A kinocilium is a special type of cilium

Cilium

A cilium is an organelle found in eukaryotic cells. Cilia are slender protuberances that project from the much larger cell body....

 on the apex of hair cell

Hair cell

Hair cells are the sensory receptors of both the auditory system and the vestibular system in all vertebrates. In mammals, the auditory hair cells are located within the organ of Corti on a thin basilar membrane in the cochlea of the inner ear...

s located in the sensory epithelium of the vertebrate

Vertebrate

Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 inner ear

Inner ear

The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...

.

Anatomy in humans

Kinocilia are found on the apical surface of hair cells and are involved in both the morphogenesis of the hair bundle and mechanotransduction. 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

Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...

 via neurotransmitter

Neurotransmitter

Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...

 release.

Role in Hair Bundle Morphogenesis

Each hair cell has a single, microtubular kinocilium. Before morphogenesis 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 moves, microvilli surrounding it begin to elongate and form actin stereocilia

Stereocilia

In the inner ear, stereocilia are the mechanosensing organelles of hair cells, which respond to fluid motion in numerous types of animals for various functions, including hearing and balance. They are about 10–50 micrometers in length and share some similar features of microvilli...

. In many mammals the kinocilium will regress once the hair bundle has matured.

Auditory system

The movement of the hair bundle, as a result of endolymph http://www.ncbi.nlm.nih.gov/sites/pubmed flow, will cause potassium channels on the stereocilia to open. This is mostly due to the 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 which K+ channels stay closed. Endolymph composition resembles that of the intracellular fluid (more K+ and less Na+) more closely compared to its counterpart, perilymph

Perilymph

Perilymph is an extracellular fluid located within the cochlea in two of its three compartments: the scala tympani and scala vestibuli. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid...

 which resembles the extracellular fluid

Extracellular fluid

Extracellular fluid usually denotes all body fluid outside of cells. The remainder is called intracellular fluid.In some animals, including mammals, the extracellular fluid can be divided into two major subcompartments, interstitial fluid and blood plasma...

 (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 ganglion

Spiral ganglion

The spiral ganglion is the group of nerve cells that serve the sense of hearing by sending a representation of sound from the cochlea to the brain...

.

Vestibular apparatus

Kinocilia are present in the crista ampullaris of the semicircular ducts and the sensory maculae of the utricle and saccule. One kinocilium is the longest cilium located on the hair cell next to 40-70 sterocilia. During movement of the body, the hair cell is depolarized when the sterocilia 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 sterocilia 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.

Anatomy in fish and frogs

The apical surface

Surface

In mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball...

 of a sensory fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 hair cell usually has numerous stereocilia

Stereocilia

In the inner ear, stereocilia are the mechanosensing organelles of hair cells, which respond to fluid motion in numerous types of animals for various functions, including hearing and balance. They are about 10–50 micrometers in length and share some similar features of microvilli...

 and a single, much longer kinocilium. Unlike mammals, the kinocilium does not regress and remains as part of the hair bundle after maturation of hair cells. Deflection

Deflection

Deflection or deflexion may refer to:* Deflection , the displacement of a structural element under load* Deflection , a technique of shooting ahead of a moving target so that the target and projectile will collide...

 of the stereocilia toward or away from the kinocilium causes an increase or decrease in the firing rate of the sensory neuron

Neuron

A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

 innervating the hair cell

Hair cell

Hair cells are the sensory receptors of both the auditory system and the vestibular system in all vertebrates. In mammals, the auditory hair cells are located within the organ of Corti on a thin basilar membrane in the cochlea of the inner ear...

 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

Cupula

The cupula is a structure in the vestibular system, providing proprioception.The cupula is located within the ampullae of each of the three semicircular canals...

. The hair cells therefore can not be seen and do not appear on the surface of skin of fish and frogs.

See also

• Fish anatomy
  Fish anatomy
  Fish anatomy is primarily governed by the physical characteristics of water, which is much denser than air, holds a relatively small amount of dissolved oxygen, and absorbs more light than air does.- Body :...
  
  
• Utricle
• Otolith
  Otolith
  An otolith, , also called statoconium or otoconium is a structure in the saccule or utricle of the inner ear, specifically in the vestibular labyrinth of vertebrates. The saccule and utricle, in turn, together make the otolith organs. They are sensitive to gravity and linear acceleration...