Because races are often set by a gunshot, the sense organ that the Olympic runner would be relying on at the start line of a race would be his ears. Specifically, the cochlea within the ear is the mechanoreceptor (a type of receptor that interprets mechanical impulses) that receives sound stimuli.
The sense organs contain receptors that are sensitive to stimuli. Receptors are cells, tissues or organs that are designed to respond to one type of environmental stimuli. For example, the sense receptors in the eyes are sensitive to the light stimuli. Receptors in the ears are sensitive to sound and position stimuli.
When the stimuli are received by the receptor, a generator potential is produced. The strength of the generator potential is directly proportional to the magnitude of the stimulus.