Muscle II.

Mechanics

Fiber Contraction.

• Tension: Force exerted by a contracting muscle on an object.

• Load: Force exerted on the muscle by the weight of an object.

• Muscle shortens (and moves a load) when tension > load.

• Isometric contraction.• Muscle develops tension but does not

shorten (L > T)• Isotonic contraction.• Muscle shortens while load remains

constant (T > L).

Isometric Contraction.

Isotonic Contraction.

Muscle twitch

• Twitch. Mechanical response of a muscle fiber to a single action potential

Length-Tension Relationship.

• Muscle prepared to record isometric contractions.

• Tension related to the amount of overlap between the thick and thin filaments.

• 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

Isotonic Contraction.Load-Velocity Relationship.

• Heavier load results in:Longer latent period.Shorter distance of shortening.Shorter duration of the twitch.Slower velocity of shortening.

• Greater the loads require more cross bridges engaged in resisting stretch by the load. Lower loads result in fewer bridges engaged in resisting the load and, therefore, more available for shortening

• Maximal Shortening velocity (zero load) determined by rate at which cross bridges undergo cyclical activity

Frequency-Tension Relationship = Temporal Summation = Wave

Summation.• AP is an electrical event

that lasts a few ms.• The tension development

is a mechanical event that last (20 ms 200 ms).

• It is possible for a second AP to be initiated during mechanical activity.

• If a second AP appears before a contraction has finished a second contraction can be added to the first.

• Temporal Summation: increase in the mechanical response of a fiber to successive APs.

• As the frequency of APs increases, the level of tension increases by summation until a maximum tension (tetanic tension) is reached.

Summation Explained.

• An AP releases enough Ca++ to saturate the troponin and activate all binding sites.

• Binding and the stretching of the elastic elements takes time.

• Before tension is fully developed [Ca++] falls with a single AP.

Summation Explained

• 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.

Motor unit: group of muscle fibers innervated by a single motor neuron.

Whole Muscle Contraction.

• Whole muscles are made up of many muscle fibers organized into motor units.

• If more motor units combine their contraction the whole muscle contraction is greater.

• Multiple Motor Unit Summation = Spatial Summation

Whole Muscle Contraction

• Muscle tone• Asynchronous motor unit summation• Treppe

• By controlling these two, the nervous system controls whole muscle tension and velocity of shortening of the whole muscle.

• Size of motor units varies from muscle to muscle.• Muscles involved in fine, delicate movements

have small motor units. Small motor units allow for the increase in tension in very small steps.

• Muscles involved in coarse movements have large motor units. Tension increases in large steps.

Recruitment.

• Process of increasing the number of motor units that are active at a given time. Spatial Summation.

• Depends on the number of motor neurons that are active.

• The greater the number of motor units active the greater the tension.