What makes up the central nervous system?

The central nervous system consists of the brain and spinal cord.  All motor neurons, sensory neurons, cranial nerves, and spinal nerves outside of the brain and spinal cord are considered to be parts of the peripheral nervous system.  The central nervous system is responsible for integrating information that it receives from incoming information from sensory neurons, processing that information, and then sending signals to effectors (muscles and glands) through motor neurons.

Within the central nervous system the neurons are organized into nuclei and tracts whereas they are organized into nerves and ganglia in the peripheral nervous system.  Nuclei (and ganglia in the peripheral nervous system) are groups of cell bodies and axon terminals.  It is within these structures that many neurons synapse or nearly meet to send signals to each other.  Tracts (and nerves in the peripheral nervous system) are bundles of axons of neurons.  Within the brain they may serve to connect regions of the brain as the corpus callosum, a major set of tracts, connects the right and left hemispheres of the brain.  Ascending tracts in the spinal cord deliver sensory information to the brain and descending tracts in the spinal cord deliver motor signals from the motor cortex of the cerebrum to effectors throughout the body.