Reflex

A reflex, or reflex action, is an involuntary and nearly instantaneous movement in response to a stimulus.[1] [2] 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.[3]

Human reflexes

Myotatic reflexes

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.[4]

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.

Tendon reflex

A tendon reflex is the contraction of a muscle in response to striking its tendon.[4] The Golgi tendon reflex is the inverse of a stretch reflex.

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
Glabellar reflex V VII
Vestibulo-ocular reflex VIII III, IV, VI +
Gag reflex IX X

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.[5] These automatic reactions to stimuli enable infants to respond to the environment before any learning has taken place. They include:

Other reflexes

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.

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.[6] 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+.

Grade Description
0 Absent
1+ or + Hypoactive
2+ or ++ "Normal"
3+ or +++ Hyperactive without clonus
4+ or ++++ Hyperactive with clonus

See also

References

  1. ↑ Purves (2004). Neuroscience: Third Edition. Massachusetts, Sinauer Associates, Inc.
  2. ↑ "Definition of REFLEX". www.merriam-webster.com.
  3. ↑ "tendon reflex". TheFreeDictionary.com.
  4. 1 2 "tendon reflex". TheFreeDictionary.com.
  5. ↑ Neurologic Exam
  6. ↑ University of Florida > Neurologic Examination Retrieved on May 9, 2009
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