Chapter 6 The Muscle Physiology

Nerve Stimulus Nerve Stimulus to Musclesto Muscles

Synaptic cleft – gap between nerve and muscle

- no contact

- filled with interstitial fluid

Transmission of Nerve Impulse to MuscleTransmission of Nerve Impulse to Muscle1. 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

Sliding Filament TheorySliding 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 MuscleContraction 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 ResponsesTypes 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 Some relaxation

occurs between contractions

The results are summed

Figure 6.9a, b

2b. Fused (complete)

tetanus No evidence of

relaxation before the following contractions

The result is a sustained muscle contraction

Tetanus can be caused by bacteriaTetanus 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 StimuliResponse 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 ContractionEnergy 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 DebtMuscle 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 ContractionsTypes 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 ToneMuscle Tone

Some fibers are contracted even in a relaxed muscle

Different fibers contract at different times

Involuntary

Effects of Exercise on MuscleEffects of Exercise on Muscle

Results of increased muscle use

- Increase size

- Increase strength

- Increase efficiency

- more fatigue resistant