Module 6: CNS Drugs and Muscle Relaxants

Muscle Relaxants

• Divided into two main classes: centrally-acting and peripherally-acting.

Centrally-Acting Muscle Relaxants

• Key drugs include baclofen, carisoprodol, and chlorphenesin.
• Mechanism of Action (MOA): Agonizes GABA_B receptors, reducing the release of excitatory neurotransmitters, inhibiting nerve signal transmission in the brain and spinal cord.
• Indications: Treatment of acute musculoskeletal conditions and management of spasticity from conditions like Multiple Sclerosis, spinal cord injuries, cerebral palsy, and strokes; off-label uses include chronic hiccups and trigeminal neuralgia.
• Adverse Reactions: CNS depression, GI disturbances, urinary issues, hypotension, arrhythmias, and potential discoloration of urine with chlorzoxazone.

Peripherally-Acting Muscle Relaxants

• Dantrolene sodium significantly affects muscle contraction by blocking calcium ion release from the sarcoplasmic reticulum.
• Indications: Manage spasticity in conditions like cerebral palsy and MS, also treats malignant hyperthermia.
• Drug Interactions: Increased CNS depression when used with other CNS depressants; potential liver toxicity risk with estrogen.

Sedatives, Anxiolytics, and Hypnotics

• Sedatives decrease activity and excitement; higher doses may induce sleep (hypnosis).
• Main classes include benzodiazepines, barbiturates, and nonbenzodiazepine-nonbarbiturate drugs.

Benzodiazepines

• Function as minor tranquilizers with anxiolytic effects; include drugs like diazepam, lorazepam, and temazepam.
• MOA: Stimulate GABA receptors, enhancing inhibition of neuron firing in the central nervous system.
• Indications: Insomnia, anxiety, alcohol withdrawal, seizure disorders.

Barbiturates

• Reduce overall CNS alertness; include amobarbital and phenobarbital.
• MOA: Depress the CNS by inhibiting neural impulse conduction and altering cerebral functions.
• Indications: Insomnia, preoperative sedation, and seizure disorders; risk of respiratory depression and dependence.

Antidepressants

• Used for affective disorders; major types include MAOIs, TCAs, and SSRIs.

MAO Inhibitors

• Increase availability of norepinephrine and serotonin by inhibiting monoamine oxidase.
• Indicated for atypical depression and some anxiety disorders; caution with tyramine-rich foods.

Tricyclic Antidepressants (TCAs)

• Effective for major depression; include amitriptyline and nortriptyline.
• MOA: Block reuptake of norepinephrine and serotonin.
• Indications: Episodes of major depression and related physical symptoms; requires careful monitoring with MAOIs to avoid severe reactions.

Selective Serotonin Reuptake Inhibitors (SSRIs)

• Second-generation antidepressants with fewer side effects.
• MOA: Prevent reuptake of serotonin.
• Indications: Major depressive episodes, OCD, panic disorders, and other mood disorders.

Antiparkinsonian Agents

• Aim to increase dopamine secretion and inhibit cholinergic activity.

Anticholinergic Drugs

• Reduce tremors caused by high acetylcholine levels by blocking its action in the CNS.

Dopaminergic Drugs

• Include levodopa and carbidopa-levodopa, enhancing motor function by increasing dopamine levels or neurotransmission.
• Indicated for severe parkinsonism and must be tapered carefully to avoid complications.

Antiepileptic Agents

• Prescribed for chronic epilepsy management and acute seizure control.
• Major classes include hydantoins, barbiturates, iminostilbenes, benzodiazepines, and valproic acid.

Hydantoins

• Phenytoin is the most commonly prescribed; stabilizes nerve cells to prevent overexcitement and manages tonic-clonic seizures.
• Adverse effects include liver toxicity and gingival hyperplasia.

Barbiturates

• Phenobarbital is used less frequently due to adverse effects; effective for various seizure types but increases risk of CNS depression and dependence.

Benzodiazepines

• Effective for absence, atonic, and myoclonic seizures by enhancing GABA effectiveness.

Antipsychotic Agents

• Used to control psychoses such as schizophrenia and mania; categorized into typical and atypical antipsychotics.

Typical Antipsychotics

• Block postsynaptic dopaminergic receptors; indications include schizophrenia and agitation.

Atypical Antipsychotics

• Cause fewer side effects, primarily weight gain; effective for treating negative symptoms in schizophrenia and less likely to cause extrapyramidal symptoms (EPS).
• Examples: clozapine, olanzapine, risperidone, primarily blocking dopamine and serotonin receptors.

Explore the various drug classes affecting the central and peripheral nervous systems, including muscle relaxants, antidepressants, and antipsychotics. This quiz covers key information related to centrally-acting and peripherally-acting muscle relaxants, with a focus on their mechanisms of action and specific examples. Test your knowledge of these essential pharmacological topics.