Essentials of Anatomy and Physiology

CONTRACTION—THE SLIDING

FILAMENT MECHANISM

All of the parts of a muscle fiber and the electrical
changes described earlier are involved in the contrac-
tion process, which is a precise sequence of events
called the sliding filament mechanism.
In summary, a nerve impulse causes depolarization
of a muscle fiber, and this electrical change enables the
myosin filaments to pull the actin filaments toward the
center of the sarcomere, making the sarcomere
shorter. All of the sarcomeres shorten and the muscle
fiber contracts. A more detailed description of this
process is the following:

1. A nerve impulse arrives at the axon terminal;
   acetylcholine is released and diffuses across the
   synapse.


2. Acetylcholine makes the sarcolemma more per-
   meable to Naions, which rush into the cell.


3. The sarcolemma depolarizes, becoming negative
   outside and positive inside. The T tubules bring
   the reversal of charges to the interior of the mus-
   cle cell.


4. Depolarization stimulates the release of Ca^2 ions
   from the sarcoplasmic reticulum. Ca^2 ions bond
   to the troponin–tropomyosin complex, which
   shifts it away from the actin filaments.


5. Myosin splits ATP to release its energy; bridges
   on the myosin attach to the actin filaments and
   pull them toward the center of the sarcomere,
   thus making the sarcomere shorter (Fig. 7–5).
   6. All of the sarcomeres in a muscle fiber shorten—
   the entire muscle fiber contracts.
   7. The sarcolemma repolarizes: K ions leave the
   cell, restoring a positive charge outside and a neg-
   ative charge inside. The pumps then return Na
   ions outside and Kions inside.
   8. Cholinesterase in the sarcolemma inactivates
   acetylcholine.
   9. Subsequent nerve impulses will prolong contrac-
   tion (more acetylcholine is released).


6. When there are no further impulses, the muscle
   fiber will relax and return to its original length.
   Steps 1 through 8 of this sequence describe a single
   muscle fiber contraction (called a twitch) in response to
   a single nerve impulse. Because all of this takes place
   in less than a second, useful movements would not be
   possible if muscle fibers relaxed immediately after
   contracting. Normally, however, nerve impulses arrive
   in a continuous stream and produce a sustained con-
   traction called tetanus, which is a normal state not to
   be confused with the disease tetanus (see Box 7–2:
   Tetanus and Botulism). When in tetanus, muscle
   fibers remain contracted and are capable of effective
   movements. In a muscle such as the biceps brachii that
   flexes the forearm, an effective movement means that
   many of its thousands of muscle fibers are in tetanus,
   a sustained contraction.
   As you might expect with such a complex process,
   muscle contraction may be impaired in many different
   ways. Perhaps the most obvious is the loss of nerve
   impulses to muscle fibers, which can occur when
   nerves or the spinal cord are severed, or when a stroke

146 The Muscular System

BOX7–2 TETANUS AND BOTULISM

tetanus the cause of death is spasm of the respira-

tory muscles.

Botulismis usually a type of food poisoning, but

it is not characterized by typical food poisoning

symptoms such as diarrhea or vomiting. The neuro-

toxin produced by the botulism bacteria (Clostridium

botulinum) prevents the release of acetylcholine at

neuromuscular junctions. Without acetylcholine,

muscle fibers cannot contract, and muscles become

paralyzed. Early symptoms of botulism include

blurred or double vision and difficulty speaking or

swallowing. Weakness and paralysis spread to other

muscle groups, eventually affecting all voluntary

muscles. Without rapid treatment with the antitoxin

(the specific antibody to this toxin), botulism is fatal

because of paralysis of the respiratory muscles.

Some bacteria cause disease by producing toxins. A

neurotoxinis a chemical that in some way dis-

rupts the normal functioning of the nervous system.

Because skeletal muscle contraction depends on

nerve impulses, the serious consequences for the

individual may be seen in the muscular system.

Tetanusis characterized by the inability of mus-

cles to relax. The toxin produced by the tetanus

bacteria (Clostridium tetani) affects the nervous sys-

tem in such a way that muscle fibers receive too

many impulses, and muscles go into spasms. Lock-

jaw, the common name for tetanus, indicates one

of the first symptoms, which is difficulty opening

the mouth because of spasms of the masseter mus-

cles. Treatment requires the antitoxin (an antibody

to the toxin) to neutralize the toxin. In untreated