The central nervous system consists of the brain and spinal cord . The brain plays a central role in the control of most bodily functions, including awareness, movements, sensations, thoughts, speech, and memory. Some reflex movements can occur via spinal cord pathways without the participation of brain structures. The spinal cord is connected to a section of the brain called the brainstem and runs through the spinal canal. Cranial nerves exit the brainstem as nerve roots exit the spinal cord to both sides of the body. The spinal cord carries signals or messages back and forth between the brain and the peripheral nerves. Cerebrospinal fluid surrounds the brain and the spinal cord, which circulates within the cavities (called ventricles) of the central nervous system. The leptomeninges surround the brain and the spinal cord. The cerebrospinal fluid circulates between 2 meningeal layers called the pia matter and the arachnoid (or pia-arachnoid membranes). The outer, thicker layer serves the role of a protective shield and is called the dura matter.
The basic unit of the central nervous system is the neuron (nerve cell). Billions of neurons allow the different parts of the body to communicate with each other via the brain and the spinal cord. A fatty material called myelin coats nerve cells to insulate them and to allow nerves to communicate quickly.