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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 are control and coordination achieved in dangerous situations?
To know how control and coordination are achieved in dangerous situations, we must know,
  1. Which part of the brain is responsible for thinking?
  2. How the signals transported from muscles to the brain and vice versa?
  3. What are input and output nerve?
  4. How does a thinking process work?
  5. How long will it take to think and take action in a situation?
The process of thinking:
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 impulses. 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 impulses. 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 a dense network of intricately arranged neurons.
Types of reflexes:
The spinal cord monitors and controls the majority of reflex actions, which is why they are also known as spinal reflexes.
There are two types of reflexes.
  1. Simple or basic reflexes
  2. Acquired or conditioned reflexes
Simple or basic reflexes: These reflexes generate responses which are automatic and unlearned. Many of the acts we do daily are simply reflexes. We do these things without even thinking about them.
Winking of eyes as dust particles enter, sneezing, coughing, yawning
Acquired or conditioned reflexes: Practice and learning have resulted in these responses.
Playing the harmonium by striking a specific key when viewing a music note is an example of conditioned reflexes that took conscious training effort to develop.
When we touch a boilingpan, the stimulus of heat is detected by heat receptors, also known as thermoreceptors, in our hands. This stimulation (heat) causes a sensory cell to fire an impulse.
The message is transmitted or conveyed to the spinal cord by the sensory neuron.
The signal is interpreted by the spinal cord, and the impulse is sent to a relay neuron, which then sends it to a motor neuron.
 From the spinal cord to our arm, motor neurons transmit commands.
Our arm muscles contract, and we quickly remove our hands from the pan.
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 a consequence, the human body or animal body is designed to act instantly in a dangerous situation as explained in the above flowchart. From the flowchart, we can say the effector organ is the hand muscle which responds to the stimuli heat. The movement of a muscle to a nerve impulse will be explained in the next object.
Process of reflex action
Reflex arc:
The video explaining the working principle of a 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  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 the 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 do not 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 the reflex arc, control and coordination can be achieved in dangerous situations.