Neurons are specialized cells that carry information through the nervous system and make mental life possible. Every sensation, decision, memory, and emotion depends on patterns of neuron activity. In psychology, neurons help explain how physical events in the brain can influence thoughts and behavior.
Understanding their structure shows how information moves from one cell to the next.
Understanding Neurons and Synapses
A neuron is not constantly firing. At rest, its membrane keeps different charged particles on each side. Sodium ions are more concentrated outside, while potassium ions are more concentrated inside.
Protein pumps and channels maintain this uneven arrangement. A strong enough change in voltage reaches a threshold and opens sodium channels. Sodium rushes inward, causing a rapid electrical spike called an action potential.
Soon after, potassium channels open and help restore the resting state. A brief recovery period follows, which helps keep each spike moving in one direction.
Action potentials follow an all or nothing rule. A signal that reaches threshold produces a full spike, while a weaker signal does not produce a smaller spike. Information is therefore carried by the timing and frequency of spikes, rather than by their size.
Some neurons can send many spikes in one second. Signal speed depends on features such as axon diameter and insulation.
Fast pathways matter when someone pulls a hand away from a hot surface or reacts to a sudden sound. Damage to myelin can slow communication and affect movement, sensation, or coordination.
At the tiny gap between cells, the electrical event triggers a chemical event. When a spike reaches the end of a neuron, calcium enters the terminal. This causes small membrane packets called vesicles to release neurotransmitter molecules into the synaptic gap.
The molecules cross the gap and attach to matching receptors on the next cell. Some receptors make that cell more likely to fire. Others make it less likely to fire.
A neuron adds together thousands of these incoming effects over a short time. Its output depends on this overall balance, not on one message alone.
Neurotransmitters do not stay in the gap forever. They may be taken back up by the releasing neuron, broken down by enzymes, or drift away. This removal process controls how long a message has an effect.
Medicines can alter communication by changing receptor activity, release, breakdown, or reuptake. For example, some antidepressants slow the reuptake of serotonin.
This does not mean that a single chemical simply causes a mood or behavior. Brain activity depends on many transmitter systems, circuits, experiences, and body states working together.
Learning changes synapses over time. Repeated use can strengthen some connections by making release more likely or by increasing receptor response. Connections that are rarely useful may weaken.
This plasticity helps explain practice, habit formation, and parts of memory. When studying diagrams, track the direction of information carefully. Separate electrical signaling within a neuron from chemical signaling across a synapse.
Pay attention to the difference between an excitatory effect and a guaranteed spike. Excitatory input raises the chance of firing, but the final result still depends on all the other inputs arriving at that moment.
Key Facts
- Dendrites receive incoming signals from other neurons.
- The soma, or cell body, integrates signals and supports cell functions.
- Axons carry action potentials away from the soma toward synapses.
- Myelin increases signal speed by insulating the axon and allowing signals to jump between nodes.
- Membrane potential is the voltage difference across the neuron membrane: V_m = V_inside - V_outside.
- Signal speed can be estimated with speed = distance/time.
Vocabulary
- Neuron
- A neuron is a nerve cell that receives, processes, and sends information through electrical and chemical signals.
- Dendrite
- A dendrite is a branch-like part of a neuron that receives signals from other neurons.
- Axon
- An axon is the long fiber of a neuron that carries electrical signals away from the cell body.
- Synapse
- A synapse is the tiny gap where one neuron communicates with another cell using chemical messengers.
- Neurotransmitter
- A neurotransmitter is a chemical released at a synapse that changes the activity of a nearby neuron.
Common Mistakes to Avoid
- Thinking neurons physically touch at synapses. Most synapses have a small gap, so communication often happens when neurotransmitters cross the synaptic cleft.
- Confusing dendrites and axons. Dendrites usually receive signals, while axons usually send signals away from the cell body.
- Assuming stronger stimuli make action potentials larger. Action potentials are all-or-none, so stronger stimuli usually increase firing rate rather than spike size.
- Ignoring inhibitory signals. Neurons do not simply add excitement, because inhibitory inputs can reduce the chance that the neuron fires.
Practice Questions
- 1 A nerve signal travels 0.80 m along a myelinated axon in 0.010 s. Calculate the signal speed using speed = distance/time.
- 2 A neuron has a resting membrane potential of -70 mV and reaches -55 mV at threshold. Calculate the change in membrane potential needed to reach threshold.
- 3 A neuron receives several excitatory inputs and one strong inhibitory input at the same time. Explain how this could affect whether the neuron fires an action potential.