Structure and Function of Skeletal Muscle

Three Muscle Types

• Skeletal- striated• Cardiac- striated, intercalated discs

• Smooth- not striated

•All muscle tissue exhibit:• Responsiveness - The ability to receive and respond to a

stimulus• Conductivity – the ability of the impulse to travel along the

plasma membrane of the muscle cell.• Contractility - The ability to shorten

• Elasticity - The ability to recoil and resume original length

Skeletal Muscle

• Human body contains over 400 skeletal muscles• 40-50% of total body weight

• Functions of skeletal muscle• Force production for locomotion and breathing• Force production for postural support• Heat production during cold stress

The Neuromuscular Junction

• Site where motor neuron meets the muscle fiber• Separated by gap called the neuromuscular cleft

• Motor end plate• Pocket formed around motor neuron by sarcolemma

• Neurotransmitter is released from the motor neuron• Causes an end-plate potential (EPP)

• Depolarization of muscle fiber• Each axonal terminal forms a junction with a single muscle

fiber (cell)• Single neuron has several connections and thus stimulate

several muscle fibers

Motor Unit

• Consist of one nerve fiber and all the muscle fibers it innervate

• Behaves as a single functional unit and contract as one

Behavior of the whole muscle• Threshold – a minimal voltage necessary to produce a

muscle contraction• Muscle twitch – a single contraction of skeletal

muscle. It is composed of latent, contraction and relaxation periods• Latent period – the time between stimulation and

the beginning of contraction• Includes all time required for excitation,

excitation-contraction coupling, and stretching of the series elastic components

• Refractory period – a phase after a twitch during which the muscle can not respond to another stimulus

Force Regulation in Muscle• 2 ways to stimulate variable contraction strength of a

muscle:• Multiple motor unit summation - activate more

motor units. Increased stimulus force • Wave summation - stimulate a muscle at a higher

rate of stimuli delivered to the muscle• Second stimulus is administered before complete

relaxation of muscle• Treppe – a response to series of stimuli of the same

strength given before the muscle completely relaxed• Fatigue – results from a prolonged period of contraction.

Muscle loose the ability to contract

An initial stimulation and resulting twitch all by itself. Then we have 2 stimuli in somewhat rapid succession. The 2nd twitch has added on to the first. This is

known as wave or temporal summation. It occurs because there is still calcium from the 1st twitch in the sarcoplasm at the time of the 2nd twitch.

Muscle Fatigue

• Physiological inability to contract• Results primarily from a relative deficit of

ATP.

Types of Contractions

• Contractions can be:• Isometric

• Iso= same, metr=measure• Isotonic

• Iso=same, ton=tension

Isotonic Contractions

• In isotonic contractions, the muscle changes in length (decreasing the

angle of the joint) and moves the load (Iso=same, ton=tension)

• The two types of isotonic contractions are:

• Concentric contractions – the muscle shortens and does work

• Eccentric contractions – the muscle contracts as it lengthens

• Example – someone pulls your arm straight while at the same time

you try to keep the arm locked in one position.

Isometric Contractions

• Tension increases to the muscle’s capacity, but the muscle neither

shortens nor lengthens (Iso= same, metr=measure); postural muscles

• Occurs if the load is greater than the tension the muscle is able to

develop

• Example - carrying an object - your hands and arms would be

opposing the gravity with equal force going upwards. Since your

arms are neither raising or lowering, your biceps will be isometrically

contracting.

Isotonic and Isometric Contractions