Neuron: The Atom of Biological Intelligence

What is the physical atom of an organism related to intelligence? It may be a neuron. There are opinions that nerve cells originate from neuromuscular cells, and opinions that epithelial cells originated independently of sensory cells, nerve cells, and muscle cells. There is also the concept of "paraneuron", in which nerve cells evolved from secretory cells.

However, even non-neuronal organisms such as eukaryotes and plants have processes corresponding to "neural information processing" such as membrane potential, transmitters for intercellular signaling, cell membrane receptors, and ion channels, and even action potentials. This mechanism has existed in life for a much longer period than the birth of neurons. But what facilitated the differentiation of neurons, a new type of cell?

The Legend of Hydra

Remember the monster named Hydra that appears in Greek mythology? Hydra is a snake with multiple heads, and the number of heads is from one to nine. It is said that the last head, or the middle head, is the head of the human being with an immortal head. One of the twelve trials he received, especially among the legends of Hercules, which everyone knew well, was to defeat this monster.

Pollaiuolo's Hercules and the Hydra (c. 1475)

In fact, there are animals of the same name that are similar in appearance and character to this monster. Hydra is classified as the phylum Cnidaria, and the same series of animals include sea anemones, corals, and jelly fish. Among them, the hydra species in freshwater evolved the first primitive nerve cell tissue. These cells are concentrated around the hydra 's mouth and peduncle, called nerve rings. Although there are no complex sensory organs, hydra can respond to mechanical, chemical, visual, and temperature stimuli. Hydra also grows to form a detachable polyp, which is similar to a jellyfish for movement to the outside world. Interestingly, this is called medusa (yes, it is also the name of a monster in Greek mythology). Hydromedusa has a similar shape to an umbrella, which has a nerve ring inside the umbrella. They have large bipolar motor neurons that can simultaneously shrink the shape of the umbrella. The exumbrella nerve ring outside the umbrella is composed of small multipolar sensory cells that respond to touch or light to the mouth or tentacle.

Fig. 1 a–c Gross morphology of the ring nerve of Tripedalia cystophora (Go gonads, NB nematocyst batteries). a Medusa fixed as for electron microscopy. The ring nerve (RN) becomes visible as a milky white string (white arrow) running around the bell. b This nerve has a sinusoid shape as it makes contact with the roof of the rhopalial niches (black arrowheads) and the pedalia (small arrow). Since the medusa has four rhopalial niches and four pedalia, the ring nerve can conveniently be divided into eight similar parts (numbered black bars approximate areas of ultra-thin sections referred to as levels 1–5). c At higher magnification, the ring nerve appears fibrous and a single giant neurite (GN) is seen. Bars 1 mm (a, b), 50 μm (c)  
Ring nerve of Tripedalia cystophora. RN: Ring Nerve, GN: Giant Neurite, Go gonads, NB nematocyst batteries

These neurons had innumerable electric synapses. Of course, neuropeptides such as acetylcholine, serotonin, dopamine, and glutamate, which are responsible for chemical transmission, and their receptors are also found depending on the species, but their importance is relatively low in the early nervous system. In other words, the first neurons evolved to react with the external environment, beyond simple communication with the cells or inside the cell, to escape from the environment unfamiliar to them, and to move the animals, which are the aggregate of whole cells, into a favorable environment.

Discovering Nervous System

The origin of the word "Nerve" was a Greek word meaning a tendon that is responsible for the physical connection of the body. Many ancient and medieval scholars thought that the senses and movements of the human body originated in the brain. They also have a relatively detailed understanding of the anatomy of the nervous system, which consists of the brain, spinal cord, and nerves. However, the first neurons were found after microscopy was introduced. More surprising discoveries were made by Luigi Galvani, an Italian anatomist and physiologist, in the days when there were no microscopes. In 1780, he discovered that the muscles of the frogs' legs twitched when struck by an electrical spark. This was the first finding that nerves and muscles moved by electrical signals.

Seventy years after the discovery of Galvani, not only simple muscle movements but also human cognitive abilities and minds have come up with amazing theories that can be explained by the function of the nervous system as if the electrical messages of the telecommunication network connecting the world are being transmitted. Long before the term "digital" came out, H. P. Bowditch, an American physiologist, suggested the nerve impulse theory, which can be described as "all-or-none". From then on, neuroscientists began to struggle to understand the world of messages that consist of dots and dashes, and various signals such as ciphers between neurons. And these studies eventually began to link with other disciplines.


Origin and Evolution of the First Nervous System
The ring nerve of the box jellyfish Tripedalia cystophora
A History of the Nervous System
Neurons and Synapses: The History of Its Discovery


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