Lecture_09_Membrane Transport II

Membrane Transport II: Ion Channels and Membrane Potential

Introduction

• Overview of ion channels and membrane potential in nerve cell signaling.

• Related to BIOL 3510: Lecture 09.

Objectives

• Cell Membrane Potential:

  • Understand factors contributing to cell membrane potential.

• Ion Channel Selectivity:

  • Explain structural selectivity of ion channels.

• Patch-Clamp Technique:

  • Describe and apply the patch-clamp technique for measuring ion channel current.

• Types of Gated Channels:

  • Compare ligand-gated, voltage-gated, and mechanically-gated channels with examples.

• Neurons Structure:

  • Describe a neuron's parts and action potential generation and propagation.

• Electrical to Chemical Signal Conversion:

  • Steps for converting action potentials to neurotransmitter release at synapses.

• Neurotransmitter Functions:

  • Differentiation between excitatory and inhibitory neurotransmitters.

• Optogenetics:

  • Explore optogenetics as a method to study neuron activity.

Ion Channels – Structure and Selectivity

• Functionality:

  • Ion channel proteins create hydrophilic pores for ions across membranes according to electrochemical gradients.

• Ion Selectivity:

  • Determined by:

    • Diameter and shape of the channel.

    • Ion-specific charged amino acids lining the channel.

• Gating Mechanism:

  • Ion channels are not continuously open and require specific stimuli for activation.

  • Faster transport than traditional transporters, but do not utilize energy for active transport.

Membrane Potential and Ion Channels

• Key Concept:

  • Membrane potential influenced by ion permeability.

• Ion Distribution:

  • Gradient and movement of specific ions, especially K+, contribute to resting potential.

  • Typical resting potential in animal cells is between -20mV and -200mV.

Patch-Clamp Technique

• Recording Ion Channel Activity:

  • Involves a microelectrode that isolates a section of the membrane to trap an ion channel.

  • Enables monitoring of ion flow through this channel.

• Channel Behavior:

  • Ion channels display an all-or-nothing behavior, alternating between open and closed states randomly.

Gating of Ion Channels

• Gating Types:

  • Ion channels show variability in selectivity and gating mechanisms, influenced by:

    • Mechanical force

    • Ligand binding

    • Membrane potential.

Mechanically-Gated Channels

• Example: Auditory hair cells.

  • Stereocilia respond to sound vibrations, leading to channel opening and ion influx.

Stretch-Activated Piezo Channels

• Functionality:

  • Open due to deformation in membrane structure, regulating physiological processes such as pressure and touch sensation.

Voltage-Gated Ion Channels

• Responsiveness:

  • Respond to changes in membrane potential, crucial for electrical signal propagation in neurons and other cells.

  • Example: Mimosa pudica’s leaf-closing response.

Neurons and Signal Propagation

• Neuron Structure:

  • Composed of cell body, axon, and dendrites.

  • Axons conduct signals away from the cell body; dendrites receive signals from other neurons.

• Action Potentials:

  • Initiated by sufficient stimulation, produces a wave of electrical activity along the axon, characterized as an action potential.

Sodium and Potassium Channels in Action Potentials

• Role:

  • Na+ Channels: Enable depolarization by opening and allowing Na+ influx.

  • K+ Channels: Induce repolarization by allowing K+ efflux post-depolarization.

• Conformational States:

  • Na+ channels oscillate between closed, open, and inactivated states influenced by membrane potential.

Synaptic Signaling

• Chemical Signal Transmission:

  • Action potentials lead to neurotransmitter release at synapses, converting electrical signals into chemical signals.

• Process:

  • Arrival of action potential opens voltage-gated Ca2+ channels, Ca2+ influx induces neurotransmitter release.

• Role of Neurotransmitters:

  • Excitatory or inhibitory effects determined by receptor type.

Ligand-Gated Channels

• Examples:

  • Receptors that respond to neurotransmitters (ACh, GABA, glycine).

  • Mechanism for converting chemical signals back into electrical ones at the postsynaptic membrane.

Optogenetics

• Application:

  • Use of light to control neuron activity via light-gated ion channels from algae in live subjects.

• Purpose:

  • Facilitates detailed study of neural circuits governing behaviors.

Conclusion

• Understanding the complex interactions of ion channels and signaling mechanisms is essential to grasp the functional dynamics of neurons and their role in signaling.