SKELETAL MUSCLE RELAXANTS
Section I. GENERAL
Some Indian tribes in South America have used muscle relaxants for centuries.
They have used curare, a potent muscle relaxant, to kill game and to protect
themselves because of curare's ultimate pharmacological effect-death. Today,
anesthesiologists use this agent to relax skeletal muscles in some surgical procedures.
This lesson will focus on skeletal muscle relaxants and their use in modern medicine.
3-2. DEFINITION OF A MUSCLE RELAXANT
A skeletal muscle relaxant may be defined as an agent that reduces skeletal
muscle tone. Even when muscles are at rest, there is a certain amount of tension or
tautness that is present. This remaining degree of contraction of skeletal muscle is
called skeletal muscle tone. It is believed that skeletal muscle tone results entirely from
nerve impulses originating from the spinal cord. If these nerve impulses are blocked in
some manner, the result is decreased skeletal muscle tone: skeletal muscle relaxation.
The degree of skeletal muscle relaxation ranges from partial to complete depending
upon the effectiveness of the skeletal muscle relaxant being used and its site of activity.
Section II. THE NEUROMUSCULAR BLOCKING AGENTS
3-3. MECHANISM OF ACTION
a. The neuromuscular blocking agents act by blocking the action of
acetylcholine (Ach) at the neuromuscular junction or at the muscle receptor site.
b. What occurs at the neuromuscular junction during normal nerve
transmission? The nerve impulse enters the terminal knob, and the neurotransmitter
acetylcholine (Ach) is released and attaches to appropriate receptor sites on the muscle
receptor site, much like a lock and key (Figure 3-1). When Ach attaches, there is a
great influx of sodium into the muscle receptor site, and potassium flows out. This
causes the receptor site to depolarize; therefore, muscle contraction results.
c. The Ach does not remain in the receptor sites forever. When it releases, it is
destroyed by acetylcholinesterase (Ache). The resultant release causes an influx of
potassium back into the muscle receptor site, and sodium is pumped out. The nerve
that stimulates the muscle receptor site repolarizes and returns to normal. Because of
repolarization, the skeletal muscle relaxes.