CHEMICAL TRANSMISSION = CONVERSION OF ELECTRIC SIGNAL TO A CHEMICAL SIGNAL

Neurons are interconnected in complex arrangements, and sometimes use electrical signals (electrical synapse) but mostly chemical signals (chemical synapse employing neurotransmitters) to transmit impulses from one neuron to the next.

Axon terminals are separated from neighboring neurons by a small gap called a synapse.

Different kind of synapses 

- The neuro-neuronal synapse : the synapse is situated between two neurons.

- The neuro-effector synapse : the synapse is situated between a pre-synaptic neuron and an effector cell (gland,musclu,etc).

- The sensory-neuronal synapse : the synapse is situated between a sensory cell and a post-synaptic neuron.

The chemical synapses

These are the type of synapses that we are going to study. The signal received by the axon terminations is electric so we are going to see how this signal is converted into a chemical signal.
  

From electric signal to neurotransmitter release

Firstly, we know that the pre-synaptic nerve-terminal synthesizes chemically messagers called neurotransmitters. These are often the size of an amino acid (examples dopamine, adrenaline, glutamate or acetylcholine) so small molecules. They are stored in vesicles near the membrane of the nerve-ending at the site of a synapse. Now that the neurotransmitters are created, they have to be released to be able to transmit their message. It is the arrival of the action potential which permits the release of the neurotransmitters by the creation of a depolarization at the axonal termination. Each action potential liberates a certain amount of neurotransmitter. The higher the frequency of the action potential, the more neurotransmitter will be released in time.

Indeed, the arriving action potential leads to the opening of the calcium channels. There is much more calcium outside the neuron than inside and the opening of the calcium channels leads to the entry of the calcium in the cell. The elevated binds proteins attached to the membrane of the synapse and causes the fusion of the vesicles membrane with pre-synaptic membrane, a process referred to as exocytosis. The liberated neurotransmitter diffuses across the synaptic cleft (space between two cells) one will then bind to specific proteins (receptors on the post-synpatic cell (see figure 3).

 A: Neuron (Presynaptic) 

 B: Neuron (Postsynaptic)

1. Mitochondria
2. Synaptic vesicle full of neurotransmitter
3. Autoreceptor
4. Synaptic cleft
5. Neurotransmitter receptor
6. Calcium Canal
7. Fused vesicle releasing neurotransmitter
8. Neurotransmitter re-uptake pump