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The usage of word—reflex:
What it means when we say, I pulled out my hand from the hot flame reflexly?, "I moved my hand reflexly when an ant bit me?"
The above sentences give a meaning that you reacted to these situations without any thinking.
So the idea is that we act on something instantly without thinking or feeling any control of our reactions. However, we managed to respond to these changes in the environment with some actions.
How control and coordination achieved in dangerous situations?
To know how control and coordination achieved in dangerous situations, we must know,
  1. Which part of the brain is responsible for thinking?
  2. How the signals are transported from muscles to the brain and vice versa?
  3. What is input and output nerve?
  4. How a thinking process works?
  5. How long will it take for thinking and taking action in a situation?
The process of thinking and tissue:
The part of the brain responsible for thinking is the frontal lobe of the brain, which is at the front end of the skull. The tissue in this part of the brain responsible for thinking is called thinking tissue.
  • Thinking tissue is connected with nerves coming from all parts of the body. As a result, it receives signals about the environment from all over the body as electrical impulse. These nerves are called input nerves.
  • Since thinking tissue has to instruct the muscular tissues to take actions, nerves must carry these instruction signals back to different parts of the body as electrical impulse. These nerves are called output nerves.
Note: The process of thinking creates the output nerve impulses.

Thinking is a complex process, so it is bound to involve complicated interactions of nerve impulses from many neurons. Consequently, the thinking tissue consists of dense network of intricately arranged neurons.

Changes in environment
Signals are sent from the sensory organs to the thinking tissue of the brain
Process of thinking happens, and a decision will be taken
 Instruction signals are sent from the thinking tissue to muscular tissues
Muscular tissues move
Imagine these steps are happening in a dangerous situation. If you touch a hot flame with your hands, the required action is to move your hand instantly. Do you think the steps mentioned above involved in the process of thinking will move the hand muscles instantly? i.e., Thinking consciously about the pain and possibility of burn, and then moving your hand.
The entire process will take some time to move the hand muscles, which will cause heavy skin burn.
To avoid such consequence, human body or animal body is designed to act instantly in a dangerous situation.
Process of reflex action
Reflex arc:
The video explaining the working principle of reflex arc

In a dangerous situation like touching a hot flame, rather than thinking about the sensation of heat and the possibility of burn, the input nerves that detect heat will directly send signals to the output nerves that move muscles in a simpler way. In order to send the signals directly, both the input and output nerves are connected by means of a connection called as reflex arc.

Reflex action
Through the reflex arc, the process of detecting the input signal and responding to it by an output action might happen instantly. Such an output action is called as Reflex action.
Reflex action is a spontaneous automatic mechanical response to a stimulus without the will of the animal.
Location of reflex arc: Nerves from all over the body meet as a bundle in spinal cord on their way to the brain, and so the input and output nerves also first meet each other here. So, reflex arc connections are formed in the spinal cord.
At the same time, input signal about the information also goes to the brain.
Why reflex arc evolved?
In animals, the thinking process is not so fast. In fact, many animals don't have the complex neuron network required for thinking. So, the evolution of reflex arc results from an efficient way of functioning in the absence of true thought processes. Furthermore, after years of evolution, even after the emergence of true thought processes and complex neuron networks, such as humans, reflex arc continues to be more efficient for quick responses.
So, through reflex arc, control and coordination can be achieved in dangerous situations.