A hiccup begins when something disturbs the sensory endings of the vagus or phrenic nerve — a stretch of the stomach wall, a sudden change in temperature, a swallow of air. That disturbance generates action potentials that travel along the afferent fibers toward the brainstem. The signals arrive at the medullary hiccup center, a region of the reticular formation that also receives input from the respiratory rhythm generator and from higher centers.
The center integrates these signals and, if the summed input crosses threshold, emits a motor command. The command descends through the spinal cord to the phrenic motor neurons in the cervical segments C3–C5, which send their axons out as the phrenic nerve to the diaphragm. At the same time, thoracic motor neurons drive the external intercostal muscles. The diaphragm contracts, the chest expands, and air is drawn in — until the glottis snaps shut, producing the characteristic sound.
The key point is that the reflex is not a simple loop from one receptor to one muscle. It is a convergent circuit: multiple sensory sources feed in, the brainstem weighs them, and the output is a coordinated motor pattern. That is why the same hiccup can be triggered by a dozen different things, and why interrupting the arc at different points produces different clinical pictures.