Structure of A Sarcomere

Fibers

Muscle cells (myocytes)

Myofibrils

Smaller units that make up muscle fibers

Sarcomeres

Smaller units that make up myofibrils

Myofilaments

Smaller units that make up sarcomeres

Myosin

Thick filaments that contain heads

Actin

Thin filaments

Contractile Proteins

Actin and myosin

Thick Filaments

Made up of contractile protein myosin

Thin Filaments

Made up of contractile protein, actin, and regulatory proteins troponin and tropomyosin

A Band

Full length of thick filaments with some overlap (middle is lighter)

Myosin Heads

Tiny knobs that project from the thick filaments towards the binding sites on the thin filaments

Cross Bridge

Joined thick and thin filaments during contraction

Z Line

Location where thin filaments attach

H Zone

Lighter area of the A band that has no cross over of filaments

I Band

Area of the sarcomere that is only comprised of thin filaments

Sliding Filament Theory

During contraction, thin filaments are pulled closer together causing the filaments to slide over each other, shortening the sarcomere (change fiber position not length)

Motor Unit (Excitation Contraction Coupling)

The motor neuron and the muscle cells it stimulates

Neuromuscular Junction

Nerve endings and muscle cells don’t touch each other

Acetylcholine

Neurotransmitter that opens the chemically gated sodium channels which generates graded potential to a threshold for action potential

Sarcoplasmic Reticulum

Store, release, and reabsorb calcium ions (Ca^2+)

Tropomyosin

Regulatory protein that wraps around actin filaments in cells to control their interaction with other proteins

Troponin

Acts as a molecular switch, allowing muscles to contract when calcium is present and causing them to relax when calcium is absent

Contraction

Shortening of the sarcomere

Relaxation

Reuptake of calcium ions into the SR occurs and the filaments return to original position

Twitch

A brief, rapid contraction of a muscle fiber in response to a single neural stimulus

Recruited

Stimulated (contraction strength increases as more fibers are stimulated)

Tetanus

Stimuli delivered so quick that no relaxation occurs, a strong sustained contraction occurs