The Axon: Sending Signals from Head to Toe
The axon is the device by which a neuron sends signals to muscles, glands, or other neurons. Axons are cablelike structures that have two functions: communicating action potentials from the cell body to the axon terminal and, at the axon terminal, releasing a neurotransmitter that binds to a membrane receptor on the postsynaptic cell and opens an excitatory or inhibitory ion channel.
To reach their postsynaptic targets, axons manage to grow long distances (a meter or more) and find those targets. Not only do axons grow long distances, but they are also able to find multiple targets in multiple brain areas and, at each target region, branch into the appropriate number of axon terminals and make contact at the right dendritic locations of the appropriate cells.
Sending a message from your head to your toe involves a two-axon link. An upper motor neuron in your primary motor cortex sends its axon down the spinal cord to synapse on a lower motor neuron that controls a muscle in your toe. That motor neuron in the spinal cord sends its axon down your leg, foot, and to your toe. The entire length of your body is traversed by two cells.
Axons are marvelous devices for conducting action potentials from a cell body to an axon terminal that may be more than a meter away. How quickly the action potential travels is roughly proportional to the diameter of the axon. Very fine axons may conduct action potentials at a rate of a few meters per second; larger caliber axons can conduct action potentials more quickly.
Making axons large for fast conduction works well in invertebrates, which have a small numbers of neurons, relatively speaking. Natural selection has produced some very large-caliber axons in some invertebrates where high speed is needed. The most famous example is the squid giant axon, which can be as much as one millimeter in diameter, big enough to see with the naked eye! The squid giant axon mediates the escape reflex by activating the siphon jet.
This squid giant axon is so large that early electrophysiologists studying the action potential used them for their studies.
But in vertebrates with hundreds of millions or billions of cells in their nervous systems, axons a millimeter in diameter aren’t going to work. The solution? Myelination, which allows small-caliber axons to conduct action potentials very rapidly.
In myelination, certain glial cells wrap many times around almost all the axon, but they leave gaps at regular intervals. These intervals are called the nodes of Ranvier, which is where all the voltage-dependent sodium channels are concentrated.
Myelinated axons conduct axon potentials in such a way that the action potential jumps from node of Ranvier to the next node. This process allows axons only a few micrometers in diameter to conduct action potentials at speeds of up to 100 meters per second.