Neurons show a wide range of biophysical and developmental properties, that can often be seen in light of adaptations to ecological needs. The axons of neurons can grow quickly and extend in space much more than any other animal cell type, and conduct signals quickly or slowly as needed.
Some of the fastest growth in nerves may be seen in deer species (family Cervidae), whose adult males grow racks of antlers each year, that get larger with increasing age. These antlers grow faster than any other mammal bones, and while they grow they are covered by a living tissue called velvet, which includes nerves for sensation. These nerves may be stretched by the developing antler more than one cm per day.
Single neurons may extend their axons more than 1.5 meters in humans, from the brain to the lumbosacral region of the spinal cord, and from there to the tips of the toes. Given that the basic developmental architecture is the same in all mammals, giraffes and whales must have much longer axons in their peripheral nervous systems, likely reaching their maximum in blue whales, which are as large as a jumbo jet.
The fastest-conducting axons are adapted to two major ecological problems: perceiving sound, and escaping danger. In humans and other terrestrial vertebrates like the barn owl, individual sound waves produce action potentials on fast axons that race into the brain to measure the time difference of sound wavefronts on left and right ears, to estimate where in space a sound came from. Many fish and shellfish species have very large, insulated axons that serve escape responses when predation seems immanent. In the shrimp Penaeus Japonicus, conduction velocities can be up to 200 m/sec, about two thirds the speed of sound.
Some axons conduct very slowly. Studies of reptiles and mammals suggest speeds as slow as four cm/sec within the cerebral cortex. Slow conduction can be useful, if the exercise is to estimate the speed of slowly-moving visual stimuli using a dx/dt calculation.
The growth and conduction properties of nerve fibers offer examples of how biological systems can reach into physical extremes to meet ecological demands.
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