receptor on the right shows the Channel when it is closed, Right when it is open

Receptors: molecules inside or on the surface of a cell that bind to a specific substance and cause a specific physiologic effect in the cell. (GHR) A protein located on the cell surface, or in the cytoplasm, that binds to a specific signaling factor, such as a “hormone,” “antigen,” or “neurotransmitter,” causing a “conformational” and functional change in the receptor molecule. (NCIt) Specialized (cell surface) proteins on the outer surface of the target cell membrane, that recognize and capture a neurotransmitter that has been released into the synaptic clef. (Kandel, 91) Large protein molecules that act as ‘biological machines.’ In each receptor there is a passageway (tunnel or ion channel) that can open into the dendrite (or into the cell body) of the receiving neuron when a neurotransmitter is detected. When the passageway opens briefly, charged ions floating in solution leak out, reducing the voltage inside the postsynaptic neuron. If the voltage change is big enough, the voltage drop triggers the postsynaptic neuron to fire (an action potential) out of its own axon to signal the next neuron in the circuit. (Fields, 20) Receptors determine whether the synaptic action will be excitatory or inhibitory by controlling different ion channels in the cell membrane, primarily sodium for excitation, and chloride for inhibition. (The Brain-Eric Kandel, 30) At least a hundred genes have been identified that code for different receptors. (Campbell, BSP210) Also referred to as ‘synaptic receptor,’ or ‘postsynaptic receptor,’ ‘receptor protein,’ ‘cell-specific protein,’ and cell-surface receptor protein.’ Editor's note - not to be confused with "sensory receptors.”

G-protein-coupled Receptor: a protein on the cell surface that binds a neurotransmitter (the "first messenger") and then “activates” a chemical inside the cell (the "second messenger") that initiates a cell-wide response. (Kandel, 442) (This is considered) slow or indirect transmission which is (usually) neuromodulatory. More complex (than ligand-gated receptors) because they use a second messenger and there’s a wide variety of effects that include indirectly opening or closing ion channels, but can go as far as turning genes on an off. (Campbell, BSP210) Also referred to as ‘metatropic receptor.’ Editor’s note - metabotropic receptors do not have ion channels, but if a neurotransmitter binds to them, it may cause the opening of an ion channel somewhere else on the postsynaptic neuron membrane.

Opiate Receptor: cell membrane proteins that bind “opioids” and trigger intracellular changes which influence the behavior of cells. The… ligands for opioid receptors in mammals include three families of peptides, the “enkephalins,” “endorphins,” and ‘dynorphins.’ (MeSH) Scientists have always assumed that all opioids—whether produced by the body (endogenously) or taken as a drug—interact in the same way with opioid receptors. Recent studies have found G-protein-coupled receptors that can be activated within the cell as well as at the cell surface. (NIH) Editor’s note - these are receptors involved in the perception and integration of “pain” and emotional experience. They are found in certain regions of the brain that bind opiate drugs. They are also concentrated in spinal cord areas. Also referred to as ‘opioid receptor.’

Muscarinic Receptor: regulates both ion channels and second messenger pathways involving enzymes. Activated by muscarine, which is a mushroom toxin. (Campbell, BSP210)

Ligand-Gated Receptor: a protein that spans the cell surface-membrane, and contains a neurotransmitter “binding site” and an “ion channel” through which ions can pass. The binding of the appropriate neurotransmitter directly opens or closes the ion channel to the movement of ions. (Kandel, 449) A ligand-bound receptor alters its interaction with target molecules, which leads to changes in cellular physiology through modification of the activity. (NCit) (This is considered) fast, or direct transmission. (Includes) a place called the ‘extracellular binding domain,’ which is where the neurotransmitter binds. Target neurons might have both ligand-gated and G-protein coupled receptors. (Campbell, BSP210) Also referred to as ‘ionotropic receptor. Editor’s note - ionotropic receptors are usually closed until a “ligand” binds to the receptor.

GABA Receptor: cell-surface protein that binds “gamma-aminobutyric acid” and triggers changes that influence the behavior of cells. GABA receptors control “chloride channels.” (MeSH) A variety of tranquilizing drugs, including “barbiturates,” “alcohol,” and general “anesthetics,” bind to GABA receptors and produce a calming effect on “behavior” by enhancing the receptors’  "inhibitory" functions. (Kandel, 99) When GABA receptors are occupied, the inside of the cell becomes more negative due to the influx of negative ions, especially "chloride," through a passage in the receptor. This makes it harder for glutamate released from other terminals to sufficiently trigger an action potential. (LeDoux, 55) Editor’s note - a GABA receptor may be a “ionotropic receptor” or a “metabotropic receptor.”

Glutamate Receptor: when glutamate binds to the outside part (of this receptor), a passage opens up through the receptor, allowing positively charged ions in the “extracellular fluid” to move inside the cell, which changes the chemical balance between outside and inside. If enough glutamate receptors are occupied at the same time, the voltage inside becomes sufficiently positive, and then an action potential occurs. (LeDoux, 55) Glutamate receptors tend to be located out on the dendrites, especially on the (dendritic) spines, whereas GABA receptors tend to be found on the cell body, or on the part of dendrites close to the cell body. (LeDoux, 56) Editor’s note - a glutamate receptor may be an “ionotropic receptor” or an “metabotropic receptor.”

NMDA Receptor: part of the family of glutamate receptors. Requires two molecules of glutamate and two molecules of glycine because it has an additional voltage channel that is blocked. (Campbell, BSP210)

Nicotinic Receptor: ionotropic receptor activated by nicotine. There is… a muscle type and a neuronal type. The neuronal type is 50 times more sensitive to nicotine. (Campbell, BSP210)

Receptor Structure: has three main components: an external ligand-bonding area called the ‘extracellular domain,’ a membrane-spanning area called a ‘transmembrane domain,’ and an intracellular domain (penetrating) the inside of the cell. (Clark, Signaling Molecules)