Muscular System

• Muscular System


  Muscles provide the force essential for movement in all animals
• Vertebrates


  • Use their endoskeleton in conjunction with muscles to move
  • Movement is usually elicited in response to the information provided by the central nervous system (CNS)
  • Skeletal muscles move the body
  • Smooth muscles move materials through tubular organs and change the size of tubular openings
  • Cardiac muscle produces the beating of the heart
• Three Principal Kinds of Animal Movement

  • Amoeboid Movement
  • Ciliary and Flagellar Movement
  • Muscular Movement
• Amoeboid Movement


  • Exhibited by amoebas, many wandering cells of higher animals (white blood cells and embryonic mesenchyme)
  • Amoeboid cells change their shape by sending out and withdrawing pseudopodia from any point on the cell surface
• Ciliary Movement


  • Exhibited by ciliated protozoans and all major groups of animals, except nematodes and arthropods
  • Cilia are responsible for moving small animals such as protozoa through their aquatic habitat or in propelling fluids and materials across the epithelial surface of larger animals
  • Cilia lining respiratory airways, usually, prevent contaminants trapped in the mucus from reaching the lungs by sweeping mucus toward the throat
  • The sweeping action of ciliated cells lining the female reproductive tract facilitates the conduction of egg cells toward the oviducts and eventually to the uterus
• Flagellar Movement


  • Exhibited by flagellated protozoa, animal spermatozoa, and sponges
  • Flagellum is a whip-like structure longer than a cilium and usually present singly or in small numbers at one end of a cell
  • Cilia and flagella are structurally the same and differ only in their beating pattern
  • A flagellum beats symmetrically with snake-like undulation; hence the water is propelled parallel to the long axis of the flagellum
  • A cilium beats asymmetrically with a fast power stroke in one direction followed by a slow recovery during which the cilium bends as it returns to its original position; the water is propelled parallel to the ciliated surface
  • Cilia and flagella are moved by microtubules
• Muscular Movement


  • Brought about by the contraction of muscle cells or fibers; the muscle fibers shorten as they contract
  • Exhibited by the rest of the members of the animal kingdom
• Types of Invertebrate Muscle

  • Bivalve molluscan muscles contain striated muscle and smooth muscle
  • Insect flight muscles are fibrillar muscle

  
• Bivalve molluscan striated muscle


  • Capable of rapid contraction due to the presence of sliding thick and thin filaments, which aid the invertebrate to snap shut its valves when under stressed or disturbed condition
• Bivalve molluscan smooth muscle

  • Capable of slow yet long-lasting contractions, due to the intermediate filaments, which aid the invertebrate to keep its valves tightly shut for hours or even days
• Insect flight muscles


  • Contract at frequencies greater than 1,000 beats per second
• Types of Vertebrate Muscle

  • Skeletal Muscle
  • Cardiac Muscle
  • Smooth Muscle

  
• Skeletal Muscle


  • Named skeletal muscle because it is attached to skeleton and makes possible the movements of the trunk, appendages and other body parts
  • Consists of skeletal muscle cells, called muscle fibers, which are large, striated, cylindrical, and multinucleated cells that develop through the fusion of many individual cells
  • The muscle fibers are bundled together by a connective tissue into a fascicle
  • Most skeletal muscles taper at their ends, where they connect to bones by tendons
  • Skeletal muscles contract powerfully and quickly but fatigue more quickly than does smooth muscle
  • Skeletal muscles are also called voluntary muscles because they are stimulated by motor fibers and are under conscious cerebral control

  
• Cardiac Muscle


  • Striated, uninucleated, and is composed of branching cell fibers that give cardiac muscle an ability to resist tearing, making the heart walls tolerant of high pressures
  • Strong mechanical adhesions between adjacent cardiac muscle cells are provided by intercalated discs containing gap junctions for rapid conduction of impulses
  • Combines certain characteristics of both skeletal and smooth muscles; it is fast acting and striated like skeletal muscle, but contraction is under involuntary autonomic control like smooth muscle
  • Muscle contraction is initiated by specialized cardiac muscle cells, called the pacemaker cells which initiate the rhythmic contractions of the heart
• Smooth Muscle


  • Non-striated with long, tapering single-nucleated cells that are found encircling the walls of hollow, internal organs
  • The contractile machinery is not obvious when cells are viewed under light microscope since filaments of actin and myosin are not as regularly arranged unlike in striated muscle type
  • Primary functions include movement of material in the internal organs, such as the stomach, by peristalsis or regulation of the opening of certain organs, such as arteries, by sustained contraction
  • Smooth muscle is usually slow acting and can maintain prolonged contractions with very little energy expenditure; it is under the control of the autonomic nervous system, thus its contractions are involuntary and unconscious

  
• Functions of Muscles


  • Movement
  • Stabilization of body positions
  • Organ volume regulation
  • Thermogenesis
• Movement


  Relies on the integrative functioning of bones, joints, and skeletal muscles
• Stabilization of body positions


  Maintenance of posture, e.g. the contraction of the sternocleidomastoid and other muscles of the neck maintains the upright position of the head
• Organ volume regulation


  Exemplified in the regulation of contents of urinary bladder, gall bladder, heart, etc.
• Thermogenesis


  Contraction generates 85% of body heat
• Muscle


  Covered by epimysium and consists of many fascicles/fasciculi

  
• Fascicles


  Joined together by perimysium and consist of many myofibers
• Myofibers


  Joined together by endomysium; a myofiber is actually a muscle cell covered by a sarcolemma; consists of many myofibrils

  
• Myofibrils


  Made up of myofilaments (thin and thick filaments)
• Thin filament


  • Made up of 3 proteins: actin, troponin, and tropomyosin
  • Actin proteins possess the myosin binding sites which are covered by the troponin-tropomyosin complex during muscle relaxation
• Thick filament


  • Made up of myosin proteins organized into heads (crossbridges) and tails
  • Myosin heads bind to the myosin binding sites on actin molecules during muscle contraction
• Sarcomere


  • The functional unit of a muscle
  • Formed from the organization of thin and thick filaments into units
  • Parts: Z lines, A bands, I bands, elastic filament

  
• Transverse (T) tubule


  An invagination of the sarcolemma covering the myofiber; this penetrates into the cell and runs between the SR; where nerve impulses are transmitted to trigger muscle contraction
• Sarcomere


  The basic unit of skeletal and cardiac muscle, formed from the organization of thin and thick filaments

  
• Sarcomere


  • Overlapping banded structures create the striations characteristic among skeletal and cardiac muscles
• Structure of a sarcomere

  1. Z lines - bind sarcomeres together on each side; boundaries of one sarcomere

  2. A bands - also called dark bands; made up of thick filaments and portions of thin filaments; contain H zone which is made up only of thick filaments and M line which bisects the A bands and connect them to one another

  3. I bands - also called light bands; made up of thin filaments only

  4. Elastic filament - composed of protein titin/connectin which anchors the thick filaments to the Z discs to stabilize their positions
• Tubular systems associated with the sarcomere

  • Transverse (T) tubule - invagination of the sarcolemma covering the myofiber; penetrates into the cell and runs between the SR; where nerve impulses travel towards the interior of the cell
  • Sarcoplasmic reticulum (SR) - releases Ca2+ during contraction, and sequesters the same during relaxation
• Sliding Filament Theory of Muscle Contraction

  1. Generation and travel of action potentials/nerve impulses to the axon terminal

  2. Nerve impulses cause entrance of Ca2+ into the axon terminal and release of neurotransmitters (Acetylcholine or ACh) from the synaptic vesicles through exocytosis

  3. Binding of ACh to receptors of the motor end plate of the sarcolemma, generation and travel of action potentials to the transverse tubule

  4. Release of Ca2+ from the terminal cisternae of the SR due to the opening of the Ca2+ release channels which results to a calcium flood

  5. Binding of Ca2+ to the troponin-tropomyosin complex which covers the myosin binding sites

  6. Regulatory proteins undergo a conformational change and slide away from the chain of actin proteins, exposing the myosin binding sites

  7. Myosin heads, "charged with energy from ATP", bind to the myosin binding sites on actin proteins

  8. Myosin molecules perform powerstroke and pull on the actin threads towards the center of the sarcomere; I bands move toward the M line and sarcomeres shorten to generate force
• Events in Muscle Relaxation

  1. AChE (acetylcholinesterase) in the synaptic cleft breaks down ACh and returns it inside the axon terminal

  2. ATP binds to myosin heads and the latter detach from the myosin binding sites on actin molecules

  3. Ca2+ detaches from the troponin-tropomyosin complex and this protein regulatory complex slides back covering the myosin binding sites on actin molecules

  4. Ca2+ active transport pumps sequester Ca2+ into the SR
• Criteria for Naming Skeletal Muscles in Vertebrates

  • Muscle fiber direction
  • Location
  • Size
  • Number of Heads/Origins
  • Shape
  • Origin and Insertion
  • Action

  
• Muscle fiber direction


  • Oblique
  • Rectus
  • Transverse

  
• Location


  • Tibialis posterior
  • Dorsalis scapula (frog)
• Size


  • Brevis
  • Longus
  • Maximus
  • Minimus
  • Major
  • Minor
• Number of Heads/Origins


  • Biceps
  • Triceps
  • Quadriceps
• Shape


  • Rhomboideus
  • Serratus
  • Deltoid
  • Trapezius