Neuroscience- neurons, electric circuits and more…
A fun take on the complicated science of neurons, from an electrical engineer’s perspective. All are true facts, but the diction used is less technical and more general.
Electrical engineering — the classic engineering subject that has brought out the best inventions. ‘Neuroscience’ — what has electrical engineering got to do with it ? It seems like they are poles apart! This was my thought when I started reading neuroscience articles. The truth is, which I understood after reading many neuroscience articles and taking a few neuroscience classes, that there is a lot of similarity. In fact, presently, in the research world, researchers use the concepts deep rooted in electrical engineering to study the working of the brain and the nervous system. With minimal jargon and less scientific definitions, this write up aims to be less technical and more relatable for electrical engineers with a curiosity for neuroscience, and new research prospects.
The brain is the most complex human organ. Its functioning has not been completely understood yet, and is estimated to take many more years to be fully comprehended. The fun begins with the basic cellular unit of the brain — the neuron. There are billions of neurons in our brain and nervous system. How are neurons different from other cells of the body ? An understanding of high school biology is enough to grasp the basics about different cell types. Neurons are special cells which pass on information, aka electrical and chemical signals, among themselves and to other organs of the body, enabling the body to function under the brain’s direction.
The biology:
A neuron has a weird shape- my older kid thinks it looks like roots attached to a long rope of connected hotdogs(see figure1 below). In my younger kid’s words, it looks like a fallen tree! There is something like a blob (shown in yellow) which is the cell body or soma, with thousands of input wires called the dendrites and a main output wire called an axon with many branches. The axonal membrane has some fatty blanket like coverings called the myelin sheath(those are the hotdogs mentioned above!). This is a very deep simplification of the true description of the neuron. But it holds true for describing the fundamentals for the flow of signals.
The link to electrical engineering:
Our body can be simplified to a machine working with input and output from the brain and other organs. Neurons connect all parts of our body to the brain. There are a cocktail of chemicals and proteins along with charges that allow the flow of information or “action potential”. The continuous flow of action potential is what makes our brain instruct our body to perform various actions, allow us to understand our sensations, pain, taste etc.
The tiny space between one neuron and the next is called a synapse. The dendrites receive input signals via these synapses — they help pass information between neurons. The cell body acts as the substation that sums up all the incoming power and the hillock(the beginning of the axon from the cell body, named for its slightly bumpy shape that appears like a hillock) produces a spike of action potential that then propagates forward. This spike is the summation of all the currents from the dendrites and the cell body. Some of those might be excitatory and some inhibitory, and the sum decides if an action potential will be triggered or not, at the hillock. Now this works on a very common electric circuit principle: the Kirchhoff’s Current Law ! The understanding that the algebraic sum of ALL the currents entering and leaving a junction must be equal to zero as: Σ IIN = Σ IOUT.[1]
Consider another very basic electrical engineering concept: the Ohm’s Law — the current flowing across a component is proportional to the potential drop, with a constant resistance. This explains the drop in voltage across different regions of the neuron, and the actual potential that is propagated across to the next neuron.
The axonal walls have leakage channels (something like leaky electric switches) and voltage gated channels.These allow the flow of ions or charges. The leakage channels don’t ever shut completely, as the name suggests. There is always some ion flow across them. The voltage gated channels, on the other hand, open and close according to a certain potential difference between the inside and outside of the cell membrane. Sodium, potassium, chloride and calcium are the main ions that flow across these gates or channels and contribute to the flow of charges. There are thousands of such gates, and each acts an individual component in the whole electric circuit. It has resistance, capacitance and conductance! See figure 2 below:
The neuronal circuit follows similar circuit rules as a normal electrical circuit. There is leakage conductance by the channels for sodium and potassium ions, the positive charges. The myelin sheath acts as an insulator, and contributes to the capacitance(Cm-membrane capacitance in figure2). The length of the axon provides an axial resistance(Ra), that causes a potential drop. The unique feature of this type of current flow is to be mentioned though. The voltage (or charges — neuroscientists seem to use these terms interchangeably, not quite sure why) jumps through areas of the ion channels. It doesn’t flow continuously like charges in a true electrical circuit. Due to the axial resistance, the voltage drops a bit (there is a mathematical formula to calculate this drop based on the values of resistance, ion concentrations inside etc., but let us stick to the basics). The regions of ion channels act like tiny relay stations — they boost the voltage during each stop, and help the signals propagate.
Transmission from one neuron to the other, via synapses, is not always purely electrical. There are some very complicated chemical processes, where calcium ions rush into the cell via the channels described above, small molecules of neurotransmitters get released into the synapse, and there are very specific receptor molecules on the surface of the receiving neuron that bind to these transmitters, and that binding creates a chain reaction inside that next neuron, and that propagates the electrical signal flow and so on. Each time the dendrites receive signals from synapses, based on the type of signal — like enhancing or inhibiting, the signals sum up in the cell body, and again propagate via the axon to the next cell. The dendrites receive signals from hundreds of other nearby cells via synapses, and this is where the networking gets complicated, as opposed to electrical circuits.
Research trends:
Understanding these fundamentals of signal propagation in the neurons has helped scientists discover a plethora of ailments and their cures- epilepsy, multiple sclerosis, and many such diseases occur due to the malfunctioning of these neuronal circuits. Current research in brain science spans a wide range of engineering applications. With imaging technologies, high precision surgical robots, the trials on surgical electrode implantation (think Neuralink !), various approaches to cure such neurological disorders, like chemical interventions with drugs, electrical stimulation therapies, all have interlinked works from various disciplines of science and engineering.
Electrical stimulation — such a simple concept, but very complicated to implement. It has actually taken a couple of centuries to finally understand how such stimulations work and to leverage its use for alleviating pain symptoms. In an excellent book named “Spark of Life- Electricity in the human body” by Frances Ashcroft, the evolution of electrical stimulation has been described in detail. What was thought to be dark magic and wizardry tricks turned out to be a normal phenomenon in the body of living beings. There are a lot of such exciting books out there, if only we took the time to explore.
Core engineering has started taking shape in a multitude of cross disciplinary aspects. An electrical engineer works with many related departments — electronics or computers or artificial intelligence or biomedical applications — the list is endless. The exciting research prospects for electrical engineers in the field of neuroscience is wide, and the awareness is still nascent. The purpose of this article is to help electrical engineers understand the fundamental unit of the brain — the neuron, and the relevance of neurobiology to engineering.
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About me: I’m an electrical and biomedical engineering graduate, and a mom of two kids interested in the wonders of neuroscience. I hope to encourage folks to think beyond the box, to relate concepts in different areas. I passionately write about a wide range of topics like music, meditation, engineering and neuroscience.
