Week 7 March 3 to 7 Membrane Potentials Part 2 Spring 2025

8.3 Electrical Signals in Neurons: Overview of Graded Potentials

Graded Potentials Defined

• A graded potential is:

  • A local electrical signal that serves as input to a neuron's dendrites or cell body.

  • Characterized by local current flow, which is a wave of depolarization moving through the neuron.

  • Diminishes in strength as it progresses due to:

    • Current leak.

    • Cytoplasmic resistance.

Comparison between Graded Potentials and Action Potentials

• Graded Potentials are variable in strength and can be:

  • Depolarizing (causing a more positive charge).

  • Hyperpolarizing (causing a more negative charge).

• Action Potentials are:

  • All-or-none responses that occur after reaching a threshold level (-55 mV or greater).

Mechanism of Graded Potentials

Role of Gated Channels

• Various types of gated ion channels facilitate the creation of graded potentials, including:

  • Mechanically gated channels

  • Chemically gated channels

  • Voltage-gated channels

• These channels alter the membrane's permeability to ions such as Na+, K+, Ca2+, or Cl-.

Depolarizing and Hyperpolarizing Potentials

• **Depolarizing Graded Potentials:

  • Occur due to:

    • Opening of ligand-gated Na+ channels (Na+ floods in).

    • Closing of K+ channels.

• Hyperpolarizing Graded Potentials:

  • Occur with:

    • Opening of K+ channels or Cl- channels (K+ floods out or Cl- floods in).

Strength of Graded Potentials

Determinants of Signal Strength

• The strength of a graded potential:

  • Is proportional to the strength of the initial stimulus.

  • More positive ions entering increases the depolarization.

  • There is no minimum threshold needed to initiate a graded potential.

• Action potential frequency indicates stimulus strength; summation occurs when multiple signals overlap in time.

  • Types of Summation:

    • Temporal Summation: Signals arriving close together in time add together leading to a larger effect.

    • Spatial Summation: Multiple signals from different neurons at the same time leading to a cumulative effect.

Threshold and Action Potential Generation

Triggering Action Potentials

• If graded potentials are strong enough to reach the axon hillock and exceed threshold (-55 mV), an action potential will occur.

• Weak graded potentials do not lead to action potentials (subthreshold).

Excitatory vs. Inhibitory Graded Potentials

• Excitatory Graded Potentials (EPSP):

  • Result when the potential reaches or exceeds threshold.

• Inhibitory Graded Potentials (IPSP):

  • Result when the potential is less than threshold.

Refractory Periods in Action Potentials

Types of Refractory Periods

• Absolute Refractory Period:

  • No action potentials can be generated regardless of stimulus strength.

  • Occurs because Na+ channels are inactivated.

• Relative Refractory Period:

  • Possible to generate an action potential only with a stronger than normal stimulus.

Function of Refractory Periods

• Refractory periods regulate the frequency of action potentials and prevent backward conduction along the axon.

Propagation of Action Potentials

Key Mechanisms

• Speed of Action Potentials:

  • Faster in larger diameter neurons.

  • Myelinated axons conduct action potentials faster due to:

    • Saltatory Conduction: Action potentials jump between nodes of Ranvier (areas without myelin).

Implications of Myelination

• Demyelinating diseases (like Multiple Sclerosis) disrupt the conduction of action potentials by damaging myelin.

Key Physiological Changes During Action Potentials

• Initial Events:

  • Graded potential must depolarize the neuron to threshold.

• Phases of Action Potential:

  • Rising Phase: Voltage-gated Na+ channels open, Na+ influx causes depolarization.

  • Falling Phase: Na+ channels close, voltage-gated K+ channels open, leading to repolarization and potential hyperpolarization.

• Return to Resting Potential:

  • Membrane sets back to resting level (-70 mV) as K+ channels close.

Discussion Questions Preparation

• List cell types that can generate action potentials.

• Create a table comparing graded potentials and action potentials based on key characteristics.

• Discuss the generation and propagation of action potentials in myelinated axons.

• Explore real-life scenarios involving pufferfish toxin effects on neuronal function and its systemic implications.