Action potential velocity
Some Signals Are Very Fast
Consider the Following:
- Pain is actually one of the slowest sensations our bodies can send. Smaller fibers without myelin, like the ones carrying pain information, carry signals at about 0.5-2.0 m/s (1.1-4.5 miles per hour). The fastest signals in our bodies are sent by larger, myelinated axons found in neurons that transmit the sense of touch or proprioception – 80-120 m/s (179-268 miles per hour). Scientists believe that this reflects the evolution of these senses - pain was among the most important things to sense, and so was the first to develop through small, simple nerves. More nuanced senses like vibration and light touch evolved later, in larger, more complex structures.
- Demyelination diseases that degrade the myelin coating on cells include Guillain-Barre syndrome and Multiple Sclerosis. Guillain-Barre syndrome is the destruction of Schwann cells (in the peripheral nervous system), while MS is caused by a loss of oligodendrocytes (in the brain and spinal column). These disorders have different causes and presentations, but both involve muscle weakness and numbness or tingling. These symptoms occur because the nerves aren’t sending information the right way. When the myelin coating of nerves degenerates, the signals are either diminished or completely destroyed. If the nerves are afferent (sensory) fibers, the destruction of myelin leads to numbness or tingling, because sensations aren’t traveling the way they should. When efferent (motor) nerves are demyelinated, this can lead to weakness because the brain is expending a lot of energy but is still unable to actually move the affected limbs. Limbs are especially affected, because they have the longest nerves, and the longer the nerve, the more myelin it has that can potentially be destroyed.