Page 197 - Anatomy and Physiology of Farm Animals, 8th Edition

182 / Anatomy and Physiology of Farm Animals

their arrowheads. These agents bind to ACh muscles may also undergo eccentric con-
traction when unsuccessfully opposing the
receptors on the postjunctional membrane
VetBooks.ir so that ACh cannot bind to the receptors actions of a prime mover.
Isotonic contraction occurs when the
and produce an end‐plate potential. Curare
is not destroyed by acetylcholinesterase. length of the muscle changes but the tension
Death can result from asphyxiation because remains the same, primarily when a muscle
of paralysis of muscles needed for respira- lifts a given weight. The weight is constant;
tion. The paralysis induced by curariform therefore, the tension does not change.
drugs depends on the concentration of the Rigor and Rigor Mortis. If most of the
drug, and such agents are used therapeuti- ATP in a muscle is depleted, the myosin
cally to produce varying amounts of paralysis, heads cannot separate from the actin in the
such as for muscle relaxation during certain thin filaments and the calcium can no longer
types of surgery. be sequestered back into the sarcoplasmic
Botulinum toxin, which is produced reticulum by the calcium pump. Therefore,
by the bacterium Clostridium botulinum, relaxation cannot occur because the actin
also acts at the neuromuscular junction. and myosin filaments are bound in a
It prevents the release of ACh from the continuous contracted state. This is termed
nerve ending by preventing synaptic rigor, and it is sustained until more ATP is
vesicles containing ACh from binding
made available. Rigor mortis is rigor that
to the cell membrane for exocytosis.
occurs a few hours after death, when ATP
Flaccid paralysis results because action
is no longer available. The muscles of the
potentials cannot be produced on the
whole animal progressively become stiff
sarcolemma to bring about muscle con-
and rigid; without ATP to sequester the
traction. The term limberneck has been
calcium and separate the cross‐bridges,
applied to the characteristic flaccid
the filaments remain locked together.
paralysis of the neck in poultry intoxi-
The rigidity continues until cell autolysis
cated with botulinum toxin.
and protein degradation break down the
muscle.
Types of Muscle Contraction
Tone. The term muscle tone refers to the
slight tension exhibited by all muscles at
The primary function of muscle is to con- rest. It is due to the continuous transmission
tract, that is, develop tension and shorten. of impulses at very low frequency from
However, contraction is often described as the spinal cord to the muscles. Tone keeps
having four types: concentric (shortening), muscles in a partially contracted state
eccentric (lengthening), isometric (same and prevents them from becoming flaccid
length but increased tension), and isotonic (flabby), as occurs in paralysis.
(same tension but length changes). When an animal becomes anxious, fear-
Concentric contraction is the usual
form, in which the muscle moves a bone ful, or excited, the muscle tone intensifies.
Therefore, the muscles become taut (tension
or segment by shortening. An example is increases) and the animal can respond
flexion of the elbow by contraction of the faster to any stimulus. This is often seen in
m. biceps brachii. the skittish, nervous, or jumpy animal.
Isometric contraction occurs naturally
whenever a limb or portion of the body is During sleep, muscle tone is low to allow
for optimal relaxation.
held stationary against equal resistance,
such as gravity. To hold the head up in a
fixed position, the dorsal neck muscles Smooth Muscle
must contract isometrically.
Eccentric contraction occurs in the Smooth muscle is sometimes called invol-
extensor muscles of the neck when an animal untary muscle because it is found in struc-
lowers its head gradually. Antagonistic tures that are not voluntarily or consciously