Chapter 6 The Muscle Physiology

Nerve Stimulus to Muscles Synaptic cleft – gap between nerve and muscle - no contact - filled with interstitial fluid

Transmission of Nerve Impulse to Muscle 1. Neurotransmitter (acetylcholine) released by nerve upon arrival of nerve impulse 2. Acetylcholine attaches to receptors on the sarcolemma 3. Sarcolemma becomes permeable to sodium (Na+) 4. Sodium rushing into the cell causes an action potential 5. Once started, muscle contraction cannot be stopped All or None!

Sliding Filament Theory Activation by nerve causes myosin crossbridges to attach to actin then bind to the next site on actin Myosin “slides” along actin

Contraction of a Skeletal Muscle Contraction is “all or none” w/in one fiber not all fibers may be stimulated during the same interval Different combinations of muscle fiber contractions give different responses Graded responses – different degrees of skeletal muscle shortening

Types of Graded Responses 1. Twitch Single, brief contraction Not a normal muscle function

2. 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

2a. Unfused (incomplete) tetanus 2b. Fused (complete) tetanus Some relaxation occurs between contractions The results are summed 2b. Fused (complete) tetanus No evidence of relaxation before the following contractions The result is a sustained muscle contraction Figure 6.9a, b

Tetanus can be caused by bacteria Clostridium tetani is a bacteria that causes tetanus in humans, also called lockjaw. Produce toxin that affects the nerves and controls the activity of muscles.

Response to Strong Stimuli Muscle force depends upon # of fibers stimulated More fibers contracting results in greater muscle tension Muscles can continue to contract until they run out of energy

Energy for Muscle Contraction Initially, muscles used stored ATP for energy - ATP broken to release energy - 4-6 sec worth of ATP stored After this time, other pathways used to make ATP

Direct phosphorylation Muscle cells contain creatine phosphate (CP) CP is a high-energy molecule After ATP is depleted, ADP is left CP transfers energy to ADP, to regenerate ATP Used up in 15- 20 sec Creatine monohydrate is a precursor to creatine phosphate. By supplementing with creatine monohydrate, your muscle creatine phosphate is maximised, and more muscular work can occur due to greater energy reserves. NOT endorsed by School!

Aerobic Respiration occur in the mitochondria Glucose is broken down to carbon dioxide and water, releasing energy slower reaction that requires oxygen Makes the most ATP Lasts for hours

Anaerobic glycolysis Reaction that breaks down glucose without oxygen Glucose is broken down to pyruvic acid to produce 2 ATP Lasts up to 1 min. Muscle fatigue – no more O2 – burning due to lactic acid

Muscle Fatigue and Oxygen Debt When muscle is fatigued, it can’t contract Common reason is oxygen debt - Oxygen needed to get rid of lactic acid Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less

Types of Muscle Contractions 1. Isotonic contractions Myofilaments slide past each other during contractions The muscle shortens 2. Isometric contractions Tension in the muscles increases The muscle does NOT shorten

Muscle Tone Some fibers are contracted even in a relaxed muscle Different fibers contract at different times Involuntary

Effects of Exercise on Muscle Results of increased muscle use - Increase size - Increase strength - Increase efficiency - more fatigue resistant