Spatial Summation & Temporal Summation

Graded Potentials and Summation

  • Graded Potentials

    • Also known as local potentials or receptor potentials.
    • Fundamental neurophysiological events that occur in neurons.

  • Types of Summation

    • Spatial Summation

    • Definition: The process of adding multiple graded potentials from different locations simultaneously at the axon hillock (trigger zone).

    • Mechanism:

      • Multiple chemically gated sodium channels may open in various locations along the neuron.
      • These openings create graded potentials that add together (constructive interference).
      • Example Analogy: Similar to throwing pebbles into the same area of a pond. Instead of multiple small ripples, it results in a larger wave-like effect as all ripples converge.

    • Temporal Summation

    • Definition: The process where a single receptor is stimulated repeatedly over a short time period, causing a series of graded potentials to occur before the previous ones decay.

    • Mechanism:

      • Continuous binding of the same chemical to a receptor causes successive openings of the channel.
      • As new graded potentials are generated, they build on the previous ones before they can decay.

  • Threshold Potential

    • Definition: The level of depolarization needed to trigger an action potential.
    • Value: Negative 55 millivolts (mV).

Action Potentials

  • Trigger Zone (Axon Hillock)

    • Location where graded potentials are transformed into action potentials.
    • Unique properties allow for initiation of action potentials due to the presence of voltage-gated channels.
    • Membrane Composition at Axon Hillock:
    • Contains sodium leak channels, potassium leak channels, and a high density of sodium voltage-gated channels.

  • Voltage-Gated Sodium Channels

    • Structure:
    • Consist of an activation gate and an inactivation gate.
    • Activation gate opens upon reaching threshold, allowing sodium (Na^+) influx.
    • Inactivation gate closes shortly after activation, preventing further ion flow.
    • Mechanism of Action:
    • At resting potential (-70 mV): Both gates are closed; sodium influx is minimal.
    • Upon depolarization (graded potential reaching threshold): Activation gate opens, allowing sodium to flow into the cell.
    • The cell becomes less negative/more positive, creating the rapid change in membrane potential fundamental to action potentials.

  • Action Potential Propagation

    • As each segment of the axon depolarizes, it influences adjacent segments, causing a wave of depolarization.
    • The forward movement of action potentials is ensured by the inactivation of previous sodium channels along the axon, preventing backward propagation.

Information Coding in Neurons

  • Graded Potentials vs. Action Potentials
    • Graded Potentials:
    • Represented by amplitude (how high the graded potential reaches).
    • Action Potentials:
    • Represented by frequency (the number of action potentials occurring in a given time).

Electrical Signal Modulation

  • Influence of Chloride Ions

    • Introduction of negatively charged chloride ions (Cl^-) into the intracellular environment can lead to hyperpolarization.
    • Result:
    • Prevents reaching the threshold potential, hence no action potential is generated (negative feedback).

  • Decisions by the Neuron

    • Incoming signals lead to either depolarization (excitatory) or hyperpolarization (inhibitory).
    • Neurons integrate signals from various sources to determine whether to fire an action potential.

Summary

  • Graded potentials can lead to action potentials through spatial and temporal summation at the axon hillock.
  • Threshold potential is critical for action potentials, defined at -55 mV.
  • Voltage-gated sodium channels play a key role in the generation and propagation of action potentials.
  • Chloride ions can negatively impact the excitability of neurons, preventing action potentials.