An axon is the long, slender cable that extends from a nerve cell to send electrical signals to other cells. While most are microscopic, single human axons can stretch over three feet from your lower spine down to your big toe. Instead of using metallic wires, they pump sodium and potassium across their membranes to push signals forward.
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A neuron is a microscopic cell, but its axon acts like a long-distance cable. In your longest neurons, the axon starts in your lower spine and terminates in your big toe.
That is a single biological structure that can be over three feet long.
We learned how nerves fire by studying the Atlantic squid in the 1930s. Their axons are 1,000 times thicker than ours, wide enough to insert an electrode inside.
We discovered they aren't copper wires; they are biological pumps swapping sodium and potassium to push a signal forward.
Electricity doesn't travel well through wet tissue, so axons need help. Many are wrapped in myelin, a fatty insulation that acts like the plastic coating on a power cord.
This allows the action potential to leap from gap to gap, moving at lightning speeds.
Axons conduct electrical impulses known as action potentials away from the neuron cell body. To speed up transmission through wet tissue, many axons are wrapped in a fatty insulating coating called myelin. In the peripheral nervous system, Schwann cells produce this myelin sheath, while oligodendrocytes provide it in the central nervous system.
The insulation is broken by evenly spaced gaps called nodes of Ranvier. Signals jump from gap to gap in a process called saltatory conduction, allowing impulses to move much faster than they would along unmyelinated fibers.
An axon is wrapped in a membrane called the axolemma and filled with cytoplasm known as axoplasm. Unlike dendrites, which taper and stay close to the cell body to receive signals, axons maintain a constant radius and can extend across long distances.
At its end, an axon divides into branches called telodendria. Each branch terminates in a swollen end-foot called an axon terminal, which forms a junction called a synapse with another neuron, muscle, or gland. A single axon can generate thousands of these terminals, allowing one cell to communicate with multiple brain regions at the same time.
When axons travel together, they form bundles. In the peripheral nervous system, these bundles are called nerves or fascicles. In the central nervous system, they are called nerve tracts.
Bundles of myelinated axons make up the brain tissue known as white matter, which gets its pale color directly from the fatty myelin. The largest white matter tract in the human brain is the corpus callosum, which connects the two cerebral hemispheres.
How does myelin help an axon carry signals quickly through wet tissue?
It lets the signal jump between gaps in the insulation. Myelin insulates the axon, so the action potential travels between exposed gaps. The signal is still produced by the axon's biological mechanisms, not metal-like conduction.
A long axon carries a signal from the spine to the foot. What best describes it?
One extended part of a single neuron. An axon can extend from a neuron's cell body to a distant target, so a long spine-to-foot axon can be one continuous cellular structure.
Do axons transmit signals like metal wires or biological pumps?
Biological pumps. Axons do not conduct current passively like copper wires. They actively swap ions using biological pumps to propagate electrical impulses.
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Dendrites are short, tapering branches that receive incoming signals near the cell body. Axons are long fibers with a constant thickness that carry outgoing signals toward other cells.
Most human axons are microscopic, measuring around one micrometer across. In contrast, the giant axon of an Atlantic squid is roughly one millimeter wide, which is about the thickness of a small pencil lead.
Neurons rarely have more than one axon. Multiple axons emerging from a single cell body are unusual but can occur in certain invertebrates, such as in lobster cardiac ganglia.
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A thought is electricity, right up until the last millimeter.
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