Excitability as a special and zoned property of individual cells of an organism is caused by the presence of special properties in them, which are determined by the structure and function of their q and toplasmic membrane. Selective permeability of the membrane to ions Na +, K +, Ca 2+ and C l provides non-equilibrium distribution yk and associated ions between the cell and the extracellular medium, which underlies the formation of the electric charge cells. The mechanism of the active tran with port ions maintains ionic asymmetry, and specializes in the system of Baths receptor proteins embedded in the membrane allows the cell into a receive electrical and chemical signals from the environment.
All excitable cells are covered outside by a membrane, which has received and the name of the cytoplasmic, or simply plasmatic, membrane. Inside the cell membrane structures are also available, for example, of membrane and us mitochondria, the cell nucleus, the endoplasmic reticulum. However, in their properties they differ from the plasma membrane
The plasma membrane of excitable cells – the thickest of the stand h GOVERNMENTAL membrane, its thickness is 7.5-11 nm. Under electron microscopy of pom she looks like a three-layer structure provided by two electric n but-dense layers, which are separated by a layer of light. Its molecular structure is described by a liquid-mosaic model , according to which it consists of a double phospholipid layer, in which the protein molecules are immersed and with which the eye is bound. Lipid molecules have a hydrophilic, or polar, head and a hydrophobic (non-polar) tail. In addition, the composition of most membranes include cholesterol. In the membrane, the hydrophobic chains face the inside of the bilayer, and the hydrophilic heads face the outside. Ele to throne dense layers match lipid arrangement hydrophilic heads. Composition lipid bilayer of each of the halves (facing the extracellular e accurate medium and into the cytoplasm, respectively) – not identical. Lipids provide the basic physicochemical properties of membranes, including their fluidity at body temperature.
Membrane proteins constitute more than 50 % by weight of the membrane and Hold and vayutsya in the lipid bilayer due to hydrophobic interactions with molec at Lamy lipids. According to their location relative to the lipid bilayer, membrane proteins are divided into two main groups – integral and peripheral. Peripheral proteins are found on the surface IU m branes and loosely associated with it. Integral proteins or fully n of the ship into the lipid bilayer or in part. In addition, many proteins permeate the entire membrane. Part of membrane proteins associated with molec at Lamy oligosaccharides which provide the formation of the glycocalyx (literally – sweet shell), the latter serves as a kind of filter for the surface membrane, as well as for the reception of chemical signals.
From a functional standpoint membrane proteins Ba operate in valued functions specific to the surface of the membrane. Some proteins are ion channels that provide passive transport of ions along a concentration gradient from the external environment into the cell or vice versa. Some proteins perform the function of active ion transport (ion pumps, for example, the Na-K pump). Thus, the proteins in s tread as passive or active carriers of ions and other hydrophilic substances, which transport across hydrophobic membranes forbidden areas. Part of the function of membrane proteins retse n torus , t. e. a specialized structure designed to recognize specific molecules and transmit a message about this event into the cell. Part membrane proteins acting as enzymes osuschest in -governing the transfer of certain groups from one molecule to another. All baa l ki membranes are synthesized in the endoplasmic reticulum, and then sent to the Golgi apparatus, where they are distributed respectively at t stvuyuschie portions membrane. Control over the synthesis of the corresponding proteins is carried out with the participation of genes located in the cell nucleus.
The plasma membrane plays a critical role in the yarn of the inactive cells, particularly excitable (neurons, muscle in a curl and others. Cells). The main function of plasma membrane bookmark th chaetsya in the creation of the necessary activities for the microenvironment cells. H e rarely this function is called a barrier-transport , and since it is of biratelnaya permeability and selective transportation enable the creation of such an environment. This function forms excitable cells and ruyut membrane potential , which change momentary pre d stavlyaet principal sign excitation – the action potential . Additionally t of the second, essential function of the plasma membrane is the reception of external signals , including those coming from other cells through cn e cially device (synapses) or through the blood, lymph or liquor. In the role of such signals are the mediator molecules, hormones, biol about logically active substances. Thus, with the participation of the membrane , the intercellular interaction in the body is carried out . Along with the ability of Stu in of wake cells generate a membrane potential, and the potential dis t Via, ability for intercellular interactions provides all the plural of manifold species CNS activity.
Membrane transport of substances (the transition of a substance from the extracellular environment into the intracellular medium or vice versa) is another important function of the membrane. Any form of transport properties determined n th tolerability substance – its ability to dissolve in water, its softening e ramie, chemical properties, and the gradient (difference) after n centration between the outer and inner surface of the plasma IU m brane. Hydrophobic substances are well passable through the plasma membrane. Therefore, their transportation is mainly determined Nalich and it and oriented concentration gradient – a substance moves from a according to the laws of thermodynamics from the region of its high concentration to the region where the concentration of this substance is lower. Hydrophilic substances cannot freely pass through the plasma membrane, even if they are small. For their transportation needed or sp e cially particles – conveyors or special arrangements, which are based on the change in cell shape. If the transfer substance proish about dit involving transport particles (carrier), in this case in the z are two possible options. First Embodiment – wrapping gradient conc a tion. This form of transport does not require for its implementation in the m of point in time energy consumption (it is spent earlier, when you create a t a whom gradient); therefore, it is conventionally called passive transport. The second option is the transfer of the substance against the gradient of its concentration. In this case, the necessary expenditure of free energy (used ene p ogy, which is released by hydrolysis of ATP, t. e. as a result of the dissociation of this molecule on ADP, and inorganic phosphate). This type of transport is called active transport.
Thus, it is possible to speak about the presence of two types of carriers inside the plasma membrane – passive and active. The process of transport of substances across the plasma membrane may regulirovat s Xia, however the permeability for specific substances – the value MOD e nyaemaya time. This principle is particularly important in respect of H and Tria, potassium, calcium and chlorine – in excitable cells are spetsial s nye mechanisms of regulation of membrane permeability of these ions, allowing it to change over a wide range, including complete cessation of ion transport. Thus there are two main fur and nism such regulation – due to changes in the level of potency of the membrane and la (potential-dependent mechanism) or due to activation of specific cellular receptors (receptor-controlled mechanism).
There are direct and indirect transport. Direct transport is carried out without the participation of carriers and without the cost of energy. It goes by diffusion or filtration, t. e. by type of passive transport. An example of such a mode of transport is the transfer of oxygen as fat about a soluble substance. Mediated transport in all cases, with about completes the involving carrier. At the same time, in some cases this type of transport goes without energy expenditure (facilitated diffusion), and in others – with energy expenditure (active transport).
Passive transport. There are two types of it – simple diffusion and light diffusion. The mechanism of simple diffusion is the transfer of small fat-soluble molecules (O 2 , CO 2 , etc.). Clothed diffusion is carried out through specific channels (including special and ficon ion channels) or with the participation of specific carrier proteins. In this and in the other case, these structures are integral s GOVERNMENTAL membrane proteins, and the transfer material is not cost ene p ology – due to chemical or electrochemical gradient. Using carrier proteins, excitable cells (like other cells) are obtained from the extracellular environment. The direction of the ion flux is determined by chemical ie skim and electrochemical gradient. In particular, it is known that cyt of ions excitable cell plasma concentration Na + is 14 mM and in the extracellular environment – 140 mM. Therefore, the passive flow of ions Na + on n and trievym channels is directed from the extracellular medium into the cytoplasm. Anal on the analogous situation for the flow of ions Ca ++ , as in the extracellular medium, their concentration is much greater than in the cytoplasm. Flow of the ions K + of potassium f vym channels sent on the contrary, from the cell into the medium, since the concentration of ions in the cytoplasm is much greater than in the extracellular medium (150 mM against 4 – 5 mM). Sodium channels in the rest conditions (in particular of STI when the membrane potential is equal to – 80 mV) closed, but when lowered e SRI membrane potential (e.g., +80 mV to +60 mV) open, whereby the flow rate of sodium, entering the cell increases. However, after a certain time (for example, 1 – 2 ms, as observed in a neuron), inactivation of sodium channels occurs. Follows d quences of this process is to reduce (nearly zero) in the incoming adhesive m ku sweat about ka sodium ions. Sodium channels play a crucial role in the activity of neurons, as they provide an initial component of sweat n tial action, ie. e. its phase of depolarization.
Active transport – transport, which is carried out by p and bots so-called pumps, which operate by hydrologo energy and for ATP.
Na , K-pump, or Na, K-ATPase , makes active transport of two ions – ions Na + , which are transferred against the gradient of the end N tration from the cytoplasm to the environment, and the ions K + , which ANSP of mouth transferred from the external environment into the cell, usually Na, the K-pump operates in the electrogenic mode — two K + ions are introduced into the cell of the Na + ion taken out of the cell into the cell .
In addition, as an option of active transport, t. e. transport with the expenditure of energy, distinguish transport with a change in the architecture of the membrane – the pattern of cytosis and endocytosis . In this case, a large molecule, such as a protein molecule, or a group of molecules is thrown out of the cell (exocytosis), or the absorption of this substance from the environment into the cell (endocytosis).