1. Section 6.2

2. 1. Sarcolemma- plasma membrane of a muscle fiber 2. Sarcoplasm- cytoplasm 3. Sarcoplasmic reticulum- smooth ER that stores Ca 2+ 4. Myofibrils- contractile unit of muscle fibers

3. 1. Myofibrils are basically bundles of myofilaments. 2. These myofilaments are aligned to give a banding appreance. 3. I band= light band 4. A band= dark band

4. 5. Sarcomeres are the functional and structural unit of myofibrils. 6. They extend from two dark lines called z lines. 7. Contain two types of filaments: actin and myosin (both are proteins) 8. Thick filaments = myosin 9. Thin filaments = actin

5. 10. Myosin filaments have heads called extensions or cross-bridges 11. Actin and myosin overlap a little= very important

6. 1. Muscles are stimulated to contract by motor neurons (nerve cells). 2. The axon of one motor neuron has several branches and can simulate several muscle fibers. 3. Motor Unit= one motor neuron and the muscle cells that it stimulates.

7. 4. Neuromuscular Junction = association site of nerve and muscle 5. Synaptic Cleft = gap between nerve and muscle (area is filled with fluid) 6. Muscle and nerve do NOT make direct contact

8. a. A neurotransmitter attaches to receptors on the sarcolemma. 1) A neurotransmitter is a chemical released by nerve cells upon the arrival of a nerve impulse. 2) Acetylcholine= neurotransmitter for skeletal muscle b. Sarcolemma becomes permeable to Na + (sodium ions). c. Sodium rushes into the cell causing an action potential. Once started it cannot be stopped.

9. 1. Activation by nerve causes myosin heads (crossbridges) to attach to binding sites on the thin filament (actin). 2. Myosin heads then bind to the next site of the thin filament. 3. This continued action causes a sliding of the myosin along the actin. 4. The result is that the muscle is shortened (contracted).

11. a. ATP is the energy source needed for muscle contraction. b. Calcium ions assists in muscle contraction by exposing the myosin binding sites. c. Muscle fiber contraction is “all or none” d. Within a skeletal muscle, not all fibers may be stimulated during the same interval e. Different combinations of muscle fiber contractions may give differing responses- called Graded Response. f. Rapid Stimulus = constant contraction or tetanus

12. f. Muscle force depends upon the number of fibers stimulated. g. More fibers contracting results in greater muscle tension. h. Muscles can continue to contract until they run out of energy.

13. Muscle Fatigue and Oxygen Debt 1. When a muscle is fatigued, it is unable to contract 2. The common reason for muscle fatigue is oxygen debt a) Oxygen must be “repaid” to the tissue to remove oxygen debt b) Oxygen is required to get rid of accumulated lactic acid 3. Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less 4. Increasing acidity also causes “muscle pain”

14. Types of Muscle Contractions 1. Isotonic contractions a) Myofilaments are able to slide past each other during contractions b) The muscle shortens 2. Isometric contractions a) Tension in the muscles increases b) The muscle is unable to shorten c) No movement at the joint

15. Muscle Tone 1. Some fibers are contracted even in a relaxed muscle 2. Different fibers contract at different times to provide muscle tone 3. The process of stimulating various fibers is under involuntary control