Lecture Notes: Resting Membrane Potentials, Graded Potentials, and Action Potentials of Neurons

Introduction

• Focus on neuronal membrane potentials: resting membrane potentials, graded potentials, and action potentials.
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Resting Membrane Potential (RMP)

• Definition: Voltage difference across the cell membrane when the cell is at rest.
• Existence: Found in all cells, but we focus on neurons.
• Voltage Range: Generally between -70mV to -90mV, with -70mV being the average.

Cellular Structure

• Zooming in on a neuron:
  • Components: Axon, cell body, axon terminal.

Mechanisms of RMP

1. Sodium-Potassium ATPases

   • Pumps 3 sodium ions out and 2 potassium ions in, making the inside of the cell slightly negative.
   • Establishes concentration gradients for sodium (higher outside) and potassium (higher inside).

2. Leaky Potassium Channels

   • Always open, allowing potassium to move out passively.
   • Higher concentration of potassium inside means potassium leaves the cell, making it more negative.
   • Leaves behind unoccupied negatively charged anions (proteins/phosphates).

3. Leaky Sodium Channels

   • Allow sodium to move in, but potassium permeability is much higher.
   • Sodium influx contributes less to RMP due to lower permeability compared to potassium.
   • Results in a final RMP around -70mV.

Nernst Potential

• Used to calculate equilibrium potentials for different ions (e.g., sodium and potassium).
• Equation: E = (61.5/Z) * log([Ion outside]/[Ion inside])
• Example:
  • Potassium: E = -90mV
  • Sodium: E = +70mV
• RMP is influenced more by potassium due to higher permeability.*

Graded Potentials

• Purpose: Move RMP closer to threshold for action potentials.
• Threshold Voltage: Generally around -55mV.
• Two types of graded potentials:
  • EPSPs (Excitatory Post-Synaptic Potentials): Depolarizing, move voltage closer to threshold.
  • IPSPs (Inhibitory Post-Synaptic Potentials): Hyperpolarizing, move voltage further from threshold.

Synaptic Activity

• Presynaptic Neurons: Release neurotransmitters that bind to receptors on Postsynaptic Neurons.
• EPSP Mechanism:
  • Stimulatory neurotransmitter (e.g., glutamate) binds to ligand-gated ion channels, allowing cations to enter and depolarize the cell.
• IPSP Mechanism:
  • Inhibitory neurotransmitter (e.g., GABA) can open channels for chloride ions (negative) or potassium ions (positive leaving), hyperpolarizing the cell.

Summation of Potentials

• Temporal Summation: One presynaptic neuron fires repeatedly to reach threshold.
• Spatial Summation: Multiple presynaptic neurons firing simultaneously to reach threshold.

Action Potentials

• Triggered once threshold potential is reached (-55mV).
• Voltage-Gated Sodium Channels:
  • Open rapidly upon reaching threshold, causing depolarization (voltage rises to +30mV).
  • Inactivation gates close at +30mV, stopping sodium influx.
• Voltage-Gated Potassium Channels:
  • Open at +30mV, allowing potassium to exit the cell, repolarizing back to resting state.

Phases of Action Potential

1. Depolarization: Rapid influx of sodium ions.
2. Repolarization: Efflux of potassium ions returning voltage to negative.
3. Hyperpolarization: Brief phase where cell becomes more negative than resting potential.

Refractory Periods

• Absolute Refractory Period: Time after action potential during which another action potential cannot occur regardless of stimulus.
• Relative Refractory Period: Following the absolute period; a stronger-than-normal stimulus is needed to elicit an action potential.

Conclusion

• Recap of resting membrane potentials, graded potentials, and action potentials.
• Importance of understanding neuronal excitability and communication.
• Encouragement to revisit key concepts and study the mechanisms discussed.