action potential- a wave of excitation moving along the membrane of a living cell in the process of transmitting a nerve signal. In essence, it represents an electric discharge - a quick short-term change in potential by small area membrane of an excitable cell (neuron, muscle fiber or glandular cell), as a result of which the outer surface of this section becomes negatively charged with respect to neighboring sections of the membrane, while its inner surface becomes positively charged with respect to neighboring sections of the membrane. The action potential is physical basis nerve or muscle impulse that plays a signal (regulatory) role.
An action potential develops on the membrane as a result of its excitation and is accompanied by a sharp change in the membrane potential.
There are several phases in an action potential:
Depolarization phase;
Fast repolarization phase;
Phase of slow repolarization (negative trace potential);
Hyperpolarization phase (positive trace potential).
phase of depolarization. The development of PD is possible only under the action of stimuli that cause depolarization of the cell membrane. When the cell membrane is depolarized to the critical level of depolarization (CDL), an avalanche-like opening of potential-sensitive Na+ channels occurs. Positively charged Na + ions enter the cell along a concentration gradient (sodium current), as a result of which the membrane potential very quickly decreases to 0, and then acquires positive value. The phenomenon of changing the sign of the membrane potential is called the reversal of the membrane charge.
Phase of fast and slow repolarization. As a result of membrane depolarization, voltage-sensitive K + channels open. Positively charged K+ ions leave the cell along a concentration gradient (potassium current), which leads to the restoration of the membrane potential. At the beginning of the phase, the intensity of the potassium current is high and repolarization occurs rapidly; towards the end of the phase, the intensity of the potassium current decreases and repolarization slows down. Ca2+ influx into the cell enhances repolarization. The hyperpolarization phase develops due to the residual potassium current and due to the direct electrogenic effect of the activated Na+/K+ pump. The entry of Cl– into the cell additionally hyperpolarizes the membrane. The change in the value of the membrane potential during the development of the action potential is primarily associated with a change in the permeability of the membrane for sodium and potassium ions.
Modern ideas about the mechanism of its generation
By fixing the membrane potential, it was possible to measure the currents flowing through the axon plasmolemma (axolemma) of the squid and make sure that at rest the current of cations (K +) is directed from the cytoplasm to the interstitium, and during excitation the current of cations (Na +) to the cell dominates. At rest, the plasmalemma almost impermeable to ions located in the intercellular space (Na + C1 - and HCO3 -,).
When excited, the permeability to sodium ions increases sharply for a time equal to several milliseconds, and then falls again.
As a result, cations (Na + ions) and anions (C1 - , HCO3) are separated on the plasma membrane: Na + enters the cytoplasm, but anions do not. The flow of positive charges into the cytoplasm not only compensates for the resting potential, but also exceeds it. There is a so-called "overshoot"(or membrane potential inversion). The incoming flow of sodium is the result of it passive movement along the opened membrane channels along the concentration and electrical gradients. The outgoing flow of this cation is provided by a potassium-sodium pump.5. Laws of irritation: The law of force. The All or Nothing Law
1. Law "all or nothing": With pre-threshold irritations of the cell, the tissue of the response does not occur. At the threshold strength of the stimulus, a maximum response develops, therefore, an increase in the strength of irritation above the threshold is not accompanied by its increase. In accordance with this law, a single nerve and muscle fiber, the heart muscle, responds to stimuli.
2.Law of power: The greater the strength of the stimulus, the stronger the response. However, the severity of the response increases only up to a certain maximum. The law of force obeys a holistic skeletal, smooth muscle, since they consist of numerous muscle cells with different excitability.
3. The law of force-duration. There is a certain relationship between the strength and duration of the stimulus. The stronger the stimulus, the less time it takes for a response to occur. The relationship between the threshold force and the required duration of stimulation is reflected in the force-duration curve. A number of excitability parameters can be determined from this curve.
The “all or nothing” law is the rule according to which an excitable cell does not give a response to subthreshold stimulation, and immediately gives a maximum response to threshold stimulation, and with a further increase in the strength of irritation, the magnitude of the response does not change.
No. 100. Action potential: graphic appearance and characteristics, mechanisms of occurrence and development.
Action potential- a wave of excitation moving along the membrane of a living cell in the process of transmitting a nerve signal. In essence, it represents an electrical discharge - a quick short-term change in potential on a small section of the membrane of an excitable cell (neuron, muscle fiber or glandular cell), as a result of which the outer surface of this section becomes negatively charged with respect to neighboring sections of the membrane, while its inner surface becomes positively charged with respect to neighboring regions of the membrane. The action potential is the physical basis of a nerve or muscle impulse that plays a signal (regulatory) role.
A - calm state; B is the membrane at which the action potential originated.
The basis of any action potential is the following phenomena:
1. The membrane of a living cell is polarized- its inner surface is negatively charged with respect to the outer one due to the fact that in the solution near its outer surface there are more positively charged particles (cations), and near the inner surface there are more negatively charged particles (anions).
2. The membrane has selective permeability- its permeability for various particles (atoms or molecules) depends on their size, electric charge and chemical properties.
3. The membrane of an excitable cell is able to quickly change its permeability for a certain type of cations, causing a transition positive charge from outside to inside.
The third phenomenon is a feature of excitable tissue cells and the reason why their membranes are able to generate and conduct action potentials.
1. prespike- the process of slow depolarization of the membrane to a critical level of depolarization (local excitation, local response).
2. peak potential, consisting of an ascending part (membrane depolarization) and a descending part (membrane repolarization).
3. Negative trace potential- from the critical level of depolarization to the initial level of membrane polarization (trace depolarization).
4. Positive trace potential- an increase in the membrane potential and its gradual return to its original value (trace hyperpolarization).
№101.Potential-dependent ion channels: structure, properties, functioning
The channels are characterized by ion specificity. Channels of one type pass only potassium ions, the other - only sodium ions, etc.
Ion voltage-gated channels are channels that open and close in response to a change in membrane potential, such as sodium channels responsible for action potential If the membrane potential is maintained at the resting potential, there is virtually no sodium current, which means that the sodium channels are closed . If we now shift the membrane potential to positive side and keep it at a constant level, the voltage-dependent sodium channels will open and sodium ions will begin to move into the cell along the concentration gradient. This sodium current will peak and after a few milliseconds, the current drops to almost zero. Once closed, the channels enter an inactivated state, different from the original closed state in which they were able to open in response to membrane depolarization. The channels remain inactivated until the membrane potential returns to its original negative value and a recovery period of several milliseconds ends.
When registering currents in very small sections of the membrane, it was found that the channel opens according to the "all or nothing" principle. Open channels have the same conductivity, but open and close independently of each other, so the total current through the membrane of the entire cell with its numerous channels is determined not by the degree of openness of the channels, but by the probability of being open for each individual channel.
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No. 102. Mechanism and rate of propagation of the action potential along the non-spasmodic nerve fiber.
Conduction velocity in nerve fibers ranges from 0.25 m/sec in very thin unmyelinated fibers
The propagation of the action potential along the nerve fiber (axon) is due to the occurrence of local currents formed between the excited and unexcited parts of the cell. At rest, the outer surface of the cell membrane has a positive potential, and the inner negative. At the moment of excitation, the polarity of the membrane changes to the opposite. As a result, a potential difference arises between the excited and unexcited sections of the membrane, and this leads to the appearance of local currents between these sections. On the surface of the cells, the local current flows from the unexcited area to the excited area, inside the cell - in the opposite direction. Local current irritates neighboring unexcited areas and causes an increase in membrane permeability. This leads to the emergence of action potentials in neighboring areas. At the same time, recovery processes of repolarization occur in the previously excited area. The newly excited area, in turn, becomes electronegative and the resulting local current irritates the area following it. This process is repeated many times and causes the propagation of excitation impulses along the entire length of the cell in both directions. In the nervous system, impulses pass only in a certain direction due to the presence of synapses with one-way conduction.
The specific resistance of biomembranes is high, but due to their small thickness, the insulation resistance is hundreds of thousands of times less than that of a technical cable. Therefore, a homogeneous nerve fiber cannot conduct an electrical signal over long distances.
λ=root of (dR/4p)
d is the diameter of the fiber, R is the surface resistance of the membrane in Ohm * m 2 and p is the specific resistance of the axoplasm in Ohm * m.
With an increase in λ (length constant), the degree of signal attenuation decreases, while the speed of the pulse increases. An increase in the constant length λ can be achieved by increasing the diameter d of the axon.
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No. 103. The mechanism and rate of propagation of the action potential along the myelinated nerve fiber.
In highly organized animals, signal attenuation is prevented by the myelin sheath around the axon. Approximately every 1-3 mm along the myelin sheath there is an intercept of Ranvier.
Its central part is the axon, along the membrane of which the action potential is conducted. The axon is filled with axoplasm, a viscous intracellular fluid.
As λ increases, the degree of signal attenuation decreases, while the rate of impulse conduction increases.
The specific resistance of myelin is much higher than that of other biological membranes. In addition, the thickness of the myelin sheath is many times greater than the thickness of a conventional membrane, which leads to an increase in the fiber diameter and, accordingly, a constant length. λ
Due to the high resistance of the myelin sheath, currents cannot flow along the surface of the axon. When one node is excited, currents arise between it and other nodes. The current approaching another node excites it, causes the appearance of an action potential in this place, and so the process spreads throughout the fiber. The energy costs for signal propagation along a fiber covered with myelin are much less than along an unmyelinated one.
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No. 104. Appointment and definition of reception. Scheme of the movement of information during reception.
reception- this is the perception by the body of the energy of the stimulus that carries information and converts it into electrical signals of nervous excitation.
reception needed for:
1. Optimizing the behavior of a living system depending on the situation in the outside world
2. Continuous regulation of the characteristics of the state of internal organs, environments and tissues of the body
The simplest block diagram (squares 1-9, 5 and 8 - above the line):
1. Source of information
2. Stimulus perceived by the body
3. Device for preparing and collecting a signal for reception
4. Directly the receptor (a device that receives a signal and converts it into electronic impulses)
5. Nerve bundles that conduct impulses to the cortical center
6. Cortical center, which perceives and analyzes primary information
7. CNS - final processing and evaluation of information
8. Efferent nerve pathways that transmit information from the central nervous system to an organ or system, that is, an effector.
9. Performer
No. 105. Receptor definition. Sense organs and analyzers. Examples of the use of reception in the life of the organism.
Receptor- a device that receives a signal and converts it into electronic impulses
Biological analyzers- These are biological systems designed to perceive, and sometimes process information from external and internal environment
Stage threshold: no sensory system is able to perceive a signal of arbitrarily low intensity. It perceives only those signals that are greater than the I stage threshold.
vice of intensity is the minimum unit that causes sensitivity
Kc = I ad.st. / I not ad. Art.
frequency response- stimuli having an oscillatory nature.
With a constant I stimulus (I st = const), but a change in its frequency, an adequate reflection of the picture occurs, but at a certain frequency range, the picture is distorted, at an even greater distance, the signal is no longer perceived.
Amplitude characteristic connects sensation I with stimulus I.
Resolution Limit: the type of difference between signal parameters (either in amplitude or frequency) that, under given conditions, still cause the sensation of change.
Sense organ- a specialized peripheral anatomical and physiological system that has developed in the process of evolution, providing, thanks to its receptors, the receipt and primary analysis of information from the outside world and from other organs of the body itself, that is, from the external and internal environment of the body.
Distant sense organs perceive stimuli at a distance (for example, organs of vision, hearing, smell); other organs (taste and touch) - only with direct contact. Some senses can complement others to a certain extent. For example, a developed sense of smell or touch can to some extent compensate for poorly developed vision.
Examples of the use of reception in the life of the body.??
No. 106. Classification of receptors.
1. According to the method of obtaining information:
Exteroreceptors (from the external environment)
Interoreceptors (from the inside)
2. By the nature of perceived stimuli:
Mechanoreceptors (lung expansion receptors)
Chemoreceptors (receptors for skin reactions, hearing, smell, taste)
Thermoreceptors (heat, cold)
Electroreceptors (lateral lines in fish)
Magnetoreceptors (navigation when moving in birds)
3. According to the degree of universality:
Monomodal - fixing the irritation of only one stimulus
Polymodal - fixing irritation of several stimuli
No. 107. The structure of the receptors.
AtoN(free nerve endings). The axon is divided into nerve endings that have lost the ability to excite, they are polymodal formations.
INO(encapsulated sensitive endings)
They were designed as sensitive specialized monomodal cells. They are modified axons of neurons, sometimes they are epithelial cells.
By internal structure receptors are both simple, consisting of a single cell, and highly organized, consisting of a large number of cells that are part of a specialized sensory organ.
The most primitive receptors are mechanical, responsive to touch and pressure. The difference between these two sensations is quantitative; touch is usually registered by the thinnest endings of neurons located close to the surface of the skin, at the bases of hairs or antennae. There are also specialized organs - Meissner's bodies. Pacinian bodies, consisting of a single nerve ending surrounded by connective tissue, react to pressure. Pulses are excited due to a change in the permeability of the membrane, which occurs due to its stretching.
No. 108. General mechanisms of reception. receptor potentials.
Stage 1: When an adequate stimulus for this receptor arrives. Interacts with the receptor substrate, which is usually found in the cell membrane.
Stage 2: In R: there is a local change in the membrane potential difference. The receptor itself is not an excitable cell, as there are no potential dependent channels! Change - receptor potential (RP), is not subject to the law "all or nothing", depends on the duration of the stimulus and on its intensity.
Stage 3: Potential generation leads to R: to the renewal of the action potential (AP).
Depolarization is called the receptor potential (or generator potential). The receptor potential is due to an increase in the Na+ - conductivity of the dendritic membrane, as a result of which the entry of sodium ions creates a depolarizing receptor potential, which propagates electrotonically to the soma. This primary transformation of a stimulus into a receptor potential is called a transformation, and the receptor is thus a transducer.
The exception is the receptor potentials of the primary visual cells of the retina, which are hyperpolarizing.
The stimulus does not serve as a source of energy for the receptor potential; it only controls, through interaction with membrane processes, the entry of ions through the membrane, based on the transmembrane difference in their concentrations.
The receptor potential electrotonically propagates from the dendrites along the soma, depolarizes the base of the axon, and if the depolarization exceeds the threshold for excitation, a series of action potentials arise in the axon, the frequency of which depends on the amplitude of the receptor potential. Action potentials are conducted in the CNS and carry all information about the magnitude and duration of stimuli in the form of a frequency code.
Action potential - a wave of excitation that moves along the membrane of a living cell in the process of transmitting a nerve signal. In essence, it represents an electrical discharge - a quick short-term change in potential in a small section of the membrane of an excitable cell (neuron, muscle fiber or glandular cell), as a result of which the outer surface of this section becomes negatively charged with respect to neighboring sections of the membrane, while its inner the surface becomes positively charged with respect to neighboring regions of the membrane. The action potential is the physical basis of a nerve or muscle impulse that plays a signal (regulatory) role.
No. 109. Coding information in the senses.
Goals of the biological system:
1. self-preservation
2. procreation
Any information coming to the receptor systems is transferred by a certain physical carrier (long-term analyzer - electromagnetic). The stimulus is converted into a receptor potential and then into an action potential.
v(nu) = k log I(st) – the frequency of the next bursts of AP is proportional to the intensity of the stimulus.
In sensory systems, the coding of the strength of the stimulus is widely used:
1) by changing the frequency of pulses in the fibers;
2) the number of nerve elements involved;
3) the encoding of the quality of the stimulus by a special structure of the response of the receptor and the fiber, the so-called pattern (pattern) of the response, is also widely used.
According to the theory of the structure of the response, the qualities of the stimulus are encoded by the pattern (pattern) of the burst of AP, i.e. the number, frequency and characteristic distribution of action potentials within each burst of impulses, as well as the number, duration, frequency of the bursts themselves, the frequency of their repetition, the duration of the interpulse intervals, etc.
No. 110. Features of light and sound perception. Weber-Fechner law.
Psychophysical law of Weber-Fechner. If an increase in irritation in geometric progression, then the sensation of this irritation increases in an arithmetic progression.
If I (sound intensity) takes on a series of successive values aI 0 ; a 2 I 0 ; a 3 I 0 , then the corresponding sensation - E 0 ; 2E0; 3E 0 ... a is a coefficient, and more than 1.
In other words, the loudness of a sound is proportional to the logarithm of the intensity of the sound. Under the action of 2 sound stimuli I0 and I (I0 - defect of hearing)
E=k*lg(I/I); k - coefficient of proportionality.
Sound reception:
Characterized by:
1. Frequency
2.amplitude
3. Spectrum
Longitudinal acoustic pressure in a certain frequency range.
The absolute threshold of hearing is the I type of sound that is picked up by the ear.
I0=10-12 W/m2 - at frequency measured in kHz
The selectivity coefficient is 10-10.
Auricle
External auditory canal
Eardrum
Light reception:
Light reception - photoreceptors
1. Cones - the implementation of color vision. The principle of operation burns like that of sticks.
2. Sticks - implementation of twilight vision. The retina is a multilayer formation, thick, there is a choroid, etc. The receptors are located at the bottom in the pigment epithelium.
A quantum of light enters the disc membrane. This visual reception is different, because. in other cases, the stimulus is in the receptors themselves, and in the visual receptor in the organelle membrane. In rods, the receptor pigment is rhodoxin; in cones, it is iodoxin. Rhodoxin consists of retinol and oxin, property - it has the ability to conformationally rearrange.
The normal state is the cis state, characterized by roundness. Having caught a quantum of light, a restructuring into a trance state occurs, while a certain amount of energy is released. The process is called photoisomerization.
There is a change in the properties of the disc membrane. An intracellular mediator is born, it transmits g / s c / p effects on the cytomembrane - there is an effect on it (hyperpolarization) - rods / cones.
Receptor potential - a biopotential that occurs when the surface membrane of the receptor is depolarized due to the action of an irritant on it. It is distributed along the cone/rod membrane and reaches the synapse. The signal passed through the synapse excites the axon membrane. Then it is distributed further and goes to the optic nerve. Hyperpolarization occurs due to the fact that the passed internal messenger contributes to the closure of sodium channels and they are called photodependent Na channels.
Color vision problems:
Color blindness (partial color blindness) is a hereditary color vision disorder in humans, consisting in the inability to distinguish between certain colors (mostly red and green). It is explained by the absence of one or more types of cones in the retina.
No. 111. The main characteristics of the auditory analyzer. Mechanisms of auditory reception.
Sound is mechanical vibration elastic medium. Has lens characteristics, i.e. does not depend on our perception.
Characterized by:
1. Frequency
2.amplitude
3. Spectrum
Intensity is the loudness of the sound.
Hearing Analyzer Specifications:
Longitudinal acoustic pressure - in a certain frequency range.
The absolute threshold of hearing is the type of sound that is picked up by the ear.
I 0 \u003d 10 -12 W / m2 - at a frequency measured in kHz
The selectivity coefficient is 10 -10 .
auditory reception. Purpose, structure and operation of sound-perceiving systems.
1. Outer ear (preparation of sound vibrations for reaction)
Auricle
External auditory canal
Eardrum
There are auditory ossicles, ligaments, muscles (middle ear), cochlea, bases. membrane.
Direct and reflected waves pass through the basement membrane. The antinode arises from the interference of these waves.
At the location of the hairs - depolarization comes to oscillation
Irritation of the auditory nerve in the lower part of the BM and through the synapse.
No. 112. The main characteristics of the visual analyzer. Mechanisms of visual reception.
The visual analyzer has an optical system that refracts and focuses incoming light rays and as a result, an image is built on the retina.
Light rays are a stream of these waves. They can be considered as waves and as analogues of some particles = light quanta.
The structure of the visual analyzer.
An adequate stimulus is waves of a certain frequency range. Sensitivity of the visual analyzer - light sensitivity threshold 10 -18 W
The eye is able to perceive light quanta starting from 10 square meters, with a transparent atmosphere, you can see a candle at a distance of 1-3 km. The selectivity coefficient is high 10 -14 .
Frequency response. (400 - 750 Nm). Amplitude response - This logarithmic relationship is performed within 100 times the stimulus measurement.
No. 113. Physical factors of ecological significance. natural background levels.
Ecology is the environmental conditions in which the biosystem is located.
Physical environmental factors (by origin):
Geophysical →Meteorological→Terrestrial
Space: solar, space
Anthropogenic
Physical environmental factors (by physical essence):
magnetic fields (force field acting on moving electric charges and on bodies that have a magnetic moment, regardless of the state of their motion.)
gravitational fields (a physical field through which gravitational interaction is carried out (Gravity is a universal fundamental interaction between all material bodies)
· electric fields→EM: radio emission, television range, locators, UV exposure (skin exposure for DNA)
2. vibration (mechanical vibrations.)
3. radiation
Infrasound (elastic waves similar to sound waves, but with frequencies below the region of frequencies audible to humans. Usually, frequencies of 16-25 Hz are taken as the upper limit of the infrasonic region)
Ultrasound (elastic sound vibrations high frequency)
4. sound factors
5. noise factors
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No. 114. Components of the natural background. Examples of anthropogenic changes in the background values of physical factors.
Background is an average value that characterizes the quantitative value of the environmental factor in a given region.
Background = E ph. (natural background) + a × s (anthropogenic state)
R f.= E f. (radiation of terrestrial rocks, cosmic radiation of radon) + a × s (arises due to testing of poisonous weapons)
M f. = E f. (geomagnetic field, cosmic component of the magnetic field from natural influences) + a×s (electric transport, household appliances, medical research)
Additionally. Forest changes. Each forest area has been or is currently exposed to certain types of anthropogenic impact - even if such impact cannot be directly detected and measured. Typical examples of such ubiquitous types of anthropogenic impacts are global atmospheric pollution, changes in the abundance of game species, or changes in the frequency of forest fires as a result of changes in the density and lifestyle of the population in forest regions.
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No. 115. The value of background radiation for human health.
Radiation radiation is one of the most studied and strong biophysical factors in terms of its impact on living systems. Behind this term lies a spectrum of radiations that are diverse in nature and effect.
One of the dangers of radioactive radiation is related to the fact that a person does not have receptors for it. Human body very sensitive to radioactive damage. Radioactive radiation as a result of influences at the cellular and subcellular level causes the appearance of a large number of free radicals (they are harmful).
There is a lesion of the blood system, the common name is radiation sickness.
Radioprotectors to some extent reduce the effects of radiation.
Penetration:
From mm to α
Up to see for β
For neurotropic radiation up to complete penetration
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No. 116. geomagnetic field. Nature, biotropic characteristics, role in the life of biosystems.
The Earth's magnetic field (geomagnetic field) is a magnetic field generated by intraterrestrial sources.
The structure and characteristics of the Earth's magnetic field
At a small distance from the Earth's surface, about three of its radii, magnetic lines of force have a dipole arrangement. This region is called the Earth's plasmasphere.
As we move away from the Earth's surface, the influence of the solar wind increases: from the side of the Sun, the geomagnetic field contracts, and from the opposite, night side, it stretches into a long "tail".
Field Options
Points of the Earth, in which the magnetic field strength has a vertical direction, are called magnetic poles. There are two such points on Earth: the north magnetic pole and the south magnetic pole.
The straight line passing through the magnetic poles is called the earth's magnetic axis. The circumference of a great circle in a plane that is perpendicular to the magnetic axis is called the magnetic equator. The magnetic field strength at the points of the magnetic equator has an approximately horizontal direction.
Magnetic fields in the free state - 0.4 Oe (Oersted)
The field strength on the Earth's surface is highly dependent on geographic location. The magnetic field strength at the magnetic equator is about 0.34 Oe (Oersted), y magnetic poles about 0.66 e. In some areas (in the so-called regions of magnetic anomalies), the tension increases sharply.
The Earth's magnetic field is characterized by disturbances called geomagnetic pulsations due to the excitation of hydromagnetic waves in the Earth's magnetosphere; the frequency range of pulsations extends from millihertz to one kilohertz.
Magnetic fields in ordinary life have little intensity. They have a high penetrating power. As a result of the study of the magnetic field, a biotropic factor was revealed.
Magnetotherapy - exposure as a magnetic factor.
The magnetic storm has a negative impact.
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No. 117. Possible mechanisms of influence of the geomagnetic field on the body.
1) If there are strongly charged particles in a substance, there is a change in the trajectory of the movement of charges
2) Zeeman Effect: Under the action of the Magnetic Field, the electronic levels of the atom are split into sublevels; weak Magnetic Fields cause this effect in those ions that are involved in metabolism.
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Irritants
By nature, stimuli are divided into:
• physical (sound, light, temperature, vibration, osmotic pressure), electrical stimuli are of particular importance for biological systems;
• chemical (ions, hormones, neurotransmitters, peptides, xenobiotics);
• informational (voice commands, conventional signs, conditioned stimuli).
According to the biological significance, stimuli are divided into:
• adequate - stimuli, for the perception of which the biological system has special adaptations;
• inadequate - irritants that do not correspond to the natural specialization of the receptor cells on which they act.
The stimulus causes excitation only if it is strong enough. Excitation threshold - the minimum strength of the stimulus sufficient to cause excitation of the cell. The expression "excitation threshold" has several synonyms: the threshold of irritation, the threshold strength of the stimulus, the threshold of strength.
Excitation as an active reaction of a cell to a stimulus
The reaction of a cell to external influence (irritation) differs from the reaction of non-biological systems in the following features:
• the energy for the reaction of the cell is not the energy of the stimulus, but the energy generated as a result of metabolism in the biological system itself;
• the strength and form of the cell's reaction is not determined by the strength and form of the external influence (if the strength of the stimulus is above the threshold).
In some specialized cells, the reaction to the stimulus is especially intense. This intense reaction is called excitation. Excitation is an active reaction of specialized (excitable) cells to external influences, manifested in the fact that the cell begins to perform its specific functions.
An excitable cell can be in two discrete states:
• a state of rest (readiness to respond to external influences, committing inner work);
• a state of excitation (active performance of specific functions, performance of external work).
There are 3 types of excitable cells in the body:
nerve cells(excitation is manifested by the generation of an electrical impulse);
• muscle cells (excitation is manifested by contraction);
• secretory cells (excitation is manifested by the release of biologically active substances into the intercellular space).
Excitability - the ability of a cell to move from a state of rest to a state of excitation when exposed to a stimulus. Different cells have different excitability. The excitability of the same cell varies depending on its functional state.
Excitable cell at rest
The excitable cell membrane is polarized. This means that there is a constant potential difference between the inner and outer surface of the cell membrane, which is called membrane potential(MP). At rest, the MF value is -60 ... -90 mV (the inner side of the membrane is negatively charged relative to the outer). The value of the MP of a cell at rest is called resting potential(PP). The MP of a cell can be measured by placing one electrode inside and the other outside the cell (Fig. 1A) .
A decrease in MP relative to its normal level (PP) is called depolarization, and an increase - hyperpolarization. Repolarization is understood as the restoration of the initial MP level after its change (see Fig. 1 B).
Electrical and physiological manifestations of arousal
Let us consider various manifestations of excitation using the example of stimulation of a cell with an electric current (Fig. 2).
Under the action of weak (subthreshold) impulses of electric current, an electrotonic potential develops in the cell. Electrotonic potential(EP) - a shift in the membrane potential of the cell, caused by the action of a direct electric current . EP is a passive reaction of the cell to an electrical stimulus; the state of ion channels and transport of ions do not change in this case. EN is not manifested by the physiological reaction of the cell. Therefore, EP is not excitation.
Under the action of a stronger subthreshold current, a more prolonged shift of the MF occurs - a local response. Local response (LO) is an active reaction of the cell to an electrical stimulus, however, the state of ion channels and ion transport does not change significantly. LO is not manifested by a noticeable physiological reaction of the cell. LO is called local excitement , since this excitation does not spread through the membranes of excitable cells.
Under the action of threshold and superthreshold current, a cell develops action potential(PD). PD is characterized by the fact that the value of the cell's MP very quickly decreases to 0 (depolarization), and then the membrane potential acquires a positive value (+20 ... +30 mV), i.e., the inner side of the membrane is charged positively relative to the outer one. Then the MP value quickly returns to the initial level. Strong depolarization of the cell membrane during PD leads to the development of physiological manifestations of excitation (contraction, secretion, etc.). PD is called spreading excitement, since, having arisen in one section of the membrane, it quickly spreads in all directions.
The mechanism of AP development is practically the same for all excitable cells. The mechanism of conjugation of electrical and physiological manifestations of excitation is different for different types excitable cells (coupling of excitation and contraction, conjugation of excitation and secretion).
The device of the cell membrane of an excitable cell
4 types of ions participate in the mechanisms of development of excitation: K+, Na+, Ca++, Cl - (Ca++ ions are involved in the processes of excitation of some cells, such as cardiomyocytes, and Cl ions are important for the development of inhibition). The cell membrane, which is a lipid bilayer, is impermeable to these ions. In the membrane, there are 2 types of specialized integral protein systems that ensure the transport of ions through the cell membrane: ion pumps and ion channels.
Ion pumps and transmembrane ion gradients
Ionic pumps (pumps)- integral proteins that provide active transport of ions against the concentration gradient. The energy for transport is the energy of ATP hydrolysis. There are Na + / K + pump (pumps Na + out of the cell in exchange for K +), Ca ++ pump (pumps Ca ++ out of the cell), Cl– pump (pumps Cl - out of the cell).
As a result of the operation of ion pumps, transmembrane ion gradients are created and maintained:
• the concentration of Na+, Ca++, Cl is lower inside the cell than outside (in the interstitial fluid);
• the concentration of K+ inside the cell is higher than outside.
ion channels
Ion channels are integral proteins that provide passive transport of ions along the concentration gradient. The energy for transport is the difference in the concentration of ions on both sides of the membrane (transmembrane ion gradient).
Non-selective channels
• pass all types of ions, but the permeability for K+ ions is much higher than for other ions;
• are always open.
selective channels have the following properties:
• pass only one kind of ions; each type of ion has its own type of channels;
• can be in one of 3 states: closed, activated, inactivated.
The selective permeability of the selective channel is provided selective filter , which is formed by a ring of negatively charged oxygen atoms, which is located at the narrowest point of the channel.
Channel state change is provided by operation gate mechanism, which is represented by two protein molecules. These protein molecules, the so-called activation gates and inactivation gates, by changing their conformation, can block the ion channel.
At rest, the activation gate is closed, the inactivation gate is open (the channel is closed) (Fig. 3). When a signal is applied to the gate system, the activation gate opens and the transport of ions through the channel begins (the channel is activated). With a 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 opening of the activation gate, selective ion channels are divided into:
chemosensitive channels
– a signal to open 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
- a signal to open the activation gate is a decrease in the MF (depolarization) of the cell membrane to a certain level, which is called critical level of depolarization
(KUD).
Resting potential formation mechanism
The resting membrane potential is formed mainly due to the release of K + from the cell through non-selective ion channels. Leakage of positively charged ions from the cell leads to the fact that the inner surface of the cell membrane is charged negatively relative to the outer one.
The membrane potential resulting from the leakage of K + is called the "equilibrium potassium potential" ( Ek). It can be calculated from the Nernst equation
where R is the universal gas constant,
T– temperature (in Kelvin),
F is the Faraday number,
[K+] nar is the concentration of K+ ions outside the cell,
[K+] ext is the concentration of K+ ions inside the cell.
PP is usually very close to Ek, but not exactly equal to it. This difference is explained by the fact that the following contribute to the formation of PP:
• entry of Na+ and Cl– into the cell through non-selective ion channels; at the same time, the entry of Cl– into the cell additionally hyperpolarizes the membrane, and the entry of Na+ additionally depolarizes it; the contribution of these ions to the formation of PP is small, since the permeability of non-selective channels for Cl– and Na + is 2.5 and 25 times lower than for K+;
• direct electrogenic effect of the Na+ /K+ ion pump, which occurs if the ion pump works asymmetrically (the amount of K+ ions transferred into the cell is not equal to the amount of Na+ ions taken out of the cell).
Mechanism of action potential development
There are several phases in the action potential (Fig. 4):
• depolarization phase;
• phase of fast repolarization;
• phase of slow repolarization (negative trace potential);
• hyperpolarization phase (positive trace potential).
Depolarization phase. The development of PD is possible only under the action of stimuli that cause depolarization of the cell membrane. When the cell membrane is depolarized to the critical level of depolarization (CDL), an avalanche-like opening of potential-sensitive Na+ channels occurs. Positively charged Na + ions enter the cell along a concentration gradient (sodium current), as a result of which the membrane potential very quickly decreases to 0, and then acquires a positive value. The phenomenon of changing the sign of the membrane potential is called reversion membrane charge.
Phase of fast and slow repolarization. As a result of membrane depolarization, voltage-sensitive K + channels open. Positively charged K+ ions leave the cell along a concentration gradient (potassium current), which leads to the restoration of the membrane potential. At the beginning of the phase, the intensity of the potassium current is high and repolarization occurs rapidly; towards the end of the phase, the intensity of the potassium current decreases and repolarization slows down.
Hyperpolarization phase develops due to the residual potassium current and due to the direct electrogenic effect of the activated Na + / K + pump.
Overshoot is the period of time during which the membrane potential has a positive value.
threshold potential is the difference between the resting membrane potential and the critical level of depolarization. The value of the threshold potential determines the excitability of the cell - the greater the threshold potential, the lower the excitability of the cell.
Changes in cell excitability during the development of excitation
If we take the level of cell excitability in a state of physiological rest as the norm, then in the course of the development of the excitation cycle, its fluctuations can be observed. Depending on the level of excitability, the following states of the cell are distinguished (see Fig. 4).
• Supernormal excitability ( exaltation ) is the state of the cell in which its excitability is higher than normal. Supernormal excitability is observed during the initial depolarization and during the slow repolarization phase. An increase in cell excitability in these phases of AP is due to a decrease in the threshold potential compared to the norm.
• Absolute refractoriness - the state of the cell in which its excitability drops to zero. No, even the strongest, stimulus can cause additional excitation of the cell. During the depolarization phase, the cell is non-excitable because all of its Na+ channels are already open.
• Relative refractoriness - a state in which the excitability of the cell is much lower than normal; only very strong stimuli can excite the cell. During the repolarization phase, the channels return to their closed state and the excitability of the cell is gradually restored.
• Subnormal excitability is characterized by a slight decrease in cell excitability below the normal level. This decrease in excitability is due to the increase in threshold potential during the hyperpolarization phase.
- prespike- the process of slow depolarization of the membrane to a critical level of depolarization (local excitation, local response).
- Peak potential, or spike, consisting of an ascending part (membrane depolarization) and a descending part (membrane repolarization).
- Negative trace potential- from the critical level of depolarization to the initial level of membrane polarization (trace depolarization).
- Positive trace potential- an increase in the membrane potential and its gradual return to its original value (trace hyperpolarization).
General provisions
The polarization of the membrane of a living cell is due to the difference in the ionic composition of its inner and outer sides. When the cell is in a calm (unexcited) state, ions different sides membranes create a relatively stable potential difference called the resting potential. If an electrode is inserted into a living cell and the resting membrane potential is measured, it will have a negative value (of the order of -70 - -90 mV). This is explained by the fact that the total charge on the inner side of the membrane is significantly less than on the outer one, although both sides contain both cations and anions. Outside - an order of magnitude more sodium, calcium and chlorine ions, inside - potassium ions and negatively charged protein molecules, amino acids, organic acids, phosphates, sulfates. It must be understood that we are talking about the charge of the membrane surface - in general, the environment both inside and outside the cell is neutrally charged.
The membrane potential can change under the influence of various stimuli. An artificial stimulus can be an electric current applied to the outer or inner side of the membrane through the electrode. Under natural conditions, the stimulus is often a chemical signal from neighboring cells, coming through the synapse or by diffuse transmission through the intercellular medium. The shift of the membrane potential can occur in the negative ( hyperpolarization) or positive ( depolarization) side.
In the nervous tissue, an action potential, as a rule, occurs during depolarization - if the depolarization of the neuron membrane reaches or exceeds a certain threshold level, the cell is excited, and an electrical signal wave propagates from its body to the axons and dendrites. (In real conditions, postsynaptic potentials usually arise on the body of a neuron, which are very different from the action potential in nature - for example, they do not obey the “all or nothing” principle. These potentials are converted into an action potential at a special section of the membrane - the axon hillock, so that action potential does not extend to dendrites).
This is due to the fact that there are ion channels on the cell membrane - protein molecules that form pores in the membrane through which ions can pass from the inside of the membrane to the outside and vice versa. Most of the channels are ion-specific - the sodium channel passes practically only sodium ions and does not pass others (this phenomenon is called selectivity). The cell membrane of excitable tissues (nerve and muscle) contains a large amount of potential-dependent ion channels capable of quickly responding to a shift in the membrane potential. Membrane depolarization primarily causes voltage-gated sodium channels to open. When enough sodium channels open at the same time, positively charged sodium ions rush through them to the inside of the membrane. The driving force in this case is provided by the concentration gradient (there are many more positively charged sodium ions on the outside of the membrane than inside the cell) and the negative charge on the inside of the membrane (see Fig. 2). The flow of sodium ions causes an even larger and very rapid change in the membrane potential, which is called action potential(in the special literature it is designated PD).
Rice. 3. The simplest circuit, showing a membrane with two sodium channels open and closed, respectively
According to all-or-nothing law the cell membrane of an excitable tissue either does not respond to the stimulus at all, or responds with the maximum possible force for it at the moment. That is, if the stimulus is too weak and the threshold is not reached, the action potential does not arise at all; at the same time, a threshold stimulus will elicit an action potential of the same amplitude as a stimulus above the threshold. This does not mean that the amplitude of the action potential is always the same - the same section of the membrane, being in different states, can generate action potentials of different amplitudes.
After excitation, the neuron for some time finds itself in a state of absolute refractoriness, when no signals can excite it again, then it enters the phase of relative refractoriness, when exceptionally strong signals can excite it (in this case, the AP amplitude will be lower than usual). The refractory period occurs due to the inactivation of the fast sodium current, that is, the inactivation of the sodium channels.
Action potential (AP) called the rapid fluctuation of the membrane potential that occurs when the excitation of nerve, muscle and some other cells. It is based on changes in the ionic permeability of the membrane. The AP amplitude depends little on the strength of the stimulus that causes it, it is only important that this strength be not less than a certain critical value, which is called irritation threshold. Having arisen at the site of irritation, AP propagates along the nerve or muscle fiber without changing its amplitude.
Under natural conditions, APs are generated in nerve fibers upon stimulation of receptors or excitation of nerve cells. The distribution of AP along nerve fibers ensures the transmission of information in the nervous system. Upon reaching the nerve endings, PD cause secretion chemical substances(mediators) that provide signal transmission to muscle or nerve cells. In muscle cells, AP initiate a chain of processes that cause a contractile act. Ions penetrating into the cytoplasm during AP generation have a regulatory effect on cell metabolism and, in particular, on the processes of protein synthesis that make up ion channels and ion pumps.
Rice. 3. Skeletal muscle fiber action potential , registered with an intracellular microelectrode: a – depolarization phase, b – repolarization phase, c – trace depolarization phase (negative trace potential). The moment of application of irritation is shown by an arrow.
It has been established that during the ascending phase (the phase of depolarization), not only the resting potential disappears (as was originally assumed), but a potential difference of the opposite sign occurs: the internal contents of the cell become positively charged with respect to the external environment, in other words, the membrane potential is reversed . During the descending phase (repolarization phase), the membrane potential returns to its original value. If we consider an example of AP recording in a skeletal muscle fiber of a frog (see Fig. 3), it can be seen that at the moment of reaching the peak, the membrane potential is +30 - +40 mV. The duration of the AP peak in various nerve and muscle fibers varies from 0.5 to 3 ms, and the repolarization phase is longer than the depolarization phase.
Changes in membrane potential following the peak of an action potential are called trace potentials. . There are two types of trace potentials - trace depolarization and trace hyperpolarization.
The ionic mechanism of PD occurrence . As noted, at rest, the permeability of the membrane to potassium exceeds its permeability to sodium. As a result, the flow of K + from the cytoplasm into the external solution exceeds the oppositely directed flow of Na + . Therefore, the outer side of the membrane at rest has a positive potential relative to the inner.
Under the action of an irritant on the cell, the permeability of the membrane for Na + increases sharply and becomes approximately 20 times greater than the permeability for K +. Therefore, the flow of Na + from the external solution into the cytoplasm begins to exceed the outward potassium current. This leads to a change in the sign (reversion) of the membrane potential: the inner side of the membrane at the site of excitation becomes positively charged with respect to its outer surface. This change in the membrane potential corresponds to the ascending phase of AP (depolarization phase).
The increase in membrane permeability to Na + lasts only a very short time. Following this, the permeability of the membrane for Na + again decreases, and for K + increases. The process leading to a decrease in the previously increased sodium permeability of the membrane is called sodium inactivation. . As a result of inactivation, the flow of Na + into the cytoplasm is sharply weakened. An increase in potassium permeability causes an increase in the flow of K + from the cytoplasm into the external solution. As a result of these two processes, membrane repolarization occurs: the inner contents of the cell again acquire a negative charge in relation to the outer side of the membrane. This potential change corresponds to the descending phase of AP (repolarization phase). Experiments on giant nerve fibers of the squid made it possible to confirm the correctness of the sodium theory of the occurrence of AP.
AP occurs when the surface membrane is depolarized . Small amounts of depolarization lead to the opening of part of the sodium channels and a slight penetration of Na ions into the cell. These reactions are subthreshold and cause only local changes on the membrane. ( local response ). With an increase in the strength of stimulation, when the threshold of excitability is reached, changes in the membrane potential reach a critical level of depolarization (CUD). For example, the value of the resting potential is -70 mV, KUD = -50 mV. To cause excitation, it is necessary to depolarize the membrane to -50 mV, i.e. by -20 mV to reduce its initial resting potential. Only when the KUD is reached, a sharp change in the membrane potential is observed, which is recorded in the form of PD. Thus, the main condition for the occurrence of an action potential is a decrease in the membrane potential to a critical level of depolarization.
The considered changes in the ionic permeability of the membrane during AP generation are based on the processes of opening and closing of specialized ion channels in the membrane, which have two important properties:
■ selectivity (selectivity) in relation to certain ions;
■ electrical excitability, i.е. the ability to open and close in response to changes in membrane potential.
Like ion pumps, ion channels are formed by protein macromolecules penetrating the lipid bilayer of the membrane.
Active and passive ion transport. In the process of recovery after PD, the operation of the sodium-potassium pump ensures that excess sodium ions are "pumped out" and the lost potassium ions are "pumped" inward, due to which the inequality of Na + and K + concentrations on both sides of the membrane, disturbed during excitation, is restored. About 70% of the energy required by the cell is spent on the operation of this mechanism.
Thus, in a living cell, there are two systems for the movement of ions through the membrane.
One of them is carried out along the ion concentration gradient and does not require energy (passive ion transport). It is responsible for the occurrence of the resting potential and AP and ultimately leads to an equalization of the concentration of ions on both sides of the cell membrane.
The second is carried out against the concentration gradient. It consists in "pumping out" sodium ions from the cytoplasm and "forcing" potassium ions into the cell. This type of ion transport is possible only if the energy of metabolism is consumed. He's called active ion transport. It is responsible for maintaining the constancy of the difference in ion concentrations between the cytoplasm and the fluid surrounding the cell. Active transport is the result of the work of the sodium pump, due to which the initial difference in ionic concentrations, which is violated with each burst of excitation, is restored.
Carrying out excitation
A nerve impulse (action potential) has the ability to propagate along the nerve and muscle fibers.
In a nerve fiber, the action potential is a very strong stimulus for neighboring sections of the fiber. The amplitude of the action potential is usually 5 to 6 times the depolarization threshold. This ensures high speed and reliability.
Between the excitation zone (which has a negative charge on the fiber surface and a positive charge on the inner side of the membrane) and the adjacent unexcited section of the nerve fiber membrane (with an inverse charge ratio), electric currents arise - the so-called local currents . As a result, depolarization of the neighboring area develops, an increase in its ion permeability and the appearance of an action potential. In the original zone of excitation, the resting potential is restored. Then the next section of the membrane is covered by excitation, and so on. Thus, with the help of local currents, excitation spreads to neighboring sections of the nerve fiber, i.e. conduction of a nerve impulse . The amplitude of the action potential does not decrease as the conduction progresses. , those. excitation does not fade even with a large length of the nerve.
In the process of evolution, with the transition from non-fleshy nerve fibers to pulpy ones (covered with a myelin sheath), there was a significant increase in the speed of nerve impulse conduction. The non-medullary fibers are characterized by continuous conduction of excitation, which sequentially covers each adjacent section of the nerve. The fleshy nerves are almost completely covered by an insulating myelin sheath. Ionic currents in them can pass only in the bare sections of the membrane - the intercepts of Ranvier, devoid of this shell. When conducting a nerve impulse, the action potential jumps from one interception to another and can even cover several intercepts. Such a conduct taught the name of somersault (Latin somersault - jump). This increases not only the speed, but also the cost-effectiveness of the implementation. Excitation captures not the entire surface of the fiber membrane, but only a small part of it. Consequently, less energy is spent on the active transport of ions across the membrane during excitation and during recovery.
The speed of conduction in different fibers is different. Thicker nerve fibers conduct excitation at a greater speed: they have greater distances between the nodes of Ranvier and longer jumps. Motor and proprioceptive afferent nerve fibers have the highest conduction speed - up to 100 m/s. In thin sympathetic nerve fibers (especially in unmyelinated fibers), the conduction velocity is low - on the order of 0.5 - 15 m/s.
During the development of the action potential, the membrane completely loses excitability. This state is called complete non-excitability, or absolute refractoriness. . It is followed by relative refractoriness, when the action potential can only occur with very strong irritation. Gradually, excitability is restored to its original level.
Laws of conducting excitation in nerves :
1. Conduction of impulses is possible only under the condition of anatomical and physiological integrity of the fiber.
2. Bilateral conduction: when a nerve fiber is irritated, excitation spreads along it in both centrifugal and centripetal directions.
3. Isolated conduction: in the peripheral nerve, impulses propagate along each fiber in isolation, i.e. without passing from one fiber to another and exerting an effect only on those cells with which the endings of this nerve fiber come into contact.
13. Define homeostasis.
14. Name the main ways of regulation of various functions in highly organized animals and humans.
15. By whom and when was "animal electricity" discovered?
16. What tissues are excitable? Why are they called that?
17. Name the main functional characteristics of excitable tissues.
18. What is called the threshold of excitability?
19. On what factors does the threshold value depend?
20. What is lability? Who put forward the concept of lability, what properties of excitable tissues does it characterize?
21. What is called the membrane potential (resting potential)?
22. What causes the presence of electrical potentials in living cells?
23. In what cases is it said about depolarization (or hyperpolarization) of the cell membrane?
24. What role does the potassium-sodium pump of the membrane play in the formation of the resting potential?
25. What is called an action potential? What is its role in the nervous system?
26. What underlies the emergence of an action potential?
27. Describe the phases of the action potential.
28. What is called membrane potential reversion?
29. Describe ionic mechanism occurrence of an action potential.
30. What is meant by sodium inactivation?
31. What is the critical level of depolarization?
32. What are the properties of the ion channels of the cell membrane?
33. Describe two types of ion transport in a cell:
■ passive;
■ active.
Module 1 GENERAL CNS PHYSIOLOGY