Reflex

A reflex action, differently known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus.^[1] Scientific use of the term "reflex" refers to a behavior that is mediated via the reflex arc; this does not apply to casual uses of the term "reflex".

Human reflexes

Stretch reflexes

The stretch reflexes (often called deep tendon reflexes, though not to be confused with Golgi tendon reflexes) provide information on the integrity of the central nervous system and peripheral nervous system. Generally, decreased reflexes indicate a peripheral problem, and lively or exaggerated reflexes a central one.

• Biceps reflex (C5, C6)
• Brachioradialis reflex (C5, C6, C7)
• Extensor digitorum reflex (C6, C7)
• Triceps reflex (C6, C7, C8)
• Patellar reflex or knee-jerk reflex L2, L3, L4)
• Ankle jerk reflex (Achilles reflex) (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 to vibration.

Reflexes involving cranial nerves

Reflexes usually only observed in human infants

Grasp reflex

Newborn babies have a number of other reflexes which are not seen in adults, referred to as primitive reflexes.^[2] These automatic reactions to stimuli enable infants to respond to the environment before any learning has taken place. They include:

• Asymmetrical tonic neck reflex (ATNR)
• Hand-to-mouth reflex^{[citation needed]}
• Moro reflex, also known as the startle reflex
• Palmar grasp reflex
• Rooting reflex
• Sucking reflex
• Symmetrical tonic neck reflex (STNR)
• Tonic labyrinthine reflex (TLR)

Other reflexes

Other reflexes found in the central nervous system include:

• Abdominal reflexes (T6-L1)
  • Gastrocolic reflex
• Anocutaneous reflex (S2-S4)
• Cremasteric reflex (L1-L2)
• Mammalian diving reflex
• Muscular defense
• Scratch reflex
• Startle reflex
• Withdrawal reflex
  • Crossed extensor reflex

Many of these reflexes are quite complex requiring a number of synapses in a number of different nuclei in the CNS (e.g., the escape reflex). Others of these involve just a couple of synapses to function (e.g., the 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.

Grading

In medicine, reflexes are often used to assess the health of the nervous system. Doctors will typically grade the activity of a reflex on a scale from 0 to 4.^[3] While 2+ is considered normal, some healthy individuals are hypo-reflexive and register all reflexes at 1+, while others are hyper-reflexive and register all reflexes at 3+.

See also

• List of reflexes (alphabetical)
• All-or-none law
• Automatic behavior
• Involuntary action
• Voluntary action
• Preflexes

References