The central nervous system which includes the brain and spinal cord is protected by the presence of three membranous connective tissue coverings collectively called the meninges. Individually, going from outside to inside they are called the dura mater which means "tough mother", the arachnoid membrane, and the pia mater.
A cross section of the spinal cord reveals that it consists of a core of grey matter (sensory) surrounded by a thicker section of white matter (motor). The pattern produced by the grey matter resembles a butterfly with outspread wings.
The upper and lower wings of grey matter refers to posterior and anterior horns, respectively. Neurons with large cell bodies located in the anterior horns give rise to motor fibers that pass out through spinal nerves to skeletal muscle.
The majority of the neurons in the grey matter of the spinal cord connect neurons with other neurons forming a nerve pathway. The white matter of the spinal cord has myelinated nerve fibers called nerve tracts that provide a two-way system of communication between the brain and body areas outside the nervous system. The tracts that conduct impulses from the body to the brain with sensory information are called ascending tracts and those that conduct motor impulses from the brain to muscles and glands are called descending tracts.