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Understanding the electrochemical process that allows neurons to communicate through electrical impulses.
How does your brain know the difference between a gentle breeze and a stinging slap if every single nerve signal is exactly the same strength?
Before a neuron fires, it sits in a state of readiness called the resting potential. Think of it as a loaded spring or a battery waiting to be used. In this state, the inside of the neuron is more negative than the outside, typically measured at . This electrical difference is maintained by the sodium-potassium pump, which actively moves three sodium ions () out for every two potassium ions () it brings in. This creates a state of polarization, where the cell is 'charged' and ready to react to an incoming stimulus.
Quick Check
What is the typical electrical charge of a neuron during its resting potential?
Answer
When a stimulus hits a neuron, it must reach a specific threshold—usually around —to trigger a response. If the stimulus is too weak, nothing happens. If it hits the threshold, the neuron fires a full electrical impulse. This is the All-or-None Law: a neuron doesn't fire 'weakly' or 'strongly'; it either fires at 100% or 0%. During depolarization, voltage-gated channels fly open, and ions rush into the cell, driven by their attraction to the negative interior. This causes the internal charge to spike rapidly toward .
Think of a neuron like a toilet handle. 1. If you push the handle only a little bit (sub-threshold), nothing happens. 2. Once you push it past the 'click' (threshold), the flush occurs with the same force every time (All-or-None). 3. You cannot make the flush 'stronger' by pushing the handle harder.
Quick Check
If a stimulus moves the membrane potential from to , will the neuron fire?
Answer
No, because it has not reached the threshold of .
After the peak of the action potential (), the cell must reset. The channels close, and channels open. During repolarization, ions rush out of the cell, carrying their positive charge with them. This makes the inside of the cell negative again. Often, the cell overshoots its target, becoming more negative than (around ), a state called hyperpolarization. This ensures the signal only travels in one direction and prevents the neuron from firing again too quickly.
Let's trace the movement of ions during a single pulse: 1. Resting: is high outside; is high inside. 2. Depolarization: channels open enters Charge becomes positive (). 3. Repolarization: channels open exits Charge becomes negative again.
The refractory period is the 'reloading' time during which a neuron cannot fire another action potential. The absolute refractory period occurs while channels are reset. This period determines the maximum frequency at which a neuron can send signals. Since the strength of an action potential is always the same, the brain encodes 'intensity' (like the difference between a touch and a punch) by the frequency of firing, not the size of the pulse.
Which ion movement is responsible for the rapid depolarization of the neuron?
A very strong stimulus will create a larger electrical spike than a weak stimulus that just barely reaches the threshold.
If a neuron's refractory period is , what is its maximum firing frequency in Hertz (Hz)?
Review Tomorrow
In 24 hours, try to sketch the action potential graph and label the threshold (), peak (), and the movement of and ions at each stage.
Practice Activity
Research how 'Novocaine' works at the dentist—hint: it involves blocking the channels we discussed today!