First you must know that the entire human body relies on the nervous system for coordination and instruction. Information of the ouside world and workings of the body, travel in the form of nerve impulses to and from the brain and spinal cord along nerves. Nerve impulses can only travel in one direction, for example if you were to burn your finger the burn would register in one nerve and travel to the brain in that nerve (a sensory nerve), the brain would then send the information for the finger to move off of the heat source, through a different nerve (a motor nerve).
Cranial nerves are a group of 12 pairs of sensory, motor, or mixed (having separate sensory and motor fibres) nerves that connect with the brain stem and the lower parts of the brain.
Autonomic nerves are motor nerves only. These nerves regulate a variety of bodily functions. The autonomic nervous system is divided into sympathetic nerves and parasympathetic nerves, normally the tow counteract eachother (i.e. if one were to cause muscle contraction, the other would trigger muscle relaxation). Sympathetic nerves prepare the body for emergancies like psyical harm, lack of water, or temerature extreme. Parasympathetic nerves commonly do the opposite, by keeping the body running calmly and smoothly with a minimum outlay of energy. The most important parasympathetic nerve is the vagus (a.k.a. cranial nerve X) because it sends fibres to many parts of the body.
Pretty much all neurons have the same general structure. Commonly they have a round of pyramyd shaped cell body, with a branched "tree" of individual dendrites and a long tube called an axon. Some of these axons can be as small as a thousandth of a centimeter or a long as several metres. In some neurons, particularly sensory neurons, dendrite-like parts link directly with the axon instead of the cell body, and the cell body is connected to a branch of the axon.
Neurons connect with other neurons in a neural circuit through synapses. A typical synapse can be made between neurons, thus it's really, really small. The human nervous system is so incredibly complex that scientists have had extreme difficulty uncoving this "wiring" plan between all the neurons- there are over in the brain alone, so it's pretty BIG!
Nerves generate their own self- amplified electrical signals (these are the nerve impulses). The electrical sign of a nerve impulse is it's action potential (or AP). The Electrically charged ions are in varying concentrations inside and outside of the cell, this causes a voltage diffence on each side of the cell membrane. The voltage in the membrane of a "resting" neuron usually ranges from -50 to -100 mV (millivolts), in this condition the nerve membrane is polarized.
An action potential travels along a nerve at speeds of up to 10 metres per second. This voltage change and depolarization at one end of the axon "flows" to the next point, where the event is duplicated, and so on until the AP reached the axon terminals.
How are these APs created? Well scientists have a fairly clear idea of how APs form in a neuron. When a portion of the membrane is depolarized, special channels in it open and allow an inward rush of sodium ions, which are in higher concentration outside the cell. Then when the positively charged sodium ions flow into the negatively charged neuron interior, they cause further depolarization by making the interior electrically more positive. This, in turn, opens more channels, allowing more sodium ions to rush in. Once opened, however, the sodium channels slowly close in spite of the depolarization that originally made them open. Channels for outgoing potassium ions are also in the neuron membrane. They, too, open during depolarization but more slowly than the sodium channels. When the potassium channels finally open, positively charged potassium ions flow to the outside of the cell, where they are in lower concentration. As they leave the cell, they hasten the return of the neuron's voltage to its resting level, and the interior again becose electrically more negative than the exterior.
These are only the very of the nervous system, extensive research is still being done to discover more about the brain and the rest of the nervous system.
