Neurons are specialized cells that receive, process, and send information throughout the nervous system. Their long shape allows signals to travel from dendrites, through the cell body, and down the axon to communicate with other cells. The action potential is the rapid electrical signal that carries information along the axon.
Understanding this process explains how the brain, muscles, senses, and reflexes work in real time.
An action potential depends on ion concentration differences across the neuron membrane, especially sodium ions and potassium ions. At rest, the inside of the neuron is more negative than the outside, creating a resting membrane potential near -70 mV. When a stimulus reaches threshold, voltage-gated sodium channels open and sodium rushes in, causing depolarization.
Potassium channels then open to restore the negative voltage, and myelin speeds the signal by making it jump between nodes of Ranvier.
Key Facts
- Resting membrane potential is about -70 mV in many neurons.
- Threshold is about -55 mV, the voltage needed to trigger an action potential.
- Depolarization occurs when voltage-gated Na+ channels open and Na+ enters the neuron.
- Repolarization occurs when voltage-gated K+ channels open and K+ leaves the neuron.
- The sodium-potassium pump helps maintain ion gradients: 3 Na+ out and 2 K+ in per ATP.
- Saltatory conduction speeds signaling because action potentials jump from node to node along a myelinated axon.
Vocabulary
- Neuron
- A nerve cell specialized to receive, process, and transmit electrical and chemical signals.
- Action potential
- A rapid, all-or-none change in membrane voltage that travels along an axon.
- Resting potential
- The voltage difference across a neuron membrane when the neuron is not firing, usually about -70 mV.
- Myelin sheath
- A fatty insulating layer around many axons that increases the speed of electrical signaling.
- Node of Ranvier
- A small gap between myelin segments where many voltage-gated ion channels are located.
Common Mistakes to Avoid
- Saying the action potential is carried by electrons, which is wrong because neuron voltage changes are produced mainly by ion movement across the membrane.
- Thinking sodium leaves during depolarization, which is wrong because Na+ enters the neuron through voltage-gated sodium channels and makes the inside less negative.
- Assuming stronger stimuli make larger action potentials, which is wrong because action potentials are all-or-none and stronger stimuli usually increase firing frequency instead.
- Forgetting the refractory period, which is wrong because it helps force the action potential to travel in one direction and limits how quickly a neuron can fire again.
Practice Questions
- 1 A neuron has a resting potential of -70 mV and a threshold of -55 mV. How many millivolts must the membrane potential change to reach threshold?
- 2 The sodium-potassium pump uses 1 ATP to move 3 Na+ out and 2 K+ in. How many Na+ ions are moved out and how many K+ ions are moved in after 6 ATP are used?
- 3 Explain why a myelinated axon conducts signals faster than an unmyelinated axon, using the role of nodes of Ranvier in your answer.