External reflex

A foreign reflex , also known as a polysynaptic reflex , is a reflex in which the reflex response does not take place in the stimulus-sensing organ. The reflex arc takes place via several synapses , which is where the name polysynaptic reflex comes from. A distinction is made between physiological and pathological external reflexes.

In contrast to self-reflexes , physiological external reflexes are habitable (weakening or absence of the reflex response due to the fact that no more vesicles with neurotransmitters are available in the presynaptic membrane of the sensory neuron). The external physiological reflex is reduced or no longer occurs, particularly in the case of spastic paralysis and sensitivity disorders.

The following list shows external physiological reflexes. The term affected nerve tracts refers to the nerves that receive the stimulus and trigger the effect. As usual, the cranial nerves are represented in Roman numerals. The abbreviations Th (thoracic nerve = pectoral nerve), L (lumbar nerve = lumbar nerve) and S (sacral nerve = cross nerve) refer to the corresponding spinal cord nerves , the following numbers to the respective segment within the corresponding spinal cord section.

External physiological reflexes

Anal reflex

• also: perineal reflex
• Triggering: stimulus next to or on the anus (e.g. inserting a finger).
• Effect: contraction of the external ani sphincter muscle
• Affected nerve tracts: S4 / S5

Abdominal skin reflex

• BHR
• Triggering: brushing the abdominal wall
• Effect: contraction of the obliquus and distortion of the navel to the side on which the stimulus was triggered.
• Affected nerve tracts: Th8-Th12
• Comment: If the pyramidal tracts are damaged , this reflex can be weakened or completely absent. In addition, a weakening can be observed in old people (> 75 years) who exercise little
• Note: The abdominal skin reflex should not be confused with the abdominal wall reflex .

Bulbocavernosus reflex

• also: bulbospongiosus reflex, ejaculation reflex
• Trigger: sensory signals from the glans ( glans penis ) or the clitoris
• Effect: muscle contraction of the bulbospongiosus and ischiocavernosus muscles
• Affected nerve tracts: pudendal nerve (pubic nerve ) S3-S4
• Comment: During the erection phase, signals from the glans trigger muscle compression at the base of the penis to increase the erection. During the ejaculation phase , the muscles synchronize with the other rhythmic contractions in the organ to expel the ejaculate.

Corneal reflex

• also: CR, blink reflex
• Triggering: mechanical, chemical or thermal stimulus of the cornea (cornea of ​​the eye)
• Effect: the eyelid closes; Long-term termination is also possible in the event of a long-term stimulus.
• Affected nerve tracts: cranial nerves V and VII
• Comment: The corneal reflex is used to test the effect of local anesthetics .

Kremaster reflex

• also: testicular elevator reflex, CrR
• Triggering: brushing the inside of the thigh
• Effect: Draws the testicle towards the body by contracting the cremaster muscle .
• Affected nerve tracts: L1 / L2
• Comment: The strength of the reaction varies and should therefore only be used with caution for diagnostic purposes.

Plantar reflex

• also: sole reflex, skin reflex, flight reflex
• Triggering: stroking the sole of the foot with a needle or the handle of the reflex hammer .
• Effect: the toes perform a grasping movement. If it is more pronounced, the tensor fasciae latae muscle is also tensed; in rare cases, the knee and hip joints flex.
• Affected nerve tracts: L5 / S2
• Comment: The plantar reflex is often absent or only weakly developed. The doctor then speaks of a mute sole . This is only of diagnostic importance if the reflex is different on the soles of the feet. It is then possibly a pyramid orbit sign . Furthermore, if the pyramidal tract is disturbed, the big toe can move in the opposite direction ( Babinski reflex or Babinski sign).

In the adult, Babinski's sign is definitely a pathological sign, in the baby it occurs physiologically.

Pupillary reflex

• also: pupillary light reflex
• Triggering: light entering the eye
• Effect: constriction of the pupil
• Affected nerve tracts: cranial nerves II and III
• Comment: Due to the interconnection of both Edinger Westphal cores at the brain stem level , the pupil constriction of the other eye occurs even when light falls into only one eye. This reflex fails when the optic nerves , the visual tracts or the brain stem nuclei are damaged and can therefore no longer be triggered in the event of brain death .

Sucking reflex

• Triggered: when the lips and the tip of the tongue are touched in the newborn
• Effect: enables food and fluid intake
• Affected nerve tracts: Maxillary nerve (V / 2), mandibular nerve (V / 3), trigeminal nerve, facial nerve and hypoglossal nerve (cranial nerves V, VII, XII)

Swallowing reflex

• Triggering: Touching the mucous membrane in the area of ​​the base of the tongue, the narrow throat (palatal arches) or the back of the throat
• Effect: enables food and fluid to be consumed without endangering the respiratory tract
• Affected nerve tracts: cranial nerves IX and X

Gag reflex

• also: palate reflex
• Triggering: touching the back wall of the throat .
• Effect: gagging or vomiting
• Affected nerve tracts: cranial nerves IX and X
• Comment: This reflex fails in brain death , so it can be tested to determine it.

Individual evidence

1. ↑ Kornhuber (Ed.), Zierz (Ed.): The neurological examination . Steinkopff Verlag Darmstadt 2005, p. 82. ISBN 3-7985-1444-5
2. ^ Gregory B. Auffenberg, Brian T. Hellfand, Kevin T. McVary: Normal Erectile Physiology. In: Kevin T. McVary: Contemporary Treatment of Erectile Dysfunction. A Clinical Guide Springer Science & Business Media, 2010, 299 pp., Pp. 11-22, ISBN 1603275363 , p. 15 (for both functions of the reflex).