Outline - 2, BIO 3360, Muscle II
I. Muscle shortens and moves a load when tension is greater than load
-Tension is the force exerted by a contracting muscle on an object
-Load is the force exerted on the muscle by the weight of an object
A. Isometric contraction - muscle develops tension but does not shorten because the load is greater than the tension
B. Isotonic contraction - muscle shortens because the tension is greater than the load
C. A muscle twitch is the mechanical response of a muscle cell to a single action potential - characterized by a latent period, a contraction period and a relaxation period.
D. Muscle twitch comparisons when varying the length of the muscle/thus the overlap between actin and myosin -
• Tension is max when overlap between actin & myosin allows the max number of cross bridges to form.
• When sarcomere is too short that thin filaments collide tension decreases.
• When sarcomere is too large no cross bridges form and no tension develops
II. Isotonic contraction: Load-Velocity relationship
A. Heavier load - longer latent period, shorter distance of shortening, shorter duration of twitch, slower velocity of shortening
B. Greater the loads require more cross bridges engaged in resisting stretch by the load.
C. Lower loads result in fewer bridges engaged in resisting the load and, therefore, more available for shortening
III. Frequency-Tension relationship AKA Temporal Summation or Wave Summation
A. A muscle twitch is a mechanical event lasting 20-200 ms as a result of an AP
B. A second AP can be initiated during this mechanical activity
1. Results in a larger twitch or more tension in the muscle when the second AP appears before the first contraction has finished
2. This results in temporal summation and a larger twitch
3. If stimuli repeat before the previous muscle twitch has completed, incomplete tetanus occurs - Contraction strength increases in amplitude with partial relaxation
4. Increasing the frequency of stimuli even more can lead to a maximum tension termed complete tetanus - Sustained contraction without any relaxation
C. Summation Explained
1. An AP releases enough Ca ++ to saturate the troponin and activate all binding sites. If another stimulus occurs while the calcium is still in the cytoplasm, additional tension can occur in the muscle and the latent period disappears.
2. Binding and the stretching of the elastic elements in muscle takes time. If another stimulus occurs before the muscle has returned to its stretched state, the power strokes can apply directly to the muscle shortening/tension.
• First filaments slide along each other and start stretching the elastic elements.
• Force developed by the cross bridges must pass through the elastic elements before is applied to the load.
IV. Whole Muscle contraction
A. Motor Unit - One motor neuron plus however many muscle cells it innervates
B. Multiple motor unit summation = Spatial summation - Individual motor units combine their activities to increase the contraction of the entire muscle.
C. Muscle tone - At any given time, some fibers are contracted which tightens muscle, but not enough to produce movement
D. Asynchronous motor unit summation - Different motor units are activated at different times
E. Treppe - Stronger contractions after muscle has contracted several times. "Staircase" phenomenon
F. Summary of factors determining muscle tension - by controlling thes following factors, the nervous system can control whole muscle tension/contraction as well as the velocity of shortening of the whole muscle
1. Tension developed by each individual muscle fiber - i.e. AP frequency, fiber length, the amount of actin & myosin in the sarcomere, ATP availability
2. Number of active fibers - ie. the make up of the motor units and how many are active
-Size of motor units vary
-Recruitment - increasing the number of active motor units through spatial summation
Return to BIO 3360 homepage