Fundamentals of Pharmacology for Veterinary Technicians Chapter 13

Fundamentals of Pharmacology for Veterinary Technicians Chapter 13 Drugs Affecting Muscle Function

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

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

Basic Anatomy and Physiology • Depolarization of the muscle occurs, calcium is released, and the muscle contracts • ACh is inactivated by acetylcholinesterase

Basic Anatomy and Physiology Insert Figure 13 -1

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)

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

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

Other Neuromuscular Blockers • Other examples: – Gallamine – Vecuronium – Metocurine • Antidote: – Atropine (because these drugs are cholinergic in nature)

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

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

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)

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