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Functions of Skeletal Muscle
• Movement of body
• Posture maintenance
• Storing and moving substances within the body
Functions of Skeletal Muscle
25 - 40 % of energy from nutrients is converted to ATP by cellular respiration
• Heat production - 85% of body heat is generated by skeletal muscle
60 - 75 % of energy from nutrients is converted to heat
Histology
Sarco = fleshy
• Muscle consists of elongated cells called muscle fibers
• 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
• 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 JunctionBotulism - 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 JunctionOrganophosphates (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)
• Not all are stimulated at same time
• One entire muscle has many motor units
• 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
Calcium in SR (terminal cisternae)
• At rest
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
Calcium is actively transported back into SR
ACh is inactivated by acetycholinesterase (from sarcolemma surface)
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
– 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: 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