The inhibitory postsynaptic potential is an inhibiting signal. It is formed by the postsynaptic termination of a synapse and leads to a hyperpolarization of the membrane potential. As a result, no new action potential is generated by this nerve cell and none is passed on.
What is the inhibitory postsynaptic 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 that are found in the human eye.
Synapses have a pre- and a postsynaptic ending. The presynaptic ending comes from the axon of the nerve cell and the postsynaptic ending is part of the dendrites of the neighboring nerve cell. The synaptic gap is created between the pre- and postsynaptic endings.
The presynaptic endings contain voltage-dependent ion channels that are permeable to calcium when they are open. Therefore these are also known 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 passed on to other cells via the synapses, an action potential is initially formed. This consists of different steps: The threshold potential of the membrane is exceeded. This also exceeds the resting potential of the membrane. This is how the depolarization follows. The electrical charge inside the cell increases. Hyperpolarization occurs before the membrane reaches the resting potential again through repolarization.
The hyperpolarization serves to ensure that no new action potential can be triggered in too short a time. The action potential is generated on the axon hill of the nerve cell and passed on via the axon to the synapses of the same cell. By releasing neurotransmitters, the signal is then transferred to another nerve cell. This signal can trigger a further action potential; it is then an excitatory postsynaptic potential (EPSP). This can also have an inhibitory effect; it is then referred to as inhibitory postsynaptic potential (IPSP).
Function & task
The calcium channels of the presynapic terminal are opened or closed depending on the membrane potential. Inside the presynaptic terminal there are vesicles that are filled with neurotransmitters. Receptor-activated ion channels are located 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 differentiated based on the neurotransmitter that they release when a signal is received. There are excitatory synapses, such as the 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 inhibiting amino acid neurotransmitters.
Another inhibiting neurotransmitter is glutamate. The membrane potential of the nerve cell is changed by a triggered action potential. Sodium and potassium channels are opened. Voltage-dependent calcium channels of the presynaptic terminal are also opened. Calcium ions reach the presynaptic terminal through the channels.
As a result, the vesicles fuse with the membrane of the presynaptic terminal and release the neurotransmitter into the synaptic gap. The neurotransmitter binds to the receptor of the postsynaptic terminal and the ion channels of the postsynaptic terminal are opened.
This changes the membrane potential at the postsynapse. If the membrane potential is reduced, an inhibitory postsynaptic potential occurs. The signal is then no longer forwarded. The main purpose of the IPSP is to control the transmission of stimuli so that there is no permanent excitation 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 release fewer transmitters to the downstream nerve cells than in the rest of the nervous system. This is converted into a light signal in the brain and enables humans and animals to see.
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If the inhibitory postsynaptic potential is disturbed, on the one hand a persistent IPSP can occur or the IPSP cannot be triggered. These disturbances can lead to incorrect transmission of signals between neurons, neurons and the muscles or between the eye and the nerve cells. It can happen that the signal cannot be forwarded as planned.
A disturbance of the inhibitory postsynaptic potential is associated with the disease of epilepsy. If there is a disruption of the inhibiting synapse, which triggers the inhibitory postsynaptic potential, this can lead to various diseases. Mutations in the receptors that bind the inhibiting neurotransmitter to the postsynaptic terminal lead to permanent excitation of the nerve cells. This also leads to epilepsy or hypereplexia. This disease describes the permanent excitation of the nerve cells.
The number of these receptors is also essential for the function of the inhibitory synapse. Mutations in the genome that result in too few of these receptors being produced by the body can lead to a disorder in the nervous system. The muscles malfunction. It has already been established in the mouse model that certain mutations of this type can lead to premature death, since the respiratory muscles can no longer be properly regulated by the nervous system.
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