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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
25 - 40 % of energy from nutrients is converted
to ATP by cellular respiration
60 - 75 % of energy from nutrients is converted
to heat
Histology
• Muscle consists of elongated cells called
muscle fibers
Sarco = fleshy
• Sarcolemma - cell membrane
• Sarcoplasm – cytoplasm
Histology
• 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
Histology
• 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)
Histology
– thick myofilaments = myosin
– thin myofilaments = actin, troponin,
tropomyosin
Histology
• 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
Histology
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 (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 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 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 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 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 means: myosin
(thick myofilaments) cross bridges pull actin
(thin myofilaments) inward during
contraction
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 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 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 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 Mechanism
Contraction will continue as an endless
repeating cycle as long as calcium and ATP
are present
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
• 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 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 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 Function
Aerobic pathway produces 20X more ATP
than anaerobic respiration but takes 2 1/2
times longer
– Anaerobic respiration causes oxygen debt to
occur
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 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