Page 192 - Anatomy and Physiology of Farm Animals, 8th Edition

Microscopic Anatomy and Physiology of Muscle / 177

Before stimulation, the Ca concentra- the permeability of the abnormal chan-
2+
tion within the sarcoplasmic reticulum is
VetBooks.ir much greater (more than 100‐fold) than nels, hence the name of the condition.
Clinical signs include muscle spasms,
within the sarcoplasm. When the Ca
2+
channels in the sarcoplasmic reticulum tremors, sweating, and weakness. This
condition is also known as Impressive
open, Ca diffuses into the surrounding sar- syndrome, because it is primarily seen
2+
coplasm of the muscle fiber and into the in American Quarter Horses and other
myofibrils. The increase in Ca in the myofi- descendants of the American Quarter
2+
brils leads to the interaction of thick and thin Horse sire Impressive.
filaments and movement (sliding) of the thin
filaments past the thick filaments toward the Myosin and Actin Filaments. Each thick
center of the sarcomere. This sliding move- filament in a sarcomere is a bundle of
ment shortens the sarcomeres, which short- myosin molecules. Each molecule has two
ens the myofibrils, which shortens the entire parts: (1) a filament‐like part that lies parallel
muscle fiber. Figure 9‐5 summarizes the to similar parts of other myosin molecules,
2+
movements of Ca during excitation– making up the length of the thick filament,
contraction coupling in skeletal muscle. and (2) a part that projects outward like an
Hyperkalemic periodic paralysis arm from the end of the filament (Fig. 9‐6).
(HYPP) is an inherited disease of horses An enlargement at the end of the arm is
caused by a genetic mutation of a trans- termed the myosin head. The arm attaching
membrane protein. The voltage‐gated the myosin head to the filament is flexible,
sodium channel (described earlier as like a hinge, where it joins the filament
participating in the generation of action segment and also where it joins the head.
potentials on the cell membranes of Myosin heads protrude from all around the
skeletal muscle) is defective in affected thick filament. They extend away from
animals, and as a result, the permea- the center in both directions, towards the
bility of the channel to sodium may be surrounding thin filaments (Fig. 9‐6).
increased inappropriately. This permits Each thin filament is made up of three
the entrance of sodium, membrane proteins: actin, tropomyosin, and troponin.
depolarization, and involuntary muscular Actin molecules are the most prominent and
contractions. Hyperkalemia is an increase are arranged in two long strands wound
in serum potassium concentration, and around each other in a spiral. Tropomyosin
this is one stimulus that can increase molecules are also joined in a strand that

Movement Active sites Actin filament

Power
stroke
Hinges

Myosin filament

Figure 9-6. The postulated “walk along” mechanism for muscular contraction. The heads of two
cross‐bridges are attaching to and disengaging from the active sites of the actin filament. The attachment
of the head to the active site is thought to cause the head to tilt towards the arm to move the actin filament
along with it, thus creating the power stroke. Source: adapted from Guyton and Hall, 2006. Reproduced
with permission from Elsevier.