MUSCULAR SYSTEM OF INVERTEBRATES:-



The muscular cells of invertebrates can be divided into three major classes on the basis of their striation pattern:

1) Transversely striated,

2) Obliquely striated, or

3) Smooth muscle.

1)Transversely striated muscles have either continuous or discontinuous Z lines and, thus, can be subdivided into two types respectively. Of all invertebrate muscles, the transversely striated muscle with continuous Z lines is present in arthropods, whose musculature (including the visceral muscles) only consists of this cell type. These muscles are multinucleate cells that contain myofibrils showing well-defined sarcomeres. Transversely striated muscles with discontinuous Z lines, consisting of multiple small electrondense patches, are found in adductor muscles of some bivalves and in the heart muscle of the gastropods. These muscles are mononucleate cells that contain a single myofibril.

2) The obliquely striated muscle appears in nematodes, annelids, molluscs, brachiopods and chaetognathes and consists of mononucleated cells with both thick and thin myofilaments which form sarcomeres delimited by Z lines. Myofilaments are not perpendicular but oblique to the Z lines.

3) Smooth muscle has been reported in coelenterates, annelids, molluscs, brachiopods and echinoderms, but is lacking in arthropods.

Muscular system of Phylum NEMATODA:-

• Roundworms (PHULUM NEMATODA) have large cell bodies on their muscle cells, but only narrow projections of the muscle cell bodies extend to the principal nerves and contact nerve cells there.

• Roundworms have obliquely striated, longitudinal muscle but no circular muscle.

• They are enclosed in a thick cuticle that allows bending but prevents swelling. Therefore, contraction of the longitudinal muscle can only bend the body.

• Roundworms do not bend from side to side like eels or snakes, but up or down (dorsal or ventral).

• By preventing swelling, the cuticle ensures that shortening of one muscle group stretches the other; thus, it makes the dorsal and ventral longitudinal muscles antagonistic to one another.

• Most crawl between soil particles or among the villi of a host’s gut by undulating waves of muscular contraction.

• Similar movements also enable some roundworms to swim.

EXAMPLE:

Ascaris:-

In Ascaris, there are four longitudinal streaks that run through the body. The muscle layer is divided into four longitudinal columns, two dorsolateral and two ventrolateral. Circular muscles are altogether absent.

Muscular System of Phylum ANNELIDA:-

• In Segmented worms (PHYLUM ANNELIDA) the fluid-filled cavity, the coelom is divided into successive segments.

• The body wall has an outer layer of circular muscle and an inner layer of longitudinal muscle.

• Earthworms crawl by peristaltic contractions of the body wall. Each segment is alternately elongated (by contraction of its circular muscles) and shortened (by contraction of its longitudinal muscles).

• The muscles of each segment contract just after those of the segment in front, so that waves of contraction pass backward along the body, enabling the worm to move slowly forward.

• The same movements also serve for burrowing. While shortened, the segments are pushed against the burrow wall; when they elongate again, the worm moves forward.

EXAMPLE:

Leech:-

It has a circular and longitudinal muscle layer, circular being the outermost muscle. in between them is the oblique muscle which is also found on humans near the stomach area. The dorsoventral muscle is the innermost layer of muscle within a leech. It is largely responsible for the posture of the leech and its locomotory activities of swimming and crawling.

Muscular System of PLATYHELMINTHES:-

• Flatworms (PHYLUM PLATYHELMINTHES)have musculo-epithelial cells, the muscle cells in most are distinct from the epithelial cells.

• There is a layer of circular muscle fibres immediately under the epidermis.

• A layer of diagonal fibres, and a still deeper longitudinal layer.

• There are also dorsoventral muscle fibres running from the upper to the lower epidermis of the flattened body. These sets of muscle fibres act in various combinations to make the body long and thin, short and fat, or bent to one side or the other.

• These muscles are also used by some of the larger flatworms to pass waves of muscular contraction along the body, enabling the worm to craw. In some flatworms the pharynx is protruded l in a snail-like fashion.

• Many flatworms have a mouth opening connected to the pharynx, a muscular tube that carries food from the mouth to the intestine.

• Although the muscle cells of flatworms are generally not musculoepithelial, their nuclei are found in large cell bodies. The muscle fibres of vertebrates and higher invertebrates, on the other hand, have no projecting cell body.

EXAMPLE:

Planaria:-

Planaria do not have a skeleton. They have an extendable crumpled muscle that allows them to move. Also, planarians have three layers of body-wall muscles that are constructed of circular muscle fibers as well as longer muscle threads.

Muscular system of Phylum ECHINODERMATA :-

• As for the muscle system, echinoderms have small tubular projections called “Tube feet” used for moving and sticking to the ocean floor.

• The two muscles involved in tube feet are ampulla and podia which both are circular suction cups that allow for Echinoderm movement.

• The muscular and water vascular system are responsible for movement . The muscular system controls the bony plates. The water vascular system is a hydraulic system composed of canals connecting many tube feet. When the tube feet contract, they push water into the tube feet allowing them to extend and move the animal.

• Sea urchins move through small muscles at the bottom of their endoskeleton.

• Sea cucumbers muscles that movement is done by the same tube feet.

EXAMPLE:

Sea Star:-

In order for starfish to use their muscles from their muscular system they mainly use their tube feet and press them against a moving object. As a result, water is withdrawn from them and make a suction effect. Later when water comes back into the canals, suction is then released.

These results make locomotion very slow but this is how their muscular system relates to their movement.

Muscular System of Phylum MOLLUSCA:-

• The mollusc's muscular system is primarily made up of the foot that allows for movement, suction, burrowing, and a hand-full of other acts. The act of the foot depends upon the particular organism.

• Squids have a foot that is essentially a base for the arms that extend from it. These arms can do multiple tasks and assist in the survival of the organism.

• Snails and clams use their foot simply for movement purposes. They both drag along slowly but surly. Without them a snail would be a boneless sack of flesh that oozed mucus.

• Clams simply use their foot to move along the ocean floors.

• Their muscular systems are not incredibly complex, yet necessary for the survival of the organism.

EXAMPLE:-

Snail:-

There are two sets of muscle fibers,each performs a different task. When moving forward one set contracts pulling the snail from the front and pushing it off toward the back.

At the same time the second set pulls the outer surface of the sole forward.The snail moves about by means of a large, muscular structure called a foot.

Waves of muscular contractions and expansions enable the snail to move forward. The snail secretes mucus from its skin, which lubricates the surface upon which it crawls.

With its muscular foot and marvelous mucus, the snail can easily glide over many surfaces.

Muscular System of Phylum ARTHROPODA:

• Insects have only striated muscles.

• The principal wing muscles are the dorsoventral muscles, which run vertically from the sternum to the tergum.

• The longitudinal muscles, which run lengthwise along the segment.

• Muscle cells are amassed into muscle fibers and then into the functional unit, the muscle.

• Muscles are attached to the body wall, with attachment fibers running through the cuticle and to the epicuticle, where they can move different parts of the body including appendages such as wings.

• The muscle fiber has many cells with a plasma membrane and sarcolemma.

• Contractile myofibrils run the length of the muscle fiber. Myofibrils comprising a fine actin filament enclosed between a thick pair of myosin filaments slide past each other instigated by nerve impulses.

EXAMPLE :

Crab:-

Crabs are primarily a system of antagonistic pairs of striated muscles that work with the exoskeleton’s system of levers.

The musculoskeletal system allows movement through the contraction of muscles Crabs .Crab has hard exoskeletons that encase their soft bodies to provide protection and the support that allows for movement through the contraction of the attached muscles. Movement is also facilitated by jointed appendages formed from a stiff cuticle.

Axons carry nerve impulses away from the cell body, and are an extension of the nerve cell. Nerve impulses are carried by action potentials down the axon, which control the force exerted by the crab’s muscles based on their frequency.

The larger force is due to a higher frequency of action potentials.

Action potentials facilitate each muscle contraction. Within the contracting crab’s muscles short, strenuous actions are powered by anaerobic respiration. Crab ’s leg muscles, used for swimming, are composed of two muscle fiber types. The red muscle fiber contains a high number of mitochondria, and it works aerobically, while the other is white due to its lack of mitochondria, and it works anaerobically.