Mind the gap

The human brain is incredibly complex and contains billions of neurons. These neurons need to communicate with each other to allow the brain to carry out its many functions. In this video, we will see how signals are transmitted from one neuron to another at a chemical synapse. Let’s first look at how information flows within a neuron. Here is a neuron, this is Brian. Brian’s hair represents processes called dendrites - these receive signals from other cells. If there is enough stimulation of Brian’s hair, the cell body will generate an action potential, which travels down the axon (Brian’s body) and the output is at the synaptic terminals at Brian’s feet.

This image shows some beautiful neurons in the cortex. It allows you to appreciate how the billions of neurons in the brain are connected in complex networks and circuits. Now let’s take a look at the events that occur when a signal is transmitted from one of these neurons to another. The flow of information within neurons I described is in the form of electrical signals, but at synapses, these electrical signals are converted into chemical signals. Synapse means ‘gap’ in Greek, and it’s not a big gap, the synaptic cleft is just 50 nm wide.

The chemical messengers that transport the signal across the synaptic cleft are neurotransmitter molecules. In the axon terminal, the neurotransmitters are packaged into synaptic vesicles. In my lab, we have taken some electron microscopy images of synapses. In this image, you can see lots of synaptic vesicles, which are like membrane-coated footballs. Each synapse is 1 µm across and synaptic vesicles are 40 nm in diameter, which is similar to the width of the synaptic cleft. A synapse contains 100-200 synaptic vesicles. So how does a synaptic vesicle deliver its neurotransmitter across the synapse?

Some synaptic vesicles are positioned near the presynaptic membrane awaiting a signal When an electrical signal arrives, the membrane depolarisation leads to the opening of voltage-gated calcium channels in the presynaptic membrane. An influx of calcium ions into the cytoplasm of the presynapse causes the vesicle membrane to fuse with the plasma membrane and release the neurotransmitter into the synaptic cleft. The vesicle membrane is now continuous with the presynaptic membrane and has to be recycled so that the vesicle can be refilled with neurotransmitter molecules. Now that we have looked at the events at the presynapse, let’s see what happens to the neurotransmitter as it diffuses across the synaptic cleft. The neurotransmitter binds specific receptors at the postsynaptic membrane.

The receptors then open and allow ion influx, causing a potential that, when summed with potentials from other synapses can cause the postsynaptic neuron to fire an action potential. So now we have a better understanding of how signals are transmitted from one neuron to another. But to give some context, each of our 86 billion neurons has many thousands of synaptic connections with other neurons, and this is what gives the brain its computational power.