(a) Reflex action. A reflex action is defined as a spontaneous, automatic and mechanical response to a stimuli without the will of a living being. In such actions there is no involvement of the brain. All reflex actions are conveyed through the spinal cord by a path called reflex arc.
(b) Synapse. A microscopic gap between a pair of adjacent neurons over which nerve impulse pass when going from one neuron to the next is called a synapse.
(c ) Phototropism. It is the response of the plant parts to the external stimulus of light. The stem shows positive phototropism as the stem of the plant grows in the direction of light while root shows negative phototropism as the root of the plant grows away from the light . This growth is controlled by the auxin hormone.
