Sneezing is a reflexive act that happens when your nasal passageways are irritated by something. Sneezing involves a reflex arc, which is the circuit traveled by impulses producing a reflex action – from the receptor organ, through the afferent nerve (sensory nerve), nerve center, efferent nerve (motor nerve), to the effector (another organ, hormone, etc.). Here are the steps:
1. An irritant is detected by nerve endings (receptors) in your nasal passageways; this irritant could be anything from a smell, dust, viruses that attack mucosa membranes, and so on.
2. Nerves carry these impulses to sneezing center in your brain stem.
3. Then the sneezing center sends the instructions down your facial nerves and nerves that lead to your lungs and diaphragm.
4. This causes your eyes to start to water and your nasal passageways to secrete fluid. This also causes you to take in a deep breath, caused by an abrupt movement of your diaphragm.
5. Next, the diaphragm and your chest muscles contract, which causes the air to leave your nose and mouth suddenly and fast.