Neuromatiq : Neuroanatomie, neurophysiologie et neuropsychopathologie - Dr Ben Brahim Mohammed Pédopsychiatre Fès Béni Mellal

05. Balance

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The balance [ 42 , 130 ] plays a crucial role in our lives. It is through this ability that you can stand, you can walk, you can locate in space ...  

The balance is the set of mechanisms that stabilize the body when the station and active or passive movements. These mechanisms help to maintain an attitude reference and allow to return whatever the circumstances.  

Maintenance of static and dynamic balance requires mechanisms seeking a driving component made ​​of neuromuscular elements but also and especially the mechanisms that constantly inform the CNS of the position of the various elements of the body and their movements.

1. Sensory modalities balance:

The sense of balance is a particular system. First it is a system that works most often unconsciously, and then it is a system that seeks several sensory modalities [ 96 ].   Among the organs involved in the sensory system of balance are:

• The vestibular system [ 67 ]: specific organ of equilibrium.
• The visual system.
• Deep sensation (proprioception).
• And the surface sensitivity of the soles.

  All these systems will synchronize their afferents to constantly keep the SNC position or movement of different parts of the body so it can then react in the most appropriate manner.

2. Reception:

The inner ear consists essentially of the bony labyrinth, it contains perilymph where the membranous labyrinth is filled with endolymph bathes. The membranous labyrinth is divided into a component before hearing (cochlear duct) and vestibular system [ 38 , 39 ], the specific organ of balance back.  

The vestibular system has a diameter of about 1 cm, it comprises two chambers: the utricle and saccule, and three semicircular canals.

2.1. Otolith organs:

The utricle and saccule are called (otolith organs) [ 38 ] because they contain otoliths (calcium carbonate crystals). They are specialized in the detection of movements and linear accelerations of the head and its static position relative to the axis of gravity.  

The utricle is arranged in a horizontal manner [ 38 ], sensory cells detect all movements made ​​in the horizontal plane. Saccule by against a vertical arrangement, it provides sensory information about the vertical movements. These two systems constantly inform the CNS about the position of the head and the displacement movements it undergoes in every way.  

Sensory cells of the vestibular system are similar to hair cells in the cochlea [ 41 ], they are buried in the epithelium with cilia on their apical pole of stereocilia and kinocils. These cilia are bathed in a gelatinous layer (the cup) [ 52 ] that at the utricle and saccule is dotted with microcrystals (otoliths) that amplify its consistency.  

When there is a movement of the head, the endolymph (because of inertia) is slow to follow, and therefore will push the cup in the opposite direction of the movement. This will cause it to the eyelashes of the hair cells which will depolarize [ 41 ] and activate afferent neurons that are associated with them.

2.2. Semicircular canals:

The three semicircular canals are each located in one of the planes in space [ 38 , 52 ] and was a former canal, posterior and lateral. Everyone ends near the utricle by a bulge (bulb channel). It contains sensory cells that operate by the same mechanism [ 38 ] than in the saccule and utricle, except that the wells at this level does not contain otoliths.  

Because of their circular arrangement, the semicircular canals report rotational movements of the head.

3. Transmission - Perception:

The nerve fibers from the hair cells of the vestibular system have their body at the Scarpa ganglion (vestibular ganglion) [ 50 ]. Thereof will emerge the vestibular nerve that joins the cochlear nerve to form the vestibulocochlear nerve (VIII).  

After reaching the vestibular nucleus [ 38 ] in the brainstem, the fibers of the vestibular branch of the eighth nerve relay will make with other neurons that win different regions of the central nervous system: The thalamus [ 41 ], the cerebellum , cerebral cortex, spinal cord, the reticular formation, and oculomotor nuclei that achieve the vestibulo-ocular reflex [ 50 ], it stabilizes images on the retina during movement.