The Muscular System: Structure and Physiology

1. The Muscular System: Structure and Physiology

2. Function of Muscles • Produce movement • Maintain posture • Stabilize joints • Generate heat

3. Characteristics of Muscles • Muscle cells are elongated • muscle cell = muscle fiber • Contraction of muscles is due to the movement of microfilaments • All muscles share some terminology • Prefix myo refers to muscle • Prefix mys refers to muscle • Prefix sarco refers to flesh

4. Characteristics of Muscle Tissue • Excitability • Muscle tissue (and nervous cells) receive and respond to stimuli by producing electrical signals • Contractability • Ability to shorten and thicken when stimulated • Extensibility • Ability to stretch without damaged • Elasticity • Ability to return to its original shape

5. Types of Muscle Tissue • Cardiac muscle • Skeletal muscle • Smooth muscle

6. Cardiac Muscle • Has striations • Usually has a single nucleus • Joined to another muscle cell at an intercalated disc • Involuntary • Found only in the heart Figure 6.2b

7. Smooth Muscle • No striations • Spindle-shaped cells • Single nucleus • Involuntary – no conscious control • Found mainly in the walls of hollow organs

8. Skeletal Muscle • Most are attached by tendons to bones • Cells are multinucleate • Striated – have visible banding • Voluntary – subject to conscious control • Cells are surrounded and bundled by connective tissue

9. Connective Tissue Wrappings of Skeletal Muscle • Tendon – cord-like structure, attaches muscle to bone • Endomysium – around single muscle fiber • Perimysium – around a fascicle (bundle) of fibers • Epimysium – covers the entire skeletal muscle Figure 6.1

10. Connective Tissue Wrappings of Skeletal Muscle • Fascia – on the outside of the epimysium • Superficial: subcutaneous tissue; made of areolar connective tissue and adipose • Deep: holds muscles together and separates them into functional groups; made of dense irregular connective tissue Figure 6.1

11. Skeletal Muscle Tissue Each skeletal muscle is a separate organ composed of hundred to thousands of skeletal muscle cells called muscle fibers because of their elongated shapes. Connective tissue surround muscle fibers and whole muscles, blood vessels, and nerves penetrate muscle.

12. Microscopic Anatomy of Skeletal Muscle • Muscle fibers(cells) are arranged parallel to one another. • Sarcomeres are the basic functional unit of striated muscle fibers; occurs at the overlap of filaments. • Sarcolemma is the plasma membrane that covers each muscle fiber • Sarcoplasm is the muscle fiber’s cytoplasm. • Tranverse (T) tubules are tunnel-like extensions of the sarcolemma that pass through muscle fiber from side to side • Sarcoplasmic reticulum is a network of membrane –enclosed tubules that stores Ca2+ ions required for muscle contractions. • Myoglobin is found in the sarcoplasm; reddish pigment; stores oxygen until needed by mitochondria for ATP production. • Myofibrils extend along the entire length of the muscle fiber; are cylindrical; consists of two types of protein filaments: light & dark

13. Microscopic Anatomy of Skeletal Muscle

14. Organization of the Scarcomere • Z-discs (lines) are zigzagging zones that separates sarcomeres • A bands (dark bands) extend the entire length of thick filaments; at end thick & thin filaments overlap • I bands (light bands) are composed of thin filaments only • H zone is found at the center of each A band; contains only thick filaments

15. Organization of the Sarcomere • Thick filaments are composed of the protein myosin. Shaped like 2 golf clubs twisted together • Thin filaments are composed of the protein actin. Twisted into helix. • Thin filaments also contains the proteins tropomyosin & troponin

16. Sliding Filament Theory of Muscle Contraction • Overall summary of what happens in a sarcomere when a muscle contracts: • Myosin heads of thick filaments pull on thin filaments • Thin filaments slide toward center of sarcomere • I bands and H zone becomes narrower • I band and H zone disappears @ maximum contraction

17. Sliding Filament Theory of Muscle Contraction Figure 6.7

18. Contraction Terminology • Neuromuscular Junction (NMJ)- area of contact between axon terminal & portion of sarcolemma • Axon terminal-branches of motor neuron that approaches, but not touch the sarcolemma • Acetylcholine (Ach)- neurotransmitter • Synaptic cleft- narrow gap that separates axon terminal of one neuron from muscle fiber • Motor end plate- part of sarcolemma that receives the neurotransmitter • Acetylcholinesterase (AChE)- enzyme that breaks down acetylcholine

19. Muscle Contraction • Nerve impulse arrives at axon terminal of motor neuron and triggers release of acetylcholine (ACh)… • ACh binds to its receptors and is activated, this causes Na/K ions to flow across membrane... • Inflow of Na ions generates a muscle action potential, which travels down sarcolemma & through T-tubules… • As the impulse moves down SR, Ca2+ isreleased from the SR to the thick and thin myofilaments… • Ca2+ binds to troponin molecules in thin filaments, causing the troponin-tropomyosin complex to change shape….

20. Muscle Contraction • This change in shape of the troponin/tropomysosin complex causes movement of the attached tropomyosin molecule…. • Allowing the myosin head to contact/bind actin, causing the myosin head to swivel (this requires ATP!) • During the swivel, the myosin head is firmly attached to the actin, so when the head moves, it pulls the actin filament forward. • This is called the ”power stroke” of contraction • Many myosin heads are swiveling simultaneously and their collective efforts are enough to pull the entire actin filament past the myosin filament into the H zone and causes shortening (contraction) of the muscle fiber

21. Muscle Relaxation • Neurotransmitter Ach (acetylcholine) is broken down by AChE. • Muscle action potentials stop • Ca2+ release channels in the SR close • As levels of calcium in sarcoplasm falls, troponin releases calcium and slides back into original position where it covers the myosin binding sites • Thin filaments slips back into their relaxed positions.