4. Conversion of signals

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Conversion of electrical signals into chemical signals

When an action potential reaches nerve terminals, the signal must be relayed to target cells. This transmission occurs at synapses (the gap between two neurons).

 For the message to be transmitted from one neuron to another (also known as from the pre-synaptic neuron to the post-synaptic neuron), the electrical signal is converted into a chemical signal in the form of a small signalling molecule known as a neurotransmitter. 

 Neurotransmitters are stored in nerve terminals in membrane bounded synaptic vesicles, and are released by exocytosis from the nerve ending when an action potential reaches the terminal.

 Depolarization of the nerve terminal plasma membrane opens voltage gated calcium channels. Since the concentration of Ca2+ ions outside the cell is greater than that in the cytosol, Ca2+ ions rush into the nerve terminal through the open channels.

 The increase in the concentration of Ca2+ ions triggers the fusion of the synaptic vesicles with the plasma membrane, releasing the neurotransmitter into the synaptic cleft. 

 The electrical signal is therefore converted into a chemical signal by the voltage gated calcium channels.

Conversion of chemical signal back into electrical signal in target cells 


The released neurotransmitter rapidly diffuses across the synaptic cleft and binds to receptors concentrated in the post synaptic membrane on the target cells. 

The chemical binding action alters the shape of the receptors, initiating a series of reactions that open channel-shaped protein molecules. Electrically charged ions then flow through the channels into or out of the neuron. This sudden shift of electric charge across the postsynaptic membrane changes the electric polarization of the membrane, producing the postsynaptic potential.

 The neurotransmitter is removed by rapid breakdown by enzymes in synaptic cleft, or re-uptake either into the nerve terminals that released it or into neighboring cells.






Excitatory and inhibitory input

Response produced by a neurotransmitter at a synapse can be either excitatory or inhibitory. The former one causes the post synaptic cell to fire action potentials whereas the latter prevents it from doing so.

Chief receptors for excitatory neurotransmitters (e.g. acetylcholine, glutamate) are ion channels which allow the passage of Na+ and Ca2+ ions. 
When the neurotransmitter binds to the channel, it causes it to open and hence resulting in an influx of Na+ and Ca2+ ions. The influx of Na+ ions depolarizes the plasma membrane towards the threshold potential required to trigger and action potential. Stimulation of these receptors therefore tends to activate the post synaptic cell.

The receptors for inhibitory neurotransmitters (e.g. gamma-aminobutyric acid, glycine) are chlorine ion channels.
When these channels are open, very little Cl− ions enter the cell. This is because the driving force for movement of Cl− ions across the membrane is close to zero at the resting potential. However, if sodium channels are open, Na+ will rush in thus causing the membrane potential to shift away from its resting value. This causes Cl− ions to move into the cell, neutralizing the effect of the Na+ influx. These neurotransmitters therefore suppress the production of an action potential by making the target cell membrane harder to depolarize.



References

The Editors of Encyclopaedia Britannica (n.d.) Synapse | anatomy. [Online]. Encyclopedia Britannica. Available from: https://www.britannica.com/science/synapse [Accessed: 11 April 2020].

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