Dating back to the drawings of Ramon y Cajal, neuroscientists have always been fascinated with the striking shapes of neurons. Today, scientists still rely heavily on microscopy—albeit with instruments far more advanced than those in 1900—in hopes of understanding how neurons work by visualizing these fantastic cells. As new imaging techniques and approaches develop, we are able to learn more and more about neurons and the brain as a whole.
The beautiful shape of a neuron is intrinsically linked to its function. If the branched dendrite of one neuron contacts the long slim axon of another neuron, a connection (synapse) may form between them. When a neuron is stimulated, little packets of neurotransmitters called vesicles are released at the synapse. Then, the neurotransmitters are taken up by the second neuron, affecting its own activity.
This process occurs continuously in the billions of neurons and trillions of synapses that make up the human brain. With these astounding numbers, how can scientists zoom in and understand how specific neurons function, let alone specific synapses?
A new study published in Nature used a trio of impressive techniques to help answer this question. By optimizing a combination of light microscopy, electrophysiology, and electron microscopy, the researchers were able to measure the activity and image the ultra-structure of synapses in a mouse brain.
They observed numerous instances of axons and dendrites from different neurons touching, suggesting synapses were at these contact sites. However, when they took ultra-structure images of these neurons by electron microscopy, they noted synapses only formed at a percentage of the contact sites.
In examining the synapses that did form, the researchers made two novel observations. First, they showed that the size of synapse correlated with the electrical activity of the neuron. They also determined numerous vesicles could be released at the synapse simultaneously, contradicting the previous belief that only one vesicle was released at a time.
This combination of various methodologies to study the same synapse opens the door for future work to further probe the relationship between neuronal structure and activity, ultimately helping the scientific community better understand how synapses form and are regulated.