Microscopic Anatomy of Skeletal Muscle MUSCLES PART 2

Microscopic Anatomy of Skeletal Muscle MUSCLES – PART 2

Anatomy of Skeletal Muscle Sarcolemma – The plasma membrane in muscle cells

Anatomy of Skeletal Muscle Myofibrils – Long ribbon-like contractile organelles found in the cytoplasm of muscle cells Composed of bundles of myofilaments Contain light (I) and dark (A) bands along the length of the perfectly aligned myofibrils. Gives the muscle cell as a whole its striped or striated appearance.

Light I and Dark A Bands Contain the Following: The Light I Band has a midline interruption, a darker area called the Z discs. The Dark A Band has a lighter central area called the H zone.

Sarcomeres – Tiny contractile units Myofibrils are actually chains of sarcomeres. Sarcomeres are aligned end-to-end like boxcars in a train.

Myofilaments – Filaments (threadlike protein) composing the myofibrils. 2 Types: 1. Thick Filaments (Myosin Filaments) 2. Thin Filaments (Actin Filaments)

Thick Filaments (Myosin Filaments) – Made mostly of bundled molecules of the protein myosin Also contain ATPase enzymes, which split ATP to generate the power for muscle contraction. Extends the entire length of the Dark A Band

Cross Bridges on Myosin Filaments Notice that the midparts of myosin filaments are smooth, but their ends are studded with small projections, or cross bridges. Cross Bridges (myosin heads) – Projections found of myosin filaments that link the thick and thin filaments together during contraction.

Thin Filaments (Actin Filaments) - Composed of the contractile protein called actin Also contains some regulatory proteins that play a role in allowing or preventing myosin heads from binding to actin. Are anchored to the Z disc When contraction occurs, the actin-containing filaments slide toward each other.

Thin Filaments The light I bands only contains thin filaments. The thin filaments overlap the ends of the thick filaments but do not extend into the H Zone (bare zone).

Sarcoplasmic Reticulum – A specialized smooth endoplasmic reticulum. The interconnecting tubules and sacs of the SR surround each and every myofibril. Similar to a sleeve surrounding your arm Major Role: Store calcium and to release it “on demand” when the muscle fiber is stimulated to contract.

What Happens During Contraction