action potential

Understanding Action Potentials

• Definition of Propagation

  • Propagation in plants refers to taking a cutting and planting it to grow a new one.

  • In neurons, an action potential is initiated in one location and spreads to adjacent areas, generating new action potentials along the way.

Characteristics of Action Potentials

• Localized Action

  • An action potential is not a wave traveling through space but a sequence of localized events, each triggering the next.

  • Long distances can be traveled by neurons, such as from the spinal cord to the foot muscles.

  • Contrast with graded potentials that only affect short distances.

• Magnitude Differences

  • Action potentials have a fixed, all-or-nothing response: they do not vary in size.

  • Graded potentials can have variable magnitudes and diminish with distance from the stimulus.

  • Analogous to pregnancy: one is either pregnant or not; there's no in-between.

Ion Movement in Action Potentials

• Depolarization vs. Hyperpolarization

  • Depolarization involves the influx of sodium ions (Na+); hyperpolarization is often a result of potassium ions (K+) leaving.

  • Graded potentials can lead to both depolarization and hyperpolarization, while action potentials are fundamentally a depolarization process.

• Graphing Action Potentials

  • Important to understand the shapes and phases of action potentials.

    • Depolarization phase: rapid influx of Na+ ions.

    • Repolarization phase: K+ ions flow out, making the inside of the cell more negative.

    • Following repolarization, after-hyperpolarization occurs where the membrane potential temporarily becomes more negative than resting potential.

Ion Channels in Action Potentials

• Voltage-Gated Ion Channels

  • Sodium ion channels have two gates: activation and inactivation gates.

  • At resting potential (-70 mV), both gates are closed.

  • At threshold potential (-55 mV), the activation gate opens, allowing Na+ ions in.

  • Once the peak is reached (+30 mV), the inactivation gate closes, stopping Na+ influx.

• Potassium Ion Channels

  • Potassium channels also have a threshold that opens them, but they're slower to respond than sodium channels.

  • Once opened, K+ ions leave the neuron, contributing to repolarization and sometimes leading to hyperpolarization due to prolonged opening.

Refractory Periods

• Definition

  • The refractory period is the recovery phase after an action potential during which the neuron cannot fire another action potential.

  • It ensures that action potentials only travel in one direction along the axon.

• Importance in Signal Transmission

  • The refractory period prevents backward propagation of the action potential, contributing to unidirectional signal transmission.

Summary of Key Concepts

• Key Distinctions Between Action and Graded Potentials

  • Action potentials are all-or-nothing, while graded potentials can vary in strength.

  • The duration and identities of ion channel gates play a crucial role in how action potentials behave and respond to stimuli.