Lecture 12: Muscle I

Muscle and Muscle Tissue

I. Background

A. Muscle types

1. Skeletal

            a. Striated

            b. Voluntary

2. Cardiac

            a. Striated

            b. Involuntary

3. Smooth

            a. Non-striated

            b. Involuntary

B. Common features

1. Elongated cells—muscle fibers

2. Myofilaments

            a. Actin

            b. Myosin

3. Terminology

            a. Myo and sacro

C. General functions

1. Movement

2. Maintain posture

3. Stabilize joints

4. Temperature homeostasis


II. Gross Anatomy of Skeletal Muscle

A. Muscles are organs comprised of:

1. Muscle fibers

2. Connective tissue

3. Blood vessels

4. Nervous tissue

B. Organization


1. Individual fibers are surrounded by endomysium

            a. Areolar connective tissue

2. Multiple fibers are bundled as fascicles

3. Fascicles are bound by collagen sheath

            a. Perimysium

4. Epimysium then surrounds all fascicles of an entire muscle

5. Deep fascia binds muscles into functional groups


III. Microscopic Anatomy

A. Terms

1. Sarcolemma—plasma membrane surface

2. Sarcoplasm—cytoplasm of muscle cells

3. Myofibrils—contractile elements of skeletal muscle

B. Striations


1. A bands—dark bands

            a. Anisotropic—polarize light

2. I bands—light bands

            a. Isotropic—nonpolarizing

3. H band (within A band)

            a. Visible only in relaxed muscle

4. M line

            a. Bisects H band

5. Z disc (membrane)

            a. Midline in I band

C. Sarcomere—region of myofibril between two successive Z discs

1. Functional unit of skeletal muscle

D. Microfilaments (myofilaments) with in bands

1. Thick filaments

            a. Run entire length of A band

            b. Myosin

2. Thin filaments

            a. Extend across I band and part of the way into A band

            b. Actin

3. Z disc—protein sheet connecting myofibrils together

E. Ultrastructure and molecular composition


1. Thick filaments

            a. Myosin

            b. Rodlike tail terminates in two globular heads

            c. Tail comprised of heavy meromysin

                        i. Polypeptide chains (2) interwoven

            d. Head comprised of ends of heavy meromysin + light meromysin

            e. During contraction heads (cross bridges) interact with thin myofilaments

2. Thin myofilaments

            a. Actin

            b. Comprised G (globular) actin

            c. Double stranded helix

3. Regulatory proteins

            a. Tropomyosin

                        i. Sprials around action

                        ii. Block myosin head binding sites during relaxed state

            b. Troponin—polypeptide complex

                        i. Binds Ca2+ (TnC)

                        ii. Binds tropomyosin (TnT)

                        iii. Inhibitory protein that binds actin (TnI)


IV. Contraction of Skeletal Muscle

Sliding Theory of Contraction: during contraction, thin filaments slide past thick ones so that actin and myosin filaments overlap to a greater degree

A. Overview




1. Prior to contraction

            a. Cross bridges are disengaged

            b. All bands distinct

2. Nerve impulse initiates contraction

3. Cross bridges engage

4. ATP splits

            a. Energy used for swinging of cross bridges

5. Actin filaments pulled together

            a. H zone and Z disc smaller or lost

6. I band reduced

7. Cross bridges disengage

8. Crossbridges and actin filaments return to original position

B. Specifics of contraction

1. During relaxed state

a. Ca2+ concentration in sarcoplasm is low

                        i. Ca2+ is stored in sacroplasmic tubules

            b. Troponin