Figure 1. Frequency coding in the nervous system: Threshold stimulus. 1  If a threshold stimulus is applied to a neuron and maintained (top, red trace), action potentials occur at a maximum frequency that is limited by the sum of the absolute and relative refractory periods (bottom, blue trace).
Phyllis, Kentucky. Phyllis is an unincorporated community located in Pike County, Kentucky, United States. U.S. Geological Survey Geographic Names Information System: Phyllis, Kentucky.
Wallis, W. R. Map of Fayette County, Kentucky. [Lexington, Ky.: s.n, 1891] Map. Retrieved from the Library of Congress, https://www.loc.gov/item/2011588005/. (Accessed April 11, 2017.)