Overview
This lecture explains the action potential process in neurons, detailing the step-by-step ionic changes that enable electrical signaling.
Basic Neuron Structure
- The neuron includes a soma (cell body), dendrites (receive messages), axon (transmits impulses), and axon terminals (release neurotransmitters).
- The neuron's membrane separates internal and external environments and controls ion exchange through channels.
Ions and Resting Potential
- Ions are charged particles; sodium (Na⁺) is prevalent outside, and potassium (K⁺) is concentrated inside the neuron ("salty banana" memory device).
- At rest, the neuron interior is at -70 millivolts (mV), termed the resting potential, meaning it's negatively charged inside.
Stimulus and Threshold
- A stimulus like acetylcholine (neurotransmitter) binds to dendrite receptors, causing sodium channels to open and Na⁺ to enter.
- If enough Na⁺ enters, the voltage reaches -55 mV, called the threshold.
- The all-or-none principle: if threshold is reached, the neuron fires an action potential; if not, no firing occurs.
Depolarization and Action Potential
- Voltage-gated sodium channels open, causing more Na⁺ influx, raising the voltage up to +30 mV.
- Depolarization refers to the neuron interior becoming more positive as action potential occurs.
Repolarization and Hyperpolarization
- At +30 mV, sodium channels close, and potassium channels open, allowing K⁺ to exit and making the neuron more negative again.
- The voltage drops below resting potential to around -90 mV (hyperpolarization), entering the refractory period when the neuron cannot immediately fire again.
- The neuron then returns to resting potential (polarized), ready to fire when stimulated anew.
Key Terms & Definitions
- Neuron — nerve cell transmitting electrical impulses.
- Axon — neuron part carrying impulses away from the soma.
- Ion — charged particle (e.g., sodium, potassium).
- Resting potential — neuron's stable negative charge when inactive (-70 mV).
- Threshold — minimum voltage to trigger action potential (-55 mV).
- Depolarization — process where neuron interior becomes more positive.
- Repolarization — process of neuron returning to a negative state.
- Hyperpolarization — temporary state more negative than resting potential.
- Refractory period — recovery phase when neuron can't fire.
- All-or-none principle — neuron fires completely if threshold is reached, otherwise does not fire.
Action Items / Next Steps
- Review the structure and function of neuron parts for better understanding.
- Memorize voltage changes and corresponding phases (depolarization, repolarization, hyperpolarization).
- Apply the “salty banana” memory technique to recall ion distributions.