In this blog post, we’ll examine the structure of the human nervous system and the principles of neural signal transmission from the perspective of the robot race known as the Decepticons, and explore the similarities and differences between the nervous systems of robots and humans.
How is the human nervous system structured?
The Autobots, a race of robots coexisting with humans on Earth, and their eternal nemesis, the Decepticons. After their defeat in the battle in Egypt, the Decepticons went into hiding to plot their revenge. At that time, the Decepticons analyzed human physiology, and that data was discovered by humans long afterward. This post reproduces a portion of that data.
To my Decepticon comrades in the Solar System and on other planets, I am Starscream. We on the planet Earth have fought several battles to eliminate the Autobots and the humans, but we were defeated. It seems this was likely due to a lack of information about the unfamiliar species known as humans. Our leader, Megatron, has ordered us to thoroughly investigate Earthlings and prepare for the next battle, so I am transmitting the data from our analysis of Earthlings to my comrades. I urge everyone to familiarize themselves with this data, analyze the Earthlings’ strengths and weaknesses, and prepare thoroughly for battle.
The first data I will transmit concerns the Earthlings’ nervous system. Unlike us, the robot race, Earthlings’ bodies are composed of various organic and inorganic materials, so their nervous system is also very different from ours. Let’s examine this point by point. We receive external stimuli through cameras, vibration sensors, and electrical sensors, whereas Earthlings are said to have “sense organs” that receive stimuli. The received stimuli are transmitted through sensory cells and nerves to a structure called the brain, where the brain issues commands in response to the stimuli. In terms of us, the robot race, this is analogous to stimuli recognized by sensors being transmitted via cables to the CPU, which then issues commands in response. While the brain, which issues commands, is important for transmitting stimuli and responding to them, the nerves—which serve as the pathways for transmitting stimuli and responses—are also extremely important.
What is the structure of a nerve cell?
First, let’s examine the structure of Earthlings’ nerves. Earthlings’ nerves are composed of small units called nerve cells (neurons), and each nerve cell consists of three main parts: dendrites, the cell body, and the axon. Dendrites are the primary components that receive stimuli; they are named for their resemblance to spreading tree branches. Dendrites contain receptor proteins capable of interacting with neurotransmitters. When a neurotransmitter binds to a receptor, a signal is transmitted through that receptor, and once the input reaches a certain threshold, the neuron can generate the next signal. In terms of our robot, receptor proteins act like switches that determine whether to transmit a stimulus or not.
Stimuli received by the dendrites are integrated in the cell body and then transmitted to the axon. The cell body is the central part of the neuron. It contains the neuron’s nucleus, which appears to function similarly to a chip that stores information about each component in our robot. The presence of a nucleus indicates that a neuron is also a structure composed of a single cell. The axon is a long process extending from the cell body, and at its tip are presynaptic terminals that branch out in multiple directions. Here, neurotransmitters are released to transmit stimuli to the next neuron or other cells.
How are signals transmitted between nerve cells?
This time, I’ll share some data related to signal transmission. While our robot has a cable running all the way to the CPU, human nerve cells do not. Because there is a very narrow gap between nerve cells, signals are not transmitted directly at a typical chemical synapse; instead, neurotransmitters are released to convey the signal. Earthlings call this connection a “synapse.” You can think of it as a short-range communication function, much like Bluetooth. The direction of signal transmission generally goes from the axon terminal of one neuron to the dendrites, cell body, or axon of the next neuron. Within a single neuron, signals—which are primarily generated or integrated in the dendrites and cell body—are transmitted along the axon; however, signal transmission within the cell does not always occur in just one direction. Nevertheless, in a typical chemical synapse, neurotransmitters are released from the axon terminal, so signals do not simply travel back toward the previous neuron through the synapse.
Let’s explore how stimuli are transmitted within a neuron. One thing we have in common with Earthlings is that we use electrical signals to transmit stimuli. In a robot’s cables, signals are transmitted by the movement of electric charge through the metal that makes up the cable; in Earthlings’ nerve cells, electrical signals are transmitted by the movement of charged ions. Inside a nerve cell, there is a relatively high concentration of K+ ions and electrically negative proteins, while outside the cell, there is a relatively high concentration of Na+ and Cl- ions. Due to this uneven distribution of ions and the selective permeability of the cell membrane, a nerve cell maintains a membrane potential in which the interior is more negatively charged than the exterior while at rest. This applies to nerve cells that are not currently receiving a stimulus; this state is referred to as “polarization.” The membrane potential of a nerve cell in a polarized state varies depending on the type and condition of the cell.
The polarized state is disrupted by stimulation, leading to a different state. The term used to describe this different state is “depolarization,” so please keep this in mind. Depolarization occurs primarily when Na+ ions move into the cell through voltage-gated Na+ ion channels. The cell membranes of humans contain passageways that allow the movement of substances between the inside and outside of the cell; channels are one such type. In the resting state, some K+ ions move through leakage channels, and the voltage-gated Na+ ion channels remain closed, maintaining the polarized state. However, when the cell receives sufficient stimulation and the membrane potential reaches a threshold, the Na+ ion channels open rapidly, allowing Na+ ions to enter the cell due to the electrochemical concentration gradient. As a result, the interior of the cell temporarily becomes less negatively charged or positively charged, resulting in a depolarized state.
How do depolarization and repolarization occur?
Since the depolarized state is unstable, the human body seeks to return to a stable membrane potential. After depolarization, the Na+ ion channels inactivate, and the K+ ion channels open, allowing K+ ions to move out of the cell, causing the membrane potential to shift back toward a negative value. This process is called repolarization. However, if an excessive amount of K+ ions flows out, the cell may enter a state of hyperpolarization, where the membrane potential becomes more negative than it is in the resting state. In this case, the membrane potential gradually returns to the resting state over time. Meanwhile, the Na⁺-K⁺ pump plays a crucial role in maintaining the ion concentration gradient across the cell membrane by using ATP to transport Na⁺ ions out of the cell and K⁺ ions into the cell. Generally, during a single cycle, the Na⁺-K⁺ pump expels three Na⁺ ions from the cell and transports two K⁺ ions into the cell.
What factors determine the speed of nerve impulse transmission?
Let’s explore the factors that influence the speed of impulse transmission. When one segment of an axon undergoes depolarization → repolarization → depolarization, the resulting electrical changes affect the membrane potential of the adjacent segment, triggering an action potential in that segment. As this process continues, the impulse is transmitted along the axon. However, if a myelin sheath is present on the axon, ion movement is restricted in the sections covered by the sheath. Between sections of the myelin sheath lie the nodes of Ranvier, where the myelin is interrupted; at these nodes, an action potential is generated anew, and the signal is transmitted to the next node. Just as when crossing a series of stepping stones, depolarization and repolarization occur intermittently, and this method of signal transmission is called saltatory conduction. When saltatory conduction occurs, there is no need to regenerate the action potential through ion movement in every section of the axon, so stimulus transmission becomes much faster.
How can we utilize Earthlings’ nerves?
We have transmitted the data analyzed so far regarding Earthlings’ nerves. It was interesting to note that the neural structure and methods of stimulus transmission in humans share many similarities with machines or our robots. It is said that mature human neurons generally have a very limited ability to divide on their own and produce new neurons. However, it is important to distinguish between the regeneration of the neuron itself and the regeneration of the axon; specifically, while damaged axons in the peripheral nervous system can regenerate to a certain extent, axonal regeneration in the central nervous system is very limited. Therefore, once we have won the battle and made Earthlings our subordinates, I believe we will be able to manage them effectively if we find ways not only to simply replace or regenerate damaged nerve cells but also to regenerate the axons of damaged nerves and restore their function. Meanwhile, considering that the ion concentrations within Earthlings’ bodies and the functioning of ion channels influence the generation and transmission of nerve impulses, I believe that conducting research and analysis to understand these mechanisms will allow us to determine how they might affect the nervous system’s functions and responses. This concludes the neural analysis report on Earthlings.
