In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Improved General Reflex Diagram

Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs via neural pathways in the nervous system called reflex arcs. A stimulus initiates a neural signal, which is carried to a synapse. The signal is then transferred across the synapse to a motor neuron which evokes a target response. These neural signals do not always travel to the brain, so many reflexes are an automatic response to a stimulus that does not receive or need conscious thought. Many reflexes are fine-tuned to increase organism survival and self-defense. This is observed in reflexes such as the

startle reflex In animals, including humans, the startle response is a largely unconscious defensive response to sudden or threatening stimuli, such as sudden noise or sharp movement, and is associated with negative affect.Rammirez-Moreno, David. "A computation ...

, which provides an automatic response to an unexpected stimuli, and the feline righting reflex, which reorients a cat's body when falling to ensure safe landing. The simplest type of reflex, a short-latency reflex, has a single synapse, or junction, in the signaling pathway. Long-latency reflexes produce nerve signals that are transduced across multiple synapses before generating the reflex response.

Types of human reflexes

Myotatic reflexes

The myotatic or muscle stretch reflexes (sometimes known as ''deep tendon reflexes'') provide information on the integrity of the central nervous system and peripheral nervous system. This information can be detected using electromyography (EMG). 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 The extensor digitorum reflex is tested as part of the neurological examination to assess the sensory and motor pathways within the C6 and C7 spinal nerves. It is also known as Braunecker-Effenberg reflex, or BER. Testing The test is performed by ...

(C6, C7) * Triceps reflex (C6, C7, C8) * Patellar reflex or knee-jerk reflex ( L2, L3, L4) *

Ankle jerk reflex The ankle jerk reflex, also known as the Achilles reflex, occurs when the Achilles tendon is tapped while the foot is dorsiflexed. It is a type of stretch reflex that tests the function of the gastrocnemius muscle and the nerve that supplies it. ...

(Achilles reflex) ( S1, S2) While the reflexes above are stimulated mechanically, the term

H-reflex The H-reflex (or Hoffmann's reflex) is a reflectory reaction of muscles after electrical stimulation of sensory fibers ( Ia afferents stemming from muscle spindles) in their innervating nerves (for example, those located behind the knee). The H ...

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. The

Golgi tendon reflex The Golgi tendon reflex (also called inverse stretch reflex, autogenic inhibition, tendon reflex) is an inhibitory effect on the muscle resulting from the muscle tension stimulating Golgi tendon organs (GTO) of the muscle, and hence it is self-ind ...

is the inverse of a stretch reflex.

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

Asymmetrical tonic neck reflex The asymmetrical tonic neck reflex (ATNR) is a primitive reflex found in newborn humans that normally vanishes around 6 months of age. It is also known as the bow and arrow or "fencing reflex" because of the characteristic position of the infant ...

(ATNR) * Palmomental reflex *

Moro reflex The Moro reflex is an infantile reflex that develops between 28 and 32 weeks of gestation and disappears at 3–6 months of age. It is a response to a sudden loss of support and involves three distinct components: # spreading out the arms (abd ...

, also known as the startle reflex * Palmar grasp reflex * Rooting reflex *

Sucking reflex Primitive reflexes are reflex actions originating in the central nervous system that are exhibited by normal infants, but not neurologically intact adults, in response to particular stimuli. These reflexes are suppressed by the development of the f ...

* Symmetrical tonic neck reflex (STNR) *

Tonic labyrinthine reflex The tonic labyrinthine reflex (TLR) is a primitive reflex found in newborn humans. With this reflex, tilting the head back while lying on the back causes the back to stiffen and even arch backwards, the legs to straighten, stiffen, and push togeth ...

(TLR)

Other kinds of reflexes

Other reflexes found in the central nervous system include: * Abdominal reflexes (T6-L1) * Gastrocolic reflex * Anocutaneous reflex (S2-S4) *

Baroreflex The baroreflex or baroreceptor reflex is one of the body's homeostasis, homeostatic mechanisms that helps to maintain blood pressure at nearly constant levels. The baroreflex provides a rapid negative feedback loop in which an elevated blood pressu ...

* Cough reflex * Cremasteric reflex (L1-L2) *

Diving reflex The diving reflex, also known as the diving response and mammalian diving reflex, is a set of physiological responses to immersion that overrides the basic homeostatic reflexes, and is found in all air-breathing vertebrates studied to date. It op ...

* Lazarus sign *

Muscular defense Muscular defense is a reflex of the abdominal muscles to contract upon mechanical force to the abdomen, and serves as protection. It is a visceromotor reflex, since the parietal peritoneum and viscera are involved in generating the reflex.A. V. Li ...

* Photic sneeze reflex * Scratch reflex *

Sneeze A sneeze (also known as sternutation) is a semi-autonomous, convulsive expulsion of air from the lungs through the nose and mouth, usually caused by foreign particles irritating the nasal mucosa. A sneeze expels air forcibly from the mouth a ...

Startle reflex In animals, including humans, the startle response is a largely unconscious defensive response to sudden or threatening stimuli, such as sudden noise or sharp movement, and is associated with negative affect.Rammirez-Moreno, David. "A computation ...

* 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 (e.g., the withdrawal reflex). Processes such as

breath Breathing (or ventilation) is the process of moving air into and from the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxygen. All aerobic creatures need oxygen for cell ...

ing,

digestion Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intest ...

, and the maintenance of the

heartbeat A heartbeat is one cardiac cycle of the heart. Heartbeat, heart beat, heartbeats, and heart beats may refer to: Computing *Heartbeat (computing), a periodic signal to indicate normal operation or to synchronize parts of a system *Heartbeat, clus ...

can also be regarded as reflex actions, according to some definitions of the term.

Grading

medicine Medicine is the science and Praxis (process), practice of caring for a patient, managing the diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment, Palliative care, palliation of their injury or disease, and Health promotion ...

, 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+.

Reflex modulation

Naively, we might imagine that reflexes are immutable. In reality, however, most reflexes are flexible and can be substantially modified to match the requirements of the behavior in both vertebrates and invertebrates. A good example of reflex modulation is the stretch reflex. When a muscle is stretched at rest, the stretch reflex leads to contraction of the muscle, thereby opposing stretch (resistance reflex). This helps to stabilize posture. During voluntary movements, however, the intensity (gain) of the reflex is reduced or its sign is even reversed. This prevents resistance reflexes from impeding movements. The underlying sites and mechanisms of reflex modulation are not fully understood. There is evidence that the output of sensory neurons is directly modulated during behavior—for example, through presynaptic inhibition. The effect of sensory input upon motor neurons is also influenced by interneurons in the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spin ...

or ventral nerve cord and by descending signals from the brain.

Other reflexes

Breathing can also be considered both involuntary and voluntary, since breath can be held through

internal intercostal muscles The internal intercostal muscles (intercostales interni) are a group of skeletal muscles located between the ribs. They are eleven in number on either side. They commence anteriorly at the sternum, in the intercostal spaces between the cartila ...

References

{{Authority control Reflexes Animal physiology