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{{BioPsy}}
 
{{BioPsy}}
   
A '''reflex arc''' is the [[neural pathway]] that mediates a [[reflex action]]. In higher animals, these pathways do not pass through the [[brain]], but synapse in the [[spinal cord]]. This characteristic allows reflex actions to occur relatively quickly.
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[[Image:ReflexArc1.jpg|right|thumb|500px|Patellar reflex. Note that this image includes an interneuron in the pathway of the patellar reflex for purposes of illustration.]]
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A '''reflex arc''' is the [[neural pathway]] that mediates a [[reflex action]]. In higher animals, these pathways do not pass through the [[brain]], but synapse in the [[spinal cord]]. This characteristic allows reflex actions to occur relatively quickly by avoiding the delay of routing signals through the brain, although the brain will receive sensory input while the reflex action occurs.
   
If a reflex arc consists of only two neurons (one [[sensory|sensory neuron]] and one [[motor neuron]]), it is defined as monosynaptic. 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. While the reflex generation of these pathways may be initiated by [[nociceptor|nociceptive]] input, extensive processing of polysynaptic reflexes can take place within the spinal cord.
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When a reflex arc consists of only two neurons (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.
   
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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.
   
 
== Classic Example: The Patellar Reflex ==
 
== Classic Example: The Patellar Reflex ==
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[[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 (see [[synapse]]) with 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.
   
[[Image:ReflexArc1.jpg|right|thumb|500px|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. The tap initiates an [[impulse]] in the [[sensory neuron]] in the leg, which travels to the spinal cord, through the relay, or [[interneuron]], and into the motor neuron where it stimulates the [[effector organ]], the leg muscle to contract.
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==Notes==
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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.
   
[[Category:Reflexes]]
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In lower animals reflex interneurons do not necessarily reside in the spinal cord, for example as in the [[lateral giant neuron]] of [[crayfish]].
[[Category:Neurophysiology]]
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==References==
   
 
==External links==
 
==External links==
*[http://www.smi.auc.dk/~oka/phdsum.html Ole K. Andersen, SMI, Aalborg University - Physiological and Pharmacological modulation of the human nociceptive withdrawal reflex]
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*[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]
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*[http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab9/Examples/exsomarc.htm Somatic Reflex Arc at vetmed.vt.edu]
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* {{eMedicineDictionary|Reflex+arc}}
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* {{Dorlands|a_56|12149511}}
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* [http://online.sfsu.edu/~psych200/unit5/54.htm Overview at sfsu.edu]
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* [http://bio.rutgers.edu/~gb102/lab_5/103ar.html Overview at rutgers.edu (with animation)]
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* {{MerckHome|06|077|c}} - "Physical Examination"
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* [http://www.wisc-online.com/objects/index_tj.asp?objid=AP11704 Tutorial at wisc-online.com]
   
Reference:
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==Reference==
Ganong, W.F. 2001. Review of Medical Physiology. McGraw-Hill Publishing, New York, p. 123.
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* Ganong, W.F. 2001. Review of Medical Physiology. McGraw-Hill Publishing, New York, p. 123.
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[[Category:Reflexes]]
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[[Category:Neurophysiology]]
   
 
[[de:Reflexbogen (Physiologie)]]
 
[[de:Reflexbogen (Physiologie)]]
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[[es:Arco reflejo]]
 
[[zh:反射弧]]
 
[[zh:反射弧]]
 
{{enWP|Reflex arc}}
 
{{enWP|Reflex arc}}

Latest revision as of 12:49, February 2, 2007

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ReflexArc1

Patellar reflex. Note that this image includes an interneuron in the pathway of the patellar reflex for purposes of illustration.

A reflex arc is the neural pathway that mediates a reflex action. In higher animals, these pathways do not pass through the brain, but synapse in the spinal cord. This characteristic allows reflex actions to occur relatively quickly by avoiding the delay of routing signals through the brain, although the brain will receive sensory input while the reflex action occurs.

When a reflex arc consists of only two neurons (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 (sensory) and efferent (motor) signals. All but the most simple reflexes are polysynaptic, allowing processing or inhibition of polysynaptic reflexes within the spinal cord.

Classic Example: The Patellar Reflex Edit

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 (see synapse) with 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.

NotesEdit

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.

ReferencesEdit

External linksEdit

- "Physical Examination"

ReferenceEdit

es:Arco reflejo zh:反射弧

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