Overview
This lecture explains how neurons generate and transmit electrical impulses (action potentials), covering the roles of membrane potential, ion channels, and the difference between graded and action potentials.
Neuronal Communication Basics
- Neurons send information as a uniform electrical signal called an action potential.
- The frequency (number per second) of action potentials encodes different messages for the brain.
- The strength and speed of each action potential are always the same; only frequency varies.
Understanding Electricity in the Body
- The body is electrically neutral overall but maintains localized separations of charge across cell membranes.
- Voltage (potential energy) is caused by separated charges, measured as membrane potential (in millivolts).
- Current is the flow of ions across membranes; resistance opposes this flow.
Resting Membrane Potential
- At rest, neurons are polarized: inside is more negative than outside, usually about -70 mV.
- Sodium ions (Na⁺) are concentrated outside, potassium ions (K⁺) inside with negatively charged proteins.
- The sodium-potassium pump maintains this gradient by moving 3 Na⁺ out and 2 K⁺ in.
Ion Channels and Gradients
- Ion channels in the membrane allow selective movement of ions when opened.
- Voltage-gated channels open at specific membrane potentials, ligand-gated channels respond to neurotransmitters, and mechanically gated channels respond to stretching.
Graded vs. Action Potentials
- Graded potentials are small, localized changes in membrane potential.
- An action potential occurs if depolarization reaches a threshold (about -55 mV).
- Action potential is "all-or-nothing": either it happens fully, or not at all.
- Depolarization causes sodium channels to open and Na⁺ to rush in, reaching +40 mV.
- Repolarization follows as K⁺ channels open and K⁺ leaves the cell, sometimes causing hyperpolarization (<-70 mV).
- The refractory period prevents the neuron from firing again immediately.
Signal Strength, Speed, and Conduction
- Stimulus strength affects the frequency, not amplitude, of action potentials.
- Myelinated axons conduct impulses faster due to saltatory conduction, where signals jump between Nodes of Ranvier.
Key Terms & Definitions
- Action Potential — a uniform electrical impulse generated by a neuron when its membrane potential reaches threshold.
- Resting Membrane Potential — the voltage difference across a neuron's membrane at rest (-70 mV).
- Sodium-Potassium Pump — a protein that actively transports 3 Na⁺ out and 2 K⁺ into a neuron.
- Graded Potential — a small, localized change in membrane potential.
- Depolarization — reduction in membrane potential difference (becomes less negative).
- Repolarization — restoration of resting membrane potential after depolarization.
- Refractory Period — time during which a neuron cannot fire another action potential.
- Myelin Sheath — insulating layer around axons that speeds up impulse conduction.
- Saltatory Conduction — action potentials jumping from one Node of Ranvier to the next.
Action Items / Next Steps
- Review how action potentials are generated and transmitted.
- Prepare to learn how action potentials lead to communication between neurons at synapses in the next lesson.