Summary
Synapse activity and related ionic mechanisms: Calcium ions flow into the presynaptique zone, through voltage-dependent channels, inducing exocytosis. Sodium ions flow into the postsynaptic neuron though receptors specific for a given neurotransmitter, inducing membrane depolarization.Click on
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Learning goals
- To understand nerve signal transmission at the level of an excitatory synapse.
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Transmission of impulses between neurons occurs at their point of junction, called the synapse. It is estimated that about 40% of the total surface of a neuron is composed of synapses. These have three main zones: the presynaptic and postsynaptic zones, and, between these, the intersynaptic gap (or intersynaptic cleft). The message is transmitted between neurons by chemical messengers known as neurotransmitters. When it reaches the end of an axon, the nerve impulse causes the release of neurotransmitters, via exocytosis, into the intersynaptic gap. These attach to the postsynaptic neuron, assuring transmission of the impulse.This transmission takes place in several stages:
- The arrival of the electrical impulse at the end of a neuron causes the depolarization of the plasma membrane in the presynaptic zone….the polarity changes bit by bit from negative to positive.
- This provokes the entry, via voltage dependent channels, of Ca2+ ions into the presynaptic zone.
- The increase in Ca2+ concentration initiates the release of neurotransmitters from synaptic vesicles into extracellular spaces via exocytosis.
- The neurotransmitters cross the synaptic gap, rich in Na+ and Cl-, and become attached to the postsynaptic receptors.
- The attachment of the neurotransmitters triggers the opening of ion channels, which provokes the entry of Na+ ions, and so the depolarization of the plasma membrane of the postsynaptic cell.
- If this depolarization attains the threshold value for initiating an action potential, the nerve impulse is transmitted onward.
- After having acted, neurotransmitters are reabsorbed by the presynaptic cell, thus ensuring a permanent stock of these substances there.
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