What is the Difference Between Excitatory and Inhibitory Synapses

Chemical synapses are connections between two neurons or between a neuron and a non-neuronal cell, such as muscle cells, glandular cells, and sensory cells. Excitatory and inhibitory synapses are two different types of chemical synapses.

The key difference between excitatory and inhibitory synapses is their function. Excitatory synapses stimulate the postsynaptic neuron to generate an action potential, while inhibitory synapses prevent the postsynaptic neuron from generating an action potential. Therefore, excitatory synapses increase the likelihood of generation of an action potential, whereas inhibitory synapses decrease the likelihood of generation of an action potential.

CONTENTS

1. Overview and Key Difference
2. What are Excitatory Synapses
3. What are Inhibitory Synapses
4. Similarities – Excitatory and Inhibitory Synapses
5. Excitatory vs Inhibitory Synapses in Tabular Form
6.  Summary – Excitatory vs Inhibitory Synapses
7.  FAQ – Excitatory and Inhibitory Synapses

What are Excitatory Synapses?

Excitatory synapses increase the likelihood of postsynaptic neurone depolarization and generation of an action potential. Excitatory synapses involve excitatory neurotransmitters.

The most common excitatory neurotransmitter is glutamate. It exerts its effects via ionotropic receptors such as kainate, AMPA, NMDA, and metabotropic receptors. This binding increases the likelihood of an action potential. This normally occurs with the influx of positively charged sodium (Na+) ions into the postsynaptic neuron cell through the ion channels activated via neurotransmitter binding mentioned above.

What are Inhibitory Synapses?

Inhibitory synapses reduce the likelihood of postsynaptic neurone depolarization and generation of an action potential. The most common inhibitory neurotransmitter is GABA, which acts via ionotropic receptors called GABAA receptors and metabotropic receptors called GABAB receptors. This binding of GABA to GABAA receptors induces the opening of ion channels that are selectively permeable to chloride ions.

GABA causes chloride ions to flow across the postsynaptic neuron membrane. The chloride ions flow into the cell since chloride ions are more abundant extracellularly. The flow of chloride ions into the cell ultimately generates a hyperpolarising current, hence generating a hyperpolarising inhibitory postsynaptic potential.

Similarities Between Excitatory and Inhibitory Synapses

1. Excitatory and inhibitory synapses are two different types of chemical synapses.
2. Ionotropic receptors and metabotropic receptors are involved in both chemical synapses.
3. Both chemical synapses are extremely important for nerve communication.
4. The problems in these chemical synapses cause nerve diseases.

Comparing the Difference Between Excitatory and Inhibitory Synapses

Definition

1. Excitatory synapses are chemical synapses that increase the likelihood of generation of an action potential.
2. Inhibitory synapses are chemical synapses that decrease the likelihood of generation of an action potential.

Neurotransmitter Involved

a. Excitatory synapses typically involve the neurotransmitter glutamate.
b. Inhibitory synapses typically involve the neurotransmitter GABA (gamma-aminobutyric acid).

Receptors Involved

a. Excitatory synapses can activate ionotropic receptors such as kainate, AMPA, NMDA, and metabotropic receptors like mGlu1-mGlu8.
b. Inhibitory synapses utilize GABA, acting via ionotropic receptors called GABAA receptors and metabotropic receptors called GABAB receptors.

Postsynaptic Potential Generated

a. Excitatory synapses generate excitatory postsynaptic potentials (EPSPs), which depolarize the postsynaptic membrane.
b. Inhibitory synapses generate hyperpolarizing inhibitory postsynaptic potentials, which make the postsynaptic membrane more negative.

Ions Flow into the Cell

a. Excitatory synapses typically allow the influx of sodium ions (Na+).
b. Inhibitory synapses typically allow the influx of chloride ions (Cl-).

The infographic below presents the differences between excitatory and inhibitory synapses in tabular form for side-by-side comparison.

Summary – Excitatory vs Inhibitory Synapses

Excitatory and inhibitory synapses are two different types of chemical synapses. However, excitatory synapses stimulate the postsynaptic neuron to generate an action potential, while inhibitory synapses prevent the postsynaptic neuron from generating an action potential. Thus, this is the key difference between excitatory and inhibitory synapses.

FAQ: Excitatory and Inhibitory Synapses

1. What are examples of excitatory and inhibitory synapses?

• Excitatory neurotransmitters include glutamate, epinephrine, and norepinephrine, which increase the likelihood of action potential generation, while inhibitory neurotransmitters block or prevent the chemical message from being passed along any further, like Gamma-aminobutyric acid (GABA), glycine, and serotonin that are part of inhibitory synapses.

2. What are excitatory and inhibitory synaptic potentials?

• PSPs (Post synaptic potentials) are called excitatory (or EPSPs) if they increase the likelihood of the occurrence of a postsynaptic action potential and inhibitory (or IPSPs) if they decrease this likelihood.

3. What is an excitatory response?

• Neurons communicate through electrical currents called action potentials, which are either excitatory or inhibitory. Excitatory responses are those that stimulate one neuron to share information with the next through an action potential, while inhibitory responses reduce the probability that such a transfer will take place.

4. What channels are excitatory synapses?

• Upon binding of glutamate molecules to an AMPA receptor, its integral ion channel opens, and ions flow across the postsynaptic neuronal membrane. AMPA receptor is a non-selective cation channel that is mostly permeable to sodium and potassium ions. Others are NMDA and metabotropic receptors.

5. What channels are inhibitory synapses?

• Upon binding of GABA to GABA_A receptors or GABA_B receptors, GABA causes chloride ions to flow across the postsynaptic neuron membrane. Therefore, GABA_A and GABA_B receptors are channels in inhibitory synapses.

Reference:

1. Lauterborn, Julie C., et al. “Increased Excitatory to Inhibitory Synaptic Ratio in Parietal Cortex Samples from Individuals with Alzheimer’s Disease.” Nature News, Nature Publishing Group.

2. “Excitatory and inhibitory receptors utilize distinct post- and trans-synaptic mechanisms in vivo.” National Library of Medicine.

Image Courtesy:

1. “Synapse Illustration2 tweaked” By Nrets – uploaded to Wikimedia Commons as Image:SynapseIllustration2.png, SVG version by User:Surachit. For further information on the image's contents, see Julien, R. M. (2005). The neuron, synaptic transmission, and neurotransmitters. In R. M. Julien, A primer of drug action: A comprehensive guide to the actions, uses, and side effects of psychoactive drugs (pp. 60-88). New York, NY, USA: Worth Publishers. (CC BY-SA 3.0) via Commons Wikimedia
2. “TAAR1 Dopamine” By Seppi333 – Own work (CC BY-SA 3.0) via Commons Wikimedia

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