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A top-down view of skeletal muscle
Muscle is the
contractile
tissue of the body and is derived from the
mesodermal layer of embryonic germ cells. Its function is to produce
force and
cause motion,
either locomotion or movement within
internal organs. Much of muscle contraction occurs without
conscious thought and is necessary for survival, like the contraction
of the heart
or
peristalsis, which pushes food through the
digestive system. Voluntary muscle contraction is used to move the
body and can be finely controlled, such as movements of the finger or
gross movements like the
quadriceps muscle of the
thigh.
Types
There are three general types of muscle:
Cardiac and skeletal muscle are "striated" in that they contain
sarcomere and are packed into highly-regular arrangements of bundles;
smooth muscle has neither. Striated muscle is often used in short, intense
bursts, whereas smooth muscle sustains longer or even near-permanent
contractions.
Skeletal muscle is further divided into several subtypes:
- Type I, slow oxidative, 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.
- Type II, fast twitch, muscle has three major kinds (Larsson
et al. 1991. Am J Physiol. 261:C93-101) that are, in order of increasing
contractile speed:
- a) Type IIa, which, like slow muscle, is aerobic, rich in
mitochondria and capillaries and appears red.
- b) Type IIx (also known as type IId), which is less dense in
mitochondria and myoglobin. This is the fastest muscle type in humans.
It can contract more quickly and with a greater amount of force than
oxidative muscle, but can sustain only short,
anaerobic bursts of activity before a build-up of
lactic acid in tissue begins to interfere with muscular contraction
and causes pain. N.B. in some books and articles this muscle in humans
was, confusingly, called type IIB (Smerdu et al. 1994. Am J Physiol.
267:C1723-8).
- c) Type IIb, which is anaerobic, glycolytic, "white" muscle that is
even less dense in mitochondria and myoglobin. In small animals like
rodents or rabbits this is the major fast muscle type, explaining the
pale color of their meat.
Anatomy
Muscle is composed of muscle
cells (sometimes known as "muscle
fibers"). Within the cells are
myofibrils; myofibrils contain
sarcomeres, which are composed of
actin and
myosin.
Individual muscle cells are lined with
endomysium. Muscle cells are bound together by
perimysium into bundles called
fascicles; the bundles are then grouped together to form muscle, which
is lined by
epimysium.
Muscle spindles are distributed throughout the muscles and provide
sensory feedback information to the
central nervous system.
Skeletal muscle is arranged in discrete groups, examples of which
include the
biceps brachii. It is connected by
tendons
to processes of the
skeleton.
In contrast, smooth muscle occurs at various scales in almost every organ,
from the skin
(in which it controls erection of
body
hair) to the
blood vessels and
digestive tract (in which it controls the caliber of a
lumen and
peristalsis).
There are approximately 650 skeletal muscles in the human body (see
list of muscles of the human body). Contrary to popular belief, the
number of muscle fibers cannot be increased through
exercise;
instead the muscle cells simply get bigger. It is however believed that
myofibrils have a limited capacity for growth through
hypertrophy and will split if subject to increased demand.
Physiology
- Main article:
muscle contraction
The ten types of muscle have significant differences. However, all but
three use the movement of
actin
against
myosin to create
contraction and relaxation. In skeletal muscle, contraction is
stimulated by
electrical impulses transmitted by the
nerves, the
motor nerves and
motoneurons in particular. All skeletal muscle and many smooth muscle
contractions are facilitated by the
neurotransmitter
acetylcholine.
Muscular activity accounts for most of the body's
energy
consumption. Muscles store energy for their own use in the form of
glycogen,
which represents about 1% of their mass. This can be rapidly converted to
glucose
when more energy is necessary.
Nervous control
Vertebrates move muscles in response to
voluntary and
autonomic signals from the
brain. Deep
muscles, superficial muscles,
muscles of the face and internal muscles all correspond with dedicated
regions in the brain.
In addition, muscles react to
reflexive nerve stimuli that do not always send signals all the way to
the brain, but most muscle activity is 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 loops 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.
Sometimes known as
muscle memory, the
sense of
where our bodies are in space is called
proprioception, the perception of body awareness. More easily
demonstrated than explained, proprioception is the "unconscious" awareness
of where the various regions of the body are located at any one time. This
can be demonstrated by anyone closing their eyes and waving their hand
around. Assuming proper proprioceptive function, at no time will the
person lose awareness of where the hand actually is, even though it is not
being detected by any of the other senses.
Several areas in the brain coordinate movement and position with the
feedback information gained from proprioception. The
cerebellum and
nucleus ruber in particular continuously sample position against
movement and make minor corrections to assure a smooth projection.
Role in health and disease
Exercise
Exercise is often recommended as a means of improving
motor skills,
fitness and muscle strength. Exercise has several effects upon
muscles,
connective tissue and
bone, and the
nerves that stimulate the muscles.
Various exercises require a predominance of certain muscle fiber
utilization over another. Aerobic events, which rely primarily on the
aerobic system, use a higher percentage of TYPE I or (slow-twitch) muscle
fibers. Shorter events, which rely on the anaerobic energy delivery
system, use predominantly TYPE II muscle fibers, or (fast-twitch) muscle
fibers.
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 adept at 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 anerobic events such as a 200 meter dash,
or weight lifting.
Disease
- For more detail, see
Neuromuscular disease
Symptoms of muscle disease may include
weakness
or
spasticity/rigidity,
myoclonus (twitching) and
myalgia
(muscle pain). 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.
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 leads to problems with movement, ranging from
cerebrovascular accident (stroke) and
Parkinson's disease to
Creutzfeldt-Jakob disease.
The strongest human muscle
Depending on what definition of "strongest" is used, many different
muscles in the human body can be characterized as being the "strongest."
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 records the achievement of a bite strength of
975 lbf (4337
N) for
two 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 at their belly. This is
because the tension exerted by an individual skeletal (striated)
muscle fiber does not vary much, either from muscle to muscle, or with
length. Each fiber can exert a force on the order of 0.3 micronewtons. By
this definition, the strongest muscle of the body is usually said to be
the
Quadriceps femoris or the
Gluteus maximus.
Again taking strength to mean only "force" (in the
physicist's sense, and as contrasted with "energy"
or "power"),
then a shorter muscle will be stronger "pound for pound" (i.e., by
weight)
than a longer muscle. The
uterus
may be the strongest muscle by weight in the human body. At the time when
an infant
is delivered, the human uterus weighs about 40 oz (1.1 kg). During
childbirth, the uterus exerts 25 to 100 lbf (100 to 400 N) 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." Eye movements, however, probably do "need" to be
exceptionally fast.
The unexplained 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
sixteen muscles, not one. The tongue may possibly be the strongest muscle
at birth.
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 can
"outwork" other muscles. An output of one watt continuously for seventy
years yields a total work output of 2 to 3 ×109
joules.
There is no such thing as
dense muscles, although some claim there are.
Efficiency
The
efficiency of human muscle has been measured (in the context of
rowing and
cycling)
at 14% to 27%. The efficiency is defined as the ratio of
mechanical work done to the total energy output (heat plus work).
Muscle evolution
According to a recent study published in 1999
[1], 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 (smooth muscle found in humans) was found to have evolved
independently from the skeletal and cardiac muscles.
References
- Costill, Jack H. and Wilmore, David L. (2004). Physiology of
Sport and Exercise. Champaign, Illinois: Human Kinetics.
ISBN 0736044892.
- Phylogenetic Relationship of Muscle Tissues Deduced from
Superimposition of Gene Trees, Satoshi OOta and Naruya Saitou, Mol.
Biol. Evol. 16(6) 856-7, 1999
External links
See also
