Ionotropic receptors, also called neurotransmitter-gated or ligand-gated channels, are specialized ion channels that mediate rapid synaptic responses by opening in response to neurotransmitter binding. These receptors are critical for processing incoming synaptic information, with their function defined by their ion selectivity and reversal potential. Excitatory and inhibitory ionotropic receptors shape the postsynaptic membrane potential, influencing neuronal communication.

Ionotropic Receptors

Ionotropic receptors are primarily located along the dendrites or cell body, but they can be present anywhere along the neuron if there is a synapse. Ligand-gated channels are important for receiving incoming information from other neurons.

Although ionotropic receptors are ion channels, they open in a different way than the voltage-gated ion channels needed for propagation of the action potential. The ionotropic receptors are ligand-gated, which means that a specific molecule, such as a neurotransmitter, must bind to the receptor to cause the channel to open and allow ion flow. As seen in previous chapters, the voltage-gated channels open in response to the membrane potential reaching threshold.

Animation 11.1. Ion channel gating mechanisms. Ligand-gated channels (ionotropic receptors) open when neurotransmitters bind, while voltage-gated channels open when membrane potential reaches threshold. Both allow ions to flow down electrochemical gradients. Glutamate receptors (teal lined), GABA receptors (yellow solid), voltage-gated sodium channels (blue dotted). 'Ion Channel Gating' by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

The receptors can only be opened by a specific ligand. Neurotransmitters and receptors fit together like a lock and key; only certain neurotransmitters are able to bind to and open certain receptors.

Animation 11.2. Receptor-ligand specificity. Neurotransmitter receptors function like locks and keys—glutamate binds to and opens glutamate receptors (teal lined) but has no effect on GABA receptors (yellow solid). This specificity ensures precise synaptic communication. 'Ligand and Receptor' by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Ion movement through ligand-gated ion channels follows the same principles covered in previous chapters

Glutamate Receptors

Glutamate causes EPSPs by opening cation channels that increase sodium permeability across the membrane

Glutamate is the primary excitatory neurotransmitter in the central nervous system and opens non-selective cation channels. There are three subtypes of glutamate receptors. The AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) and kainate receptors allow both sodium and potassium to cross the membrane. Although potassium can leave the cell when the receptors open, the electrochemical gradient driving sodium ion movement is stronger than the gradient driving potassium movement, resulting in a depolarization of the membrane potential.

Animation 11.3. AMPA and kainate receptor function. These glutamate receptors are non-selective cation channels allowing both sodium and potassium flow. When glutamate binds, the stronger sodium gradient produces net inward current and depolarization (EPSP). AMPA receptors (teal lined), kainate receptors (teal checkered). 'AMPA and Kainate' by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

The NMDA (N-methyl-D-aspartate) receptor requires the binding of glutamate to open, but it is also dependent on voltage. When the membrane potential is below, at, or near rest, a magnesium ion blocks the open NMDA receptor and prevents other ions from moving through the channel. Once the cell depolarizes, the magnesium block is expelled from the receptor, which allows sodium, potassium, and calcium to cross the membrane. The voltage change needed to open the NMDA receptor is usually a result of AMPA receptor activation. Released glutamate binds to both AMPA and NMDA receptors, sodium influx occurs through open AMPA channels, which depolarizes the cell enough to expel the magnesium ion and allow ion flow through the NMDA receptors.

Animation 11.4. NMDA receptor dual-gating mechanism. NMDA receptors require both glutamate binding and membrane depolarization. At rest, magnesium blocks the channel despite glutamate binding. AMPA receptor activation depolarizes the membrane, expelling the magnesium block and allowing sodium, potassium, and calcium (important for plasticity) to flow through NMDA receptors. AMPA receptors (teal lined), NMDA receptors (violet dotted). 'AMPA and NMDA' by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Nicotinic Acetylcholine Receptors

Like glutamate receptors, nicotinic acetylcholine receptors are non-selective cation channels. Nicotinic receptors are found throughout the nervous system but are especially prominent and are best know for their role at the neuromuscular junction outside the CNS.

GABA and Glycine Receptors

GABA and Glycine cause IPSPs by opening chloride channels that increase chloride permeability across the membrane

GABA and glycine receptors are chloride channels. Since an increase in chloride permeability across the membrane is inhibitory, the binding of GABA or glycine to their respective ionotropic receptor will cause inhibition.

Animation 11.5. GABA and glycine receptor function. These inhibitory receptors are chloride-selective channels. When GABA or glycine binds, chloride permeability increases, producing IPSPs that reduce neuronal excitability. GABA receptors (yellow solid), glycine receptors (yellow patterned). 'GABA and Glycine' by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Ionotropic Receptors Cause Postsynaptic Potentials

Postsynaptic potentials (Chapter 5) are a result of ionotropic receptors opening. Excitatory ionotropic receptors increase sodium permeability across the membrane, whereas inhibitory ionotropic receptors increase chloride permeability. Ion flow through the ionotropic receptors follows the same principles as other ion channels covered so far.

Equilibrium Potential Review

Previously, we covered ion movement through voltage-gated channels and discussed that electrochemical gradients will drive ion movement toward equilibrium. The neuron’s membrane potential at which the chemical and electrical gradients balance and equilibrium occurs is the ion’s equilibrium potential.

Animation 11.6. Equilibrium potential review. When voltage-gated channels open, ions flow until the membrane potential reaches the ion's equilibrium potential, where electrical and concentration gradients balance. Sodium influx through voltage-gated sodium channels (blue dotted) drives the membrane toward +60 mV, sodium's equilibrium potential. 'Equilibrium Potential' by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Reversal Potential

This same principle is used for ion movement through ionotropic receptors. The membrane potential at which ion flow through a receptor is at equilibrium is called the reversal potential of the receptor. The direction of ion movement can be predicted if the reversal potential of the receptor is known.

You can think of a reversal potential as being the equilibrium potential for a receptor, which may allow more than one ion to move across the membrane

GABA and Glycine - Receptors Selective to One Ion

When an ionotropic receptor that is selective to only one ion opens, the reversal potential of the receptor is the same as the equilibrium potential of the ion. GABA and glycine receptors only allow chloride ions to cross the membrane. Therefore, the reversal potential of a GABA or glycine receptor is equal to the equilibrium potential of chloride, and the binding of GABA or glycine to their respective ionotropic receptor will cause an inhibitory postsynaptic potential (IPSP).

Animation 11.7. When an ionotropic receptor is selective to one ion, its reversal potential equals that ion's equilibrium potential. GABA receptors (yellow checkered) are chloride-selective, so their reversal potential equals chloride's equilibrium potential of -65 mV, and chloride influx produces an IPSP. 'GABA Reversal Potential' by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Glutamate - Reversal Potential for Receptors that are Non-Selective

However, if the ionotropic receptor allows the flow of more than one ion, or is non-selective, the reversal potential of the receptor does not equal the equilibrium potential of either ion but is somewhere in between. The equilibrium potential of sodium is approximately +60 mV, and the equilibrium potential of potassium is approximately -80 mV. A glutamate receptor is a non-selective cation channel that allows the flow of both ions, and the reversal potential of the receptor is 0 mV. This means that if the neuron’s membrane potential is negative, the driving forces acting on sodium are stronger than the driving forces acting on potassium, so more sodium will flow in than potassium will flow out, and the membrane potential will depolarize, causing an excitatory postsynaptic potential (EPSP).

Animation 11.8. Glutamate receptor reversal potential. Non-selective channels have reversal potentials between the equilibrium potentials of permeable ions. Glutamate receptors (teal lined) are permeable to sodium (+60 mV) and potassium (-80 mV), giving a reversal potential of 0 mV. From rest, stronger sodium influx produces net depolarization and EPSPs. 'Glutamate Reversal Potential' by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

If the membrane potential reached the reversal potential of the glutamate receptor, the electrochemical gradients acting on sodium and potassium would balance, so overall ion flow in both directions would be equal, and the membrane potential would not change.

Animation 11.9. Reversal potential equilibrium. At the reversal potential (0 mV for glutamate receptors), electrochemical gradients balance so equal numbers of sodium ions enter and potassium ions exit. No net current flows, and the membrane potential remains stable at the reversal potential if the channel stays open. 'Glutamate Reversal Potential - 0 mV' by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Conclusion

Ionotropic receptors are essential for fast synaptic transmission, integrating chemical and electrical signals at the synapse. By allowing specific ions to flow through their channels, these receptors generate postsynaptic potentials that influence neuronal firing and circuit activity, underscoring their importance in neural communication and plasticity.

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