Muscle Microstructure/ Contraction & Relaxation Muscle

Muscle Tissue Three types: – Smooth, Involuntary Arteries, veins, gastrointestinal tract – Striated, Involuntary Cardiac muscle – Striated, Voluntary Skeletal muscle

Skeletal Muscle Fiber Sarcolemma – membrane surrounding the muscle fiber – Composed of protein and lipids – Relatively elastic – Periodically along the length and around its entire circumference are invaginations that form a network of tubules called transverse tubules (t – tubules)

Skeletal Muscle Fiber Sarcolemma – Motor nerve fibers terminate on the sarcolemma at the myoneural junction Structures present at the myoneural junction form a raised mound called the motor end plate

Structure of a Muscle Fiber

Skeletal Muscle Fiber Sarcoplasmic Reticulum (SR) – Membranous system of tubules and cisternae (flattend reservoirs) – Site of calcium storage – Forms a closely meshed network around each myofibril – SR and T-tubules work together T-tubules part of sarcolemma SR part of the intracellular membrane structure

Skeletal Muscle Fiber Myofibrils – Organelle unique to muscle fiber – 1 to 2  m diameter – Parallel to long axis of the fiber – Bathed in sarcoplasm – Extend entire length of fiber – Comprised of myofilaments

Skeletal Muscle Fiber Myofilaments – Comprised of thick and thin filaments – Thick and thin filaments have a precise and exact alignment within the myofibril and are parallel to each other – Locations where thick and thin filament overlap results in dark bands, which give skeletal muscle its striated appearance

STRUCTURE OF MUSCLE (CONT’D)

Sarcomere

Sliding Filament Theory ACTIN MYOSIN

Additional elements required for muscle contraction and relaxation 1.Calcium 2.Adenosine triphosphate (ATP) a)Derived from glycolysis and Kreb’s Cycle 3.Acetylcholine and Cholinesterase

Contraction 1. Nerve pulse/impulse transmitted through action potential

Contraction Continued 2. Acetylcholine is released at neural juncture

Contraction Continued 3. Action potential transmitted to muscle fiber via the T-tubles to the sarcoplasmic reticulum (SR)

Contraction Continued 4. Calcium is released from SR into sarcoplasm

Contraction Continued 5. Calcium binds to troponin C 6. causes a shift in tropomyosin and actin binding site is exposed

Contraction Continued 7. Actin-myosin cross bridge forms (cross bridge is termed actomyosin)  8. ATP hydrolyzed  9. Myosin head rotates  10. Repeated over and over; filaments slide causing shortening of sarcomere

Contraction Continued

1. Cholinesterase released  2. Calcium pump activated by SR to sequester calcium  3. Actin-myosin cross bridge terminated  4. Tropomyosin shifts covering the binding site on actin Relaxation

3. Actin-myosin cross bridge terminated  4. Tropomyosin shifts covering the binding site on actin

Relaxation Continued 5. Passive sliding of filaments  6. Sarcomere returns to resting state

Review of Contraction and Relaxation

Lit. Cited Romans et al. The Meat We Eat Miller M.F., et al. Selection, Care, and Processing of Meats Notebook Miller, M.F., et al. Advanced Meats Notebook Savell et al. Meat science Lab Manaul Aberle et al. Principles of Meat Science

Questions