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What Is a Reflex Arc?

Tapping the petellar tendon with a reflex hammer will trigger the knee jerk reflex.
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  • Written By: Victoria Blackburn
  • Edited By: Bronwyn Harris
  • Last Modified Date: 03 April 2014
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A reflex is an automatic response to some kind of stimulus. As well as being automatic, these types of actions cannot be prevented. For example, if dust or dirt blows into someone's eye, that person cannot stop his or her eye from blinking, or if a piece of food moves into the windpipe, he or she cannot stop the coughing that results. Reflexes do not follow the same pathways as other nerve impulses. Instead, the nervous pathway for reflexes is called the reflex arc.

Usually, when cells within the body are stimulated, they send a message, called a nerve impulse, to the brain. The brain receives the message and then sends back another message in response to this initial stimulation. This takes place very quickly, but it is not automatic like the response seen with reflexes.

Reflexes can be spinal reflexes or brain reflexes. The difference in the two types of reflexes is where the reflex arc is found. With spinal reflexes, it occurs in the spinal cord and for brain reflexes, within the brain. No matter where the reflex arc is located, there is no conscious control involved. An example of a spinal reflex is the knee-jerk reaction and brain reflexes including blinking, coughing and iris contraction within the eye.

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The key difference between a reflex action and any other action is the involvement of the brain and the lack of conscious control. By following a reflex arc, nerve impulses travel along sensory neurons from the site of stimulation to the spinal cord or brain and then back to the area of the response along motor neurons. In some reflex arcs, the sensory neurons are connected to the motor neurons by connector neurons, but either way, there is no control by the brain.

The nervous pathway for the knee-jerk reaction is well-known and documented. It provides a clear example of the action of following a simple reflex arc. During the knee-jerk test, one leg is crossed over the other and kept completely relaxed. Sharply tapping the tendon just below the kneecap will result in the leg swinging upward.

The pathway for this reflex arc starts at a stretch receptor within the tendon. Hitting this receptor stimulates it, which causes it to send a nerve impulse along a sensory neuron to the spinal cord. Within the spinal cord, the nerve impulse passes from the sensory neuron to a motor neuron and travels back to the thigh muscle. When the impulse arrives at the thigh muscle, it causes it to contract and jerk the lower part of the leg upward. The person is aware that this is happening, so sensory impulses do travel from the spinal cord to the brain, but there is nothing that can be done to stop it happening.

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Discuss this Article

anon941474
Post 11

Do reflexes follow the sensory-interpret-motor function of the nervous system?

anon353801
Post 10

Reflex arcs offer the advantage of rapid, automatic responses to a particular stimulus. Such a response produced will prevent further injury to the individual as information is transmitted to the spinal cord before the brain, reducing reaction time and thus, the (mostly) negative effects of the stimulus.

anon337113
Post 9

When is the reflex arc useful in life?

aarons
Post 2

@bobcat60- I think that the difference may be in the reflex arc stimulus. The reflex arc is physically stimulated, like with the dirt in the eye, the food in the windpipe, or hammer on the tendon below the knee. A tick may be stimulated, but usually it is an outside stimulus, say stress in general, or something visual or audible.

bobcat60
Post 1

So, what is the difference in a reflex and a tick?

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