Illustration of neural exo and endo cytosis. Credit: U of Utah
Many people, including neuroscientists, refer to the patterns of neurons in the brain and elsewhere in the body as “wiring.” It’s a metaphor, which makes it seem almost axiomatic that our nervous system operates on electricity and is akin to the electrical systems of, say, a house or a computer. Actually, for all but a small percentage of neurons in the human body, ‘wiring’ is not a good metaphor. Wiring, in the usual understanding, implies a flow of electrons through a wire, usually metallic such as copper or fiber-optic. “Electron flow” is hardly the best descriptor for the way axon bodies (the long form of the neuron) transmit electricity – and the synapses (the gaps between neurons); they’re something else again.
In most neurons, the axons transmit signals via “action potentials,” which involves channeling ions of sodium and potassium in a complex “impulse” consisting of passing ionic charges along the axon. This does not sound like house wiring…it works more like a burning fuse. For another thing, it’s a lot slower. Electron flows in copper wire can travel at a reasonable approximation of light speed. Action potentials in a neuron vary, but an average speed is about 10 meters per second, a lame tortoise’s view of the speed of light. Then the action potential inevitably comes to the end of an axon and encounters an actual physical gap – the synaptic cleft.
At the synapses, in order to cross the gap between neurons, transmission is mainly by chemical reaction. Of course, it’s an electrochemical process and electrical charge is involved, but its relationship to electron flow or even ion exchange is distant. The principal actors are neurotransmitters, specialized chemicals such as norepinephrine and dynorphin. The method of transmission involving neurotransmitters is, not to put too fine a point on it, elaborate. Only recently have scientists begun to understand how it works.
When two neurons meet at a synapse, the one carrying the action potential (nerve impulse) generates the appropriate neurotransmitter chemicals (this is a variable and highly complex process). The neurotransmitters package in ‘bubbles’ – vesicles – and are sent to the neuron’s outer membrane at the synapse. There, the vesicles fuse with the membrane and dump their load of neurotransmitters into the synaptic cleft. The neurotransmitters quickly reach the outer membrane of the receiving axon, which is loaded with neurotransmitter receptors, chemical points that attract specific molecules of the neurotransmitters. The pattern and strength of the electrochemical charge generated in the receptors activates a new action potential in the receiving axon, and the impulse or ‘message’ continues.
Many decades of research went into building the above rough description of what happens at the synapses, and the work continues. This includes the 2013 Nobel Prize in Physiology or Medicine for work by Rothman, Schekman and Südhof on the machinery regulating vesicle traffic. As you probably noticed, vesicles are a key factor in the functioning of the nervous system. The questions involving how, why and when vesicles generate, fill with the appropriate neurotransmitters, transport to the terminal membrane and dump into the synapse have generated some truly amazing biochemical explanations. Many questions remain. For example, it’s obvious that in some way the vesicles are recycled, the question is how that works and how does it affect the speed of neuron transmission.
To find answers, two researchers, Erik Jorgensen and Shigeki Watanabe at the University of Utah (Salt Lake City, USA) and a team of neuroscience researchers at the Charity University of Medicine (Berlin, Germany) starting digging into the process known as endocytosis, the recycling of vesicles at the nerve ends.
The current state of knowledge identifies three mechanisms for vesicle recycling, which Jorgensen illustrates with a machine gun analogy:
1. Clathrin mediated – the clathrin coating of the vesicles disintegrates after the vesicle deposits its neurotransmitters into the synaptic cleft, and the vesicle material is re-used from scratch to make new vesicles. This is like making rounds of new bullets to rapidly feed a machine gun.
2. Kiss and run – re-uses existing, sometimes partially filled vesicles. This is like refilling used shells one at a time.
3. Ultrafast – “grabs” (chemically) a batch of vesicles at one time and refills them, something like an endless conveyer belt of bullets for a machine gun.
To-date each of these mechanisms has its proponents and detractors. What Jorgensen and Watanabe wanted to do is develop some evidence behind the mechanism. For this, they had to invent new investigation techniques – in this case photographic.
They started with growing hundreds of brain cells (neurons) from the hippocampus of mice. It’s an area associated with memory formation, where neurotransmission and synaptic integration are most likely to be optimized. The researchers grew the neurons on sapphire disks one-quarter inch wide and placed them in a petri dish with a growth medium.
As the neurons were grown, the researchers inserted a gene taken from algae that forced the mouse brain cells to produce an “ion channel” that would switch on the neurotransmitter process with a light signal (from a laser) and not from an electrical impulse. They did this because the next step in the technique involved super-freezing the cells, and an electrical wire could not be used for stimulus. The cooling was done in a high-pressure chamber – set at 310 degrees below zero Fahrenheit and 2000 times Earth barometric pressure.
In this chamber, the researchers flashed a blue laser light, making them “fire” neurotransmitter nerve signals. Each firing was frozen by injecting a blast of liquid nitrogen. This was repeated for various time intervals (15, 30,100 milliseconds and 1, 3 and 10 seconds). Watanabe called it the “flash and freeze” technique. The sapphire disks containing the frozen neurons were put into liquid epoxy, hardened and thin sliced to be photographed under an electron microscope. Roughly, 3,000 mouse neuron synapses were photographed this way. About 20% of them were firing at the time, which provided a basis for examining the behavior of the vesicles.
In the images, it was clear that large numbers of vesicles were in different stages of formation, a continuous cycle of refilling batches and sending them to the neuron’s terminal membrane for transmission. In short, they were looking at the ultrafast mechanism, which they believe is the most common (and efficient) method for recycling vesicles – and an explanation for some the of high speeds (milliseconds) observed for synaptic transmission.
The recycling of vesicles is but one step in a relatively long chain of steps involved in “synaptic transmission” (perhaps better named as “synaptic integration,” because transmission isn’t always the result). It’s representative of the work scientists are doing to pick apart this absolutely crucial process (our brains, or indeed, our entire nervous system wouldn’t work without it). Almost every component, whether it’s the chemistry of neurotransmitters, the vesicle formation process, neurotransmitter receptors, the role of astrocytes and other glia in neurotransmitter control (and more), is the subject of intense research.
From this new research, neurotransmission is turning out to be dauntingly complex. The big question is why, why is it so complex? It’s a question thus far eliciting guesses and partial evaluations. Why is synaptic integration so complicated? What is actually happening at the synapses (and there may be multiple answers), and how does it affect the processes of the body? For example, some researchers believe that the complex nature of chemical neurotransmission plays an important role in memory. They just don’t know exactly how it works, or if it applies to all neurons or just specialized neurons (or groups of neurons in special locations).
Part of the reason so many important questions remain is that neuroscience is still very much into the process of explaining “what is” – discovering and verifying an accurate description of what is involved with synaptic integration and how it works. Until there are some substantial and substantiated models, it’s premature to make anything but educated guesses about the “why” question. Not that scientists won’t speculate, but most of them are reluctant to put much weight on the speculation. They know that neuroscience has a long way to go before it can not only describe the functioning of neurons (especially at the synapses) and from that explain how basic neurological processes, such as memory and consciousness, take place.