5.7 bio notes

Electrophysiology Introduction

• Muscle and nerve cells transmit electrical signals by managing ion diffusion along chemical gradients.
• Ion diffusion generates an electrical charge across the plasma membrane.
• The electrical charge travels across the cell membrane by regulating membrane permeability to specific ions.
• Ion flow alters membrane potential:
  • Sodium or calcium influx makes the membrane potential more positive.
  • Potassium efflux makes the membrane potential more negative.
  • Chloride influx makes the membrane potential more negative.

Changing the Membrane Potential

• Types of changes:
  • Depolarization:
    • The membrane potential shifts towards a more positive voltage.
    • The inside of the membrane becomes closer to positive than its resting state.
    • On a membrane potential trace, depolarization causes an upward movement.
    • Opening ion channels (e.g., Na^+ channels) causes depolarization.
  • Repolarization:
    • The membrane potential returns to a more negative value after depolarization.
    • On a trace, repolarization causes a downward movement towards the resting potential.
  • Hyperpolarization:
    • The membrane potential becomes more negative than the resting potential.
    • On a trace, hyperpolarization causes the line to move below the resting potential.
    • Opening ion channels (e.g. K^+ or Cl^- channels) causes hyperpolarization.

Nerve Cells (Neurons)

• Three major portions:
  • Dendrites:
    • Receptors that receive chemical or physical signals.
  • Cell body:
    • Carries out essential life functions.
    • Integrates all incoming electrical signals.
  • Axon (effector):
    • Generates and transmits electrical signals called action potentials along its membrane.

Nerve Cells and Synapses

• Each axon terminal connects with other cells at synapses.
• The synapse facilitates neuron communication.
• Chemical synapses:
  • The axon releases chemical signals (neurotransmitters).
  • Neurotransmitters diffuse across the synaptic cleft (a small gap).
  • Neurotransmitters bind to receptors on the post-synaptic neuron.

Action Potentials

• Dendrites receiving signals cause small disturbances in membrane potential.
• If strong enough, these disturbances trigger an action potential.
• This occurs if the membrane potential reaches a threshold potential.
• Above the threshold, the nerve cell initiates action potentials.

Key Physiological Behaviors of Action Potentials

• Triggered by a stimulus:
  • Dependent on the nerve cell receiving a chemical or physical signal.
  • The signal, usually received by dendrites, must be strong enough to exceed the threshold.
• All or none:
  • Action potentials are identical in strength and size.
• Non-decremental:
  • Action potentials do not weaken with distance.
  • They travel long distances along the cell membrane without losing signal strength.
• Irreversible:
  • Once triggered, an action potential continues until completion.
  • Cannot be stopped once initiated.

How an Action Potential is Made

• Action potentials result from a chain reaction of opening and closing voltage-gated ion channels.
• The opening and closing sequence is consistent for each action potential.
• Two key channels:
  • Voltage-gated sodium channel.
  • Voltage-gated potassium channel.

Voltage-Gated Potassium Channels

• States: Open or closed.
• Open:
  • Potassium diffuses out of the cell.
  • Membrane potential becomes more negative.
• Closed: No potassium diffusion.

Voltage-Gated Sodium Channels

• States: Open, closed, or inactivated.
• Open:
  • Sodium diffuses into the cell.
  • Membrane potential becomes more positive.
• Closed: No sodium diffusion.
• Inactivated:
  • No sodium diffusion.
  • The channel cannot reopen until reset.

Stages of an Action Potential

• Resting stage (membrane potential around -60 to -70 mV).
• Depolarization.
• Repolarization.
• Hyperpolarization.

Action Potential Stages in Detail

• Resting Stage:
  • All voltage-gated channels are closed.
  • Channels remain closed until a signal exceeds the threshold.
  • Leak channels maintain resting potential.
• Depolarization Stage:
  • Above threshold, voltage-gated sodium channels open.
  • Sodium diffuses into the cell, making the membrane potential more positive.
  • The stage ends when the membrane potential reaches +35mV.
• Repolarization Stage:
  • Voltage-gated sodium channels inactivate.
  • Voltage-gated potassium channels open.
  • Potassium diffuses out of the cell, making the membrane potential more negative.
  • The stage ends when the membrane potential reaches the resting potential.
• Hyperpolarization Stage:
  • Potassium channels remain open, causing the potential to drop below the resting potential.
  • Voltage-gated sodium channels reset.
  • Voltage-gated potassium channels close.
  • The membrane potential slowly rises back to the resting state.

Sodium Channel Inactivation and Refractory Period

• Sodium channels inactivate to ensure action potentials are irreversible.
• Inactivation makes it necessary for the action potential to fully complete before the channels can reset.
• Resetting only occurs during hyperpolarization.
• The absolute refractory period is the time during which a cell cannot initiate a new action potential until the previous one completes.