2-2 Membrane Potential

Overview of Membrane and Neuronal Signaling

Neurotransmission Mechanisms

• Ion Channels and Signaling

  • Ligand-gated Na+ channel: Opens in response to specific neurotransmitters (ligands).

  • Voltage-gated Na+ channel: Opens when a specific electrical threshold is reached.

Electrical vs Chemical Signals

• Electrical signal: Transmitted within the same neuron.

• Chemical signal: Involves the transfer of neurotransmitters between different neurons at the synapse.

• Graded Potentials: Local changes in membrane potential; vary in strength and may lead to action potentials based on summation.

Neuron Structure and Function

Key Components

• Neurons vs. Glial Cells: Neurons are responsible for signaling, while glial cells support and protect neurons.

• Resting Membrane Potential (-RMP): The difference in charge across the plasma membrane when the neuron is at rest, primarily determined by ion gradients.

Ionic Concentrations

• Concentrations inside vs. outside of the neuron:

  • [K+]: Inside: 150mM, Outside: 5mM

  • [Na+]: Inside: 15mM, Outside: 150mM

  • [Cl-]: Inside: 7mM, Outside: 110mM

  • Ion Channels: K+ channels (open), Na+ channels (closed at rest).

Causes of Resting Membrane Potential

• Na+/K+ ATPase Pump: Maintains concentration gradients of K+ and Na+.

• K+ Leak Channels: Allow some K+ to exit the cell, contributing to negativity inside the cell.

Changes in Membrane Potential

Types of Changes

• Depolarization:

  • Occurs when Na+ enters the neuron, making the inside less negative.

  • Characterized as excitatory.

• Hyperpolarization:

  • Results from K+ leaving or Cl- entering, making the inside more negative.

  • Characterized as inhibitory.

Channel Gating Mechanisms

• Ligand-gated channels: Open upon binding of neurotransmitter.

• Voltage-gated channels: Open in response to changes in membrane potential.

Graded Potentials and Action Potentials

Characteristics of Graded Potentials

• Magnitude Variability: Magnitude depends on the size of the stimulus or amount of neurotransmitter.

• Distance Decay: Signal strength decreases with distance from the origin.

• Summation: Multiple graded potentials can combine to create a larger change in membrane potential leading to firing an action potential depending on the sum of depolarizing or hyperpolarizing signals.

Action Potentials

• Initiated when depolarization reaches a threshold level (around -55mV).

• Sequential events: Voltage-gated channels allow a rapid influx of Na+, creating a positive feedback loop until peak voltage is reached.

Conclusion

Important Takeaways

• Membrane potentials are critical for neuronal signaling through action potentials and graded potentials.

• Rapid changes in membrane potential rely on the interplay of various ion channels and pumps in response to chemical signals (neurotransmitters) and electrical changes.