Graded Potentials

Graded Potentials

• Definition: Local changes in the membrane potential associated with the dendrites of a neuron.

• Magnitude: The change in membrane potential correlates directly with the size of the stimulus.

  • Example: Testing water temperature; warm water causes minimal change, while hot water induces a significant change.

Classification of Graded Potentials

• Graded potentials are classified as depolarizing or hyperpolarizing.

• Depolarizing Graded Potentials:

  • Occur through the influx of Na+ or Ca2+ into the cell.

  • Increases the positive charge inside the cell, shifting the voltage from -70 mV towards less negative values.

• Hyperpolarizing Graded Potentials:

  • Result from K+ exiting or Cl- entering the cell.

  • Makes the inside of the cell more negative, shift below -70 mV.

Membrane Potential Dynamics

• Depolarization and hyperpolarization depend on the specific ion channels activated.

• Example illustrated in Figure 1, which shows how graded potentials manifest based on stimulus strength:

  • A small stimulus results in minimal depolarization.

  • A larger stimulus leads to greater depolarization.

  • Prolonged stimulation maintains depolarization without enhancing its strength.

Types of Graded Potentials

1. Generator Potential:

   • Graded potentials in sensory neurons (e.g., unipolar cells).

   • Influence the generation of action potentials in the same neuron.

2. Receptor Potential:

   • Found in special sensory cells (e.g., taste cells, photoreceptors) and leads to neurotransmitter release with sensory neurons.

3. Postsynaptic Potentials (PSPs):

   • Graded potentials in dendrites receiving inputs from other cells.

   • Can be:

     • Excitatory Postsynaptic Potential (EPSP): Depolarization driving closer to action potential threshold.

     • Inhibitory Postsynaptic Potential (IPSP): Hyperpolarization shifting away from threshold.

Summation of Graded Potentials

• Summation of graded potentials adds small changes in membrane voltage leading to potential action generation.

  • A positive total change (e.g., from -70 mV to -55 mV) can reach the threshold for action potential.

• Generator Potentials: Initiate action potentials directly in sensory neurons.

• Initial Segment: Site at the beginning of the axon where graded potentials summate to generate an action potential; this includes axon hillock areas rich in voltage-gated Na+ channels.

Types of Summation

• Spatial Summation: Multiple graded potentials occur at different neuron locations collectively contributing to threshold.

• Temporal Summation: Single neuron firing multiple times may lead to significant changes in membrane potential.

• Both types can work together to facilitate action potential generation.

Key Terms

• Axon Hillock: Region where axon originates; key site for potential summation.

• Threshold Potential: The level of depolarization necessary for action potential initiation.