The balance system or vestibular system, is the sensory system that provides the dominant input about our movement and orientation in space. Together with the cochlea, the auditory organ, it is situated in the vestibulum in the inner ear.

As our movements consist of rotations and translations, the vestibular system comprises two components:

• The semicircular canals: Indicate rotational movements.
  
• The Otoliths: Indicate linear translations.

The semicircular canals are three half-circular, interconnected tubes located inside each ear that are the equivalent of three gyroscopes located in three planes perpendicular. The vertical canals are positioned at an angle of about 100 degrees relative to one another, while the horizontal canal makes an angle of about 95 degrees with the posterior canal and an angle of about 110 degrees with the anterior canal. Deviations up to 10-15 degrees between individuals are normal. Because the angles between the canals are not perpendicular, movements of the head stimulate horizontal and vertical canals simultaneously.

The three canals are:

• Superior semicircular canal
• Posterior semicircular canal
• Horizontal semicircular canal

Each canal is filled with a fluid called endolymph and contains a motion sensor with little hairs (cilia) whose ends are embedded in a gelatinous structure called the cupula.
The Semicircular canals are a component of the Labyrinth.

Humans have the ability to control posture and movements of the body and eyes relative to the external environment. The vestibular system mediates these motor activities through a network of receptors and neural elements. This system integrates peripheral sensory information from vestibular, somatosensory, visceromotor, and visual receptors, as well as motor information from the cerebellum and cerebral cortex. Central processing of these inputs occurs rapidly, with the output of the vestibular system providing an appro priate signal to coordinate relevant movement reflexes.

Although the vestibular system is considered to be a special sense, most vestibular activity is conducted at a subconscious level. However, in situations producing unusual or novel vestibular stimulation, such as rough air in a plane flight or wave motion on ships, vestibular perception becomes acute, with dizziness, vertigo, or nausea often resulting.

The vestibular system is an essential component in the production of motor responses that are crucial for daily function and survival. Throughout evolution, the highly conserved nature of the vestibular system is re vealed through striking similarities in the anatomic or ganization of receptors and neuronal connections in fish, reptiles, birds, and mammals.

The vestibular system can be divided into five components:

• The peripheral receptor apparatus resides in the inner ear and is respon sible for transducing head motion and position into neural information.
• The central vestibular nuclei comprise a set of neurons in the brainstem that are responsible for receiving, integrating, and distributing information that controls motor activities such as eye and head movements, postural reflexes, and gravity-dependent autonomic reflexes and spatial orientation.
• The vestibulo-ocular network arises from the vestibu lar nuclei and is involved in the control of eye move ments.
• The vestibulospinal network coordinates head movements, axial musculature, and postural reflexes.
• The vestibulo-thalamo-cortical network is responsible for the conscious perception of movement and spatial orientation.

Vestibular Receptor Organs. The five vestibular receptor organs in the inner ear complement each other in function. The semicircular canals (horizontal, anterior, and posterior) transduce rotational head movements (angular accelerations). The otolith organs (utricle and saccule) respond to translational head movements (linear accelerations) or to the orientation of the head relative to gravity. Each semicircular canal and otolith organ is spatially aligned to be most sensi tive to movements in specific planes in three-dimen sional space.

In humans, the horizontal semicircular canal and the utricle both lie in a plane that is slightly tilted anterodorsally relative to the naso-occipital plane. When a person walks or runs, the head is normally declined (pitched downward) by approximately 30 degrees, so that the line of sight is directed a few meters in front of the feet. This orientation causes the plane of the horizontal canal and utricle to be parallel with the earth horizontal and perpendicular to gravity. The anterior and posterior semicircular canals and the sac cule are arranged vertically in the head, orthogonal to the horizontal semicircular canal and utricle. The two vertical canals in each ear are positioned orthogonal to each other, whereas the plane of the anterior canal on one side of the head is coplanar with the plane of the contralateral posterior canal.