The Muscular System

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).

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

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.

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

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.

Types of Graded Responses Figure 6.9b

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.

Types of Graded Responses Figure 6.9c

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.

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

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

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.

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.

Energy for Muscle Contraction Figure 6.10a

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).

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

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 30-60 seconds.

Energy for Muscle Contraction Figure 6.10c

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.

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.

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.

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.

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