An insect's nervous system is a network of specialized cells (called neurons) that serve as an "information highway" within the body. These cells generate electrical impulses (action potientials) that travel as waves of depolarization along the cell's membrane. Every neuron has a nerve cell body (where the nucleus is found) and filament-like processes (dendrites, axons, or collaterals) that propagate the action potential. Signal transmission is always unidirectional -- moving toward the nerve cell body along a dendrite or a collateral and away from the nerve cell body along an axon.
Neurons are usually divided into three categories, depending on their function within the nervous system:
1. Afferent (sensory) neurons -- these bipolar or multipolar cells have dendrites that are associated with sense organs or receptors. They always carry information toward the central nervous system.
2. Efferent (motor) neurons -- unipolar cells that conduct signals away from the central nervous system and stimulate responses in muscles and glands.
3. Internuncial (association) neurons -- unipolar cells (often with several collaterals and/or branching axons) that conduct signals within the central nervous system.
Individual nerve cells connect with one another through special junctions, called synapses. When a nerve impulse reaches the synapse, it releases a chemical messenger (neurotransmitter substance) that diffuses across the synapse and triggers a new impulse in the dendrite(s) of one or more connecting neurons. Acetylcholine, 5-hydroxytryptamine, dopamine, and noradrenaline are examples of neurotransmitters found in both vertebrate and invertebrate nervous systems.
Nerve cells are typically found grouped in bundles. A nerve is simply a bundle of dendrites or axons that serve the same part of the body. A ganglion is a dense cluster of interconnected neurons that process sensory information or control motor outputs.
The Central Nervous System
Like most other arthropods, insects have a relatively simple central nervous system with a dorsal brain linked to a ventral nerve cord that consists of paired segmental ganglia running along the ventral midline of the thorax and abdomen. Ganglia within each segment are linked to one another by a short medial nerve (commissure) and also joined by intersegmental connectives to ganglia in adjacent body segments. In general, the central nervous system is rather ladder-like in appearance: commissures are the rungs of the ladder and intersegmental connectives are the rails. In more "advanced" insect orders there is a tendency for individual ganglia to combine (both laterally and longitudinally) into larger ganglia that serve multiple body segments.
An insect's brain is a complex of six fused ganglia (three pairs) located dorsally within the head capsule. Each part of the brain controls (innervates) a limited spectrum of activities in the insect's body:
Protocerebrum: The first pair of ganglia are largely associated with vision; they innervate the compound eyes and ocelli.
Deutocerebrum: The second pair of ganglia process sensory information collected by the antennae.
Tritocerebrum: The third pair of ganglia innervate the labrum and integrate sensory inputs from proto- and deutocerebrums. They also link the brain with the rest of the ventral nerve cord and the stomodaeal nervous system (see below) that controls the internal organs. The commissure for the tritocerebrum loops around the digestive system, suggesting that these ganglia were originally located behind the mouth and migrated forward (around the esophagus) during evolution.
Located ventrally in the head capsule (just below the brain and esophagus) is another complex of fused ganglia (jointly called the subesophageal ganglion). Embryologists believe this structure contains neural elements from the three primitive body segments that merged with the head to form mouthparts. In modern insects, the subesophageal ganglion innervates not only mandibles, maxillae, and labium, but also the hypopharynx, salivary glands, and neck muscles. A pair of circumesophageal connectives loop around the digestive system to link the brain and subesophageal complex together.
In the thorax, three pairs of thoracic ganglia (sometimes fused) control locomotion by innervating the legs and wings. Thoracic muscles and sensory receptors are also associated with these ganglia. Similarly, abdominal ganglia control movements of abdominal muscles. Spiracles in both the thorax and abdomen are controlled by a pair of lateral nerves that arise from each segmental ganglion (or by a median ventral nerve that branches to each side). A pair of terminal abdominal ganglia (usually fused to form a large caudal ganglion) innervate the anus, internal and external genitalia, and sensory receptors (such as cerci) located on the insect's back end.

The Stomodaeal Nervous System
An insect's internal organs are largely innervated by a stomodaeal (or stomatogastric) nervous system. A pair of frontal nerves arising near the base of the tritocerebrum link the brain with a frontal ganglion (unpaired) on the anterior wall of the esophagus. This ganglion innervates the pharynx and muscles associated with swallowing. A recurrent nerve along the anterio-dorsal surface of the foregut connects the frontal ganglion with a hypocerebral ganglion that innervates the heart, corpora cardiaca, and portions of the foregut. Gastric nerves arising from the hypocerebral ganglion run posteriorly to ingluvial ganglia (paired) in the abdomen that innervate the hind gut.
In comparison to vertebrates, an insect's nervous system is far more de-centralized. Most overt behavior (e.g. feeding, locomotion, mating, etc.) is integrated and controlled by segmental ganglia instead of the brain. In some cases, the brain may stimulate or inhibit activity in segmental ganglia but these signals are not essential for survival. Indeed, a headless insect may survive for days or weeks (until it dies of starvation or dehydration) as long as the neck is sealed to prevent loss of blood!