Graded Potential Notes

Graded Potential

Definition
  • A graded potential is a local change in membrane potential.
  • This change can be either positive or negative.
    • If the resting membrane potential is -70 mV, a graded potential can make it more negative (e.g., -90 mV) or more positive (e.g., -60 mV).
Characteristics
  • Travels a very short distance (a few millimeters).
  • It is decremental, meaning it doesn't last very long.
  • Varies in magnitude and duration.
  • Its purpose is to either get the cell to an action potential or prevent one.
Location
  • Occurs on the dendrites or the cell body of a neuron.
Triggering Stimuli
  • Graded potentials are triggered by a stimulus.
    • Types of stimuli:
      • Chemical stimulus: Involves neurotransmitters.
      • Mechanical stimulus: For example, touching the skin, which mechanically deforms sensory receptors.
Types of Graded Potentials
  • EPSP (Excitatory Post-Synaptic Potential):
    • Makes the resting membrane potential more positive (depolarization).
  • IPSP (Inhibitory Post-Synaptic Potential):
    • Makes the resting membrane potential more negative (hyperpolarization).
Terminology
  • Depolarization:
    • The process of the membrane potential becoming more positive, moving from negative to positive.
  • Repolarization:
    • The process of the membrane returning to its resting membrane potential from a positive value.
  • Hyperpolarization:
    • The process of the membrane becoming more negative than its resting membrane potential.
EPSP and IPSP in Relation to Membrane Potential
  • An EPSP causes depolarization.
  • An IPSP causes hyperpolarization.
  • Repolarization is specifically defined as going from a depolarized state back to the resting membrane potential.
Detailed Look at EPSPs and IPSPs
  • Occur in the cell body.
  • Involve extracellular and intracellular fluid separated by a membrane.
  • Chemical stimulus involves neurotransmitters binding to ligand-gated channels.
EPSP
  • Neurotransmitter binds to a ligand-gated channel.
  • Sodium ions (Na+) flood inside the cell, making the cell more positive.
  • If the depolarization is not enough to reach the threshold potential (typically -55 mV), an action potential will not occur.
  • The sodium-potassium pump helps restore the resting membrane potential by pumping three sodium ions out and two potassium ions in.
  • Na+Na^+ flows in
  • Sodium-potassium pump contributes 20% to resting membrane potential.
IPSP
  • Neurotransmitter binds to a ligand-gated channel.
  • Potassium ions (K+) leave the cell, or chloride ions (Cl-) enter the cell, making the cell more negative (hyperpolarization).
  • The sodium-potassium pump helps restore the resting membrane potential.
Factors Influencing Graded Potential
  • Strength of the stimulus: A stronger stimulus results in a greater change in membrane potential.
  • Duration of the stimulus: A longer stimulus results in a longer-lasting graded potential.

GradedPotential∝Stimulus<em>strength,Stimulus</em>durationGraded Potential \propto Stimulus<em>{strength}, Stimulus</em>{duration}

  • The presence of the neurotransmitter determines whether the gate of the ligand-gated channel remains open or not.
  • If the neurotransmitter leaves, the gate closes, and the flow of ions stops.
Graphical Representation
  • Graphs illustrate membrane potential over time, showing how different strengths and durations of stimuli affect graded potentials.
  • Higher stimulus strength leads to a greater magnitude of change in membrane potential.
  • Longer stimulus duration leads to a longer-lasting graded potential.
Ionotropic vs. Metabotropic Receptors
  • There are different kinds of ligand-gated channels:
    • Ionotropic: Has a direct effect on the membrane potential.
      • Lets ions pass directly through itself.
    • Metabotropic: Has an indirect effect on the membrane potential.
      • Activate secondary messengers inside of the cell.
  • Both types can cause EPSPs and IPSPs depending on the neurotransmitter that binds.
Synaptic Integration
  • A postsynaptic neuron may receive inputs from multiple presynaptic neurons, some inhibitory and some stimulatory.
  • Inhibitory neuron: Lowers the resting membrane potential (IPSP).
  • Stimulatory neuron: Increases the resting membrane potential (EPSP).
  • The combination of all EPSPs and IPSPs on a postsynaptic neuron is called the grand postsynaptic potential.
Detailed Mechanisms
  • Ionotropic Channels: Neurotransmitter binds, the gate opens, and ions flow through (e.g., K+ leaves or Cl- enters for IPSP; positive charges (e.g. Na+Na^+) enter for EPSP).
  • Metabotropic Channels: Neurotransmitter binds, the G-protein separates and activates secondary messengers, which then stimulate the opening of channels on the membrane.
Importance of Summation
  • One graded potential may not be enough to reach the threshold for an action potential.
  • Synaptic integration: The process of summing all inputs on the postsynaptic membrane.
Types of Summation
  • Temporal Summation: Occurs when one presynaptic neuron continuously fires action potentials on a single postsynaptic neuron.
  • Spatial Summation: Occurs when multiple presynaptic neurons each fire one action potential, releasing neurotransmitters that bind to the postsynaptic membrane.
  • Spatial-Temporal Summation: A combination of both, where multiple presynaptic neurons fire multiple times on a postsynaptic neuron. The sum of all EPSPs and IPSPs determines whether the postsynaptic neuron hits threshold.
The Role of Chloride
  • The resting membrane potential is an average, and some neurons have different potentials.
  • Chloride (Cl-) can play a crucial role in IPSPs and EPSPs depending on the neuron's resting membrane potential and the presence of specific transporters.
Chloride Movement and IPSPs/EPSPs
  • Leaky Chloride Channels: In some neurons, these channels help stabilize the resting membrane potential by counterbalancing the leak of other ions.
  • Chloride Ligand-Gated Channels: When a neurotransmitter binds, the channel opens, and chloride ions flow in, making the cell more negative (IPSP).
  • In cells where the membrane potential is different from the equilibrium potential of chloride (-70 mV), chloride movement may require active transport.
    • To cause an IPSP, chloride must be actively transported in.
    • To cause an EPSP, chloride must be actively transported out.
GABA and Chloride
  • GABA (gamma-aminobutyric acid) is a neurotransmitter that binds to GABA-A ligand-gated channels.
  • This channel opens and allows chloride to flow in, causing an IPSP.