Neuronal Action Potentials Overview

Overview

This lecture explains the mechanisms of neuronal action potentials, covering resting, graded, and action potentials, and the roles of ion channels and synaptic integration.

Resting Potential

• Resting potential is a neuron's baseline voltage, typically -70 mV, created by the separation of charges across the membrane.
• The sodium-potassium pump uses ATP to maintain more sodium ions outside and more potassium ions inside the neuron.
• Leak channels allow limited K+ to move out and Na+ to move in, helping establish the electrochemical gradient.
• Negatively charged proteins and chloride ions also contribute to the inside-negative resting potential.

Action Potential

• Action potentials are rapid voltage changes due to ion movement across the membrane.
• Voltage-gated sodium channels open at -55 mV (threshold), causing a rapid influx of Na+ and neuron depolarization.
• At +30 mV, sodium channels inactivate and voltage-gated potassium channels open, allowing K+ to exit and repolarize the neuron.
• The outflow of K+ can cause an undershoot below resting potential, then leak channels and pumps restore the original state.
• Action potentials propagate by triggering depolarization in adjacent axon segments, moving only forward due to repolarization behind.

Myelination & Speed

• Myelination insulates axons, allowing action potentials to leap between gaps (nodes), increasing signal speed.

Graded Potentials & Synaptic Integration

• Graded potentials are small voltage changes from neurotransmitter binding at synapses and can be excitatory or inhibitory.
• Chemically gated channels at synapses open when neurotransmitters bind, allowing specific ions to flow.
• Excitatory neurotransmitters (e.g., open Na+ channels) move the neuron toward threshold; inhibitory (e.g., open K+ or Cl- channels) move it away.
• The neuron sums all incoming signals to decide if it reaches threshold for firing an action potential.

All-or-None Law

• An action potential occurs fully if the threshold is reached; if not, there is no action potential.
• The action potential's size and shape remain constant regardless of stimulus strength, provided the threshold is met.

Key Terms & Definitions

• Resting Potential — the baseline electrical charge difference across the neuron's membrane (-70 mV).
• Action Potential — a rapid, all-or-none electrical signal along the neuron.
• Threshold — the membrane voltage (-55 mV) that triggers an action potential.
• Depolarization — movement toward a positive membrane potential.
• Repolarization — return to a negative membrane potential after depolarization.
• Graded Potential — small, variable membrane voltage changes at dendrites or cell body.
• Voltage-Gated Channel — ion channels that open/close in response to membrane voltage changes.
• Chemically Gated Channel — channels that open when neurotransmitters bind.
• Sodium-Potassium Pump — a membrane protein using ATP to exchange 3 Na+ out for 2 K+ in.
• Myelin Sheath — insulating layer around axons that accelerates action potential transmission.

Action Items / Next Steps

• Review and diagram the steps of action potential generation and propagation.
• Study key ion channel functions and their sequence during action potentials.
• Prepare to explain how synaptic inputs influence neuron firing.