3B MICROanatomy Muscle Fiber Manuel D'instructions page 4

3B MICROanatomy™ Muscle Fiber
English
The model illustrates a section of a skeletal muscle fiber and its neuromuscular end plate magnified
approx. 10,000 times.
The muscle fiber is the basic element of the diagonally striped skeletal muscle. It is a giant cell (1 – 10 cm
long and up to 0.1 mm thick) with many nuclei. Its chief functional element is formed by myofibrils. The
myofibrils are made of the myofilaments myosin and actin and are surrounded by the sarcoplasmic reticu-
lum. The characteristic longitudinal striping of the skeletal muscle is caused by the specific arrangement of
the myofilaments. The thick myosin filaments, which are optically bi-refracted, form the A (transverse)
band. The thin actin filaments, by contrast, are uni-refracted and form the I (isotropic) band. The Z line
(intermediate stripe) runs through its center. The section between two Z lines is called the sarcomere.
Starting from the cell membrane, the cytoplasm is run through by membranous tubes, called the transver-
sal tubuli, which together with the terminal cisterns of the sarcoplasmic reticulum form a triad. The nuclei
are situated in the cell periphery. Mitochondria, the "power plants of the cells" run parallel to the myofi-
brils.
The neuromuscular end plate is the name of the area in which a motor nerve ending is in contact with the
skeletal muscle cell. The nerve ending is covered by a Schwann cell. The synaptic gap, which is filled by a
common basal lamina of the muscle and Schwann cell, lies between the muscle cell and the nerve ending.
The nerve ending contains mitochondria as well as presynaptic vesicles that are filled with transmitter sub-
stances (usually acetylcholine).
1 Myofibrils
2 Mitochondrium
3 Postsynaptic membrane ®
4 Synaptic gap with basal lamina
5 Presynaptic membrane
6 Presynaptic vesicle
7 Schwann cell
8 Nucleus
9 Myosin and actin filament
1 0 Sarcomere
1 1 Actin filament
1 2 Myelin sheath
1 3 Neurofibers
1 4 Cell membrane (sarcolemma)
1 5 Transverse membrane tube
1 6 Triad
1 7 Sarcoplasmic reticulum
1 8 Basal lamina
1 9 Reticular fibers
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