Editing Nervous system (section)

===Neural circuits and systems===
The basic neuronal function of sending signals to other cells includes a capability for neurons to exchange signals with each other. [[Neural network (biological)|Networks]] formed by interconnected groups of neurons are capable of a wide variety of functions, including feature detection, pattern generation and timing,<ref name=Dayan>{{Cite book |title=Theoretical Neuroscience: Computational and Mathematical Modeling of Neural Systems |publisher=MIT Press |year=2005 |isbn=978-0-262-54185-5 |vauthors=Dayan P, Abbott LF}}</ref> and there are seen to be countless types of information processing possible. [[Warren Sturgis McCulloch|Warren McCulloch]] and [[Walter Pitts]] showed in 1943 that even [[artificial neural network]]s formed from a greatly simplified mathematical abstraction of a neuron are capable of [[universal computation]].<ref name=McCullochPitts/>
[[File:Descartes-reflex.JPG|thumb|right|Illustration of pain pathway, from [[René Descartes]]'s ''Treatise of Man'']]

Historically, for many years the predominant view of the function of the nervous system was as a stimulus-response associator.<ref name=Sherrington1906/> In this conception, neural processing begins with stimuli that activate sensory neurons, producing signals that propagate through chains of connections in the spinal cord and brain, giving rise eventually to activation of motor neurons and thereby to muscle contraction, i.e., to overt responses. Descartes believed that all of the behaviors of animals, and most of the behaviors of humans, could be explained in terms of stimulus-response circuits, although he also believed that higher cognitive functions such as language were not capable of being explained mechanistically.<ref name=Descartes>{{Cite book |title=Passions of the Soul |author=Descartes R |publisher=Hackett |year=1989 |isbn=978-0-87220-035-7 |others=Voss S}}</ref> [[Charles Scott Sherrington|Charles Sherrington]], in his influential 1906 book ''The Integrative Action of the Nervous System'',<ref name=Sherrington1906>{{Cite book |author=Sherrington CS |title=The Integrative Action of the Nervous System |publisher=Scribner |year=1906 |url=https://books.google.com/books?id=6KwRAAAAYAAJ}}</ref> developed the concept of stimulus-response mechanisms in much more detail, and [[behaviorism]], the school of thought that dominated [[psychology]] through the middle of the 20th century, attempted to explain every aspect of human behavior in stimulus-response terms.<ref name=Baum>{{Cite book |author=Baum WM |year=2005 |title=Understanding behaviorism: Behavior, Culture and Evolution |publisher=Blackwell |isbn=978-1-4051-1262-8}}</ref>

However, experimental studies of [[electrophysiology]], beginning in the early 20th century and reaching high productivity by the 1940s, showed that the nervous system contains many mechanisms for maintaining [[Membrane potential#Cell excitability|cell excitability]] and generating patterns of activity intrinsically, without requiring an external stimulus.<ref name=Piccolino>{{Cite journal |author=Piccolino M |title=Fifty years of the Hodgkin-Huxley era |journal=Trends Neurosci. |volume=25 |issue=11 |pages=552–553 |date=November 2002 |pmid=12392928 |doi=10.1016/S0166-2236(02)02276-2 |s2cid=35465936}}</ref> Neurons were found to be capable of producing regular sequences of action potentials, or sequences of bursts, even in complete isolation.<ref name=Johnston>{{Cite book |title=Foundations of cellular neurophysiology |vauthors=Johnston D, Wu SM |publisher=MIT Press |year=1995 |isbn=978-0-262-10053-3}}</ref> When intrinsically active neurons are connected to each other in complex circuits, the possibilities for generating intricate temporal patterns become far more extensive.<ref name=Dayan/> A modern conception views the function of the nervous system partly in terms of stimulus-response chains, and partly in terms of intrinsically generated activity patterns—both types of activity interact with each other to generate the full repertoire of behavior.<ref name=Simmons>{{Cite book |title=Nerve cells and animal behaviour |url=https://archive.org/details/nervecellsanimal02essimm |url-access=registration |chapter=Ch 1.: Introduction |publisher=Cambridge Univ. Press |year=1999 |isbn=978-0-521-62726-9 |vauthors=Simmons PJ, Young D}}</ref>

====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.

====Intrinsic pattern generation====
Although stimulus-response mechanisms are the easiest to understand, the nervous system is also capable of controlling the body in ways that do not require an external stimulus, by means of internally generated rhythms of activity. Because of the variety of voltage-sensitive ion channels that can be embedded in the membrane of a neuron, many types of neurons are capable, even in isolation, of generating rhythmic sequences of action potentials, or rhythmic alternations between high-rate bursting and quiescence. When neurons that are intrinsically rhythmic are connected to each other by excitatory or inhibitory synapses, the resulting networks are capable of a wide variety of dynamical behaviors, including [[attractor]] dynamics, periodicity, and even [[chaos theory|chaos]]. A network of neurons that uses its internal structure to generate temporally structured output, without requiring a corresponding temporally structured stimulus, is called a [[central pattern generator]].

Internal pattern generation operates on a wide range of time scales, from milliseconds to hours or longer. One of the most important types of temporal pattern is [[circadian rhythm]]icity—that is, rhythmicity with a period of approximately 24 hours. All animals that have been studied show circadian fluctuations in neural activity, which control circadian alternations in behavior such as the sleep-wake cycle. Experimental studies dating from the 1990s have shown that circadian rhythms are generated by a "genetic clock" consisting of a special set of genes whose expression level rises and falls over the course of the day. Animals as diverse as insects and vertebrates share a similar genetic clock system. The circadian clock is influenced by light but continues to operate even when light levels are held constant and no other external time-of-day cues are available. The clock genes are expressed in many parts of the nervous system as well as many peripheral organs, but in mammals, all of these "tissue clocks" are kept in synchrony by signals that emanate from a master timekeeper in a tiny part of the brain called the [[suprachiasmatic nucleus]].