The term brainstem reflex includes all reflexes that, bypassing consciousness, are directed from the brainstem via efferent fibers of the corresponding cranial nerves directly to effector organs – usually specific muscles. Brainstem reflexes, which serve to protect against impending injury, play a critical role in determining brain death prior to organ removal. If only one of the reflexes is functioning when brainstem reflexes are tested, brain death does not exist and no organ may be removed for organ donation.
What are brainstem reflexes?
Brainstem reflexes are triggered by the brainstem after receiving appropriate sensory messages. Brainstem reflexes are triggered by the brainstem upon receipt of appropriate sensory messages. In most cases, motor “instructions” are directed to effector organs, i.e., specific muscles. Characteristic of brainstem reflexes is their short latency from receipt of sensory messages to execution of the reflex. The short reaction time is achieved by bypassing consciousness to the greatest extent possible. The incoming sensor signals are not first processed by specific brain regions and summarized for adequate assessment to initiate a voluntary response, but are converted into direct action potentials without detours. Since consciousness is largely bypassed in this process, the reflexes function even in deep unconsciousness, so that the protective function of the body is maintained even during temporary unconsciousness. Specifically, these reflexes are the pupillary reflex, the eyelid closure reflex, the vestibulo-ocular reflex (VOR), and the cough and gag reflexes. The pupillary and lid closure reflexes are usually routinely checked in an unconscious casualty. When either pupil is illuminated with a diagnostic light, both pupils should constrict conspicuously, and eyelid closure is expected as an automatic response when the cornea is touched.
Function and task
The most important function and task is to protect the affected organs from imminent injury by sudden strong incidence of light (pupillary reflex) or by foreign body impact on the cornea by insects or other small flying objects (corneal reflex). The tracheal reflex (cough reflex) protects the respiratory tract from unintentional inhalation of larger or smaller foreign bodies, and the pharyngeal reflex (gag reflex) protects the esophagus and digestive tract from objects that are too large or have unintentionally entered the mouth and become inedible. Crucial to the effectiveness of the protective function of the brainstem reflexes is an extremely short reaction time in the range of milliseconds. This means that consciousness cannot be involved within the reflex arc, as the time required to process the incoming signals would be too high. At the same time, bypassing consciousness has the advantage that the protective function is maintained even if the patient is unconscious. The gag reflex and the vestibulo-ocular reflex occupy a certain exceptional position. The gag reflex can be influenced or suppressed voluntarily to a certain degree. This usually requires the person to be able to concentrate on it before the gag reflex sets in, for example when a feeding tube is inserted. The vestibulo-ocular reflex plays an even greater special role. It has less of a direct protective function than practical tasks that are important for coping with daily movement requirements. Upright walking, running and similar movement sequences can only be mastered with an intact vestibular ocular reflex. It has the task of being able to continue to see an object in focus despite rapid head movements. The VOR ensures that the eyes track against the head movement, which is roughly comparable to a gyro-stabilized camera. This means that when we are jogging, for example, we can continue to see the surroundings relatively sharply in the field of vision, despite the rapid up and down movements of the head. The VOR is controlled by the arcuate and otolith organs of the sense of balance. The arcuates respond to rotational accelerations and the otolith organs respond to linear accelerations. In each case, the eyes are moved in opposition to the acceleration experienced. This also works when not only the head but the whole body is subject to acceleration.
Diseases and ailments
The correct course of a brainstem reflex requires that the muscles involved and the afferent and efferent fibers of the corresponding nerves are functional and that the brainstem can provide the necessary wiring of the nerve impulses. The three reflexes involving the eyes, or eyelids, are consensual. If a stimulus is applied to only one eye, the unaffected eye also follows the reflex. If only one eye performs the reflex, alternating stimuli to both eyes can determine which afferent or efferent nerve fibers are affected. An overriding – usually reversible – disturbance of the reflexes can occur due to nerve toxins or even consumption of alcohol or other drugs. If the pupillary reflex and the eyelid closure reflex fail in one or both eyes, the protective function is lost, so that the affected person must be extremely careful with regard to possible corneal injuries and with regard to glare from strong light sources. A failure of the cough reflex has more serious consequences for the affected person than a failure of the gag reflex, since an inactivated cough reflex can easily lead to the penetration of foreign bodies – including liquids – into the trachea, with sometimes serious consequences up to the danger of suffocation. Because the VOR does not so much perform a protective function as it is constantly needed to support movement, a functional impairment has a particularly serious effect. Even normal walking is then only possible under difficult conditions. If the vestibular organs themselves show functional disturbances, e.g. as a result of circulatory disturbances or heavy alcohol consumption, not only spinning attacks and nausea may occur, but the VOR then follows the misleading sensory messages of the vestibular organs and unconscious, disturbing eye movements or eye tremors (nystagmus) occur, which significantly aggravate the situation. In unconscious accident victims suspected of brain death, testing of brainstem reflexes serves as an important criterion for determining brain death if a decision must be made regarding removal of an organ for donation.
