The arrangement and function of octopus arm musculature and connective tissue

Abstract

The morphology of the musculature and connective tissues of the arms of Octopus bimaculoides was analyzed with light microscopy. We also studied O. briareus and O. digueti, which possess relatively more elongate and less elongate arms, respectively. The morphology of the arms was found to be remarkably uniform among species. The arms consist of a densely packed three-dimensional arrangement of muscle fibers and connective tissue fibers surrounding a central axial nerve cord. Three primary muscle fiber orientations were observed: 1) transverse muscle fibers oriented in planes perpendicular to the long axis of the arm; 2) longitudinal muscle fibers oriented parallel to the long axis; and 3) oblique muscle fibers arranged in helixes around the arm. The proportion of the arm cross section occupied by each of these muscle fiber groups (relative to the total cross sectional area of the musculature) remains constant along the length of the arm, even though the arm tapers from base to tip. A thin circular muscle layer wraps the arm musculature on the aboral side only. Much of this musculature has its origin and insertion on several robust connective tissue sheets including a layer surrounding the axial nerve cord and crossed-fiber connective tissue sheets located on the oral and the aboral sides of the arm. An additional thin layer of connective tissue wraps the arm musculature laterally and also serves as a site of origin and insertion of some of the muscle fibers. The fibers of the oral and aboral crossed-fiber connective tissue sheets are arranged oblique to the long axis of the arm with the same fiber angle as the oblique muscle layers that originate and insert on the sheets. The oblique muscle layers and the crossed-fiber connective tissue sheets thus form composite right- and left-handed helical fiber arrays. Analysis of arm morphology from the standpoint of biomechanics suggests that the transverse musculature is responsible for elongation of the arms, the longitudinal musculature is responsible for shortening, and the oblique muscle layers and associated connective tissues create torsion. Arm bending may involve unilateral contraction of longitudinal muscle bundles in combination with resistance to arm diameter increase due to contraction of the transverse musculature or passive stiffness of the arm tissues. The arms may also be bent by a combination of decrease in diameter due to contraction of the transverse musculature and maintenance of constant length on one side of the arm by unilateral activity of longitudinal muscle bundles. An increase in flexural stiffness of the arm may be achieved by cocontraction of the transverse and longitudinal muscle. Torsional stiffness may be increased by simultaneous contraction of both the right- and left-handed oblique muscle layers.

Behavioral evidence for the evolution of walking and bounding before terrestriality in sarcopterygian fishes

“…S1). This organization appears to be a novel mechanism for locomotor force generation, and perhaps is more similar to the limbs of invertebrate taxa (31) than to those of other extant vertebrates; however, fossil sarcopterygians (8, 10) also have similar, serially repeated limb segmentation, suggesting that the locomotor system of extant lungfish may have at one time been more prevalent. Future work on morphology, mechanical modeling, and force production by these limbs will be necessary to understand how lungfish limbs function and may have evolved.…”

On the preservation of fibre direction during axisymmetric hyperelastic mass-growth of a finite fibre-reinforced tube

“…If A Z or, equivalently, A Θ is considered constant in (i 3 ), then that class specifies helical fibres, such as those met often in plants, arterial wall, muscle and other types of soft tissue (e.g. [1][2][3][4][5][6][7][8][9][10][11]). However, by considering that A Z may further depend on the cylindrical polar coordinate parameters, class (i 3 ) includes also possible families of fibres that may grow in the form of irregular helices.…”

“…An elephant's trunk and the arm of an octopus may be referred to as additional examples of tube-like soft tissue containing families of fibres organised along several different orientations (e.g. [10,11]). In this context, particular mention is made of a set of experimental and theoretical investigations that aim to clarify the role of collagen fibre reinforcement as well as its influence on the mechanical behaviour of arterial wall (see [5][6][7] and relevant references therein).…”

“…7 considers and discusses the aforementioned case of particular practical interest [1][2][3][4][5][6][7][8][9][10][11], where fibres grow with and within the tube in a helical manner. A similar kind of interest is shown afterwards for the families of plane fibres considered in Sect.…”

Design and development of a soft robotic gripper for manipulation in minimally invasive surgery: a proof of concept

“…In the octopus arm, an active bending requires selectively contracting the longitudinal muscle fibers along one side of the arm, thus creating an asymmetrical longitudinal compressional force that shortens one side of the arm and thus causes bending [12,15]. Inside an elastomer segment, a longitudinal and eccentric arrangement of sheathed wires along its length is assimilable to a muscle longitudinal fiber and the pulling of a cable produces the selective contraction of the side subject to the action of the same cable.…”