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Muscular system

The neuromuscular junction

At an age of 9 weeks (begin of the fetal period) the first neuromuscular junctions appear on the newly created myotubes. With the fusing of several myotubes a syncytium is constructed in which a certain striation (i.e., ordered contractile elements) can be recognized. Soon the myotubes receive the first impulse from the motoneurons. This causes the muscle fibers to begin contracting.
Originally the myotubes are innervated by different nerves (polyneurally) on different portions of their surfaces (multiply). Only after birth the superfluous connections are undone and the innervation becomes mononeural.

Fig. 6 - Neuromuscular junction
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  1. Myelinated axons
  2. Neuromuscular junction
  3. Capillaries
  4. Muscle fiber nucleus

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Fig. 6

At the end of the embryonic period the nuclei move into the myotubes in the periphery below the basal membrane. Now the striation is recognizable. The myotubes are initially multiply and polyneurally innervated.

© Dr. T. Voigt, Fribourg

Now the muscle fibers have attained their adult structure in that they are united through connective tissue sheaths to fascicles and the fascicles to muscles. These sheaths stem from fibroblasts that surround these muscle fibers. They form the internal and external perimysium as well as the epimysium. The endomysium stems from the muscle fibers themselves.

Adult musculature and its structures

With the differentiation to muscle fibers through the fusing of several myotubes the muscle cells lose their ability to divide. However, satellite cells remain as stem cells at the surface of the muscle fibers.

Fig. 7 - Skeletal musculature, 20000 times enlarged (longitudinal)
media/module14/m2d_skeletmusk.gif

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Fig. 7

In the electron-microscopic image one recognizes the ordered complexes of the actin and myosin muscle proteins.
The myofiber-unit located between each two succeeding Z-stripes are called sarcomeres. During a contraction the actin filaments slip further into the A-bands of the myosin section.

More info

A muscle consists of various types of muscle fibers. Roughly, one distinguishes between slow twitch, "red" (type I) muscle fibers and fast twitch, "white" (type II) ones. Through their high content in myoglobin as well as mitochondria (in which the oxidative burning of glucose and fatty acids occurs) and oxidative enzymes (for aerobic burning of glucose and fat), type I fibers are specialized in aerobic energy supply.
Fast twitch muscle fibers, on the other hand, are characterized by a high content of energy-rich phosphates and enzymes that decompose these as well as also being able to decompose glycogen without oxygen and are thereby specialized in anaerobic energy supply. Somewhat more precisely:

  1. Type I fibers
    "red" or "slow twitch", small diameter muscle fibers with high resistance to fatigue, higher concentration of ATPase, relatively low glycogen content and lower concentration of SDH (succinatdehydrogenase) as well as - besides the above mentioned high myoglobin content - a large number of mitochondria. They are mainly found in the "red" musculature and possess a good energy supply due to being well capillarized. They are employed in long-lasting movements with limited development of force.

  2. Type II fibers
    "white" or "fast twitch", large diameter muscle fibers
    • Type IIA fibers:
      "fast" or "fast twitch" fibers with a high fatigue tendency, high content of glycolytic and oxidative enzymes that are needed with longer lasting contractions with relatively higher development of force.
    • Type IIB fibers:
      fast, easily fatigued fibers with high glycogen and low mitochondria content. Their energy supply occurs very rapidly, mainly via glycolysis, which is important for short or intermittent strain with a high amount of force development.
    • Type IIC fibers:
      so-called intermediary fibers, which can be ordered between types I and II and, depending on the training, develop more type I or more type II characteristics.
Fig. 8a - Skeletal muscle
(longitudinal section)
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  1. Myofibrille
  2. I-Band
  3. A-Band

Fig. 8b - Skeletal muscle
(cross section)
media/module14/m2f_histoquerquer1.gif

  1. Red muscle fiber
    (glycogen poor)
  2. White muscle fiber
    (glycogen rich)
  3. Myelinated axons
  4. Muscle fiber nucleus
  5. Capillaries

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Fig. 8a

The striations of adult muscle tissue in a longitudinal section are shown in a light microscopic image.

Fig. 8b

Red and white muscle fibers of adult muscle tissue are shown in a light microscopic cross section.

© Dr. T. Voigt, Fribourg