Hair cell
Hair cells or hair cells are a type of secondary sensory cells (receptors ) in the nervous system of vertebrates that convert mechanical stimuli into neural activity. They are in fact the class of mechanoreceptors. Hair cells can be excited depending on the type of sound, water currents, rotary or linear acceleration. The best studied are the hair cells of the inner ear of mammals.
Construction
Hair cells are made from the cell body and the eponymous hair-like structures that are used for receiving the stimulus. This hair bundle is located at the upper end of the cell and is composed of a plurality of cilia and Stereovilli, the cilia of the hair cells of the cochlea, in contrast to those of the equilibrium apparatus are re-formed after birth. Each Stereovilli are connected together at the tips, these compounds are called " tip- left". At the bottom, the hair bundle opposite end of the cell there is a region in which the excitation of the hair cell leads to a release of chemical messengers, the neurotransmitters. Here hair cells form synapses with interneurons that carry information in the form of action potentials on the central nervous system (CNS).
Function
Receiving stimuli - transduction
The decisive stimulus for the receiving structure of the inner hair cells form the hair bundle. The individual Stereovilli are connected at the tops by the tip links. At the bottom of these compounds ( at the shorter Stereovilli ) is the ion channel, the so-called Transduktionskanal, which is opened or closed depending on the voltage of the tip - link. However, it was the molecule which forms the Transduktionskanal not yet been identified. The opening of the channels leading to an influx of positive potassium ions that depolarize the cell with it. Without a deflecting force acting on the hair bundle, the channels are only partially open - the cell is so excited mediocre at rest. For deflections of Stereovilli toward the cilia, the channels are open and slide it over the influx of potassium to an excitation of the hair cell. Deflections against the cilium close the channels. Movements on a different axis than the determined by Zilienanordnung not lead to a change of the channel opening and thus do not play any role in the excitation state of the cell.
Forwarding the excitation
Unlike most of the sensory cells of the hair cells do not form action potentials that could not be generated at the high speed of the moving acoustic receptor. The amount of the distributed transmitter is rather determined by the height of the receptor potential, which depends on the time average of the deflection of Stereovilli. When hair cells in the inner ear of the people, therefore we also speak of a " microphone potential."
Transduction of hair cells in the inner ear
In the cochlea of the human inner ear are three rows of outer and one row of inner hair cells are found. The sensory receiving mechanical movements in the cochlea occurs almost exclusively by the inner hair cells, while the outer hair cells receive efferent innervation esp. by higher centers of the CNS. In principle, the transduction of mechanical deflection of the (inner) hair cells in the inner ear in an electrical signal as described above by Kaliumioneneinstrom. However, there are some special features.
Ion distribution
The lower basal part of the hair cell is surrounded by Corti lymph, which is located in the inner and outer tunnel and Nuel space of the Corti organ and which is similar in its composition to the perilymph - the liquid that which vestibuli Scala ( and scala tympani ) fills. The tip of the hair cell with the Stereovilli located in the endolymph of the scala media. The perilymph has a high concentration of sodium and a low concentration of potassium ions. In the endolymph, this ratio is reversed ( many potassium ions, sodium ions bit ). A voltage difference exists between these two outer regions of the hair cell: the endolymph (top) is compared with the perilymph (bottom) 85 mV positively charged. In the rest position (when no deflection of the Stereovilli done ), the cytoplasm of the hair cell to the perilymph is negatively charged. In the upper part of the hair cells, which is surrounded by the endolymphen liquid exists between the cell interior and the environment, a potential difference of -155 mV. In the lower range of cells, which is surrounded by the perilymph, there is a voltage difference of around -70 mV.
Depolarization
The hair cells Stereovilli deflected by mechanical oscillations of the basilar membrane of the cochlea in the direction of the longest stereocilium this causes ( as described above ) via tip- link connections, the opening of potassium channels in the hair cells. At the top of the hair cell ( Endolymphflüssigkeit ) leads to K- Ioneneinstrom. This influx comes from the fact that the cell interior is loaded -155 mV more negative than the endolymph. This means that positive charges flow in the form of K ions. The chemical equilibrium potential of potassium is 0 mV, as intracellularly the same concentration as in the endolymph prevails, but with the electric potential of -155 mV " committed " to the voltage difference between positivize Zelläußerem and heart. The potassium ions inside the cell cause the opening of calcium channels, causing calcium flows. This leads, as in other neurons to depolarize and thus to increased secretion of neurotransmitters to downstream neurons.
Hyperpolarization
The specificity of the transduction is that potassium is responsible both for the de - and for the repolarization. The flowed into the upper part of the hair cell potassium ions in turn lead to further opening of potassium channels in the whole cell membrane. The increasingly available through the depolarization of calcium leads, inter alia, also to the opening of K channels. At the bottom, surrounded by perilymph cell range but is at -45 mV lower voltage difference to the environment and at the top. The above flowed potassium flows through potassium channels in the lower part of the cell again as
- There is a very low concentration of potassium in the perilymph in comparison to the cell interior
- Potassium is striving to establish its equilibrium potential of -80 mV
The latter means that positive charges must escape in the form of K ions in order to reduce the voltage difference of -45 mV to -80 mV. The potassium efflux through it comes to the hair cell repolarization.