But the grasshopper mouse is completely resistant to the bark scorpion's venom. In fact, it actively preys upon scorpions and other poisonous creatures. As the film clip below shows, it responds to the bark scorpion's sting by licking its paw for a second or two, before resuming its attack, then killing and eating the scorpion, starting with the stinger and the bulb containing the venom. Researchers have now established exactly why this is – paradoxically, the venom has an analgesic, or pain-killing, effect on the grasshopper mouse.
The animal's secret lies in two proteins, the sodium channels Nav1.7 and Nav1.8, which are found in a subset of sensory nerve fibres called nociceptors. These cells express numerous other proteins that are sensitive to damaging chemicals, excessive mechanical pressure, and extremes in temperature, and have fibres that extend from just beneath the skin surface into the spinal cord.
The sensor proteins relay these signals to Nav1.7 and Nav1.8, which then change their structure in response, so that their pores, which span the nerve cell membrane, open up, allowing sodium ions to flood into the cell. This causes the nociceptors to generate nervous impulses, which are transmitted along the fibre into the spinal cord. From there, the signals are relayed to second-order sensory neurons, which then carry the signals up into the brain, where they are interpreted as pain.