This is the central nervous system. It consists of the brain and the spinal cord.
Tracts are bundles of axons that go up and down the spinal cord. Ascending tracts start from the periphery and go up the spinal cord to the brain. Descending tracts start from the brain and go down the spinal cord. Nerves are bundles of axons outside of the central nervous system. Tracts and nerves are made up of the same “material;” the difference is solely based on their location in the body.
Sensory information (which is made from various forms of energy) comes in from the outside world and stimulates receptors. Receptors send this information to the spinal cord through nerves. The information is then carried to the brain up the spinal cord through the ascending tracts. The brain integrates this information and eventually sends motor information down the spinal cord through the descending tracts. This information than reaches the muscles and glands though the nerves.
The ascending, sensory system is also called the afferent system while the descending, motor/glandular system is also called the effector system. Muscles and glands are often called effectors.