A sneeze, or sternutation, is a powerful, involuntary reflex action designed to protect the respiratory system. This convulsive expulsion of air from the lungs clears the upper airways. The primary biological purpose of this sudden event is to rid the nasal passages of foreign particles and irritants.
How the Sneeze is Triggered
The sneeze reflex is initiated by a variety of stimuli that irritate the sensitive mucous membranes lining the nasal cavity. The most common triggers are mechanical or chemical irritants, such as dust, pollen, smoke, or strong chemical odors. These airborne particles land on the nasal mucosa, where they are detected as foreign invaders.
A number of non-irritant stimuli can also activate the reflex, including the sudden exposure to bright light, known as the photic sneeze reflex. This is thought to involve a cross-activation between the optic nerve and the trigeminal nerve. Abrupt changes in temperature, such as encountering a blast of cold air, can also trigger a sneeze.
The Neural Pathway
The sneeze reflex arc begins when specialized sensory nerve endings, called pruriceptors, in the nasal lining detect the presence of an irritant. This sensation sends an electrical signal along the afferent pathway of the trigeminal nerve, which transmits facial sensory information, toward the brainstem. The sensory information converges in the medulla oblongata, commonly referred to as the “sneeze center.” This center integrates the input and initiates the efferent, or motor, response, sending coordinated motor signals to muscles throughout the body to execute the expulsion phase.
The Physical Stages of the Sneeze
The physical event of a sneeze follows a precise, three-stage sequence involving the respiratory and musculoskeletal systems.
Inhalation Phase
The process begins with the involuntary Inhalation Phase, where the diaphragm and the intercostal muscles between the ribs contract. This action draws a substantial volume of air deep into the lungs, maximizing the available force for the subsequent expulsion.
Compression Phase
Next is the Compression Phase, which builds up the necessary pressure to clear the airways. The vocal cords, or glottis, close tightly while the muscles of the abdomen and chest powerfully contract. This simultaneous contraction against a closed glottis causes a rapid and significant increase in pressure within the respiratory system.
Expulsion Phase
The final and most dramatic event is the Expulsion Phase, which begins with the sudden, explosive opening of the glottis. The compressed air, along with mucus and the original irritant, is violently ejected through the nose and mouth. The expelled air travels at a high velocity, reaching maximum speeds around 4.5 meters per second (10 miles per hour).
During this forceful expulsion, the eyes reflexively close as a protective action to prevent irritants from entering the ocular surface. The soft palate also depresses and the back of the tongue elevates, which helps to channel the air through the nasal cavity to maximize the cleansing effect.