Edmund has spent the last ten years working in clinical research. He has written many articles on human anatomy and physiology.
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Reflex Action and Reflex Arc
Reflex is a special ability that evolution gifted us to facilitate our survival. Whenever part of your body comes in contact with an object capable of causing you harm, you tend to quickly withdraw that part of the body. This happens before your brain gets the time it needs to process the threat.
If you accidentally touch a hot pot on your stove while cooking, you would involuntarily (and nearly instantaneously) snatch your hand away from the pot. This response is called a ‘reflex action'.
Contact with the hot pot triggers the start of a series of events in the body to evoke a response.
At the point of contact with the hot pot, skin receptors quickly send nerve impulses (electrical) to the spinal cord (central nervous system) via sensory neurons. In the spinal cord, the impulses are processed and a response is relayed back.
In the spinal cord, the interneurons (also known as relay neurons) make the connections between the sensory neurons (bringing the message from hand) and the correct motor neurons (taking the response back to the hand). It would not be useful if the response was sent to the wrong part of the body—in this case, a response sent to the leg wouldn't be too helpful as the stimulus is coming from the hand.
From the interneurons, the response is relayed to the motor neurons which project out of the spinal cord to stimulate your muscles (effector) to contract, causing you to snatch your hand away from the hot pot. This pathway taken by nerve impulses to elicit a response is known as a ‘reflex arc’.
This process happens so fast that the response occurs before the message reaches the brain. This results to a quicker time-to-response as the thinking process of the brain may be relatively time consuming.
Components of a Reflex Arc
In the above example, the stimulus is the contact with the hot pot. This contact causes a nerve impulse that will travel to the spinal cord via the sensory neurons. Another example of a stimulus is an object (e.g. an insect) approaching your eye causing you to blink before you know it.
These neurons carry the nerve impulse to the spinal cord. Similar to the interneuron and motor neuron, sensory neurons receive incoming impulses at the dendrites. The impulses move away from the cell body along the axon to the synaptic terminal where the impulse is sent to the next neuron (the interneuron) with the help of a neurotransmitter (acetylcholine).
The spinal cord
Interneurons (also known as relay neurons) are fully contained in the central nervous system (the spinal cord and the brain). The interneuron serves as the connection between the sensory neurons and the motor neurons.
The synapse is a tiny space between two neurons. When an impulse gets to the end of one neuron and has to be sent down the next neuron, the synapse acts as a bridge. The signal arrives at the end of one neuron (close to the synapse) as an electrical signal, crosses the synapse as a chemical signal (with the help of a neurotransmitter known as acetylcholine released by the synaptic vesicles at the synaptic terminal) and continues as an electrical signal in the next neuron.
In the 'hot pot' example above, motor neurons send nerve impulses away from the central nervous system to effector organs or muscle fibers. This causes the muscle fibers to contract, resulting in you snatching your hand away from the hot pot.
If the stimulus was at the sight of an insect flying towards your eyeball, then the motor neurons would relay the response back to your eyelids (to close) to protect them from the approaching threat.
This occurs when the motor neurons deliver nerve impulses from the spinal cord to the part of the body where a response to the stimulus is needed. In the above example, the response is the muscle contraction to quickly pull the hand away from the hot pot. In the second example, the response is to blink to prevent the insect from making contact with the eye.
A reflex arc, an article on how a stimulus elicits a response
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Unknown on April 16, 2020:
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hi on February 03, 2020:
Hi the same time as the 50th anniversary
Vic on October 27, 2019:
Hi I'm new
Janeth on October 15, 2019:
Katalo j on October 07, 2019:
Bhat rashid on July 05, 2019:
Thanks this article had helped me lot
Kanav sharma on May 22, 2019:
What will happen if you touch the hot pot ?
It is very useful to every one thanks to the author
पटेल Sunil on March 26, 2019:
It is very useful and easy language for me...
Sheemal on March 09, 2019:
Nganda Nadesh on February 12, 2019:
Wow so wonderful thanks, it has really helped me with my studies.
Satya prakash on January 06, 2019:
kasirye William on December 14, 2018:
this is really very good article, l 'd teach my fellows about reflex arc.
Rekha on December 02, 2018:
It is very useful.
Amon on November 14, 2018:
Anam on October 28, 2018:
Nithya shree on October 20, 2018:
Very good it helps me know in class 5
yug gandhi on July 23, 2018:
Very good , help full for my precious class 10
Poonam Arora on June 15, 2018:
Thanks this help me in my science holiday homework
finally on June 04, 2018:
thx so much
Nalungwe b on May 29, 2018:
Thanks u make my work to be complete
Yeee on April 26, 2018:
Helped me a lot with my Science homework. Thanks!!!
.... on March 23, 2018:
But good article! I forgot to say that for the last comment I submitted.
.... on March 23, 2018:
God gave us reflexes, not "evolution"
cc on February 07, 2018:
Namrata on January 14, 2018:
Sukhpreet kaur on December 07, 2017:
Can u show me this answer in small size
Nathan Allen. on November 20, 2017:
Thanks, This piece of writing helped me with my Science work. Yes, this post by a year 10 student. Anyway, it helped a lot. Thank you again.
Nathan Allen. on November 20, 2017:
Thanks,it helped me with my Science work.
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zzzzzzzz on November 17, 2017:
Gujjar bhai on October 31, 2017:
Debasrita das on August 06, 2017:
Bibin on August 06, 2017:
Why do you immediately remove your hand on touching a very hot object?
Pankaj Kumar Thakur on July 05, 2017:
This is a very helpful
Yoram Tumwiine on June 22, 2017:
Edmund Custers (author) on June 19, 2017:
Hi Arjun, thank you reading and leaving a comment. I'll try to answer your question. Pain that is still detected much later is most likely because there was significant damage done during the contact. In this case nerve fibers would still be sending messages to the brain from the location of the injury. The brain is kind of telling you to treat the affected area with care.