Overview

The complexity and diversity in nervous systems is dependent on the interconnections between neurons, which rely on a limited number of different signals transited within the neurons to other neurons or to muscles and glands. The signals are produced and propagated by chemical ions that produce an electrical charge that moves along the neuron.

Neurons exist in a number of different shapes and sizes and can be classified by their morphology and function. The anatomist Camillo Golgi grouped neurons into two types; type I with long axons used to move signals over long distances and type II without axons. type I cells can be further divided by where the cell body or soma is located. The basic morphology of type I neurons, represented by spinal motor neurons, consists of a cell body called the soma, a long thin axon which is covered by the myelin sheath, one end has a tree branch like dendritic tree that receives signals from other neurons, the other end has branching terminals that release transmitter secretions into an area called the gap junction between the terminals and the dendrites of next neuron.

The anatomy and the properties of the surface membrane determine the behavior of a neuron. The surface membrane is not uniform over the entire length of a neuron, but is modified in specific areas, some regions secrete transmitter substances while other areas respond to transmitter molecules. Other areas of the neuron membrane have passive electrical properties that effect capacitance and resistance. Within the neuron membrane there are gated ion channels that vary in type, the membrane of the axons include fast response sodium channels that are voltage-gated and are used to send rapid signals. Neurons communicate by chemical and electrical synapses, in a process known as synaptic transmission. The fundamental process that triggers synaptic transmission is the action potential, a propagating electrical signal that is generated by exploiting the electrically excitable membrane of the neuron. This is also known as a wave of depolarization.

Fully differentiated neurons are permanently amitotic; however, recent research shows that additional neurons throughout the brain can originate from neural stem cells found in high concentrations in (but throughout the brain) the subventricular zone and subgranular zone through the process of neurogenesis.

History

The neuron's place as the primary functional unit of the nervous system was first recognized in the early 20th century through the work of the Spanish anatomist Santiago Ramón y Cajal. Cajal proposed that neurons were discrete cells that communicated with each other via specialized junctions, or spaces, between cells. This became known as the neuron doctrine, one of the central tenets of modern neuroscience. To observe the structure of individual neurons, Cajal used a silver staining method developed by his rival, Camillo Golgi. The Golgi stain is an extremely useful method for neuroanatomical investigations because, for reasons unknown, it stains a very small percentage of cells in a tissue, so one is able to see the complete micro structure of individual neurons without much overlap from other cells in the densely packed brain.