The inhibitory postsynaptic potential is an inhibitory signal. It is formed by the postsynaptic terminal of a synapse and leads to hyperpolarization of the membrane potential. As a result, no new action potential is generated by that neuron and none is transmitted.
What is the inhibitory postsynaptic potential?
The inhibitory postsynaptic potential is an inhibitory signal. It is formed by the postsynaptic terminal of a synapse and results in hyperpolarization of the membrane potential. Synapses represent the connections between different nerve cells or between nerve cells and the muscles or those cells that enable vision. These are the so-called cone and rod cells, which are found in the human eye. Synapses have a presynaptic and a postsynaptic termination. The presynaptic termination originates from the axon of the nerve cell and the postsynaptic termination is part of the dendrites of the neighboring nerve cell. The synaptic cleft is formed between the presynaptic and postsynaptic terminals. The presynaptic terminals contain voltage-gated ion channels that are permeable to calcium when they are open. Therefore, these are also referred to as calcium channels. Whether these channels are closed or open depends on the state of the membrane potential. If a nerve cell is excited and forms a signal that is to be transmitted to other cells via the synapses, an action potential is first formed. This consists of several steps: The threshold potential of the membrane is exceeded. Thus, the resting potential of the membrane is also exceeded. This is followed by depolarization. The electric charge inside the cell increases. Hyperpolarization occurs before the membrane returns to the resting potential through repolarization. The hyperpolarization serves to prevent another action potential from being triggered in too short a time. The action potential is formed at the axon hillock of the nerve cell and transmitted via the axon to the synapses of the same cell. The signal is then transmitted to another nerve cell by the release of neurotransmitters. This signal can trigger another action potential, which is then an excitatory postsynaptic potential (EPSP). This can also have an inhibitory effect, it is then called an inhibitory postsynaptic potential (IPSP).
Function and task
The calcium channels of the presynaptic terminal are open or closed depending on the membrane potential. Within the presynaptic terminal are vesicles filled with neurotransmitters. Receptor-activated ion channels are localized at the postsynaptic terminal. The binding of the ligand, in this case the neurotransmitter, regulates the opening and closing of the channel. There are different types of synapses. These are distinguished on the basis of the neurotransmitter they release in response to a signal. There are excitatory synapses, such as chonlinergic synapses. There are also synapses that release inhibitory neurotransmitters. These neurotransmitters include gamma aminobutyric acid (GABA) or glycine, taurine and beta alanine. These belong to the group of inhibitory amino acid neurotransmitters. Another inhibitory neurotransmitter is glutamate. A triggered action potential alters the membrane potential of the nerve cell. Sodium and potassium channels are opened. Voltage-dependent calcium channels of the presynaptic terminal are also opened. Calcium ions pass through the channels into the presynaptic terminal. This results in the vesicles fusing with the membrane of the presynaptic terminal and releasing the neurotransmitter into the synaptic cleft. The neurotransmitter binds to the postsynaptic terminal receptor and the ion channels of the postsynaptic terminal are opened. This changes the membrane potential at the postsynapse. If the membrane potential is decreased, an inhibitory postsynaptic potential occurs. The signal is then no longer transmitted. The IPSP serves primarily to control stimulus transmission so that no permanent excitation occurs in the nervous system. It also plays an important role in the visual process. Certain cells in the retina, the rods, generate an inhibitory postsynaptic potential when exposed to light.This measures the degree to which these cells send out less transmitter to the downstream nerve cells than in the rest of the nervous system. This is converted in the brain as a light signal and thus enables humans and animals to see.
Diseases and ailments
When the inhibitory postsynaptic potential is disturbed, on the one hand, IPSP may persist or IPSP may not be triggered. These disturbances can lead to misrouting of signals between neurons, neurons and the musculature, or the eye and neurons. It may happen that the signal cannot be transmitted as planned. A disturbance of the inhibitory postsynaptic potential is associated with the disease of epilepsy. If there is a disruption of the inhibitory synapse that triggers the inhibitory postsynaptic potential, this can lead to various diseases. Mutations in the receptors that bind the inhibitory neurotransmitter at the postsynaptic terminal lead to permanent excitation of the neurons. This also leads to epilepsy or hyperekplexia. This disorder describes the permanent excitation of the nerve cells. The number of these receptors is also essential for the function of the inhibitory synapse. In the case of mutations in the genome that result in too few of these receptors being produced by the body, a disorder occurs in the nervous system. Muscle dysfunction occurs. In mouse models, it has already been found that certain mutations of this type can lead to premature death because the respiratory muscles can no longer be properly regulated by the nervous system.
