Pharm Skel Muscle Relaxants Part 2

Learning objective: describe the adverse effects of succinylcholine.
Answer: B
Succinylcholine activates Nm receptors This can account for the involuntary contraction of
group of muscle fibers (called fasciculations or twitching), that occurs before the muscle
paralysis. Nondepolarizing neuromuscular blockers do not activate Nm receptors and so
contractions cannot occur before paralysis.
A, C, D, E, F) (see explanation above)

Learning objective: explain the mechanism of action of tubocurarine.
Answer: B
Tubocurarine blocks competitively Nm receptors. The activation of these ionotropic receptors
opens Na+ channels, so initiating the cascade that ultimately leads to skeletal muscle
contraction. By blocking Nm receptors the Na+ channels postreceptor pathway is blocked and
muscle relaxations occurs.
A, C, D, E, F) These postreceptor pathway are not involved in the mechanism of action of
nondepolarizing neuromuscular blockers

Learning objective: describe the mechanism of action of baclofen.
Answer: A
Baclofen is an agonist at GABA B receptors. Activation of these receptors by baclofen results in
hyperpolarization of presynaptic terminals, probably by increased K+ conductance. This
hyperpolarization reduces the release of excitatory transmitters in both the brain and the spinal
cord. Baclofen is at least as effective as diazepam in reducing spasticity and is frequently used
for this purpose.
B) This would be the action of nondepolarizing neuromuscular blockers.
C) Actually baclofen can cause blockade of substance P release from spinal cord and this seem
one of the reasons of its analgesic activity in patients with spasticity.
D, E) Baclofen can indirectly block Ca++ channels as a consequence of membrane
hyperpolarization, and can increase k+ conductance, but these actions occur in neuronal, nor in
skeletal muscle membrane.

Learning objective: explain the mechanism of the spasmolytic action of benzodiazepines
Answer: D
Benzodiazepines facilitate the action of GABA in the CNS by increasing the affinity of GABAA
receptors for GABA. Their action in reducing spasticity however seems at least partly mediated
in the spinal cord because they are effective also in patients with cord transection.
A) This would be the mechanism of action of dantrolene.
B) This would be the mechanism of action of botulinum toxin.
C) This would be the mechanism of action of succinylcholine.
E) This would be the mechanism of action of baclofen.

Learning objective: describe the sequence of muscle paralysis induced by tubocurarine.
Answer: A
The skeletal muscle paralysis induced by nondepolarizing neuromuscular blocking drugs follows
a sequence that is related to the innervation of the skeletal muscles. Muscles that are small,
rapidly moving and richly innervated (like extrinsic eye muscles, small muscles of the face and
pharynx) are affected first. The paralysis of the extrinsic eye muscles causes a lack of
parallelism of the visual axes of the eyes which leads to double vision. As a rule, large muscles
(like limb muscles) are paralyzed after small muscles. Ultimately the intercostal muscles and
finally the diaphragm are paralyzed
B, C, D, E) (see explanation above)

Learning objective: describe the therapeutic uses of baclofen.
Answer: B
The stiff-man syndrome is a disorder of neuromuscular transmission characterized by the
insidious onset of progressive stiffness in the trunk, abdomen, legs and arms. The cause of the
syndrome is unknown but an autoimmune pathogenesis is suspected. Only symptomatic therapy
is available and central spasmolytic drugs (benzodiazepines, baclofen) consistently relieve the
muscle stiffness.
A, C, D, E; F) (see explanation above)

Learning objective: describe the therapeutic uses of botulin toxin.
Answer: B
Spasmodic torticollis is an involuntary tonic contraction of the neck muscles causing painful
spasm and abnormal head position. In severe cases the selective denervation of the neck
muscle is the most successful surgical approach but injection of botulin toxin into the dystonic
muscles of the neck can reduce the symptoms for 1 to 3 months in about 70% of patients .
Botulin toxin inhibits acetylcholine release from cholinergic terminals, so weakening skeletal
muscle contractions.
A, B, C) Neuromuscular blocking drugs are of no value in this disease mainly because of their
short duration of action.
E) Spasmolytic drugs (dantrolene, benzodiazepines, tizanidine, etc.) are sometimes used but
they decrease spasms in only about 25% of cases.

Learning objective: describe the actions which mediate the tubocurarine-induced
hypotension.
Answer: E
Tubocurarine induced hypotension can be due either to histamine release or to blockade of
nicotinic receptors of autonomic ganglia or, most likely, to both.
A, B, C, D) (see explanation above)

Learning objective: explain the mechanism of action of dantrolene
Answer: D
The clinical picture of the patient suggest the diagnosis of malignant hyperthermia, a rare but
potentially life-threatening disorder that can be triggered by a variety of stimuli, including the
administration of certain anesthetics and neuromuscular blocking agents. The disorder is
related to an hereditary impairment in the ability of sarcoplasmic reticulum to sequester calcium
and therefore a prolonged release of calcium occurs after the triggering event. This causes
massive muscle contraction, hyperthermia and lactic acidosis. Dantrolene is a drug of choice in
this disorder because it blocks Ca++ channels in sarcoplasmic reticulum of skeletal muscle.
Cardiac and smooth muscle are minimally affected by dantrolene since they have a different
type of Ca++ channel in their sarcoplasmic reticulum.
A, B, C, E) (see explanation above)

Learning objective: explain the mechanism of action of botulin toxin
Answer: D
Botulin toxin must get inside the axon terminals in order to cause paralysis. Following the
attachment of the toxin to proteins on the surface of axon terminals, the toxin can be taken into
neurons by endocytosis. There it is able to cleave endocytotic vesicles and reach the cytoplasm.
The light chain of the toxin has protease activity and proteolytically degrades a specific protein
that is required for vesicle fusion that releases acetylcholine from the axon endings. In this way
it prevents neurosecretory vesicles from fusing with the nerve synapse plasma membrane and
releasing the neurotransmitter.
A, B, C, E, F) (see explanation above)

Learning objective: describe the pharmacokinetics of mivacurium.
Answer: D
Mivacurium and succinylcholine are two neuromuscular blockers metabolized by plasma
cholinesterase. In patients with a genetically induced deficiency of the normal plasma
cholinesterase these drug can cause apnea of several hours and are therefore contraindicated.
A, B, C ,E, F) (see explanation above)

Learning objective: describe the adverse effects of succinylcholine.
Answer: E
Succinylcholine can release rapidly potassium from intracellular sites. In a normal patient this
results in a small, transient hyperkalemia. In patients with extensive soft-tissue trauma or burns,
cerebral vascular accident and prolonged stays in an intensive care unit, this hyperkalemia can
be much more pronounced when succinylcholine is given at least several days after the
underlying condition has become established. The hyperkalemia can be life threatening since it
can lead to cardiac arrest like in the present case. The cause of succinylcholine-induced
hyperkalemia in these patients seems related to up regulation of Nm receptors in the skeletal
muscle. Patients with the aforementioned conditions are deprived of neural influence or activity
because of the prolonged immobilization. Such deprivation stimulates the synthesis of new Nm
receptors across the muscle membrane. When these receptors are activated by succinylcholine
an action potential occurs, ion channels open allowing Na+ to flow inside and K+ to flow outside
of the cell. Since succinylcholine is metabolized more slowly than acetylcholine the receptor
activation is more prolonged, causing a larger influx of Na+ and a vigorous efflux of K+. When
Nm receptors are increased in skeletal muscle this K+ efflux can cause a life-threatening
hyperkalemia.
A, B, C) Nondepolarizing neuromuscular blockers cause blockade, not activation, of Nm
receptors. Therefore they do not induce hyperkalemia.
D) Dantrolene has no effect on Nm receptors.

Learning objective: describe the adverse effects of baclofen.
Answer: E
Baclofen is a GABA B receptor agonist that is currently used to treat various forms of spasticity.
The drug causes hyperpolarization of neuron membrane, which in turn reduced the release of
excitatory neurotransmitters in the brain as well as in the spinal cord. The most common
adverse effects of baclofen are relate to this central inhibitory activity. Drowsiness is by far the
most common, affecting about one half of patients.
A, B, C, D) All these listed effects can occur with baclofen but they are very rare at standard
therapeutic doses.

Learning objective: describe the biotransformation of mivacurium.
Answer: D
Mivacurium is the only nondepolarizing neuromuscular blocker that is metabolized by plasma
pseudocholinesterase. Because of this rapid metabolism the action of the drug is quite short.
A) succinylcholine is metabolized by plasma cholinesterase, but it is a depolarizing, not a
nondepolarizing neuromuscular blocker.
B, C, E) (see explanation above)