentially Different Functions

Abstract

The proximal leg muscles of decapod crustaceans, controlling movements at the first two joints, are anatomically more complex than the better-studied distal leg muscles. Despite extensive research on their involvement in diverse behaviors, no complete descriptions of the anatomy and innervation of these muscles for any species have been published. We describe the anatomy and innervation of the depressor muscle in the second leg of the squat lobster Munida quadrispina and compare its anatomy with that of its homologue in the crayfish Procambarus clarkii and its antagonist, the levator, in both species. Of the six anatomically distinct heads comprising M. quadrispina’s depressor muscle, one arises in the coxa (coxal head) and five are bi-articular (cross two joints), arising from widely dispersed sites on the thoracic endophragmal skeleton (dorsal, sternal, caudal, ventral-rostral, ventral-caudal heads). The heads’ widely divergent force vectors are accommodated by the depressor apodeme’s bifurcation at a thin flexible point. In total, eighteen neurons with central somata were backfilled from nerve branches to the heads. The common inhibitor and at least one neuron of unknown function with rostro-lateral soma and extremely sparse neurites innervate all heads. The sixteen excitatory motoneurons’ somata are clustered in two locations, five rostral and eleven caudal to the neuropil. Rostral motoneurons innervate the two ventral heads (rostral and caudal). Their integrating segments lie rostral to those of the caudal group motoneurons and are straight or ‘Y’-shaped, the latter longer and larger in diameter. Both morphological types have one prominent medial neurite that crosses the midline and could allow direct interaction between bilateral pairs of rostral motoneurons. The caudal motoneurons provide partially shared innervation to the remaining four heads. Six provide exclusive innervation, one to the caudal head, two to the sternal head, and three to the bi-articular dorsal head and uni-articular coxal head which share innervation. Of the remaining five caudal motoneurons, two are shared by the caudal head and the dorsal-coxal pair of heads and three are shared by the caudal and sternal heads. P. clarkii has two depressor muscles; the rostral depressor has a single head morphologically similar to the ventral rostral head in M. quadrispina; the caudal depressor muscle has four heads (dorsal, coxal, caudal, sternal) that insert on the large caudal depressor apodeme. The overall organization of depressor muscle heads in P. clarkii and M. quadrispina is similar, taking into consideration the different internal and external thoracic anatomies and the different resting stances, horizontal and tilted, respectively. As in other species, M. quadrispina and P. clarkii have two levator muscles, each with an apodeme of complex structure attributable to the levators’ role in leg autotomy. The caudal levator arises in the coxa; it has two heads in M. quadrispina, but only one in P. clarkii. In both species the rostral levator ha...

Serotonergic and octopaminergic systems in the squat lobster Munida quadrispina (anomura, galatheidae)

“…In T5, the thoracico‐coxal nerves leave the ganglion dorsal to the distal leg nerves, as they do in crayfish (Elson, 1996). However, the thoracico‐coxal nerves of the more anterior pereiopod ganglia are displaced increasingly to the posterior relative to the distal leg nerves (Antonsen and Paul, 2000). The second segmental nerves of these ganglia emerge just posterior to the distal leg nerves, and the third emerge dorsally from the posterior region of the ganglia or from the remnant of the connectives just posterior to the ganglion (Antonsen and Paul, 2000).…”

“…However, the thoracico‐coxal nerves of the more anterior pereiopod ganglia are displaced increasingly to the posterior relative to the distal leg nerves (Antonsen and Paul, 2000). The second segmental nerves of these ganglia emerge just posterior to the distal leg nerves, and the third emerge dorsally from the posterior region of the ganglia or from the remnant of the connectives just posterior to the ganglion (Antonsen and Paul, 2000). In A2 through A5, the first and second segmental nerves leave laterally, but due to the position of A1, these nerves leave this ganglion posteriorly.…”

The effect of sensory feedback on crayfish posture and locomotion: I. Experimental analysis of closing the loop

“…Active movement of the leg around the CB joint is produced in the crayfish by the levator and depressor muscles, which contain four and five muscle heads, respectively, including both phasic and tonic muscle fibers (Antonsen and Paul 2000). Separate muscle models were used to represent the tonic and phasic portion of each Lev and Dep muscle head (Fig.…”

“…1, Crayfish leg and body model). They were attached in parallel to a common origin and insertion in the model placed according to the origin and insertion of the corresponding muscle head in vivo (Antonsen and Paul 2000).…”

Recruitment in a heterogeneous population of motor neurons that innervates the depressor muscle of the crayfish walking leg muscle

“…Within the proximal limb, functional muscles, defined by the direction of movement they cause, may consist of multiple muscle bundles with multiple origins and insertions (Bévengut et al, 1983). In the crayfish, the depressor muscle consists of two bundles: a rostral bundle with a single origin and a caudal bundle with four origins (Antonsen and Paul, 2000). In order to have smooth movement around the pivot point of the basipodite-coxopodite joint, it is necessary to activate the separate muscle bundles differentially since some will have to contract more in length than others to contribute force equally at the apodeme.…”