Myograms and Muscle Contraction

1. Myograms and Muscle Contraction

2. Myogram of Muscle Twitch

3. Muscle Twitch Terminology • Latent Period: period of time between when the stimulus is applied and contraction begins. Action potential is sweeping across the sarcolemma and Ca2+ is being released from the SR. • Contraction phase: Muscle begins contracting and continues until it reaches it’s peak contraction. Calcium is binding to troponin, moving the tropomyosin from the actin active sites, cross bridges between the myosin heads and actin form, and power stroke occurs. • Relaxation phase: when the action potential has passed and the SR is collecting the Ca2+ from the sarcoplasm, ending contraction and allowing muscle to return to resting tension.

4. Myograms

5. Myogram Terminology • Muscle Twitch: One stimulus is being applied to the muscle and therefore one contraction cycle occurs. • Treppe: A second stimulus is applied once the muscle has ended its relaxation phase. Results in a slightly more forceful contraction. Multiple stimuli applied in this manner will result in a higher and higher force of contraction until a plateau is reached. “Treppe” is German for step.

6. Myogram Terminology • Wave Summation: A second stimulus is applied during the relaxation phase of a contraction cycle. Results in a more forceful second contraction. • Incomplete or Unfused Tetanus: Stimuli applied during relaxation phase and applied frequently enough to raise the force of contraction to near maximum force and to a level approximately 4 times more forceful than during treppe. • Complete or Fused Tetanus: Stimuli applied so frequently that muscle has no time to enter a relaxation phase and results in a smooth, sustained contraction at maximum force.

7. Motor Unit • A motor unit is comprised of a motor neuron and all the muscle fibers under the control of that motor neuron. • Each muscle contains many motor units. • The size of the motor unit is an indicator for how finely controlled the movements are. Very small motor units have very fine control while very large motor units have more gross control.

8. Motor Unit Reinforcement

9. Recruitment • When you decide to make a movement, first smaller, slower, less powerful motor units are stimulated. If it is not enough to result in the desired movement, larger, faster, more powerful motor units will gradually be recruited until the movement can be made. This occurs until peak force is attained. • When peak force is attained, and all motor units in a muscle are contracting, the contraction is short lived because muscle fatigue will set in quickly due to lack of energy and Calcium to keep a sustained contraction. • When sustained contractions are needed, motor units are stimulated in a sort of relay system: while some motor units are contracting, others are resting, which allows for longer sustained contractions and staves off muscle fatigue. This is called asynchronous motor unit summation.

10. Muscle Tone: the resting tension in a muscle. • Some motor units in a skeletal muscle are always contracting, but not forcefully enough to produce movement. • Helps maintain constant tension on the tendons. • Stabilizes the position of the bones and joints to maintain posture and body position.

11. Isotonic Contraction

12. Isotonic Contractions • Isotonic Contractions: Contraction in which as tension rises, the muscle shortens. (ex: lifting an object off the desk, walking, running, etc) • Types of Isotonic Contractions • Concentric Contraction • Muscle tension exceeds resistance and muscle shortens. • Eccentric Contraction • Peak tension is less than the load and the muscle elongates owing to the contraction of another muscle or the pull of gravity.

13. Isometric Contraction

14. Isometric Contractions • Isometric Contractions: the muscle as a whole never changes length because the force of contraction never overcomes the resistance. • Examples: carrying a bag of groceries, holding a baby, holding your head up. • During isometric contractions, the muscle bulges as tension rises, but never shortens. Cross bridges still form and contract, but as the muscle contracts, the tendons lengthen so the overall length does not change.

15. Resistance and Speed of Contraction • The lower the resistance, the faster the speed of contraction. • The heavier the resistance, the slower the movement because tension has to gradually build within the muscle as more and more motor units are recruited. • Also at a molecular level, myosin head pivoting is slower with heavy resistance, which means the contraction itself is also slower.