Reflex
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus.[1] In most contexts, in particular those involving humans, reflex actions are mediated via the reflex arc; this is not always true in other animals, nor does it apply to casual uses of the term 'reflex'.
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Reaction time
For a reflex, reaction time or latency is the time from the onset of a stimulus until the organism responds.
In animals, reaction time to visual stimuli is typically 150 to 300 milliseconds.[2]
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 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 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
| Name | Sensory | Motor |
| Pupillary light reflex | II | III |
| Accommodation reflex | II | III |
| Jaw jerk reflex | V | V |
| Corneal reflex, also known as the blink reflex | V | VII |
| Vestibulo-ocular reflex | VIII | III, IV, VI + |
| Gag reflex | IX | X |
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.[3] These include:
- Asymmetrical tonic neck reflex (ATNR)
- Grasp reflex
- Hand-to-mouth reflex
- Moro reflex, also known as the startle reflex
- Rooting reflex
- Sucking
- Symmetrical tonic neck reflex (STNR)
- Tonic labyrinthine reflex (TLR)
Other reflexes
Other reflexes found in the central nervous system include:
- Abdominal reflexes (T6-L1)
- 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 (eg., 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
The activity of a reflex is usually graded on a scale from 0 to 4:[4]
| Grade | Description |
| 0 | Absent |
| 1+ or + | Hypoactive |
| 2+ or ++ | "Normal" |
| 3+ or +++ | Hyperactive without clonus |
| 4+ or ++++ | Hyperactive with clonus |
See also
- List of reflexes (alphabetical)
- All-or-none law
- Automatic behavior
- Involuntary action
- Voluntary action
- Preflexes
References
- ↑ Purves (2004). Neuroscience: Third Edition. Massachusetts, Sinauer Associates, Inc.
- ↑ "Human Benchmark: Reaction Time Statistics". http://www.humanbenchmark.com/tests/reactiontime/stats.php. Retrieved 2007-10-11.
- ↑ Neurologic Exam
- ↑ University of Florida > Neurologic Examination Retrieved on May 9, 2009
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