Neurons are very important to life and as such are extraordinarily protected. The CNS is encased in bone with three layers of protective coverings underneath the skull cap. In addition, the brain and spinal cord are surrounded and cushioned by cerebral spinal fluid (CSF). The brain has a lower specific gravity than the CSF so it actually floats in the skull. Beyond the physical barriers there are also physiological barriers. Neurons in the CNS have cells that support them known as neuroglial cells. There are specific cell types that nourish the neurons, insulate them electrically from one another and protect them from toxins and microbiological invaders. They create, along with the less permeable capillaries in the brain, what is known as the blood-brain barrier which acts to keep toxic substances and microbiological invaders out of the CNS but also makes delivering medications to the brain problematic.
Figure 4- Schwann cells & Nodes of Ranvier
While those neuroglial cells are important in the CNS we are concerned with nerves in this course and they are in the PNS. In the PNS each nerve fiber, whether dendrite or axon, is surrounded by a layer of connective tissue known as the endoneurium. These fibers are collected into bundles known as fascicles and surrounded by a connective tissue sheath known as the perineurium. If this is reminding you of the nomenclature used for the tissue surrounding muscle fibers you have already deduced that the entire nerve has a connective tissue sheath called the epimysium. Nerves go one step further and have a series of cells that wrap around the individual fibers. These cells are known as Schwann cells and come in two types depending on whether they contain a fatty substance known as myelin. Myelin is used as an electrical insulator and its presence in the Schwann cells increases the speed of nerve conduction. When myelin surrounds the fiber, depolarization of the membrane is restricted only to the small gaps known as the nodes of Ranvier where one Schwann cell ends and the next begins. This causes the electrical signal to leap along the fiber from gap to gap rather than flowing along the whole length of the fiber. This greatly speeds conduction on these fibers.
There are other fibers that do not have myelin surrounding them and these transmit the action potential more slowly. This difference in transmission rates is the basis of the gate control theory of pain perception. We use this by applying pressure to the injection point prior to a palatal injection to lessen the pain of the needle insertion. The pressure stimulates nerves that have a slow conduction path but act to inhibit the quicker pain signals that the needle will cause. Pressure at the time of, or after, the injection is much less effective as the pain signal is already through the gate.