MECHANICS OF SKELETAL MUSCLE

Dr. Ayisha Qureshi

Assistant Professor

MBBS, MPhil

A MUSCLE TWITCH

The Muscle Twitch

A single action potential causes a brief contraction followed by relaxation in the muscle. This is called a

single Muscle twitch.

• Electrical and mechanical events in a muscle always occur in relation to one another: The electrical event (Action potential) is followed by the mechanical events (contraction). The whole process is called Excitation-contraction coupling.

• Twitch starts 2 ms after depolarization of the membrane, before repolarization is complete-----Why the delay?

Contractile activity and electrical activity in

skeletal muscle:

A single action potential in a skeletal muscle fiber lasts only 1 to 2 msec, while a skeletal muscle contraction and relaxation lasts for about 100 msec.

The onset of the resulting contractile response lags behind the action potential because the entire excitation–contraction

coupling must occur before cross-bridge activity begins. In fact, the action potential is completed before the contraction even begins.

Time is take for the following processes:

• AP to spread down the t-tubule.

• Release of Ca2+

• Ca2+ to attach to Troponin C

• Power stroke

• Ca2+ uptake by the ATPase pump in the SR.

MOTOR UNIT

Definition: All the muscle fibers innervated by a single nerve fiber are called a MOTOR

UNIT.OR

Each single motor neuron plus all the muscle fibers it innervates is called a MOTOR UNIT.

• One motor neuron innervates a number of muscle fibers, but each muscle fiber is supplied by only one motor neuron. When this neuron is stimulated, all the muscle fibers supplied by it contract together.

• Each muscle consists of a number of mixed motor units.• For a weak contraction of the whole muscle, only one or a few of

its motor units are activated. • The number of muscle fibers per motor unit and the number of motor

units per muscle vary widely, depending on the specific function of themuscle. E.g. the kind of work that the muscle performs…..

Force of Contraction Summation:

Summation: is the process of adding together of individual twitch contractions to increase the

intensity of whole muscle contraction.

There are 2 types of summation:

1. Multiple Fiber Summation (No. of motor units stimulated)

2. Frequency Summation

Multiple Fiber Summation

Definition:It is the summation of individual muscle fiber contractions

by increasing the number of motor units contracting simultaneously.

• Initially, with a weak signal from the CNS-only smaller units are stimulated.

• Later, when signal from CNS becomes stronger, larger motor units are excited.

Importance: It allows gradation of force to occur for weak & strong contractions.

FREQUENCY SUMMATION

Definitions:

Force of contraction increases by increasing the frequency of contractions. Two twitches from 2 action potentials add together to

produce greater tension in the fiber than produced by a single action potential. This is called twitch summation or frequency summation.

• If repeated APs are separated by long intervals of time, muscle fibers have time to relax completely between stimuli.

• If interval of time between APs is shortened, the Muscle fiber will not have relaxed completely at time of 2nd stimulus, resulting in a more forceful contraction.

• A single action potential in a muscle fiber produces only a twitch. What happens when a second action potential occurs immediately again in the same muscle fiber?

If the muscle fiber has completely relaxed before the next action potential takes place, a second twitch of the same magnitude as the first one occurs. The same excitation-contraction events take place each time, resulting in identical twitch responses. If, however, the muscle fiber is stimulated a second time before it has completely relaxed from the first twitch, a second action potential causes a second contractile response, which is added “piggyback” on top of the first twitch. (see diagram on the next slide)

FREQUENCY SUMMATION

When APs come one after the other after the relaxation of the

muscle is complete…….

When APs come one after the other before relaxation of the

muscle is complete…

FREQUENCY SUMMATION

If APs continue to stimulate the muscle repeatedly at short intervals, there is no time for complete relaxation between

contractions↓

Individual twitches fuse into one continuous contraction↓

Whole muscle contraction appears to be smooth, sustained & of maximal strength

↓This is called TETANIZATION or TETANUS

(A tetanic contraction is usually three to four times stronger than a single twitch.)

• Physiologic basis of twitch summation & Tetanus:

The main reason is the sustained elevation in cytosolicCa2+ permitting greater cross-bridge cycling. As thefrequency of action potentials increases, the duration ofelevated cytosolic Ca2+ concentration increases, andcontractile activity likewise increases until a maximumtetanic contraction is reached. With tetanus, themaximum number of cross-bridge binding sites remainuncovered so that cross-bridge cycling, and consequentlytension development, is at its peak.

FREQUENCY SUMMATION & TETANUS

Two types of Tetanus:1. COMPLETE or FUSED TETANUS: If repeated stimuli are

applied at fast rate, then no relaxation occurs between the stimuli, muscle reaches max. tension and remains there & a sustained contraction phase is obtained.

2. INCOMPLETE or UNFUSED TETANUS: if repeated stimuli at a slower rate, then muscle fiber relaxes slightly/incompletely between summated stimuli but the relaxation remains incomplete.

CAUSE: Enough Ca2+ ions are maintained in the muscle sarcoplasm so that contractile state is sustained without allowing relaxation between AP.

THE STAIRCASE/ TREPPE EFFECT

• DEFINITION:When a series of maximal stimuli are delivered to the muscle at

a frequency just below tetanizing frequency (when muscle twitch due to previous stimulus has just

completed), the tension/amplitude developed during each twitch increases till a max. height is reached & a plateau is

formed. This is called the Treppe/ staircase effect.Because the tension rises in stages, like the steps in a staircase,

this phenomenon is called treppe, a German word meaning "stairs."

• CAUSE: The rise is thought to result from a gradual increase in the concentration of calcium ions in the sarcoplasm, in part because the ion pumps in the sarcoplasmic reticulum are unable to recapture them in the time between stimulations.

Treppe Effect

ISOTONIC VS. ISOMETRIC CONTRACTION

ISOTONIC CONTRACTION

Not all muscle contractions shorten muscles. They can generate force or tension and at the same time stay the same length. There are two primary types of contraction, depending on whether the muscle changes length during contraction. They are:• Isotonic contraction: occurs when muscle contracts with

shortening of length but against a constant load, thus, the tension on the muscle remains constant (iso= same, tonic= tension)

ORA contraction that creates force & moves a load.

Isotonic contractions are used for body movements and for moving external objects. E.g. picking up a book, a box.

ISOMETRIC CONTRACTION

• Isometric contraction: occurs when muscle contracts without shortening in length.

(iso= same, metric= measure or length)OR

A contraction that creates force without movement.Isometric contractions can be seen in 2 cases:1. If the object you are trying to lift is too heavy.2. If the tension developed in the muscle is deliberately

less than needed to move the load. E.g. standing for long time or holding up a glass of water while taking sips.

FAST vs SLOW FIBERS

• The skeletal muscle fibers are mainly of 2 types:

1. SLOW or RED or TYPE I MUSCLE FIBERS

2. FAST or WHITE or TYPE II MUSCLE FIBERS

• Every muscle of the body is composed of a mixture of both fast & slow fibers.

• Simply: Muscle that reacts rapidly is composed of Fast fibers & muscles that react slowly with long contractions are composed of Slow fibers

• Color is determined by the protein myoglobin

FAST vs SLOW FIBERS

SLOW-TWTCH/ RED/ Type I

Small diameter

More myoglobin

Fatigue resistant

Mostly Oxidative

Slow rate of contraction

Myosin ATPase activity LOW

↓ no. of myofilaments

Red

Posture maintenance

FAST-TWITCH/ WHTE/Type II

Large diameter

Less myoglobin

Easily fatigue

Mostly glycolytic & oxidative

Fast rate of contraction

Myosin ATPase activity HIGH

↑ no. of myofilaments

White

Forceful & rapid movements

MUSCLE HYPERTROPHY

Definition:

When the total mass of a muscle increases, this is called Muscle Hypertrophy. The resulting muscle enlargement comes from an increase in diameter of the muscle fibers. It is in response

to a regular & intensive use of that particular muscle.Cause: An increase in the number of actin and myosin, causing an

increase in thickness of individual muscle fibers. Example: Body building

MUSCLE ATROPHY

Definition: When the total mass of a muscle decreases, it is called Muscle

Atrophy. If a muscle is not used, its actin and myosin content decreases, its filaments become smaller and the muscle decreases

in mass and becomes weaker. Physiologic Basis: 1. When the muscle is prevented from doing work even though the

nerve supply is intact. e.g. in bed-ridden patients, in a limb in a plaster of Paris cast. This type is thus called Disuse Atrophy.

2. When nerve supply to the muscle is lost. This can be due to an accident or when motor neurons supplying a muscle are destroyed .e.g. Poliomyelitis.

• Muscle fiber becomes thin & low in proteins, glycogen and ATP.• When muscle continuously shortened then sarcomeres at the end

of the muscle fiber actually disappear