That’s right, it’s the startle reflex. Basically, when your arm is grasped suddenly by a stranger, the sudden contact sends a signal to the brain that an alarm has been triggered because of a potentially dangerous situation. 


As a result, signals in your nerves and muscles go into overdrive. This is why you feel the jumpy feeling in your chest and have uncontrollable jerky movements when you have this type of reflex triggered.


The startle reflex is also linked to what’s called “mirror neurons.” Mirror neurons are basically sensory receptors devoted to observing other members in social groups so as to better communicate with them socially even from afar.


Recently, however, it’s been discovered that mirror neurons aren’t limited to observing only others and are also sensitive to what’s going on with your own body. For example, when someone else is poked in the arm, mirror neurons fire in your own arm. The same thing happens when you pick up an object with your left or right hand.


Do you know which reflex is triggered when a stranger suddenly grasps your arm?


Here some points are discussed-

1. Strange grasp by a stranger


A person or an experimenter can grasp a person’s arm or leg. The event triggers the startle reflex, which sends electrical signals to the brain and the visual system that are interpreted as the body is reacting to danger. 


It requires very little motor output from the motor cortex to move away from an object that feels too close. The motor cortex is a part of the brain that controls voluntary movements and needs very little input from our brains to move around.


2. Monkey’s arm: 


A monkey’s arm is grabbed by an experimenter. In the video, you can see how the monkey’s arm moves away from the grabber which makes the grabber laugh because of this response. 


There are many experiments done on monkeys which show that in spite of being wired differently, the way they respond to grabbing on their arms is almost similar to humans when they feel something is wrong or dangerous. They jump too and even have jerky movements when feeling threatened or afraid.


3. The startle reflex is felt in the legs too


The startle reflex is triggered by sudden movements, such as when someone punches your stomach or you get punched. As discussed earlier, it only takes very little movement from the brain to move away from an object that feels too close. 


The moment someone punches your chest, the leg muscles contract quickly and you feel a jerk sensation. This occurs because there are many sensory receptors in the leg muscles related to touch and proprioception (where your muscles sense where they’re located in space).


The startle reflex is also triggered by sudden noises. For example, a loud noise in the vicinity of a person can trigger the startle reflex in that person. It also occurs when there is flickering light or sudden changes in lighting. Also, when you walk down stairs and you feel something bump against your legs.


4. Startle reflex and sleep


As mentioned earlier, the startle reflex is triggered very quickly in response to sudden events such as being touched in an unexpected way, seeing someone fall down or hearing a loud sound that seems dangerous to you like a gun going off nearby. 


It’s essential for human survival because it allows people to avoid danger by moving away from it quickly without thinking about it. Although this reaction happens automatically, the brain can consciously override it by thinking about what’s dangerous to them or their needs.


5. When is the startle reflex triggered?


The startle reflex is triggered when there is a sudden, unexpected movement in your space. Although this may seem obvious, there are many instances where people have not been aware of it happening to them as it happens so quickly. 


The most common example of this occurs when somebody suddenly grabs your arm or leg or you see someone fall off a cliff and you jump because of that. When you do this, your brain sends a signal to your spinal cord which then triggers the startle reflex in the muscles and nerves in your arms and legs.


6. Signaling motor neurons connected to the startle reflex are very fast


There is a large network of neurons called collaterals from sensory receptors all over the body to motor neurons. These are like electrical wires that send signals from one part of your brain to another, also known as relay cells. The faster these signals travel between two parts of your brain, the more important that information is that it carries.


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