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.
Understanding Biology: The Neuron and the Action Potential
A neuron membrane acts like a selective barrier. Charged particles cannot all cross it freely, so the cell controls movement through protein channels. Some channels are always partly open.
Others open only when the electrical state of the membrane changes. This control gives the signal its clear sequence. A small local change in voltage may fade away if it is too weak.
Once the trigger level is reached, however, many sodium channels open in a rapid chain reaction. The resulting impulse follows an all or nothing rule.
Its size does not grow with a stronger stimulus. Instead, stronger stimulation usually makes the neuron fire more often.
The signal moves in one direction because recently active sections of the axon need time to reset. During this refractory period, sodium channels cannot immediately open again. Potassium movement continues briefly, which can make the inside of the cell more negative than its usual resting state.
This short recovery period prevents a new impulse from travelling backward into the section that has just fired. It also places a limit on how rapidly a neuron can send impulses. Different neurons have different firing rates because their channel types and recovery times differ.
An action potential carries information along an axon, but communication between cells usually occurs at a synapse. When the impulse reaches the axon ending, calcium channels open. Calcium entering the cell causes tiny sacs called vesicles to release chemical messengers.
These neurotransmitters cross a very small gap and bind to receptors on the next cell. Some receptors make that cell more likely to fire. Others make firing less likely.
The next neuron adds together many incoming signals. This is important because brain activity depends on patterns of excitation and inhibition, not on a single message alone. Muscles use this same basic process when nerve endings release acetylcholine and start contraction.
Myelin is a fatty insulating layer made by support cells around certain axons. It reduces ion movement across covered sections of membrane. The impulse is renewed mainly at the uncovered gaps, which saves time and energy.
Damage to myelin can slow or block communication. Multiple sclerosis is one condition in which the immune system damages myelin in the central nervous system. Symptoms can include weakness, altered sensation, or poor coordination, depending on which pathways are affected.
When studying this topic, keep the stages in order and separate electrical events inside the neuron from chemical signaling at synapses. It helps to trace the movement of each ion, then connect that movement to the change in membrane voltage and the cell response.
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.