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• Energy Sources for Contraction

  1. Creatine phosphate
  2. Cellular respiration

  • Creatine phosphate – stores energy that quickly converts ADP to ATP
• Oxygen Supply and Cellular Respiration

  Cellular respiration:

  Anaerobic Phase

  • Glycolysis

  • Occurs in cytoplasm

  • Produces little ATP

  Aerobic Phase

  • Citric acid cycle

  • Electron transport system

  • Occurs in the mitochondria

  • Produces most ATP

  • Myoglobin stores extra oxygen
• Oxygen debt – amount of oxygen needed by liver cells to use the accumulated lactic acid to produce glucose

  • Oxygen not available

  • Glycolysis continues

  • Pyruvic acid converted to lactic acid

  • Liver converts lactic acid to glucose
• Muscle Fatigue

  • Inability to contract muscle

  Commonly caused from:

  • Decreased blood flow

  • Ion imbalances across the sarcolemma

  • Accumulation of lactic acid

  Cramp – sustained, involuntary muscle contraction
• Heat Production

  • By-product of cellular respiration

  • Muscle cells are major source of body heat

  • Blood transports heat throughout body core
• Muscular Responses

  • Muscle contraction can be observed by removing a single skeletal muscle fiber and connecting it to a device that senses and records changes in the overall length of

  the muscle fiber.

  Threshold Stimulus

  • Minimal strength required to cause contraction
• Recording a Muscle Contraction

  • Twitch

  • Latent period
  • Period of contraction
  • Period of relaxation

  • Refractory period

  • All-or-none response
• Summation

  • Process by which individual twitches combine

  • Produces sustained contractions

  • Can lead to tetanic contractions
• Recruitment - increase in the number of motor units activated

  • Whole muscle composed of many motor units

  • More precise movements are produced with fewer muscle fibers within a motor unit

  • As intensity of stimulation increases, recruitment of motor units continues until all motor units are activated
• Sustained Contractions

  • Smaller motor units (smaller diameter axons) - recruited first

  • Larger motor units (larger diameter axons) - recruited later

  • Produce smooth movements

  Muscle tone– continuous state of partial contraction
• Types of Contractions

  • Isotonic– muscle contracts and changes length

  • Concentric – shortening contraction

  • Eccentric– lengthening contraction

  • Isometric– muscle contracts but does not change length
• Fast Twitch and Slow Twitch Muscle Fibers

  Slow-twitch fibers (Type I)

  • Always oxidative

  • Resistant to fatigue

  • Red fibers

  • Most myoglobin

  • Good blood supply

  Fast-twitch glycolytic fibers (Type IIa)

  • White fibers (less myoglobin)

  • Poorer blood supply

  • Susceptible to fatigue

  Fast-twitch fatigue-resistant fibers (Type IIb)

  • Intermediate fibers

  • Oxidative

  • Intermediate amount of myoglobin

  • Pink to red in color

  • Resistant to fatigue
• Smooth Muscle Fibers

  Visceral Smooth Muscle

  • Single-unit smooth muscle

  • Sheets of muscle fibers

  • Fibers held together by gap junctions

  • Exhibit rhythmicity

  • Exhibit peristalsis

  • Walls of most hollow organs

  Multi-unit Smooth Muscle

  • Less organized

  • Function as separate units

  • Fibers function separately

  • Iris of eye

  • Walls of blood vessels
• Smooth Muscle Contraction

  Resembles skeletal muscle contraction in that:

  • Interaction between actin and myosin

  • Both use calcium and ATP

  • Both are triggered by membrane impulses
• Smooth Muscle Contraction

  Different from skeletal muscle contraction in that:

  • Smooth muscle lacks troponin

  • Smooth muscle uses calmodulin

  • Two neurotransmitters affect smooth muscle

  • Acetlycholine (Ach) and norepinephrine (NE)

  • Hormones affect smooth muscle

  • Stretching can trigger smooth muscle contraction

  • Smooth muscle slower to contract and relax

  • Smooth muscle more resistant to fatigue

  • Smooth muscle can change length without changing tautness
• Origin – immovable end

  Insertion– movable end
• Interaction of Skeletal Muscles

  • Prime mover (agonist) – primarily responsible for movement

  • Synergists– assist prime mover

  • Antagonist– resist prime mover’s action and cause movement in the opposite direction of the prime mover
• Lifespan Changes

  • Myoglobin, ATP, and creatine phosphate decline

  • By age 80, half of muscle mass has atrophied

  • Adipose cells and connective tissues replace muscle tissue

  • Exercise helps to maintain muscle mass and function