Muscles and Muscle Tissue PDF

**Muscles and muscle tissue** **Muscle tissue has four main properties:** 1. Excitability: an ability to respond to stimuli 2. Contractibility: an ability to contract 3. Extensibility: an ability to be stretched without tearing 4. Elasticity: an ability to return to its normal shape Struct...

**Muscles and muscle tissue** **Muscle tissue has four main properties:** 1. Excitability: an ability to respond to stimuli 2. Contractibility: an ability to contract 3. Extensibility: an ability to be stretched without tearing 4. Elasticity: an ability to return to its normal shape Structure Of Three Basic Muscle Types - Muscle System - MCAT Content **Structure of muscle tissue** Regardless of its morphology or type, muscle tissue is composed of specialized cells known as muscle cells or [[myocytes]](https://www.kenhub.com/en/library/anatomy/types-of-muscle-cells) , commonly referred to as muscle fibers; this is due to their extensive length and appearance. Myocytes are characterized by protein filaments known as actin and myosin, producing contractions that move body parts, including internal organs. Interestingly, these proteins are not exclusive to muscle cells; actin and myosin are commonly found as cytoskeletal elements in many cell types and are involved in cellular functions relating to the changing of cell shape (e.g. cell movement, phagocytosis etc.), muscle tissue is classified as either [[striated]](https://www.kenhub.com/en/library/anatomy/striated-musculature) or [[non-striated/smooth]](https://www.kenhub.com/en/library/anatomy/smooth-musculature) based on the presence or absence of 'striations' seen at a microscopic level; these are formed due to a particular arrangement of actin and myosin filaments within the myocytes. 1. Skeletal muscle [[Skeletal muscle]](https://www.kenhub.com/en/library/anatomy/histology-of-skeletal-muscle) is the most common type of muscle tissue found in the body and consists of highly elongated, multinucleate, non-branching cells which are arranged in a parallel manner. Skeletal myocytes often measure several centimeters, or tens of centimeters in length, with the number of nuclei contained within being proportional to their length. They are biologically classified as '*syncytia*'; cells which are formed by fusion of several smaller, mononuclear cells. In the case of skeletal muscle, the cells which merge to form myocytes are known as myoblasts. Skeletal muscle is often referred to as 'voluntary' muscle due to the fact that we think of its contraction being under conscious control; this is a misconception however, as skeletal muscle is involved in various movements which are under a subconscious level (e.g. breathing). 2. **Cardiac muscle** Like skeletal muscle, [[cardiac muscle]](https://www.kenhub.com/en/library/anatomy/cardiac-tissue) is striated and hence is composed of similar contractile proteins which are also structurally arranged into sarcomeres. It is here however, where most similarities between these muscle tissue types ends. Cardiac muscle cells/fibers (cardiomyocytes) are much shorter and broader compared to those found in skeletal muscle and are branched at their ends. They are generally uninucleate (i.e. have one nucleus each), however sometimes may be binucleate. The nucleus is centrally located compared to those seen peripherally in skeletal myocytes, with the myofibrils passing on either side, leaving a clear zone of perinuclear sarcoplasm around the nucleus. Striations in cardiac muscle are not as defined as those seen in skeletal muscle as they are slightly obscured by relatively large amounts of mitochondria and other organelles present in the cell (reflecting the higher metabolic demands of this tissue compared with skeletal muscle). Cardiomyocytes are surrounded by fine, [[loose connective tissue]](https://www.kenhub.com/en/library/anatomy/loose-connective-tissue), similar to endomysium seen in skeletal muscle although less organized. Condensations of perimysium-like dense connective tissue can be observed dividing groups of cardiac muscle cells/fibers into fascicles which, unlike that seen in skeletal muscle, whirl and spiral in a multidirectional manner (except in the case of the papillary muscles). As a result, cut sections of cardiac muscle tissue will usually present various orientations of muscle fibers adjacent to one another. 3. **Smooth muscle** [[Smooth muscle]](https://www.kenhub.com/en/library/anatomy/smooth-musculature) is most commonly found in the walls of tubular structures (e.g. vessels, gut, ducts, [[bronchi]](https://www.kenhub.com/en/library/anatomy/bronchi) etc.) as well as hollow organs (e.g. [[urinary bladder]](https://www.kenhub.com/en/library/anatomy/urinary-bladder), [[uterus]](https://www.kenhub.com/en/library/anatomy/the-uterus)) When compared to skeletal/cardiac muscle, smooth muscle is morphologically and functionally much more diverse and is subject to subconscious/involuntary control. Therefore, its arrangement varies from organ to organ. Smooth muscle cells are generally uninucleate and are much smaller and shorter than those seen in skeletal muscle. They are spindle-shaped with long tapered ends and are usually packed together with their long axes parallel to neighboring cells in an 'interdigitating' manner. Each cell is enveloped by a [[basement membrane]](https://www.kenhub.com/en/library/anatomy/basement-membrane) and other connective fibers which bridge the spaces between adjacent cells; condensations of these extracellular structures, known as dense plaques, provide a region of attachment for the smooth muscle cells. Two adjacent dense plaques allow for cell--to-cell attachment, providing mechanical stability to the tissue. The components of the extracellular matrix are produced by the smooth muscle cells themselves, rather than [[fibroblasts]](https://www.kenhub.com/en/library/anatomy/fibroblast) as seen in skeletal muscle, Actin and myosin filaments in smooth muscle. One of the primary differences between smooth muscle and skeletal/cardiac muscle cells is the fact that the contractile proteins (actin/myosin) are not organized into sarcomeres; therefore they lack striations as seen in other muscle tissue types. Instead, actin (thin) and myosin (thick) filaments are scattered across the sarcoplasm of the cell. Actin filaments are attached to condensations of cytoskeletal intermediate filaments known as dense bodies (which therefore are functionally equivalent to Z-discs seen in skeletal muscle) as well as dense plaques mentioned above. Myosin filaments are located between actin filaments. During contraction they cause actin filaments to slide past each other, causing the cell to shorten mainly along its long axis.