MECHANICAL PROPERTIES OF SKELETAL MUSCLE

Motor Unit

• A motor unit consists of a somatic motor neuron plus all the skeletal muscle fibers it stimulates

• A single somatic motor neuron makes contact with an average of 150 skeletal muscle fibers, and all of the muscle fibers in one motor unit contract in unison

Motor Unit

A twitch contraction is the brief contraction of all the muscle fibers in a motor unit in response to a single action potential in its motor neuron

Q. In muscle physiology, the latent period refers toa. the period of lost excitability that occurs when two stimuli are applied immediately one after the other.b. the brief contraction of a motor unit.c. the period of elevated oxygen use after exercise.d. an inability of a muscle to contract forcefully after prolonged activity.e. a brief delay that occurs between application of a stimulus and the beginning of contraction

• Note that a brief delay occurs between application of the stimulus (time zero on the graph) and the beginning of contraction. The delay, which lasts about two milliseconds, is termed the latent period.

• During the latent period, the muscle action potential sweeps over the sarcolemma and calcium ions are released from the sarcoplasmic reticulum.

• The second phase, the contraction period, lasts 10–100 msec. During this time, Ca2 binds to troponin, myosin-binding sites on actin are exposed, and crossbridges form. Peak tension develops in the muscle fiber.

• During the third phase, the relaxation period,also lasting 10–100 msec, Ca2is actively transported back into the sarcoplasmic reticulum, myosin-binding sites are covered by tropomyosin, myosin heads detach from actin, and tension in the muscle fiber decreases

PROPERTIES OF SKELETAL MUSCLE

• Extensibility & elasticity • Excitability• Conductivity• Contractility • Tonicity • Refractory period• Fatigue

• Extensibility:Ability of the muscle to elongate when stretched.

• Elasticity: Ability of the muscle to return to its original length when stretch is removed.

• Excitability: Ability of the muscle to respond to a stimulus.(electrical ,thermal ,chemical,mechanical).

• Contractility: Ability of the muscle to shorten or contract in response to a stimulus.

• Conductivity: Ability of the muscle to transmit an impulse (AP) from one part of the fibre to another part.

• The condution velocity of an impulse in skeletal

muscle fiber is slow. it is about 5 meters/sec

• Tone:The state of partial sustained contraction seen in all muscles.

REFRACTORY PERIOD: It is about 30-50millisec . It is a period of action potential in which another stimulus applied will not produce a response in a muscle.

Types: Absolute Relative

• The refractory period is short in skeletal muscle, but very long in cardiac muscle – 250 msec

• This means that skeletal muscle can undergo summation and tetanus, via repeated stimulation

• Cardiac muscle CAN NOT sum action potentials or contractions and can’t be tetanized

• CONTRACTILITY: Ability to contract in response to a stimulus.

• Types :-• Isotonic contraction• Isometric contraction

Isotonic and Isometric Contractions

• Isotonic contraction:The contraction that occurs when the muscle is allowed to freely shorten,so that tension in the muscle is kept constant.

• Isometric contraction: the contraction that occurs without any shortening of the muscle, so that the tension increases, but the length of the muscle remains constant.

Isotonic Contraction

1 Same Tension In The Muscle

2 Muscle Length Decreases3 Work Is Done –Weight Lifted4 Extra Heat

produced.Relaxation Heat produced After Contraction

5 Greater Energy Is Used6 Eg-1. Muscles of upperlimb

while lifting weight ,lifting the leg while walking

Isometric Contraction1same Length-muscle Length

Remains Constant2 Increase In Tension3 Work Performed Is Not Seen

4 Less Heat Produced

5 Less Energy Is Used6 Eg:- Calf muscles on

standing

How do we control the strength of contraction?

1. Large Motor unit involved2. More motor units recruited3. More fast type II b types of fiber4. Increasing the rate of stimulation

SUMMATION

• When the strength of the stimulus is increased the contractile response also increases.The increase in response is due to recruitment of motor units. This is called quantal summation or multimotor unit summation

Twitch, Summation, and Tetanus

• Twitch:– Muscle is stimulated with a single electrical

shock (above threshold).• Quickly contracts and then relaxes.

• Summation:– If second electrical shock is administered

before complete relaxation of muscle.

Twitch, Summation, and Tetanus (continued)

• Incomplete tetanus:– Stimulator delivers an increasing frequency of

electrical shocks.• Relaxation period shortens between twitches.• Strength of contraction increases.

• Complete tetanus:– Fusion frequency of stimulation.– No visible relaxation between twitches.– Smooth sustained contraction.

Simple Twitch, Summation, and Tetanus

TETANUS

• When the number of stimuli are more than two but less than tetanizable frequency then partial tetanus is obtained –clonus 30/sec

• Tetanus- when stimuli are applied at a higher frequency, tetanus-40/sec (continuous contraction) is produced.

• Infection by clostridium tetani is due to synchronous discharge from all the nerve fibers-leading to tetanic contraction of all skeletal muscles .

Preload

• Preload is the load on a muscle in a relaxed state, that is, prior to contraction.

• Applying preload to muscle does two things:• Causes the muscle to stretch. The greater the preload

added, the greater the stretch of the muscle. Along with stretching the muscle, preload stretches the sarcomere. The greater the preload, the greater the pre- stretch of the sarcomere.

• Causes the muscle to develop passive tension. If a 2-g weight is suspended from a muscle, a 2-g force also develops within that muscle. This force is the passive tension. The greater the preload, the greater the passive tension in the muscle.

Afterload

• Afterload is the load the muscle is working against or trying to move during stimulation.

• If the muscle is trying to lift 100 lb. during stimulation, then the afterload is 100 lb.

• During contraction, the muscle does not necessarily lift or move the afterload.

LENGTH-TENSION CURVESPreload-length Tension Curve

– resting skeletal muscle acts as a simple spring. As preload is added, the muscle stretches and develops a passive tension. The passive tension can be considered an internal force that opposes and equals the preload force.

ISOMETRIC CONTRACTION OF THE ISOLATED SKELETAL MUSCLE

• During an isometric (same length) contraction, the overall muscle length will not change.

• the cross-bridge cycling will produce active tension

Active Tension Development • The active tension developed during an isometric

contraction is proportional to the number of these cross-bridges that cycle. The more cross-bridges that cycle, the greater the developed active tension.

• the active tension is maximal when there is maximal overlap of thick and thin filaments and maximal possible cross-bridges which is at resting state.

• When the muscle is stretched to longer lengths, the number of possible cross-bridges is reduced, and active tension is reduced.

• When muscle length is decreased, the thin filaments don’t have enough space to slide on thick filaments so more active tension can’t be generated.

Total Tension• The preload creates a passive tension prior to

contraction, and cross-bridge cycling during contraction adds an active tension component.

• The total tension in the active muscle is the passive or preload tension plus the active tension.

• Length-tension relationship in skeletal muscle. Maximal active tension occurs at muscle lengths where there is maximal overlap of thick and thin filaments.

Q. All of the following will occur when an unstimulated muscle is stretched except:A. increased preloadB. increased afterloadC. increased muscle lengthD. increased passive tension

Q. The figure depicts the isometric length-tension relationship of skeletal muscle. Identify the region where actin and myosin overlap is the least

FORCE-VELOCITY RELATIONSHIP

• describes the velocity of shortening when the force against which the muscle contracts i.e. the afterload, is varied

• Isotonic (same tone) contraction - The force, rather than the length, is fixed i.e. the muscle contracts with the same force

• The velocity of shortening reflects the speed of cross-bridge cycling.

• the velocity of shortening will be maximal (Vmax) when the afterload on the muscle is zero.

• As the afterload on the muscle increases, the velocity will be decreased because cross-bridges can cycle less rapidly against the higher resistance.

• As the afterload increases to even higher levels, the velocity of shortening is reduced to zero.

Classification of Fiber Types in Skeletal Muscles

Examples:• Type I Red fibers: in postural muscles

– Large myoglobin content and many mitochondria• Type IIa Red fibers: in muscles needed for

activities like middle distance running, swimming, etc.– Large myoglobin content and many mitochondria

• Type IIb White fibers: needed for activities like sprinting• Low myoglobin content and few mitochondria