Centrally Acting Muscle Relaxants

Centrally Acting Muscle Relaxants are drugs that reduce muscle spasm and pain by inhibiting the CNS, often used for back pain and injuries.

Definition of Centrally Acting Muscle Relaxants:

• Centrally acting muscle relaxants are a class of drugs that reduce muscle tone and help relieve muscle spasms through actions on the central nervous system (CNS) rather than directly on skeletal muscles.

Mechanism of Action:

• These drugs work primarily by:
• Depressing neuronal activity in the brain and/or spinal cord.
• They often enhance inhibitory pathways, such as GABAergic transmission, or inhibit excitatory signals, leading to reduced muscle spasticity or spasms.

Classification of Centrally Acting Muscle Relaxants

1. GABAergic Drugs

   • Drug: Diazepam
   • Class: Benzodiazepine
   • Mechanism: Facilitates GABA-A receptor activity → increases inhibitory neurotransmission
   • Uses: Muscle spasms, anxiety, seizures

2. GABA-B Agonists

   • Drug: Baclofen
   • Mechanism: Activates GABA-B receptors in the spinal cord → reduces excitatory transmission to skeletal muscles
   • Uses: Spasticity due to multiple sclerosis, spinal cord injury

3. α2-Adrenergic Agonists

   • Drug: Tizanidine
   • Mechanism: Centrally acting α2-adrenergic agonist → inhibits motor neurons in spinal cord
   • Notes: Less sedating than clonidine
   • Uses: Muscle spasticity, chronic pain

4. Others

   1. Cyclobenzaprine
      • Structure: Related to tricyclic antidepressants (TCAs)
      • Mechanism: Acts at brainstem level to reduce tonic somatic motor activity
      • Use: Acute muscle spasm (e.g., trauma, strain)
   2. Methocarbamol, Carisoprodol, Chlorzoxazone
      • Mechanism: Unclear; likely general CNS depressants
      • Uses: Acute musculoskeletal conditions (e.g., low back pain, injuries)

Therapeutic Uses:

• Relief of painful musculoskeletal conditions, e.g., back pain, sprains
• Spasticity management in neurological disorders (e.g., cerebral palsy, multiple sclerosis, spinal cord injuries)

Adverse Effects:

• Sedation, dizziness, drowsiness (common due to CNS depression)
• Weakness, fatigue
• Dependency/abuse potential (notably with carisoprodol)
• Hypotension (notably with tizanidine)

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