Muscle (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.

Types

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 esophagus, 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.

Cardiac and skeletal muscle are "striated" in that they contain sarcomere and are packed into highly-regular arrangements of bundles; smooth muscle has neither. While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects at branching, irregular angles. Striated muscle contracts and relaxes 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 incorrectly attributed to a build-up of lactic acid). N.B. in some books and articles this muscle in humans was, confusingly, called type IIB.
  • 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 hyperplasia if subject to increased demand.

Physiology

Main article: muscle contraction

The three (skeletal, cardiac and smooth) types of muscle 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 motor nerves and motoneurons in particular. Cardiac and smooth muscle contractions are stimulated by internal pacemaker cells who regularly contract, and propogate contractions to other muscle cells they are in contact with. 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 energy is required for sustained, powerful contractions. Within the voluntary skeletal muscles, the glucose molecule is split into two-three carbon molecules known as Lactic acid, producing two Adenosine triphosphate (ATP) molecules in the process, which are then used to power the movement of the myosin heads. Muscle cells also contain globules of fat, which are used for energy during aerobic exercise. The aerobic energy systems takes longer to produce the ATP and reach peak efficiency, and requires many more biochemical steps, but produces significantly more ATP than a glucose molecule. Cardiac muscle on the other hand, can readily consume any of the three macronutrients (protein, glucose and fat) without a 'warm up' period and always extracts the maximum ATP yeild from any molecule involved. The heart will also consume lactic acid produced and excreted by skeletal muscles during exercise.

Nervous control

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 brian, 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 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

The afferent leg of the peripheral nervous system is responsible for conveying sensory information to the brain, primarily from the sense organs like the skin. In the muscles, the 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. 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 smooth motion.

Role in health and disease

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 and bone, and the nerves that stimulate the muscles.

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 muscle fibers, or (fast-twitch) muscle fibers, rely mainly on Adenosine triphosphate or glucose for fuel, consume relatively little oxygen, protein and fat, produces larger amounts of lactic acid and can not be sustained for as long a period as 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 anaerobic events such as a 200 meter dash, or weight lifting. People with high overall musculation and balanced muscle type percentage engage in sports such as Rugby, or Boxing, and often engage on other sports just to increase their performance on the former.

Delayed onset muscle soreness is the pain or discomfort often felt 24 to 76 hours after exercising and subsides generally within 2 to 3 days. Once thought to be caused by lactic acid buildup, a more recent theory is that it is caused by tiny tears in the muscle fibres caused by eccentric movements, or unaccustomed training levels. The reason for the demise of the lactic acid theory was that since lactic acid disperses fairly rapidly, it could not explain pain felt the next day.

Disease

Main article: Neuromuscular disease

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.

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 micronewton. 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 does "outwork" other muscles. An output of one watt continuously for seventy years yields a total work output of 2 to 3 ×10⁹ joules.

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). 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.

Muscle evolution

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 (smooth muscle found in humans) was found to have evolved independently from the skeletal and cardiac muscles.

References

• Costill, David L and Wilmore, Jack H. (2004). Physiology of Sport and Exercise. Champaign, Illinois: Human Kinetics. ISBN 0-7360-4489-2.
• 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

External links

• Physics factbook (Heart output 1.3 to 5 watts, lifetime output 2 to 3 ×10⁹ joules)
• University of Dundee article on performing neurological examinations (Quadriceps "strongest")
• Muscle efficiency in rowing
• "Gatorade Sports Science Institute" on muscle efficiency in cyclists (PDF)
• Human Muscle Tutorial (clear pictures of main human muscles and their latin names, good for orientation)
• MUSCLE STRUCTURE & FUNCTION

See also

• Bodybuilding
• 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)
• Characteristics of muscle