Experiment:The Beginning of Modern Neuroscience - The Galvani/Volta Debate
One of our first experiments was the "dancing cockroach leg" in which nerves and muscles of a cockroach leg could be electrically excited by the output of an mp3 player. But let's go further back. Way back. To 1780. To Northern Italy.
What will you learn?
In this lab you will recreate the famous experiments of Galvani and Volta. You will build a replica of the very first battery, invented by Volta and which began the electronics revolution that continues today, and you will use it to to stimulate the nerves of a cockroach leg, in an adaptation of Luigi Galvani's famous frog leg experiments that began the NeuroRevolution still manifesting.
- You are starting from zero. You are beginning the field of neuroscience. There are no previous experiments to do.
In our previous microsimulation experiment we plugged the audio cable of an mp3 player directly into pins inserted into a cockroach leg. The electrical waves of the music then excited the nerves and muscles of the leg, causing the leg to appear to "dance" to the low frequencies (the beat) of pop music. This worked because an mp3 player is capable at max volume of sending out oscillating voltage signals at 1.3 V, which is sufficient to excite nerves and muscle at the music frequencies below 200 Hz (the bass). This was a modern interpretation of Luigi Galvani's famous frog leg experiments, but we have now found the experiment can be made more educationally and emotionally compelling by building our own voltage source (battery) out of common materials - a potato, a sheet of aluminum, and a sheet of copper. In this variation, we are building a replica of Volta's "Voltaic Pile" and using it to stimulate the cockroach leg, as a replica of the famous debate between the two scientists.
Our differences are minor: 1) We won't need to sacrifice frogs like Galvani did. Rather, we will remove a hind leg from a cockroach, which the cockroach can easily tolerate, continue walking, eating, being a cockroach, and, moreover, will regrow a new hind leg within 125 days, and 2) Volta's "Pile" used alternating layers of copper and zinc separated by cardboard soaked with salty water. We will use alternating layers of copper and aluminum, as aluminum sheeting is easier to find in art/hardware supply stores. Also, using cardboard soaked with salty water (a salt battery) can be leaky and difficult to manage, and thus we will use slices of fresh potatoes as a weakly acidic conductor (acid battery), which maintains moisture quite well and does not drip.
Aluminum, potatoes, electricity... what? Yes, placing a slice of fresh potato between a plate of copper and a plate of aluminum will enable electron flow that we can harvest. Aluminum, being the anode, loses electrons (oxidation). The electrons flow through the acidic bridge (the potato) to the surface of the copper plate, which, being the cathode, gains electrons (reduction ). What is the potato doing? By being electrically conductive and slightly acidic, it permits electron transfer from the aluminum to the copper via the H+ ions (the hydrogen ions are reduced to hydrogen gas H2). The voltage that this homemade battery generates is not very high (0.5 volts), and the current is very low (~1 mA - our lab voltmeters are not even sensitive enough to reliably measure it) but the battery is sufficient to stimulate a cockroach leg or turn on an LED if multiple copper-potato-aluminum cells are "piled" up.
But let us consider the historical context. What motivated Galvani to even examine the electrical excitability of nerves in the first place? What was the knowledge of his time? Systematic study of electricity as "something" was just beginning in the 18th century in Europe. Some clues were eternally present. The ancient Greeks had observed odd properties of substances living and material. For instance, Amber is an excretion of certain trees in response to wounds and was made famous by the movie Jurassic park. When rubbed with a piece of cloth or hair, hardened amber will attract small particles of dust, and indeed the Greek work for amber is "elektron". Mediterranean peoples had also long known of the strange properties of the torpedo fish that in lived in coastal areas and caused stinging sensations when bothered (the fish uses electrical shocks for attack and defense behaviors ).
But what is happening exactly when the torpedo fish "shocks" someone? And how is amber able to attract dust? Are the two phenomena even related? And do they have anything to do with the lightning of electrical storms? For that matter, what is even going on in lodestones (naturally occurring magnets). Such questions were very of the age in 18th century Europe. The Leyden Jar (the first capacitor) had been invented in the 1740's, and reliable electrostatic friction machines (machines capability of generating electrical sparks) had recently just been invented as well.
Interested in the possible connection between electricity and living beings, Bologna physician and professor Luigi Galvani began studying "electrophysiology" in 1780. He was aware of previous experiments by other Italian scientist Tommaso Laghi, who observed in 1757 that electrical stimulation of nerves caused muscle contraction. Using a common frog leg preparation in which the muscle and nerve tissue are viable for a number of hours, Galvani decided to systematically study this phenomenon in more detail, leading to the recognition that we give him today.
The majority of Luigi's experiments involved connecting the frog legs to capacitors and metallic loops during the 1780's, which are described in detail in his 1791 magnum opus: "De viribus electricitatis in motu musculari commentarius" - translated from Latin to "Commentary on the Force of Electricity on Muscular Motion." Beginning his general experiments on exactly November 6th, 1780 (noted from his lab book), he connected a type of capacitor - a "Franklin Square" (yes, an invention of Benjamin Franklin) to the nerve of a frog leg, causing the leg to famously twitch.
Luigi's next step was connecting nerve to muscle, or nerve to nerve, with metallics arcs and examining leg contraction . These experiments revealed that an external voltage source (be it in a spark generator or a capacitor) was not necessary to cause leg contraction, providing evidence to Galvani's hypothesis that the nerve and muscle tissue itself was a generator of electrical energy that it used for its own proper functioning.
After Luigi published his results in 1791, he gained a reader turned scientific rival from nearby Pavia. Alessandro Volta had previously invented an instrument called the electrophorus - a capacitor that could be used multiple times without recharging, and the "weak electricity" of the frog nerve experiments intrigued Volta. He began examining the preparation as well, and he importantly found that using using bimetallic arcs of two different metals worked much better in stimulating frog legs than arcs composed of just one metal, and he published this observation in 1792 in "Memoria seconda sull'elettricita animale" (-Italian - Second Memoir on Animal Electricity). He believed that Galvani's use of metallic arcs and organic tissue created a sort of electrical disequilibrium that caused contraction, and the animal did not generate its own electricity.
In response to this criticism, Galvani did a further experiment in which he connected one severed nerve on one leg to another severed nerve on another leg, causing both legs to twitch. The experiment was essential to show that external metal wasn't needed to cause contraction and was his strongest evidence against Volta. Galvani published this in 1794 in "Dell'uso e dell'attivita dell'arco conduttore nelle contrazioni dei usoli" (Of the Use and Activity of the Conducting Arc in the Contractions of the Muscles) .
The vigorous debate between Galvani and Volta continued for three years more in another round of letters and manuscripts, but then Napoleon arrived, disrupted Northern Italy, and Luigi died in 1798. Volta continued investigating, going on to build a prototype based on the two metal disequilibrium called "the pile" - which was the invention of what we now call the battery . His prototype contained alternating layers of copper, salt-water impregnated cardboard, and zinc, and when he sent his battery to the Royal Society in England in 1800, Volta gained enormous international recognition. The use of the battery as a continuous controllable source of voltage and current allowed many more experiments by many more scientists, such as Humphrey Davy and Michael Faraday who used the battery for electrolysis experiments that led to the discovery of new elements (sodium, potassium, chlorine, calcium, etc), and Georg Ohm using the battery to derive the law after his namesake.
This excellent "Galvani/Volta" debate centers on 1) whether nerve or muscle tissue generates its own electricity or if 2) externally applied electricity merely excites the tissue. The debate was not resolved until the mid-19th century when electrical impulses in nerves were observed through indirect methods with the first primitive amplifiers , and finally unequivocally in the early 20th century with the invention of the vacuum tube amplifier. But, being residents of the future, we can now see that both Italian scientists were correct. Volta's suggestion that two metals connected with a salty medium (the frog) could generate electricity was correct. Galvani's suggestion that the nerves and muscle generated their own internal electricity for proper functioning was also correct.
Which would you think if you were present in that epoch? Let's see.
Here are the tools with which you will begin the fields of neuroscience and electronics.
We need about 2-2.5V Volts to do our experiment, so we will build a "pile" of 5 cells.
...which we can use to stimulate a cockroach leg or turn on an LED.
Step by step details continue below. It is important to attempt to imagine a world where we do not have a system to understand this phenomenon. What is going on the pile? What is going on in the leg? The experiment strikes us oddly still today even when we understand the science. Imagine how it must have been perceived 235 years ago.....
Congratulations, you have just begun electronics and neuroscience! We emphasize again, as you do this experiment, imagine that as of yet 1) there is not any unifying theory of how electricity works, and 2) physiology is still in its infancy . What would you think was occurring if you connected a primitive battery to a nerve of muscle, saw twitching, and had no background knowledge?
We close with the words of what Galvani himself thought: "Such a disequilibrium in the animal either must be there naturally or should result from artifice. If it is there naturally, we should admit that in the animal there is a particular machine capable of generating such a disequilibrium, and it will be convenient to refer to this form of electricity as to an animal electricity in order to denote, not an electricity whatsoever, but a particular one referred to a particular machine.…But what will it be this animal machine? We cannot establish it with certitude; it remains totally occult to the most acute sight; we can do nothing else than figure out its properties, and, from these, somewhat envision its nature”
Well said, Professor Galvani.