How Nerves Fire

Whack! OUCH!

Ever wonder how pain from a whacked hand gets to your brain so quickly? Also, what do people mean when they say we have electrical impulses inside our bodies? In this post, we’ll take a look at how nerves send messages throughout your body. Though, let’s not call it “firing nerves”, in biology we use the term “action potential”.

Anatomy of a Neuron

A neuron is a nerve cell. It resembles a smashed spider with a long, frayed tail. The spider part is analogous to the cell body, the long tail is the axon, and the frayed end is the axon terminal. Messages go into the cell body via the “spider hairs”, run along the axon, and move to the next neuron via the axon terminal.

A neuron, amongst others. Cell body on the right, axon terminal on the left.

Resting Potential

The electricity that powers your iPhone is the result of moving electrons. The “electricity” that transmits messages in the neuron is the result of moving ions, specifically, K+, Na+ and Cl- ions (periodic table for reference). There is an equilibrium between Cl- going in and out of the cell; this sets up the resting potential of the neuron at about -70 mV. Sodium concentration is high inside the cell and potassium is high outside. Because these ions move across cell membranes easily, the cell uses active transport pumps to maintain this concentration gradient difference.

Message Transmission

When a stimulus is received (like the hammer whacking your hand), it causes sodium channels on the cell membrane to open . The influx of positive ions causes the voltage inside the cell to go up and stimulates other sodium ion channels to open in succession, all the way down the axon. This is the electrical “message” that everyone is talking about! When sodium channels close, potassium channels open up. Since positive ions are now moving out, the inside of the cell goes back to a negative voltage.

Axon Terminal

Once the “message” gets to the axon terminal, vesicles carrying neurotransmitters fuse with the cell membrane, releasing their contents into the extracellular space. The next neuron or cell (for example, a muscle cell) picks up these neurotransmitters and either fires another action potential or reacts (contract, for example, if we’re talking about a muscle cell).

Action potential is what causes you to feel, see, hear, and understand your environment. It’s also how your brain bosses the other organs around. Just like how we have wires and satellites that connect people, cities, and countries to each other, your nerves play a similar role in your body. Cut a nerve? You lose connection to a part of your body.