Stimulus-Response and Sensor-Effector

What ability should life have to survive in nature? Perhaps the most important thing is to recognize and adapt to changes in the external environment. In order to do this, life must be able to recognize the change of the external environment. From the viewpoint of life, the change of external environment can be regarded as 'stimulus'. At this time, the life reacts to 'stimulus', which is called 'response'.

Stimulus-Response in Bacteria

Bacteria need to recognize nutrient-rich substances such as sugars for survival, as well as toxins such as heavy metals that can kill themselves. To this end, they have developed a number of chemoreceptors and mechanoreceptors that can detect changes in the external environment. The collected diverse information is integrated through the chain of complex chemical reactions, and control the motion of the flagella. For example, Escherichia coli has six flagella, each has a proton-driven motor, but they are integrated into a single superflagellum and are responsible for cell movement. If the toxin substances are detected by the cell, the whole chemical-reaction chain reaction leads to the movement of the superflagellum to avoid them, and the nutrients are detected, it is controlled to move in that direction.

Receptor detect the nutrients (A), release of a signal molecule (B), reacts with proteins associated with the bacterium's flagellum (C), causing a change in its direction of rotation to the nutrients (D) Created by Emily Harrington 

If there is no special nutrients or toxin gradient, the superflagellum disintegrates, in which case each flagella moves independently. The cells then move in a direction that is difficult to predict, and they continue to move in arbitrary directions until they detect changes in the nutrients or toxin gradient. When a new nutrient or toxin gradient is detected, six flagella combine to construct a superflagellum and start the movement according to the gradient. Does not it remind you of the exploitation vs. exploration process in the last post?

The bacteria also have very little memory so you can at least compare the current incoming information with the previous one. And 'knowledge' obtained through these comparisons determines the behavior of bacteria. The most primitive life forms also have three basic components for adaptive control of behavior. That's the sensor, effector, and controller that stores and processes information.

Chemical Regulation vs. Nervous Regulation

How about simple organisms? In multi-cellular organisms, such as fungi or protozoa, a response that is received by a particular cell will have to propagate to other adjacent cells. This is called taxis. If the size of an organism grows and becomes complicated, these responses will need to be synchronized and integrated across different parts of the body, and will require a controller to adjust it. This controller should be between stimulus and response. Usually, multicellular organisms use a mechanism called chemical regulation and nervous regulation to appropriately respond to these stimuli. For example, humoral regulation using hormones and neural regulation using the brain and nervous system simultaneously work in humans, and they respond to external stimuli with harmoniously appropriate timelines.

A hormone is a substance that is generated inside a living organism and causes a chemical reaction to respond to stimuli to the organism. Hormones are secreted by a specific group of well-differentiated cells and spread or transported to other parts of the body through blood or body fluids, and they induce cells that influence metabolism or produce certain other substances.

Apart from chemical regulation, living organisms that require a more complex and immediate response evolve specific cells so that the whole body reacts to them, which is called the nervous system. The specialized unit cells that make up the nervous system are called neurons, which are specialized for conduction of impulses collected from sensory receptors that recognize external stimuli. It is the purpose of neuron to make such an impulse reach the effectors that are appropriately to produce a response through conduction of the nervous system.

It's like connecting a number of sensors to recognize the external environment when we build a robot and connecting computers and controllers that can determine the data the sensors are delivering. And the commands that the computers and the controllers have decided are transmitted to the actuator, and the robot responds appropriately.


Receptors guide bacterial navigation
Evolution of Nervous Systems and Brains in the Book, <Neurosciences - From Molecule to Behavior: A University Textbook>


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