Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

III. Heterogeneity of Skeletal Muscle
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IIB
I
IIA
IIB
I
IIA
(a)
(b)
IIB
IIB
I
I
IIA
IIA
(c)
(d)
FIGURE 15-5 Schematic representation of the homogeneity of skeletal
muscle motor units. Motor units have a homogeneous myofiber-type
composition whereby slow-twitch motor units are composed of only type
1 myofibers (light staining for myosin ATPase), whereas fast-twitch units
are composed of only type 2 a (fast-twitch, fatigue resistant) myofibers, or
only type 2b (fast-twitch, fatigable) myofibers (dark staining for myosin
ATPase).
of (1) the motor neuron, consisting of its cell body located
within the central nervous system (selected cranial nerve
nuclei or the ventral horn of the spinal cord), and its axon,
which extends along the ventral root and peripheral nerve;
(2) the neuromuscular junctions; and (3) the myofibers
innervated by the motor neuron.
Motor neurons may differ based on their rates of discharge:
(1) phasic motor neurons with a fast discharge rate and
(2) tonic motor neurons with a slow discharge rate. In addition,
the phasic motor neurons are characterized by shorter
after hyperpolarization potentials, faster conduction velocities,
and larger axons than the tonic motor neurons. Investigations
of these parameters in motor neurons of slow-contracting
and fast-contracting muscles indicate that tonic motor neurons,
which discharge at rates of 10 to 20 per second, innervate
slow-contracting muscles, and phasic motor neurons,
which discharge at rates of 30 to 60 per second, innervate
fast-contracting muscle. Thus, there are at least two types
of motor units, which differ in their physiological properties
and type of motor neuron innervation ( Eccles et al. , 1958 ).
Physiological measurements performed on isolated
motor units in the cat have revealed two types of fasttwitch
motor units and one type of slow-twitch unit ( Burke,
1975 ). Some fast-twitch motor units are resistant to fatigue
and designated FR units (i.e., fast-twitch, resistant to
fatigue); others fatigue rapidly and are designated FF units
(i.e., fast-twitch, fatigable). All of the slow-twitch units are
resistant to fatigue and are therefore designated as S units
(i.e., slow-twitch). The average number of muscle fibers
per motor unit in cats ranges from 550 to 650. Subsequent
immunohistochemical studies (discussed later) reveal that
slow-twitch motor units contain type I myosin isoforms,
and most FR motor units contain type IIa or IIa/x myosin
isoforms and most FF motor units contain type IIx (mammals)
or IIb (rodent) myosin isoforms.
FIGURE 15-6 Serial sections of cat medial gastrocnemius muscle incubated
for the histochemical demonstration of (a) myofibrillar ATPase,
incubated at pH 9.4; (b) myofibrillar ATPase, preincubated at pH 4.5;
(c) NADH-tetrazolium reductase. Type 1 myofibers comprise slow-twitch
motor units that are resistant to fatigue. Type IIA myofibers comprise
fast-twitch motor units that are resistant to fatigue, whereas type IIB
myofibers comprise fast-twitch motor units that fatigue rapidly. Dark
NADH staining indicates high mitochondrial content; and (d) myofibrillar
ATPase, preincubated at pH 4.2.
D . Histology and Histochemistry
1 . Histochemical Properties of Myofi bers
a . Histoenzymic Properties Associated with
Myosin-ATPase
Myofibers have been differentiated histochemically into
type I and type II myofibers based on their staining reaction
for myosin ATPase ( Fig. 15-6 ) ( Dubowitz and Brooke,
1973 ). Furthermore, type II myofibers, classified by the
myosin ATPase staining reaction, may be further subdivided
into type IIA, type IIB, and type IIC myofibers based
on the lability of their ATPase activity following preincubation
in acid and alkaline media ( Fig. 15-6 ) ( Brooke and
Kaiser, 1970 ). If the actin-activated myosin ATPase activity
is rate limiting in the speed of contraction, it follows
that the histochemical method for myosin ATPase would
be a specific method for the differentiation of myofiber
types based on their speed of contraction. This classification
scheme is generally applicable to all mammalian
species except for canine muscles, which do not contain
classical type IIB myofibers ( Braund et al. , 1978 ; Orvis
and Cardinet, 1981 ).
b . Immunocytochemical Identification of Myosin
Isoforms
More recently, immunohistochemical differentiation of
fiber types based on antibodies directed against myosin
heavy chain isoforms have been used to identify contractile
muscle fiber types ( Gorza, 1990 ). To henceforth distinguish
between the two methods for fiber-type identification
roman numerals will be used for histochemical fiber types