How is IPSP formed? (Stages of transmission)
The process of inhibitory signal transmission at the synapse includes the following sequential stages:
- Arrival of excitation: The action potential (AP) reaches the presynaptic ending.
- $Ca^{2+}$ influx: The permeability of the presynaptic membrane to calcium ions increases, and they enter the terminal.
- Exocytosis: Under the influence of $Ca^{2+}$, vesicles containing the inhibitory neurotransmitter move toward the presynaptic membrane, after which the neurotransmitter is released into the synaptic cleft.
- Receptor interaction: The neurotransmitter binds to receptors on the postsynaptic membrane of the target neuron.
- Ionic shift: Ion channels open, increasing membrane permeability to chloride ($Cl^-$) and potassium ($K^+$) ions. Chloride enters the cell along its concentration gradient, while potassium leaves it.
- IPSP generation: As a result of ion movement, hyperpolarization of the membrane occurs, which constitutes the inhibitory postsynaptic potential.
Which neurotransmitters cause IPSP?
Depending on the region of the nervous system, IPSP is formed under the action of various chemical agents:
- In the central nervous system (CNS): The main inhibitory neurotransmitters are GABA and glycine.
- In the autonomic nervous system: Acetylcholine and norepinephrine act in inhibitory synapses.
- In sympathetic ganglia (slow IPSP): Mediated by the action of dopamine. Dopamine is secreted by small interneurons of the ganglion (SIF cells), which are activated via muscarinic cholinergic receptors (M-cholinergic receptors). It is hypothesized that the slow IPSP itself is mediated by $\beta_2$-receptors, as it is cAMP-independent.
What is the physiological effect of IPSP?
IPSP causes inhibition of excitation. The occurrence of postsynaptic inhibition requires the obligatory participation of an inhibitory interneuron.
- Change in excitability: IPSP prevents reaching the depolarization threshold. The stimulation threshold of the main neuron increases, and its excitability drops.
- Signal blocking: Unlike an excitatory synapse, where EPSP summation triggers an action potential and the signal propagates further, during IPSP development, the generation of an action potential becomes impossible. Conduction of excitation is blocked.
Role of IPSP in physiological processes
- Termination of short-term memory: The basis of short-term memory is the circulation (reverberation) of excitation through a closed neural loop, which is maintained by excitatory potentials (EPSP). This process stops when an IPSP develops in one of the neurons of the chain under the influence of an inhibitory neuron.
- Generation of electrical potentials in ganglia: Upon stimulation of preganglionic fibers of the sympathetic nervous system, IPSP is part of a sequential shift in potentials. First, a fast EPSP is recorded, then a slow IPSP appears, followed by a slow EPSP (which can last for minutes).
Class and exam questions
Which ions are involved in IPSP formation?
Inhibitory neurotransmitters increase the permeability of the postsynaptic membrane to potassium ($K^+$) and chloride ($Cl^-$) ions. Potassium leaves the cell, and chloride enters it along its concentration gradient, causing hyperpolarization.
What is the difference between the outcomes of EPSP and IPSP?
EPSP summation leads to reaching the threshold level of depolarization, generation of an action potential, and further conduction of excitation. IPSP, in contrast, causes hyperpolarization, increases the excitation threshold, and blocks the conduction of excitation.
How is slow IPSP generated in the autonomic nervous system?
It arises in sympathetic ganglia under the action of dopamine. Dopamine is released by small interneurons (SIF cells) that are preliminarily activated via muscarinic cholinergic receptors.
What condition is mandatory for the occurrence of postsynaptic inhibition in the CNS?
The development of postsynaptic inhibition and the formation of IPSP on the membrane of the target neuron require the obligatory participation of an inhibitory interneuron.
What else to study on this topic
- Excitatory postsynaptic potential (EPSP)
- Mechanisms of synaptic transmission
- Postsynaptic inhibition in the CNS
- Reverberation of excitation and memory
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