A kinocilium is a special type of

cilium The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike pr ...

on the apex of

hair cell Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...

s located in the sensory

epithelium Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...

of the

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...

. Contrasting with stereocilia, which are numerous, there is only one kinocilium on each hair cell. The kinocilium can be identified by its apical position as well as its enlarged tip. Together with stereocilia, the kinocilium regulates depolarization and hyperpolarization of the hair cell, which is a

neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

that can generate

action potentials An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific cell rapidly rises and falls. ...

. When the stereocilia and kinocilium move further apart, the cell hyperpolarizes. When they move closer together, the cell depolarizes and may fire an action potential.

Anatomy in humans

Kinocilia are found on the apical surface of

s and are involved in both the morphogenesis of the hair bundle and

mechanotransduction In cellular biology, mechanotransduction ('' mechano'' + '' transduction'') is any of various mechanisms by which cells convert mechanical stimulus into electrochemical activity. This form of sensory transduction is responsible for a number o ...

. Vibrations (either by movement or sound waves) cause displacement of the hair bundle, resulting in depolarization or hyperpolarization of the hair cell. The depolarization of the hair cells in both instances causes

signal transduction Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a biochemical cascade, series of molecular events. Proteins responsible for detecting stimuli are generally termed receptor (biology), rece ...

via

neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a Chemical synapse, synapse. The cell receiving the signal, or target cell, may be another neuron, but could also be a gland or muscle cell. Neurotra ...

release.

Role in hair bundle morphogenesis

Each hair cell has a single,

microtubular Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nanometer, nm and have an inner diameter bet ...

kinocilium. Before

morphogenesis Morphogenesis (from the Greek ''morphê'' shape and ''genesis'' creation, literally "the generation of form") is the biological process that causes a cell, tissue or organism to develop its shape. It is one of three fundamental aspects of deve ...

of the hair bundle, the kinocilium is found in the center of the apical surface of the hair cell surrounded by 20-300 microvilli. During hair bundle morphogenesis, the kinocilium moves to the cell periphery dictating hair bundle orientation. As the kinocilium does not move, microvilli surrounding it begin to elongate and form actin stereocilia. In many mammals, but not in humans, the kinocilium will regress once the hair bundle has matured.

Auditory system

The movement of the hair bundle, as a result of

endolymph Endolymph is the fluid contained in the membranous labyrinth of the inner ear. The major cation in endolymph is potassium, with the values of sodium and potassium concentration in the endolymph being 0.91 mM and 154 mM, respectively. ...

flow, will cause potassium channels on the stereocilia to open. This is mostly due to the pulling force stereocilia exerts on its neighboring stereocilia via interconnecting links that hold stereocilia together (usually from tallest to shortest) and this leads to the depolarization of the hair cell. This pattern of depolarization should not be confused with the more common depolarization which involves the influx of Na+ into the cell while K+ channels stay closed. Endolymph composition resembles that of the

intracellular fluid The human body and even its individual body fluids may be conceptually divided into various fluid compartments, which, although not literally fascial compartment, anatomic compartments, do represent a real division in terms of how portions of the ...

(more K+ and less Na+) more closely compared to its counterpart, perilymph which resembles the

extracellular fluid In cell biology, extracellular fluid (ECF) denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 50–60% (range 45 to 75%) of total body weight; women and the obese typically ha ...

(more Na+ and less K+ compared to intracellular matrix). This depolarization will open voltage gated calcium channels. The influx of calcium then triggers the cell to release vesicles containing excitatory neurotransmitters into a synapse. The post-synaptic neurite then sends an action potential to the Spiral Ganglia of Gard. Unlike the hair cells of the crista ampullaris or the maculae of the saccule and utricle, hair cells of the cochlear duct do not possess kinocilia.

Vestibular apparatus

Kinocilia are present in the crista ampullaris of the semicircular ducts and the sensory maculae of the utricle and

saccule The saccule (Latin: sacculus) is a bed of sensory cells in the inner ear that detects linear acceleration and head tilting in the vertical plane, and converts these vibrations into electrical impulses to be interpreted by the brain. When the he ...

. One kinocilium is the longest cilium located on the hair cell next to 40–70 stereocilia. During movement of the body, the hair cell is depolarized when the stereocilia move toward the kinocilium. The depolarization of the hair cell causes neurotransmitter to be released and an increase in firing frequency of cranial nerve VIII. When the stereocilia tilt away from the kinocilium, the hair cell is hyperpolarized, decreasing the amount of neurotransmitter released, which decreases the firing frequency of cranial nerve VIII.Ross. 2006. Histology: A Test and Atlas

Anatomy in fish and frogs

The apical surface of a sensory

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

hair cell usually has numerous stereocilia and a single, much longer kinocilium. Deflection of the stereocilia toward or away from the kinocilium causes an increase or decrease in the firing rate of the sensory

innervating the

at its basal surface. Hair cells in fish and some frogs are used to detect water movements around their bodies. These hair cells are embedded in a jelly-like protrusion called cupula. The hair cells therefore can not be seen and do not appear on the surface of skin of fish and frogs.

References

External links

* https://web.archive.org/web/20120414213627/http://www.unmc.edu/physiology/Mann/pix_4b/hair_cell.gif {{Authority control Vestibular system Auditory system Fish anatomy