52 yrs male presented with muscular stiffness and painful cramps. The face was drawn into a forced grimace (“sardonic grin,” or risus sardonicus). Strychnine Poisoning was suspected; Strychnine is supposed to antagonize competitively
## **Core Concept**
Strychnine poisoning is a condition caused by the ingestion of strychnine, a highly toxic alkaloid found in the seeds of the Strychnos nux-vomica tree. The toxin acts on the nervous system, causing muscle convulsions and stiffness. The primary mechanism of action involves the inhibition of an inhibitory neurotransmitter in the spinal cord.
## **Why the Correct Answer is Right**
The correct answer involves understanding the mechanism of action of strychnine. Strychnine competitively antagonizes **glycine**, an inhibitory neurotransmitter in the spinal cord. By blocking glycine receptors, strychnine reduces the inhibitory effects on spinal neurons, leading to an increase in excitatory neurotransmission. This results in muscle convulsions, stiffness, and the characteristic symptoms observed in the patient, such as a "sardonic grin" or risus sardonicus.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because strychnine's action is not primarily associated with acetylcholine, a neurotransmitter involved in muscle activation, among other functions.
- **Option B:** This option is incorrect because GABA (gamma-aminobutyric acid) is an inhibitory neurotransmitter in the brain, but strychnine's toxic effects are specifically related to the spinal cord and glycine receptors, not GABA receptors.
- **Option C:** This option might seem plausible because glutamate is an excitatory neurotransmitter. However, strychnine's mechanism does not involve directly antagonizing glutamate; instead, it involves blocking an inhibitory neurotransmitter.
## **Clinical Pearl / High-Yield Fact**
A key clinical pearl in this context is that strychnine poisoning classically presents with muscle convulsions and stiffness without affecting the level of consciousness until very late stages. The "sardonic grin" is a hallmark sign. Treatment involves supportive care and sometimes the use of benzodiazepines to control muscle spasms.
## **Correct Answer:** . **Glycine**