**Core Concept**
Respiratory centre depression refers to the reduction in the activity of the medulla oblongata's respiratory control centers, leading to decreased respiratory rate and depth. This can be caused by various substances that either directly or indirectly affect the central nervous system, particularly the medulla oblongata.

**Why the Correct Answer is Right**
Strychnine is a central nervous system stimulant that acts as a competitive antagonist at the inhibitory neurotransmitter glycine receptors. Unlike other substances listed, strychnine does not cause respiratory center depression. Instead, it causes an increase in muscle tone and can lead to muscle convulsions due to its excitatory effects on the nervous system. This is because strychnine blocks the inhibitory effects of glycine, leading to unopposed excitatory neurotransmission.

**Why Each Wrong Option is Incorrect**
**Option A:** Opium, an opioid alkaloid, causes respiratory center depression by binding to opioid receptors in the brainstem, specifically the medulla oblongata. This leads to decreased respiratory rate and depth, as well as decreased cough reflex.
**Option C:** Barbiturates are central nervous system depressants that can cause respiratory center depression by enhancing the activity of inhibitory neurotransmitters, such as GABA, and decreasing the activity of excitatory neurotransmitters. This leads to sedation, decreased respiratory rate, and decreased respiratory drive.
**Option D:** Gelsemium is a plant-based substance that causes respiratory center depression by affecting the central nervous system, particularly the medulla oblongata. It is thought to act as a GABA agonist, leading to increased sedation and decreased respiratory drive.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that strychnine poisoning primarily presents with muscle convulsions, muscle rigidity, and hyperreflexia, rather than respiratory depression. This is due to its unique mechanism of action as a glycine receptor antagonist.

**✓ Correct Answer: B. Strychnine**