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Muscle Tissue. Chapter 8 Bio201. Functions of Skeletal Muscle. Movement of body. Posture maintenance . Storing and moving substances within the body. Functions of Skeletal Muscle. Heat production - 85% of body heat is generated by skeletal muscle.

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muscle tissue

Muscle Tissue

Chapter 8


functions of skeletal muscle
Functions of Skeletal Muscle
  • Movement of body
  • Posture maintenance
  • Storing and moving substances within the body
functions of skeletal muscle1
Functions of Skeletal Muscle
  • Heat production - 85% of body heat is generated by skeletal muscle

25 - 40 % of energy from nutrients is converted to ATP by cellular respiration

60 - 75 % of energy from nutrients is converted to heat

  • Muscle consists of elongated cells called muscle fibers

Sarco = fleshy

  • Sarcolemma - cell membrane
  • Sarcoplasm – cytoplasm
  • Transverse (T) tubule - tubular invagination of sarcolemma that surrounds each myofibril
  • Sarcoplasmic reticulum (SR) - smooth endoplasmic reticulum that stores Ca2+, has enlarged portions called cisternae that surround the transverse tubules
  • Myofibrils – cross section of muscle cell consists of small cylinders called myofibrils which may number several 100 to several 1000/cell (exercise increases myofibril production; lack of exercise decreases myofibrils (atrophy))

Each myofibril consists of myofilaments (protein)

  • thick myofilaments = myosin
  • thin myofilaments = actin, troponin, tropomyosin
  • Sarcomere - myofilaments don't extend entire length of muscle fiber; they are stacked into compartments called sarcomeres

Sarcomeres are the functional unit of a skeletal muscle (contractile unit)

Sarcomere extends from Z disc to Z disc


Parts of sarcomere

  • A band - myosin + overlapping actin
  • I band - only actin, troponin, tropomyosin (2 I bands / sarcomere)
  • Z disc – through center of I band
neuromuscular junction
Neuromuscular Junction
  • Neuromuscular junction (NMJ) - (one per muscle fiber and usually in middle) = axon terminal (synaptic end bulb) + motor end plate (sarcolemma under motor neuron)
  • Acetylcholine (Ach), a neurotransmitter, is released at the NMJ by a motor neuron causing a muscle impulse, which in turn will cause the muscle to contract
neuromuscular junction1
Neuromuscular Junction
  • Problems at the NMJ

Curare - binds to ACh receptors in skeletal muscle membrane; competes with ACh but does not stimulate the ACh receptor; therefore muscle paralysis

neuromuscular junction2
Neuromuscular Junction

Botulism - toxin inhibits ACh release (from the bacteria Clostridium botulinum); therefore: muscle paralysis

  • A dilute solution of botulinum toxin can be injected into a muscle that is in spasm to help it relax

Myasthenia gravis - antibodies destroy ACh receptors; therefore muscle paralysis

neuromuscular junction3
Neuromuscular Junction

Organophosphates (in some pesticides) - inhibits acetylcholinesterase; therefore muscle spasms

Tetanus - affects nervous system (from the bacteria Clostridium tetani) - this anaerobic bacteria produces a toxin that blocks an inhibitory neurotransmitter in the central nervous system, causing spasms and painful convulsions; therefore tetanus shots immunize against the toxin

motor units
Motor Units
  • Motor unit - motor neuron (densely branched) + all the skeletal muscle fibers it services (5 fibers to 2000 muscle fibers)
  • One entire muscle has many motor units
  • Not all are stimulated at same time
  • The smaller the number of muscle fibers/motor unit, the more precise the control of the muscle fibers
sliding filament mechanism
Sliding Filament Mechanism
  • Sliding Filament Mechanism means: myosin (thick myofilaments) cross bridges pull actin (thin myofilaments) inward during contraction
sliding filament mechanism1
Sliding Filament Mechanism
  • Sliding Filament Mechanism means: myosin (thick myofilaments) cross bridges pull actin (thin myofilaments) inward during contraction
  • At rest

Calcium in SR (terminal cisternae)

sliding filament mechanism2
Sliding Filament Mechanism

Troponin-tropomyosin prevents myosin from binding to sites on actin

ATP bonded to myosin cross bridges (concentration of ATP is high in relaxed muscle)

sliding filament mechanism3
Sliding Filament Mechanism
  • Excitation-Contraction Coupling

Motor neuron releases acetylcholine at the NMJ causing a muscle impulse (excitation)

In response to the muscle impulse, the SR releases calcium into the sarcoplasm

sliding filament mechanism4
Sliding Filament Mechanism

Calcium interacts with troponin and tropomyosin in the thin filament changing their shape, exposing binding sites for myosin (thick filament) on actin

Myosin breaks down ATP and uses the energy released to pull the thin filament toward the middle of the sarcomere, contraction

sliding filament mechanism5
Sliding Filament Mechanism

Contraction will continue as an endless repeating cycle as long as calcium and ATP are present

sliding filament mechanism6
Sliding Filament Mechanism
  • To relax following contraction

ACh is inactivated by acetycholinesterase (from sarcolemma surface)

Calcium is actively transported back into SR

Troponin-tropomyosin reattach to actin preventing attachment of myosin cross bridges to actin

ATP attaches to myosin cross bridge

atp and muscle function
ATP and Muscle Function
  • Sources of ATP

Stored ATP - lasts only 6 seconds during bursts of muscle contraction

ATP generated from creatine phosphate (CP) (CP + ADP → creatine + ATP) - together ATP that is stored and CP provide muscle power for 10-15 sec (CP replenished during resting periods)

atp and muscle function1
ATP and Muscle Function

Even as ATP and CP are being used, ATP is generated by aerobic respiration and anaerobic respiration

  • Resting and slowly contracting muscles obtain bulk of ATP via aerobic respiration of fatty acids
atp and muscle function2
ATP and Muscle Function
  • In actively contracting muscles, glucose (from blood and breakdown of glycogen) is primary fuel supply

Aerobic pathway: glucose + O2 → CO2 + H2O + 36ATP

Anaerobic pathway: glucose → lactic acid + 2 ATP

atp and muscle function3
ATP and Muscle Function
  • Anaerobic respiration causes oxygen debt to occur

Aerobic pathway produces 20X more ATP than anaerobic respiration but takes 2 1/2 times longer

oxygen debt
Oxygen Debt
  • Oxygen Debt: Amount of oxygen needed to metabolize the accumulated lactic acid and to restore ATP levels

Muscle fatigue is result of ATP depletion and accumulation of lactic acid

Oxygen debt results in labored breathing in order to pay back the O2 debt

all or none principal
All or None Principal
  • All or None Principle - individual muscle fibers of a motor unit will contract to its fullest extent of its immediate ability when stimulated by a nerve impulse of threshold level - the principle does not apply to the entire muscle but only to motor units