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From Wikipedia

Reflex arc

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. There are two types of reflex arc - autonomic reflex arc (affecting inner organs) and somatic reflex arc (affecting muscles). 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) 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. In lower animals reflex interneurons do not necessarily reside in the spinal cord, for example as in the lateral giant neuron of crayfish.


From Yahoo Answers

Question:What is a reflex arc? How does it function?

Answers:- a stimulus is detected by a receptor cell, which synapses with a sensory neurone. - The sensory neurone carries the impulse from site of the stimulus to the central nervous system (the brain or spinal cord), where it synapses with an interneurone. - The interneurone synapses with a motor neurone, which carries the nerve impulse out to an effector, such as a muscle, which responds by contracting. The neural path of a reflex is called reflex arc.

Question:A. from effector to dorsal root to interneuron to sensory receptors B. from sensory receptors to ventral root to interneuron to effector C. from sensory receptors to dorsal root to interneuron to ventral root to effector D. from sensory receptor to dorsal root to interneuron to brain. please help. i'm trying to study for finals, and I can't seem to find this anywhere.

Answers:I'm pretty sure its C because sensory information goes through the dorsal root and passes to the ventral root thru an interneuron before leaving.

Question:Hey guys, well we are studying the nervous system. However,i am a little confused. I don't get the difference between the reflex circuit and just the normal nervous system response. They both have the sensor, the sensory neuron, the inter neuron, the motor neuron and finally the effector. What is the difference? Am i missing something here cause they look the same. I know one is voluntary and the other is not. Is there more to it? Thank yoouu!

Answers:A couple of examples: "Normally," when you touch something like a pencil, the sensory data is transmitted along the arm, up the spinal column, to the brain, is processed, then fed back to the muscles through the spine and arm to write. Reflexively, when something painful (e.g. hot) is touched, the sensory data is transmitted along the arm, but then immediately fed back to muscles to withdraw from the stimulus before processing by the brain. The withdrawal motion is uncontrolled, sometimes violent, and the brain is not even "aware" of it until after it happens. It is possible to overcome certain reflexes through training. I'm not what the state-of-art scientific understanding is of the whole process. The generally accepted model used to be that the feed back reflex was generated at the spine, but I'm not sure about the current best model.

Question:Which of the following sequences describes the passage of a nerve impulse through a simple reflex arc in humans? a. receptor, sensory neuron, motor neuron, association neuron, effector. b. receptor, motor neuron, association neuron, sensory neuron, effector. c. receptor, sensory neuron, association neuron, motor neuron, effector. d. effector, motor neuron, association neuron, sensory neuron, receptor e. receptor, association neuron, sensory neuron, motor neuron, effector.

Answers:c. receptor, sensory neuron, association neuron, motor neuron, effector.