Page 188 - Anatomy and Physiology of Farm Animals, 8th Edition

Microscopic Anatomy and Physiology of Muscle / 173

enzyme that splits adenosine triphosphate
(ATP) to yield energy for muscle contraction,
VetBooks.ir reveals that some muscle fibers stain darkly

(type II fibers), others lightly (type I fibers)
(see Fig. 2‐6). These histochemical results
correlate with the physiologic properties of
the muscle fibers themselves: type I fibers
contract slowly (slow twitch) but can con-
tract for long periods. Type II fibers contract
quickly (fast twitch) but are more suscepti-
ble to fatigue. Speed of contraction is there-
fore a property of the activity of myosin
ATPase and the rate of ATP hydrolysis;
endurance is related to the intracellular con-
tent and activity of mitochondria and the Figure 9-2. Light micrograph (A) and electron
ability to generate ATP for contraction by micrograph of longitudinal skeletal muscle (B).
Source: Dellman and Eurell, 1998. Reproduced
oxidative or aerobic metabolism. with permission of John Wiley & Sons, Inc.
The specific types of muscle fibers that
compose the muscle determine the functional
characteristics of a whole muscle. Muscles regions, called A bands, are composed
that require sustained contraction, such as of overlapping thick and thin filaments
the antigravity muscles, typically contain (Fig. 9‐2). Thus, alternating A and I bands
more slow‐twitch endurance fibers than do produce the banding pattern on the myofi-
the muscles that contract briefly but quickly bril. Within the A band is a region of thick
and with great force. It is even possible to filaments not superimposed by thin fila-
show a difference among breeds of horses ments, called the H zone. At the very center
with regard to the muscle fiber composition of the H zone is the M line, which is another
of the same muscle; the middle gluteal muscle feature visible as a result of the cross‐
of the sprinting American Quarter Horse is connection of thick filaments (Fig. 9‐2).
usually characterized by more of the large, A dense line, the Z line, bisects each I
fast‐twitch, but low‐endurance type II fibers band (in fact, one end of each thin fila-
than is the same muscle of the slower but ment is attached to the Z line; the opposite
long‐winded Arabian. end of each thin filament is free). The seg-
On casual examination with a light ment of myofibril between adjacent Z lines
microscope, the cross‐striations of skeletal is the sarcomere, the fundamental unit of
muscle appear to be disks throughout the contraction in striated muscle (Figs. 9‐2
entire fiber. However, the electron micro- and 9‐3).
scope shows the striations only in the Each striated muscle fiber contains hun-
myofibrils and not in the sarcoplasm (cyto- dreds or thousands of myofibrils, and each
plasm of muscle cell). The alternate light myofibril contains approximately 1500 thick
and dark bands of all myofibrils appear at and 3000 thin myofilaments (Fig. 9‐3). Each
the corresponding places in the fiber thick filament is composed of hundreds of
(Figs. 9‐2 and 9‐3). The fact that corre- molecules of myosin, a golf club‐shaped
sponding bands of adjacent myofibrils are protein molecule with a molecular weight of
in register makes it appear that these bands 332,000 (by comparison, hydrogen has a
extend across the whole fiber (Figs. 9‐1, 9‐2, molecular weight of 2). A thin filament is
and 9‐3). The apparent bands of myofibrils composed principally of chains of molecules
are due to relative density and partial over- of actin, a globular protein of molecular
lapping of thick and thin filaments. Letters weight 70,000. The structure of these fila-
are used to designate the different bands. ments and details of their actions in con-
The light zones, or bands, called I bands, traction are described in more detail in a
consist of thin filaments only. The darker later section.