Contraction of Skeletal muscle

Chapter 6

Ouabain• A plant alkaloid from Strophantus gratus, that specifically binds

to and inhibits the sodium potassium ATPase• Ouabain Inhibition of Energy Dependent Ion Transport• The difference between the external and internal sodium and

potassium concentrations is important in maintaining the membrane potential and in the generation of the action potential. Without a gradient of sodium and potassium ions there would be no action potential. The maintenance of these gradients are maintained by an ion pump, the Na+/K+

ATPase, that uses the energy from ATP to pump sodium out of the cell and potassium into the cell. This pump can be inhibited by a small amount of the chemical, ouabain.

Skeletal muscle

• Diagram and label the parts of a skeletal muscle fiber

• Sarcolemma, Myofibrils, sarcoplasm, sarcoplasmic reticulum

Anatomy• Sacrolemma- cell membrane of the muscle

cell, fuses with tendon fiber which insert into bones

• MyofibrilsI band, A band , Cross bridges, z disc, sarcomere

Sarcoplasm- salts and mitochondriaSarcoplasmic reticulum-vesicular tubules

containing Ca in high concentration, connects to t-tubules to pass potentials to the SR via junctional feet.

Contractile filaments• Myosin

– Polymer (480,000mw)– 6 chains 4 heavy and

two light– Head = 1 light 1 heavy– 2 lie side by side– Tail– Body – arm– Crossbridges– ATPase activity!

Contractile filamentsActin– Actin– Tropomyosin– Troponin– DS f- actin

backbone– G actin-ADP

activity– 1µm inserts – into z disc

Contractile filamentsTropomyosin– 70,000 MW– Wrapped around f-

actin– Resting- cover

active sites on actin– Troponin– I/T/C subunits– Affinities

Sliding filament theory

• What are the phases of sliding filament mechanism?

Sliding Filament mechanism

• Cross bridges bind with ATP, Hyddolyze extended

• Binding of Calcium and uncovering of active sites on actin. Binding of myosin head.

• Power stroke, tilting of head, movement of actin filament,

• ADP and Pi released, and refreshed

• Overlap!• Maximum contraction occurs with maximum

overlap of myosin and actin

Excitation-contraction coupling

• Muscle action potential– App. –80 mv– 1-5ms– Initiated by myelinated nerve fibers through

the neuromuscular junction– T-tubules spread the current flow to the

sacroplasmic reticulum which release calcium

10-7 relaxed

10-5-10-4

contracted

Motor Units

Twitches, summation and tetany

• Muscle twitches• Motor units

– Control• Summation

– Multiple motor unit– Frequency summation

• Tetany– Critical frequency

• Fatigue– O2, glycogen

Muscle Atrophy

• The polio virus is anenterovirus which in most subjects only causes a non-specific influenza-like illness with or without diarrhoea.

• Damage to lower motor neurons which causes the peripheral nerve fibres or axons supplying the musclefibres to die back. This leaves the muscles fibres 'orphaned'

• The muscle fibres cannot then function and begin to atrophy. Denervation

Muscle Hypertrophy

• Increase in # of actin and myosin filaments

Pop Quiz

• Sketch a skeletal muscle fiber and label the parts

Principles of

Anatomy and PhysiologyTortora and GrabowskiISBN 0-471-36692-7