Page 343 - Color Atlas of Biochemistry

P. 343

334 Tissues and organs

Control of muscle contraction enter the sarcoplasm, where they lead to a
rapid increase in Ca 2+ concentrations. This in
turn causes the myofibrils to contract (C).
A. Neuromuscular junction
Muscle contraction is triggered by motor
neurons that release the neurotransmitter C. Regulation by calcium ions
acetylcholine (see p. 352). The transmitter dif- In relaxed skeletal muscle, the complex con-
fuses through the narrow synaptic cleft and sisting of troponin and tropomyosin blocks
binds to nicotinic acetylcholine receptors on the access of the myosin heads to actin (see
the plasma membrane of the muscle cell p. 332). Troponin consists of three different
(the sarcolemma), thereby opening the ion subunits (T, C,and I). The rapid increase in
channels integrated into the receptors (see cytoplasmic Ca 2+ concentrations caused by
+
p. 222). This leads to an inflow of Na ,which opening of the calcium channels in the SR
triggers an action potential (see p. 350) in the leads to binding of Ca 2+ to the C subunit of
sarcolemma. The action potential propagates troponin, which closely resembles calmodulin
from the end plate in all directions and con- (see p. 386). This produces a conformational
stantly stimulates the muscle fiber. With a change in troponin that causes the whole tro-
delay of a few milliseconds, the contractile ponin–tropomyosin complex to slip slightly
mechanism responds to this by contracting and expose a binding site for myosin (red).
themusclefiber. This initiates the contraction cycle. After con-
traction, the sarcoplasmic Ca 2+ concentration
is quickly reduced again by active transport
B. Sarcoplasmic reticulum (SR)
back into the SR. This results in troponin los-
The action potential (A) produced at the neu- ing the bound Ca 2+ ions and returning to the
romuscular junction is transferred in the initial state, in which the binding site for my-
muscle cell into a transient increase in the osin on actin isblocked. It isnot yet clear
Ca 2+ concentrationinthe cytoplasm of the whether the mechanism described above is
muscle fiber (the sarcoplasm). the only one that triggers binding of myosin
In the resting state, the Ca 2+ level in the to actin.
sarcoplasm is very low (less than 10 –7 M). By
contrast, the sarcoplasmic reticulum (SR), When triggering of contraction in striated
muscle occurs, the following sequence of
which corresponds to the ER, contains Ca 2+
ions at a concentration of about 10 –3 M. The processesthustakes place:
SR is a branched organelle that surrounds the 1. The sarcolemma is depolarized. 2+
2. The action potential is signaled to Ca
myofibrils like a net stocking inside the
channels in the SR.
muscle fibers (illustrated at the top using 3. The Ca 2+ channels open and the Ca 2+ level
the example of a heart muscle cell). The high
2+
Ca 2+ level in the SR is maintained by Ca - in the sarcoplasm increases.
2+
transporting ATPases (see p. 220). In addition, 4. Ca bindstotroponin C andtriggersa
conformational change.
the SR also contains calsequestrin, aprotein
(55 kDa) thatisabletobindnumerousCa 2+ - 5. Troponin causes tropomyosin to slip, and
the myosin heads bind to actin.
ions via acidic amino acid residues.
The transfer of the action potential to the SR 6. The actin–myosin cycle takes place and the
is made possible by transverse tubules (T tu- muscle fibers contract.
bules), which are open to the extracellular space Conversely, at the end of contraction,the fol-
and establish a close connection with the SR. lowing processes take place:
There is a structure involved in the contact be- 1. The Ca 2+ level in the sarcoplasm declines
tween the T tubule and the SR that was formerly due to transport of Ca 2+ back into the SR.
known as the “SR foot” (it involves parts of the 2. Troponin C loses Ca 2+ and tropomyosin re-
ryanodine receptor; see p. 386). turns to its original position on the actin
At the point of contact with the SR, the molecule.
action potential triggers the opening of the 3. The actin–myosin cycle stops and the
Ca 2+ channels on the surface of the sarco- muscle relaxes.
lemma. Calcium ions then leave the SR and

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