The nervous system can be divided into two groups: the central nervous system (CNS), which comprises of the brain the spinal cord, and the peripheral nervous system (PNS), which consists of the nerves. Nervous tissue consists of two types of cells: the nerve cells and supporting glial cells (or simply glia). Glia have different types and outnumber nerve cells. In the CNS, astrocytes provide structural support for neurons, regulate the extracellular concentrations of neurotransmitters and ions, and facilitate information transfer at synapses. Radial glia is critical in the development of the nervous system. Both astrocytes and radial glia can act as stem cells to generate neurons and additional glia. Ependymal cells promote the circulation of cerebrospinal fluid (CSF) through their cilia, while microglia protect against microorganisms in the brain. Oligodendrocytes deposit myelin on some neurons’ axons. In the PNS, the primary supporting glial cells are the Schwann cells, which deposit the myelin sheath and promote neural repair (Campbell and Reece 2008, pp. 858, 1064-1068).
Nerve cells, or neurons, operate by generating electric signals that are relayed from cell to cell. Neurons vary in shape and size, but their basic organization includes three parts: cell body, dendrites, and axon. The nucleus and the organelles are located in the cell body. Dendrites are highly branched extensions that receive signals from sensory receptors and other neurons. The axon, also called nerve fiber, is a single long process extending from the cell body to the target cells and is responsible for signal transmission. The portion of the axon where it joins the cell body is called the initial segment or axon hillock, and this is where the signals that travel down the axon are generated. Some axons are covered by a myelin sheath, layers of lipid-rich multilamellar membrane deposited by neuroglia called neurolemmocytes or Schwann cells in the PNS. Gaps between adjacent sections of myelin are called neurofibril nodes or nodes of Ranvier. Each branched end of an axon transmits information to another cell at a specialized junction called a synapse. A synaptic terminal is a part of the axon branch that forms this junction. Chemical messengers called neurotransmitters pass signals from the transmitting neuron to the receiving cell. A presynaptic cell can be a neuron, muscle, or gland cell that transmits the signal and the recipient of this signal is called the postsynaptic cell (Vander, Sherman and Luciano 2001, pp. 176-177; Mader 2004, p. 142; Campbell and Reece 2008, p. 1048).
Figure 1. Structural organization of neurons (Cambell and Reece 2008, p. 1049).
Changes in the membrane potential of neurons produce electric signals called action potentials or nerve impulses that are conducted along neurons' axons to process and transmit information. When the nerve fiber is active, a series of changes in polarity occurs across the axon’s membrane. First, the membrane becomes depolarized, wherein the inside of the axon becomes more positive compared to the outside. Then, it becomes repolarized and the inside becomes negative again. Action potentials are generated through the opening and closing of channels that allow the movement of ions in and out of the membrane (Mader 2004, p. 143; Campbell and Reece 2008, p. 1053).
Figure 2. Generation of action potential (Norries and Siegfried 2011).
The conduction speed of action potential is affected by several factors, one of which is the axon diameter. Wider axons conduct impulses more rapidly than narrower ones. The physical principle behind this is that resistance to electrical current flow is inversely proportional to the cross-sectional area of a conductor (in this case the axon). Thus, in wider axons, the resulting depolarization can spread farther along the interior and brings more distant regions of the membrane to the threshold sooner. The myelin sheath in vertebrate axons is an evolutionary adaptation that allows rapid conduction of impulses despite axons being narrow. It provides electrical insulation, and has the same effect as increasing the axon's diameter. Consequently, myelinated axons conduct impulses (through saltatory conduction) much faster than unmyelinated axons (Campbell and Reece 2008, pp. 1055-1056).
References
Campbell, NA & Reece, JB 2008, Biology, 8th edn, Pearson Education, California, CA.
Norries, M & Siegfried, DR 2011, Anatomy and Physiology for Dummies, 2nd edn, John Wiley & Sons, New Jersey, NJ.
Mader, SS 2004, Understanding Human Anatomy & Physiology, 5th edn, McGraw-Hill, New York, NY.
Vander, AJ, Sherman, JH & Luciano, DS 2001, Human Physiology, 8th edn, McGraw-Hill, New York, NY.
