Lecture Notes: Membrane Potential and Neuronal Signals

Resting Membrane Potential

• Defined as the difference in charge between the extracellular fluid and intracellular fluid.
• Typically -70 millivolts (mV) when a neuron is at rest.
• Neuron at rest is not sending signals.
• Altering membrane permeability through gated channels and leaky channels can change membrane potential.

Types of Neuronal Signals

Graded Potentials

• Occur mostly at dendrites and cell body.
• Short-range signals that decay with distance from the stimulus.
• Rely on chemically gated channels.
• Can result in depolarization (more positive) or hyperpolarization (more negative).

Action Potentials

• Occur along axons, initiated at the axon hillock (trigger zone).
• Use voltage-gated channels.
• Always involve depolarization followed by repolarization.
• Require achieving a threshold to initiate.
• Known as "all or nothing" signals.
• Maintain strength regardless of distance.

Changes in Membrane Potential

Depolarization

• Membrane potential becomes more positive relative to resting potential.
• Often due to sodium (Na+) influx.

Hyperpolarization

• Membrane potential becomes more negative relative to resting potential.
• Often due to potassium (K+) efflux.

Repolarization

• Return to resting membrane potential after depolarization.
• Typically associated with potassium efflux.

Graded Potentials: Detailed Look

• Graded potentials involve chemically gated sodium and potassium channels at the dendrites.
• Sodium influx through open chemically gated channels causes depolarization.
• The amount of neurotransmitter affects the duration the channel remains open, influencing the strength of depolarization.
• Strength of the graded potential is highest at the stimulus site and decreases with distance.

Comparison: Graded Potentials vs. Action Potentials

• Channels Used:
  • Graded Potentials: Chemically gated channels.
  • Action Potentials: Voltage-gated channels.
• Location:
  • Graded Potentials: Dendrites and cell body.
  • Action Potentials: Axons.
• Signal Characteristics:
  • Graded Potentials: Short-range, can be depolarizing or hyperpolarizing.
  • Action Potentials: Depolarization and repolarization, all-or-nothing.

Conclusion

• Action potentials involve a significant change in membrane potential (from -70 mV to +30 mV).
• Action potentials maintain strength and require a threshold to be reached.
• Next video will explore the conversion of graded potentials into action potentials.