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'.)

Contents

Mechanism

Reaction time

Human reflexes

Reflexes involving cranial nerves

Reflexes in infants only

Other

See also

References

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 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.

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.''

Human reflexes

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

'Name'	'Sensory'	'Motor'
Pupillary reflex	II	III
Accommodation reflex	II	III
Corneal reflex, also known as the blink reflex	V	VII
Caloric reflex test/Vestibulo-ocular reflex	VIII	III, IV, VI
Gag reflex	IX	X

Reflexes in infants only

Newborn babies have a number of other reflexes which are not seen in adults.^[2]. These include:

★ sucking

★ 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)
See also main article at primitive reflexes.

Other

★ Anocutaneous reflex

★ Conditioned reflex

★ Crossed extensor reflex

★ Escape reflex

★ Mammalian diving reflex

★ Photic sneeze reflex

★ Scratch reflex

★ Withdrawal reflex

★ Oculocardiac reflex

★ Optokinetic reflex

★ Jaw jerk 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

★ Automatism

★ Automatism (case law)

★ Autonomic nervous system

★ Reflexology

★ Reflex hammer

★ Sensory system

★ Voluntary action

★ Blink

★ Neurological examination

References

1. Purves (2004). ''Neuroscience: Third Edition''. Massachusetts, Sinauer Associates, Inc.
2.