Muscle contraction 1
Muscle contraction
Muscle fiber generates tension through the
action of actin and myosin cross-bridge
cycling. While under tension, the muscle
may lengthen, shorten or remain the same.
Although the term 'contraction' implies
shortening, when referring to the muscular
system, it means muscle fibers generating
tension with the help of motor neurons (the
terms twitch tension, twitch force and fiber
contraction are also used).
Voluntary muscle contraction is controlled
by the central nervous system. Voluntary
muscle contraction occurs as a result of
conscious effort originating in the brain. The
brain sends signals, in the form of action A top-down view of skeletal muscle
potentials, through the nervous system to the
motor neuron that innervates several muscle fibers. In the case of some reflexes, the signal to contract can originate
in the spinal cord through a feedback loop with the grey matter. Involuntary muscles such as the heart or smooth
muscles in the gut and vascular system contract as a result of non-conscious brain activity or stimuli proceeding in
the body to the muscle itself.
Contractions, by muscle type
For voluntary muscles, contraction occurs as a result of conscious effort originating in the brain. The brain sends
signals, in the form of action potentials, through the nervous system to the motor neuron that innervates several
muscle fibers [1] . In the case of some reflexes, the signal to contract can originate in the spinal cord through a
feedback loop with the grey matter. Involuntary muscles such as the heart or smooth muscles in the gut and vascular
system contract as a result of non-conscious brain activity or stimuli endogenous to the muscle itself. Other actions
such as locomotion, breathing and chewing have a reflex aspect to them: the contractions can be initiated
consciously or unconsciously.
There are three general types of muscle tissues:
• Skeletal muscle responsible for movement
• Cardiac muscle responsible for pumping blood
• Smooth muscle responsible for sustained contractions in the blood vessels, gastrointestinal tract, and other areas
in the body
Skeletal and cardiac muscles are called striated muscle because of their striped appearance under a microscope,
which is due to the highly organized alternating pattern of A band and I band.
While nerve impulse profiles are, for the most part, always the same, skeletal muscles are able to produce varying
levels of contractile force. This phenomenon can be best explained by Force Summation. Force Summation describes
the addition of individual twitch contractions to increase the intensity of overall muscle contraction. This can be
achieved in two ways [2] : (1) by increasing the number and size of contractile units simultaneously, called multiple
fiber summation, and (2) by increasing the frequency at which action potentials are sent to muscle fibers, called
frequency summation.
, Muscle contraction 2
• Multiple fiber summation – When a weak signal is sent by the CNS to contract a muscle, the smaller motor
units, being more excitable than the larger ones, are stimulated first. As the strength of the signal increases, more
motor units are excited in addition to larger ones, with the largest motor units having as much as 50 times the
contractile strength as the smaller ones. As more and larger motor units are activated, the force of muscle
contraction becomes progressively stronger. A concept known as the size principle allows for a gradation of
muscle force during weak contraction to occur in small steps, which then become progressively larger when
greater amounts of force are required.
• Frequency summation - For skeletal muscles, the force exerted by the muscle is controlled by varying the
frequency at which action potentials are sent to muscle fibers. Action potentials do not arrive at muscles
synchronously, and, during a contraction, some fraction of the fibers in the muscle will be firing at any given
time. In a typical circumstance, when a human is exerting a muscle as hard as he/she is consciously able, roughly
one-third of the fibers in that muscle will be firing at once, yet can be affected by various physiological and
psychological factors (including Golgi tendon organs and Renshaw cells). This 'low' level of contraction is a
protective mechanism to prevent avulsion of the tendon - the force generated by a 95% contraction of all fibers is
sufficient to damage the body.
Skeletal muscle contractions
Skeletal muscles contract according to
the sliding filament model:
1. An action potential originating in
the CNS reaches an alpha motor
neuron, which then transmits an
action potential down its own axon.
2. The action potential propagates by
activating voltage-gated sodium
channels along the axon toward the
synaptic cleft. Eventually, the
action potential reaches the motor
neuron terminal and causes a
calcium ion influx through the
voltage-gated calcium channels.
3. The Ca2+ influx causes vesicles
containing the neurotransmitter
acetylcholine to fuse with the plasma membrane, releasing acetylcholine out into the extracellular space between
the motor neuron terminal and the motor end plate of the skeletal muscle fiber.
4. The acetylcholine diffuses across the synapse and binds to and activates nicotinic acetylcholine receptors on the
motor end plate of
Muscle contraction
Muscle fiber generates tension through the
action of actin and myosin cross-bridge
cycling. While under tension, the muscle
may lengthen, shorten or remain the same.
Although the term 'contraction' implies
shortening, when referring to the muscular
system, it means muscle fibers generating
tension with the help of motor neurons (the
terms twitch tension, twitch force and fiber
contraction are also used).
Voluntary muscle contraction is controlled
by the central nervous system. Voluntary
muscle contraction occurs as a result of
conscious effort originating in the brain. The
brain sends signals, in the form of action A top-down view of skeletal muscle
potentials, through the nervous system to the
motor neuron that innervates several muscle fibers. In the case of some reflexes, the signal to contract can originate
in the spinal cord through a feedback loop with the grey matter. Involuntary muscles such as the heart or smooth
muscles in the gut and vascular system contract as a result of non-conscious brain activity or stimuli proceeding in
the body to the muscle itself.
Contractions, by muscle type
For voluntary muscles, contraction occurs as a result of conscious effort originating in the brain. The brain sends
signals, in the form of action potentials, through the nervous system to the motor neuron that innervates several
muscle fibers [1] . In the case of some reflexes, the signal to contract can originate in the spinal cord through a
feedback loop with the grey matter. Involuntary muscles such as the heart or smooth muscles in the gut and vascular
system contract as a result of non-conscious brain activity or stimuli endogenous to the muscle itself. Other actions
such as locomotion, breathing and chewing have a reflex aspect to them: the contractions can be initiated
consciously or unconsciously.
There are three general types of muscle tissues:
• Skeletal muscle responsible for movement
• Cardiac muscle responsible for pumping blood
• Smooth muscle responsible for sustained contractions in the blood vessels, gastrointestinal tract, and other areas
in the body
Skeletal and cardiac muscles are called striated muscle because of their striped appearance under a microscope,
which is due to the highly organized alternating pattern of A band and I band.
While nerve impulse profiles are, for the most part, always the same, skeletal muscles are able to produce varying
levels of contractile force. This phenomenon can be best explained by Force Summation. Force Summation describes
the addition of individual twitch contractions to increase the intensity of overall muscle contraction. This can be
achieved in two ways [2] : (1) by increasing the number and size of contractile units simultaneously, called multiple
fiber summation, and (2) by increasing the frequency at which action potentials are sent to muscle fibers, called
frequency summation.
, Muscle contraction 2
• Multiple fiber summation – When a weak signal is sent by the CNS to contract a muscle, the smaller motor
units, being more excitable than the larger ones, are stimulated first. As the strength of the signal increases, more
motor units are excited in addition to larger ones, with the largest motor units having as much as 50 times the
contractile strength as the smaller ones. As more and larger motor units are activated, the force of muscle
contraction becomes progressively stronger. A concept known as the size principle allows for a gradation of
muscle force during weak contraction to occur in small steps, which then become progressively larger when
greater amounts of force are required.
• Frequency summation - For skeletal muscles, the force exerted by the muscle is controlled by varying the
frequency at which action potentials are sent to muscle fibers. Action potentials do not arrive at muscles
synchronously, and, during a contraction, some fraction of the fibers in the muscle will be firing at any given
time. In a typical circumstance, when a human is exerting a muscle as hard as he/she is consciously able, roughly
one-third of the fibers in that muscle will be firing at once, yet can be affected by various physiological and
psychological factors (including Golgi tendon organs and Renshaw cells). This 'low' level of contraction is a
protective mechanism to prevent avulsion of the tendon - the force generated by a 95% contraction of all fibers is
sufficient to damage the body.
Skeletal muscle contractions
Skeletal muscles contract according to
the sliding filament model:
1. An action potential originating in
the CNS reaches an alpha motor
neuron, which then transmits an
action potential down its own axon.
2. The action potential propagates by
activating voltage-gated sodium
channels along the axon toward the
synaptic cleft. Eventually, the
action potential reaches the motor
neuron terminal and causes a
calcium ion influx through the
voltage-gated calcium channels.
3. The Ca2+ influx causes vesicles
containing the neurotransmitter
acetylcholine to fuse with the plasma membrane, releasing acetylcholine out into the extracellular space between
the motor neuron terminal and the motor end plate of the skeletal muscle fiber.
4. The acetylcholine diffuses across the synapse and binds to and activates nicotinic acetylcholine receptors on the
motor end plate of
