2-3 Action Potential

Action Potential and Membrane Potential

Action Potential Phases:

Resting Membrane Potential: -70 mV

Threshold: -55 mV

Depolarization: +30 mV (Voltage-gated Na+ channels open)

Repolarization: Returns toward -70 mV (Voltage-gated K+ channels open)

Hyperpolarization: Membrane potential goes below -70 mV

Graded Potential: Small changes in membrane potential that can summate to trigger an action potential

Neuron vs. Glial Cells

Overview of Nervous System:

Neuron: Primary signaling cell

Glial Cells: Supportive role, maintaining homeostasis, forming myelin, and protecting neurons

Membrane Potential Basics

Resting Membrane Potential: Caused by differences in ion concentration (Na+, K+) across the membrane

Na+/K+ Pump: Maintains resting potential by pumping 3 Na+ out and 2 K+ in

Ionic Basis of Action Potential

Na+ Channels:

Ligand-gated Na+ channels: Allow a few Na+ ions in each time

Voltage-gated Na+ channels: Open extensively and rapidly at threshold (-55 mV) allowing Na+ influx, leading to depolarization

States of Voltage-Gated Na+ Channels:

Resting (closed)

Open (activation gate open)

Inactivated (inactivation gate closes at +30 mV)

K+ Channels:

Voltage-gated K+ channels: Open at +30 mV allowing K+ to rush out, repolarizing the cell

Phases of Action Potential

Depolarization: Membrane potential rises toward +30 mV

Repolarization: Return towards resting potential, initiated by K+ efflux

Hyperpolarization: Membrane potential slightly goes below resting level before stabilizing back to -70 mV

Refractory Periods

Absolute Refractory Period: No action potential can be generated (Na+ channels inactivated)

Relative Refractory Period: Stronger stimulus can initiate action potential due to hyperpolarization (K+ channels still open)

Conduction of Nerve Impulses

Action potentials propagate along the axon due to sequential opening of voltage-gated Na+ channels

The absolute refractory period ensures one-way conduction of the impulse

All-or-None Principle of Action Potential

Once threshold (-55 mV) is reached, action potentials occur with full magnitude (not affected by stimulus size)

Action potentials follow the all-or-none law; size and duration remain constant regardless of stimulus strength.

Conduction Velocity

Unmyelinated Neurons: Slower due to continuous action potentials at each membrane segment

Myelinated Neurons: Faster due to saltatory conduction (AP jumps between nodes of Ranvier)

Increased Diameter: Reduces resistance, increasing conduction speed

Myelinated speeds: Thick, myelinated neurons can conduct at 100 m/sec vs 1.0 m/sec for thin, unmyelinated neurons

Summary of Key Points

Action potentials consist of rapid depolarization, repolarization, and hyperpolarization phases

The absolute and relative refractory periods play crucial roles in the unidirectional propagation of nerve signals

The nature of ionic channels (Na+ and K+) is fundamental to understanding neuronal signaling and action potential generation.