3. Summarize the steps that occur when a muscle relaxes?

4. .

5. STEP 2 With the absence of the muscle action
Relaxed Muscle.
After ACh generates a muscle action potential, and no new motor neuron action potential is generated the muscle will relax.
STEP 1 The enzyme acetylcholinesterase breaks down ACh, which is recycled. It is found on the surface of the sarcolemma. With no ACh binded to the gated channels they are closed to sodium ions, and no muscle action potential is generated.
STEP 2 With the absence of the muscle action

6. calcium ions are taken up by S R.
Step 3 The tropomyosin-troponin complex blocks the myosin binding site.
STEP 4 Thin myofilaments return to their relaxed positions.
J. Motor Units
A motor unit is a motor neuron and all the muscle fibers it supplies.
The number of muscle fibers per motor unit can vary from four to several hundred.
Muscles that control fine movements have small motor units.

7. Large weight-bearing muscles have large motor units.
Muscle fibers from a motor unit are spread throughout the muscle, therefore, contraction of a single motor unit causes weak contraction of the entire muscle.
Tension can be varied by adjusting the number of motor units that are activated this is known as recruitment.
Not all motor units are contracted some are excited others are inhibited, this prevents fatigue.

8. K. All-or-none principle
1. Subthreshold stimulus a weak stimulus that cannot initiate a contraction.
2. Threshold stimulus the stimulus strength at which the first observable muscle contraction occurs.
Beyond threshold, muscle contracts more vigorously as stimulus strength is increased.
3. Myogram-record of a muscle contraction.
4. Muscle tone-a constant, slight contracted state of all muscles, which does not produce active movement.

9. 5. Muscle twitch-is a response of a muscle to a single, brief threshold stimulus.
L. Muscle Tension
1. Frequency of stimulation :Tension generated by a muscle fiber depends partially on the frequency of stimulation it receives from its motor neuron. Impulses are transmitted in bursts, one impulse closely following another.
2. Number of muscle fibers contracting : Overall sustained muscle contraction can be maintained for long periods while the tension generated process is shared at different moments in turn by the motor unit.

12. What is the difference between fast-twitch muscles and slow-twitch muscles, and explain the functions for which each type is best adapted?

13. 3. Components of muscle fibers.
a. Contractive elements :Components that are actively involved in muscle contraction. They produce active tension.
b. Elastic elements :Structures capable of being stretched. Includes connective tissue and tendons. They produce passive tension.
M. Muscular atrophy and hypertrophy.
1. Atrophy when not used muscle fibers decrease in size by the loss of myofibrils. Can occur when the nerve supply is cut or damaged, this is denervation atrophy. Muscle fibers are replaced by fibrous tissue.

14. N. Types of Muscle Fibers.
2. Hypertrophy increased diameter of muscle fibers. Produces more myofibrils, mitochondria, sarcoplasmic reticulum.
N. Types of Muscle Fibers.
Muscle varies by the content of myoglobin a redish pigment that stores oxygen until needed.
1. Red muscles are high in myoglobin, have smaller diameter, more mitochondria, and more blood capillaries.
2. White muscle are low in myoglobin, with more extensive sarcoplasmic reticulum.

15. 3. Skeletal Muscle Fibers
a. Slow-twitch (high-oxidative muscle fibers) contract slowly, smaller in diameter, have better developed blood supply, have more mitochondria, and are more fatigue resistant. These type muscles split ATP slowly.
b. Fast-twitch (low-oxidative muscle fibers) These muscle fatigue easily and has large amounts of glycogen, a poor blood supply, fewer mitochondria, and little myoglobin.
c. Fast-twitch (fatigue-resistant fibers) have a well-developed blood supply, more mitochondria, and more myoglobin
People who are good sprinters have a greater

16. percentage of fast-twitch muscle fibers, and people who are good long-distance runners have a higher percentage of slow-twitch muscle fibers.
O. Regeneration of Muscle Tissue.
Muscle cannot divide after the first year of life, but enlargement of existing cells. Fibers are replaced on an individual basis. New fibers are derived from satellite cells which are dormant stem cells. They are associated with muscle fibers. Satellite cells lengthen existing fibers, fusing with them. Significant

17. skeletal muscle damage is replaced by scar tissue.
P. Homeostasis of Muscle.
ATP is the only source of energy for muscle contraction.
As soon as available stores of ATP are hydrolyzed (4-6 seconds). ATP is regenerated by ADP and creatine phosphate, by aerobic respiration, and anaerobic glycolysis.

18. a. After exercise the oxygen dept must be repaid
a. After exercise the oxygen dept must be repaid. Extra oxygen is needed to replenish oxygen reserves. Lactic acid is converted to pyruvic acid. Glycogen stores are then replaced.
b. ATP and creatine phosphate reserves must be resynthesized.
c. Lactic acid is removed from muscle, diffuses into the blood stream, travels to the liver and is converted into pyruvic acid.

19. d. Creatine phosphate releases stored energy to convert ADP into ATP.
e. Aerobic metabolism provides most ATP needed for contraction.
f. At peak activity, anerobic glycolysis needed to generate ATP.

21. 2. Muscle fatigue is the inability of muscle to maintain strength, the muscles no longer respond.
Muscle fatigue occurs when ATP production fails to keep pace with ATP use.
Lactic acid accumulates in the muscle.
3. Heat Production
Only 40% of energy from muscle contraction

22. Is used as work. The remaining 60% is given off as heat
Is used as work. The remaining 60% is given off as heat. This heat is then removed from the skin by sweating.

23. 1. How do muscles contribute to the heat responsible for body temperature before, during, and after exercise?
2. What is accomplished by shivering?
3. What is the function of creatine phosphate?
4. When does lactic acid production increase in a muscle cell?

24. Compare oxygen dept, fatigue, and heat production as examples of muscular homeostasis.

25. What are the events that lead to an oxygen debt and recovery from it, and what occurs during muscle regeneration?