EPSP

Where is the EPSP formed?

Generation of the potential occurs in the chemical synapse, which consists of several structural elements:

• Presynaptic membrane — characterized by the presence of lattice thickenings and hexagonal spaces between them.
• Exocytosis zone — the site of fusion of the synaptic vesicle with the presynaptic membrane, through which the neurotransmitter is released into the synaptic cleft.
• Synaptic cleft — the intercellular space through which neurotransmitter molecules pass via passive diffusion.
• Postsynaptic membrane — the receptive part of the synapse containing specific receptors. It is here that ionic currents cause a change in the membrane potential and the formation of an EPSP.
• Synaptic vesicles — store neurotransmitter molecules in the presynaptic terminal.

Stages and mechanism of generation

The process of synaptic transmission in an excitatory synapse involves a strict sequence of events:

1. Arrival of excitation: The action potential (AP) reaches the presynaptic ending.
2. Calcium influx: Calcium channels open, and $Ca^{2+}$ ions enter the terminal.
3. Exocytosis: Under the influence of $Ca^{2+}$, synaptic vesicles fuse with the presynaptic membrane, releasing portions (quanta) of the excitatory neurotransmitter into the synaptic cleft.
4. Reception: The neurotransmitter binds to protein-lipid receptors on the postsynaptic membrane.
5. Depolarization: Pores for sodium ($Na^+$) ions open. The influx of $Na^+$ (and in some cases $Ca^{2+}$) into the cell causes local depolarization — an EPSP arises.

The mechanism of excitation largely depends on the specifics of postsynaptic chemical reactions, which can extend all the way to the genetic apparatus of neuronal nuclei (gene expression).

Receptor types and transmission speed

The rate of EPSP development depends on the mechanism of action of the receptors involved:

• Ligand-gated (neurotransmitter-gated) ion channels — provide rapid action. The channel opens immediately upon binding to the ligand. Example: rapid EPSP upon the action of acetylcholine on nicotinic (N-) cholinergic receptors.
• G-protein-coupled receptors — act much slower, triggering a cascade of intracellular signaling. Example: slow EPSP in sympathetic ganglia, which can last for minutes.

A special mechanism is observed in NMDA receptors in glutamatergic synapses: they are under a magnesium block. The resulting membrane depolarization removes this block, making the receptors permeable.

Physiological role and effects

The main result of excitatory synapse function is the propagation of the signal further through the neural network. Local currents from EPSPs summate, and upon reaching the threshold level, trigger an action potential in adjacent areas of the membrane.

In addition, EPSP generation underlies crucial CNS functions:

• Short-term memory: Based on the circulation (reverberation) of excitation through a closed neural circuit, supported by EPSP generation in the circuit neurons.
• Aftereffect: The continuation of a reflex response after the cessation of the stimulus can be ensured by high-amplitude polysynaptic EPSPs.
• Electrical processes in ganglia: In the sympathetic ganglia of the autonomic nervous system, a characteristic shift of potentials is recorded: a rapid EPSP is replaced by a slow IPSP, followed by a prolonged slow EPSP.

Class and exam questions

What else to study on this topic

• Inhibitory postsynaptic potential (IPSP)
• Action potential (AP)
• Synaptic transmission
• Reverberation of excitation
• NMDA receptors

Other topics in "Physiology concepts"

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