The sensory system is responsible for detecting stimuli from the outside world and transferring nervous impulses to the correct portion of the brain or spinal column to allow the body to react. The sensory system consists of the eyes, ears, nose, tongue, skin and their associated nerves.
Six senses are commonly accepted as being accessible to humans and many animals: sight, hearing, taste, smell, touch and proprioception. That last sense is the ability to know where the parts of the body are in relation to one another at all times. Each sensory organ has nerve receptors attached to it that are designed to transmit signals. For example, the eyes contain photoreceptors for light, the nose and tongue contain chemoreceptors for chemicals, and the skin contains mechanoreceptors for movement, pressure and related sensations. Sensory neurons operate on the all-or-nothing principle; they are either on or off. No varying levels of signal exist. Instead, a stronger stimulus causes more of the receptors to fire or the same amount of receptors to fire more rapidly.
When the brain receives sensory input, it sends the appropriate response along motor neuron pathways to respond to the stimulus. For example, an excessively bright light causes the brain to direct the pupil of the eye to contract and let in less light.