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Muscle Tissue

 


Skeletal muscle tissue and the Muscular System

Three types of muscle

      Skeletal attached to bone

      Cardiac found in the heart

      Smooth lines hollow organs

 

Skeletal muscle functions

      Produce skeletal movement

      Maintain posture and body position

      Support soft tissues

      Guard entrances and exits

      Maintain body temperature


Anatomy of Skeletal Muscle

Organization of connective tissues

      Epimysium surrounds muscle

      Perimysium sheathes bundles of muscle fibers

    Epimysium and perimysium contain blood vessels and nerves

      Endomysium covers individual muscle fibers

      Tendons or aponeuroses attach muscle to bone or muscle

 

Skeletal muscle fibers

      Sarcolemma (cell membrane)

      Sarcoplasm (muscle cell cytoplasm)

      Sarcoplasmic reticulum (modified ER)

      T-tubules and myofibrils aid in contraction

      Sarcomeres regular arrangement of myofibrils

 

Myofibrils

      Thick and thin filaments

      Organized regularly

 

Muscle Fiber

Thin filaments

      F-actin

      Nebulin

      Tropomyosin

    Covers active sites on G-actin

      Troponin

    Binds to G-actin and holds tropomyosin in place

 

Thick filaments

      Bundles of myosin fibers around titan core

    Myosin molecules have elongate tail, globular head

    Heads form cross-bridges during contraction

    Interactions between G-actin and myosin prevented by tropomyosin during rest

 

Sliding filament theory

      Explains the relationship between thick and thin filaments as contraction proceeds

      Cyclic process beginning with calcium release from SR

    Calcium binds to troponin

    Trponin moves, moving tropomyosin and exposing actin active site

    Myosin head forms cross bridge and bends toward H zone

    ATP allows release of cross bridge

 

 

The Contraction of Skeletal Muscle

Tension

      Created when muscles contract

      Series of steps that begin with excitation at the neuromuscular junction

    Calcium release

    Thick/thin filament interaction

    Muscle fiber contraction

    Tension

 

Control of skeletal muscle activity occurs at the neuromuscular junction

      Action potential arrives at synaptic terminal

      ACh released into synaptic cleft

      ACh binds to receptors on post-synaptic neuron

    Action potential in sarcolemma

 

Excitation/contraction coupling

      Action potential along T-tubule causes release of calcium from cisternae of SR

      Initiates contraction cycle

   Attachment

   Pivot

   Detachment

   Return

 

Relaxation

      Acetylcholinesterase breaks down ACh

      Limits the duration of contraction


Tension Production

Tension production by muscle fibers

      All or none principle

      Amount of tension depends on number of cross bridges formed

      Skeletal muscle contracts most forcefully over a narrow ranges of resting lengths

      Twitch

    Cycle of contraction, relaxation produced by a single stimulus

      Treppe

    Repeated stimulation after relaxation phase has been completed

 

Summation

      Repeated stimulation before relaxation phase has been completed

    Wave summation = one twitch is added to another

    Incomplete tetanus = muscle never relaxes completely

    Complete tetanus = relaxation phase is eleminated

 

Tension production by skeletal muscles

      Internal tension generated inside contracting muscle fibers

      External tension generated in extracellular fibers

      Motor units

    All the muscle fibers innervated by one neuron

    Precise control of movement determined by number and size of motor unit

      Muscle tone

    Stabilizes bones and joints

 

 

Tension production by skeletal muscles

      Internal tension generated inside contracting muscle fibers

      External tension generated in extracellular fibers

      Motor units

    All the muscle fibers innervated by one neuron

    Precise control of movement determined by number and size of motor unit

      Muscle tone

    Stabilizes bones and joints

 

Contractions

      Isometric

    Tension rises, length of muscle remains constant

      Isotonic

   Tension rises, length of muscle changes

      Resistance and speed of contraction inversely related

      Return to resting lengths due to elastic components, contraction of opposing muscle groups, gravity


Energy Use and Muscle Contraction

Muscle Contraction requires large amounts of energy

      Creatine phosphate releases stored energy to convert ADP to ATP

      Aerobic metabolism provides most ATP needed for contraction

      At peak activity, anaerobic glycolysis needed to generate ATP

 

Energy use and level of muscular activity

      Energy production and use patterns mirror muscle activity

      Fatigued muscle no longer contracts

    Build up of lactic acid

    Exhaustion of energy resources

 

Recovery period

      Begins immediately after activity ends

      Oxygen debt (excess post-exercise oxygen consumption)

    Amount of oxygen required during resting period to restore muscle to normal conditions


Muscle Performance

Types of skeletal muscle fibers

      Fast fibers

      Slow fibers

      Intermediate fibers

 

Fast fibers

      Large in diameter

      Contain densely packed myofibrils

      Large glycogen reserves

      Relatively few mitochondria

      Produce rapid, powerful contractions of short duration

 

Slow fibers

      Half the diameter of fast fibers

      Take three times as long to contract after stimulation

      Abundant mitochondria

      Extensive capillary supply

      High concentrations of myoglobin

      Can contract for long periods of time

 

Intermediate fibers

      Similar to fast fibers

      Greater resistance to fatigue

 

Muscle performance and the distribution of muscle fibers

      Pale muscles dominated by fast fibers are called white muscles

      Dark muscles dominated by slow fibers and myoglobin are called red muscles

      Training can lead to hypertrophy of stimulated muscle

 

Physical conditioning

      Anaerobic endurance

    Time over which muscular contractions are sustained by glycolysis and ATP/CP reserves

      Aerobic endurance

    Time over which muscle can continue to contract while supported by mitochondrial activities


Cardiac Muscle Tissue

Structural characteristics of cardiac muscle

      Located only in heart

      Cardiac muscle cells are small

    One centrally located nucleus

    Short broad T-tubules

    Dependent on aerobic metabolism

      Intercalated discs where membranes contact one another

 

Functional characteristics of cardiac muscle tissue

      Automaticity

      Contractions last longer than skeletal muscle

      Do not exhibit wave summation

    No tetanic contractions possible


Smooth Muscle Tissue

Structural characteristics of smooth muscle

      Nonstriated

    Lack sarcomeres

    Thin filaments anchored to dense bodies

      Involuntary

 

Functional characteristics of smooth muscle

      Contract when calcium ions interact with calmodulin

    Activates myosin light chain kinase

      Functions over a wide range of lengths

    Plasticity

      Multi-unit smooth muscle cells are innervated by more than one motor neuron

      Visceral smooth muscle cells are not always innervated by motor neurons

    Neurons that innervate smooth muscle are not under voluntary control