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1.4. Synapses
Introducing the Synapse
When the action potential arrives at the presynaptic terminal, it needs to communicate with another neuron (or, in some cases, muscles or glands). However, in most neurons the action potential doesn't jump electrically from one neuron to the other. Instead, these synapses are chemical. In summary, this means that the electrical action potential is converted into a chemical messenger, sent across a short gap to the next neuron, and then reconverted back into a second electrical action potential. This process of communication is vital throughout the body, including in the spinal cord and brain, and when it breaks down, medical problems, such as depression (see Section 4.2. Depression) can occur. Check out the explanations and accompanying diagrams below.
The Five Stages of Synaptic Transmission
Stage 1
An action potential travels along the presynaptic neuron. It eventually reaches the presynaptic terminal (also called presynaptic knob). The presynaptic terminal, like the rest of the neuron, is depolarised.
Stage 2
Depolarisation of the neuron stimulates voltage-gated calcium ion (Ca2+) channels in the presynaptic terminal to open. As a result, Ca2+ ions flood into the presynaptic terminal. In close proximity to the presynaptic membrane lie vesicles which contain neurotransmitter molecules. The Ca2+ ions stimulate the vesicles to migrate towards the presynaptic membrane and fuse with it.
Stage 3
The neurotransmitter molecules are released into the synaptic cleft – this is simply the space between the two neurons. The release of neurotransmitter molecules is called 'exocytosis'. The neurotransmitter molecules that have just been released are initially present in a high concentration. So, they diffuse across the synaptic cleft towards the postsynaptic membrane to an area of low concentration.
Stage 4
The neurotransmitter molecules travel towards the postsynaptic membrane. There are receptors in the postsynaptic membrane that have a complementary shape to the neurotransmitter molecules, and so the neurotransmitter molecules bind to the receptor sites perfectly. This binding stimulates the opening of the Na+ ion channels in the postsynaptic membrane. Thus, Na+ ions rush into the postsynaptic membrane and…
Stage 5
…this Na+ influx generates an action potential in the postsynaptic neuron that continues along the entire length of the postsynaptic neuron. At this stage, the neurotransmitter molecules will be recycled and used again in the presynaptic terminal.
An action potential travels along the presynaptic neuron. It eventually reaches the presynaptic terminal (also called presynaptic knob). The presynaptic terminal, like the rest of the neuron, is depolarised.
Stage 2
Depolarisation of the neuron stimulates voltage-gated calcium ion (Ca2+) channels in the presynaptic terminal to open. As a result, Ca2+ ions flood into the presynaptic terminal. In close proximity to the presynaptic membrane lie vesicles which contain neurotransmitter molecules. The Ca2+ ions stimulate the vesicles to migrate towards the presynaptic membrane and fuse with it.
Stage 3
The neurotransmitter molecules are released into the synaptic cleft – this is simply the space between the two neurons. The release of neurotransmitter molecules is called 'exocytosis'. The neurotransmitter molecules that have just been released are initially present in a high concentration. So, they diffuse across the synaptic cleft towards the postsynaptic membrane to an area of low concentration.
Stage 4
The neurotransmitter molecules travel towards the postsynaptic membrane. There are receptors in the postsynaptic membrane that have a complementary shape to the neurotransmitter molecules, and so the neurotransmitter molecules bind to the receptor sites perfectly. This binding stimulates the opening of the Na+ ion channels in the postsynaptic membrane. Thus, Na+ ions rush into the postsynaptic membrane and…
Stage 5
…this Na+ influx generates an action potential in the postsynaptic neuron that continues along the entire length of the postsynaptic neuron. At this stage, the neurotransmitter molecules will be recycled and used again in the presynaptic terminal.
More Info
It is worth noting that there are a few electrical synapses (not chemical), an example being found between the secretory neurons in the hypothalamus (see Section 2.1. Overview of the Brain). These electrical synapses are essentially gap junctions, and they transmit action potentials at a faster rate compared to the chemical synapses described above. The distance between the neurons connected by electrical synapses is much smaller too, meaning a faster transmission speed. It should also be noted that some neurotransmitters are not 'excitatory' but instead they are 'inhibitory'. When these inhibitory neurotransmitters bind to their receptors in the postsynaptic membrane, they stimulate the opening of K+ or Cl- ion channels. When K+ or Cl- ions flood into the postsynaptic neuron, they induce a state of hyperpolarisation and no action potential is generated.
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