"Gee Wiz!" Facts About the Brain

Size

• The cortex has 15 million neurons per sq. cm, or 146000 per sq. mm. This holds throughout the brain, regardless of cortical thickness, except in vision areas, where there is 2 1/2 times this number. [C 51]
• A typical neuron may have one to ten thousand input connections, but may have as many as 200 thousand. [A 40]
• A typical neuron may make 1000 connections to other neurons.
• There are approximately 100 billion (i.e. 10¹¹) synapses per sq. cm, or 10 million per sq. mm. This estimate is based on a nominal 7000 synapses per neuron.
• The area of the cortex is 2200 sq. cm = 0.22 sq. m. The neocortex is 700 sq. cm. [C 45]
• Therefore the human cortex contains approximately 30 billion (3 X 10¹⁰) neurons. [C 51] Other estimates run from 10 billion to 100 billion (10¹⁰ - 10¹¹) neurons and 10¹⁴ to 10¹⁵ synapses. [A 5]
• The chimp and gorilla have about 500 sq. cm of cortex and therefore 7-8 billion neurons. [C 51]
• A rat has 4-5 sq. cm of cortex, and therefore about 65 million neurons. [C 51]
• There are approximately 100 million receptor cells in the retina. They feed into approximately one million ganglion cells (also in the retina).
• After age 40, about 1000 neurons die per day. (I seem to have forgotten the source of this...)

Speed

• Typical spikes (action potentials) are 1 - 10 msec. long.
• Maximum spike rate is several hundred per second.
• It takes at least N msec. to distinguish N values by rate coding (due to the Gabor uncertainty principle). Therefore, in 100 msec. a value can be transmitted with 1-2 digits of precision. [M 166]
• The synaptic delay is about 1/2 msec. [A]
• A typical postsynaptic potential has a rise time of 1 - 2 msec. and a decay time of 3 - 5 msec. Much longer decay times occur, tens to hundreds of msec. [A 39-40]
• The membrane time constant is typically 1 - 2 msec. [A 30]
• The membrane length constant is typically 2 - 5 mm. [A 30]
• A typical mental rotation rate is 450 degrees per second. [G 515]

Miscellaneous