Nonsynaptic Communication, Postsynaptic Potentials, and Neural Integration

Nonsynaptic Chemical Communication

  • Definition: Nonsynaptic chemical communication refers to the chemical modulation of the cell membrane at locations other than the synapse.

  • Neuromodulators:

    • Definition: Naturally secreted substances that act like a neurotransmitter except that they are not restricted to the synaptic cleft, but instead diffuse through the extracellular fluid.

    • Chemical Composition: Most neuromodulators are chains of amino acids known as peptides.

    • Functions:

    • Alter the firing rate of neurons.

    • Raise or lower the required potential to activate voltage-dependent ion channels.

    • Representative Example:

    • Endorphins: Decrease pain by inhibiting pain signals.

    • Pharmacological Parallel: Morphine works in the exact same way as endorphins.

  • Hormones:

    • Definition and Secretion: Secreted by endocrine glands or specialized cells in various organs throughout the body, including:

    • Stomach

    • Intestines

    • Kidneys

    • Brain

    • Distribution Mechanism: Distributed throughout the body via the bloodstream.

    • Target Cells: Contain specific receptors for particular hormones.

    • Representative Example:

    • Leptin: Decreases hunger by inhibiting neurons that signal low blood sugar.

Excitatory Postsynaptic Potentials (EPSPs)

  • Definition: An excitatory depolarization of the postsynaptic membrane.

  • Ionic Mechanism: Caused by neurotransmitters that open sodium ion channels, allowing positive sodium ions into the cell.

  • Summation and Threshold Activation:

    • Individual postsynaptic potentials cause only small changes relative to the overall membrane potential.

    • If enough excitatory synapses are active at the same time, the EPSPs (represented visually in red) add up to an amount that reaches the threshold of excitation.

    • Once the threshold of excitation is reached at the axon hillock, an action potential is triggered down the axon.


Diagram showing activity of excitatory synapses producing EPSPs (red) in a postsynaptic neuron, reaching the threshold of excitation at the axon hillock and triggering an action potential

Inhibitory Postsynaptic Potentials (IPSPs)

  • Definition: An inhibitory hyperpolarization of the postsynaptic membrane of a synapse.

  • Ionic Mechanisms: Caused by neurotransmitters which:

    • Release potassium out of the cell.

    • Admit chloride or calcium into the cell.

  • Interaction with Excitatory Signals:

    • If inhibitory synapses are active at the same time as excitatory synapses, the IPSPs (represented visually in blue) diminish the overall size of the EPSPs.

    • Reduces the likelihood of the postsynaptic membrane potential reaching the threshold of excitation.

    • Counteracts EPSPs to block action potential generation completely.


Diagram showing activity of inhibitory synapses producing IPSPs (blue) that counteract EPSPs in a postsynaptic neuron, preventing an action potential from being triggered in the axon

Neural Integration

  • Definition: The process by which inhibitory and excitatory postsynaptic potentials sum together to control the rate of firing of a neuron is known as neural integration.

  • Key Features:

    • Potential Magnitude: Postsynaptic potentials represent only small relative changes in the context of the whole membrane potential.

    • Convergence: A single neuron receives incoming signals simultaneously from thousands of other neurons.

    • Synaptic Locations:

    • Synapses on the soma (cell body).

    • Synapses on dendrites.

    • Mechanism: The algebraic summation of opposing EPSPs and IPSPs across the cell body and dendrites determines whether the threshold of excitation is reached at the axon hillock, thereby controlling the overall firing rate of the neuron.