Can nanoscopy play an important role in brain sciences comparable to microscopy, as was outlined in our brief history of optical methods in the neurosciences? One central question of neuroscience today is just how memories and experiences can be stored in the brain. In what form are they preserved – chemical deposits, electrical patterns, synaptic networks or morphological changes? This question becomes even
1.4. Nanoscopic details in the brain
more complex when considering that there is not just one but several different types of memories. We can distinguish between short- and long-term memory, declarative memory (recollection of events and facts), procedural memory (commonly learned tasks, muscle-memory, primed responses) autobiographic memory and many other subcategories. How are they processed, where and when are they stored, when are they forgotten? In current opinion, short-term memories are saved in the form of chemical and electrical signals passed from synapse to synapse. At some point they are either forgotten or else processed and stored elsewhere in the brain, probably in the form of synaptic plasticity. That means in the creation of new synaptic junctions between neurons or in the modification of the strength of existing synapses. How can the strength of an individual synaptic connection be altered? For this, we need to know what a typical synapse looks like. Most excitatory synapsesXIIIare located on dendritic spines, which are small protrusions on the neuronal dendrites. Most dendritic spines consist of a bulbous head connected to the parent dendrite through a slender spine neck, but the proportions of spine head and neck vary considerably from spine to spine. A lasting synapse can be formed when a dendritic spine contacts a varicosity (or: bouton) on a nearby axon. At such a synapse, an electrical signal arriving from the axon can be relayed via a chemical signal to the dendritic spine, which can itself create an electrical signal in the adjoining dendrite. The axonal bouton is said to form thepresynapticpart of the connection and dendritic spines the postsynapticpart. So if synapses are junctions that transmit electric and chemical signals, then how can their precise functionality be modified? There are many different pathways by which this might be accomplished, for example by increasing the number of proteins or synaptic vesicles at the synaptic active zone, increasing the surface contact area of the synapse or modifying the overall size and geometry of the dendritic spine.
While there is general agreement that a number of different factors contribute to changes in synaptic plasticity, it has become quite clear that morphological changes of nanoscopic structures especially in dendritic spines play an important role, too.
However, these changes are very difficult to observe for numerous reasons. Dendritic spines are quite small; they typically measure between0.2µm to2µm in length and
XIIIActivated excitatory synapses make it more likely that a neuron will produce an action potential, whereas activated inhibitory synapses do the opposite.
can measure ≤40nm at their thinnest spot. Because of the small dimensions of dendritic spines, even subtle changes can potentially lead to functional changes.
To observe such subtleties we require good spatial resolution, since distorted or blurry images reveal few details. A good time resolution would also be beneficial if we want to observe the changes as they happen and if we are interested in whether they are long-lasting functional changes or merely temporary fluctuations, and whether they occur as response to certain stimuli of situations. Changes can occur over seconds, minutes or hours and, of course, can last for a few minutes (the phone number you are trying to remember) or an entire lifetime (your parents’
voices). This requires the capacity to observe fast changes as well as to monitor these changes continuously over hours or repeatedly over days, months or years without causing damage to the brain. If we intend to observe memory(-like) processes in action, then there is no real point in observing isolated neuronsin vitro.
Instead we should examine neurons that are still integrated in a functional neural network and can emit and receive signals from other brain cells. This requires that neurons must be embedded in neural tissue and surrounded by other functional neurons. Unfortunately, neural tissue is a fairly dense, inhomogeneous material that causes light scattering and beam aberrations, thereby distorting any signal passing through. Such obstacles would need to be overcome or at least alleviated to enable detailed examinations. A further consideration is the region of the brain in which we would like to observe the changes. If we leave the brain intact, then we can access the outer layers of the cerebral cortex, but this bars us from investigating regions that are embedded deeper inside the brain, such as the hippocampus. Yet contemporary research suggests that the hippocampal formation plays a pivotal role in memory formation, possibly acting as mediator and processor that collects short-term memories, interconnects them with various other memories and then sends them off to be stored in various locations in the brain. A method to examine hippocampal neurons without completely destroying the functional network entails cutting (300nm to 400nm) thick hippocampal slices and cultivating them under appropriate conditions. Hippocampal brain slices can be sustained over weeks and even months in this fashion, while preserving large parts of the synaptic network and the natural consistency of the brain tissue.