Reflex

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A reflex action is an automatic (involuntary) neuromuscular action elicited by a defined stimulus.^[1] In most contexts, especially involving humans, a reflex action is mediated via the reflex arc (although this is not always true in other animals, or in more casual usage of the term 'reflex'.)

Mechanism

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 learnt. For example, a person stepping on a sharp object would initiate the reflex action through the creation of a nociceptive stimulus within specialized sense receptors located in the skin tissue of the foot. The resulting stimulus would be transmitted through an afferent nerve to the spinal cord. This stimulus is usually processed by an interneuron to create an immediate response to nociception by initiating a motor response to withdraw from the pain-producing object. This retraction would occur as the sensation is arriving in the brain and producing the subjective perception of pain, which would result in a more cognitive evaluation of the situation.

Reflexes are tested as part of a neurological examination to assess damage to or functioning of the central and peripheral nervous system.

Reflexes may be trained, such as during repetition of motor actions during sport practice, or the linking of stimuli with autonomic reactions during classical conditioning.

Reaction time

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

In humans, reaction time to visual stimuli is typically 150 to 300 milliseconds.^[2]

Human reflexes

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

Reflex actions include:

Tendon reflexes and stretch reflexes

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.

• Biceps stretch reflex (C5, C6)
• Brachioradialis reflex (C5, C6)
• Triceps stretch reflex (C7, C8)
• Patellar reflex or knee-jerk reflex (L3, L4)
• Achilles reflex (S1, S2)
• Plantar reflex or Babinski reflex (L5, S1, S2)

While the reflexes above are stimulated mechanically, the term H-reflex refers to the analogous reflex stimulated electrically, and Tonic vibration reflex for those stimulated by vibration.

Reflexes involving cranial nerves

Reflexes in infants only

Main article: Primitive reflexes

Newborn babies have a number of other reflexes which are not seen in adults, referred to as primitive reflexes.^[3] These include:

• Asymmetrical tonic neck reflex (ATNR)
• Grasp reflex
• Hand-to-mouth reflex
• Moro reflex, also known as the startle reflex
• Sucking
• Symmetrical tonic neck reflex (STNR)
• Tonic labyrinthine reflex (TLR)

Other reflexes

Other reflexes found in the human nervous system include:

• Anocutaneous reflex
• Crossed extensor reflex
• Escape reflex
• Jaw jerk reflex
• Mammalian diving reflex
• Oculocardiac reflex
• Optokinetic reflex
• Photic sneeze reflex
• Scratch reflex
• Withdrawal reflex

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.

See also

• Automatic behavior
• Voluntary action

References

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