By Francesco Rivetti
Biology and engineering rarely intersect: biology deals with understanding the world, and engineering shapes it. Nevertheless, over time scientists have learnt how to shape the world before really understanding what they discovered. This is exactly how the basics of neuroscience were cracked. In the 18th and 19th centuries, scientists were exploring the science of electricity, but only a hundred years later these concepts were applied to biology. This provided science with the first steps to understand the intricacies that makes up our most complex organ: the brain.
Electrical resistance is not just useful in electronics: it also spans into the molecular phenomena that enable neurons to send messages to each other. Resistance in electrical engineering is as a property that describes the opposition to flow of charge in a conductor (Tool et al., 2020). However, in the world of neuroscience we must differentiate two types of resistances: axial (ra) and membrane (rm). The former refers to the resistance that the fluid of the axon exhibits against the flow of charge down its path (ScienceDirect, 2020). The latter represents the opposition that the axonal membrane shows to the flow of charge through it, and it is proportional to the concentration of ion-channels (University of Texas, 2020). Therefore, we can assume that in order to maximize speed of communication one must decrease axial resistance, as ion flow will be opposed less, facilitating travel of the action potential (Experiment, 2020). Conversely, membrane resistance should be maximized, as this minimizes the dissipation of the action potential (ScienceDirect, 2020), as the ions cannot exit the axon. This simple assumption enables us to derive the length constant: λ=rmra , which is defined as the distance it takes for a potential charge to fall to ~37% of its initial value.
Another important concept is capacitance, which is the ability to build up charge at either side of an insulator. In neuroscience, capacitance (cm) refers to the charge-build up across the membrane that separates the cytosol from the extracellular environment Presti and Rusconi, 2019). Good capacitors (which are bad insulators) act as buffers, as they slow down change; this can be visualized at the cellular level by the fact that oppositely charged ions can interact through the membrane, and so oppose the movement of charge across axons (What is, 2020). This connects us to the time constant (τ=rmcm), which is how long it takes for a piece of membrane to charge up to 63% of its final value. In other words, it is the time it takes for adjacent areas of the membranes to charge – the smaller the value, the faster the neuron-firing (Experiment, 2020). As stated prior, a membrane with higher capacitance would act as a buffer, and therefore increase the time it takes for voltage to change. Similarly, a membrane with more ion-channels (lower resistance) would obviously be able to respond faster to a stimulation (Presti and Rusconi, 2019).
Even though these concepts were discovered by humankind just a century ago, nature has known how to maximize the length constant and how to minimize the time constant for more than half a billion years. The first method that evolved is known as “Axonal Gigantism”, found in the nervous system of cephalopods (see Figure 1). For example, the neurons of North Atlantic Squids can be 1-1.5 mm in diameter, making them around 1000 times larger than human ones (Neuron, 2020). Axonal gigantism evolved because axial resistance decreases with axon diameter, because as the diameter increases the probability of an ion to bounce against something that will push it back decreases (Action potential, 2020). Nevertheless, it is not extremely efficient: in order to double the speed, the diameter needs to be quadrupled. Regardless of its inefficiency, this gigantism is at times beneficial since it does not require the specialization of certain cells (Experiment, 2020).
Another (more efficient) way to increase the speed of neuronal communication is the myelination of axons, which seems to have evolved independently many times in vertebrates, crustaceans and annelids (Hartline and Coleman, 2020). Myelin is a fatty-rich substance (hence why the brain is white), that wraps around the axon and is synthesized by specialized glial cells (Presti and Rusconi, 2019). Myelination increases speed of signaling as it increases membrane resistance and significantly decreases the capacitance of the axon: increasing the length constant and shortening the time constant (Figure 3). Even if the time constant increases with membrane resistance, computer simulations have shown that the decrease in capacitance compensates for this (Experiment, 2020). In electronics, this is a parallel to coating metal wires with plastic. Myelination enables saltatory conduction, meaning instead of having a Mexican wave-like continuous formation of action potentials, the action potential is only reached in the nodes of Ranvier, where the myelin does not wrap around the axon (Presti and Rusconi, 2019). Myelination is so effective because enables the minimization of the opening and closing of ion-channels, which are slow-acting, and instead it maximizes the wire-like fast conduction. As shown in Figure 4, the nodes of Ranvier are extremely permeable to ions since they have a very high concentration of ion-channels, which when stimulated re-establishes the action potential (Presti and Rusconi, 2019). Between the nodes, the action potential is not renewed, but the insulatory myelin increases the length-constant so that the change in voltage can reach the next node and still activate the ion-channels and re-start a new action potential (Experiment: Comparing Speeds of Two Nerve Fiber Sizes, 2020). Thanks to this simple alteration, the action potential in human touch-perception neurons travels at a speed of 80 to 120 m/s, which is actually faster than most high-speed trains (Neuron, 2020).
All of these concepts, equations and phenomena were first discovered by physicists and engineers of the 18th century, and then they were applied centuries later by the biologists who pioneered in neuroscience. This is testimony to the fact that in the real world, there are no real distinctions between biology, chemistry and physics – it is not enough to know one of these disciplines in depth in order to understand the world around us.
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