A reflex, or reflex action, is an involuntary and nearly instantaneous movement in response to a stimulus. A reflex is made possible by neural pathways called reflex arcs which can act on an impulse before that impulse reaches the brain. The reflex is then an automatic response to a stimulus that does not receive or need conscious thought.
Reflexes in the Human body can be classified by: A] Complexity of the circuit: - Monosynaptic Reflex - Polysynaptic Reflex. B] Response : - Somatic Reflex - Autonomic Reflex. C] Development : - Innate Reflex - Aquired Reflex. D] Processing site : - Spinal Reflex - Cranial Reflex.
The myotatic reflexes (also known as deep 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. A stretch reflex is the contraction of a muscle in response to its lengthwise stretch.
- 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.
A tendon reflex is the contraction of a muscle in response to striking its tendon. The Golgi tendon reflex is the inverse of a stretch reflex.
Reflexes involving cranial nerves
|Pupillary light 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 +|
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. 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)
- Palmomental reflex
- 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 found in the central nervous system include:
- Abdominal reflexes (T6-L1)
- Anocutaneous reflex (S2-S4)
- Cough reflex
- Cremasteric reflex (L1-L2)
- Diving reflex
- Muscular defense
- Photic sneeze reflex
- Scratch reflex
- Startle reflex
- Withdrawal 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.
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. 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 +||Hypoactive|
|2+ or ++||"Normal"|
|3+ or +++||Hyperactive without clonus|
|4+ or ++++||Hyperactive with clonus|
- Purves (2004). Neuroscience: Third Edition. Massachusetts, Sinauer Associates, Inc.
- "Definition of REFLEX". www.merriam-webster.com.
- "tendon reflex". TheFreeDictionary.com.
- "Neurologic Exam". fpnotebook.com.
- University of Florida > Neurologic Examination Retrieved on May 9, 2009