Hair Cell: Structure, Function & Diseases

Hair cells are sensory cells located in the inner ear in the cochlea and in the vestibular organs. They are included in the mechanoreceptor category because they translate sounds and vestibular messages arriving as mechanical stimuli into electrical nerve impulses by means of sensory cilia and can transmit them to the brain via the vestibulocochlear nerve (VIII cranial nerve) and also receive signals from there.

What is the hair cell?

Hair cells are sensory cells that can convert mechanical stimuli into electrical nerve impulses via the deflection of their kinocilia and are therefore classified as mechanoreceptors. The term hair cell is somewhat misleading because the “sensory hairs”, the stereocilia and stereovilli, are not hair-like or cilia-like structures. Rather, they are composed of actin filaments, a structural protein commonly found in the body, and unlike cilia (e.g., in the external auditory canal), they do not have a basal corpuscle. Once the stereocilia are mechanically deflected, they generate an electrical signal that is transmitted to the brain via the vestibulocochlear nerve (VIII cranial nerve). Simultaneously, as the stereocilia are deflected, neurotransmitters are released at the end of the hair cell opposite to them, allowing them to communicate with intermediate interneurons via synapses. Hair cells are mainly located in the cochlea, where they mechanically transform incoming sound waves into electrical impulses and simultaneously release neurotransmitters. Other hair cells are located in the vestibular organs, where they “translate” mechanical translational and rotational accelerations of all possible directions in three-dimensional space into electrical nerve impulses and into messenger substances.

Anatomy and structure

Hair cells, which are located in the cochlea, must be distinguished between the total of 3,500 inner hair cells and the approximately 12,000 outer hair cells, each of which has different functions. Hair cells consist of the cell body, from which “hair bundles” protrude at the upper end, consisting of a stereocilia and stereovilli. Each individual hair cell is connected to the fibers of the vestibulocochlear nerve. The inner hair cells are predominantly afferent fibers that conduct messages from the cells to the appropriate brain centers. The outer hair cells have mainly efferent fibers, receiving instructions and information from the brain. The individual stereovilli of the hair cells located in the cochlea are connected at the tips (tip links). This is not the case with the hair cells in the vestibular organs. The hair cells of each of the 3 arcuates for the perception of rotational accelerations are located in a thickening at the base of each arcuate. The tips of the stereocilia project into the gelatinous cupula and are bent and excited by rotational acceleration in the respective plane due to inertia. In the otolith organs sacculus and utriculus, the gelatinous substance into which the stereocilia protrude is weighted down by so-called otoliths, which consist of calcium carbonate crystals, and is brought out of its rest position during linear accelerations due to inertia, so that the stereocilia are bent and convert the mechanical stimulus into electrical signals.

Function and tasks

The basic function and task of hair cells is to convert mechanical stimuli into electrical nerve impulses while releasing neurotransmitters to communicate with interneurons through synapses. The hair cells, located in the cochlea, have the task of transforming incoming sound signals into electrical impulses in such a way that pitch, timbre and volume can be defined by the auditory centers. To define the pitch, the sensory system uses an active amplifier. In simple terms, the outer hair cells can amplify the incoming tone via self-resonance and self-activity, which is then transformed into an electrical signal and transmitted by the inner hair cells. The hair cells in the vestibular organs have the task of translating accelerations into electrical impulses. Here, too, each individual hair cell is connected to afferent fibers of the vestibulocochlear nerve, so that the movement centers in the brain can “calculate” in which direction the body is currently accelerating translationally and/or rotationally and to what extent.Uniform movements (speed) cannot be detected by the vestibular organs. Decelerations are felt like accelerations in the opposite direction. Due to the principle of inertia, which the vestibular organs make use of, there is a brief false signal after each stopping of a stronger acceleration, because the endolymph in the arcades needs more than one second to come to rest again after an abrupt stopping, for example of a pirouette, due to the inertia. The phenomenon manifests itself in a dizziness that lasts only a short time after a rapid body rotation.

Diseases

Causes of functional impairment up to total functional failure of the hair cells are basically problems in the upstream mechanical processing of the incoming stimuli or problems with the hair cells themselves or functional disorders in the downstream nervous processing of the hair cell signals. In the case of the sense of hearing, temporary functional impairments at the mechanical upstream stage can typically be caused by damage to the eardrum, obstruction of the external auditory canals, or inflammation of the middle ear. In the case of the sense of balance, unusual sensations of movement may occur at the “mechanical” precursor stage due to a viscosity change of the endolymph in the vestibular organs, in addition to an inner ear infection affecting the vestibular organs due to medications or drugs (especially alcohol). Diseases originating in the hair cells themselves are extremely rare and virtually unknown. However, hair cells in the cochlea may experience temporary or permanent damage depending on the duration and intensity of extraordinary noise. Unlike some avian species, hair cells in humans are not regenerable. In addition, hair cells can be irreversibly damaged due to circulatory disturbances caused by lack of oxygen. Dysfunction in the nervous processing of hair cell signals can be caused by lesions of the vestibulocochlear nerve or by hematomas in the brain or by brain tumors or other nervous impairments.

Typical and common ear disorders

  • Ear flow (otorrhea)
  • Otitis media
  • Ear canal inflammation
  • Mastoiditis
  • Ear furuncle