7.11 : Spasmolytic Agents: Chemical Classification

Spasmolytic agents are drugs used to alleviate muscle spasms and spasticity. They can be categorized into different chemical groups based on their mechanisms of action. Centrally acting spasmolytics primarily affect the spinal cord, while others directly target skeletal muscle cells.

A major class of centrally acting spasmolytics is the α₂-agonist, such as tizanidine. These drugs bind to α₂-adrenoceptors, inhibiting the release of the excitatory neurotransmitter glutamate. They also promote glycine release, increasing chloride influx into postsynaptic neurons and reducing excitability.

GABA mimetics, such as baclofen, activate GABA_B receptors, leading to hyperpolarization of neuronal membranes and reduced excitability. Baclofen suppresses the release of excitatory transmitters in the brain and spinal cord, suppressing sensory afferents, interneurons, and motor neurons.

Benzodiazepines like diazepam bind to the allosteric site of GABA_A receptors and facilitate GABA's action in the CNS. Diazepam reduces spasticity, particularly in the spinal cord, but sedation can occur at doses required to reduce muscle tone.