A sneeze (sternutation) is a sudden, involuntary, protective reflex that clears the nasal passages. This rapid expulsion of air removes foreign particles, irritants, and pathogens from the nasal mucosa, protecting the lower respiratory tract. It is a complex physiological event coordinated by the nervous system to maintain the cleanliness and health of the airways.
The Triggering Mechanism
The sneeze reflex begins when sensory receptors lining the nasal cavity detect an irritant, such as dust, pollen, strong odors, or smoke. These specialized nerve endings belong to the trigeminal nerve, the largest cranial nerve, which is responsible for sensation in the face. Upon contact with the irritant, the trigeminal nerve sends an afferent (incoming) signal directly to the brainstem.
The signal travels specifically to an area in the medulla oblongata, known as the sneeze center or sneeze-evoking zone. This region acts as the central integration node for the sensory input, where the nervous system determines the appropriate motor response. Once a specific threshold of irritation is reached, the medulla coordinates the complex, synchronized muscle movements that constitute the sneeze.
Less common is the photic sneeze reflex, sometimes called the sun sneeze. In this neurological variation, an estimated 18 to 35 percent of the population experiences a sneeze triggered not by nasal irritants, but by sudden exposure to bright light. This response is thought to be caused by a cross-talk between the optic nerve and the trigeminal nerve pathways within the brainstem.
The Physiological Process
The physical act of sneezing is a coordinated motor sequence involving numerous muscles across the body, executed in three distinct phases. The process begins with a preparatory phase, marked by a deep, involuntary inhalation that fills the lungs with air. This large intake is necessary to provide the volume required for the forceful expulsion that follows.
Next, the compression phase begins as the muscles of the chest, abdomen, and diaphragm contract powerfully. This simultaneous contraction rapidly builds up significant pressure within the lungs and the upper airway. During this moment, the soft palate and the back of the tongue momentarily seal off the throat, ensuring the pressure is contained.
The final and most noticeable phase is the expulsion phase. The built-up pressure is suddenly released as the glottis opens, allowing a high-velocity stream of air to rush out. The soft palate then rapidly lifts, directing the explosive blast primarily through the nose and mouth. This forceful expulsion successfully clears the nasal passages of the initial irritant.
Common Questions and Immediate Aftermath
The air and expelled droplets from a sneeze are released at a high velocity, ranging from approximately 10 to over 100 miles per hour. This powerful burst creates a cloud of mucus and saliva droplets capable of traveling a considerable distance, highlighting the importance of covering the nose and mouth.
The involuntary closing of the eyes during a sneeze is a protective reflex. This reflex is believed to shield the eyes from the high-velocity expulsion of infectious particles and mucus. While urban legends suggest the pressure could cause the eyes to pop out, the eye closure is instead a coordinated muscle response involving the eyelid muscles.
Trying to stop or suppress a sneeze by pinching the nose and closing the mouth forces the immense pressure to redirect internally. This sudden redirection can channel the pressure into the Eustachian tubes, potentially causing a ruptured eardrum or inner ear damage. In extremely rare cases, the pressure has been known to cause damage to blood vessels in the eyes or even lead to soft tissue tears in the throat.