Structure and Function: The Sense of Balance

The ability to stand upright and move without falling over depends on our sense of balance. Structures in the inner ear, known as the vestibular apparatus, contribute to balance by detecting the position and movements of the head. The vestibular apparatus is composed of three semi-circular canals and the two-chambered vestibule.

The role of hair cells

The head’s movements are detected by hair cells found in structures called cristae in the semicircular canals and in two structures called maculae in the vestibule.

Location

Inside a macula

This magnified image shows hair cells surrounded by supporting cells inside a macula. When the hairs move, the hair cells send electrical impulses to the brain.

Linear movement and static position

The two maculae within the vestibule of the inner ear sense linear movements – for example, when travelling by car or using a lift – and the orientation of the head relative to gravity. Detecting the head’s position in relation to gravity helps us, for example, to know instantly which way is up when we dive into deep water.

Head upright

When the head is held upright, the gelatinous membrane of the macula is stable and the hairs on the hair cells remain in an upright position.

Head in tilted position

If the head is held in a tilted position, the pull of gravity displaces the gelatinous membrane. The embedded sensory hairs are bent, triggering the hair cells to produce electrical signals.

Rotational movement

Rotational movements of the head are detected by the cristae in the fluid-filled semicircular canals. The three semicircular canals are at right angles to each other, so head rotation in any direction is detected by at least one canal. The information is used both to maintain balance and to keep vision stable when the head moves.

Head stationary

The fluid in the semicircular canals is not moving. The cupula is upright and the hair cells of the crista are not being stimulated.

Head turning

As the head turns, the fluid pressure in the semicircular canals displaces the cupula, bending the sensory hairs, which create electrical impulses.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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