A molecule that acts like a transmitter and binds to a receptor, causing a response similar to one caused by a transmitter. These molecules are usually drugs.

Agonist

A molecule that blocks a transmitter from binding with a receptor. These molecules are usually drugs.

Antagonist

A ligand is a substance that binds to receptor molecules, like agonists and antagonists when they mimic neurotransmitters. Agonists and antagonists are usually drugs; because they originate outside of the body, they are referred to as exogenous ligands. If an agonist or antagonist were to originate inside of the body, however, they would be referred to as endogenous ligands. 

Agonist and Antagonist Characterization 

Processes like up-regulation and down-regulation (see "Agonist" and "Antagonist") are evidence of the plasticity of receptors in the brain (i.e., the fact that each subtype of neurotransmitter receptor can vary in number and distribution in the brain). Receptor plasticity occurs during development and throughout adulthood. Therefore, a neuron's sensitivity to different transmitters can vary throughout the lifespan. 

Receptor Plasticity

After the transmitters have been released due to exocytosis, they bind to proteins called receptor molecules (or receptors). This binding is similar to how a key fits into a lock. If a transmitter, known as a ligand during this process, is not the correct shape, it will not be able to bind with a receptor. If a transmitter is the correct shape, it will be able to bind with the receptor at ligand-binding sites. Each transmitter is able to bind to a group of different receptors.

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The passing of information received from an action potential through the synapse. It occurs in the following steps:
- Exocytosis of neurotransmitter-containing vesicles occurs as the incoming action potential depolarizes the presynaptic membrane, leading to a rush of Ca 2+ entering the terminal
- As a result of exocytosis, transmitters are released into the synaptic cleft and bind with receptor molecules in the postsynaptic membrane
- These bindings cause ion channels in the postsynaptic membrane to open, creating a local EPSP or IPSP in the postsynaptic neuron
- The effects of the released transmitters are halted through either degradation or reuptake
- Some transmitters bind to autoreceptors, which further regulates the release of transmitters

Synaptic Transmission

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020, April 22). Psychology (2nd ed.). OpenStax. https://openstax.org/books/psychology-2e/pages/1-introduction

OpenStax Psychology (2nd ed.) Textbook

A process where neurotransmitters contained in a vesicle are released into the synaptic cleft due to the fusion of the vesicle and the presynaptic membrane. The arrival of the action potential at the axon terminal depolarizes the presynaptic terminal. This depolarization leads to the opening of voltage-gated calcium ion channels to allow Ca 2+ to enter the terminal.  The influx of calcium ions leads to exocytosis.

Exocytosis

Receptor Molecules

The binding of transmitters to receptors causes the ion channels to open.

Ion Channels Open

A receptor in the presynaptic membrane that regulates how much transmitter is released. It tells the presynaptic cell the amount of transmitter that has been released into the synaptic cleft, and it can use this information to adjust the amount of transmitter it releases in the future.

Autoreceptors

The classic synapses are axo-dendritic synapses (i.e., synapses that forms between a dendrite and an axon) or axo-somatic synapses (i.e., synapses that form between the axon and the cell body or soma). However, there are many other different types of synapses. Synapses can vary by location on the neuron as well as by the mechanism of synaptic transmission (i.e., electrical or chemical transmission). 

Types of Synapses

Ectopic transmission is a form of cell-cell communication between neurons that involves the release of neurotransmitters at locations other than traditional synapses. In this form of neural communication, neurotransmitters are act on regions other than traditional synapses. 

Ectopic Transmission

Once a neural signal is delivered, excess neurotransmitters must be removed from the synaptic cleft in a process known as clearing the synapse. This clearance occurs through three primary mechanisms: the neurotransmitters may drift away, be broken down into inactive fragments, or be reabsorbed into the presynaptic neuron via reuptake. Clearing the synapse is essential because it establishes a distinct "on" and "off" state between signals and helps regulate the overall production of neurotransmitters.

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