Muscle
is 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.
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Types
There are three types of muscle:
- Smooth muscle or "involuntary muscle" is found within the walls of organs and structures such as the oesophagus, stomach, intestines, bronchi, uterus, ureters, bladder, and blood vessels.
- Cardiac muscle is also an "involuntary muscle" but it's a specialized kind of muscle found only within the heart.
- Skeletal muscle or "voluntary muscle" is anchored by tendons to the bone and is used to affect skeletal movement such as locomotion.
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 that are, in
order of increasing contractile speed:[1]
- 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 muscle contraction becomes painful (often attributed to a build-up of lactic acid). N.B. in some books and articles this muscle in humans was, confusingly, called type IIB.[2]
- 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, which involves muscles from the skeletal tissue, 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
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
Efferent leg
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.
Afferent leg
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
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. There are many diseases and
conditions which cause a decrease in muscle
mass, known as
atrophy. For example diseases such as cancer
and AIDS induce a body wasting syndrome called "cachexia",
which is notable for the severe
muscle atrophy seen. Other syndromes or
conditions which can induce skeletal
muscle atrophy are congestive heart disease
and liver disease. During aging, there is a
gradual decrease in the ability to maintain
skeletal muscle function and mass. This
condition is called "sarcopenia". The exact
cause of sarcopenia is unknown, but it may be
due to a combination of the gradual failure in
the "satellite cells" which help to regenerate
skeletal muscle fibers, and a decrease in
sensitivity to or the availability of critical
secreted growth factors which are necessary to
maintain muscle mass and satellite cell
survival. In addition to the simple loss
of muscle mass (atrophy),
or the age-related decrease in muscle function (sarcopenia),
there are other diseases which may be caused by
structural defects in the muscle (the
dystrophies), or by inflammatory reactions in
the body directed against muscle (the myopathies). 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. 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. 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). 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.Disease
The strongest human muscle
Efficiency
Muscle evolution
References
Notes
