Muscle
 |
A top-down view of skeletal muscle |
Muscle (from
Latin musculus "little mouse", referring to muscles like the biceps which pop up as though a mouse were scurrying about under the skin [
1]) 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.
There are three types of muscle:
*
Skeletal muscle or "voluntary muscle" is anchored by
tendons to the
bone and is used to affect
skeletal movement such as
locomotion.
*
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, and unlike skeletal muscle, smooth muscle is not under conscious control.
*
Cardiac muscle is also an "involuntary muscle" but it's a specialized kind of muscle found only within the
heart. The biggest difference between cardiac muscles and other muscles is that cardiac muscles create their own electrical shock, giving the heart complete control over itself.
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:
**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.
[ Note: Access to full text requires subscription; abstract freely available]**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.
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.
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.
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.
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 anaerobic events such as a 200 meter dash, or weight lifting.
Disease
For more detail, also see Neuromuscular diseaseThere 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 ×10
9 joules.
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). This can be improved by using muscles with machines, a human riding a
bicycle is very efficient (more than a combustion powered vehicle), travelling great distances with relatively little fuel.
According to a recent study published in 1999 [
2], 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.
* Costill, David L and Wilmore, Jack H. (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
*
Johnson George B. (2005) "Biology, Visualizing Life."Holt, Rinehart, and Winston. ISBN: 0-03-016723-X
Notes
*
Physics factbook (Heart output 1.3 to 5 watts, lifetime output 2 to 3 ×10
9 joules)
*
Alaska optometric association (External eye muscles "100 times as strong as they need to be")
*
course notes for a Virginia Commonwealth dance course (Quadriceps "strongest")
*
University of Dundee article on performing neurological examinations (Quadriceps "strongest")
*
Muscle efficiency in rowing*
"Gatorade Sports Science Institute" on muscle efficiency in cyclists (PDF)*
Trigger Point Therapy for Myofascial Pain*
Human Muscle Tutorial (clear pictures of main human muscles and their latin names, good for orientation)
*
MUSCLE STRUCTURE & FUNCTION*
List of muscles of the human body*
Myopathy (Pathology of muscle cells)
*
Myotomy*
Rapid plant movement*
Atrophy*
Muscle atrophy *
Muscle tone (residual muscle tension)
*
Skeletal Muscle *
Electroactive polymers (materials that behave like muscles, used in robotics research)
