Editing Nervous system (section)

====Reflexes and other stimulus-response circuits<!-- This section is linked from [[Pain#Evolutionary and behavioral role]] -->====
[[File:Nervous system organization en.svg|thumb|right|400px|Simplified schema of basic nervous system function: signals are picked up by sensory receptors and sent to the spinal cord and brain, where processing occurs that results in signals sent back to the spinal cord and then out to motor neurons]]

The simplest type of neural circuit is a [[reflex arc]], which begins with a [[sensory system|sensory]] input and ends with a motor output, passing through a sequence of neurons connected in [[Series and parallel circuits|series]].<ref name=KandelCh36/> This can be shown in the "withdrawal reflex" causing a hand to jerk back after a hot stove is touched. The circuit begins with [[sensory receptor]]s in the skin that are activated by harmful levels of heat: a special type of molecular structure embedded in the membrane causes heat to change the electrical field across the membrane. If the change in electrical potential is large enough to pass the given threshold, it evokes an action potential, which is transmitted along the axon of the receptor cell, into the spinal cord. There the axon makes excitatory synaptic contacts with other cells, some of which project (send axonal output) to the same region of the spinal cord, others projecting into the brain. One target is a set of spinal [[interneuron]]s that project to motor neurons controlling the arm muscles. The interneurons excite the motor neurons, and if the excitation is strong enough, some of the motor neurons generate action potentials, which travel down their axons to the point where they make excitatory synaptic contacts with muscle cells. The excitatory signals induce contraction of the muscle cells, which causes the joint angles in the arm to change, pulling the arm away.

In reality, this straightforward schema is subject to numerous complications.<ref name=KandelCh36/> Although for the simplest [[reflex]]es there are short neural paths from sensory neuron to motor neuron, there are also other nearby neurons that participate in the circuit and modulate the response. Furthermore, there are projections from the brain to the spinal cord that are capable of enhancing or inhibiting the reflex.

Although the simplest reflexes may be mediated by circuits lying entirely within the spinal cord, more complex responses rely on signal processing in the brain.<ref name=KandelCh38/> For example, when an object in the periphery of the visual field moves, and a person looks toward it many stages of signal processing are initiated. The initial sensory response, in the retina of the eye, and the final motor response, in the [[oculomotor nuclei]] of the [[brainstem]], are not all that different from those in a simple reflex, but the intermediate stages are completely different. Instead of a one or two step chain of processing, the visual signals pass through perhaps a dozen stages of integration, involving the [[thalamus]], [[cerebral cortex]], [[basal ganglia]], [[superior colliculus]], [[cerebellum]], and several brainstem nuclei. These areas perform signal-processing functions that include [[Feature detection (nervous system)|feature detection]], [[perception|perceptual]] analysis, [[memory recall]], [[decision-making]], and [[motor planning]].<ref name=KandelCh39/>

[[feature detection (nervous system)|Feature detection]] is the ability to extract biologically relevant information from combinations of sensory signals.<ref name=KandelCh21/> In the [[visual system]], for example, sensory receptors in the [[retina]] of the eye are only individually capable of detecting "points of light" in the outside world.<ref name=KandelCh25/> Second-level visual neurons receive input from groups of primary receptors, higher-level neurons receive input from groups of second-level neurons, and so on, forming a hierarchy of processing stages. At each stage, important information is extracted from the signal ensemble and unimportant information is discarded. By the end of the process, input signals representing "points of light" have been transformed into a neural representation of objects in the surrounding world and their properties. The most sophisticated sensory processing occurs inside the brain, but complex feature extraction also takes place in the spinal cord and in peripheral sensory organs such as the retina.