Download MUSCLES

MUSCLES
Your basic physiology class discussed the roles of smooth and striated
muscle and that striated muscle was found in the heart and skeletal muscle.
Our focus here will be the locomotor muscles. That skeletal muscle comprises
something around 60-70% of the body mass makes this tissue an obvious
system for study. The primary power comes from the hindlimb (though not to
take away from the forelimbs) and as a consequence, receive the most
attention. Of these, the gluteus has been examined most closely. The
semitendinosus and semimembranosus provide for the power stroke.
The force a muscle can generate is a function of: 1)whether that muscle
is undergoing short periods of stimulation or tetanic stimuli 2)the length of the
muscle when it is stimulated to contract and 3)the number of actin and myosin
filaments acting. Tetanic stimuli provide for more calcium to be present in the
muscle and thus a longer period of time to overcome elastic elements. If the
stimulus is only of short duration, some of the force is taken up in overcoming
the elastic elements of the tendons and other connective tissue. Muscle force is
also a function of the number of actin and myosin crossbridges formed. This
relationship between actin and myosin is altered by the length of the muscle. If
the muscle is stretched, the number of cross bridges will decrease. When the
muscle is stimulated to contract, the amount of force produced is decreased.
The number of cross s bridges is also affected by the number of actin and
myosin molecules in the muscle fiber. These filaments are arranged in parallel,
and the greater number of filaments in parallel, the greater the force of
contraction. This may be recognized as either recruitment of more muscle fibers
(thereby increasing the number of filaments in cross section) or by actually
increasing the number of filaments in cross section. This later event occurs
when muscles hypertrophy. Increased force requirements of an exercise regime
produce an increase in the number of filaments in cross section. The phenotypic
response is "bulking up" or larger muscles.
Mammalian skeletal muscle is composed of 3 different types of fibers.
The first major distinction is the rate of ATP cleavage by the myosin. Some
myosin rapidly cleave ATP and this would produce a rapid contraction. These
fibers are labeled as "fast" fibers or given the confusing appellation of type "II"
fibers. The other category of fiber are those that split ATP slowly. These fibers
are called type "I" fibers. Type I fibers are found in postural muscles such as
the psoas in the back musculature, or the soleus in the leg. If one needs rapid
contraction for movement, then type II fibers would be the adaptive choice.
This "adaptive choice" is reflected in the organization of these fiber types within
a muscle. Portions of the muscle close to the bone tend to be high in type I
fibers. Superficial portions of the muscle are high in type II.
Fibers can also be segregated on the basis of oxidative capacity.
Although oxidative capacity is more like a continuum than categorical, muscle
physiologists tend to lump fibers into low oxidative (few mitochondria) and high
oxidative (lotsa mitochondria). Those fibers with few mitochondria must be
relegated to favoring anaerobic metabolism. Indeed, there are high levels of
glycolytic enzymes in these fibers and there are large concentrations of
glycogen.. These fibers will function for short, high intensity events. For longer,
more sustained activity, more mitochondria are needed. These fibers can use
not only carbohydrates, but lipids as well. There high aerobic capacity means
they resist fatigue.
Combining the myosin ATPase speed (Type I and II) with oxidative
capacity reveals three basic fiber types: fast, low oxidative; fast, high oxidative;
and slow, high oxidative. The terms, FG (fast glycolytic), FOG (fast,oxidative
and glycolytic) and SO (slow, oxidative) are also terms used by some. The
short-hand indications are IIB, IIA, and I, respectively.
There are a number of correlates to these fiber types.
I
IIA
IIB
diameter
small
intermediate
large
myoglobin
high [ ]
high
low
glycogen [ ]
low
high
high
cap. density
high
intermediate
low
mitochondria
large numbers intermediate
low
lipid [ ]
high
intermediate
low
glycolytic nz
low
intermediate
high
The technique of fiber typing requires either a surgical or percutaneous
needle biopsy. The needle biopsy technique can provide a relatively safe sample
from 50-200 mg.
There is a relationship between fiber types and breed with respect to
performance. Horse that have been bred for high intensity, short duration
events (e.g. Quarter horse) have an increased percentage of fast fibers. Horses
bred for slower, but longer duration events, such as the Arabian horse, have a
higher percentage of slow oxidative fibers. Different breeds will generally fall
out where one would expect. Thoroughbreds are intermediate in their
distribution of fast and slow fibers, large heavy hunters have a higher
percentage of slow fibers...etc. In establishing these fiber type distributions,
care must be taken in sampling from the same muscle and the same relative
depth. even if these precautions are followed, variation in fiber distribution in a
population is relatively large. The reliance on fiber types for picking a
performance animal can quickly lead to spurious results. The fiber type
distribution in Australia's number one endurance horse in 1990 had a muscle
fiber type characteristic of a great Quarter horse!
Fibers can change their characterization with training. There are
instances in which fibers may change from type I to II or vice versa, but in
general, there is little conversion on the basis of speed. It appears that the
percentage of fast and slow fibers is genetically set (hence the great interest in
using fiber typing to predict performance). However, muscle fibers show great
plasticity in their oxidative capacity. If muscle are conditioned to long, slow
workouts, mitochondrial enzymes, such as citrate synthase and HOAD, are
increased without a concomitant change in the fiber size. In contrast, high
intensity workouts stress the anaerobic machinery and will not produce
increases in the mitochondria. In these fibers, the stimulus is to increase the
amount of myosin and actin. These proteins are added to existing muscle
fibers. The result of this "power" training is a muscle hypertrophy not a
hyperplasia. Compare the bulk of muscular from the hind limb in Arabians and
Quarter Horses..
Pathology
Athletic events pose the potential of tissue damage which may range
from simple to severe. One means of assessing this trauma is measurement of
CK.
Fibrotic myopathy: Muscle possesses some capacity for regeneration.
Minor tears and strains associated with physical activity serve as a stimulus for
regrowth and rebuilding. Certainly in these instances, the muscle comes back
bigger (hypertrophy) and better able to resist further trauma. However, if the
trauma is too severe, damage is irrevocable. That means that connective tissue
(scar tissue) replaces the muscle. This investiture of scar tissue severely limits
motion. This fibrotic myopathy is seen more commonly in those animals asked
to produce severe excesses in power output. Commonly Quarter horses in
cutting and sliding activities experience such trauma.
Exertional rhabdomyolysis, Monday Morning Disease, Tying-Up
The severity of this disease varies considerably. Originally, it was
identified in farm horses and mules. Animals were worked hard 6 days a week
and then rested on the 7th (usually the Sabbath) so that on Monday, when they
returned to work, there was severed bouts of pain associated with muscle
damage. The inciting cause was correlated to a high level of nutrition that was
continued even over the nonworking day. The actual mechanistic cause of the
myolysis is uncertain, but may be due to excessive lactate production. The high
plain of nutrition during the off day leads to high levels of glycogen. When
activity is resumed, the glycogen breaks down rapidly leading to acidosis.
Damaged muscle releases not only enzymes, but the respiratory pigment,
myoglobin. The myoglobin is filtered by the kidneys and appears in the urine.
Horses with this affliction invariably pee dark colored urine. A major concern is
the deposition of myoglobin in the kidney tubules leading to kidney failure. This
is a serious emergency situation. Treatment is directed at stabilizing
membranes and preventing kidney damage. First the animals must be
rehydrated, then treated with NSAIDS. NSAID treatment to a dehydrated
animal can potentiate the nephrotoxicity. Vit E and selenium have been
associated with muscle necrosis and some individuals treat (and try to prevent)
exertional rhabdomyolysis with supplements.