Garbuzova V.Yu., Obukhova O.A.
The General and Cellular Basis
of Medical Physiology
CONTENTS
P.
1 PHYSIOLOGY AS A SCIENCE……… 4
2 PHYSIOLOGY OF THE CELL. FUNCTIONAL
PROPERTIES OF CELL MEMBRANES…………. 17 3 RESTING POTENTIAL OF THE NERVOUS FIBRES AND MUSCLE FIBERS………….……...….. 37
4 ACTION POTENTIAL………..………. 46
5. EXCITABILITY……….…... 57
6. ELECTRIC CURRENT AS REASONS EXCITA- TION OF THE NERVOUSAND MUSCLE FIBERS…
65 7 MECHANISMS CONDUCTING EXCITATION
(AP) ALONG NERVE AND MUSCLE FIBERS.……. 76 8. CONDUCTION OF EXCITATION THROUGH
NEUROMUSCULAR SYNAPSE..………... 86 9 CONTRACTION OF SKELETAL MUSCLE……… 100 10 PHYSIOLOGY OF SMOOTH MUSCLE….……… 126
1 PHYSIOLOGY AS A SCIENCE
Physiology (physis – nature, logos – science) – is the science about general laws of functioning of living organisms and their parts.
There are many different species of living organisms in the nature, which functioning is significantly different.
Therefore, the physiology is divided into several independent sciences: plant physiology, physiology of microorganisms, animal physiology and human physiology (fig. 1.1). The object of the study of human physiology is the human body, features of functioning of which depends on how healthy or sick this organism. So human physiology can be divided into 2 sciences:
normal and pathological.
Physiology studies the vital functions of a healthy human body and pathological - sick.
Figure 1.1 – Areas of physiology
Link normal physiology with other sciences Physiological patterns of functioning of living organisms based on data about macro-and microscopic structure of organs
Physiology
Depolariza tion of
Physiology of microorganisms
Animal physiology
Human physiology
Pathological physiology Normal
physiology
and tissues, biochemical and biophysical processes that are carried out in cells, tissues and organs.
In the structures of the body are carried out physical and chemical processes that are the subject of study of biophysics and biochemistry. Physical and chemical processes are the basis of the functions that they study physiology (fig. 1.2).
Therefore, to clarify the physiological mechanisms is based on the anatomy, histology, biochemistry, biophysics and other disciplines. Physiology synthesizes the knowledge of other disciplines, combining them into a unified system of knowledge about the body. On the other hand, the physiology is the basis for the study of other health sciences such as pathophysiology and clinical disciplines. Revealing the basic mechanisms that ensure the existence of the whole organism and its interaction with the environment, physiology makes it possible to determine the causes, conditions and nature of violations of these mechanisms under conditions of disease. It helps to identify ways and means of influence on the body, which allow normalizing impaired function, restoring health.
Figure 1.2 – Link normal physiology with other sciences
Biophysics Biochemistry
Morphological disciplines S t r u c t u r e Physical Chemical F u n c t i o n s
Normal physiology
The value of normal physiology
A general theoretical importance of physiology is that man (even an average) was always curious to know how to work her heart, how she breathes, how digested nutrients and how energy is produced. Answers to these questions gave her physiology. In medicine, physiology general theoretical significance lies in the fact that it is the basis for the study of other disciplines (such as pathophysiology).
1. The practical significance. Physiology is one of the most important disciplines in the preparation of a doctor. Most diseases are manifested primarily by the dysfunction, because without knowledge about the functioning of a healthy body can not diagnose illness, identify ways of treatment, preventive measures, to correct and prevent disease.
2. The work of physician assesses the severity of illness by the largest deviation from the normal physiological functions. Physiological research is the basis of clinical diagnosis, an important method of evaluating the effectiveness of treatment and disease prognosis. Studying the functions of various organs and systems allow to simulate these functions using the devices and machines (apparatus of artificial respiration and blood circulation, the apparatus for hemodialysis, apparatus for defibrillation, the device for hyperbaric oxygenation, etc.).
Basic concepts of normal physiology 1. Function – is a form of living structure.
For example, the function of muscle tissue is contraction, function of nervous tissue - the generation of impulses, etc.
2. The functional unit – is the smallest group of cells, united to perform a specific function (nephron, motor unit, etc.).
The value of functional units:
• provide continuous operation without fatigue (until some functional units are working, the rest - rest. In this mode of authority may work for a long time, without fatigue);
• changes in the intensity of the function being performed, depending on the needs of the body (the work involved different numbers of functional units);
• provide compensation for damage (intact functional units are beginning to work actively).
3. Physiological systems - the union of the organs for a specified function (blood system, circulatory system, the system of external respiration, etc.). The physiological system is the concept of sustainable. Once disturbed the functioning of one of the bodies, which is part of the system is disturbed or even impossible the functioning of the system.
4. Operating System - a temporary union of the organs and physiological systems for biologically useful effect to the body (gas transportation system - combining the systems of blood, circulatory, respiratory; excretory system - combining the systems of external respiration, digestion, kidneys, skin, etc.;
thermoregulatory system - combining the systems of external respiration, circulation, skin, muscles, etc.). A functional system is a dynamic concept. Once achieved the desired result and the biological needs of the body is satisfied, the functional system breaks down.
5. A functional state - the state of biological structures and functions of the organism as a whole at a particular time (fig.
1.3).
Figure 1.3 – Functional state of biological structures
The function is performed The function is
notperformed
State of rest State of activity
State of calm
FUNCTIONAL STATE
The function is not performed;
there is recovery of energy and structures
The concept of a function at different levels of biological organization
Function – is a form of activity which is typical for a living structure.
Elementary structure of a living organism is a cell. So, actually, physiological functions begin from the cellular level.
However, the term "function" is often used when dealing with the activities of those structures that make up the cell.
The basic level of biological organization:
1) pre cell 2) cell 3) tissue; 4) organ, 5) system, 6) the body as a whole.
Pre cell (subcellular) level – is a level of structures that make up the cell. This level, in turn, has the following hierarchy:
1 Micromolecular level form electrolytes (Na+, K+, Ca2+, Cl-, HCO-, SO2-, PO3-, etc.), water, microelements (Cu, Fe, Zn, Mn, Co, Mo, Se, etc.), simple organic compounds (monosaccharides , amino acids, lipids, nitrogenous bases, etc.).
2Macromolecular level is form proteins, nucleic acids, polysaccharides.
The role of these substances is studied in detail in the course of physical and colloid chemistry, biochemistry.
3 Supramolecular level is forms membranes, ribosomes, chromatin, microtubules and microfilaments.
The role of these components of the cells studied in detail in the histology course
4 Organoid level is represented by cellular organoids - mitochondria, Golgi apparatus, endoplasmic reticulum, lysosomes, nucleus, and others.
Cellular level. It is at this level begins to realize the concept of "function".
Elementary cell function:
1) Generation and conduction of electrical impulses (neurons, muscle cells, some types of secretory cells).
2) Contraction (muscle cells, endotheliocytes).
3) Migration and mobility (leukocytes).
4) Endocytosis.
5) Exocytosis.
6) Division.
7) Transportation substances.
8) Biochemical work (hepatocytes, adipocytes, leukocytes).
Tissue level. There are four types of the tissue in the organism: nervous, muscular, epithelial, connective. Details of their functions are studied in the course of histology, we note only the basic.
The functions of nervous tissue:
1. Generation and conduction of the electrical impulses.
2. The formation of signaling substances (neurotransmitters, neuromodulators, neurohormones).
The function of muscle tissue is contraction.
The functions of epithelial tissue:
1) Barrier function. Separation of environments is by establishing barriers of epithelial cells connected by close contacts (such as between the epithelial cells of the mucous membrane of the stomach, intestines).
2) Transport function. Oxygen and carbon dioxide is transported throughout the lung epithelium. A gut epithelium absorbs amino acids, glucose and other substances.
3) Secretory function. Epithelial cells make exocytosis of the mucus, which is formed, for example, by the special mucous cells of the epithelium of the stomach, genital tract, epithelial cells of intestines, trachea, bronchi, and proteins (hormones, enzymes, growth factors) that are formed by
the endocrine cells.
4) Endocytosis. Most epithelial cells are able to absorb cholesterol and lipoproteins, transferrin by the way of the receptor-mediated endocytosis. Renal tubular epithelium is involved in pinocytosis.
5) Protective. Epithelial tissue protects the body from damaging action of physical and chemical environmental factors.
The functions of connective tissue:
1) Trophic. Ensuring the supply of elements of the parenchyma.
2) Protective. Connective tissue is involved in the creation of biological barriers, phagocytosis, reaction of cell and humoral immunity.
3) Support. Connective tissue forms the stroma for histological elements of the parenchyma, provides strength of the skin, forms capsules of the organs and can withstand significant mechanical loads.
Organ level includes separate organs: heart, blood vessels, kidneys, lungs, stomach and others.
The system level consists with physiological systems:
blood, circulatory, respiratory, digestive, selection, reproduction, nervous system, musculoskeletal and endocrine.
The functions of these organs and physiological systems will be the subject of detailed study in the physiology course.
The organism level as a whole. At this level characteristic functions is that ensure the interaction of the organism with the environment. The most important among them are:
1) Functions that ensure receipt of information about environment;
2) Functions that provide analysis of this information;
3) Functions that provide behavioral reactions that underlie in the base of adaptation to environmental conditions.
Basic functional properties of the body:
1 Metabolism and energy between the organism and the surrounding environment is a phenomenon that is the basis of life. In detail this property of the organism being studied in the course of biochemistry.
2 Self-regulation – is the body's ability to carry out the regulation of physiological functions.
There are two mechanisms of regulation of functions:
• nerve (by means of the nervous system);
• humoral (by means of chemicals dissolved in body fluids).
3 Homeostasis – is the constancy of internal environment.
In 1878 Bernard C. proposed postulate that all life processes have only one purpose - to maintain the sustainability of living conditions in our domestic environment.
In 1929 William Cannon launched the term "homeostasis".
Translated from the Greek homois - like, the same; stasis - a condition, fixity.
The normal life and activity of cells in multicellular organisms requires the sustainability of the conditions of the internal environment, the environment surrounding the cell (blood, lymph, interstitial fluid).
The need for sustainable living conditions was, in fact, the factor that caused the association of individual cells in multicellular organisms.
This association had a number of consequences for cells.
On the one hand, the body created for the cells optimal conditions for existence - homeostasis, which has played a
positive role with regard to the possibility of survival. On the other hand, the cell itself was forced to accept a share of care to establish homeostasis for the entire organism. This led to the specialization of cells and, thus, the loss of their "freedom."
The main parameters of homeostasis:
• Constancy of body temperature (thermal homeostasis).
• Sustainability of osmotic pressure (osmotic homeostasis).
• Sustainability of the ion (ion homeostasis).
• Sustainability indicators of acid-base balance (acid-base homeostasis).
• Sustainability of the water in the body.
• Sustainability gas composition (gas homeostasis).
• Sustainability of the chemical composition (chemical homeostasis).
• Sustainability antigenic structure (antigenic homeostasis).
4 Adaptation – is adaptation of an organism to environmental conditions.
The purpose of adaptation – is maintaining homeostasis of the organism in an environment that is constantly changing.
By the mechanism are distinguished:
• Immediate adaptation;
• Long-term adaptation.
Immediate adaptation is very fast due by the mechanisms and structures that exist at this time. For example, the vessels are narrowed at low temperatures, heart rate increases during the exercise.
Long-term adaptation is carried out gradually by increasing the number of structures involved in adaptation. For example, regular physical exercise increases skeletal muscles mass, the number of red blood cells are increasing during the stay in the mountains.
5 Growth, development, and reproduction
This physiological property provides self-healing and self- reproduction of organisms.
An irritability - the ability of biological structures to move from a state of calm in the active state under the influence of various factors (irritants).
Irritant – is a factor that causes the transition of the biological structure from physiological rest in active status.
Classification of the irritants:
By the nature of power:
- Physical;
- Chemical;
- Biological;
- Social.
By the biological features:
- Adequate – stimuli to which the biological structure is adapted. For example, a light to the eye, the sound to the ear;
- Inadequate – stimuli to action of which biological structures are not adapted. For example, the effect of mechanical factors (impact) on the receptors of the eye, the effect of chemicals on tactile skin receptors.
By the power, the intensity of the action:
- sub-threshold - stimuli that do not cause biological reactions;
- threshold – stimuli that are beginning to cause a biological response;
- suprathreshold – stimuli whose power exceeds the power threshold stimuli.
Irritation – the process of effect of the stimulus on the biological structure.
The biological response – a response of biological structures to the action of the stimulus.
There are such types of biological reactions:
• Local (biological reaction that occurs at the site of irritation and does not extend to adjacent biological structures);
• Common (biological response that extends to adjacent structures).
A common biological response called excitation.
Excitability – is ability of biological structures to excitation.
Excitable structures – is structure which is characterized by excitability.
In excitable structures include:
1) Nerve cells and nerve fibers;
2) Muscle fibers;
3) Some types of glandular cells.
The laws of the irritation І The law of power relations (the law of power)
If is greater power of the stimulus, then is greater (up to certain limits) biological response.
Biological
reaction а
b c
0 1 2 Power of the stimulus
Figure 1.4 – The law of power relations 0 - 1 – subthreshold is range of the powers.
1 - the minimum stimulus power, able to cause minimal reaction - irritation
threshold.
For excitable structures characteristic of this pattern: the smaller the threshold of irritation, the greater excitability of the structure and vice versa.
1 - 2 – submaximal range of the powers, which employs law of thepower relations.
2 - maximum power of the stimulus can cause the biggest reaction.
2 - – supermaximum range of the powers.
In this range of powers available such variants of the answer of the biological structures:
a - stored maximum response;
b - the intensity of biological response is reduced;
c - the biological structure is destroyed, any response is absent.
ІІ The law "All or Nothing"
On the effect of subthreshold stimulus biological structure does not answer ("nothing"). On the effect of stimulus of the threshold power occurs once the maximum response ("all"). Further increase in stimulus force did not cause increased biological response.
Biologycal reactions
irritation Power of the threshold stimulus
Figure 1.5 – The Law of "All or Nothing"
ІІІ The law of duration of the stimulation (the law "Power of Time")
The greater power of the stimulus, the less time is needed so that there was a biological reaction.
Figure1.6 – The law of “Power of Time”
2 PHYSIOLOGY OF THE CELL. FUNCTIONAL PROPERTIES OF CELL MEMBRANES
The cell is an elementary biological unit. The cell level provides an independent existence and implementation of all major biological functions. Most of the physiological processes in the cell occurring involve the cell membrane. Membranes perform the following functions in the cell:
Structural. Create the structure of cells and their organoids.
Isolation. Provide selective permeability of cells to substances.
Create a gradient of concentration of substances between the structures and environment that surrounds them.
Regulate the activity of processes that occur in specific structures and cells in general.
Cell membrane (plasmolemma) – is a membrane that separates contents of cells from the extracellular fluid. Its thickness is 7.5 - 10 mkm.
Structure of cell membrane
Modern model of the cell membrane is a liquid-mosaic model, proposed in 1972 by Singer and Nicholson (fig. 2.1).
The authors of model called the membrane "lipid sea, where protein icebergs float."
So, according to the mosaic model membrane consists of the following components:
Lipid component (42%);
Protein component (55%);
Carbohydrate component (3%).
Figure 2.1 – Liquid-mosaic model of cell membrane
Lipid component is based membrane. It performs two main functions:
barrier function (separation of intracellular contents of cells from their microenvironment, transport of substances);
matrix function (is the matrix in which there are many membrane proteins).
Lipid film is a double layer of lipids, the so-called bilayer, represented by phospholipids and cholesterol.
Phospholipids molecule is consisting of hydrophilic heads and hydrophobic tails (fig. 2.2). The head is ¼ of molecules of phospholipids. It can be negatively charged or neutral (often neutral, as neutral head easily packed in a film, and negative repel). The structure of the head is including the nitrogen base and phosphoric acid. Tails constitute ¾ of the length of phospholipids. One molecule of phospholipids has two tails. The composition of tails are higher fatty acids – saturated (palmitic, stearic) and unsaturated (linoleic, linolenic,
arachidonic).
Figure 2.2 – Schematic representation of the phospholipids molecule
Phospholipids’ film – is a liquid, which for normal functioning must have a certain viscosity. Normally, the membrane viscosity is the viscosity of olive oil. Normal viscosity is provided by certain ratio of saturated and unsaturated fatty acids: saturated increase viscosity while unsaturated – reduce viscosity.
Figure 2.3 – Formula of the phospholipids
Phospholipids within the cell membrane are not rigidly
fixed. They move or within one monolayer (lateral diffusion), or from one monolayer to another (flip-flop).
А B
Figure 2.4 – Types of movements in membrane phospholipids: A - lateral diffusion, B - flip-flop.
Cholesterol - monatomic alcohol, derivative of the cyclopentanperhydrofenantren. Its molecule doesn’t contain any long straight chains, but consists of 4 rings (fig. 2.5).
Cholesterol molecule, like other lipid molecules have polar and non-polar parts so well embedded in the lipid cell membrane ensembles. Plasma membranes contain a significant amount of cholesterol. For example, in plasma membranes of liver cells it constitute about 30% of membrane lipids
Figure 2.5 – Formula of the Cholesterol
The values of cholesterol for membrane function are very versatile.
Cholesterol regulates the aggregate state of the bilipid film. If the density of the cell membrane increases, it dilutes it. If the membrane becomes liquid, it rather makes it denser.
Cholesterol is a membrane damper. Lipid bilayer chains are in state order so random movements of one of them are inevitably transmitted to the others. Cholesterol arranged between phospholipids, blocks this transfer, so co-operative movements quickly decay and the order in membrane is stored.
Cholesterol provides membrane electroisolation properties.
Protein component. Molecules of the membrane proteins are floating in the lipid membrane matrix like icebergs. They are divided into 2 groups:
1) Integral proteins - are proteins that pass through the membrane (by chemical structure is mainly glycoproteins).
2) Peripheral proteins – are proteins that do not penetrate inside the membrane but attached to its inner or outer surface.
Functions of membrane proteins:
Transport. Implementation of the transport of substances through the membrane is provided by the protein- channels, proteins-carriers and protein-pumps;
Catalytic. Catalysis of biochemical reactions is performed usually by the peripheral proteins: endoenzymes, which act on the inner surface of the membrane and ectoenzymes which act on its outer surface;
Receptor. It is based on specific interactions of membrane proteins with different ligands: mediators, biologically active substances, hormones, immunoglobulins,
complement components, etc.;
Antigenic. Lies in the implementation of immune reactions;
Structural. Proteins provide the support of the some structures in the cells. For example, spectrin, glycophorin et al.;
Implementation of intercellular interactions is provided by adhesive proteins, integrin, selectin et al.
Carbohydrate component of cell membranes is represented by glycocalyx which is composed of carbohydrate residues of membrane glycoproteins and glycolipids and extracellular proteoglycans. Glycocalyx thickness is about 50 nm. Carbohydrates of the glycocalyx have a large number of anionic groups which determines their basic functions.
Functions of glycocalyx:
Creates a negative charge outside the cell. There are repulsive forces between cells that are in fluid (e.g.
blood) with this charge , and they do not stick to each other;
Provides the intercellular interaction. In tissues glycocalyx of one cell can merge with another glycocalyx, forming intercellular contacts;
Deposition of extracellular cations includes Ca2+. Thanks to polyanionic nature of the glycocalyx, it can bind large amounts of Ca2+ and thus serve its depot.
The main differences between the chemical composition of the cell content and extracellular fluid
There are significant differences in chemical composition of intracellular environment and extracellular fluids. These differences are reflected in Table 2.1.
Table 2.1 Extracellular fluid Intracellular fluid
Na+ 142 mEq/L 10 mEq/L
K+ 4 mEq/L 140 mEq/L
Ca2+ 2,4 mEq/L 0,0001 mEq/L
Mg2+ 1,2 mEq/L 58 mEq/L
Cl- 103 mEq/L 4 mEq/ L
HCO3- 28 mEq/L 10 mEq/L
РО43- 4 mEq/L 75 mEq/L
SO42- 1 mEq/L 2 mEq/L
Proteins 5 mEq/L 40 mEq/L
Glucose 90 mg% from 0 to 20 mg%
Amino acids 30 mg% 200 mg%
рО2 35 mm Hg 20 mm Hg
рСО2 46 mm Hg 50 mm Hg
рН 7,4 7,0
There is a constant exchange through the cell membrane of substances between the intracellular and extracellular sectors. The basis of this exchange is the mechanisms of transport of substances through the cell membrane.
Figure 2.6 – Exchange of substances between the cell and its microenvironment
Exchange of substances between the cell and its microenvironment
Trans‐membrane transport Vesicular transport
Passive Active Endocytosis Exocytosis
Vesicular transport – is a transport of substances by the vesicles. Depending on the direction there are of two kinds:
• endocytosis (transport in the cell);
• exocytosis (transport out of the cells).
There are two types of endocytosis:
- Phagocytosis (absorption of solids);
- Pinocytosis (absorption of liquids in the form of drops).
Figure 2.7 – Endocytosis
A substance that is close to the surface of the cell membrane is absorbed in it. Then the membrane is drawn inward and its edges merge. As a result, is formed endocytosis vesicle that breaks away from the membrane and migrates into the cell (fig. 2.7). Often endocytosis vesicles cluster together in one big vesicle and merge with lysosomes containing enzymes for digestion of substances that are transported. Hydrolysis products are used by cells for their own needs. The first two stages of endocytosis occur without energy consumption while the following require ATP energy.
Most cells synthesize macromolecules (hormones, blood proteins, enzymes) "for export". In addition, in the process of metabolism metabolites are formed. Elimination of these secretions and excretions by means of vesicles is called exocytosis.
There are two types of exocytosis:
- Secretion (discharge from the cells of hormones, proteins, enzymes);
- Excretion (discharge from the cell metabolic products).
Figure 2.8 – Exocytosis
Exocytosis vesicles approach the inner surface of the cell membrane and contact with it by using special proteins. Shell of the vesicle merges with the membrane and its contents placed in the extracellular space (fig. 2.8).
Transmembrane transport - a transport of substances through the membrane and through all its layers.
Transmembrane transport is of two kinds:
• passive;
• active.
Passive transport – is a form of transport carried out according to the gradients that exist in the cell.
There are 3 types of passive transport:
• filtering (transport by hydrostatic pressure gradient).
• osmosis (transport by osmotic pressure gradient)
• diffusion (transport by the concentration gradient, electric charges, etc.).
The main diffusion is lightweight and simple.
Simple diffusion – diffusion without using carriers. It may be through lipid bilayer and through the protein-channels.
Fat-soluble compounds (alcohols, oxygen, carbon dioxide, nitrogen) and water well diffuse through lipid bilayer.
Water is not fat-soluble compound but it is well diffuses through the lipid film because its molecule has a small size and high kinetic energy. The intensity of diffusion of water across
cell membranes is very high. For example, through the erythrocyte membrane every second diffuses into both sides the volume of water that is 100 times greater the volume of the erythrocyte. Transport of water through the lipid bilayer is due to "the theory of temporary voids." According to it by the constant movement of tails of phospholipids in the membrane formed temporary voids through which water molecules pass.
NB: fat-soluble compounds whose diameter is greater than the diameter of water molecules and ions are not well diffused through the lipid bilayer. With increasing size of molecule diffusion ability of the substance decreases sharply.
Thus, the diameter of a molecule of glucose is greater than the diameter of water molecules in 3 times and the rate of diffusion of glucose is less than that of water in 100 thousand times. Ions are practically not diffused through the lipid bilayer as in the aquatic environment having the hydration shell, which significantly increases their diameter. The presence of the charge of ions prevents diffusion; this charge interacts with the electric charge of the polar heads of phospholipids.
There are protein-channels for the transport of ions in membrane (integral proteins embedded in the membrane).
Protein-channels have the following properties:
1) Selectivity – the ability to selectively pass through a certain compounds. Depending on the characteristics of the protein-channels, they are not selective (can pass different compounds) and selective (mostly miss one type of molecules).
Selectivity of the channel may be absolute when the channel passes through one type of molecule and relative when the channel can pass through some other types of molecules. The selected channels are sodium, potassium, calcium, chloride and some other channels.
2) Presence and condition of gate mechanisms. Condition
of the channel depends on condition of the gate of the channel (gate closed - closed channel, gate opened - opened channel).
The basis of opening and closing of the channel is a conformational change of the protein molecule. The reason for these changes can be two types of regulatory influences (electrical potential and chemicals). According to this we can distinguished two types of control of condition of the gates.
1 Potential dependent mechanism. Condition of the portal mechanism of the channel is controlled by the membrane electric potential. For example, when the nerve fiber membrane has a charge of -90 mV, sodium and potassium channels are closed and when this charge begins to decrease, the channels open (fig.2.9).
Figure 2.9 – Potential dependent mechanism of control of the gate
Channels in which the state of the gate depends on the potential of the membrane are called potential dependent channels
2 Ligand dependent mechanism. Condition of portal mechanism of the channel is controlled by certain chemical compounds – ligands.
During the interaction of the channel with chemical changes conformation of protein and the channel opens.
For example, during the interaction of acetylcholine with sodium channels in muscle fibers, these channels are opened (fig. 2.10).
Figure 2.10 – Ligand dependent mechanism of control of the gate
Channels in which the state of the gate depends on the presence of certain chemicals are called ligand dependent (chemo- sensitive).
3 Kinetics of the channels is characterized by the speed of the passage of substances through the channel. Depending on these characteristics the channels are divided into fast and slow channels.
Facilitated diffusion - the movement of compounds within their concentration gradient with the participation of protein-carriers. The intensity of the facilitated diffusion is limited by the number of molecules of protein-carriers and kinetics of their binding with substances that are transferred (fig. 2.11).
Figure 2.11 – Dependence of the intensity of the facilitated diffusion of number of protein carriers
Stages of facilitated diffusion:
• Specific binding molecule of the substance with the protein-carrier;
• Conformational changes of protein-carrier;
As a result of these changes disrupted communication of molecules with place of the binding occurs and compounds move freely on the other side of the membrane (fig. 2.12).
Figure 2.12 – The mechanism of facilitated diffusion
Glucose and most amino acids are transported by the mechanism of facilitated diffusion
Factors affecting the intensity of diffusion of substances through the cell membrane
1 Factors associated with the membrane through which the diffusion are:
a) Membrane permeability for some substance - speed of diffusion of the substance through the unit of area of membrane per unit of difference of concentration of substance (in the absence of electrical gradient or pressure gradients).
Membrane permeability depends on:
• Membrane thickness (the thicker the membrane the lower the permeability);
• Physical-chemical condition of the lipid membrane layer. This condition is determined by the chemical
composition of lipids of the membrane: saturated and unsaturated fatty acids. Unsaturated fatty acids provide the membrane liquid condition and increase permeability.
Physical-chemical condition of the membrane is very sensitive to the temperature. When there is hypothermia, the membrane becomes "hardened" and their permeability decreases and vice versa.
• The number of protein-channels and protein-carriers per unit of area of membrane and their functional status (closed or open them);
b) the total area of the membrane through which the diffusion takes place.
2 Factors associated with qualities and condition of substance which diffuses through the membrane:
a) Solubility in lipids;
b) Temperature;
c) The presence of electric charge;
d) Molecular weight.
Influence of molecular weight on the speed of diffusion is ambiguous. If is higher the molecular of weight substances, then is the greater the velocity of the molecules and the greater the intensity of diffusion. On the other hand, increasing the diameter of molecules complicates the diffusion through protein channels (if the diameter of the molecule is larger than the diameter of the channel, diffusion through the channel stops).
3 Factors that drives the diffusion.
- Concentration gradient on both sides of the membrane;
The intensity of the diffusion of the substances is described by the Fick’s equation
C1 C2
l D S dt
dm
dt
dm- intensity of the diffusion ;
D – coefficient of the diffusion;
S – surface area of membrane through which the diffusion occurs;
l - membrane thickness;
С1-С2 – difference in concentrations of substances on both sides of the membrane;
- electric gradient;
- Gradient of hydrostatic pressure. Transport of substances which is by the gradient of hydrostatic pressure is called filtering. Since the pressure in the middle of the cell and extracellular environment is almost the same, filtering does not play an essential role in transport of substances through the cell membrane but is essential when it comes about the transport
through the vascular wall;
- Osmotic pressure gradient. This gradient is of great importance for the transport of solvents, especially water.
Transportation for the osmotic pressure gradient is called osmosis.
Active transport – is a form of transport carried out against the existing gradients (concentration, electrical charge, pressure).
Depending on the source of energy used to carry out an active transport, it is divided into primary and secondary.
Primary active transport – is a mechanism of active transport that uses the energy of ATP or other macroergic compounds. It is through protein-pumps.
Each protein-pump consists of two components:
- protein-carrier which binds to a substance and transports it through the membrane;
- protein-enzyme - ATPase which can liberate energy of ATP and use it for conformational changes of the protein-carrier.
Examples of primary active transport is the Na+-K+-pump, Ca2+-pump and H+-pump. Consider the principle of the pump on the example of Na+-K+-pump.
Sodium-potassium pump – is a protein that carries Na+ transport out of cells and K+ transport in the cell. This mechanism operates in all cells.
Na+-K+-pump consists of two subunits: large (100 000) and small (55 000). Large subunit has 3 receptor site for binding of Na+ ions on the inner surface of the membrane, 2 receptor binding sites of K+ ions on the outer surface of the membrane and ATPase on the inner surface. Once the binding of 3 Na+ ions and 2 K+ ions occurs, the inner part begins the ATPase activity. The energy is released when ATP splitting goes to the
conformational changes of the protein-transporter and Na+ is released from cells and K+ goes into the cell (fig. 2.13).
Figure 2.13 – Sodium-potassium pump
The value of Na +-K + - pump:
1 Provides a difference of the concentrations of Na+ and K+ in the cell and extracellular environment.
2 Creates an electric potential on the cell membrane.
Electrogenesis of the pump associated with the unequal transfer of charge during its work (3 positive charges (3 Na+) taken out with cells, and made 2 (2 K+)). For one cycle of the pump cell loses one positive charge.
3 Compliance the sustainability of the cell’s volume. How this pump would do not worked, the majority of cells would be edematous.
Secondary active transport (cotransport) – is a mechanism of active transport which directly uses the energy of concentration gradient of certain ions (usually ions Na+) to transport substances. This gradient in turn creates primary active transport mechanisms and therefore also depends on the energy of ATP.
Secondary active transport is divided into two types:
• symport (transport of two substances in one direction);
• antyport (transport of substances in opposite directions).
Secondary active transport is carried out by specific protein-
carriers.
Symport. Consider the mechanism of symport on example of the sodium cotransport of the glucose. Protein- carrier which carries out this transport has two specific binding sites – for Na+ and glucose. When one Na+ ion and one molecule of glucose join, protein undergoes conformational changes resulting in Na+ and glucose finding them in the cell (Fig. 2.14).
By the same mechanism occurs sodium cotransport of the amino acids occurs. There are 5 types of protein-carriers for transport of amino acids. Sodium cotransport of the glucose and amino acids is particularly intense in epithelial cells of intestinal and renal tubular epithelium.
In addition to these mechanisms of the cotransport in the cells, other forms of symport spread. For example, Na+-K+-Cl- cotransport, K+-Cl- cotransport. In some cells there is a symport of ions of iodine, iron and urate ions.
Figure 2.14 – Mechanism of the symport
Antyport (ion exchange mechanism). Consider a mechanism of antyport on the example of Na+-Ca2+ exchange mechanism that is in almost of all cells. Protein-carrier which carries out this transport has two specific binding sites - on the outer surface for Na+ and on the inside – for Ca2+. When these ions join, protein undergoes conformational changes resulting
to the Na+ to be placed in the cell and Ca2+ - outside the cell (fig. 2.15).
Figure 2.15 – Mechanism of the antyport
By the same mechanism, Na+-H+ exchange mechanism occurs which is in renal tubular epithelial cells, and Na+-K+, Ca2+-Mg2+, Cl--HCO3-, Cl--SO42- - exchange mechanisms.
3 RESTING POTENTIAL OF THE NERVOUS FIBRES AND MUSCLE FIBERS
There is a difference of electric potentials between the inner and outer surfaces of the plasma membrane of all cells. It is called the membrane potential and in excitable cells – resting potential.
Membrane potential (MP) – is a trans-membrane potential difference that exists between the inner and outer surfaces of the plasma membrane.
Resting potential (RP) – is a membrane potential of excitable cells that are at rest. In other words, RP – is a special case of membrane potential.
Methods of registering of the RP
Registration of RP is engaged by the electrodes. There are two methods of registering RP depending on their location.
I. Intracellular removal of RP. Carried is out by means of a glass electrode and a tip diameter of which is less than 1 mkm. Such electrodes pierce the cell membrane and enter in the cytoplasm. At the time of puncture the membrane potential difference is recorded by the device (voltmeter) (fig. 3.1).
А B
C
Figure 3.1 – Registration of intracellular membrane potential: А – before, B - after the entering of electrodes in the cell; C – recording of RP on the oscillograph.
II Extracellular removal of RP. Both electrodes are placed outside the cell in this method. The named method allows to register the potential damage and potential difference between intact and damaged areas of tissue. The damaged area is depolarized with respect to the area of intact (fig. 3.2).
A B
Figure 3.2 – Extracellular registration of membrane potential: А – intact nerve fiber, B - nerve fiber of the damaged area (shaded)
The main physical characteristics of the RP 1) Polarity. The inner surface of the membrane resting
potential is electronegative with respect to "zero" of the Earth. In other words, the outer surface of the membrane is charged positively and the inner surface of the membrane - negatively.
2) Sustainability of magnitude. Value of the RP for particular structures (nerve fiber, muscle cells, neurons) is constant.
3) Absolute value. RP has the following values for different body structures: nerve fibers are -90 mV, skeletal muscle fibers are -90 mV, smooth muscles are -50-60 mV, and neurons of the central nervous system are -40-60 mV.
Under the influence of some factors the absolute value of RP is subjected to some changes. There are two types of changes the value of the RP - depolarization and hyperpolarization (fig.3.3).
Figure 3.3 – Changes in the absolute value of the RP: А – resting potential;
B – depolarization;C – hyperpolarization
Membrane depolarization – is a decrease and hyperpolarization - increasing the absolute value of the RP.
Ionic mechanisms for the origin of resting potential The first hypothesis about ionic mechanisms of the origin of the membrane potential was proposed in 1896 by a representative of the Ukrainian school of physiologists Chagovets V.Y., who was the head of the physiology department at Kiev University of St. Vladimir.
Today, it is finally proved that the occurrence of membrane potential is associated with the diffusion of ions. To understand the phenomenon, imagine a cell in which the cytoplasm is replaced by the electrolyte solution, which consists of small particles of cations (e.g. potassium ions) and anions of larger particles (e.g. protein). Small particles of cations can easily diffuse through the pores of the membrane while the cell membrane is impermeable to anions. Place a cell filled with this electrolyte in an environment where it is missing or its concentration is much smaller than in the cell.
Figure 3.4 – Experience, which explains the ionic mechanisms of the RP.
In this case the share of cations (K+) under the laws of diffusion will leave the cells in the environment by the concentration gradient and anions will remain in the cell because the membrane is impermeable to them. The transition of the cations from cells in the extracellular environment leads to what is on the inner surface of the membrane and creates an excess of K+ ions ("+"sign appears), on the outer surface exactly the same number of anions increases (sign "-" ). In other words, difference of the potential (i.e. the membrane potential) arises between the inner and outer surfaces of the membrane.
The emergence between extracellular environment and cytoplasm of the cell potential difference - resting potential
Figure 3.5 – Scheme, which explains the ionic mechanisms of the RP
How long will continue diffusion of cations continue from cells in the extracellular environment?
The fact that the positive charge is created on the outer surface of the membrane will prevent further diffusion of K+ ions by the concentration gradient. Potassium will go into a cell by the electric gradient (negatively charged membrane from the inside and positively charged from outside). Finally, at the certain membrane potential state of the equilibrium reached.
This means that the number of K+ ions which comes from the cell by concentration gradient is equal to the number of K+ ions
Major ionic mechanisms of the RP
Availability in the cells transmembrane
gradients of concentrations of ions
The high permeability of the
membrane at rest for potassium ions
Impermeability of the membrane at rest for intracellular protein-
anion
Diffusion of potassium ions by the concentration gradient out of the cells
The relative is increase in concentration of cations in an
extra-cellular environment.
The relative increase of concentration of anions in the
cytoplasm
which enters the cell by the electrical gradient. Membrane potential value at which occur states of equilibrium is called the equilibrium potential for this ion.
Thus, the equilibrium potential - a level of electrical membrane potential at which ion diffusion by concentration gradient is balanced by the oppositely directed diffusion of this ion by the electric gradient. Or, in other words, it is a level of electric potential on the membrane that completely stops the flow of ion diffusion by concentration gradient.
Size equilibrium potential calculated by the Nernst’s formula, which after the series of transformations takes the form:
KK oiЕ
61 lg ,
Е – equilibrium potential (Nernst's potential);
К і,
К о –concentration of potassium ion inside and outside the cell respectivelySubstituting in the Nernst’s formula the value of the concentration of K+ ions inside the cell (140 mEq/L) and in the extracellular environment (4 mEq/L), we obtain the value of potassium equilibrium potential which is equal to – 94 mV.
If there are not one but several types of ions in the cell (e.g. potassium, sodium, chlorine) that penetrate through the membrane, the membrane potential value is calculated by the Goldmann-Hodgkin-Katz formula :
K
Na
i Cl Cl i Nai K
P Cl P
Na P
K
P Cl
P Na P
Е K
0 0
lg 0
1 ,
6 ,
Р – membrane permeability to this ion.
Resting potential of nerve and skeletal fibers Intracellular removal of membrane potential of the large nerve and skeletal muscle fibers suggests that the resting potential in these structures is equal to -90 mV. This level of resting potential is explained by the existence of basic and additional factors that affect the membrane potential in these structures.
Basic factors:
1 Difference of concentrations of K+ ions in the cytoplasm and in the extracellular fluid. Thus, in the cytoplasm of nerve fiber, the content of K+ ions is in 35 times higher than in the extracellular environment.
2 High membrane permeability to K+ ions, low permeability to Na+ ions and impermeability to intracellular proteins – anions.
That K+ ions and cell membrane permeability characteristics determine the value of resting potential. This becomes evident by the fact that the resting potential of nerve and skeletal muscle fibers (-90 mV) is very close to the value of the potassium equilibrium potential (-94 mV).
Additional factors:
1) A passive entry of Na+ ions into the cell. Although at the state of rest, membrane permeability to Na+ ions is many times smaller than for K+ ions (about 100), there is a growing diffusion of Na+ into the cell. It is due on one hand that very large electrochemical gradient for the Na+ ions is aimed into the cell (concentration of Na+ outside the cell is 10 times higher than inside, in addition, Na+ ions are trying to enter the cell, inside which the "-") and on the other hand - the presence of proteins in the cell membrane allow the Na+ to pass through when the cell is at rest. These proteins are called "potassium- sodium leakage channels." They are non-selective ion channels through which ions diffuse through the membrane which is at rest. These proteins are much easier to pass through K+ ions
channels than Na+ (approximately 100).
Permanent passive entry of Na+ into the cell reduces its membrane potential, therefore its value is actually smaller than the potassium equilibrium potential.
Calculations by Goldmann-Hodgkin-Katz formula show that the existing entrance of Na+ into the cell resting potential would be equal to -86 mV but actually it is -90 mV. Why? This explains the following additional factors that affect the RP.
2) Working of the Na+, K+-pump.
Na+, K+-pump – a cell membrane proteins that carry out active transport of Na+ and K+ against their concentration gradients. Operation of this pump has two consequences:
• maintained concentration gradient of Na+ and K+ on the both sides of the membrane despite the passive input of Na+ and K+ out of cells;
• Makes an immediate impact on the value of the RP due electro genesis of pump. For one cycle of working of the pump is unequal exchange of Na+ and K+ (three ions of Na+ is removed from the cell, and only 2 ions of K+ come into the cell), resulting to the slight hyperpolarization of the membrane and also the resting potential of nerve and skeletal muscle fibers is -90 mV but not -86 mV as should be expected from calculations performed by the Goldmann-Hodgkin-Katz formula.
The physiological importance of resting potential The presence of the RP on the membrane of cells determines their rice such as anxiety, i.e. the ability to be excited in response to stimulus.
In terms of electrophysiology, this means that the presence of RP is a prerequisite for the emergence of the action potential (AP).
Changes of RP in terms of pathology
The pathology changes of the RP of the excitable cells most often caused by these violations:
1) Changes of extracellular concentration of K+ ions.
There are two variants of violations:
• hyperkalemia. K+ ions enter into the cell and membrane depolarization is developing;
• hypokalemia. In this case, by contrast, K+ ions go out of cells leading to membrane hyperpolarization.
2) Changes in intracellular concentrations of K+ ions at:
• enhanced breakdown of proteins (catabolism increase speed).
In this case, by reducing the concentration of intracellular protein results to a decrease in intracellular concentrations of K+ ions which leads to depolarization of the membrane;
• enhanced protein synthesis (anabolic increase speed). Thus, in contrast, increased protein content within the cell is accompanied by an increase of intracellular concentration of K+ ions. This causes the hyperpolarization of the membrane.
3) Increased cell membrane permeability to Na ions+. A similar situation occurs when:
• There is damage of cell membranes (violation of its barrier function);
• Appearance (adsorption) of new proteins or other compounds that are passed through Na+ ions (e.g., adsorption complexes of antigen + antibody, antibiotics – ionophors).
4) Violation of Na+-K+-pumps.
The most common cause of such violations is the lack of ATP.
During the disorders of function Na+-K+-pump decreases the RP due to passive Na+ entering into the cell.
Regardless of the reason for the change of the RP in terms of pathology (depolarization or hyperpolarization) leads to a steady decrease of excitability of cell which manifested a wide range of disorders of the nervous system, heart, skeletal muscles and organs, which include the smooth muscles.
4 ACTION POTENTIAL
The action potential (AP) – is a rapid change of membrane potential that occurs in excitable structures in response to stimulus.
Actually the ability to generate the AP is the main feature which distinguishes the excitable structures (nerve, muscle and some types of secretory cells) from others.
Structure of the AP
AP of large nerve fibers during intracellular removal has the form shown below fig. 4.1.
Figure 4.1 – Action Potential at intracellular removal: I – phase of rest; II - phase of depolarization, ІІІ – phase of repolarization, IV – phase of the after potential
There are such phases of the AP:
1) Phase of the rest. It is represented by the resting potential. Development of the AP is impossible without resting potential.
2) Phase of depolarization (upward phase). It is very fast (0.1 ms) change of membrane potential is from -90 mV to + 35 mV. In large nerve fibers positive values of the AP are called overshoot. There is no overshoot in small nerve fibers and neurons.
3) Phase of repolarization (downward phase). This is the phase of the restoration of the negative charge on the inner surface of the membrane. Its duration exceeds the duration of depolarization phase.
4) Phase of after potential. Membrane potential for some time deviates from the level of RP after the AP. There are traces of depolarization and traces of hyperpolarization. After potential amplitude never exceeds 15-20% to that of AP.
The main physical characteristics of AP
1) Polarity of the AP. AP is electropositive on the inner surface of the membrane, and on external - electronegative in respect to "zero" of the Earth. In other words, during the development of AP, it is the potential reversion that changes the polarity of charge on the inner and outer surfaces of the membrane (Fig. 4.2).
Area with AP
+ + + + + + + + + + + - - - + + + + + + + + + + + + - - - + + + + + + - - - - - - + + + + + + - - - + + + + + + + + + + + - - - + + + + + + + + + + +
Figure 4.2 – reversion of membrane potential during the AP
2) Value of membrane potential on the top of the AP (value of the overshoot). This value is from +30 to +40 mV in large nerve and skeletal muscle fibers.
3) The amplitude of the AP. It is the distance from the resting potential level to the highest point of the overshoot.
This figure is 110 - 130 mV in large nerve and skeletal muscle fibers.
4) Duration of the AP. This characteristic of the AP is significantly different in different types of excitable structures.
Thus, in large nerve and skeletal muscle fibers, this figure is 0,5 – 5 ms, whereas in cardiac muscle fibers it is 300 ms.
5) Wavelength of the AP. AP is able to spread throughout the nerve and muscle fibers and its wave length is 0.1 to 5 cm in different structures.
6) Speed distribution of the AP. Depending on the type of fibers, the figure is 0.5 - 120 m / sec.
The main physiological characteristics of the AP 1. Obeys the law of "all or nothing." This means that:
• AP occurs when the stimulus, the power which is no less than certain thresholds;
• Physical characteristics of the AP (amplitude, duration, shape) do not depend on the power of stimulus.
2. Ability to auto spread along the cell membrane without damping, i.e. without changing their physical characteristics.
3. AP is accompanied with the refractory period.
4. AP is not capable of summation, i.e., to overlap.
Ionic mechanisms of the AP
The emergence of AP is associated with the existence in the plasma membrane of cells of two types of protein channels.
Let us make a brief discussion on their characteristics.
І Voltage-gated sodium channel proteins
These channels have two properties: selectivity and electrical excitability.
Selectivity – the ability of the channel to allow only Na+ ions to pass through, therefore these channels are called sodium.
Electrical excitability – the ability of the channel opened and closed in response to changes of the membrane potential (hence the name – voltage-gated).
Na+ channel consists of two parts:
1) Actually a transport system – a protein that transmits the Na+ ions through itself.
2) Gate – part of the channel, which defines its opening and closing condition.
Voltage-gated sodium channel has two gates:
• Activation (fast). They are located on the outer surface of the protein-channel, have the ability to rapidly open and close;
• Inactivation (slow). They are on the inner surface of the protein-channel and are characterized by being slow to open and close.
There are three functional states of the voltage-gated sodium channels (fig. 4.3).
• State of the rest. Activation gates are closed, inactivation - open. Channel is impenetrable for Na+ ions. The channel is in this state when the membrane potential is -90 mV (resting potential level).
• Activated state. Both gates (activation and inactivation) are opened, causing the channel to be penetrable for Na+ ions.
These ions pass from outside into the cell by the concentration gradient by simple diffusion mechanism. The reason for opening of the activation gate is due to changes in membrane
potential from -90 mV to +35 mV (depolarization).
• Inactivated state. Activation gates are open, inactivated - closed. Channel in impenetrable for Na+ ions. This state occurs because of changes of the membrane potential from -90 mV to +35 mV. Conformational changes of protein-channel result in depolarization leading not only to the rapid opening of the activation gate but also to the closing of the inactivation gates.
However, the latter is much slower
Figure 4.3 – Voltage-gated sodium channels
ІІ Voltage-gated potassium channels
They have the same properties as the sodium channels - selectivity and electrical excitability.
K+ - channel consists of two parts:
1) Its own transport system - a protein that transmits the K+ ions through itself;
2) Gate. Unlike sodium channels, there is one gate in the potassium channels located on the inside of the membrane.
There are two functional states of potassium channels (fig.
4.4):
• State of rest. Gates are closed. Channel is impermeable for K+ ions. The channel is in this state when the membrane potential is -90 mV.
• Active state. Gates are open. Channel is permeable to K+ ions. This state occurs during changing the membrane potential from -90 to +35 mV as a result of conformational changes in protein-channel. As these changes occur slowly, then the gate opens slowly.
Figure 4.4 – Voltage-gated potassium channel
Thus, there are two main differences of the potassium channels:
- Potassium channels are not inactivated because they have no inactivation gates;
- Opening and closing of potassium channels is slow.
Given the structure and functional characteristics of ion channels consider the origin of the major phases of AP:
depolarization and repolarization.
