Electrical information is conducted through the body along neurons. But neurons are not infinitely long (although some can be pretty long and measure above 1 m in length) and they “synapse” on each other.
The problem is that at the point of synaptic junction there is a “gap” between the first neuron and the subsequent one. This gap is called the synaptic cleft (it’s 10nm~20nm large).
5 Steps to Chemical Synaptic Transmission
- The electrical signal cannot “jump” over that gap. Instead, at the point of synaptic junction between two neurons, the electrical signal is “translated” into a chemical message (the neurotransmitter) by the presynaptic neuron (at the presynaptic terminal) – Step 1 & 2.
- That chemical diffuses (“swims”) across the synaptic cleft until it reaches the other neuron – Step 2 & 3.
- The other neuron then “translates” the chemical signal back into an electrical one – Step 3 & 4.
- The chemical message is degraded – Step 5.
This “new” electrical message can now travel down the neuron until it reaches a new synaptic junction.
Let’s look at these 5 steps in more detail.
Step 1 – Neurotransmitter Synthesis
There are two general classes of neurotransmitters: large neuropeptides or smaller amines/amino acids.
The large peptides are synthesized in the cell body of the neuron and are transported to the synaptic terminal through the axon.
The smaller amines/amino acids can generally be synthesized at the presynaptic terminal itself.
Step 2 – Neurotransmitter Packaging
Once the neurotransmitters are synthesized, they need to be put into “small groups” ready to be “launched” across the synaptic cleft.
In neurobiological terms, we say that the neurotransmitters need to be packaged into vesicles.
Here is what happens:
The small groups of neurotransmitters are released into the synaptic cleft when they receive an order from Ca 2+ ions to do so.
When the electrical signal reaches the presynaptic terminal, it opens some channels in the membrane (these are called voltage gated Ca 2+ channels). Once these channels are open, calcium ions from the surrounding extracellular environment rush into the presynaptic terminal.
As the calcium ions encounter the vesicles, the membrane of the vesicles fuse with the membrane of the presynaptic terminal, right at the synaptic cleft.
As the vesicles fuse with the membrane, the neurotransmitters are “expelled” into the synaptic cleft.
Step 4 – Neurotransmitter Binding
The neurotransmitters can now “swim” (diffuse) through the synaptic cleft, until they reach the postsynaptic neuron.
The membrane of the postsynaptic neuron contains a few channels (receptors) that control how a neurotransmitter can be translated into an electrical signal.
Step 5 – Stopping the Chemical Signal
Once the chemical / neurotransmitter signal has been translated into an electrical signal, the postsynaptic receptors need to be “cleared” very quickly so that they can receive new transmitters from new signals (otherwise you’d end up with neurotransmitter traffic worse than any traffic you’ve ever experienced on the freeway!).
Some neurotransmitters will be degraded, some will be transported back to the presynaptic terminal to be recycled, and sometimes they are “absorbed” by the postsynaptic terminal…
Main To Remember
- In order to “jump” across two neurons, the electrical signal needs to be converted into a chemical one then back into an electrical one.
- Synaptic transmission is a 5 step process.
- Synthesis of the chemical message – Neurotransmitter synthesis
- Grouping of the chemical message – Neurotransmitter packaging
- Release to the neurotransmitter
- Translating the chemical message back into an electrical one at the postsynaptic membrane
- “Clearing” of the postsynaptic receptors.
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