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'.
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.
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
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:
Other reflexes found in the central nervous system include:
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.
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+.
|1+ or +
|2+ or ++
|3+ or +++
||Hyperactive without clonus
|4+ or ++++
||Hyperactive with clonus