Reflex arc
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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.
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.
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[edit] Classic Example: 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 (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.
[edit] 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.
[edit] References
[edit] External links
- Ole K. Andersen, SMI, Aalborg University - Physiological and Pharmacological modulation of the human nociceptive withdrawal reflex at smi.auc.dk
- Somatic Reflex Arc at vetmed.vt.edu
- Dictionary at eMedicine Reflex+arc
- Dorlands/Elsevier a_56/12149511
- Overview at sfsu.edu
- Overview at rutgers.edu (with animation)
- Merck Home 06-077c
- "Physical Examination"
[edit] Reference
- Ganong, W.F. 2001. Review of Medical Physiology. McGraw-Hill Publishing, New York, p. 123.de:Reflexbogen (Physiologie)
| This page uses content from the English-language version of Wikipedia. The original article was at Reflex arc. The list of authors can be seen in the page history. As with Psychology Wiki, the text of Wikipedia is available under the GNU Free Documentation License. |
