All of the body systems do not function independently of one another. Instead, they are linked by the nervous system, which connects all organs, muscles, glands, and tissues with the spinal cord and the brain.
The nervous system itself has been divided into two distinct parts, the central nervous system and the peripheral nervous system. The peripheral nervous system, as you will read, is further divided into the Somatic and Autonomic Nervous Systems.
The central nervous system is comprised of the brain and the spinal cord. All actions that occur within the human body are executed from here. When a stimulus is detected by any of the sensory organs (eyes, ears, and nose) or from another source (nerve in your arm) it is here that the body decides how to deal with the stimulus. The central nervous system relies on information gathered from the peripheral nervous system to make split second decisions, with or without input from your brain. In fact, many split-second reactions are done completely in the spinal cord, having bypassed the brain completely!
The peripheral nervous system relies on nerves to gather information about changes in environment both inside and outside the body, and relies on a completely different set to send this information in the form of electrochemical messages to the central nervous system. Sensory nerves receive information from their environment and translate these stimuli to messages for the central nervous system. Motor nerves are relied upon to carry instructions back from the central nervous system.
There are several different kinds of receptors, which receive information from their environment and translate them into electrical impulses, which can be understood by the brain. For instance, the vision receptors in your eyes translate light waves your eyes capture, and turn those light waves into electrical (nerve) impulses that your brain interprets as an image. In your ears, a sound receptor turns sound waves into nerve impulses. In your nose and on your tongue another set of entirely different receptors allow your brain to interpret chemical stimuli in your environment as smells and tastes.