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

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

– 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 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

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