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Fundamentals of Pharmacology for Veterinary Technicians
Chapter 7
Drugs Affecting the Nervous System
Basic Anatomy and Physiology
• Basic unit of the nervous system = neuron– Sensory– Associative– Motor
• Parts of the neuron– Cell body– Dendrite– Axon
• Two parts of the nervous system– CNS (central):
brain and spinal cord
– PNS (peripheral): cranial nerves, spinal nerves, autonomic nervous system
Autonomic Nervous System
• Sympathetic (Adrenergic): “fight or flight”– Increases heart rate, respiration rate, and
blood flow to muscles; decreases GI function; causes pupillary dilation
– Preganglionic synapse: ACh; postganglionic synapse: epi or norepi
• Parasympathetic (Cholinergic): “homeostatic”– Brings heart rate, respiration rate, and blood
flow to muscles back to normal levels; returns GI function to normal; constricts pupils to normal size
– Pre- and postganglionic synapse: ACh
CNS Drugs
• Anticonvulsants: help prevent seizures by suppressing the spread of abnormal electric impulses from the seizure focus to other areas of the cerebral cortex– All anticonvulsants are CNS depressants and
may cause ataxia, drowsiness, and hepatotoxicity
– Examples:• Phenobarbital (short-acting barbiturate)• Primidone (structurally similar to phenobarbital)• Diazepam (used IV to treat status epilepticus)• Clorazepate (adjunct anticonvulsant)• Potassium bromide (adjunct anticonvulsant)
CNS Drugs
• Tranquilizers: used to calm animals; reduce anxiety and aggression
• Sedatives: used to quiet excited animals; decrease irritability and excitement
• Anti-anxiety drugs: lessen anxiousness, but do not make animals drowsy
• Examples in these groups:– Phenothiazine derivatives (acepromazine,
chlorpromazine)– Benzodiazepines (diazepam)– Alpha-2 agonists (xylazine, detomidine, medetomidine)
CNS Drugs
• Analgesics: drugs that relieve pain
• Analgesics are categorized as non-narcotic (Chapter 16) or narcotic
• Narcotic analgesics are used for moderate to severe pain
• Narcotic refers to opioid (natural) or opioid-like (synthetic) products
CNS Drugs
• Opioids:– Do not produce anesthesia; patients still
respond to sound and sensation– Produce analgesia and sedation, and relieve
anxiety– Side effects: respiratory depression,
excitement if given too rapidly– Produce their effects by the action of opioid
receptors• Mu = found in the brain• Kappa = found in the cerebral cortex and spinal cord• Sigma = found in the brain
CNS Drugs
• Examples of opioids:– Opium– Morphine sulfate– Meperidine– Hydromorphone– Butorphanol– Hydrocodone– Fentanyl– Etorphine– Buprenorphine– Pentazocine
CNS Drugs
• Opioid antagonists:– Block the binding of opioids to their
receptors– Used to treat respiratory and CNS
depression of opioid use– Examples include naloxone and
naltrexone
CNS Drugs
• Neuroleptanalgesics:– Combination of an opioid and a
tranquilizer or sedative– Can cause a state of CNS depression
and analgesia and may or may not produce unconsciousness
– Combination products may be prepared by veterinarian
– Examples include acepromazine and morphine; xylazine and butorphanol
CNS Drugs
• Anesthetics:– Anesthesia means without sensation– Anesthetics interfere with the conduction of
nerve impulses– Anesthetics produce loss of sensation and
muscle relaxation, and may cause loss of consciousness
– General anesthetics affect the CNS, produce loss of sensation with partial or complete loss of consciousness
– Local anesthetics block nerve transmission in the area of application with no loss of consciousness
Local Anesthetics
• Local anesthetics:– Block pain at the site of administration or
application in the PNS and spinal cord– May be used as nerve blocks, aid in
endotracheal tube placement, and ease skin irritation
– Applied topically to mucous membranes and the cornea by infiltration of a wound or joint, by IV, and around nervous tissue
– Examples include lidocaine, proparacaine, tetracaine, mepivacaine, bupivacaine
General Anesthetics
• Injectable general anesthetics:– Barbiturates: CNS depressants derived from
barbituric acid. Used mainly as anticonvulsants, anesthetics, and euthanasia solutions
– Side effects: potent cardiovascular and respiratory depression
– May be long-acting, short-acting, or ultra-short acting
– May vary in structure and be classified as an oxybarbiturate or thiobarbiturate
– Examples: phenobarbital, pentobarbital, thiopental, methohexital
General Anesthetics
• Injectable general anesthetics (cont.):– Dissociatives: belong to the cyclohexamine
family– Cause muscle rigidity (catalepsy), amnesia,
and mild analgesia– Work by altering neurotransmitter activity– Used for restraint, diagnostic procedures, and
minor surgical procedures– Side effects: cardiac stimulation, respiratory
depression, and exaggerated reflexes– Examples include ketamine and tiletamine
General Anesthetics
• Injectable general anesthetics (cont.):– Miscellaneous:
• Guaifenesin: skeletal muscle relaxant used in combination with an anesthetic drug to induce general anesthesia in horses
• Propofol: short-acting injectable anesthetic agent that produces rapid and smooth induction when given IV (lasts 2–5 minutes)
General Anesthetics/Analgesics
• Inhalant general anesthetics: Inhalant anesthetics are halogenated hydrocarbons– Halothane:
• Nonflammatory, inhalant anesthetic administered via a precision vaporizer
• Can cause hepatic problems, malignant hyperthermia, cardiac problems, and tachypnea
• Contraindicated in cases of gastric dilatation, pneumothorax, and twisted intestines
• Leave animals on 100% oxygen following surgery to prevent diffusion hypoxia
General Anesthetics/Analgesics
• Inhalant general anesthetics (cont.):– Isoflurane:
• Nonflammatory, inhalant anesthetic administered via a precision vaporizer
• Causes rapid induction of anesthesia and short recoveries following anesthetic procedures
• Does not cause the cardiac arrhythmia problems of halothane
• Vigilant monitoring is needed because the animal can change anesthetic planes quickly
• Masking of animals with isoflurane is difficult because it irritates the respiratory system
• Side effects include respiratory depression and malignant hyperthermia
General Anesthetics/Analgesics
• Inhalant general anesthetics (cont.):– Isomers of isoflurane:
• Nonflammable and have fewer cardiovascular side effects than other inhalants
• Quickly enter the bloodstream and escape to the brain, making them good for mask inductions
• Examples:– Enflurane: increases intracranial pressure (do
not use if animal has seizure history)– Desflurane: cannot be delivered by standard
vaporizers and can reduce blood pressure– Sevoflurane: profound respiratory depressant;
close monitoring is needed
General Anesthetics/Analgesics
• Inhalant general analgesics (cont.):– Nitrous oxide:
• Inhalant analgesic that diffuses rapidly throughout the body
• Can enter gas-filled body compartments (increases pressure in these compartments)
• Contraindicated in cases of gastric dilatation, pneumothorax, and twisted intestines
• Leave animals on 100% oxygen following surgery to prevent diffusion hypoxia
CNS Drugs
• CNS stimulants:– Reverse CNS depression caused by
CNS depressants• Doxapram: stimulates brainstem to
increase respiration in animals with apnea or bradypnea. Commonly used when animals have C-sections
• Methylxanthines: bronchodilators that have adverse effect of CNS stimulation. Include caffeine, theophylline, and aminophylline. Side effects include gastrointestinal irritation and bronchodilation
CNS Drugs
• Euthanasia solutions:– Used to humanely end an animal’s life– Usually contain pentobarbital– When pentobarbital is the only narcotic
agent present, it is a C-II controlled substance
– When pentobarbital is in combination with other agents, it is a C-III controlled substance
Autonomic Nervous System Drugs
• Autonomic nervous system drugs work either by acting like neurotransmitters or by interfering with neurotransmitter release
• Two groups of drugs affect the parasympathetic nervous system
• Two groups of drugs affect the sympathetic nervous system
Cholinergic Drugs
• Mimic the action of the parasympathetic nervous system (see Table 7-5)
• Examples include bethanechol, metoclopramide, pilocarpine, drophonium, neostigmine, demecarium, and organophosphates
Anticholinergic Drugs
• Inhibit the actions of acetylcholine by occupying the acetylcholine receptors (see Table 7-6)
• Examples include atropine, glycopyrrolate, aminopentamide, and propantheline
Adrenergic Drugs
• Simulate the action of the sympathetic nervous system (see Table 7-7)
• Side effects include tachycardia, hypertension, and cardiac arrhythmias
• Examples include epinephrine, norepinephrine, isoproterenol, dopamine, dobutamine, phenylpropanolamine, isoetharine, albuterol, terbutaline, ephedrine, and xylazine
Adrenergic Blocking Agents
• Block the effects of the adrenergic neurotransmitters (see Table 7-7)
• Examples of alpha-blockers include phenoxybenzamine, prazosin, and yohimbine
• Examples of beta-blockers include propranolol, metoprolol, and timolol