Chapters > Sensory systems > Balance

Balance

Category: Sensory systems• Published on: April 18, 2012• Hits: 14587•

Balance [42, 130] plays a vital role in our lives. It is thanks to this ability that we can stand, walk, and orient ourselves in space.

Balance is the set of mechanisms that stabilize the body during standing and during active or passive movement. These mechanisms help maintain a reference posture and allow the body to return to it regardless of the circumstances.

Maintaining static and dynamic balance requires mechanisms involving a motor component made of neuromuscular elements, but also, and most importantly, mechanisms that constantly inform the CNS of the position of different body parts and their movements.

Sensory modalities of balance :

The sense of balance is a unique system. First, it's a system that functions mostly unconsciously, and second, it involves several sensory modalities [96].

Among the organs participating in the sensory system of balance are:

  • The vestibular system [67]: a specific organ of balance.
  • The visual system.
  • Deep sensitivity (proprioception).
  • And the superficial sensitivity of the soles of the feet.

All these systems synchronize their afferent signals to constantly inform the CNS of the position or movements of different body parts so that it can react in the most appropriate manner.

Reception :

The inner ear consists of a bony labyrinth filled with a fluid called perilymph. Floating within this fluid is the membranous labyrinth, which is filled with its own separate fluid called endolymph. The membranous labyrinth is divided into an anterior auditory component (cochlear duct) and a posterior vestibular system [38, 39], the specific organ for balance.

The vestibular system is about 1 cm in diameter and comprises two chambers: the utricle and the saccule, as well as three semicircular canals.

Otolithic organs :

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

The utricle is oriented horizontally [38]; its sensory cells detect all movements occurring in the horizontal plane. The saccule, on the other hand, has a vertical orientation; it provides sensory information on vertical movements. These two systems constantly inform the CNS about the position of the head and the displacement movements it undergoes in all directions.

The sensory cells of the vestibular system are similar to the hair cells of the cochlea [41]; they are embedded in the epithelium with cilia on their apical pole: stereocilia and kinocilia. These cilia are bathed in a gelatinous layer (the cupula) [52] which, in the utricle and saccule, is peppered with microcrystals (otoliths) that increase its consistency.

When the head moves, the endolymph (due to its inertia) takes time to follow, and thus pushes the cupula in the opposite direction of the movement. This carries the cilia of the sensory cells with it, causing them to depolarize [41] and activate the associated afferent neurons.

Semicircular canals :

The three semicircular canals are each arranged in one of the planes of space [38, 52]; there is an anterior, a posterior, and a lateral canal. Each canal ends near the utricle with a swelling (the ampulla of the canal). The latter contains sensory cells that function according to the same mechanism [38] as those in the saccule and utricle, except that the cupulae at this level do not contain otoliths.

Due to their circular arrangement, the semicircular canals signal the rotational movements of the head.

Transmission - Perception :

The nerve fibers originating from the hair cells of the vestibular system have their cell bodies in Scarpa's ganglion (vestibular ganglion) [50]. From this, the vestibular nerve emerges and joins the cochlear nerve to form the vestibulocochlear nerve (CN VIII).

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