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Encyclopedia > Motor system
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Structure of a skeletal muscle

Muscle is one of the four tissue types. The other three types are: epithelium, connective tissue and nervous tissue. The primary purpose of muscle tissue is to contract. Muscle contraction is used to move parts of the body, as well as to move substances within the body.


There are three general types of muscle. The first two are "striated", they contain sarcomeres; the third type is "smooth":

The differences in characteristics of the smooth muscles and the striated muscles include: the fibers of the smooth muscles are not arranged regularly as the ones of striated muscles, smooth muscles are use to sustain longer contraction or even near permanent whereas the striated muscles are often used for short, burst activities.


Muscles within the skeletal muscle are also divided in to two types of muscles: Type I and Type II muscles. Type I has a lot of myoglobin, hence the red color. It is good for endurance, because it is a place where lots of aerobic metabolism takes place. It is a site where there is lots of mitochondria. Type II muscles are good for short-term activities that require lots of energy because it is good at generating power. However, this type of muscle fatigues quickly due to anaerobic metabolism.

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Contraction and relaxation

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 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.


Muscle composition

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 fascicules; the bundles are then grouped together to form muscle, which is lined by epimysium. Muscle spindles are distributed throughout the muscles and provide sensory information to the central nervous system.


Exercise

Exercise is often recommended as a means of improving motor skills. Exercise has several effects upon muscles, connective tissue, and the nerves that stimulate the muscles. Many common myths have arisen surrounding exercise, some of which have a basis in reality, and some which are completely false.

  • Exercise does not increase the number of muscle cells.

This is generally false. While additional cell formation is not the primary reason for short term strength gains, strenuous exercise does trigger the release of low levels of anabolic steroids. Over time this does cause a net increase in muscle cell count.

  • No Pain No Gain.

This is generally false. Depending upon the type of exercise you are engaged in and the underlying cause of the pain, pain is generally a sign that you are causing more harm than good. It is common to experience sore muscles the day after a workout following the start of an exercise program. This is due to a buildup of lactic acid and is known either as Post Exercise Muscle Stiffness or Delayed Onset Muscle Soreness. This can last for a few days or up to a month depending on your initial physical condition. You should NOT feel pain during or immediately after a workout. Pain at these times can indicate a serious condition that requires immediate medical attention. It is uncommon to experience pain the next day after you have been engaged in a program for a month or more. If this is the case you should seek medical advice.

  • Only overweight people need a physical before beginning an exercise program.

Absolutely false. Only a physician can determine your ability to engage in an exercise program. Apparently healthy people can still have unknown medical conditions, such as a heart murmur, that can cause severe injury or death not only to themselves, but also to others that are dependent upon them, such as someone they are spotting.


Motor system

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 unique regions in the brain. 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 muscles in mammals correspond with neuron groups along the primary motor area of the brain's cerebral cortex. Commands are routed to basal ganglia and to the cerebellum before being relayed to connections in the pons and medulla for transmission to synapses 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 inervated from origins in the brain stem.


Muscle sense

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).


See also


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