Ion channels physiology. Ion channels, their structure. Classification of ion channels. Sodium and potassium channels. Ligand-gated ion channels

The excitable membrane model according to the Hodgkin-Huxley theory assumes the controlled transport of ions across the membrane. However, the direct passage of an ion through the lipid bilayer is very difficult, and therefore the ion flow would be small.

This and a number of other considerations gave reason to believe that the membrane must contain some special structures - conducting ions. Such structures were found and called ion channels. Similar channels have been isolated from various objects: the plasma membrane of cells, the postsynaptic membrane of muscle cells and other objects. Ion channels formed by antibiotics are also known.

Basic properties of ion channels:

1) selectivity;

2) independence of operation of individual channels;

3) discrete nature of conductivity;

4) dependence of channel parameters on membrane potential.

Let's look at them in order.

1. Selectivity is the ability of ion channels to selectively allow ions of one type to pass through.

Even in the first experiments on the squid axon, it was discovered that Na+ and Kt ions have different effects on the membrane potential. K+ ions change the resting potential, and Na+ ions change the action potential. The Hodgkin-Huxley model describes this by introducing independent potassium and sodium ion channels. It was assumed that the former allow only K+ ions to pass through, and the latter only pass through Na+ ions.

Measurements have shown that ion channels have absolute selectivity towards cations (cation-selective channels) or anions (anion-selective channels). At the same time, various cations of various chemical elements can pass through cation-selective channels, but the conductivity of the membrane for the minor ion, and therefore the current through it, will be significantly lower, for example, for the Na + channel, the potassium current through it will be 20 times less. The ability of an ion channel to pass different ions is called relative selectivity and is characterized by a selectivity series - the ratio of channel conductivities for different ions taken at the same concentration. In this case, for the main ion, selectivity is taken as 1. For example, for the Na+ channel this series has the form:

Na + : K + = 1: 0.05.

2. Independence of the operation of individual channels. The flow of current through an individual ion channel is independent of whether current flows through other channels. For example, K + channels can be turned on or off, but the current through the Na + channels does not change. The influence of channels on each other occurs indirectly: a change in the permeability of some channels (for example, sodium) changes the membrane potential, and this already affects the conductivity of other ion channels.

3. Discrete nature of the conductivity of ion channels. Ion channels are a subunit complex of proteins that span the membrane. In its center there is a tube through which ions can pass. The number of ion channels per 1 μm 2 membrane surface was determined using a radioactively labeled sodium channel blocker - tetrodotoxin. It is known that one TTX molecule binds to only one channel. Then, measuring the radioactivity of a sample with a known area made it possible to show that there are about 500 sodium channels per 1 µm2 squid axon.

Those transmembrane currents that are measured in conventional experiments, for example, on a squid axon 1 cm long and 1 mm in diameter, that is, an area of ​​3 * 10 7 μm 2, are due to the total response (change in conductivity) of 500 3 10 7 -10 10 ion channels. This response is characterized by a smooth change in conductivity over time. The response of a single ion channel changes over time in a fundamentally different way: discretely for Na+ channels, and for K+-, and for Ca 2+ channels.

This was first discovered in 1962 in studies of the conductivity of lipid bilayer membranes (BLMs) when microquantities of a certain excitation-inducing substance were added to the solution surrounding the membrane. A constant voltage was applied to the BLM and the current I(t) was recorded. The current was recorded over time in the form of jumps between two conducting states.

One of the effective methods for experimental study of ion channels was the method of local fixation of membrane potential (“Patch Clamp”), developed in the 80s (Fig. 10).

Rice. 10. Method of local fixation of membrane potential. ME - microelectrode, IR - ion channel, M - cell membrane, SFP - potential clamp circuit, I - single channel current

The essence of the method is that the ME microelectrode (Fig. 10), with a thin end having a diameter of 0.5-1 μm, is suctioned to the membrane so that the ion channel enters its inner diameter. Then, using a potential-clamp circuit, it is possible to measure currents that pass only through a single channel of the membrane, and not through all channels simultaneously, as happens when using the standard potential-clamp method.

The results of experiments performed on various ion channels showed that the conductivity of an ion channel is discrete and it can be in two states: open or closed. Transitions between states occur at random times and obey statistical laws. It cannot be said that a given ion channel will open at exactly this moment in time. You can only make a statement about the probability of opening a channel in a certain time interval.

4. Dependence of channel parameters on membrane potential. Nerve fiber ion channels are sensitive to membrane potential, such as the sodium and potassium channels of the squid axon. This is manifested in the fact that after the start of membrane depolarization, the corresponding currents begin to change with one or another kinetics. This process occurs as follows: The ion-selective channel has a sensor - some element of its design that is sensitive to the action of the electric field (Fig. 11). When the membrane potential changes, the magnitude of the force acting on it changes, as a result, this part of the ion channel moves and changes the probability of opening or closing the gate - a kind of damper that operates according to the “all or nothing” law. It has been experimentally shown that under the influence of membrane depolarization, the probability of the sodium channel transitioning to the conducting state increases. The voltage surge across the membrane created during potential clamp measurements causes a large number of channels to open. More charges pass through them, which means, on average, more current flows. It is important that the process of increasing channel conductivity is determined by an increase in the probability of the channel transitioning to an open state, and not by an increase in the diameter of the open channel. This is the modern understanding of the mechanism of current passage through a single channel.

Smooth kinetic curves of currents recorded during electrical measurements on large membranes are obtained due to the summation of many stepwise currents flowing through individual channels. Their summation, as shown above, sharply reduces fluctuations and gives fairly smooth time dependences of the transmembrane current.

Ion channels can also be sensitive to other physical influences: mechanical deformation, binding of chemicals, etc. In this case, they are the structural basis, respectively, of mechanoreceptors, chemo-receptors, etc.

The study of ion channels in membranes is one of the important tasks of modern biophysics.

Structure of the ion channel.

The ion-selective channel consists of the following parts (Fig. 11): immersed in the bilayer of the protein part, which has a subunit structure; a selective filter formed by negatively charged oxygen atoms, which are rigidly located at a certain distance from each other and allow ions of a certain diameter to pass through; gate part.

The gate of the ion channel is controlled by the membrane potential and can be in either a closed state (dashed line) or an open state (solid line). The normal position of the sodium channel gate is closed. Under the influence of an electric field, the probability of an open state increases, the gate opens and the flow of hydrated ions is able to pass through the selective filter.

If the ion fits in diameter, it sheds its hydration shell and jumps to the other side of the ion channel. If the ion is too large in diameter, such as tetraethylammonium, it is not able to fit through the filter and cannot cross the membrane. If, on the contrary, the ion is too small, then it encounters difficulties in the selective filter, this time associated with the difficulty of shedding the hydration shell of the ion.

Ion channel blockers either cannot pass through it, getting stuck in the filter, or, if they are large molecules like TTX, they sterically match some entrance to the channel. Since blockers carry a positive charge, their charged part is drawn into the channel to the selective filter as an ordinary cation, and the macromolecule clogs it.

Thus, changes in the electrical properties of excitable biomembranes are carried out using ion channels. These are protein macromolecules that penetrate the lipid bilayer and can exist in several discrete states. The properties of channels selective for K + , Na + and Ca 2+ ions may depend differently on the membrane potential, which determines the dynamics of the action potential in the membrane, as well as the differences in such potentials in the membranes of different cells.

Rice. 11. Cross-sectional diagram of the structure of the sodium ion channel of the membrane

Feedback.

Ion channels (IC) are membrane molecular structures formed by integral (transmembrane) proteins that penetrate the cell membrane across the cell membrane in the form of several loops and form a through channel (pore) in the membrane. Channel proteins consist of subunits that form a structure with a complex spatial configuration, in which, in addition to the pore, there are usually additional molecular systems: opening, closing, selectivity, inactivation, reception and regulation. ICs can have not one, but several sites (sites) for binding to control substances (ligands).

ICs consist of proteins of complex structure (channel-forming proteins).

IR proteins have a certain conformation, forming a transmembrane pore, and are “sewn” into the lipid layer of the membrane. A channel protein complex can consist of either one protein molecule or several protein subunits, identical or different in structure. These subunits can be encoded by different genes, synthesized separately on ribosomes, and then assembled into a complete channel. In another case, the channel may be a single polypeptide that crosses the membrane several times in the form of loops. At the beginning of the 21st century, more than 400 channel-forming proteins are known, for the biosynthesis of which 1-2% of the human genome is used.

Domains are individual, compactly formed parts of a channel protein or subunits. Segments are parts of a channel-forming protein that are coiled and stitch the membrane. The terminal domains of the channelformer protein (N- and C-terminal domains) can protrude from the membrane both outside and inside the cell.

Almost all ICs contain in their subunits regulatory domains, capable of binding to various control substances (regulatory molecules) and thereby changing the state or properties of the channel. In voltage-activated IR, one of the transmembrane segments contains a special set of amino acids with positive charges and works as electric potential sensor membranes. When the potential changes, such a sensor changes the state of the channel from open to closed or vice versa. Thus, IC can be controlled by certain external influences, this is their important property.

IC may also include accessory subunits, performing modulatory, structural or stabilizing functions. One class of such subunits is intracellular, located entirely in the cytoplasm, and the second is membrane-bound, because they have transmembrane domains that span the membrane.

Classification of ion channels:

By activation type
-Voltage dependent
-Ligand dependent

Mechanically activated

By selectivity
-Selective (Na, K, Ca, Cl)
-Non-selective

By kinetics
-Fast
-Slow

Mixed

Sodium channel

It is a voltage-gated ion channel that provides a rapid increase in sodium conductance, responsible for the depolarization phase during the development of action potentials in nerve and muscle cells. Channels isolated from mammalian tissues have a molecular weight of ~335,000. Na+ channels interact with various toxins, in particular tetrodotoxin, saxitoxin and scorpion α-toxin, which bind very strongly to channel proteins and can be used in quantitative biochemical measurements.

Potassium channel

Voltage-gated K channels are located in both the plasma membrane and the sarcoplasmic reticulum. A common property of these channels is sensitivity to the inhibitory effects of tetraethylammonium, 4-aminopyridine and Cs, although the effectiveness of these inhibitors on different channel subtypes varies significantly. These channels are activated by depolarization and repolarize the membrane during an action potential. The speed of their inactivation is low - from 100 ms to several seconds.

5. The concept of excitability. Parameters of excitability of the neuromuscular system: threshold of irritation (rheobase), useful time (chronaxy). Dependence of the strength of irritation on the time of its action (Goorweg-Weiss curve). Refractoriness.

Excitability- this is the ability (property) of some physiological systems to respond to external or internal influence with a specialized response - the generation of an action potential.

Cells capable of excitation (in the human body) - muscle, nerve, secretory - are called excitable. All other cells are irritable. It follows from this that irritability is a more general property of living systems, while excitability is a particular and specialized manifestation of irritability.

For any excitable system there is its own minimum stimulus strength that causes excitation. She got the name threshold or rheobase.

Any stimulus must act for at least a certain time to cause an arousal reaction; this time is called latent or useful time.

Chronaxia - This is a special case of the useful time of a stimulus with a value of 2 thresholds (2 rheobases).

Lability– a measure of excitability or the maximum rhythm of impulses that an excitable system can reproduce per unit time. The magnitude of lability is inversely proportional to the duration of the absolute refractory phase, i.e. 1/ARF (sec).

Law duration of irritation. The tissue response depends on the duration of stimulation, but is carried out within certain limits and is directly proportional. There is a relationship between the strength of irritation and the time of its action. This relationship is expressed as a force-time curve. This curve is called the Goorweg-Weiss-Lapik curve. The curve shows that no matter how strong the stimulus, it must act for a certain period of time. If the time period is short, then the response does not occur. If the stimulus is weak, then no matter how long it acts, a response does not occur. The strength of the stimulus gradually increases, and at a certain moment a tissue response occurs. This force reaches a threshold value and is called rheobase (the minimum force of stimulation that causes a primary response). The time during which a current equal to the rheobase operates is called useful time.

Force-time/duration graph (Goorweg-Weiss-Lapick curve)

Refractoriness– (physiological property of excitable tissues) a temporary decrease in excitability simultaneously with excitation occurring in the tissue. Refractoriness can be absolute (there is no response to any stimulus) and relative (excitability is restored, and the tissue responds to a subthreshold or suprathreshold stimulus);

6. Ion pumps (ATPases): K+-Na+-e, Ca2+-e (plasmolemma and sarcoplasmic reticulum). N+-K+-lbmennik.

12. Ion channels...

An ion channel consists of several subunits; their number in an individual ion channel ranges from 3 to 12 subunits. In terms of their organization, the subunits included in the channel can be homologous (of the same type); a number of channels are formed by subunits of different types.

Each of the subunits consists of several (three or more) transmembrane segments (non-polar parts twisted into α-helices), extra- and intracellular loops and terminal sections of domains (represented by the polar regions of the molecules that form the domain and protrude beyond the bilipid layer of the membrane) .

Each of the transmembrane segments, extra- and intracellular loops and terminal sections of domains performs its own function.

Thus, transmembrane segment 2, organized in the form of an α-helix, determines the selectivity of the channel.

The terminal sections of the domain act as sensors for extra- and intracellular ligands, and one of the transmembrane segments plays the role of a voltage-dependent sensor.

The third transmembrane segments in the subunit are responsible for the operation of the gating channel system, etc.

Ion channels operate by the mechanism of facilitated diffusion. The movement of ions through them when the channels are activated follows a concentration gradient. The rate of movement through the membrane is 10 ions per second.

Specificity of ion channels.

Most of them are selective, i.e. channels that allow only one type of ion to pass through (sodium channels, potassium channels, calcium channels, anion channels).

Channel selectivity.

The selectivity of the channel is determined by the presence of a selective filter.

Its role is played by the initial section of the channel, which has a certain charge, configuration and size (diameter), which allows only a certain type of ions to pass into the channel.

Some of the ion channels are non-selective, such as leak channels. These are membrane channels through which K + ions leave the cell at rest along a concentration gradient, but through these channels a small amount of Na + ions also enters the cell at rest along a concentration gradient.

Ion channel sensor.

The ion channel sensor is the sensitive part of the channel that perceives signals, the nature of which can be different.

On this basis, the following are distinguished:

voltage-gated ion channels;

receptor-gated ion channels;

ligand-gated (ligand-dependent);

mechanically controlled (mechanically dependent).

Channels that have a sensor are called controlled. Some channels do not have a sensor. Such channels are called uncontrollable.

Ion channel gate system.

The channel has a gate that is closed when at rest and opens when exposed to a signal. Some channels have two types of gates: activation (m-gate) and inactivation (h-gate).

There are three states of ion channels:

a state of rest when the gate is closed and the channel is inaccessible to ions;

activation state when the gate system is open and ions move through the membrane along the channel;

a state of inactivation when the channel is closed and does not respond to stimuli.

Conduction speed (conduction).

There are fast and slow channels. “Leak” channels are slow, sodium channels in neurons are fast.

The membrane of any cell contains a large set of various (in terms of speed) ion channels, the activation of which determines the functional state of the cells.

Voltage controlled channels.

The voltage-controlled channel consists of:

pores filled with water;

• selective filter;

  activation and inactivation gates;

  voltage sensor.

The diameter of the channel is significantly larger than the diameter of the ion; in the selective filter zone it narrows to atomic sizes, this ensures that this section of the channel performs the function of a selective filter.

The opening and closing of the gate mechanism occurs when the membrane potential changes, with the gate opening at one value of the membrane potential and closing at a different level of membrane potential.

It is believed that changes in the electric field of the membrane are sensed by a special section of the channel wall, which is called a voltage sensor.

A change in its state, caused by a change in the level of membrane potential, causes the conformation of the protein molecules that form the channel and, as a consequence, leads to the opening or closing of the gate of the ion channel.

Channels (sodium, calcium, potassium) have four homologous domains - subunits (I, II, III, IV). The domain (using the example of sodium channels) consists of six transmembrane segments organized in the form of α-helices, each of which plays a different role.

Thus, transmembrane segment 5 plays the role of a pore, transmembrane segment 4 is a sensor that responds to changes in membrane potential, other transmembrane segments are responsible for the activation and inactivation of the channel’s gate system. The role of individual transmembrane segments and subunits is not fully understood.

Sodium channels (internal diameter 0.55 nm) are present in cells of excitable tissues. The density per 1 micron 2 is not the same in different tissues.

Thus, in non-myelinated nerve fibers it is 50-200 channels, and in myelinated nerve fibers (nodes of Ranvier) - 13,000 per 1 µm 2 membrane area. At rest they are closed. The membrane potential is 70-80 mV.

Exposure to the stimulus changes the membrane potential and activates the voltage-gated sodium channel.

It is activated when the membrane potential shifts from the resting potential level towards the critical level of depolarization.

A strong sodium current shifts the membrane potential to a critical depolarization level (CDL).

Changing the membrane potential to -50-40 mV, i.e. to the level of a critical level of depolarization, causes the opening of other voltage-gated Na + channels, through which the incoming sodium current is carried out, forming the “peak” of the action potential.

Sodium ions along the concentration gradient and chemical gradient along the channel move into the cell, forming the so-called incoming sodium current, which leads to further rapid development of the depolarization process.

The membrane potential changes sign to the opposite +10-20 mV. A positive membrane potential causes the closure of sodium channels, their inactivation.

Voltage-gated No+ channels play a leading role in the formation of the action potential, i.e. process of excitation in the cell.

Calcium ions impede the opening of voltage-gated sodium channels, changing response parameters.

TO + -channels

Potassium channels (internal diameter 0.30 nm) are present in cytoplasmic membranes; a significant number of potassium “leak” channels from the cell have been detected.

At rest they are open. Through them, in a state of rest, potassium “leaks” from the cell along a concentration gradient and an electrochemical gradient.

This process is referred to as an outgoing potassium current, which leads to the formation of a resting membrane potential (-70-80 mV). These potassium channels can only be conditionally classified as voltage-gated.

When the membrane potential changes during depolarization, the potassium current is inactivated.

During repolarization, an incoming K+ current is formed through potential-gated channels, which is called the K+ delayed rectification current.

Another type of voltage-gated K + channels. A fast outgoing potassium current appears along them in the subthreshold region of the membrane potential (positive trace potential). Inactivation of the channel occurs due to trace hyperpolarization.

Another type of voltage-gated potassium channels is activated only after preliminary hyperpolarization; it forms a fast transient potassium current that is quickly inactivated.

Calcium ions facilitate the opening of voltage-gated potassium channels, changing response parameters.

Sa + -channels.

Voltage-gated channels make a significant contribution to both the regulation of calcium entry into the cytoplasm and electrogenesis.

The proteins that form calcium channels consist of five subunits (al,a2,b,g,d).

The main subunit al forms the channel itself and contains binding sites for various calcium channel modulators.

Several structurally distinct α-calcium channel subunits have been discovered in mammalian nerve cells (designated A, B, C, D, and E).

Functionally, different types of calcium channels differ from each other in activation, kinetics, single channel conductance, and pharmacology.

Up to six types of voltage-gated calcium channels have been described in cells (T - , L - , N - , P - , Q - , R - channels).

The activity of plasma membrane voltage-gated channels is regulated by various intracellular second messengers and membrane-bound G proteins.

Voltage-gated calcium channels are found in large quantities in the cytoplasmic membranes of neurons, myocytes of smooth, striated and cardiac muscles, and in the membranes of the endoplasmic reticulum.

The SPR Ca 2+ channels are oligomeric proteins embedded in the SPR membrane.

Sa 2+ - controlled by Sa 2+ - SPR channels.

These calcium channels were first isolated from skeletal and cardiac muscle.

It turned out that the SPR Ca 2+ channels in these muscle tissues have molecular differences and are encoded by different genes.

Ca 2+ channels of the SPR in cardiac muscles are directly connected to high-threshold Ca 2+ channels of the plasmalemma (L-type) through calcium-binding proteins, thus forming a functionally active structure - a “triad”.

In skeletal muscle, depolarization of the plasma membrane directly activates the release of Ca 2+ from the endoplasmic reticulum due to the fact that the Ca 2+ channels of the plasma membrane serve as voltage-sensitive transmitters of the activating signal directly to the Ca 2+ channels of the SPR through binding proteins.

Thus, the Ca 2+ stores of skeletal muscles have a mechanism for Ca 2+ release caused by depolarization (RyRl type).

Unlike skeletal muscles, endoplasmic Ca 2+ channels of cardiomyocytes are not connected to the plasmalemma, and to stimulate the release of Ca 2+ from the store, an increase in the concentration of cytosolic calcium (RyR2 type) is required.

In addition to these two types of Ca 2+ -activated Ca 2h channels, a third type of Ca 2+ channels SPR (RyR3 type) has recently been identified, which has not yet been sufficiently studied.

All calcium channels exhibit slow activation and slow inactivation compared to sodium channels.

When a muscle cell depolarizes (protrusions of cytoplasmic membranes - T-tubules approach the membranes of the endoplasmic reticulum), a voltage-dependent opening of calcium channels of the membranes of the sarcoplasmic reticulum occurs.

Since, on the one hand, the calcium concentration in the SPR is high (calcium depot), and the calcium concentration in the cytoplasm is low, and on the other hand, the area of ​​the SPR membrane and the density of calcium channels in it are large, the level of calcium in the cytoplasm increases 100 times.

This increase in calcium concentration initiates the process of myofibril contraction.

Calcium channels in cardiomyocytes are located in the cytoplasmic membrane and belong to L-type calcium channels.

They are activated at a membrane potential of +20-40 mV, forming an incoming calcium current. They remain in an activated state for a long time, forming a “plateau” of the action potential of the cardiomyocyte.

Anion channels.

The largest number of chlorine channels in the cell membrane. There are fewer chlorine ions in the cell compared to the intercellular environment. Therefore, when the channels open, chlorine enters the cell along a concentration gradient and an electrochemical gradient.

The number of channels for HCO 3 is not so large; the volume of transport of this anion through the channels is significantly less.

Ion exchangers.

The membrane contains ion exchangers (transport proteins), which facilitate facilitated diffusion of ions, i.e. accelerated coupled movement of ions through the biomembrane along a concentration gradient; such processes are ATP-independent.

The best known are Na + -H + , K + -H + , Ca 2+ -H + exchangers, as well as exchangers that provide the exchange of cations for anionsNa + -HCO- 3, 2CI-Ca 2+ and exchangers that provide the exchange of cation for cation (Na + -Ca 2+) or anion to anion (Cl- HCO3).

Receptor-gated ion channels.

Ligand-gated (ligand-dependent) ion channels.

Ligand-gated ion channels are a subtype of receptor-gated channels and are always combined with a receptor for a biologically active substance (BAS).

The receptors of the channels under consideration belong to the ionotropic type of membrane receptors, the interaction of which with biologically active substances (ligands) results in rapid reactions.

A ligand-gated ion channel consists of:

pores filled with water;

selective filter;

activation gate;

ligand binding site (receptor). A highly energetically active biologically active substance has a high

affinity (affinity) for a certain type of receptor. When ion channels are activated, certain ions move along a concentration gradient and an electrochemical gradient.

In a membrane receptor, the ligand binding site may be accessible to the ligand from the outer surface of the membrane.

In this case, hormones and parahormones and ions act as ligands.

Thus, when N-cholinergic receptors are activated, sodium channels are activated.

Calcium permeability is initiated by neuronal acetylcholine-gated, glutamate-gated (NMDA and AMPA/kainate-type) receptors and purine receptors.

GABA A receptors are coupled to chloride ion channels, and glycine receptors are also coupled to chloride channels.

In a membrane receptor, the ligand binding site may be accessible to ligands from the inner surface of the membrane.

In this case, protein kinases activated by second messengers or the second messengers themselves act as a ligand.

Thus, protein kinases A, C, G, phosphorylating cation channel proteins, change their permeability.

Mechanically controlled ion channels.

Mechanically gated ion channels change their conductance to ions either by changing the tension of the lipid bilayer or through the cell cytoskeleton. Many mechanically controlled channels are associated with mechanoreceptors; they exist in auditory cells, muscle spindles, and vascular endothelium.

All mechanically controlled channels are divided into two groups:

stretch-activated cells (SAC);

stretch-inactivated cells (SIC).

Mechanically controlled channels have all the main channel characteristics:

time filled with water;

gate mechanism;

stretch sensor.

When a channel is activated, ions move along a concentration gradient.

Sodium, potassium ATPase.

Sodium, potassium ATPase (sodium-potassium pump, sodium-potassium pump).

Consists of four transmembrane domains: two α-subunits and two β-subunits. The α subunit is the large domain, and the β subunit is the small domain. During ion transport, large subunits are phosphorylated and ions move through them.

Sodium, potassium ATPase plays a critical role in maintaining sodium and potassium homeostasis in the intra- and extracellular environment:

maintains a high level of K + and a low level of Na + in the cell;

participates in the formation of the resting membrane potential and the generation of action potentials;

provides Na + coupled transport of most organic substances across the membrane (secondary active transport);

significantly affects H2O homeostasis.

Sodium, a potassium ATPase, makes the most important contribution to the formation of ionic asymmetry in extra- and intracellular spaces.

The step-by-step operation of the sodium and potassium pump ensures an unequal exchange of potassium and sodium across the membrane.

Sa + -ATPase (pump).

There are two families of Ca 2+ pumps responsible for the removal of Ca 2+ ions from the cytoplasm: Ca 2+ pumps of the plasmalemma and Ca 2+ pumps of the endoplasmic reticulum.

Although they belong to the same family of proteins (the so-called P-class ATPases), these pumps show some differences in structure, functional activity and pharmacology.

Found in large quantities in the cytoplasmic membrane. In the cytoplasm of the cell at rest, the calcium concentration is 10-7 mol/l, and outside the cell it is much higher - 10-3 mol/l.

Such a significant difference in concentrations is maintained due to the work of the cytoplasmic Ca ++ -ATPase.

The activity of the plasma membrane Ca 2+ pump is controlled directly by Ca 2+: an increase in the concentration of free calcium in the cytosol activates the Ca 2+ pump.

At rest, there is almost no diffusion through calcium ion channels.

Ca-ATPase transports Ca from the cell to the extracellular environment against its concentration gradient. Along a gradient, Ca+ enters the cell due to diffusion through ion channels.

The membrane of the endoplasmic reticulum also contains a large amount of Ca ++ -ATPase.

The endoplasmic reticulum calcium pump (SERCA) ensures the removal of calcium from the cytosol into the endoplasmic reticulum - a calcium “depot” due to primary active transport.

In the depot, calcium binds to calcium-binding proteins (calsequestrin, calreticulin, etc.).

At least three different isoforms of SERCA pumps have now been described.

SERCA1 subtypes are concentrated exclusively in fast skeletal muscles; SERCA2 pumps are widely distributed in other tissues. The significance of SERCA3 pumps is less clear.

SERCA2 proteins are divided into two different isoforms: SERCA2a, characteristic of cardiomyocytes and smooth muscles, and SERCA2b, characteristic of brain tissue.

An increase in cytosolic Ca 2+ activates the uptake of calcium ions into the endoplasmic reticulum, while an increase in free calcium within the endoplasmic reticulum inhibits SERCA pumps.

H+ K+ -ATPase (pump).

With the help of this pump (as a result of the hydrolysis of one ATP molecule), two potassium ions are transported from the extracellular space into the cell and two H+ ions from the cytosol into the extracellular space during the hydrolysis of one molecule in the parietal cells of the gastric mucosa. This mechanism underlies the formation of hydrochloric acid in the stomach.

Ion pump classF.

Mitochondrial ATPase. Catalyzes the final stage of ATP synthesis. Mitochondrial crypts contain ATP synthase, which couples oxidation in the Krebs cycle and phosphorylation of ADP to ATP.

Ion pump classV.

Lysosomal H + -ATPases (lysosomal proton pumps) - proton pumps that provide transport of H + from the cytosol to a number of organelles - lysosomes, Golgi apparatus, secretory vesicles. As a result, the pH value decreases, for example, in lysosomes to 5.0, which optimizes the activity of these structures.

Features of ion transport

1. Significant and asymmetrical transmembrane! gradient for Na + and K + at rest.

Sodium outside the cell (145 mmol/l) is 10 times more than in the cell (14 mmol/l).

There is approximately 30 times more potassium in the cell (140 mmol/l) than outside the cell (4 mmol/l).

This feature of the distribution of sodium and potassium ions:

homeostatic by the work of Na + /K + -nacoca;

forms an outgoing potassium current at rest (leakage channel);

forms resting potential;

the work of any potassium channels (voltage-dependent, calcium-dependent, ligand-dependent) is aimed at the formation of an outgoing potassium current.

This either returns the state of the membrane to its original level (activation of voltage-dependent channels during the repolarization phase), or hyperpolarizes the membrane (calcium-dependent, ligand-dependent channels, including those activated by second messenger systems).

Please keep in mind that:

the movement of potassium across the membrane is carried out by passive transport;

the formation of excitation (action potential) is always due to the incoming sodium current;

activation of any sodium channel always causes an inward sodium current;

the movement of sodium across the membrane is almost always carried out by passive transport;

in epithelial cells that form the walls of various tubes and cavities in tissues (small intestine, nephron tubules, etc.), in the outer membrane there is always a large number of sodium channels that provide an incoming sodium current upon activation, and in the basement membrane there is a large number of sodium, potassium pumps that pump sodium out of the cell. This asymmetric distribution of these transport systems for sodium ensures its transcellular transport, i.e. from the intestinal lumen, renal tubules into the internal environment of the body;

passive transport of sodium into the cell along an electrochemical gradient leads to the accumulation of energy, which is used for secondary active transport of many substances.

2. Low level of calcium in the cytosol of the cell.

In the cell at rest, the calcium content (50 nmol/l) is 5000 times lower than outside the cell (2.5 mmol/l).

Such a low level of calcium in the cytosol is not accidental, since calcium in concentrations 10-100 times higher than the initial one acts as a second intracellular messenger in the implementation of the signal.

Under such conditions, a rapid increase in calcium in the cytosol is possible due to the activation of calcium channels (facilitated diffusion), which are present in large quantities in the cytoplasmic membrane and in the membrane of the endoplasmic reticulum (the endoplasmic reticulum is the “depot” of calcium in the cell).

The formation of calcium fluxes, which occurs due to the opening of channels, provides a physiologically significant increase in the concentration of calcium in the cytosol.

The low level of calcium in the cytosol of the cell is maintained by Ca 2+ -ATPase, Na + /Ca 2+ -exchangers, and calcium-binding proteins of the cytosol.

In addition to the rapid binding of cytosolic Ca 2+ by intracellular Ca 2+ -binding proteins, calcium ions entering the cytosol can be accumulated by the Golgi apparatus or the cell nucleus and captured by mitochondrial Ca 2+ stores.

3. Low chlorine levels in the cage.

In the cell at rest, the chlorine content (8 mmol/l) is more than 10 times lower than outside the cell (110 mmol/l).

This state is maintained by the work of the K + /Cl- transporter.

The change in the functional state of the cell is associated (or caused) with a change in the permeability of the membrane to chlorine. When voltage- and ligand-gated chlorine channels are activated, the ion enters the cytosol through the channel by passive transport.

In addition, the entry of chlorine into the cytosol is formed due to the Na + /K + /2CH cotransporter and the CH-HCO 3 exchanger.

The entry of chlorine into the cell increases the polarity of the membrane up to hyperpolarization.

Features of ion transport play a fundamental role in the formation of bioelectric phenomena in organs and tissues that encode information, determine the functional state of these structures, their transition from one functional state to another.

All channels present in living tissues, and now we know several hundred types of channels, can be divided into two main types. The first type is rest channels, which spontaneously open and close without any external influences. They are important for generating the resting membrane potential. The second type is the so-called gate channels, or portal channels(from the word "gate") . At rest, these channels are closed and can open under the influence of certain stimuli. Some types of such channels are involved in the generation of action potentials.

Most ion channels are characterized selectivity(selectivity), that is, only certain ions pass through a certain type of channel. Based on this feature, sodium, potassium, calcium, and chloride channels are distinguished. The selectivity of channels is determined by the size of the pore, the size of the ion and its hydration shell, the charge of the ion, as well as the charge of the inner surface of the channel. However, there are also non-selective channels that can pass two types of ions at once: for example, potassium and sodium. There are channels through which all ions and even larger molecules can pass.

There is a classification of ion channels according to activation method(Fig. 9). Some channels specifically respond to physical changes in the neuron's cell membrane. The most prominent representatives of this group are voltage-activated channels. Examples include voltage-sensitive sodium, potassium, and calcium ion channels on the membrane, which are responsible for the formation of the action potential. These channels open at a certain membrane potential. Thus, sodium and potassium channels open at a potential of about -60 mV (the inner surface of the membrane is negatively charged compared to the outer surface). Calcium channels open at a potential of -30 mV. The group of channels activated by physical changes includes

Figure 9. Methods for activating ion channels

(A) Ion channels activated by changes in membrane potential or membrane stretch. (B) Ion channels activated by chemical agents (ligands) from the extracellular or intracellular side.

Also mechanosensitive channels that respond to mechanical stress (stretching or deformation of the cell membrane). Another group of ion channels open when chemicals activate special receptor binding sites on the channel molecule. Such ligand-activated channels are divided into two subgroups, depending on whether their receptor centers are intracellular or extracellular. Ligand-activated channels that respond to extracellular stimuli are also called ionotropic receptors. Such channels are sensitive to transmitters and are directly involved in the transmission of information in synaptic structures. Ligand-activated channels, activated from the cytoplasmic side, include channels that are sensitive to changes in the concentration of specific ions. For example, calcium-activated potassium channels are activated by local increases in intracellular calcium concentration. Such channels play an important role in repolarizing the cell membrane during the termination of an action potential. In addition to calcium ions, typical representatives of intracellular ligands are cyclic nucleotides. Cyclic GMP, for example, is responsible for the activation of sodium channels in the retinal rods. This type of channel plays a fundamental role in the operation of the visual analyzer. A separate type of modulation of channel operation by binding an intracellular ligand is the phosphorylation/dephosphorylation of certain sections of its protein molecule under the action of intracellular enzymes - protein kinases and protein phosphatases.

The presented classification of channels by activation method is largely arbitrary. Some ion channels can only be activated by a few stimuli. For example, calcium-activated potassium channels are also sensitive to changes in potential, and some voltage-activated ion channels are sensitive to intracellular ligands.

Ion channels are integral proteins that provide passive transport of ions along a concentration gradient. The energy for transport is the difference in ion concentration on both sides of the membrane (transmembrane ion gradient).

Non-selective channels have the following properties:

· allow all types of ions to pass through, but the permeability for K+ ions is significantly higher than for other ions;

• are always open.

Selective channels have the following properties:

· allow only one type of ion to pass through; for each type of ion there is its own type of channel;

• can be in one of 3 states: closed, activated, inactivated.

The selective permeability of the selective channel is ensured selective filter, which is formed by a ring of negatively charged oxygen atoms, which is located at the narrowest point of the channel.

Changing the state of the channel is ensured by the operation of the gate mechanism , which is represented by two protein molecules. These protein molecules, so-called. activation gates and initiation gates can block the ion channel by changing their conformation.

In the resting state, the activation gate is closed, the inactivation gate is open (the channel is closed) (Fig. 2.3). When a signal acts on the gate system, the activation gate opens and ion transport through the channel begins (the channel is activated). With significant depolarization of the cell membrane, the inactivation gate closes and ion transport stops (the channel is inactivated). When the MP level is restored, the channel returns to its original (closed) state.

Depending on the signal that causes the activation gate to open, selective ion channels are divided into:

· chemosensitive channels – the signal for the opening of the activation gate is a change in the conformation of the receptor protein associated with the channel as a result of the attachment of a ligand to it.

• voltage-sensitive channels - the signal to open the activation gate is a decrease in MP (depolarization) of the cell membrane to a certain level, which is called the critical level of depolarization (CLD);

According to the activation method, they are distinguished:

· voltage-activated ion channels (the transition from a closed to an open state and back is carried out by the conformation of the protein molecule when the membrane potential changes). An example is the voltage-gated sodium channel, which determines the depolarization of the cell during the generation of an action potential.

· mechanosensitive ion channels (open when a mechanical stimulus is applied to the cell membrane, for example, when skin mechanoreceptors are activated).

· ligand-activated ion channels. According to the method of activation, they are divided into two groups (extracellular and intracellular), depending on which side of the membrane the ligand acts on. If a stimulus (for example, acetylcholine) during synaptic transmission of excitation at the neuromuscular synapse acts on a receptor (in this example, a cholinergic receptor, which is one of several protein subunits of an ion channel) located on the outer surface of the muscle cell membrane, an ion channel will open, permeable for cations. If ligand-activated channels depend on second messengers in the cell, their transition to the open state occurs when the concentration of certain ions in the cytoplasm changes. An example is the calcium-activated potassium channel, which is activated when the concentration of calcium ions in the cell increases. Such channels take part in membrane repolarization at the end of an action potential.

The concept of membrane potential, equilibrium ionic potential and resting potential. Conditions and causes of the existence of poten rest. The equation is constant of the field. Memb potential function.

Conditions and reasons for the existence of the resting potential.

Calculations and experimental data indicate that all cells of the body in a state of “operative” rest are characterized by a certain degree of polarization. The plasmalemma of each cell is charged, and at rest, a negative potential relative to the intercellular medium is maintained on its inner surface. The transmembrane potential difference in different cells is different, but everywhere reaches several tens of millivolts. Using microelectrode technology, it was possible in an experiment to directly measure the real potential difference on both sides of the cell membrane.

What ions and ion channels provide bioelectrogenesis? It is now known that four ions make the main contribution to the resting potential and action potential. Na + K + Ca ++ Cl - are able to penetrate (or not penetrate) under certain conditions through the corresponding ion channels.

In order for a certain ion (having a charge) to penetrate the membrane, it is necessary that the following conditions be present:

1. The presence of a concentration gradient (created by the work of ion pumps)

2. The presence of an electrochemical gradient (created by the sum of the concentrations of charged particles and the properties of ion channels to separate cations and anions on both sides of the membrane).

3. Availability of suitable channels in an open state.

At resting potential the inner side of the cell membrane has a charge, the sign of which (negativity) is determined by presence of organic anions in the cytoplasm(proteins and amino acids) unable to pass through ion channels, and deficiency of counterions– potassium cations that can penetrate through potassium ion channels, as a result of which an excess of negative ions is created in the cell, and an excess of positive charge is created in the interstitium. The magnitude of the negative charge in the cell and the positive charge in the intercellular space can be predicted mathematically, but only for relatively simple cases, for example, for the giant squid axon.

The magnitude of the rest potential is described to a known approximation by the constant field equation proposed by Hodgkin, Goldman and Katz.

Vм=RT/zFln ((pko+pNa o +pCl i) / (pki+pNa i +pCl i))

The concepts should not be confused membrane potential, equilibrium potential And resting potential.

The membrane potential is determined by the sum of the charges acting on both sides of the membrane, which determines the ability of certain ions to penetrate through ion channels.

Equilibrium potential is the potential of the cell plasmalemma at which the total current of a certain ion through the membrane is zero, despite the ability of individual ions to penetrate through open channels in exchange for the same ions flowing in the opposite direction. Determined by the Nernst equation.

Functions of the resting membrane potential:

1. Membrane polarization is a condition for excitation and inhibition.

2. Polarization determines the volume of transmitter release from the presynaptic ending.

3. The PP creates conditions for voltage-gated channels to be in a closed state (membrane polarization creates conditions for the formation of an action potential).

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM

The concept of the nerve center.

Nerve center- a set of structures of the central nervous system, the coordinated activity of which ensures the regulation of individual functions of the body or a certain reflex act. The idea of ​​the structural and functional basis of the nerve center is determined by the history of the development of the doctrine of the localization of functions in the central nervous system. Properties of nerve centers:

2. Slow conduction of excitation through neural complexes of the central nervous system. The synaptic delay T syn of one intercellular contact is approximately 0.5-2 ms. If there are n neurons in the network, the total latency period of the signal in the brain corresponds to n×T syn and can be quite significant. Indirectly, knowing the time of signal transmission through the central nervous system (calculated taking into account the total time of the reflex and the time spent on transmission along the nerve trunks), one can estimate the number of synaptic switches (n) in the arc of a particular reflex.

4. The one-sided conduction of excitation, as well as the divergence and convergence of synaptic inputs create a morphological substrate for the circulation of excitation (reverberation) along closed neural circuits. This phenomenon is believed to underlie short-term memory.

5. Certain neurons associated in nuclei are characterized by background activity. It is determined by the properties of the membrane and depends on spontaneous depolarization. Other neurons are “silent” and generate APs only when synaptic inputs are activated.

6. Neurons and synapses located on their surface are characterized by sensitivity to various substances, signaling molecules and metabolites contained in the cerebrospinal fluid.

7. characterized by fatigue, one of the reasons for which is a decrease in the reserves of the available mediator and the low rate of its synthesis.

8. plasticity. Relief, potentiation (tetanic post-tetanic, long-term), depression are determined by the properties of receptors, trace processes and the appearance of new synaptic contacts or receptors on the surface of neurons.

The nerve networks of the brain are characterized by directional, one-sided (linear) excitation conduction. If there is a chain of neurons connected to each other by synaptic contacts, then due to the property of chemical synapses to release a transmitter from the presynaptic ending into the synaptic cleft and receive it by a receptor localized on the postsynaptic membrane, the vector of excitation propagation in the neural network is directed towards the subsequent postsynaptic neuron. A common example of this principle is the law Bella–Magendie(afferent fibers enter the spinal cord through the dorsal roots, motor fibers leave the spinal cord through the ventral roots).

Processes convergence consist in the convergence of various impulse flows from several nerve cells to the same neuron (see section 4.1.4). The process of convergence is characteristic not only of nerve cells of the same type. For example, on motor neurons of the spinal cord, in addition to primary afferent fibers, fibers of various descending tracts from supraspinal and spinal centers proper, as well as from excitatory and inhibitory interneurons converge. As a result, spinal cord motor neurons function as a common final pathway for numerous nerve structures, including the suprasegmental apparatus of the brain, related to the regulation of motor function.

Divergence is the ability of a nerve cell to establish numerous synaptic connections with various nerve cells. Thanks to this, one nerve cell can participate in several different reactions, transmit excitation to a significant number of other neurons, which can excite a larger number of neurons, providing a wide irradiation of the excitatory process in the central nervous formations.

The structure of a neuron.

Functionally, spinal cord neurons can be divided into 4 main groups:

1) motor neurons, or motor neurons, are cells of the anterior horns, the axons of which form the anterior roots;

2) interneurons - neurons that receive information from the spinal ganglia and are located in the dorsal horns. These neurons respond to pain, temperature, tactile, vibration, proprioceptive stimulation;

3) sympathetic and parasympathetic neurons are located predominantly in the lateral horns. The axons of these neurons exit the spinal cord as part of the ventral roots;

4)) association cells - neurons of the spinal cord’s own apparatus, establishing connections within and between segments.

Motor neurons. The motor neuron axon innervates hundreds of muscle fibers with its terminals, forming a motor neuron unit

Interneurons. These intermediate neurons, generating impulses with a frequency of up to 1000 per second, are background active and have up to 500 synapses on their dendrites. The function of interneurons is to organize connections between the structures of the spinal cord and ensure the influence of ascending and descending pathways on the cells of individual segments of the spinal cord. A very important function of interneurons is inhibition of neuronal activity, which ensures that the direction of the excitation pathway is maintained.

Neurons of the sympathetic division of the autonomic system. Located in the lateral horns of the thoracic spinal cord segments. These neurons are background active, but have a rare firing rate (3-5 per second).

Neurons of the parasympathetic division of the autonomic system. They are localized in the sacral spinal cord and are background active.

Neuroglia, or glia, is a collection of cellular elements of nervous tissue, formed by specialized cells of various shapes. Neuroglial cells fill the spaces between neurons, making up 40% of the brain volume. Glial cells 3-4 times smaller in size than nervous ones; As a person ages, the number of neurons in the brain decreases, and the number of glial cells increases. Classification:

Astrocytes are multi-processed cells with oval-shaped nuclei and a small amount of chromatin. The size of astrocytes is 7-25 microns. located mainly in the gray matter of the brain. The nuclei of astrocytes contain DNA, the protoplasm has a lamellar complex, a centrisome, and mitochondria. astrocytes serve as a support for neurons, provide reparative processes of nerve trunks, isolate nerve fibers, and participate in the metabolism of neurons. Astrocyte processes form “legs” that envelop the capillaries, almost completely covering them. As a result, only astrocytes are located between neurons and capillaries. Apparently, they ensure the transport of substances from the blood to the neuron and back. Astrocytes form bridges between the capillaries and the ependyma lining the cavities of the ventricles of the brain. It is believed that this ensures exchange between the blood and the cerebrospinal fluid of the ventricles of the brain, i.e., astrocytes perform a transport function.

Oligodendrocytes have a small number of processes. They are smaller in size than astrocytes. In the cerebral cortex, the number of oligodendrocytes increases from the upper layers to the lower. There are more oligodendrocytes in the subcortical structures and in the brain stem than in the cortex. Oligodendrocytes are involved in the myelination of axons (therefore there are more of them in the white matter of the brain), in the metabolism of neurons, as well as in the trophism of neurons.

Microglia are represented by the smallest multi-processed glial cells belonging to the wandering cells. The source of microglia is the mesoderm. Microglial cells are capable of phagocytosis.

14.Modern ideas about intercellular contacts.

Synapses are the contacts that establish neurons as independent entities. The synapse is a complex structure and consists of a presynaptic part (the end of the axon that transmits the signal), a synaptic cleft and a postsynaptic part (the structure of the receiving cell).

Classification of synapses. Synapses are classified by location, nature of action, and method of signal transmission.

Based on location, they are classified into neuromuscular, synapses and neuroneuronal, the latter in turn divided into axo-somatic, axoaxonal, axodendritic, dendrosomatic.

According to the nature of the effect on the perceptive structure, synapses can be excitatory or inhibitory.

According to the method of signal transmission, synapses are divided into electrical, chemical, and mixed.

The nature of the interaction of neurons. It is determined by the method of this interaction: distant, adjacent, contact.

Distant interaction can be ensured by two neurons located in different structures of the body. For example, in the cells of a number of brain structures, neurohormones and neuropeptides are formed, which are capable of influencing humoral neurons on neurons of other parts.

Adjacent interaction between neurons occurs when the membranes of neurons are separated only by intercellular space. Typically, such interaction occurs where there are no glial cells between the membranes of neurons. Such contiguity is characteristic of axons of the olfactory nerve, parallel fibers of the cerebellum, etc. It is believed that contiguous interaction ensures the participation of neighboring neurons in the performance of a single function. This occurs, in particular, because metabolites, products of neuron activity, entering the intercellular space, affect neighboring neurons. Adjacent interaction can, in some cases, ensure the transfer of electrical information from neuron to neuron