A reflex action, also known as a reflex, is an involuntary and almost instant movement in response to stimulus. 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'.)
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.
For a reflex, reaction time
or latency is the time from the onset of a stimulus until the organism
In animals, reaction time to visual stimuli is typically 150 to 300 milliseconds.
Reflex actions include:
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 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)
- 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
Newborn babies have a number of other reflexes which are not seen in adults, referred to as primitive reflexes. These include:
Other reflexes found in the human nervous system include:
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.