Page 195 - Anatomy and Physiology of Farm Animals, 8th Edition

180 / Anatomy and Physiology of Farm Animals

in the whole muscle. Temporal summation produces spasm of the masseter muscles
occurs when the frequency of stimulation
VetBooks.ir to one or more motor units is increased. (lockjaw) followed by a spasm of other
muscles, is also specifically referred to as
That is, the frequency of stimulation is
such that the first contraction is not over tetanus. The Clostridium toxin produces
skeletal muscle tetany by inhibiting the
by the time the second contraction begins. release of inhibitory neuromediators
The two become additive, which increases within the central nervous system. These
the contraction strength (Fig. 9‐7). Ordi- inhibitors normally act within the spinal
narily, in normal muscle function both types cord to regulate the activity of motor
of summation occur at the same time. neurons to skeletal muscle. Without

Tetany (Tetanus). When the frequency these inhibitors, any motor activity can
of stimulation becomes so rapid that result in spastic or tetanic contractions
further increases in frequency will not of the skeletal muscle. Loud noises or
increase the strength of contraction, the sudden movements can cause affected
greatest force that the muscle can develop animals to tense their muscles and may
has been reached. This is called tetany, induce generalized spasms.
tetanus, or tetanization. All of these terms Fatigue. Fatigue is a decrease in work
are defined as a continuous tonic spasm of capacity caused by work itself. Fatigue
muscle or a steady state of contraction may occur at the level of the individual
(Fig. 9‐8). The disease caused by the muscle fiber, or it may be a generalized
toxin from Clostridium tetani, which state affecting the animal as a whole. On a

Force

S S S S
S=Stimulus Time
Figure 9-7. Temporal summation of muscle twitches. The first two contractions illustrate the force
generated by a single stimulus (action potential). When a second stimulus is applied before the muscle
can completely relax, summation occurs, and the force of contraction is greater (third recording).

Tetany

Force

= Stimulus Time

Figure 9-8. Tetany. The interval between stimuli (action potentials) steadily decreases until no relaxation
can occur between action potentials.