Protective reflexes are autonomic muscle movements triggered by external factors to protect a specific body part. The muscles involved are usually skeletal muscles that normally serve conscious, voluntary movement. Protective reflexes are triggered bypassing consciousness in favor of much higher reaction rates, as in the case of the eyelid closure reflex, which protects the eye from injury by foreign bodies or from excessive glare.
What are protective reflexes?
Protective reflexes are autonomic muscle movements that are triggered by external factors to protect a specific body part. E.g., the gag reflex. Protective reflexes occur involuntarily and serve to protect specific organs or body regions. Protective reflexes are triggered by sensory messages that exceed certain thresholds. These can be triggering stimuli such as pressure or traction, acceleration, light, sound, temperature, pain, or chemical stimuli. The connection between the sensory organs, which report a threshold crossing via their afferent sensory fibers, with the executing efferent motor nerve fibers occurs via a single or via multiple synapses. Accordingly, it is a monosynaptic or polysynaptic reflex. The circuitry itself is called a reflex arc. In the simplest case, monosynaptic circuitry, the reaction time between the triggering stimulus and the onset of stimulus execution is only 30 to 40 milliseconds. In principle, protective reflexes can be executed as intrinsic or extrinsic reflexes. An extraneous reflex occurs when the execution of the reflex is not intended to protect the muscle or body part in question, but another organ, such as the eyeball in the case of the eyelid closure reflex. Stretch reflexes, which serve to protect muscles from overstretching, are typical intrinsic reflexes because the stretch sensors, the muscle spindles, are located in the very muscle that is protected by the contraction reflex.
Function and task
The main function of the protective reflexes is to protect the muscles themselves, in the form of an intrinsic reflex, or other organs, in the form of an extrinsic reflex, from imminent damage by thermal, mechanical, chemical, or by extreme light incidence, through certain muscular reactions. The benefit for humans is primarily the short reaction time from the triggering of the stimulus to the execution of the protective movement, which is achieved by bypassing consciousness. The brevity of the reaction time can be critical to the success of the protective reflex. For example, an approaching insect or foreign object may damage the eye, which the rapid eyelid closure reflex is designed to prevent. In this case, the shortest possible reaction time from perception of the object to eyelid closure is crucial for the protective effect. The “short-circuited” reaction arcs of the various protective reflexes have developed in the course of evolution and are genetically fixed. Protective reflexes can therefore not be “acquired” or trained by training. In addition to the eyelid closure reflex, the swallowing, gagging, coughing and sneezing reflexes as well as withdrawal reactions are the best-known protective reflexes. Withdrawal reactions can also be triggered by nociceptors (pain sensors). A typical withdrawal reaction is, for example, the reflex-like withdrawal of the hand from the hot stove. In the case of most protective reflexes, the reason for their creation is easily recognizable, as in the case of the sneezing reflex, which is intended to prevent allergens or other problematic substances from initially remaining in the nasal cavity or even being inhaled into the lungs. A relatively complex protective reflex is the vomiting reflex, which can be triggered by a wide variety of causes and primarily protects against food recognized as harmful that is already in the stomach from causing further damage by being carried back. However, the nausea reflex can also be triggered by problems with the transmission of the stomach contents in the digestive tract or by hormonal problems as well as by unusual vestibular feedback. The cough reflex is intended to prevent blockage of the airway by bronchial secretions or foreign bodies. This is in contrast to conditioned or conditioned reflexes, which can be acquired. Ultimately, all learned complex movement sequences that occur unconsciously after intensive training are based on conditioned reflexes.This includes, for example, movement sequences such as walking upright, balancing, artistic gymnastics or steering a car, as well as many other movement sequences.
Diseases and ailments
Impairments of the protective reflexes may be due to neuronal disorders or caused by injuries or acute diseases of the affected muscle parts. Neurologic disturbances may be present at the sensors themselves or at the afferent nerve branches of the sensors, or at the synapse(s) or ganglia at which switching to the efferent motor nerve fibers occurs. The motor fibers themselves may also have disruptions. This means that a disruption at just a single link in the reflex arc can lead to impairment or total failure of the corresponding unconditional protective reflex. For example, Parkinson’s disease is accompanied by a decrease in certain protective reflexes associated with movement coordination. All other nerve diseases that are associated with restrictions in the transmission of nerve impulses or neurotransmitters also affect protective reflexes. In most cases, a slowing and weakening of the reflexes occurs in the early stages. When unconsciousness sets in, there is a disturbance of the protective reflexes, which, depending on the depth of unconsciousness, can go as far as complete failure of the reflex. Conversely, a check of certain protective reflexes, such as the eyelid closure reflex, can provide clues to the depth of unconsciousness. Failure of the swallowing and coughing reflex, with concomitant flaccidity of the palate and pharyngeal muscles, can be particularly dangerous, as there is a risk of tracheal obstruction by the muscles or by vomit, which is not relieved by the cough reflex and can lead to death by asphyxiation. A temporary restriction of the protective reflexes occurs as a result of alcohol consumption, which leads to a reduced sensitivity of sensors such as thermo- and nociceptors and to an impairment of the overall nervous processing of impulses, including in the coordination of movement. In addition, with increasing alcohol concentrations above 2.5 per mille, irreversible neurotoxic intoxication symptoms and increasing failure of all reflexes set in.
