This isn't quite a limitation because evolution has corrected for it, but.... The neurons responsible for sending sound information to the brain do so by creating action potentials. These action potentials represent the voltage spike observed within a neuron when it fires. Since this is a physical process, there are limitations to how many times/second a single neuron can fire, and that limit is about 300-500 times per second, equivalent to 300-500Hz. By audio equipment standards, that kind of frequency response is laughable. The reason you can hear sounds above 500Hz is that multiple neurons fire in a "volley" mimicing the higher frequency signals from the ear, and thus producing higher frequency sounds for the brain to perceive. This is not a limitation of the cilia/cochlea/ear, it is a limitation of neurons, and can be compared to the effect that lower sampling rates have on frequency. So I guess in spec terms: the frequency response of a single neuron is limited to 500Hz