肌肉

肌肉是能夠收縮的人體組織，其為四種主要組織之一，其他則是上皮組織、結締組織與神經組織。肌肉組織作用於身體不同部分的移動，並且運送身體內的養分。

種類

人體的肌肉有三種：

心肌是一种在心脏及近心大血管壁中发现的特殊肌肉。
骨骼肌也稱為“橫紋肌”或“随意肌”，是通过肌腱固定到在骨骼上，其伸缩可以带动骨骼的的移动。
平滑肌或是“非自主肌肉”，见于小肠，喉和血管等器官。

Cardiac and skeletal muscle are "striated" in that they contain sarcomeres 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 two 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, glycolytic, "fast twitch", or "white" muscle is less dense in mitochondria and myoglobin. It can contract more quickly and with a greater amount of force than Type I muscle, but can only sustain short, anaerobic bursts of activity before a build-up of lactic acid in tissue begins to interfere with muscular contraction and causes pain.

解剖學

肌肉由肌肉細胞組成(有時以"肌肉纖維著名") 。在細胞之內是肌原纖維；肌原纖維包含肌節，而肌節由肌動蛋白和肌球蛋白組成。各自的肌肉細胞於肌內膜內排列成行。肌肉細胞由肌束膜捆綁在一起叫做肌束；這些束聚集在一起然後形成肌肉，由肌外膜排行。肌肉紡錘被分散至遍佈在肌肉裡，並對中央神經系統提供反饋知覺資訊。

骨骼肌被安排在不同的小組, 其中一個例子包括肱二頭肌。它由腱連結到骨骼的突起。相反, 光滑肌出現以各種各樣的比例出現在幾乎每種器官。從皮膚(在那它控制體毛的舉起) 到對血管和消化道(在那它控制內腔和蠕動的大小) 。

生理學

The three types of muscle have significant differences, but all use the movement of actin against myosin to produce 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.

Muscles and muscular activity account 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.

神經控制

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 哺乳類 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 大腦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.

對健康與疾病的影響

運動

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.

疾病

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).

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.

Diseases of the motor end plate include myasthenia gravis, a form of muscle weakness due to antibodies to the acetylcholine receptor, and its related condition Lambert-Eaton myasthenic syndrome (LEMS). Tetanus and botulism are bacterial infections in which bacterial toxins cause increased or decreased muscle tone, respectively.

The myopathies are all diseases affecting the muscle itself, rather than its nervous control.

Muscular dystrophy is a large group of diseases, many of them hereditary, where the muscle integrity is disrupted. It leads to progressive loss of strength, high dependence and decreased life span.

Inflammatory muscle disorders:

Polymyalgia rheumatica (or "muscle rheumatism") is an inflammatory condition that mainly occurs in the elderly; it is associated with giant-cell arteritis. It often responds dramatically to glucocorticoids (e.g. prednisolone).
Polymyositis, dermatomyositis and inclusion body myositis are autoimmune conditions in which the muscle is affected.

Rhabdomyolysis is the breakdown of muscular tissue due to any cause. While it may not lead to any muscular symptoms at all, the myoglobin thus released may cause acute renal failure.

Tumors of muscle include:

Smooth muscle: leiomyoma (benign, very common in the uterus), leiomyosarcoma (malignant, very rare)
Striated muscle: rhabdomyoma (benign) and rhabdomyosarcoma (malignant) - both very rare
Metastasis from elsewhere (e.g. lung cancer)

Smooth muscle has been implicated to play a role in a large number of diseases affecting blood vessels, the respiratory tract (e.g. asthma), the digestive system (e.g. irritable bowel syndrome) and the urinary tract (e.g. urinary incontinence). These disease processes are not usually confined to the muscular tissue.

人類最強壯的肌肉

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, are and probably "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 technically the tongue consists of sixteen muscles, not one. The tongue may possibly be the strongest muscle at birth.

The 心臟 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.

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).

肌肉的演化

According to a recent study published in 1999 ^*, 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.

參考資料

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

外部連結

The Straight Dope (Masseter "strongest;" 975 pounds for 2 seconds; gluteus maximus and quadriceps strongest if leverage not included)
Physics factbook (Heart output 1.3 to 5 watts, lifetime output 2 to 3 ×10⁹ 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")
a body-building equipment website (Quadriceps "strongest")
University of Dundee article on performing neurological examinations (Quadriceps "strongest")
- course notes from a Hartnell College course (Uterus "strongest pound for pound")
the Pregnancy Today website (Uterus "strongest")
Muscle efficiency in rowing
"Gatorade Sports Science Institute" on muscle efficiency in cyclists (PDF)

參見

肌肉系統 | 組織

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種類

解剖學

生理學