1. The Muscular System

2. Contraction of Skeletal Muscle
Muscle fiber contraction is “all or none”
Within a skeletal muscle, not all fibers may be stimulated during the same interval
Different combinations of muscle fiber contractions may give differing responses
Graded responses—different degrees of skeletal muscle shortening
The all or none principle states that a muscle will contract to its fullest extent when it is stimulated adequately (enough- reaches action potential).

3. Contraction of Skeletal Muscle
Graded responses can be produced by changing
The frequency of muscle stimulation
The number of muscle cells being stimulated at one time

4. Types of Graded Responses
Twitch
Single, brief contraction
Not a normal muscle function
A muscle twitch is a single, brief, jerky contraction. This movement is not normal. In a twitch, a single stimulus is delivered and the muscle very quickly contracts and relaxes without sustaining or withholding a contraction for an adequate amount of time.

5. Types of Graded Responses
The arrow indicates the stimuli being delivered. The slope shows a quick single contraction and almost immediate relaxation.
Figure 6.9a

6. Types of Graded Responses
Tetanus (summing of contractions)
One contraction is immediately followed by another
The muscle does not completely return to a resting state
The effects are added
Summing of contractions can be described as adding up the twitches. These twitches are produced when stimuli is delivered rapidly so rapidly that the muscle does not have time to relax. The contractile forces increase due to the sum of the twitches. This is described at tetanus which can be incomplete (unfused) or complete (fused). Fused tetanus is the end goal and ideal muscle contraction.

7. Types of Graded Responses
Figure 6.9b

8. Types of Graded Responses
Unfused (incomplete) tetanus
Some relaxation occurs between contractions
The results are summed
Stimuli are delivered at a faster rate but not rapidly enough to prohibit relaxation and sustain the contraction; therefore some relaxation occurs between contractions and stimuli being delivered resulting in an incomplete tetanus.

9. Types of Graded Responses
Figure 6.9c

10. Types of Graded Responses
Fused (complete) tetanus
No evidence of relaxation before the following contractions
The result is a sustained muscle contraction
Fused or complete tetanus is a smooth continuous contraction without an relaxation between contractions or delivered stimuli. This is due to the fact that stimuli are being delivered so rapidly that it prevents this relaxation from occurring. A fused or complete tetanus is the ideal muscle contraction and allows us to sustain complete muscle movement.

11. Types of Graded Responses
The rapid delivery of stimuli is represented by the vast amount of arrows. This allows the plateau shape of the slope, representing the smooth continuous and sustained contraction.
Figure 6.9d

12. Muscle Response to Strong Stimuli
Muscle force depends upon the number of fibers stimulated
More fibers contracting results in greater muscle tension
Muscles can continue to contract unless they run out of energy

13. Energy for Muscle Contraction
Initially, muscles use stored ATP for energy
ATP bonds are broken to release energy
Only 4–6 seconds worth of ATP is stored by muscles
After this initial time, other pathways must be utilized to produce ATP
The phosphate bond is broken to release energy.

14. Energy for Muscle Contraction
Direct phosphorylation of ADP by creatine phosphate (CP)
Muscle cells store CP
CP is a high-energy molecule
After ATP is depleted, ADP is left
CP transfers energy to ADP, to regenerate ATP
CP supplies are exhausted in less than 15 seconds
ADP is left when the phosphate bond of ATP is broken and ADP is the end result. CP puts phosphate back into ADP to form ATP and use its energy.

15. Energy for Muscle Contraction
Figure 6.10a

16. Energy for Muscle Contraction
Aerobic respiration
Glucose is broken down to carbon dioxide and water, releasing energy (ATP)
This is a slower reaction that requires continuous oxygen
A series of metabolic pathways occur in the mitochondria
Aerobic=oxygen. Glucose is broken down with oxygen. This type of energy is used for long lasting activities including those that take up to hours to complete (such as running a marathon or biking/swimming long distances).

17. Energy for Muscle Contraction
Known as the Krebs cycle
Figure 6.10b

18. Energy for Muscle Contraction
Anaerobic glycolysis and lactic acid formation
Reaction that breaks down glucose without oxygen
Glucose is broken down to pyruvic acid to produce some ATP
Pyruvic acid is converted to lactic acid
This reaction is not as efficient, but is fast
Huge amounts of glucose are needed
Lactic acid produces muscle fatigue
Anaerobic=no oxygen. This energy is used for fast short bursts of intense activity. An example of this is running the 100m dash, running to home base, or blocking an opponent. This type of energy is only used for activities from seconds.

19. Energy for Muscle Contraction
Figure 6.10c

20. Muscle Fatigue and Oxygen Deficit
When a muscle is fatigued, it is unable to contract even with a stimulus
Common cause for muscle fatigue is oxygen debt
Oxygen must be “repaid” to tissue to remove oxygen deficit
Oxygen is required to get rid of accumulated lactic acid
Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less
Oxygen is needed to breakdown the lactic acid in your body, but if there is no oxygen available, you will be unable to break it down and this causes actual muscle fatigue.

21. Types of Muscle Contractions
Isotonic contractions
Myofilaments are able to slide past each other during contractions
The muscle shortens and movement occurs
Isometric contractions
Tension in the muscles increases
The muscle is unable to shorten or produce movement
Isotonic allows for both muscle contraction and movement. Imagine picking up a heavy box, motion of picking up the box in your arms is isotonic. When you carry the box to the desired location, you are not moving the box with your arms, yet your muscles are still contracted, this is an isometric contraction.

22. Muscle Tone Some fibers are contracted even in a relaxed muscle
Different fibers contract at different times to provide muscle tone
The process of stimulating various fibers is under involuntary control
Muscle tone is both a visible and invisible concept.

23. Effect of Exercise on Muscles
Exercise increases muscle size, strength, and endurance
Aerobic (endurance) exercise (biking, jogging) results in stronger, more flexible muscles with greater resistance to fatigue
Makes body metabolism more efficient
Improves digestion, coordination
Resistance (isometric) exercise (weight lifting) increases muscle size and strength
The more muscle endurance, the less likely a muscle is to fatigue. Aerobic exercise allows for an increase of muscle endurance and flexibility. Resistance exercise like strength training increases muscle size and strength and these changes are usually more visual.

24. Effect of Exercise on Muscles
The runner has a different muscle tone than the weight lifter. The runner has more muscular endurance and most likely is more flexible. The weight lifter however has stronger and bigger muscles. He may not be able to lift heavy weights for hours, unlike the runner who can last for hours.
Figure 6.11