Which part of our body is responsible for doing actions without the brains order?For example, If we hurt our hand means suddenly tears will come...

The only thing that your body does without direction of
some sort from the brain is called a reflex. Reflexes occur in a variety of areas, and
they do involved the nervous system, but they bypass the brain itself. In fact,
creatures without a brain, for example clams and oysters, still possess
reflexes.

In your example, a person who hurts their hand
will stimulate a withdrawal reflex. The pain of the injury stimulates sensors which pass
the message to sensory nerves. These nerves go up the arm and into the spinal cord,
where the message is passed to an outgoing nerve fiber which causes the arm muscles to
contract, pulling the hand in toward the body to remove it from the source of injury.
Watery eyes, crying, or calling out come from the response of the sympathetic nervous
system, which provides the fight-or-flight response when we are hurt or
frightened.