Neurons are fascinating, widely diverse devices. Some like the giraffe primary afferent, have axons that are at least 15 feet long. Others, like the typical granule cell, have axons that are only about 100 microns long. Some, like the common pyramidal cells, produce and transmit stereotypical action potentials (neural spikes) down their axon. Others, like the retinal horizontal cells, communicate without such spikes. Some send action potentials at speeds over 400 km/h. Others transmit spikes at a mere 2 km/h. Some, like the purkinje cells, have about 200,000 input connections. There are literally hundreds and hundreds -- by some estimates thousands -- of different kinds of neurons in the brain.1 And, taken together their numbers are staggering: 1010 neurons in the brain; at least 1013 synapses; 45 miles of fiber; 100 different kinds of neurotransmitters.
-- Chris Eliasmith , Charles H. Anderson
from "Neural Engineering: Computation, Representation, and Dynamics in Neurobiological Systems"
Quoted on Sat Dec 15th, 2012