Since I was a child I’ve wondered why an hour feels like an hour and not a second or a year, and, relatedly, what sets the speed of our thoughts. Like every aspect of life, the answer is governed by the physical phenomena that underlie biological processes. In the case of signals traveling through our nervous system, there are a handful of different phenomena and processes, each of which has been, and continues to be, the subject of intense and immensely useful biophysical investigation — hence this installment of the “What is Biophysics?” series.
Perhaps best known are the “action potentials” that race along neurons — pulses of voltage across cell membranes that induce neighboring membrane regions to open the gates to electrically charged atoms, altering the voltage across the membrane, inducing the neighboring region to do the same, and so on, generating an electrical pulse that zips along at about the speed of a car on a highway.
This propagation of signals along the threadlike projections of neurons, and its surprising character as a signal located at the membrane rather than the core of the neural fiber, was discovered by Alan Hodgkin and Andrew Huxley in the mid-twentieth century. Hodgkin and Huxley used clever instruments largely of their own design and also devised a mathematical model of these propagating pulses. Not surprisingly, physical properties of the membranes and charged atoms are the determinants of the propagation speed — for example, the larger the fiber diameter, the faster the speed.
Action potentials aren’t the only mechanism for sending signals in the nervous system. At chemical synapses, one neuron meets another (shown), releasing neurotransmitters that cross the cleft between them. What ferries the neurotransmitters across? Absolutely nothing. Simply the fact that everything, everywhere is in constant, random motion suffices to allow neurotransmitters to reach their targets. I find this beautiful in its simplicity, and I wrote about chemical synapses in my pop-science biophysics book. (Links: My description, Publisher, Amazon.) It’s an easy exercise to calculate how much time the neurotransmitters take, on average, to wander across the cleft; it’s about a millionth of a second. You can’t transmit a signal much faster (or much slower) than that!
I’ll note for completeness that there’s also another type of synapse, the “electrical synapse” that forms rigid, stable conduits between cells. It too is governed by the flows of atoms and molecules.
In all these cases, with variations and embellishments invented by all sorts of organisms, the speed of neural signals and therefore the speed of thought forms a fascinating topic for biophysics.
In all these cases, with variations and embellishments invented by all sorts of organisms, the speed of neural signals and therefore the speed of thought forms a fascinating topic for biophysics.
Today’s illustration
A chemical synapse. I’m not too fond of it — it’s not as good as the one in the book — but I thought I’d try something different, using a lot of paint and not thinking much about it. It looks vaguely like a coughing monster.
— Raghuveer Parthasarathy. March 22, 2023
The Eighteenth Elephant 
Always enjoy your art and your message. Drawing mechanisms routinely increases my understanding and retention of new concepts. Student’s minds are underutilized when they don’t take hand-written notes and make sketches of new concepts and ideas. The intersection of verbal and visual domains create new dimensions for comprehension.
Thank you!