Muscle is a soft tissue found in most animals.
Muscle cells contain
protein filaments of actin and myosin that slide past one another,
producing a contraction that changes both the length and the shape of
the cell. Muscles function to produce force and motion. They are
primarily responsible for maintaining and changing posture,
locomotion, as well as movement of internal organs, such as the
contraction of the heart and the movement of food through the
digestive system via peristalsis.
Muscle tissues are derived from the mesodermal layer of embryonic germ
cells in a process known as myogenesis. There are three types of
muscle, skeletal or striated, cardiac, and smooth.
Muscle action can
be classified as being either voluntary or involuntary. Cardiac and
smooth muscles contract without conscious thought and are termed
involuntary, whereas the skeletal muscles contract upon command.
Skeletal muscles in turn can be divided into fast and slow twitch
Muscles are predominantly powered by the oxidation of fats and
carbohydrates, but anaerobic chemical reactions are also used,
particularly by fast twitch fibers. These chemical reactions produce
adenosine triphosphate (ATP) molecules that are used to power the
movement of the myosin heads.
The term muscle is derived from the
Latin musculus meaning "little
mouse" perhaps because of the shape of certain muscles or because
contracting muscles look like mice moving under the skin.
1.3 Gross anatomy
1.4 Muscular system
2.2 Nervous control
2.4.1 Physiological strength
2.4.2 The "strongest" human muscle
4 Clinical significance
6 See also
8 External links
The anatomy of muscles includes gross anatomy, which comprises all the
muscles of an organism, and microanatomy, which comprises the
structures of a single muscle.
The body contains three types of muscle tissue: (a) skeletal muscle,
(b) smooth muscle, and (c) cardiac muscle. (Same magnification)
Muscle tissue is a soft tissue, and is one of the four fundamental
types of tissue present in animals. There are three types of muscle
tissue recognized in vertebrates:
Skeletal muscle or "voluntary muscle" is anchored by tendons (or by
aponeuroses at a few places) to bone and is used to effect skeletal
movement such as locomotion and in maintaining posture. Though this
postural control is generally maintained as an unconscious reflex, the
muscles responsible react to conscious control like non-postural
muscles. An average adult male is made up of 42% of skeletal muscle
and an average adult female is made up of 36% (as a percentage of body
Smooth muscle or "involuntary muscle" is found within the walls of
organs and structures such as the esophagus, stomach, intestines,
bronchi, uterus, urethra, bladder, blood vessels, and the arrector
pili in the skin (in which it controls erection of body hair). Unlike
skeletal muscle, smooth muscle is not under conscious control.
Cardiac muscle (myocardium), is also an "involuntary muscle" but is
more akin in structure to skeletal muscle, and is found only in the
Cardiac and skeletal muscles are "striated" in that they contain
sarcomeres that are packed into highly regular arrangements of
bundles; the myofibrils of smooth muscle cells are not arranged in
sarcomeres and so are not striated. While the sarcomeres in skeletal
muscles are arranged in regular, parallel bundles, cardiac muscle
sarcomeres connect at branching, irregular angles (called intercalated
discs). Striated muscle contracts and relaxes in short, intense
bursts, whereas smooth muscle sustains longer or even near-permanent
Skeletal (voluntary) muscle is further divided into two broad types:
slow twitch and fast twitch:
Type I, slow twitch, or "red" muscle, is dense with capillaries and is
rich in mitochondria and myoglobin, giving the muscle tissue its
characteristic red color. It can carry more oxygen and sustain aerobic
activity using fats or carbohydrates as fuel. Slow twitch fibers
contract for long periods of time but with little force.
Type II, fast twitch muscle, has three major subtypes (IIa, IIx, and
IIb) that vary in both contractile speed and force generated.
Fast twitch fibers contract quickly and powerfully but fatigue very
rapidly, sustaining only short, anaerobic bursts of activity before
muscle contraction becomes painful. They contribute most to muscle
strength and have greater potential for increase in mass. Type IIb is
anaerobic, glycolytic, "white" muscle that is least dense in
mitochondria and myoglobin. In small animals (e.g., rodents) this is
the major fast muscle type, explaining the pale color of their flesh.
The density of mammalian skeletal muscle tissue is about
1.06 kg/liter. This can be contrasted with the density of
adipose tissue (fat), which is 0.9196 kg/liter. This makes
muscle tissue approximately 15% denser than fat tissue.
Myocyte and Sarcomere
A skeletal muscle fiber is surrounded by a plasma membrane called the
sarcolemma, which contains sarcoplasm, the cytoplasm of muscle cells.
A muscle fiber is composed of many fibrils, which give the cell its
Skeletal muscles are sheathed by a tough layer of connective tissue
called the epimysium. The epimysium anchors muscle tissue to tendons
at each end, where the epimysium becomes thicker and collagenous. It
also protects muscles from friction against other muscles and bones.
Within the epimysium are multiple bundles called fascicles, each of
which contains 10 to 100 or more muscle fibers collectively sheathed
by a perimysium. Besides surrounding each fascicle, the perimysium is
a pathway for nerves and the flow of blood within the muscle. The
threadlike muscle fibers are the individual muscle cells (myocytes),
and each cell is encased within its own endomysium of collagen fibers.
Thus, the overall muscle consists of fibers (cells) that are bundled
into fascicles, which are themselves grouped together to form muscles.
At each level of bundling, a collagenous membrane surrounds the
bundle, and these membranes support muscle function both by resisting
passive stretching of the tissue and by distributing forces applied to
the muscle. Scattered throughout the muscles are muscle spindles
that provide sensory feedback information to the central nervous
system. (This grouping structure is analogous to the organization of
nerves which uses epineurium, perineurium, and endoneurium).
This same bundles-within-bundles structure is replicated within the
muscle cells. Within the cells of the muscle are myofibrils, which
themselves are bundles of protein filaments. The term "myofibril"
should not be confused with "myofiber", which is a simply another name
for a muscle cell. Myofibrils are complex strands of several kinds of
protein filaments organized together into repeating units called
sarcomeres. The striated appearance of both skeletal and cardiac
muscle results from the regular pattern of sarcomeres within their
cells. Although both of these types of muscle contain sarcomeres, the
fibers in cardiac muscle are typically branched to form a network.
Cardiac muscle fibers are interconnected by intercalated discs,
giving that tissue the appearance of a syncytium.
The filaments in a sarcomere are composed of actin and myosin.
See also: List of muscles of the human body
Bundles of muscle fibers, called fascicles, are covered by the
Muscle fibers are covered by the endomysium.
The gross anatomy of a muscle is the most important indicator of its
role in the body. There is an important distinction seen between
pennate muscles and other muscles. In most muscles, all the fibers are
oriented in the same direction, running in a line from the origin to
the insertion. However, In pennate muscles, the individual fibers are
oriented at an angle relative to the line of action, attaching to the
origin and insertion tendons at each end. Because the contracting
fibers are pulling at an angle to the overall action of the muscle,
the change in length is smaller, but this same orientation allows for
more fibers (thus more force) in a muscle of a given size. Pennate
muscles are usually found where their length change is less important
than maximum force, such as the rectus femoris.
Skeletal muscle is arranged in discrete muscles, an example of which
is the biceps brachii (biceps). The tough, fibrous epimysium of
skeletal muscle is both connected to and continuous with the tendons.
In turn, the tendons connect to the periosteum layer surrounding the
bones, permitting the transfer of force from the muscles to the
skeleton. Together, these fibrous layers, along with tendons and
ligaments, constitute the deep fascia of the body.
Main article: Muscular system
On the anterior and posterior views of the muscular system above,
superficial muscles (those at the surface) are shown on the right side
of the body while deep muscles (those underneath the superficial
muscles) are shown on the left half of the body. For the legs,
superficial muscles are shown in the anterior view while the posterior
view shows both superficial and deep muscles.
The muscular system consists of all the muscles present in a single
body. There are approximately 650 skeletal muscles in the human
body, but an exact number is difficult to define. The difficulty
lies partly in the fact that different sources group the muscles
differently and partly in that some muscles, such as palmaris longus,
are not always present.
A muscular slip is a narrow length of muscle that acts to augment a
larger muscle or muscles.
The muscular system is one component of the musculoskeletal system,
which includes not only the muscles but also the bones, joints,
tendons, and other structures that permit movement.
Main article: Myogenesis
A chicken embryo, showing the paraxial mesoderm on both sides of the
neural fold. The anterior (forward) portion has begun to form somites
(labeled "primitive segments").
All muscles are derived from paraxial mesoderm. The paraxial mesoderm
is divided along the embryo's length into somites, corresponding to
the segmentation of the body (most obviously seen in the vertebral
column. Each somite has 3 divisions, sclerotome (which forms
vertebrae), dermatome (which forms skin), and myotome (which forms
muscle). The myotome is divided into two sections, the epimere and
hypomere, which form epaxial and hypaxial muscles, respectively. The
only epaxial muscles in humans are the erector spinae and small
intervertebral muscles, and are innervated by the dorsal rami of the
spinal nerves. All other muscles, including those of the limbs are
hypaxial, and inervated by the ventral rami of the spinal nerves.
During development, myoblasts (muscle progenitor cells) either remain
in the somite to form muscles associated with the vertebral column or
migrate out into the body to form all other muscles. Myoblast
migration is preceded by the formation of connective tissue
frameworks, usually formed from the somatic lateral plate mesoderm.
Myoblasts follow chemical signals to the appropriate locations, where
they fuse into elongate skeletal muscle cells.
Main article: muscle contraction
The three types of muscle (skeletal, cardiac and smooth) have
significant differences. However, all three use the movement of actin
against myosin to create contraction. In skeletal muscle, contraction
is stimulated by electrical impulses transmitted by the nerves, the
motoneurons (motor nerves) in particular. Cardiac and smooth muscle
contractions are stimulated by internal pacemaker cells which
regularly contract, and propagate contractions to other muscle cells
they are in contact with. All skeletal muscle and many smooth muscle
contractions are facilitated by the neurotransmitter acetylcholine.
When a sarcomere contracts, the Z lines move closer together, and the
I band becomes smaller. The A band stays the same width. At full
contraction, the thin and thick filaments overlap.
The action a muscle generates is determined by the origin and
insertion locations. The cross-sectional area of a muscle (rather than
volume or length) determines the amount of force it can generate by
defining the number of "sarcomeres" which can operate in parallel.
Each skeletal muscle contains long units called myofibrils, and each
myofibril is a chain of sarcomeres. Since contraction occurs at the
same time for all connected sarcomeres in a muscles cell, these chains
of sarcomeres shorten together, thus shortening the muscle fiber,
resulting in overall length change. The amount of force applied to
the external environment is determined by lever mechanics,
specifically the ratio of in-lever to out-lever. For example, moving
the insertion point of the biceps more distally on the radius (farther
from the joint of rotation) would increase the force generated during
flexion (and, as a result, the maximum weight lifted in this
movement), but decrease the maximum speed of flexion. Moving the
insertion point proximally (closer to the joint of rotation) would
result in decreased force but increased velocity. This can be most
easily seen by comparing the limb of a mole to a horse - in the
former, the insertion point is positioned to maximize force (for
digging), while in the latter, the insertion point is positioned to
maximize speed (for running).
Simplified schema of basic nervous system function. Signals are picked
up by sensory receptors and sent to the spinal cord and brain via the
afferent leg of the peripheral nervous system, whereupon processing
occurs that results in signals sent back to the spinal cord and then
out to motor neurons via the efferent leg.
The efferent leg of the peripheral nervous system is responsible for
conveying commands to the muscles and glands, and is ultimately
responsible for voluntary movement. Nerves move muscles in response to
voluntary and autonomic (involuntary) signals from the brain. Deep
muscles, superficial muscles, muscles of the face and internal muscles
all correspond with dedicated regions in the primary motor cortex of
the brain, directly anterior to the central sulcus that divides the
frontal and parietal lobes.
In addition, muscles react to reflexive nerve stimuli that do not
always send signals all the way to the brain. In this case, the signal
from the afferent fiber does not reach the brain, but produces the
reflexive movement by direct connections with the efferent nerves in
the spine. However, the majority of muscle activity is volitional, and
the result of complex interactions between various areas of the brain.
Nerves that control skeletal muscles in mammals correspond with neuron
groups along the primary motor cortex of the brain's cerebral cortex.
Commands are routed though the basal ganglia and are modified by input
from the cerebellum before being relayed through the pyramidal tract
to the spinal cord and from there to the motor end plate at the
muscles. Along the way, feedback, such as that of the extrapyramidal
system contribute signals to influence muscle tone and response.
Deeper muscles such as those involved in posture often are controlled
from nuclei in the brain stem and basal ganglia.
Main article: Proprioception
In skeletal muscles, muscle spindles convey information about the
degree of muscle length and stretch to the central nervous system to
assist in maintaining posture and joint position. The sense of where
our bodies are in space is called proprioception, the perception of
body awareness, the "unconscious" awareness of where the various
regions of the body are located at any one time. Several areas in the
brain coordinate movement and position with the feedback information
gained from proprioception. The cerebellum and red nucleus in
particular continuously sample position against movement and make
minor corrections to assure smooth motion.
Main article: Bioenergetic systems
(a) Some ATP is stored in a resting muscle. As contraction starts, it
is used up in seconds. More ATP is generated from creatine phosphate
for about 15 seconds. (b) Each glucose molecule produces two ATP and
two molecules of pyruvic acid, which can be used in aerobic
respiration or converted to lactic acid. If oxygen is not available,
pyruvic acid is converted to lactic acid, which may contribute to
muscle fatigue. This occurs during strenuous exercise when high
amounts of energy are needed but oxygen cannot be sufficiently
delivered to muscle. (c) Aerobic respiration is the breakdown of
glucose in the presence of oxygen (O2) to produce carbon dioxide,
water, and ATP. Approximately 95 percent of the ATP required for
resting or moderately active muscles is provided by aerobic
respiration, which takes place in mitochondria.
Muscular activity accounts for much of the body's energy consumption.
All muscle cells produce adenosine triphosphate (ATP) molecules which
are used to power the movement of the myosin heads. Muscles have a
short-term store of energy in the form of creatine phosphate which is
generated from ATP and can regenerate ATP when needed with creatine
kinase. Muscles also keep a storage form of glucose in the form of
Glycogen can be rapidly converted to glucose when energy is
required for sustained, powerful contractions. Within the voluntary
skeletal muscles, the glucose molecule can be metabolized
anaerobically in a process called glycolysis which produces two ATP
and two lactic acid molecules in the process (note that in aerobic
conditions, lactate is not formed; instead pyruvate is formed and
transmitted through the citric acid cycle).
Muscle cells also contain
globules of fat, which are used for energy during aerobic exercise.
The aerobic energy systems take longer to produce the ATP and reach
peak efficiency, and requires many more biochemical steps, but
produces significantly more ATP than anaerobic glycolysis. Cardiac
muscle on the other hand, can readily consume any of the three
macronutrients (protein, glucose and fat) aerobically without a 'warm
up' period and always extracts the maximum ATP yield from any molecule
involved. The heart, liver and red blood cells will also consume
lactic acid produced and excreted by skeletal muscles during exercise.
At rest, skeletal muscle consumes 54.4 kJ/kg (13.0 kcal/kg)
per day. This is larger than adipose tissue (fat) at 18.8 kJ/kg
(4.5 kcal/kg), and bone at 9.6 kJ/kg (2.3 kcal/kg).
The efficiency of human muscle has been measured (in the context of
rowing and cycling) at 18% to 26%. The efficiency is defined as the
ratio of mechanical work output to the total metabolic cost, as can be
calculated from oxygen consumption. This low efficiency is the result
of about 40% efficiency of generating ATP from food energy, losses in
converting energy from ATP into mechanical work inside the muscle, and
mechanical losses inside the body. The latter two losses are dependent
on the type of exercise and the type of muscle fibers being used
(fast-twitch or slow-twitch). For an overall efficiency of 20 percent,
one watt of mechanical power is equivalent to 4.3 kcal per hour. For
example, one manufacturer of rowing equipment calibrates its rowing
ergometer to count burned calories as equal to four times the actual
mechanical work, plus 300 kcal per hour, this amounts to about 20
percent efficiency at 250 watts of mechanical output. The mechanical
energy output of a cyclic contraction can depend upon many factors,
including activation timing, muscle strain trajectory, and rates of
force rise & decay. These can be synthesized experimentally using
work loop analysis.
Muscle is a result of three factors that overlap: physiological
strength (muscle size, cross sectional area, available crossbridging,
responses to training), neurological strength (how strong or weak is
the signal that tells the muscle to contract), and mechanical strength
(muscle's force angle on the lever, moment arm length, joint
Main article: Physical strength
Grading of muscle strength
Trace of contraction, but no movement at the joint
Movement at the joint with gravity eliminated
Movement against gravity, but not against added resistance
Movement against external resistance, but less than normal
Vertebrate muscle typically produces approximately 25–33 N
(5.6–7.4 lbf) of force per square centimeter of muscle
cross-sectional area when isometric and at optimal length. Some
invertebrate muscles, such as in crab claws, have much longer
sarcomeres than vertebrates, resulting in many more sites for actin
and myosin to bind and thus much greater force per square centimeter
at the cost of much slower speed. The force generated by a contraction
can be measured non-invasively using either mechanomyography or
phonomyography, be measured in vivo using tendon strain (if a
prominent tendon is present), or be measured directly using more
The strength of any given muscle, in terms of force exerted on the
skeleton, depends upon length, shortening speed, cross sectional area,
pennation, sarcomere length, myosin isoforms, and neural activation of
motor units. Significant reductions in muscle strength can indicate
underlying pathology, with the chart at right used as a guide.
The "strongest" human muscle
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Since three factors affect muscular strength simultaneously and
muscles never work individually, it is misleading to compare strength
in individual muscles, and state that one is the "strongest". But
below are several muscles whose strength is noteworthy for different
In ordinary parlance, muscular "strength" usually refers to the
ability to exert a force on an external object—for example, lifting
a weight. By this definition, the masseter or jaw muscle is the
strongest. The 1992
Guinness Book of Records
Guinness Book of Records records the achievement
of a bite strength of 4,337 N (975 lbf) for 2 seconds. What
distinguishes the masseter is not anything special about the muscle
itself, but its advantage in working against a much shorter lever arm
than other muscles.
If "strength" refers to the force exerted by the muscle itself, e.g.,
on the place where it inserts into a bone, then the strongest muscles
are those with the largest cross-sectional area. This is because the
tension exerted by an individual skeletal muscle fiber does not vary
much. Each fiber can exert a force on the order of 0.3 micronewton. By
this definition, the strongest muscle of the body is usually said to
be the quadriceps femoris or the gluteus maximus.
Because muscle strength is determined by cross-sectional area, a
shorter muscle will be stronger "pound for pound" (i.e., by weight)
than a longer muscle of the same cross-sectional area. The myometrial
layer of the uterus may be the strongest muscle by weight in the
female human body. At the time when an infant is delivered, the entire
human uterus weighs about 1.1 kg (40 oz). During childbirth,
the uterus exerts 100 to 400 N (25 to 100 lbf) of downward force
with each contraction.
The external muscles of the eye are conspicuously large and strong in
relation to the small size and weight of the eyeball. It is frequently
said that they are "the strongest muscles for the job they have to do"
and are sometimes claimed to be "100 times stronger than they need to
be." However, eye movements (particularly saccades used on facial
scanning and reading) do require high speed movements, and eye muscles
are exercised nightly during rapid eye movement sleep.
The statement that "the tongue is the strongest muscle in the body"
appears frequently in lists of surprising facts, but it is difficult
to find any definition of "strength" that would make this statement
true. Note that the tongue consists of eight muscles, not one.
The heart has a claim to being the muscle that performs the largest
quantity of physical work in the course of a lifetime. Estimates of
the power output of the human heart range from 1 to 5 watts. This is
much less than the maximum power output of other muscles; for example,
the quadriceps can produce over 100 watts, but only for a few minutes.
The heart does its work continuously over an entire lifetime without
pause, and thus does "outwork" other muscles. An output of one watt
continuously for eighty years yields a total work output of two and a
Main article: Physical exercise
Exercise is often recommended as a means of improving motor skills,
fitness, muscle and bone strength, and joint function. Exercise has
several effects upon muscles, connective tissue, bone, and the nerves
that stimulate the muscles. One such effect is muscle hypertrophy, an
increase in size. This is used in bodybuilding.
Various exercises require a predominance of certain muscle fiber
utilization over another.
Aerobic exercise involves long, low levels
of exertion in which the muscles are used at well below their maximal
contraction strength for long periods of time (the most classic
example being the marathon). Aerobic events, which rely primarily on
the aerobic (with oxygen) system, use a higher percentage of Type I
(or slow-twitch) muscle fibers, consume a mixture of fat, protein and
carbohydrates for energy, consume large amounts of oxygen and produce
little lactic acid.
Anaerobic exercise involves short bursts of higher
intensity contractions at a much greater percentage of their maximum
contraction strength. Examples of anaerobic exercise include sprinting
and weight lifting. The anaerobic energy delivery system uses
predominantly Type II or fast-twitch muscle fibers, relies mainly on
ATP or glucose for fuel, consumes relatively little oxygen, protein
and fat, produces large amounts of lactic acid and can not be
sustained for as long a period as aerobic exercise. Many exercises are
partially aerobic and partially anaerobic; for example, soccer and
rock climbing involve a combination of both.
The presence of lactic acid has an inhibitory effect on ATP generation
within the muscle; though not producing fatigue, it can inhibit or
even stop performance if the intracellular concentration becomes too
high. However, long-term training causes neovascularization within the
muscle, increasing the ability to move waste products out of the
muscles and maintain contraction. Once moved out of muscles with high
concentrations within the sarcomere, lactic acid can be used by other
muscles or body tissues as a source of energy, or transported to the
liver where it is converted back to pyruvate. In addition to
increasing the level of lactic acid, strenuous exercise causes the
loss of potassium ions in muscle and causing an increase in potassium
ion concentrations close to the muscle fibres, in the interstitium.
Acidification by lactic acid may allow recovery of force so that
acidosis may protect against fatigue rather than being a cause of
Delayed onset muscle soreness is pain or discomfort that may be felt
one to three days after exercising and generally subsides two to three
days later. Once thought to be caused by lactic acid build-up, a more
recent theory is that it is caused by tiny tears in the muscle fibers
caused by eccentric contraction, or unaccustomed training levels.
Since lactic acid disperses fairly rapidly, it could not explain pain
experienced days after exercise.
Jogging is one form of aerobic exercise.
Humans are genetically predisposed with a larger percentage of one
type of muscle group over another. An individual born with a greater
percentage of Type I muscle fibers would theoretically be more suited
to endurance events, such as triathlons, distance running, and long
cycling events, whereas a human born with a greater percentage of Type
II muscle fibers would be more likely to excel at sprinting events
such as 100 meter dash.
Independent of strength and performance measures, muscles can be
induced to grow larger by a number of factors, including hormone
signaling, developmental factors, strength training, and disease.
Contrary to popular belief, the number of muscle fibres cannot be
increased through exercise. Instead, muscles grow larger through a
combination of muscle cell growth as new protein filaments are added
along with additional mass provided by undifferentiated satellite
cells alongside the existing muscle cells.
Biological factors such as age and hormone levels can affect muscle
hypertrophy. During puberty in males, hypertrophy occurs at an
accelerated rate as the levels of growth-stimulating hormones produced
by the body increase. Natural hypertrophy normally stops at full
growth in the late teens. As testosterone is one of the body's major
growth hormones, on average, men find hypertrophy much easier to
achieve than women. Taking additional testosterone or other anabolic
steroids will increase muscular hypertrophy.
Muscular, spinal and neural factors all affect muscle building.
Sometimes a person may notice an increase in strength in a given
muscle even though only its opposite has been subject to exercise,
such as when a bodybuilder finds her left biceps stronger after
completing a regimen focusing only on the right biceps. This
phenomenon is called cross education.
Prisoner of war exhibiting muscle loss as a result of malnutrition.
Muscles may atrophy as a result of malnutrition, physical inactivity,
aging, or disease.
Inactivity and starvation in mammals lead to atrophy of skeletal
muscle, a decrease in muscle mass that may be accompanied by a smaller
number and size of the muscle cells as well as lower protein
Muscle atrophy may also result from the natural aging
process or from disease.
In humans, prolonged periods of immobilization, as in the cases of bed
rest or astronauts flying in space, are known to result in muscle
weakening and atrophy.
Atrophy is of particular interest to the manned
spaceflight community, because the weightlessness experienced in
spaceflight results is a loss of as much as 30% of mass in some
muscles. Such consequences are also noted in small hibernating
mammals like the golden-mantled ground squirrels and brown bats.
During aging, there is a gradual decrease in the ability to maintain
skeletal muscle function and mass, known as sarcopenia. The exact
cause of sarcopenia is unknown, but it may be due to a combination of
the gradual failure in the "satellite cells" that help to regenerate
skeletal muscle fibers, and a decrease in sensitivity to or the
availability of critical secreted growth factors that are necessary to
maintain muscle mass and satellite cell survival.
Sarcopenia is a
normal aspect of aging, and is not actually a disease state yet can be
linked to many injuries in the elderly population as well as
decreasing quality of life.
There are also many diseases and conditions that cause muscle atrophy.
Examples include cancer and AIDS, which induce a body wasting syndrome
called cachexia. Other syndromes or conditions that can induce
skeletal muscle atrophy are congestive heart disease and some diseases
of the liver.
Main article: Neuromuscular disease
In muscular dystrophy, the affected tissues become disorganized and
the concentration of dystrophin (green) is greatly reduced.
Neuromuscular diseases are those that affect the muscles and/or their
nervous control. In general, problems with nervous control can cause
spasticity or paralysis, depending on the location and nature of the
problem. A large proportion of neurological disorders, ranging from
cerebrovascular accident (stroke) and
Parkinson's disease to
Creutzfeldt–Jakob disease, can lead to problems with movement or
Symptoms of muscle diseases may include weakness, spasticity,
myoclonus and myalgia. Diagnostic procedures that may reveal muscular
disorders include testing creatine kinase levels in the blood and
electromyography (measuring electrical activity in muscles). In some
cases, muscle biopsy may be done to identify a myopathy, as well as
genetic testing to identify
DNA abnormalities associated with specific
myopathies and dystrophies.
A non-invasive elastography technique that measures muscle noise is
undergoing experimentation to provide a way of monitoring
neuromuscular disease. The sound produced by a muscle comes from the
shortening of actomyosin filaments along the axis of the muscle.
During contraction, the muscle shortens along its longitudinal axis
and expands across the transverse axis, producing vibrations at the
The evolutionary origin of muscle cells in metazoans is a highly
debated topic. In one line of thought scientists have believed that
muscle cells evolved once and thus all animals with muscles cells have
a single common ancestor. In the other line of thought, scientists
believe muscles cells evolved more than once and any morphological or
structural similarities are due to convergent evolution and genes that
predate the evolution of muscle and even the mesoderm - the germ layer
from which many scientists believe true muscle cells derive.
Schmid and Seipel argue that the origin of muscle cells is a
monophyletic trait that occurred concurrently with the development of
the digestive and nervous systems of all animals and that this origin
can be traced to a single metazoan ancestor in which muscle cells are
present. They argue that molecular and morphological similarities
between the muscles cells in cnidaria and ctenophora are similar
enough to those of bilaterians that there would be one ancestor in
metazoans from which muscle cells derive. In this case, Schmid and
Seipel argue that the last common ancestor of bilateria, ctenophora,
and cnidaria was a triploblast or an organism with three germ layers
and that diploblasty, meaning an organism with two germ layers,
evolved secondarily due to their observation of the lack of mesoderm
or muscle found in most cnidarians and ctenophores. By comparing the
morphology of cnidarians and ctenophores to bilaterians, Schmid and
Seipel were able to conclude that there were myoblast-like structures
in the tentacles and gut of some species of cnidarians and in the
tentacles of ctenophores. Since this is a structure unique to muscle
cells, these scientists determined based on the data collected by
their peers that this is a marker for striated muscles similar to that
observed in bilaterians. The authors also remark that the muscle cells
found in cnidarians and ctenophores are often contests due to the
origin of these muscle cells being the ectoderm rather than the
mesoderm or mesendoderm. The origin of true muscles cells is argued by
others to be the endoderm portion of the mesoderm and the endoderm.
However, Schmid and Seipel counter this skepticism about whether or
not the muscle cells found in ctenophores and cnidarians are true
muscle cells by considering that cnidarians develop through a medusa
stage and polyp stage. They observe that in the hydrozoan medusa stage
there is a layer of cells that separate from the distal side of the
ectoderm to form the striated muscle cells in a way that seems similar
to that of the mesoderm and call this third separated layer of cells
the ectocodon. They also argue that not all muscle cells are derived
from the mesendoderm in bilaterians with key examples being that in
both the eye muscles of vertebrates and the muscles of spiralians
these cells derive from the ectodermal mesoderm rather than the
endodermal mesoderm. Furthermore, Schmid and Seipel argue that since
myogenesis does occur in cnidarians with the help of molecular
regulatory elements found in the specification of muscles cells in
bilaterians that there is evidence for a single origin for striated
In contrast to this argument for a single origin of muscle cells,
Steinmetz et al. argue that molecular markers such as the myosin II
protein used to determine this single origin of striated muscle
actually predate the formation of muscle cells. This author uses an
example of the contractile elements present in the porifera or sponges
that do truly lack this striated muscle containing this protein.
Furthermore, Steinmetz et al. present evidence for a polyphyletic
origin of striated muscle cell development through their analysis of
morphological and molecular markers that are present in bilaterians
and absent in cnidarians, ctenophores, and bilaterians. Steimetz et
al. showed that the traditional morphological and regulatory markers
such as actin, the ability to couple myosin side chains
phosphorylation to higher concentrations of the positive
concentrations of calcium, and other MyHC elements are present in all
metazoans not just the organisms that have been shown to have muscle
cells. Thus, the usage of any of these structural or regulatory
elements in determining whether or not the muscle cells of the
cnidarians and ctenophores are similar enough to the muscle cells of
the bilaterians to confirm a single lineage is questionable according
to Steinmetz et al. Furthermore, Steinmetz et al. explain that the
orthologues of the MyHc genes that have been used to hypothesize the
origin of striated muscle occurred through a gene duplication event
that predates the first true muscle cells (meaning striated muscle),
and they show that the MyHc genes are present in the sponges that have
contractile elements but no true muscle cells. Furthermore, Steinmetz
et all showed that the localization of this duplicated set of genes
that serve both the function of facilitating the formation of striated
muscle genes and cell regulation and movement genes were already
separated into striated myhc and non-muscle myhc. This separation of
the duplicated set of genes is shown through the localization of the
striated myhc to the contractile vacuole in sponges while the
non-muscle myhc was more diffusely expressed during developmental cell
shape and change. Steinmetz et al. found a similar pattern of
localization in cnidarians with except with the cnidarian N. vectensis
having this striated muscle marker present in the smooth muscle of the
digestive track. Thus, Steinmetz et al. argue that the pleisiomorphic
trait of the separated orthologues of myhc cannot be used to determine
the monophylogeny of muscle, and additionally argue that the presence
of a striated muscle marker in the smooth muscle of this cnidarian
shows a fundamentally different mechanism of muscle cell development
and structure in cnidarians.
Steinmetz et al. continue to argue for multiple origins of striated
muscle in the metazoans by explaining that a key set of genes used to
form the troponin complex for muscle regulation and formation in
bilaterians is missing from the cnidarians and ctenophores, and of 47
structural and regulatory proteins observed, Steinmetz et al. were not
able to find even on unique striated muscle cell protein that was
expressed in both cnidarians and bilaterians. Furthermore, the Z-disc
seemed to have evolved differently even within bilaterians and there
is a great deal diversity of proteins developed even between this
clade, showing a large degree of radiation for muscle cells. Through
this divergence of the Z-disc, Steimetz et al. argue that there are
only four common protein components that were present in all
bilaterians muscle ancestors and that of these for necessary Z-disc
components only an actin protein that they have already argued is an
uninformative marker through its pleisiomorphic state is present in
cnidarians. Through further molecular marker testing, Steinmetz et al.
observe that non-bilaterians lack many regulatory and structural
components necessary for bilaterians muscle formation and do not find
any unique set of proteins to both bilaterians and cnidarians and
ctenophores that are not present in earlier, more primitive animals
such as the sponges and amoebozoans. Through this analysis the authors
conclude that due to the lack of elements that bilaterians muscles are
dependent on for structure and usage, nonbilaterian muscles must be of
a different origin with a different set regulatory and structural
In another take on the argument, Andrikou and Arnone use the newly
available data on gene regulatory networks to look at how the
hierarchy of genes and morphogens and other mechanism of tissue
specification diverge and are similar among early deuterostomes and
protostomes. By understanding not only what genes are present in all
bilaterians but also the time and place of deployment of these genes,
Andrikou and Arnone discuss a deeper understanding of the evolution of
In their paper Andrikou and Arnone argue that to truly understand the
evolution of muscle cells the function of transcriptional regulators
must be understood in the context of other external and internal
interactions. Through their analysis, Andrikou and Arnone found that
there were conserved orthologues of the gene regulatory network in
both invertebrate bilaterians and in cnidarians. They argue that
having this common, general regulatory circuit allowed for a high
degree of divergence from a single well functioning network. Andrikou
and Arnone found that the orthologues of genes found in vertebrates
had been changed through different types of structural mutations in
the invertebrate deuterostomes and protostomes, and they argue that
these structural changes in the genes allowed for a large divergence
of muscle function and muscle formation in these species. Andrikou and
Arnone were able to recognize not only any difference due to mutation
in the genes found in vertebrates and invertebrates but also the
integration of species specific genes that could also cause divergence
from the original gene regulatory network function. Thus, although a
common muscle patterning system has been determined, they argue that
this could be due to a more ancestral gene regulatory network being
coopted several times across lineages with additional genes and
mutations causing very divergent development of muscles. Thus it seems
that myogenic patterning framework may be an ancestral trait. However,
Andrikou and Arnone explain that the basic muscle patterning structure
must also be considered in combination with the cis regulatory
elements present at different times during development. In contrast
with the high level of gene family apparatuses structure, Andrikou and
Arnone found that the cis regulatory elements were not well conserved
both in time and place in the network which could show a large degree
of divergence in the formation of muscle cells. Through this analysis,
it seems that the myogenic GRN is an ancestral GRN with actual changes
in myogenic function and structure possibly being linked to later
coopts of genes at different times and places.
Evolutionarily, specialized forms of skeletal and cardiac muscles
predated the divergence of the vertebrate/arthropod evolutionary
line. This indicates that these types of muscle developed in a
common ancestor sometime before 700 million years ago (mya).
Vertebrate smooth muscle was found to have evolved independently from
the skeletal and cardiac muscle types.
Electroactive polymers—materials that behave like muscles, used in
Rohmert's law—pertaining to muscle fatigue
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Look up muscle in Wiktionary, the free dictionary.
Media related to muscles at Wikimedia Commons
University of Dundee article on performing neurological examinations
Muscle efficiency in rowing
Muscle Physiology and Modeling
Scholarpedia Tsianos and Loeb (2013)
Muscle Tutorial (clear pictures of main human muscles and their
Latin names, good for orientation)
Microscopic stains of skeletal and cardiac muscular fibers to show
striations. Note the differences in myofibrilar arrangements.
Dermal tissue: Epidermis
Vascular tissue: Phloem
Ground tissue: Parenchyma
Meristematic tissue: Primary
Anatomical terms of muscle
List of muscles of the human body
Vascular smooth muscle
Laminin, alpha 2
(a, i, and h bands;
z and m lines)
Connective tissue in skeletal muscle
Sliding filament mechanism
Muscles of the head
Levator palpebrae superioris
Depressor septi nasi
Levator labii superioris
Levator labii superioris alaeque nasi
Levator anguli oris
Levator labii superioris
Depressor anguli oris
Depressor labii inferioris
Muscles of the neck
Rectus capitis anterior muscle
Rectus capitis lateralis muscle
Rectus capitis posterior
Deep cervical fascia
Muscles of the thorax and back
Muscles and ligaments of abdomen and pelvis
Abdominal external oblique
Abdominal internal oblique
Fascia of Camper
Fascia of Scarpa
Deep inguinal ring
Superficial inguinal ring
Crura of superficial inguinal ring
Muscles of the arm
flexor carpi radialis
flexor carpi ulnaris
flexor digitorum superficialis
flexor digitorum profundus
flexor pollicis longus
extensor carpi radialis longus and brevis
extensor digiti minimi
extensor carpi ulnaris
anatomical snuff box
abductor pollicis longus
extensor pollicis brevis
extensor pollicis longus
flexor pollicis brevis
abductor pollicis brevis
opponens digiti minimi
flexor digiti minimi brevis
abductor digiti minimi
Muscles of the hip
Muscles of the hip and human leg
psoas major/psoas minor
tensor fasciae latae
lateral rotator group:
Lateral intermuscular septum of thigh
Medial intermuscular septum of thigh
extensor hallucis longus
extensor digitorum longus
flexor hallucis longus
flexor digitorum longus
extensor hallucis brevis
extensor digitorum brevis
flexor digitorum brevis
abductor digiti minimi
flexor hallucis brevis
flexor digiti minimi brevis