The temporal theory of pitch perception proposes that the frequency of a sound wave is encoded by the activity level of a sensory neuron, meaning a given hair cell fires action potentials at a rate related to the sound's frequency. However, this theory is limited because humans can detect a broad range of frequencies (20–20,000 Hz), which is much wider than the maximum possible firing rate of an individual hair cell. This constraint exists because neurons have a limit to how fast they can fire action potentials, a physiological limitation determined by the properties of sodium channels located on the neuronal membrane (Shamma, 2001).