Reflex arc

Reflex arc

A reflex arc is a neural pathway that controls an action reflex. 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.

There are two types of reflex arc - autonomic reflex arc (affecting inner organs) and somatic reflex arc (affecting muscles).

Contents

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 (sensory) and efferent (motor) signals. All but the most simple reflexes are polysynaptic, allowing processing or inhibition of polysynaptic reflexes within the spinal cord.

The Patellar Reflex (knee jerk)

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.

In lower animals reflex interneurons do not necessarily reside in the spinal cord, for example as in the lateral giant neuron of crayfish.

References

  • Ganong, W.F. 2001. Review of Medical Physiology. McGraw-Hill Publishing, New York, p. 123.

External links


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См. также в других словарях:

  • reflex arc — n the complete nervous path that is involved in a reflex * * * the nervous circuit involved in a reflex, being at its simplest a sensory nerve with a receptor, linked at a synapse in the brain or spinal cord with a motor nerve, which supplies a… …   Medical dictionary

  • reflex arc — n. Physiol. the entire nerve path involved in a reflex action …   English World dictionary

  • reflex arc — the nervous circuit involved in a reflex, being at its simplest a sensory nerve with a receptor, linked at a synapse in the brain or spinal cord with a motor nerve, which supplies a muscle or gland. In a simple reflex (such as the patellar… …   The new mediacal dictionary

  • reflex arc — re′flex arc n. phl the nerve pathways followed by an impulse during a reflex • Etymology: 1880–85 …   From formal English to slang

  • reflex arc — noun Date: 1882 the complete nervous path involved in a reflex …   New Collegiate Dictionary

  • reflex arc — Physiol. the nerve pathways followed by an impulse during a reflex. [1880 85] * * * …   Universalium

  • reflex arc — noun The neural pathway that mediates a reflex …   Wiktionary

  • reflex arc — noun Physiology the nerve pathway involved in a reflex action …   English new terms dictionary

  • reflex arc — /riflɛks ˈak/ (say reefleks ahk) noun the path taken by neural impulses within the body from stimulus to reflex action …   Australian-English dictionary

  • reflex arc — noun the neural path of a reflex • Hypernyms: ↑neural structure …   Useful english dictionary


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