Animals and plants respond to stimuli for survival. In plants, IAA act as a plant growth factor and promotes growth in the shoots and inhibits growth in the roots. This causes phototropism and gravitropism as an effect.
Receptors are responsible to convert stimuli into action potentials. The Panician Corpuscle is an example of a pressure receptor within your skin to detect touch. As pressure is applied, it deforms the membrane with stretch-mediated sodium ion channels. This causes more Na+ ions to enter the sensory neurone, causing a generator potential which will cause an action potential if a threshold voltage is reached.
The heart is myogenic and contracts by itself. When the baroreceptors detect there is a low blood pressure in the carotid arteries, impulses are sent to the medulla oblongata. More impulses are sent along the sympathetic nerve to the SAN. The SAN spreads a wave of electrical activity which causes atrial systole. The atria is allowed the fill before ventricles contract. This is because electrical impulses are sent to the AVN, down the Bundle of His and Purkyne Fibres. This causes ventricular systole from the bottom up.