Nerves in General

Neuron (Wiki) 

A neuron is a special type of cell that is found in the bodies of most animals (all members of the group Eumetazoa, to be precise—this excludes only sponges and a few other very simple animals). The features that define a neuron are electrical excitability and the presence of synapses, which are complex membrane junctions used to transmit signals to other cells.

File:Complete neuron cell diagram en.svg

"Neurons are highly specialized for the processing and transmission of cellular signals. Given the diversity of functions performed by neurons in different parts of the nervous system, there is, as expected, a wide variety in the shape, size, and electrochemical properties of neurons. For instance, the soma of a neuron can vary from 4 to 100 micrometers in diameter.[1]
  • The soma is the central part of the neuron. It contains the nucleus of the cell, and therefore is where most protein synthesis occurs. The nucleus ranges from 3 to 18 micrometers in diameter.[2]
  • The dendrites of a neuron are cellular extensions with many branches, and metaphorically this overall shape and structure is referred to as a dendritic tree. This is where the majority of input to the neuron occurs.
  • The axon is a finer, cable-like projection which can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. The axon carries nerve signals away from the soma (and also carries some types of information back to it). Many neurons have only one axon, but this axon may—and usually will—undergo extensive branching, enabling communication with many target cells. The part of the axon where it emerges from the soma is called the axon hillock. Besides being an anatomical structure, the axon hillock is also the part of the neuron that has the greatest density of voltage-dependent sodium channels. This makes it the most easily-excited part of the neuron and the spike initiation zone for the axon: in electrophysiological terms it has the most negative action potential threshold. While the axon and axon hillock are generally involved in information outflow, this region can also receive input from other neurons.

Evolution of the mechanisms of connection between neurons: electrical, mixed, and chemical synapses.  
Investigation of the mechanisms of transmission of stimuli in synapses of isolated perfused spinal cord of cyclostomes, amphibia, reptiles, and mammals demonstrated that the ratio between electrical and chemical synapses decreased progressively in favor of the latter in the transition from primitive toward more highly developed forms. Electrical transmission was not detected in synapses of spinal cords of reptiles and mammals. On the basis of the data, the result of analysis of elementary synaptic processes in synapses uniting electrical and chemical mechanisms of transmission and dendrodendrite electronic interdependences of the motor neurons, a hypothesis is formulated that the number of electrical connections characterizes the union of similar neurons, while in a sequential union of nerve cells of various functions and origins, there are mixed or chemical synapses. The possible cellular mechanisms which are the basis of this phenomenon are discussed."