Anatomy of the nervous system

The basic structural and functional unit of the nervous system is the nerve cell or neuron, illustrated at figure .a ^4.1 . All neurons have a cell body which contains the usual cellular organelles common to all cells in the body. Most nerve cells have processes called dendrites, which act like antennae, in that they receive input to the cell. Most neurons also have a single long process called an axon, which is capable of transmitting a pulse of electricity from the cell body to some distant target either in the brain or the periphery. These axons may be quite long, up to a meter or more for the axons connecting the spinal cord to the foot. Axons usually break up into terminal branches near their target. These branches end in swellings which make a specialized contact with the target cell. If the target cell is another neuron, the swelling is called a bouton, and the specialized contact a synapse. If the target is a muscle fiber, the bouton is a motor endplate and the synapse is a neuro-muscular junction.

  

Figure 4.1: The neurons. a. a neuron. b. a myelinated fiber. c. a myelin sheath cell. d. terminology for the central and peripheral nervous systems.

The nervous system also contains cells which are not neurons and which do not directly participate in the task of sending and receiving electrical signals. These supporting cells are called glia. We are particularly interested in those that form myelin sheaths around axons in the central and peripheral nervous systems.
Indeed, axons are generally not naked, as in figure .a. Rather, they are wrapped into an insulating material called myelin. The presence of a myelin sheath around an axon increases the velocity at which it conducts a nerve impulse down its length. The myelin sheath (see figure .b) is formed by flattened out cells that wrap themselves jelly-roll style around the axon. In the central nervous system the cells that form the myelin sheath are called oligodendroglia; in the peripheral nervous system they are simply called axon sheath cells. Figure .c is a highly schematic drawing of a flattened axon sheath cell. To the right of it is a cross-section of a myelinated axon, on which one can see how the axon sheath cell wraps around the axon.
The sheath itself is essentially composed of flattened cell membrane, with all of the cytoplasm squeezed out except in the outermost layer. The major component of a cell membrane is the phospholipid bilayer. With many layers of membrane stacked on top of one another, it has a fatty appearance due to the presence of this phospholipid. Myelinated axons therefore have a glistening white appearance in the central and peripheral nervous systems, and are referred to as white matter. Areas containing mainly cell bodies tend to lack myelin and are referred to as gray matter. The terminology for both the central and the peripheral nervous system is found at figure .d.
A nerve is a bundle of axons traveling together in the periphery. If the nerve contains sensory axons only, it is called a sensory nerve. If it contains motor axons (going to muscles) only, it is called a motor nerve. Most nerves in the body contain both sensory and motor axons and are therefore called mixed nerves.
Most axons in any nerve are myelinated, which gives nerves their glistening white appearance. In addition, there are some connective tissue elements associated with nerves (see figure ): individual axons are enveloped in a connective tissue wrapping called endoneurium. Bundles, or fascicles, of axons are wrapped in a connective tissue covering called perineurium. The nerve as a whole is enveloped in a connective tissue sheath called the epineurium.

  

Figure 4.2: A nerve