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Chapter 13
Drugs Affecting Muscle Function
Copyright © 2011 Delmar, Cengage Learning
Basic Anatomy and Physiology
• Muscles are tissues that contract to produce
movement
• Muscles are made up of long, slender cells
called muscle fibers
• Muscle fibers are encased in a fibrous sheath
• Muscle cells are categorized into three types
based on their appearance and function:
– Skeletal
– Smooth
– Cardiac
Copyright © 2011 Delmar, Cengage Learning
Basic Anatomy and Physiology
• Motor nerves that originate in the spinal cord
and terminate in fibers connected to muscle cells
activate skeletal muscle
• A neuromuscular junction is where a motor
nerve fiber connects to muscle cells
• Electrical impulses from the spinal cord travel to
the neuromuscular junction, causing release of
the neurotransmitter acetylcholine (ACh)
• ACh binds to specialized receptor sites on the
muscle opposite the nerve ending
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Basic Anatomy and Physiology
• Depolarization of the muscle occurs,
calcium is released, and the muscle
contracts
• ACh is inactivated by acetylcholinesterase
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Muscle Disorders
• Usually associated with traumatic injuries,
neuromuscular disease, or chronic debilitating
disorders
• Signs such as weakness, muscle atrophy, and
pain may need to be treated to prevent other
complications such as pressure sores, skin
infection, and further debilitation
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Drugs That Affect
the Skeletal System
• Anti-inflammatories (Chapter 16)
• Neuromuscular blockers (drugs that
produce paralysis)
• Skeletal muscle spasmolytics (drugs that
reduce muscle spasms)
• Anabolic steroids (drugs whose tissue
building effects can reverse muscle
atrophy or wasting)
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Neuromuscular Blockers
• Relax the muscles of animals that are undergoing
surgery; used to prevent or treat muscle spasms
• Work by disrupting the transmission of nerve impulses
from the motor nerves to skeletal muscle fibers
• Most are competitive nondepolarizers (compete with
ACh for the same receptor sites)
• Are also called curarizing agents
• Examples:
– Pancuronium
– Atracurium
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Neuromuscular Blocker Antidotes
• Competitive nondepolarizers require antidotes
once their effects are no longer needed
• Examples:
– Neostigmine: competes with ACh for
acetylcholinesterase, allowing ACh to accumulate in
the neuromuscular junction
– Pyridostigmine: competes with ACh for attachment to
acetylcholinesterase
– Edrophonium: attaches to acetylcholinesterase,
thereby hindering the breakdown of ACh
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Other Neuromuscular Blockers
• Other examples:
– Gallamine
– Vecuronium
– Metocurine
• Antidote:
– Atropine (because these drugs are cholinergic
in nature)
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Depolarizing Neuromuscular
Blockers
• Depolarizing neuromuscular blockers
mimic the action of ACh in muscle fibers,
and because they are not destroyed by
acetylcholinesterase, their action is
prolonged
• Example:
– Succinylcholine: binds to the cholinergic
receptors
• No antidote needed; wears off by itself
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Spasmolytics
• Break down the muscle spasticity
• Used to treat acute episodes of muscle spasticity
associated with neurological and musculoskeletal
disorders (malignant hyperthermia, equine
postanesthetic myositis, and traumatic injury)
• Examples:
– Methocarbamol: used as an adjunctive therapy for inflammatory
and traumatic conditions of skeletal muscle
– Guaifenesin: used as an adjunct to anesthesia because it
relaxes both laryngeal and pharyngeal muscles, thereby making
intubation easier
– Diazepam: used for muscle relaxation
– Dantrolene: used to prevent and treat malignant hyperthermia
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Anabolic Steroids
• Have tissue-building effect (they increase muscle mass)
• Used to promote growth, counteract postsurgical debility,
and treat diseases such as muscular atrophy and
orthopedic conditions
• Examples:
– Nandrolone (stimulates erythropoiesis and appetite)
– Stanozolol (improves appetite and weight gain)
– Boldenone (improves appetite and musculature)
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Anabolic Steroids
• To obtain optimal results when giving
anabolic steroids, adequate and wellbalanced dietary intake is essential
• Can cause electrolyte imbalance, liver
toxicity, behavioral changes, and
reproductive abnormalities
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