Muscle

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Muscle (from Latin musculus "little mouse" ^[1]) is contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. It is classified as skeletal, cardiac, or smooth muscle^[2], and 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, like movements of the eye, or gross movements like the quadriceps muscle of the thigh. There are two broad types of voluntary muscle fibers, slow twitch and fast twitch. Slow twitch fibers contract for long periods of time but with little force while fast twitch fibers contract quickly and powerfully but fatigue very rapidly.

Contents 1 Types 2 Anatomy 3 Physiology 4 Nervous control 4.1 Efferent leg 4.2 Afferent leg 5 Role in health and disease 5.1 Exercise 5.2 Disease 6 Strength 6.1 The 'strongest' human muscle 7 Efficiency 8 Muscle evolution 9 References 9.1 Notes 10 External links 11 See also

[edit] Types

There are three types of muscle:

• Skeletal muscle or "voluntary muscle" is anchored by tendons to bone and is used to affect skeletal movement such as locomotion and in maintaining posture. Though this postural control is generally maintained as a subconscious reflex, the muscles responsible react to conscious control like non-postural muscles. An average adult male is made up of 40-50% of skeletal muscle and an average adult female is made up of 30-40%.
• Smooth muscle or "involuntary muscle" is found within the walls of organs and structures such as the esophagus, stomach, intestines, bronchi, uterus, urethra, bladder, and blood vessels, and unlike skeletal muscle, smooth muscle is not under conscious control.
• Cardiac muscle is also an "involuntary muscle" but is a specialized kind of muscle found only within the heart.

Cardiac and skeletal muscle are "striated" in that they contain sarcomeres 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:^[3]
  • Type IIa, which, like slow muscle, is aerobic, rich in mitochondria and capillaries and appears red.
  • 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.^[4]
  • 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.

[edit] Anatomy

Muscle is mainly composed of muscle cells. Within the cells are myofibrils; myofibrils contain sarcomeres, which are composed of actin and myosin. Individual muscle fibres are surrounded by endomysium. Muscle fibers are bound together by perimysium into bundles called fascicles; the bundles are then grouped together to form muscle, which is enclosed in a sheath of epimysium. Muscle spindles are distributed throughout the muscles and provide sensory feedback information to the central nervous system.

Skeletal muscle is muscle attached to skeletal tissue, distinct from heart or smooth muscle. It is arranged in discrete muscles, an example of which is 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 the lumen and peristalsis). Cardiac muscle is the muscle tissue of the heart, and is similar to skeletal muscle in both composition and action, being comprised of myofibrils of sarcomeres. Cardiac muscle is anatomically different in that the muscle fibers are typically branched like a tree branch, and connect to other cardiac muscle fibers through intercalcated discs, and form the appearance of a syncytium.

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 fibres cannot be increased through exercise; instead the muscle cells simply get bigger. Muscle fibres have a limited capacity for growth through hypertrophy and some believe they split through hyperplasia if subject to increased demand.°

[edit] 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 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 contain an ATP store 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. Glycogen can be rapidly converted to glucose when energy is required for sustained, powerful contractions. Within the voluntary skeletal muscles, the glucose molecule is metabolized in a process called glycolysis which produces two ATP and two lactic acid molecules in the process. 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) without a 'warm up' period and always extracts the maximum ATP yield from any molecule involved. The heart and liver will also consume lactic acid produced and excreted by skeletal muscles during exercise.

[edit] Nervous control

[edit] 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 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.

[edit] 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. 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 red nucleus in particular continuously sample position against movement and make minor corrections to assure smooth motion.

[edit] Role in health and disease

[edit] 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.

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. 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. The ability of the body to export lactic acid and use it as a source of energy depends on training level.

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 in other sports to increase their performance in the former.^{[citations needed]}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 contraction, or unaccustomed training levels. Since lactic acid disperses fairly rapidly, it could not explain pain experienced days after exercise.^[5]

[edit] 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.

[edit] Strength

A display of "strength" (e.g lifting a weight) 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 capabilities).

[edit] The 'strongest' human muscle

Since three factors affect muscular strength simultaneously and muscles never work individually, it is unrealistic to compare strength in individual muscles, and state that one is the "strongest". Accordingly, no one muscle can be named 'the strongest', but below are several muscles whose strength is noteworthy for different reasons.

• 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 4337 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.

• • 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 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.

• 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 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 two to three gigajoules.

[edit] 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 output to the total metabolic cost.

[edit] Muscle evolution

Evolutionarily, specialized forms of skeletal and cardiac muscles predated the divergence of the vertebrate/arthropod evolutionary line.^[6] 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.

[edit] 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

[edit] Notes

1. ^ Definition and origin of the word 'muscle'
2. ^ KMLE Medical Dictionary. KMLE Medical Dictionary Definition of muscle. Retrieved on 17 Feb 2006.
3. ^ Larsson, L; Edstrom L, Lindegren B, Gorza L, Schiaffino S (July 1991). "MHC composition and enzyme-histochemical and physiological properties of a novel fast-twitch motor unit type". The American Journal of Physiology 261 (1 pt 1): C93–101. PMID 1858863. Retrieved on 11 June 2006. 
4. ^ Smerdu, V; Karsch-Mizrachi I, Campione M, Leinwand L, Schiaffino S (December 1994). "Type IIx myosin heavy chain transcripts are expressed in type IIb fibers of human skeletal muscle". The American Journal of Physiology 267 (6 pt 1): C1723–1728. PMID 7545970. Retrieved on 11 June 2006.  Note: Access to full text requires subscription; abstract freely available
5. ^ Robergs R, Ghiasvand F, Parker D (2004). "Biochemistry of exercise-induced metabolic acidosis.". Am J Physiol Regul Integr Comp Physiol 287 (3): R502-16. PMID 15308499. 
6. ^ Evolution of muscle fibers

[edit] External links

• Flash animation of muscle contraction
• 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
• Human 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.

[edit] See also

• Bodybuilding
• Physical exercise
• 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)
• Muscle memory

v • d • e Muscles of upper limbs

VERTEBRAL COLUMN: trapezius - latissimus dorsi - rhomboid major - rhomboid minor - levator scapulae ANTERIOR AND LATERAL THORACIC WALLS: pectoralis major - pectoralis minor - subclavius - serratus anterior SHOULDER: deltoid - rotator cuff (supraspinatus, infraspinatus, teres minor, subscapularis) - teres major ARM: anterior compartment: coracobrachialis - biceps brachii - brachialis posterior compartment: triceps brachii - anconeus FOREARM: anterior compartment super. - pronator teres - palmaris longus - flexor carpi radialis - flexor carpi ulnaris - flexor digitorum superficialis deep - pronator quadratus - flexor digitorum profundus - flexor pollicis longus posterior compartment super. - mobile wad (brachioradialis, extensor carpi radialis longus and brevis) - extensor digitorum, - extensor digiti minimi - extensor carpi ulnaris deep - supinator - anatomical snuff box (abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus) - extensor indicis HAND: Lateral volar - abductor pollicis brevis - thenar (opponens pollicis, flexor pollicis brevis, adductor pollicis) Medial volar - palmaris brevis - hypothenar (abductor minimi digiti, flexor digiti minimi brevis, opponens digiti minimi) Intermediate - lumbrical - dorsal interossei - palmar interossei

v • d • e Muscles of lower limbs

ILIAC REGION/ILIOPSOAS: psoas major - psoas minor - iliacus BUTTOCKS: gluteals: (maximus, medius, minimus) - tensor fasciae latae lateral rotator group: piriformis - obturator externus - obturator internus - inferior gemellus - superior gemellus - quadratus femoris THIGH: anterior compartment: sartorius - quadriceps (rectus femoris, vastus lateralis, vastus intermedius, vastus medialis) - articularis genu medial compartment/adductor: gracilis - pectineus - adductor brevis - adductor longus - adductor magnus posterior compartment/hamstring: biceps femoris - semitendinosus, semimembranosus LEG: anterior compartment: tibialis anterior - extensor hallucis longus - extensor digitorum longus - fibularis tertius posterior compartment: superficial - calf/triceps surae (gastrocnemius, soleus) - plantaris deep - popliteus - tarsal tunnel (flexor hallucis longus - flexor digitorum longus - tibialis posterior) lateral compartment: fibularis muscles (longus - brevis) FOOT: dorsal - extensor digitorum brevis - extensor hallucis brevis plantar first layer - abductor hallucis - flexor digitorum brevis - abductor digiti minimi plantar second layer - quadratus plantae - lumbrical muscle plantar third layer - flexor hallucis brevis - adductor hallucis - flexor digiti minimi brevis plantar fourth layer - dorsal interossei - plantar interossei