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
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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
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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
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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
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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
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Contractions can be:
• Isometric
• Iso= same, metr=measure
• Isotonic
• Iso=same, ton=tension
Isotonic Contractions
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In isotonic contractions, the muscle changes in length (decreasing the
angle of the joint) and moves the load (Iso=same, ton=tension)
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The two types of isotonic contractions are:
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Concentric contractions – the muscle shortens and does work
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Eccentric contractions – the muscle contracts as it lengthens
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Example – someone pulls your arm straight while at the same time
you try to keep the arm locked in one position.
Isometric Contractions
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Tension increases to the muscle’s capacity, but the muscle neither
shortens nor lengthens (Iso= same, metr=measure); postural muscles
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Occurs if the load is greater than the tension the muscle is able to
develop
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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