The human body is a complex machine . It often carries out apparently simple tasks; but they require the coordination of a very precise interlocking of organs, muscles, nerves and ligaments.
This is particularly relevant in the process from the perception of a stimulus (in any sensory mode) to its arrival at the port of the brain where it is to be processed.
In this article we will deal with one of these phenomena: barognosia . The truth is that we all make use of it in our daily activities, but… did you know it?
What is barognosia?
The term barognosia can be dissected for a more precise etymological analysis, providing information on its origin and meaning. The elements that form it, “baro” and “gnosia”, come from the classical Greek. The first of these refers to weight or pressure (hence the word barometer for the measurement of this atmospheric condition) and the second alludes to the ability to recognize an aspect of reality after its reception through the senses.
Therefore, barognosy is the capacity that we have to recognize the weight of the objects with which we interact in our daily lives , with total independence of other visual or acoustic parameters that could suggest such a property of these objects. We would make use of this capacity, included within the functions of sensory recognition, at the moment in which we hold an object with one hand and calculate its weight in contrast to a different one.
In fact, this is one of the essential mechanisms through which the preservation of barognosia is explored in any neurological examination, something very common, since it is a function that can be affected as a consequence of certain pathologies of the central or peripheral nervous system. More specifically, objects with similar morphology (balls, for example) are placed in each of the two hands, so that an estimate can be made of which of them has the greatest or least relative weight (varying the magnitude of this variable to speed up the precision of the measurement).
It is a capacity included within the general category of sterognosies, a modality of perceptive recognition that is related to the sense of touch (haptic), and from which the physical properties of objects are extracted without the need to resort to the rest of the senses. Although aspects such as texture or temperature can be included here, with which a more precise knowledge of the stimulus with which one interacts is obtained, for barognosia, deep sensitivity is alluded to by avoiding receptors of another nature (superficial).
How does barognosia occur?
Barognosia is made possible by a group of mechanoreceptors located in the skin, periosteum, joint capsule, ligaments and menisci; called Pacini’s corpuscles.
At an anatomical level they can be described as nerve endings that arise from the cellular axon and whose end acquires the appearance of an encapsulated cone, which permeates the central space. This cone is formed by the succession of some 40-80 plates, made up of connective tissue cells and separated by collagen. They are receptors sensitive to vibration and tissue deformation.
It is precisely this same capsule, which deforms when pressure is exerted on the tissues, that informs the nervous system of the magnitude of such external force so that it can be interpreted in terms of weight. Since its distribution is concentrated in the deep layers of the skin (dermis), as well as in the hypodermis of the fingers and palms of the hands (glabrous areas), these are usually the parts of the body that are most frequently used to assess the preservation of barognosia.
The detection of alterations in baryognosia has an essential clinical component, as these are discrimination tasks that can only be reported by the subject and for which there is a lack of objective evidence. However, once this symptom has been detected, attention must be focused on a thorough examination of the skin tissues and the central and peripheral nervous system, with the aim of identifying an organic cause that is susceptible to intervention.
How is barognosia evaluated?
Tests to evaluate baryognosia are included in the neurological examination protocol for tactile discrimination, along with a variety of procedures aimed at assessing deep sensitivity. In all cases the person is asked to close their eyes, so that interference from any adjuvant perception is minimized. After that, the person is asked to recognize which is the object that has been left on his hand, the intensity of its vibration or the location of different points when they are stimulated by pressure.
In the particular case of barognosia assessment, the examinee is urged to remain in a seated position while placing the palms of his hands on the thighs (without contact) and positioned upwards. Two pieces with different weights, whose size and shape are comparable, are placed on top of them, and he is asked to discriminate which one is heavier. The scan starts with a significant dissonance, and progressively the parameter is adjusted until a differential threshold is detected.
The differential threshold refers to the minimum weight, expressed in grams, above which a difference between two objects can be recognised . Thus, it is not always a function whose commitment is absolute, but sometimes there are different degrees within a spectrum that ranges from normal to completely affected. It is informative to explore the lower and upper limits, so all the possible alternatives must be considered at both ends.
It may be useful to further explore sensitivity, stereognosia (recognition of objects such as coins, balls or dice), tactile pressure (non-painful), graphesthesia (recognition of signs drawn on the palm of the hand), textures, kinesthesia (movement), proprioception (location of the body in relation to the space surrounding it) and vibration; all of which depend (like weight discrimination) on the lemnisch-medial dorsal pathway.
If the subject is able to respond adequately, his baryognosia is considered to be perfectly preserved. Otherwise, the presence of a problem can be inferred at any of the points through which sensitive information passes from the receptors to the brain areas where it is processed. The detection of the cause is essential to elaborate the treatment and the differential diagnosis of any basic pathology.
In the event that the scan detects an alteration limited to the deep sensitivity and with conservation of the superficial one, a dorsal tabes may be suspected. This would be a degeneration of the dorsal cords of the spinal cord, which would disturb the sensory discrimination ability by maintaining the action of nociceptors (widely distributed in the skin) and thermoreceptors (Ruffini corpuscles).
How does barognosia express itself when it is committed?
Barognosia is a concrete form of recognition that requires the involvement of Pacini’s corpuscles and many other structures located in the nervous system.
When an object is picked up by hand, the cells send the information to the spinal cord through sensory nerves, in afferent ascendancy to the spinal cord and finally to the thalamus (from which a large part of the sensory information is managed). It is from this point that the signal can be processed and interpreted, mediated by the parietal cortex (which integrates sensation into perception).
Any point of the journey can be altered by different circumstances, which would produce an abarognosia. This is expressed as a severe difficulty in estimating, recognizing and discriminating the weight of objects located in areas of the body susceptible to assessment. It most frequently occurs as a result of an injury to the parietal cortex, whose clinical manifestation is at a contralateral level (if it is in the right hemisphere, the left hand would be affected, for example).
There is evidence that lesions in the post-central gyrus are the most frequently detected in people with abarognosia , who also have tactile/haptic agnosia (inability to recognize textures and shapes), agrafestesia (inability to interpret numbers or letters that are drawn on the affected surface) and atopognosia (difficulty in recognizing contact with one or more stimuli on the skin).
When abarognosia is present without apparent damage to tissues, joints, or bones (burns, fractures, etc.), diagnostic procedures are needed, among which neuroimaging techniques play an essential role (especially structural). Through these, the initial exploration carried out by the neurological specialist can be complemented, determining the state of the spinal cord and the brain regions potentially involved.
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