2026-09-28 04:20 #neuroscience-and-engineering #neuron #action-potential #synapse #resting-membrane-potential

The neuron as the brain's building block

Lesson overview

A neuron is a cell specialized to receive, integrate, and transmit information. It does this through electrical signals inside the cell and chemical signals across the small gap between neurons. This lesson covers the main parts of a neuron, the resting membrane potential, the action potential, and synaptic transmission.

1. The main parts of a neuron

Cell body (soma) — contains the nucleus and organelles; the cell’s metabolic center.
Dendrites — branching input structures that collect signals from other neurons.
Axon — a single outgoing fiber that carries the signal away from the cell body toward other neurons.
Axon terminals — the endings of the axon that release signals across a synapse.
Synapse — the gap between the axon terminal of one neuron and a dendrite or cell body of another neuron.

The basic information flow is: dendrites → soma → axon → axon terminals → synapse → next neuron.

2. Resting membrane potential

The inside of a neuron is more negative than the outside. This difference is called the resting membrane potential, usually about −70 mV.

Two main mechanisms create this:
1. Ion concentration gradients — more sodium ions (Na+) outside, more potassium ions (K+) inside.
2. Selective ion channels — the membrane is much more permeable to K+ than to Na+ at rest, so K+ leaks out and leaves negative charge behind.
3. Sodium-potassium pump — actively moves 3 Na+ out and 2 K+ in, keeping the gradients stable.

The membrane potential is the battery that lets neurons produce electrical signals.

3. Action potential

When a neuron receives enough excitatory input, the membrane potential rises from negative toward a threshold, often around −55 mV. At that point, voltage-gated sodium channels open and Na+ rushes in, making the inside briefly positive. This is the depolarization phase of the action potential.

Then:
- Voltage-gated potassium channels open, and K+ leaves the cell, bringing the membrane back down (repolarization).
- The voltage may briefly overshoot the resting level, producing hyperpolarization.

Key properties of action potentials:
- All-or-none — they fire fully once threshold is reached, or not at all.
- Self-propagating — the depolarization spreads down the axon, triggering new channels at each point.
- Frequency coded — stronger stimuli produce more action potentials per second, not bigger ones.

4. Synaptic transmission

When an action potential reaches the axon terminals, it triggers the release of neurotransmitters into the synaptic cleft. These chemical messengers diffuse across the gap and bind to receptors on the next neuron’s membrane.

Depending on the neurotransmitter and receptor type, this can:
- Excite the postsynaptic neuron (make it more likely to fire), by letting positive ions in.
- Inhibit the postsynaptic neuron (make it less likely to fire), by letting negative ions in or blocking positive ions.

After binding, the neurotransmitter is removed or recycled, ending the signal. This whole process lets a neuron decide whether to fire by combining thousands of tiny inputs from its synapses.

5. Why this matters for engineering

Understanding neurons as signal-processing units is the foundation for building neural models, reading brain signals, and designing brain-inspired systems. The same principles — integration, thresholding, all-or-none output, and frequency coding — show up in artificial neural networks and in neurotechnology devices.

Summary

A neuron collects inputs through dendrites, integrates them in the soma, and fires an all-or-nothing action potential down its axon. That electrical signal is converted into a chemical signal at the synapse, which then influences the next neuron. This simple building-block rule is repeated billions of times to produce the brain’s complex behavior.

Text diagrams (for visual learners)

Neuron structure

        dendrites (receive)
       , , , , , ,
        \ | | | /
       [ soma ]
          |
          |  axon (sends)
          |
         / \__ axon terminals
        (synapse → next neuron)

Signal flow: dendrites receive → soma integrates → axon sends → synapse passes on.

Action potential shape

 voltage (mV)
   +40 |           ___
     0 |__________/   \__________  ← returns to resting level (-70 mV)
   -70 |_________/
         depolarize  repolarize
             ↑
        Na⁺ rushes in

Rising phase: voltage-gated Na⁺ channels open, Na⁺ flows in, membrane depolarizes.
Falling phase: Na⁺ channels inactivate, K⁺ channels open, K⁺ flows out, membrane repolarizes.