Lecture on Synaptic Integration

Overview of Lecture on Synaptic Integration

  • The primary focus of today's lecture is synaptic integration and its role in neuronal function.

  • Relevant reading is from SilverForm, Chapter 8.5.

Graded Potentials

  • Definition: Graded potentials are changes in membrane potential that vary in size, as opposed to being all-or-nothing.

    • Properties of Graded Potentials:

    • Graded: The magnitude of the potential depends on the amount of neurotransmitter released.

      • Small Release of Neurotransmitter: Opens a small number of ion channels, resulting in a small postsynaptic potential (PSP).

      • Large Release of Neurotransmitter: Opens a larger number of channels, leading to a larger PSP.

    • Decremental: Graded potentials decay in strength as they travel across the cell membrane.

    • Types of Potentials:

      • Excitatory Postsynaptic Potential (EPSP):

      • A depolarizing graded potential that brings the neuron closer to the threshold for firing an action potential.

      • Inhibitory Postsynaptic Potential (IPSP):

      • A hyperpolarizing graded potential that moves the neuron away from the threshold, making it less likely to fire an action potential.

    • Summation:

      • Graded potentials can combine; two smaller graded potentials can combine to form a larger graded potential.

Synaptic Integration

  • Purpose of Synaptic Integration:

    • The process of integrating multiple synaptic inputs to determine whether the neuron's initial segment will reach the threshold necessary to fire an action potential.

  • Diagram Discussion:


Two inputs, A and B, both are excitatory inputs causing EPSPs.

  • Mechanism of A and B:

  • At input A:

    • Stimulation results in the release of an excitatory neurotransmitter.

    • The neurotransmitter diffuses across the synaptic cleft and binds to ionotropic receptors, specifically non-specific monovalent cation channels, leading to rapid sodium influx and depolarization, resulting in a fast EPSP.

  • At input B:

    • The same process occurs, resulting again in an EPSP.

  • Comparison of A and B:

  • A's EPSP is smaller due to the decremental property of graded potentials; it travels further, thus degrading more compared to the closer input B.

Threshold and Action Potentials

  • Initial Segment and Axon Hillock:

    • The axon hillock decides whether an action potential will be fired based on membrane potential reaching threshold.

    • Threshold:

    • If the membrane potential reaches the critical threshold of -55 millivolts, an action potential is initiated.

  • Reaching Threshold:

    • Input B alone does not reach threshold due to its small EPSP.

    • Two methods to reach threshold:

    • Temporal Summation:

      • Firing input B twice in rapid succession results in the addition of two EPSPs, potentially elevating the potential to threshold.

    • Spatial Summation:

      • Firing inputs A and B simultaneously adds their EPSPs together to be enough to reach threshold.

    • Important Note:

    • Temporal and spatial summation are crucial, but the timing interval is critical for effective summation; if separated by too much time, they do not sum effectively.

Inhibitory Synapses

  • Inhibition via Input C:

    • Stimulation of input C releases an inhibitory neurotransmitter.

    • This neurotransmitter binds to different ionotropic receptors with integral chloride channels, leading to rapid hyperpolarization, which is a fast IPSP.

  • Adding EPSPs and IPSPs:

    • When input B (EPSP) and input C (IPSP) are stimulated:

    • The EPSP from B rises the potential toward threshold, while the IPSP from C counters it, moving the potential away from threshold, thus inhibiting action potential firing.

  • Presynaptic Inhibition via Input D:

    • Input D does not synapse directly on the neuron but on the presynaptic terminal of input B.

    • Effect of Stimulation:

    • When D is stimulated alongside B, it reduces the amount of neurotransmitter released from B, resulting in a smaller EPSP due to reduced depolarization. This is termed presynaptic inhibition.

Types of Synapses

  • Axoaxonic Synapse (D):

    • Terminology can be misleading; it synapses onto the presynaptic terminal not the axon directly.

    • Typically inhibitory, limits neurotransmitter release.

  • Axosomatic Synapse (C):

    • Synapses directly onto the cell body (soma); typically serves inhibitory functions.

  • Axodendritic Synapse (A & B):

    • Synapses onto the dendritic spines, generally excitatory, allowing the neuron to receive inputs effectively.

Implications of Synaptic Integration

  • Physiological Relevance:

    • Neurons possess hundreds of excitatory and inhibitory synapses each evoking small EPSPs and IPSPs.

    • Individual postsynaptic potentials (PSPs) are small (usually a few millivolts) and not enough alone to reach action potential threshold.

    • Summation of these potentials is necessary:

    • Depolarization from EPSPs moves the cell towards threshold, increasing action potential likelihood.

    • Hyperpolarization from IPSPs moves the cell away from threshold, reducing action potential likelihood.

  • Summary of Synaptic Integration Role:

    • Determines if the initial segment reaches the threshold of -55 millivolts to trigger an action potential.

Conclusion and Activities

  • Learning Outcome Review:

    • The objective is to explain the role of synaptic integration in neuronal function thoroughly.

  • Preparation for Next Lecture: Discussions will continue on synaptic functions and their broader implications.