Neurons consist of a cell body (or soma), which contains the cell nucleus, short processes called dendrites for receiving input from other neurons, and long processes called axons, which conduct signals away from the cell body (eTable 9.2). Depending on their location, neuronal morphology can be quite variable. The majority of mammalian neurons are multipolar, indicating that there are several dendrites from one end and a single axon that branches extensively at its terminus (eFig. 9.7). Some additional neuronal types are bipolar, unipolar, and pseudounipolar (eFig. 9.8).
To prevent the loss of linear signal propagation, glial cells form a phospholipid-based layer of insulation called the myelin sheath along the length of the axon (eFig. 9.7). The myelin sheath is formed by oligodendrocytes in the CNS and Schwann cells in the PNS. Interspersed between the segments of myelin are exposed segments of the axon called nodes of Ranvier, which have a large population of voltage-gated ion channels. Presence of the ion channels facilitates rapid conduction of the action potential (a transient voltage change in the axonal membrane) from node to node in a process called saltatory conduction (eFig. 9.7).
Functionally, the nervous system is organized into a somatic nervous system and visceral nervous system. The somatic nervous system consists of nerves that carry conscious sensation from peripheral regions back to the CNS and nerves that exit the CNS to innervate voluntary (skeletal) muscles. In contrast, the visceral nervous system consists of nerves that carry sensory information into and motor (autonomic) innervation out of the CNS to regulate homeostatic functions. Further discussion of the somatic and visceral nervous systems will be presented within the context of the subsequent “Spinal Cord” section.
Part II: Brain
Externally, the outer surface of the brain, or cerebral cortex, is composed of six layers of cell bodies referred to as gray matter. Internally, the myelinated axonal processes of these cells extend into the cerebral hemispheres. Because of the whitish appearance of these large bundles of myelinated axons, they are referred to as white matter. In the brain, gray matter is predominantly located on the cortical surface and the white matter runs deep inside the cerebral hemispheres; the opposite is true for the spinal cord, where the white matter is superficial to the gray matter. Topographically, the surface of the cerebral hemispheres has a series of elevations called gyri and infoldings referred to as sulci, both of which significantly increase the surface area of the brain.
Structurally, each cerebral hemisphere is divided into four major anatomical lobes: frontal, parietal, occipital, and temporal (eFig. 9.9A). The frontal lobes are located anteriorly and are separated from the more posterior parietal lobe by the central sulcus (sulcus of Rolando) (eFig. 9.9A). Laterally, the frontal lobe is separated from the temporal lobe by the lateral sulcus (fissure of Sylvius). Although there is no specific demarcation between the parietal and occipital lobe laterally, along the medial aspect of the hemispheres the two lobes are separated by the parieto-occipital sulcus (eFig. 9.9B). Along the midline, the cerebral hemispheres are separated from one another by the longitudinal fissure (interhemispheric fissure, sagittal fissure). Concealing a small area of cortex called the insula laterally are portions of the frontal, parietal, and temporal lobes collectively referred to as the operculum (Latin for “lid”) (eFig. 9.10). The insula represents fusion of the telencephalon and diencephalon and can be seen by gently prying open the lateral sulcus.