Muscular System

Types of Muscles

  • Body movements are determined by 1 or more of the 3 principal types of muscles
  • Skeletal Muscle 
    • Attached to the bones of the skeleton
    • Called striped or striated
    • Cross bandings of alternating lights and dark bands which run perpendicular to the length of the muscle
    • Consists of bundles of multinucleated muscle cells
    • Each muscle cell is known as a muscle fiber
    • Sarcolemma Cell Membrane
    • Sarcoplasm Cytoplasm
    • These muscles can be controlled at will

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Smooth Muscle (Involuntary)

  • Smooth Muscle (Visceral)
    • Small and spindle-shaped
    • One nucleus located at the center of the cell
    • Unmarked by any distinct striations
    • Unattached to bones
    • Act slowly
    • Do not tire easy
    • Can remain contracted for a long time
    • Controlled by autonomic nervous system
    • Found in walls of the internal organs 

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Cardiac Muscle (Involuntary)

  • Found only in the heart
  • Striated and branched
  • Joined in a continuous network
  • Membranes of adjacent cells joined at intercalated discs 
  • Communication will not allow for independent cell contraction
  • One cell receives a signal to contract, all neighboring cells are stimulated and they contract together to produce a heartbeat 
  • Require continuous supply of oxygen

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Sphincter or Dilator Muscle

  • Special circular muscles in the openings between the esophagus and the stomach, and the stomach and the small intestine
  • Also found on the walls of the anus, urethra, and the mouth
  • They open and close to control the passage of substances

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4 Characteristics of Muscles

  • Contractility
    • A muscle shortens or contracts, it reduces the distance between the parts of its contents, or space it surrounds
  • Examples 
    • Contraction of skeletal muscle brings points of attachment together causing bones to move
    • Contraction of cardiac muscle reduces area in the heart chamber aids in pumping blood
    • Contraction of smooth muscle, like blood vessels, causes a decrease in diameter 
  • Excitability or Irritability 
    • Characteristics of both muscle and nervous cells
    • It is the ability to respond to certain stimuli by producing electric signals called Action potential (impulses)
  • Extensibility
    • The ability to be stretches
    • When bend, the muscles are extended or stretched
  • Elasticity
    • The ability to return to original length when relaxing

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Muscle Attachments 

  • Muscles only pull, never, ever push!!
  • Muscles attach to bones by tendons
  • Attachment may be to bones, cartilage, ligaments, tendons, skin, and sometimes each other
  • Origin is where the skeletal muscle is attached to a fixed structure or bone 
    • It moves least during contraction
  • Insertion is the other end, attached to a movable part 
    • It moves during muscle contraction
  • Belly is the central body of the muscle

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Prime Mover, Antagonist, Synergist 

  • Prime Mover: produces movement in a single direction
  • Antagonist: Opposite direction
  • Synergist: Helps to steady movement or joint activity 

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Energy and Heat 

  • Muscles not only move, but produce heat as well

  • In order to contract, muscles need energy

  • ATP (Adenosine Triphosphate)

  • ATP requires:

  • Oxygen 

  • Glucose

  • Other material brought by blood through circulation

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How does the Muscle Contract? 

  • Contraction starts with a nerve impulse (action potential)
  • Motor unit: a motor neuron plus all the muscle fiber it stimulates
  • An impulse travels down axon to the neuromuscular junction
    • Point between the motor axon and sarcolemma 
  • The end of the axon is the synaptic cleft
  • When the impulse reaches the end of the axon, acetylcholine is released
    • Acetylcholine: chemical released when a nerve impulse is transmitted
  • Acetylcholine diffuses across the synaptic cleft and attach receptors on the srcolemma 
  • Sarcolemma becomes temporarily permeable to sodium ions [NA+] which rush into the muscle cell
  • Potassium exists as NA+ goes into the cell
  • An action potential then occurs over the sarcolemma

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All or None Law

  • A muscle cell, when stimulated properly, contracts all the way 

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Muscle Fatigue

  • Muscle Fatigue: caused by an accumulation of lactic acid in the muscles
  • Vigorous exercises causes muscles to contract anaerobically (without oxygen)
  • The lactic acid generally moves out in the bloodstream, but the high level of exercise cause a sharp rise in lactic acid
  • It cannot move out quick enough
  • This impedes muscle contraction causing muscle fatigue and cramps
  • Oxygen must be restored to the bloodstream through respirations
  • Lactic Acid converts back to Glucose and other substances
  • Oxygen Debt is the amount of oxygen needed to restore normal respirations

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Muscle Tone

  • Muscle Tone: muscles always in the state of partial contraction
    • Achieved through nutrition and regular exercise
  • Isotonic: when muscles contract and shorten
    • Walking, talking 
  • Isometric: Muscle tension increases but muscles does not shorten
    • Tensing abdominal muscles
  • Atrophy: Muscle shrinks from disuse 
  • Hypertrophy: muscle size increases due to over exercise
    • The muscle fiber (cell) enlarges 

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Changing Muscle

  • Muscle must be changed through training
    • Training can improve muscle efficiency
    • Coordination
    • Respiratory and circulatory system
    • Elimination of excess fat
    • Joint movement
  • Training improves strength
    • Strength: The capacity to do work
    • Increases muscle size: the sarcoplasm increases, not the number of fibers
    • Improves coordination
    • Improves functioning in the cortical brain region, where impulse to contract originates