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THE MUSCULAR SYSTEM
SKELETAL MUSCLE

     There are two major proteins responsible for the contraction of a muscle cell, actin and myosin.  Myosin proteins possess a pivoting head which, after binding to ATP molecules for energy, can attach to specific sites on actin molecules, pivot, and return to their original position.  Although these proteins are essential components of vertebrate muscle, they evolved in early cells are conserved throughout the groups of eukaryotes.  Actin proteins are highly conserved proteins which constitute the majority of the eukaryotic cytoskeleton and can compose 10-20% of the total cellular protein of a non-muscle eukaryotic cell.  These thin filaments are involved in organelle transport, cell motility, and cytokinesis (OMIM; Hoar, 1983).

     The shape of all eukaryotic cells is determined by the shape of the protein cytoskeleton.   These thin filaments are involved in organelle transport, cell motility, and cytokinesis.  While yeast possess only one known actin gene, multiple genes are known from all protozoa, plants, and animals studied (Hightower, 1986).  There see to be around 20 actin genes in the human genome. 

      Although actin was formerly thought to be unique to eukaryotes, it is now evident that bacteria possess several homologs of actin, such as MreB and ParM, which can polymerize into filaments. 

BACTERIA
NOTOCHORD

MreB determines the shape of the cell and ParM functions in the movement of intracellular structures, such as plasmids (Amos, 2004; Egleman, 2003).  MreB seems to form a bacterial filamentous cytoskeleton under the cell membrane (Egelman, 2001). 

     Amphioxus uses muscle fibers in its notochord, using this structure for locomotion in a way unlike that of vertebrate embryos.  The actin in amphioxus notochord is intermediate between the forms of actin used in muscle and non-muscle cells in vertebrates (Suzuki, 2000).

     Myosin molecules are not limited to muscle cells.  Many types of cell create some type of movement, such as the intracellular cytoplasmic streaming which helps to distribute materials evenly throughout a cell when myosin molecules pull on the actin filaments of the cytoskeleton.  There are 11 classes of myosin molecules in the myosin superfamily that are known from animals, plants, fungi, and protists.   Some of the predominant myosins in amoeba are conserved in vertebrates.   In addition to the conventional myosins found in vertebrate muscle, many are referred to as unconventional myosins which act as molecular motors which move along actin molecules.  Seven of the known 11 classes of unconventional myosin molecules are found in vertebrates.  (Mooseker, 1995).

     Vertebrates possess multiple copies of muscle genes which are expressed in different types of muscle and the duplications which produced these genes seems to have occurred after the separation of urochordates from the lineage of higher chordates.  Since tunicate smooth muscle lacks specific proteins expressed only in smooth muscle, smooth muscle may be considered as unique to vertebrates.  Tunicates do possess homologs of vertebrate genes involved in heart formation, but typically fewer members than contained in vertebrate gene families (Dehal, 2002).

     Additional protein muscles are also conserved among animals.  For example, troponin and tropomyosin are known in invertebrates as primitive as nematodes (Hoar, 1983, p. 325).  Titin is known in arthropods and a homolog of titin is known in nematodes (Kenny, 1999; Champagne, 2000). 

     Placozoans are among the simplest animals and they possess no nervous tissue or muscle tissue.  Their bodies are composed of only 4 types of cells which use actin, microtubules, and intermediate filaments in their cytoskeleton (Grell, from Harrison, Vol.2).  Sponges, like placozoans, lack tissues, including muscle tissue.  Some sponges possess contractile myocytes similar to smooth muscle and these cells are associated with AChE, an enzyme important in the function of neuromuscular junctions in vertebrates (Harrison, Vol. 2).  These contractile cells have been referred to as “almost muscles” (Mackie, 1990).

     Cnidarians possess primitive muscle cells in circular, longitudinal, and sometime oblique layers (Fretter, p.56-8). A significant amount of cnidarian muscle is striated. Some invertebrate muscle possesses oblique striations while others possess striations on one half of the cell and mitochondria on the other (Prosser, 1973, p. 723; Hoar, 1983).  These contractile cells are not organized into prime movers and antagonists, as in vertebrates (Hickman, p. 137; Fretter, p. 66).  Ctenophores, which some analyses identify as the group of cnidarians which are most closely related to bilateran animals, possess true muscle cells (Hickman, p. 181).  Skeletal muscle is mononucleated in cnidarians and worms but multinucleated in vertebrates and other coelomates such as insects and leeches (Castanon, 2002).  The following slides are of the muscle tissue of a sea anenome

ANENOME ANENOME

     Cnidarians are usually classified as diploblastic (having only two embryonic tissue layers, ectoderm and endoderm), while all bilateran animals are classified as triploblastic (because in addition to ectoderm and endoderm, they possess a third tissue layer, mesoderm, from which muscle is derived).  Although cnidarians are classified as diploblastic, in some cnidarians the muscle cells of the oral disk are covered by epithelia, not unlike the location of the mesodermal muscle of flatworms (Barrington, p. 62).  The gene Twist functions in the differentiation of mesoderm in tribloblast bilaterans but it is also present in diploblast cnidarians.  Twist expression first occurs in the entocodon, the mesoderm-like layer from which muscle tissue differentiates in cnidarians (Castanon, 2002).   The development of striated and smooth muscle from this third cell layer have led some to consider cnidarians as tribloblastic (Muller, 2003, Seipel, 2005).

    In bilateran animals, some of the genes which are essential for the embryonic formation of muscle are members of the basic helix-loop-helix gene family (bHLH).  Jellyfish are known to possess at least four bHLH transcription factors.  The sequence, dimerization, and expression of the JellyD1 protein in striated muscle indicate that it is a homolog of MyoD in bilaterans.  Vertebrate MyoD genes, which can form both homodimers and heterodimers, can form dimers with JellyD.  This indicates that the striated muscle of jellyfish is homologous to that of bilaterans (Muller, 2003).

     Flatworms (such as the planarian in the following image) possess subepidermal and mesodermal muscle which is more similar to the muscle of higher animals (Hickman).

FLATWORM
Flatworms classified as Acoela are the most primitive bilateran animals.  The cutaneous musculature of these worms still possesses radial symmetry (Beklemishev, vol.  1, p. 108) and even include musculo-epithelial cells similar to those found in cnidarians (Barrington, p. 71). The most primitive and smallest flatworms still depend on cilia as their primary means of locomotion; the role of muscle in locomotion increases in larger flatworms which utilize waves of muscle contraction.  Cilia covering a worm’s epidermis are evident in the following photo.
CILIA ON WORM

  In addition to enabling the movement of the body, muscle functions in both the digestive and reproductive systems in worms.  (Beklemishev, vol.  2). In nemertine worms waves of muscle contraction are referred to as peristalsis, as are the waves of muscle contraction used in a number of internal tracts in vertebrates (Barrington, p. 73).  The development of a coelom in higher worms had a great impact on the muscular system in that it increased the flexibility and speed of possible movements.

     Most hemichordate muscle is smooth but some striated muscle also exists (Benito, form Harrison 1997, p. 44).  Although ascidians possess skeletal muscle in the larval tail, the cells are single and do not fuse to form the large syncytial cells as occurs in vertebrate skeletal muscle (Burighel, from Harrison, 1997, p. 316).  

     Many invertebrates possess hydrostatic skeletons.  Muscle contraction in animals which possess a hydrostatic skeleton has several disadvantages which include the changing of pressure throughout the entire body when one area moves, the tendency for slower movement, and the wasted energy expenditure if the animal moves quickly (Barrington).

     In arthropods, neurotransmitters can be released from any point in which a neural axon contacts the muscle cell and muscle cells do not require specialized regions (such as the motor end plate of vertebrates) to interact with neurons (Prosser, 1973, p. 753).  Invertebrate visceral muscles may be striated (arthropods) or smooth (mollusks).  Smooth muscle may be stimulated by stretch alone in both invertebrates and vertebrates. (Prosser, 1973,  779-80).  As is discussed in the material on the nervous system, the visceral muscle of the gastrointestinal tract of all vertebrates and many invertebrates is innervated by a diffuse nerve net, similar to the nerve nets observed in the most primitive nervous systems of coelenterates.

     Metamerism (the repetition of anatomical units along the longitudinal axis of the body) exists in both protostomes and deuterostomes, although it may have evolved separately in each lineage.  It is observed in the repetition of gill slits, gonads, and digestive pouches of hemichordates (basal deuterostomes) and is especially obvious in annelids and arthropods among the protostomes (Barrington).  The repetition of blocks of muscle (known as myomeres) separated by connective tissue at a angle (forming a chevron shape) is unique to chordates (Stach, 2000).   In Amphioxus (pictured below), most muscle cells form V-shaped myotomes similar to those observed in fish.  Myotomes first evolved in cephalochordates (Cameron, 2000).

SOMITES

Additional muscles in Amphioxus surround the oral and pharyngeal areas and form an anal sphincter.  A pair of trapezius muscles on the dorsal surface helps to expel water from the body. (Ruppert, from Harrison, 1997, p. 397-9).

     Fish possess different kinds of muscle fibers including aerobic and anaerobic fibers, dark fibers and light fibers (Hoar, 1970).