Glabellar reflex: what it is and how it occurs

Reflexes are quick reactions that occur when in contact with a specific stimulus, or in a specific way. The glabellar reflex is one of these reactions. which do not involve in any measure the higher functions of consciousness. Let's see how it is.

• Related article: "The 12 primitive reflexes of babies"

What is the glabellar reflex?

As with all reflexes, the glabellar reflex is a process in which the sensory neuron conducts the impulse directly to the motor neuron, which is responsible for immediately sending the response to a muscle, which executes the action of the reflex act.

This reflex happens when the center of the forehead is struck repeatedly, a little above where the nose ends, so that the eyes cannot come into contact with the examiner or with the object that hits that area.

Once the glabellar zone is struck, as described above, briefly but energetically, it is when the glabellar reflex occurs, which consists of a constant and abnormal blinking on the part of the examined subject.

• You may be interested in: "Neuropsychology: what is it and what is its object of study?"

What nerve does this reflex involve?

During this reaction, the trigeminal nerve, which is the V cranial nerve, is mobilized. This nerve is located in the pons of the brainstem, and from there it extends to the Gasserian ganglion. This ganglion is the largest sensory root, and is located in the middle cranial fossa, hence its bodies. Neuronal branches are divided into three parts: The ophthalmic branch (V1), the maxillary branch (V2), and finally, the mandibular branch. (V3).

Depending on which branch is stimulated, by means of a contact or a visual stimulus we can observe a different reflection in the eyes or in some area of ​​the subject's face.

When probing the sector of the forehead above the nose (glabellar area), with the blow and the shape described above what we are doing is stimulate the ophthalmic branch (V1) of the Gasserian ganglion, which is linked to the trigeminal nerve (5th cranial nerve).

All of these connections must be working properly for mirroring to happen, so there is some interference or anomaly at some point, then the synapse could not occur properly.

Therefore, the afferent neuron of this reflex, which belongs to the facial nerve (III cranial nerve) would not send any signal to the muscle for it to move, and the reflex would not occur. Taking into account that the III cranial nerve (facial nerve) also intervenes in this reflex, through the efferent neuron, then it is understood that the reflex of the glabellar zone It is of trigemino-facial origin..

• Related article: "Cranial nerves: the 12 nerves that exit the brain"

The importance of this physiological phenomenon

This reflex is essential to protect against exaggerated stretching of the muscles, in this case the muscles of the eyelids. The reflex of the glabellar zone is responsible for preventing the eyelids from being damaged and, in turn, this represents important protection for the eyes.

This assessment is really simple for the examiner to do, as it is non-invasive. It is part of the person's routine physical exam, and allows to detect with great accuracy if there is any damage in the spinal cord.

medical evaluation

The examiner should be attentive to the reflex response in both eyelids; if it happened that the blinking is not symmetrical on both sides, then is an indicator of neurological damage. If so, it will be up to determine where the damage is located exactly, if in the afferent pathway or sensory, in the reflex processing center (interneuron), the efferent or motor pathway, or in the muscle effector.

In order for the evaluation of this reflex to be carried out satisfactorily, it is necessary for the person evaluated to be in a state of complete muscle relaxation. Otherwise the muscle will not react to the stimulus in the expected way.

Sometimes it is difficult for the evaluator to make the patient relax enough To apply the evaluation method, when these complications occur, a series of techniques can be used to achieve the favorable state of relaxation in the subject.

Examples of techniques

Some of these techniques are as follows.

1. Using the Jendrassik Maneuver

This procedure helps to obtain the reflex action through sudden movements in the area where the response is sought.

For example, in the case of the glabellar zone, the examiner will ask the subject to blink rapidly before proceeding with the assessment.

2. relaxation techniques

It is convenient to use them before applying sudden percussion on the muscle tendon.

Bibliographic references:

• Purves (2004). Neuroscience: Third Edition. Massachusetts, Sinauer Associates, Inc.
• Derrickson (2006). Principles of anatomy and physiology.