Excitation and Contraction of Skeletal Muscle
The area of contact between the end of a motor nerve and a skeletal
muscle cell is called the motor end plate. Small
branches of the motor nerve form contacts (boutons) with the muscle cell in a
roughly eliptical area. The excitatory transmitter at the motor end plate is acetylcholine. The space between the boutons and the
muscle fibre is called primary synaptic cleft. Numerous infoldings of the
sarcolemma in the area of the motor end plate form secondary synaptic clefts.
Motor end plates typically concentrate in a narrow zone close to the middle of
the belly of a muscle. The excitable sarcolemma of skeletal muscle cells will
allow the stimulus to spread, from this zone, over the entire muscle cell.
The spread of excitation over the sarcolemma is
mediated by voltage-gated ion channels.
Invaginations of the sarcolemma form the T-tubule system which "leads" the excitation
into the muscle fibre, close to the border between the A- and I-bands of the
myofibrils. Here, the T-tubules are in close apposition with cisternae formed
by the sarcoplasmatic reticulum. This association is called a triad. The narrow gap between the T-tubule and the
cisternae of the sarcoplasmatic reticulum is spanned by proteins which mediate
the excitation-contraction coupling.
Proteins in the sarcolemma which forms the wall of
the T-tubule (dihydropyridine (DHP) receptors)
change conformation, i.e. they change their shape, in response to the
excitation travelling over the sarcolemma. These proteins are in touch with
calcium channels (ryanodine receptors) which are
embedded in the membrane of the cisternae of the sarcoplasmatic reticulum. The
change in the shape of the proteins belonging to the T-tubule opens the calcium
channels of the sarcoplasmatic reticulum. Calcium can now move from stores in
the sarcoplasmatic reticulum into the cytoplasm surrounding the myofilaments.
Sites of interaction between actin and myosin are
in resting muscle cells "hidden" by tropomyosin.
Tropomyosin is kept in place by a complex of proteins collectively called troponin. The binding of calcium to troponin-C induces a conformational change in the
troponin-tropomyosin complex which permits the interaction between myosin and
actin and, as a consequence of this interaction, contraction.
ATP-dependent calcium pumps in the
membrane of the sarcoplasmatic reticulum typically restore the concentration of
Ca to resting levels within 30 milliseconds after the activation of the muscle
fibre.
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Title:
Excitation and Contraction of Skeletal Muscle
by:
om
at
2013-02-15T03:09:00+07:00
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Excitation and Contraction of Skeletal Muscle