How Neurons Send Messages Through Your Body

A neuron has a cell body, branching dendrites and a long fibre called an axon. Dendrites receive signals from nearby cells, while the axon carries a signal away from the cell body. Some axons are wrapped in a fatty covering called myelin, which helps messages travel more quickly and efficiently.

Neurons do not all have the same shape or job. Sensory neurons carry information from sense receptors toward the brain and spinal cord. Motor neurons carry instructions toward muscles and glands. Interneurons connect neurons inside the central nervous system, helping information move through networks rather than along one lonely wire.

The nervous system works because huge numbers of neurons communicate in organised pathways. A single neuron can receive signals from many others, combine that information and then either send its own message or remain quiet. This simple choice, repeated across billions of cells, supports movement, memory, attention and thought.

When a neuron is resting, charged particles are distributed unevenly across its cell membrane. If incoming signals are strong enough, channels in the membrane open and the electrical balance changes. This produces an action potential, a brief electrical event that moves along the axon.

The action potential does not fade like a weak sound travelling down a hallway. Each section of membrane activates the next, so the signal is renewed as it moves. Myelin leaves small gaps along the axon, and the signal appears to jump from gap to gap. This greatly increases speed without requiring the entire membrane to activate at once.

After the signal passes, the membrane returns to its resting condition. For a short time, that section cannot immediately fire again. This recovery helps keep the message moving in one direction and prevents the axon from becoming a confused electrical roundabout.

Neurons usually do not touch. A tiny gap called a synapse separates the end of one neuron from the next cell. When an action potential reaches the axon ending, it triggers the release of chemical messengers called neurotransmitters. Different neurotransmitters can increase, reduce or adjust the next cell's response, allowing nervous pathways to handle far more than simple on-and-off commands.

A useful sequence to remember is:

• •Dendrites receive incoming information.
• •A strong enough signal starts an action potential.
• •The action potential travels along the axon.
• •Neurotransmitters cross the synapse.
• •Receptors on the next cell respond.

The hot pan example in fact skips the brain for the first decision. Sensory neurons in your hand carry the signal to the spinal cord, where an interneuron passes it straight to motor neurons that tell the arm muscles to contract. This short route is called a reflex arc. At the same time, other neurons carry the message up to the brain, which is where the sensation of pain is produced. Because that journey is longer, your hand is often already moving before you consciously notice the burn.

You might expect a stronger stimulus to create a bigger action potential, but it does not. Action potentials are all or nothing: once the threshold is reached, each one has roughly the same size. Intensity is coded in other ways. A stronger touch or hotter surface makes neurons fire more often and recruits more neurons at once. Speed also varies widely. The fastest myelinated fibres carry signals at more than a hundred metres per second, while thin unmyelinated fibres may manage only about one metre per second, which is why a dull ache can arrive after a sharp first sting.

The idea that nerves carry electrical signals began with experiments on frogs. In the eighteenth century the Italian physician Luigi Galvani found that frog leg muscles twitched when touched with metals or connected to electrical sources. He argued that animals produce their own electricity. His fellow countryman Alessandro Volta disagreed and believed the metals themselves produced the current, a dispute that helped lead Volta to invent the battery. Both were partly right. Animal tissue does generate electrical signals, though Galvani could not explain how.

The detailed mechanism came much later, from an unexpected animal. Squid have a giant axon that can be up to about a millimetre wide, large enough for researchers to insert fine electrodes inside it. In the late 1940s and early 1950s, Alan Hodgkin and Andrew Huxley used these axons to measure how the voltage across the membrane changes during an action potential. They showed that the signal is produced by sodium ions rushing into the axon followed by potassium ions flowing out, and they built a mathematical model of the process. The work earned them a Nobel Prize.

The importance of myelin becomes clear when it is damaged. In multiple sclerosis, the immune system attacks the myelin around nerve fibres in the brain and spinal cord. Where the coating is stripped away, the signal can no longer leap quickly from gap to gap. It may slow down, become distorted or fail to get through at all. The name of the condition refers to the patches of hardened scar tissue that form at damaged sites, and doctors can often see these areas on MRI scans of the brain and spinal cord.

Because the damage can occur almost anywhere in the brain and spinal cord, symptoms differ from person to person. Blurred vision, numbness, tingling, muscle weakness, problems with balance and tiredness are all common, depending on which pathways are affected. Many people have periods when symptoms flare and then partly or fully ease, as inflammation settles and the nervous system adapts. A different condition, Guillain-Barré syndrome, affects myelin in the nerves outside the brain and spinal cord, often after an infection. Both show how much ordinary movement and sensation depend on a fatty coating most people never think about.

It was once unclear whether the nervous system was made of separate cells at all. In 1873 the Italian scientist Camillo Golgi found that soaking nerve tissue in silver salts stained a few cells completely black, revealing their branches against a pale background. Golgi himself concluded that nerve fibres fused into one continuous network, like a mesh of pipes. The Spanish scientist Santiago Ramón y Cajal used and improved Golgi's stain and reached the opposite view. After studying countless slides, and drawing them in exquisite detail, he argued that each neuron is a distinct unit whose endings come very close to other cells without merging.

The two men shared the Nobel Prize in 1906 and used their lectures to disagree with each other. Cajal's view, called the neuron doctrine, became the foundation of modern neuroscience, but final proof had to wait for the electron microscope. In the 1950s images of synapses showed a narrow gap, a few tens of nanometres wide, between one cell and the next. Golgi was not entirely wrong, though. Some neurons are linked by electrical synapses, where channels called gap junctions connect neighbouring cells directly, letting current pass almost instantly. These are common where very fast, synchronised firing is needed.

Neurons communicate with an electrical signal inside the cell and a chemical signal between cells. Follow the path from dendrites to axon to synapse, and the nervous system becomes a chain of understandable steps rather than a mysterious flash of information.