Protective reflexes

Protective reflexes are autonomic muscle movements that are triggered by external factors to protect a certain part of the body. The muscles involved are mostly skeletal muscles that are normally used for conscious, voluntary movement. The protective reflexes are triggered by bypassing the consciousness in favor of significantly higher reaction speeds, as in the case of the eyelid closing reflex, which protects the eye from injury from foreign bodies or from glaring light.

What are protective reflexes?

Protective reflexes take place involuntarily and serve to protect certain organs or body regions. Protective reflexes are triggered by sensory messages that exceed certain threshold values. These can be triggering stimuli such as pressure or tension, acceleration, light, sound, temperature, pain or chemical stimuli.

The interconnection between the sensory organs, which report a threshold value exceedance via their afferent sensory fibers, with the executing efferent motor nerve fibers takes place via a single synapse or several synapses. Accordingly, it is a monosynaptic or polysynaptic reflex. The interconnection itself is called a reflex arc. In the simplest case, the monosynaptic connection, the reaction time between the triggering stimulus and the beginning of the stimulus execution is only 30 to 40 milliseconds.

In principle, protective reflexes can be implemented as internal or external reflexes. An external reflex is present if the execution of the reflex is not intended to protect the muscle or part of the body in question, but rather another organ, such as the eyeball during the blink reflex. Stretch reflexes, which serve to protect the muscles from overstretching, are typical self-reflexes, since the stretch sensors, the muscle spindles, are located in exactly the muscle that is protected by the contraction reflex.

Function & task

The main task of protective reflexes is to use certain muscle reactions to protect the muscles themselves, in the form of a self-reflex, or other organs, in the form of an external reflex, against the threat of damage from thermal, mechanical and chemical or from extreme incidence of light.

The benefit for humans lies above all in the short reaction time from the triggering of the stimulus to the execution of the protective movement, which is achieved by bypassing the consciousness. The short reaction time can be decisive for the success of the protective reflex. For example, a flying insect or a foreign body can damage the eye, which is to be prevented by the quick blink reflex. In this case, the shortest possible reaction time from perceiving the object to closing the eyelid is decisive 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 through training.

In addition to the blink reflex, the most well-known protective reflexes are swallowing, choking, coughing and sneezing reflexes as well as withdrawal reactions. Withdrawal reactions can also be triggered by nociceptors (pain sensors). A typical withdrawal reaction is the reflexive withdrawal of the hand from the hot stove.

With most protective reflexes, the reason for their nature is easily recognizable, as with the sneezing reflex, which is supposed to prevent allergenic 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 that is recognized as harmful and already in the stomach from causing no further damage when it is returned. The nausea can also be triggered by problems with the transmission of stomach contents in the digestive tract or by hormonal problems and unusual vestibular feedback. The aim of the cough reflex is to prevent the airways from being blocked by bronchial secretions or foreign bodies.

In contrast, there are conditioned or conditioned reflexes that can be acquired. Ultimately, all complex movement sequences learned, which occur unconsciously after intensive training, are based on conditioned reflexes. This includes, for example, motion sequences such as walking upright, balancing, artistic gymnastics or driving a car, as well as many other motion sequences.

Illnesses & ailments

Impairments to the protective reflexes can be reduced by neuronal disorders or caused by injuries or acute diseases of the affected muscle areas. Neurological disorders can be present on the sensors themselves or on the afferent nerve branches of the sensors or on the synapse or synapses or ganglia at which the switchover to the efferent motor nerve fibers takes place.

The motor fibers themselves can also exhibit disorders. This means that a disturbance in just a single limb of the reflex arc can lead to impairment or total failure of the corresponding unconditional protective reflex. For example, Parkinson's is accompanied by a decrease in certain protective reflexes in connection with movement coordination. All other nerve diseases that are associated with restrictions in the transmission of nerve impulses or neurotransmitters also have an influence on protective reflexes.

In the early stages, the reflexes usually slow down and weaken. When unconsciousness occurs, the protective reflexes are disturbed, which, depending on the depth of the unconsciousness, can go as far as the complete failure of the reflex. Conversely, checking certain protective reflexes, such as the blink reflex, can provide information about the depth of the unconsciousness.

The failure of the swallowing and coughing reflexes, with simultaneous relaxation of the palate and throat muscles, can be particularly dangerous, as there is a risk of the trachea being blocked by the musculature or by vomit, which cannot be eliminated by the cough reflex and can lead to death by suffocation.

A temporary restriction of the protective reflexes occurs through alcohol consumption, which leads to a reduced sensitivity of sensors such as thermo and nociceptors and to an impairment of the entire nervous processing of impulses, including movement coordination. With an increasing alcohol concentration of over 2.5 per mille, irreversible neurotoxic symptoms of poisoning and an increasing loss of all reflexes occur.