Peripheral Nervous System
Nerve fibers that exit the brainstem and spinal cord become part of the peripheral nervous system. Cranial nerves exit the brainstem and function as peripheral nervous system mediators of many functions, including eye movements, facial strength and sensation, hearing, and taste.
The optic nerve is considered a cranial nerve but it is generally affected in a disease of the central nervous system known as multiple sclerosis, and, for this and other reasons, it is thought to represent an extension of the central nervous system apparatus that controls vision. In fact, doctors can diagnose inflammation of the head of the optic nerve by using an ophthalmoscope, as if the person's eyes were a window into the central nervous system.
Nerve roots leave the spinal cord to the exit point between two vertebrae and are named according to the spinal cord segment from which they arise (a cervical eight nerve root arises from cervical spinal cord segment eight). Nerve roots are located anterior with relation to the cord if efferent (for example, carrying input toward limbs) or posterior if afferent (for example, to spinal cord).
Fibers that carry motor input to limbs and fibers that bring sensory information from the limbs to the spinal cord grow together to form a mixed (motor and sensory) peripheral nerve. Some lumbar and all sacral nerve roots take a long route downward in the spinal canal before they exit in a bundle that resembles a horse's tail, hence its name, cauda equina.
The spinal cord is also covered, like the brain, by the pia matter and the arachnoid membranes. The cerebrospinal fluid circulates around the pia and below the outer arachnoid, and this space is also termed the subarachnoid space. The roots of the cauda equina and the rootlets that make up the nerve roots from higher segments are bathed in cerebrospinal fluid. The dura surrounds the pia-arachnoid of the spinal cord, as it does for the brain.
The neuroanatomical basis for multiple brain functions is oversimplified in the above summary. A good example is the neuroanatomical substrate for memory function. Damage to multiple areas of the brain can affect memory. These include structures such as the frontal and temporal lobes, the thalamus, the cerebellum, the putamen, mamillary bodies and fornix, and a convolution above the corpus callosum known as the cingulate gyrus. These structures are variably involved in complex processes such as the storing, processing, or retrieval of memories.