Reflex action

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This article is about biological reflex. For use in linguistics, see reflex (linguistics).

The mechanism of the reflex arc. Note that this image includs an interneuron in the monosynaptic patellar reflex for purposes of illustration.

A reflex action is an automatic (involuntary) neuromuscular action elicited by a defined stimulus. ¹ A reflex action is mediated via the reflex arc.

A reflex action or reflex is a biological control system linking stimulus to response and mediated by a reflex arc. Reflexes can be built-in or learned. For example, a person stepping on a sharp object would initiate the reflex action through the creation of a stimulus, (pain) within specialized sense receptors located in the skin tissue of the foot. The resulting stimulus would be transmitted through afferent, or sensory neurons to the spinal cord. This stimulus is usually processed by an interneuron to create an immediate response to pain by initiating a motor (muscular) response which is acted upon by muscles of the leg, retracting the foot away from the object. This retraction would occur as the pain sensation is arriving in the brain which would process a more cognitive evaluation of the situation.

1 Simple reflex
2 Conditioned reflex
3 Reaction time
4 Human reflexes
5 Significance
6 See also
7 External links
8 References

[edit] Simple reflex

An example is the escape reflex (e.g., the sudden withdrawal of a hand in response to a pain stimulus), or the patellar reflex (the jerking of a leg when the kneecap is tapped). Sensory cells (receptors) in the stimulated body part send signals to the spinal cord along a sensory nerve cell. Within the spine a reflex arc switches the signals straight back to the muscles of the body (in this case the arm or the leg) (effectors) via a motor nerve cell, and the muscle contracts (the arm or leg jerks upwards). In all but the most primitive reflexes, an interneuron in the spinal cord communicates signals from the sensory nerve to the motor nerve. Only two or three nerve cells are involved, and the brain is only aware of the initial reflexive response after it has taken place. Such reflex arcs are particularly common in animals, and have a high survival value, enabling organisms to take rapid action to avoid potential danger.

[edit] Conditioned reflex

A conditioned reflex involves the modification of a reflex action in response to experience (learning). A stimulus that produces a simple reflex response becomes linked with another, possibly unrelated, stimulus. For example, a dog may salivate (a reflex action) when it sees its owner remove a tin-opener from a drawer because it has learned to associate that stimulus with the stimulus of being fed.

[edit] Reaction time

For a reflex, reaction time or latency is the time from the onset of a stimulus until the organism responds.

Hold a dollar bill in your right hand, at about chest level. Have someone place his left hand around the middle of the bill with the thumb and index finger about an inch apart. No part of his hand may touch the bill. Now, tell him that when you let go of the bill he won't be able to catch it before it passes through his fingers.

This was an original problem that was stated and though it looks easy to catch the dollar bill, it's impossible to do it more than once in ten tries.

[edit] Human reflexes

grasp reflex

Reflex actions seen in adult humans include:

Accommodation reflex
Achilles reflex
Anocutaneous reflex
Biceps stretch reflex
Brachioradialis reflex
Corneal reflex (also known as the blink reflex)
Crossed extensor reflex
Gag reflex
Gastroc-Soleus reflex
Gladipudendal reflexes
H-reflex
Mammalian diving reflex
Patellar reflex (knee-jerk reflex)
Photic sneeze reflex
Plantar reflex (Babinski reflex)
Pupillary reflex
Quadriceps reflex
Salivation
Scratch reflex
Sneeze
Tendon reflex
Triceps stretch reflex
Vestibulo-ocular reflex
Withdrawal reflex
Yawn

Processes such as breathing, digestion and the maintenance of the heartbeat can also be regarded as reflex actions, according to some definitions of the term.

Newborn babies have a number of other reflexes which are not seen in adults, including:

suckling
hand-to-mouth reflex
Moro reflex, also known as the startle reflex
grasp reflex
Asymmetrical tonic neck reflex (ATNR)
Symmetrical tonic neck reflex (STNR)
Tonic labyrinthine reflex (TLR)

[edit] Significance

The deep tendon reflexes provide information on the integrity of the central and peripheral nervous system. Generally, decreased reflexes indicate a peripheral problem, and lively or exaggerated reflexes a central one.