- Distance – The farther signals need to travel, the longer the reaction time is going to be. Reaction times for movements of the foot are longer than for movements of the hand, in large part because the signals traveling to and from the brain have a longer distance to cover. This principle is readily demonstrated through reflexes (note, however, that reflexes are responses that occur without “thought” because they do not involve neurons that engaged in conscious thought). The key observation for the present purpose is that the same reflexes evoked in taller individuals tend to have longer response times than for shorter individuals. By way of analogy, if two couriers driving to New York leave at the same time and travel at exactly the same speed, a courier leaving from Washington, DC will always arrive before one leaving from Los Angeles.
Neuron characteristics – The width of the neuron is
important. Signals are carried more quickly in neurons with larger
diameters than those that are narrower – a courier will generally travel
faster on wide multi-lane highways than on narrow country roads.
How much myelination a neuron has is also important. Some nerve cells have myelin cells that wrap around the neuron to provide a type of insulation sheath. The myelin sheath isn’t completely continuous along a neuron; there are small gaps in which the nerve cell is exposed. Nerve signals effectively jump from exposed section to exposed section instead of traveling the full extent of the neuronal surface. So signals move much faster in neurons that have myelin sheaths than in neurons that don’t. The message will get to New York sooner if it passes from cellphone tower to cellphone tower than if the courier drives the message along each and every inch of the road. In the human context, the signals carried by the large-diameter, myelinated neurons that link the spinal cord to the muscles can travel at speeds ranging from 70-120 miles per second (m/s) (156-270 miles per hour[mph]), while signals traveling along the same paths carried by the small-diameter, unmyelinated fibers of the pain receptors travel at speeds ranging from 0.5-2 m/s (1.1-4.4 mph). That’s quite a difference!
- Complexity – Increasing the number of neurons involved in a thought means a greater absolute distance the signal needs to travel – which necessarily means more time. The courier from Washington, DC will take less time to get to New York with a direct route than if she travels to Chicago and Boston along the way. Further, more neurons mean more connections. Most neurons are not in physical contact with other neurons. Instead, most signals are passed via neurotransmitter molecules that travel across the small spaces between the nerve cells called synapses. This process takes more time (at least 0.5 ms per synapse) than if the signal was continually passed within the single neuron. The message carried from Washington, DC will take less time to get to New York if one single courier does the whole route than if multiple couriers are involved, stopping and handing over the message several times along the way. In truth, even the “simplest” thoughts involve multiple structures and hundreds of thousands of neurons.