An overview of the major structures of the nervous system is necessary to discuss its functional pathways. The structure of the nervous system is divided into two parts: the central nervous system, which is mainly the brain and spinal cord, and the peripheral nervous system, which is almost everything else.
Some parts of the olfactory (smell) and visual pathways outside the brain are also central nervous system tissue.
The peripheral nervous system is mostly made up of nerves, which contain bundles of axons from motor, sensory, and autonomic neurons. The peripheral nervous system also contains ganglia, which are small lumps attached to many nerves that contain neuron somas.
Each spinal nerve is formed from anterior and posterior spinal nerve roots that are attached to the spinal cord, which then fuse to become spinal nerves that pass through the spine to the periphery. Efferent axons exit the spinal cord in the anterior roots, and afferent axons enter the spinal cord in the posterior roots. Posterior roots have attached ganglia, called posterior root ganglia.
The cranial nerves pass from the intracranial space (the space inside the skull) through the skull to the periphery; some of these also have attached ganglia.
Branches of most cranial and spinal nerves get progressively smaller as they continue branching and spread out through the tissues in the periphery of the body.
The spinal cord is divided into 31 horizontal levels, each of which has bilateral spinal nerves.
There are 12 bilateral pairs of cranial nerves, which are attached to the brain or upper spinal cord.
They are referred to by name or number (traditionally with Roman numerals): I is olfactory, II is optic, III is oculomotor, IV is trochlear, V is trigeminal, VI is abducens, VII is facial, VIII is vestibulocochlear, IX is glossopharyngeal, X is vagus, XI is accessory, and XII is hypoglossal.
The tissue of the central nervous system is divided into areas called white matter, which contains many myelinated axons, and areas called gray matter, which contains many neuron somas. The spinal cord has gray matter on the inside and white matter on the outside. On axial sections, the spinal cord gray matter is shaped like a butterfly, with bilateral anterior and posterior parts called horns. Some spinal cord levels also have small lateral gray horns. The spinal cord white matter is divided into posterior, lateral, and anterior parts called columns.
The brain is divided into the cerebrum superiorly, the brainstem inferior to the cerebrum and superior to the spinal cord, and the cerebellum posterior to the brainstem.
The cerebrum is divided into bilateral cerebral hemispheres.
The brainstem is divided into the midbrain inferior to the cerebrum, the pons inferior to the midbrain, and the medulla inferior to the pons.
The brain has more gray matter on the outside, called cortex, and more white matter on the inside. The parts of the brain covered in cortex are the cerebrum, which is called the cerebral cortex, and the cerebellum, which is called the cerebellar cortex. The terms cortical and subcortical are often used as shorthand to describe structures in the cerebral cortex versus structures “under” the cerebral cortex in the deep cerebrum or brainstem. Subcortical areas of gray matter, located deep in the cerebrum, cerebellum, or brainstem, which are collections of neuron somas, are mostly called nuclei.
Bundles of axons in the central nervous system are called tracts, and the axons in a tract usually carry similar types of information. In addition to neurons carrying motor, sensory, or autonomic information, the central nervous system also contains many interneurons involved in either the lower or higher neural functions.
There are loosely-organized areas of gray matter deep in the brainstem called the reticular formation, in addition to more discrete areas of gray matter called nuclei.
Some of the subcortical nuclei deep in the cerebrum are grouped together into more complex structures, including the particularly important nuclear groups called the thalamus, the hypothalamus, and the basal ganglia.
The cerebral cortex is divided into structural areas called the frontal, parietal, temporal, and occipital lobes, as well as an area called the insula.
The cerebral cortex is also divided into functional areas, with areas called primary cortex and areas called association cortex. Primary cortical areas perform more basic processing of motor or sensory information. Association cortical areas perform more complex processing of one kind of motor or sensory information, or they process multiple types of motor or sensory information, or they perform higher neural functions.