Page 193 - Anatomy and Physiology of Farm Animals, 8th Edition

178 / Anatomy and Physiology of Farm Animals

spirals around the strands of actin. The Relaxation. Muscle contraction will con-
tinue as long as there is an excess of Ca
third protein, troponin, is found attached
2+
VetBooks.ir to tropomyosin at specific sites along in the sarcoplasm, but when the effect of
the strand. Together they are called the
the action potentials on the sarcolemma
troponin–tropomyosin complex. ends, the Ca is sequestered back into the
2+
The strands of tropomyosin lie over sites sarcoplasmic reticulum (Fig. 9‐5). Ion
on the actin strand where myosin heads pumps in the membrane of the sarcoplasmic
can bind. Calcium ions released from the reticulum use the energy of ATP to pump
sarcoplasmic reticulum bind to the tro- the Ca from the sarcoplasmic fluid back
2+
ponin part of the troponin–tropomyosin into storage so that it can be ready for the
complex and induce a molecular change in next depolarization. (Without ATP the
the tropomyosin strand. This change muscle cannot relax.) Only a small amount
uncovers myosin binding sites on the actin of Ca is left out in the sarcoplasm of the
2+
strands so that the myosin head can attach. relaxed resting muscle, not enough to act
Binding of the myosin head to actin on the troponin–tropomyosin complex.
leads to the release of adenosine diphos- Therefore, during relaxation the thin and
phate (ADP) and phosphate, which were thick filaments are dissociated, allowing the
bound to the myosin head. The myosin elasticity of the muscle to return it to its
head also rotates from its resting position resting length, which pulls the Z lines and
toward the center of the sarcomere; this thin filaments back to their original positions.
movement pulls the actin chain to which it
is bound past the thick filament (Fig. 9‐6). Replenishment of ATP. A great amount
The myosin head remains at its final of ATP is needed because the energy for
angle and bound to the actin of the thin contraction is derived from hydrolysis
filament until an intact ATP molecule (dephosphorylation) of ATP to ADP. Also,
binds to another site on the myosin head. the muscle fiber uses ATP to sequester
This site on the myosin head is ATPase, Ca back into the sarcoplasmic reticulum.
2+
which also promotes the hydrolysis of ATP It is also needed for complete recovery of
prior to movement of the head. With the the membrane after depolarization – the
binding of a new ATP, the myosin head Na –K –ATPase system.
+
+
detaches from the actin chain and resumes The concentration of ATP in resting
its resting angle. It is ready to repeat the skeletal muscle is relatively small, supplying
process of attaching to actin, moving from only enough energy to maintain contraction
resting to final angle and pulling the for a brief period. Since muscles continue
attached thin filament farther toward the to contract after the initial supply of ATP
center of the sarcomere, detaching, then has been used, the resulting ADP is phos-
binding still another ATP molecule. phorylated again from another source,
The cycle of events that produces the creatine phosphate (CP).
shortening of each sarcomere, the sliding There is normally about five times as
filament model of muscle contraction, is much CP as ATP stored in the sarcoplasm
summarized in Figure 9‐6. Each pull of of skeletal muscle. When the ATP is
the actin filament creates a new power used for contraction and relaxation,
stroke and adds to the “walk along” or transphosphorylation occurs from CP to
“ratchet‐like” action that shortens the the resulting ADP, forming ATP again.
sarcomere and shortens the I bands. The This replenishment reaction occurs almost
A band always remains the same length. as fast as ATP is used. Therefore, the ATP
During shortening, the thin filaments slide level changes little until the concentration
over the thick filaments as they are drawn of CP gets low. CP acts as the immediate
from both ends, pulling the Z lines closer energy source for the resynthesis of ATP.
together (Fig. 9‐2). The filaments them- This is a convenient means of transferring
selves do not shorten. energy for muscle contraction.