Reflex arc 569418 219685358 2008-06-16T12:14:12Z 99.255.34.152 /* Classic Example: The Patellar Reflex */ [[Image:ReflexArc1.jpg|right|thumb|250px|Patellar reflex. Note that this image includes an interneuron in the pathway of the patellar reflex for purposes of illustration. The inhibitory component of the reflex involving the hamstring muscle is not shown.]] A '''reflex arc''' is the [[neural pathway]] that mediates a [[reflex action]]. In higher animals, most sensory neurons do not pass directly into the [[brain]], but synapse in the [[spinal cord]]. This characteristic allows reflex actions to occur relatively quickly by activating spinal motor neurons without the delay of routing signals through the brain, although the brain will receive sensory input while the reflex action occurs. ==Monosynaptic vs. polysynaptic== When a reflex arc consists of only two neurons in an animal (one [[sensory neuron]] and one [[motor neuron]]), it is defined as '''monosynaptic'''. Monosynaptic refers to the presence of a single chemical synapse. In the case of peripheral muscle reflexes ([[patellar reflex]], [[achilles reflex]]), brief stimulation to the [[muscle spindle]] results in contraction of the agonist or effector muscle. By contrast, in '''polysynaptic''' reflex pathways, one or more interneurons connect [[afferent nerve|afferent]] ([[sensory system|sensory]]) and [[efferent nerve|efferent]] ([[motor system|motor]]) signals. All but the most simple reflexes are polysynaptic, allowing processing or inhibition of polysynaptic reflexes within the spinal cord. == The Patellar Reflex == [[Patellar reflex]]: when the [[patellar tendon]] is tapped just below the knee, the [[patellar reflex]] is initiated and the lower leg kicks forward (via contraction of the quadriceps). The tap initiates an [[action potential]] in a specialised structure known as a [[muscle spindle]] located within the quadriceps. This action potential travels to the spinal cord, via a sensory axon which chemically communicates by releasing glutamate (see [[synapse]]) onto a [[motor nerve]]. The result of this motor nerve activity is contraction of the [[quadriceps muscle]], leading to extension of the lower leg at the knee. The sensory input from the quadriceps also activates local interneurons that release the inhibitory neurotransmitter glycine onto motor neurons, blocking the innervation of the antagonistic (hamstring) muscle. The relaxation of the opposing muscle facilitates extension of the lower leg. ==Notes== It should be noted that 'simple' monosynaptic reflexes do also have additional components. As the sensory axon enters into the spinal cord, it sends out a ''collateral axon'' which synapses onto an ''inhibitory interneuron''. When activated, this IA inhibitory interneuron releases [[glycine]] which inhibits the motor activity of the antagonist muscle. The result is reinforced activity of the agonist muscle by removing tonic activity. In lower animals reflex interneurons do not necessarily reside in the spinal cord, for example as in the [[lateral giant neuron]] of [[crayfish]]. ==External links== <!-- Wikipedia is NOT a link depository. Considering all the commercial links on this site before (and the POV status that has ''still'' not been removed), links added to this entry will be watched RATHER CLOSELY. --> <!--===========================({{NoMoreLinks}})===============================--> <!--| DO NOT ADD MORE LINKS TO THIS ARTICLE. WIKIPEDIA IS NOT A COLLECTION OF |--> <!--| LINKS. If you think that your link might be useful, do not add it here, |--> <!--| but put it on this article's discussion page first or submit your link |--> <!--| to the appropriate category at the Open Directory Project (www.dmoz.org)|--> <!--| and link back to that category using the {{dmoz}} template. |--> <!--| |--> <!--| Links that have not been verified WILL BE DELETED. |--> <!--| See [[Wikipedia:External links]] and [[Wikipedia:Spam]] for details |--> <!--===========================({{NoMoreLinks}})===============================--> *[http://www.smi.auc.dk/~oka/phdsum.html Ole K. Andersen, SMI, Aalborg University - Physiological and Pharmacological modulation of the human nociceptive withdrawal reflex at smi.auc.dk] *[http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab9/Examples/exsomarc.htm Somatic Reflex Arc at vetmed.vt.edu] * {{eMedicineDictionary|Reflex+arc}} * {{Dorlands|a_56|12149511}} * [http://online.sfsu.edu/~psych200/unit5/54.htm Overview at sfsu.edu] * [http://bio.rutgers.edu/~gb102/lab_5/103ar.html Overview at rutgers.edu (with animation)] * {{MerckHome|06|077|c}} - "Physical Examination" * [http://www.wisc-online.com/objects/index_tj.asp?objid=AP11704 Tutorial at wisc-online.com] ==References== * Ganong, W.F. 2001. Review of Medical Physiology. McGraw-Hill Publishing, New York, p. 123. [[Category:Reflexes]] [[Category:Neurophysiology]] [[de:Reflexbogen (Physiologie)]] [[es:Arco reflejo]] [[pl:Łuk odruchowy]] [[zh:反射弧]]