Synaptic Update

Wow, what a fascinating take on the synapse!  Compare these two images of the active zone of a synapse.  One (Figure 2 below) is easily recognized as a typical textbook rendition of a synapse, but the other not (Figure 1).  The image that looks like hallucinatory spaghetti comes from a relatively recent paper and depicts our current understanding the appearance of the active zone of a synapse.  Astonishing!!

Image of active synaptic zone
Figure 1. Zooming in on the active zone of a synaptic bouton. The redish area at the bottom of the image is the active zone. Colours code different proteins (see Figure 4).
A classis textbook redition of a synapse.
Figure 2. A classis textbook redition of a synapse. (Young, KA., Wise, JA., DeSaix, P., Kruse, DH., Poe, B., Johnson, E., Johnson, JE., Korol, O., Betts, JG., & Womble, M.)

Published in 2014 by Wilhelm et al in Science, I don’t know why it took me so long to encounter this paper 😊.  Their research is a revelation in terms of the physical structure and complexity of the synapse.

Structure of the Synapse

An average synaptosome (synaptic bouton) is made up of about 300,000 proteins, far more than we ever thought.  The bouton only has a volume of about 0.37 µm3; it’s miniscule, yet packed into that space are about 350-400 tiny vesicles that contain neurotransmitters, waiting to be released and to play their role in the persistence of consciousness and mental life.  Estimates of the number of synapses in the human brain vary considerably, ranging between about 100 trillion to around 1,000 trillion.  There are more synapses in your brain than stars in the Milky Way.  Without synapses, you wouldn’t be reading this.  In fact, you wouldn’t exist.

Image of a synaptosome
Figure 3. A synaptosome, or synaptic bouton. The small circles are vesicles.  The red rectangle at the bottom is enlarged in Figure 1.  Dark red are microtubules that transport neurotransmitters and nutrients from the cell body to the bouton.

Synaptic Proteins

The researchers identified about 65 major proteins that play a major role in the functioning of the synapse, as illustrated in Figure 4.  The colours in the Figure 4 map onto the colours used in Figures 1 and 3.

An image of the major proteins involved in synaptic function
Figure 4. Major proteins involved in synaptic function.

This research comes from the Rizzoli Lab, headed up by a genius in neurobiology, Prof Silvio O. Rizzoli from the Department of Neuro- and Sensory Physiology at the University of Göttingen Medical Center.  Have a look at his movie of a 3D reconstruction of the contents of a typical synaptosome available on the Rizzoli Lab website.

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