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 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+ is released 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.