Thanks to this "alarm hotline", Shushu was able to "run away as the best strategy"

Produced by: Science Popularization China

Author: Wang Fei (PhD in Neuroscience)

Producer: China Science Expo

As the series of rat videos on major short video platforms have become popular, we have discovered that not all rats are "despised by everyone". Rats can help human science reach a new level. Rats such as chinchillas and guinea pigs can even become "guests of honor" for humans, turning into beloved pets, waiting to be fed by their "poop scoopers".

In the conventional impression, some rats are often looked down upon and driven away by humans due to their destructive power and their tendency to steal food. However, people have discovered that rats are very quick in dodging and escaping, and can be called "escape experts". When encountering natural enemies, they will also adopt a "playing dead" strategy, that is, using rigid defensive behavior to "save their lives", which can be described as superb "acting skills".

Most of the time, humans cannot catch this "escape expert" directly, but need to use some mouse-catching tools. So why do mice react so quickly? What is the secret behind their agile and nimble escape behavior?

The mouse ran away quickly when it saw the cat

(Image source: AmazinglyTimedPhotos.com)

Mice - natural escape experts

It is a fact that everyone knows that mice react extremely quickly when they encounter danger. Whether it is a cat catching a mouse or a human chasing a mouse, we often only see it running away without even knowing what color it is. Moreover, they are very agile when running away, and can change direction, climb, jump, and jump up and down instantly, which gives mice a natural advantage in movement flexibility over some large animals.

More importantly, mice are particularly alert, with some special structures in their brains that allow them to predict the presence of danger before escaping. Mice are called "escape geniuses" because they can quickly detect the arrival of danger and initiate defensive behaviors such as escape and freezing through a fast pathway.

Studies have shown that it only takes mice 0.05 seconds to detect danger information, and this information can be quickly transmitted to the brain and produce behavior.

A fox surprises a groundhog

(Photo credit: Bao Yongqing, winners of the 2019 Wildlife Photographer of the Year Competition)

"Alarm Line" - detect predators and send danger signals

There is a special "alarm line" in the mouse's retina that allows the mouse to quickly detect danger information. This "alarm line" is the alpha ganglion cell in the retina. It can detect the approaching predator and send an electrical signal to the brain - danger is coming!

So how did the researchers find these special neurons? The first step in scientific discovery is guessing.

After analyzing and comparing different ganglion cells in the mouse retina, the researchers found that alpha ganglion cells have a signal transduction ability that is unmatched by other cells. It can send visual signals to the brain in the first place. Because of this, this cell naturally becomes the first choice for this alarm line.

Different types of ganglion cells in the retina

(Image source: Volgi et al., 2009)

Although researchers had already guessed about this type of cell, it is scattered throughout the retina. Researchers can only randomly grab one cell each time, and whether they can encounter the next one depends entirely on luck.

Thanks to in-depth genetic research, the researchers found a gene that is specifically expressed in alpha cells. After that, the researchers bred transgenic mice, through which they could light up this cell and make it stand out from the crowd of cells. And they could control this group of cells through special methods to prove what the function of this type of cell is.

Recording from fluorescently labeled alpha retinal ganglion cells

(Image source: Fei Wang et al. 2021, Current Biology)

So how did the researchers prove that alpha cells are the cells that report danger information to the brain?

First, they silenced these cells while allowing other cells in the mouse's eyes to function normally. The mice were no longer afraid of approaching predators and would even continue to wander around.

Then, the researchers activated the cells, causing them to send a false alarm signal to the brain, and the mice began to run away, seemingly in fear, even though there was no approaching predator.

Controlling fear behavior in mice by manipulating alpha cells in their retina

(Image source: Fei Wang et al. 2021, Current Biology)

Now that it has been confirmed that alpha cells are cells that report danger information to the brain, the question is, how do alpha cells see dangerous stimuli?

The "behind-the-scenes hero" of the "alarm hotline" to detect danger - the special structural advantages

In fact, whether it is a cat rushing towards it or an owl flying down from the sky, the mouse only sees a dark shadow approaching it. Researchers have found that just by showing the mouse such a black circle that changes from small to large, the mouse will trigger the escape and freezing response.

Alpha cells can see this approaching shadow stimulus. When recording alpha cells and providing them with the stimulus of approaching shadows, the researchers found that alpha cells would respond to the approaching shadows, and the intensity of the response could be adjusted to the size of the approaching visual stimulus, that is, mice were able to judge the distance of predators through alpha cells.

The dendrites of alpha cells (structures that receive signals) are like radars facing the space, with long antennas forming a wide net. There are thousands of such cells in the entire retina, overlapping each other, not missing any point in the visual field. This type of cell also has a thick axon (structure that transmits signals), which can transmit signals like a cable. The special thing about this "cable" is that it is wrapped in an insulating coat, which can transmit signals to the brain faster than other cells.

Alpha cells in the mouse retina

(Image source: Zhang Yifeng Laboratory, Institute of Neuroscience, Chinese Academy of Sciences)

When alpha cells see the stimulus of an approaching shadow, they send an electrical signal to the brain. This electrical signal can go directly to the brain area responsible for defensive behavior without passing through the mouse's cerebral cortex. This is a highway from the retina to the subcortical pathway, directly converting danger into action.

This highway starts from the retina and transmits danger signals to a place in the brain called the "superior colliculus", and then "splits into two routes". One route goes to the brain area that commands movement and initiates defensive behavior; the other route reaches the brain area responsible for fear - the amygdala (which looks like an almond), causing the mice to feel fear.

In this way, the danger information does not need to be processed by the cerebral cortex (where thinking consciousness is generated) first, and then output behavior through the movement-related brain area, and the information transmission pathway is greatly shortened. At this time, the animal can take action without thinking, and has a "subconscious" reaction.

Such a dedicated alarm line and a rapid response system help animals quickly avoid sneak attacks by predators.

Visual pathway responsible for approximating dark shadow stimuli in the mouse brain

(Image source: Julieta E. Lischinsky and Dayu Lin, 2019, Trends in Neurosciences)

"Alarm Line" that is superior to traditional visual perception

The brain's visual system provides the circuitry that allows us to see the world clearly, but less well known is that it also has other, more conservative functions, such as helping us sense circadian rhythms, regulating our emotions, and initiating some instinctive behaviors.

This "alarm line" in the mouse retina is an example of the visual system triggering instinctive defensive behavior. This research adds to our understanding of the function of the visual system.

Seeing this, some people may ask, is there any difference between the information encoding strategy used by the visual system to provide alarm function and the encoding strategy in traditional visual perception?

Generally speaking, combined coding is the traditional way of visual perception, which can provide our brain with clear and complete images. However, this method has disadvantages such as slow response time and redundant information processing. It is still unknown whether there is another fast coding method in the visual system. This "alarm line" may use a fast coding method to quickly transfer specific key information to the brain and trigger behavior.

Imaging and other functions of the visual pathway

(Image source: Silvia E. Braslavsky, 2020)

Conclusion

This rapid response mechanism of the biological world also brings enlightenment to human society. For specific dangerous stimuli, we not only need extensive information input, but also need a special line to make quick decisions. This is like the army leader cannot obtain enemy information through daily news, and the army must set up a special reconnaissance system at the forefront.

This is mainly because the massive amount of information may overwhelm us with various redundant information, causing us to ignore important content. Only by keeping the alarm phone number can we specifically detect dangerous stimuli and transmit relevant information directly to relevant departments without confusing other useless information.

Through the study of this "alarm line" in the mouse brain, we can also try to answer a series of questions we encounter in life. For example, in real life, do driverless cars really need lidar? Is it enough to install a few more cameras on the car?

Automatic detection function for driverless cars

(Image credit: Bernard Marr)

Through research we can get the answer - in the future cars, not only cameras are needed to collect various information, but also a lidar channel dedicated to reporting dangerous information must be reserved, so as to maximize the safety of drivers and cars.

Editor: Ying Yike