In this article we will look at the types of muscle tissue. This is a very important topic in biology, because everyone should know how our muscles function. They represent a complex system, which we hope you will find interesting to study. And the pictures that you will find in this article will help you better imagine the types of muscle tissue. First of all, we will give a definition that is necessary when studying this topic.
This is a special group of animals, the main function of which is its contraction, causing the movement of the organism or its constituent parts in space. This function corresponds to the structure of the basic elements that make up various types of muscle tissue. These elements have a longitudinal and elongated orientation of myofibrils, which include myosin and actin. Muscle tissue, like epithelial tissue, is a composite tissue group, since its main elements develop from embryonic rudiments.
Contraction of muscle tissue
Its cells, like nerve cells, can be excited when exposed to electrical and chemical impulses. Their ability to contract (shorten) in response to a particular stimulus is associated with the presence of myofibrils, special protein structures, each of which consists of microfilaments, short protein fibers. In turn, they are divided into myosin (thicker) and actin (thin) fibers. In response to nervous stimulation, various types of muscle tissue contract. Contraction to the muscle is transmitted along the nerve process through the neurotransmitter, which is acetylcholine. Muscle cells in the body perform energy-saving functions, since the energy consumed during the contraction of various muscles is then released in the form of heat. That is why, when the body is exposed to cooling, trembling occurs. This is nothing more than frequent muscle contractions.
The following types of muscle tissue can be distinguished, depending on the structure of the contractile apparatus: smooth and striated. They consist of histogenetic types that differ in structure.
Muscle tissue is striated
Myotome cells, which are formed from the dorsal mesoderm, are the source of its development. This fabric consists of elongated cylinders, the ends of which are pointed. These formations reach 12 cm in length and 80 microns in diameter. Symplasts (multinuclear formations) are contained in the center of muscle fibers. Adjacent to them are cells called “myosatellites”. The sarcolemma is limited by the fibers. It is formed by the plasmolemma simplast and the basement membrane. Myosatelliotocytes are located under the basement membrane of the fiber - so that the plasmalemma simplast touches their plasmalemma. These cells are the cambial reserve of muscle skeletal tissue, and it is due to it that fiber regeneration occurs. Myosymplasts, in addition to the plasmalemma, also include sarcoplasm (cytoplasm) and numerous nuclei located along the periphery.
The importance of striated muscle tissue
When describing the types of muscle tissue, it should be noted that striated muscle tissue is the executive apparatus of the entire motor system. It forms In addition, this type of tissue is included in the structure of internal organs, such as the pharynx, tongue, heart, upper esophagus, etc. Its total mass in an adult is up to 40% of body weight, and in elderly people, as well as newborns , its share is 20-30%.
Features of striated muscle tissue
Contraction of this type of muscle tissue, as a rule, can be done with the participation of consciousness. It is slightly faster than the smooth one. As you can see, the types of muscle tissue differ (we will talk about smooth tissue very soon and note some other differences between them). In striated muscles, nerve endings perceive information about the current state of muscle tissue, and then transmit it along afferent fibers to the nerve centers responsible for the regulation of motor systems. Control signals come from regulators in the form of nerve impulses along motor or autonomic efferent nerve fibers.
Smooth muscle tissue
Continuing to describe the types of human muscle tissue, we move on to smooth tissue. It is formed by spindle-shaped cells, the length of which ranges from 15 to 500 microns, and the diameter ranges from 2 to 10 microns. Unlike striated muscle fibers, these cells have one nucleus. In addition, they do not have transverse striations.
The importance of smooth muscle tissue
The functioning of all body systems depends on the contractile function of this type of muscle tissue, since it is part of the structure of each of them. For example, smooth muscle tissue is involved in controlling the diameter of the respiratory tract, blood vessels, contraction of the uterus, bladder, and in the implementation of the motor functions of our digestive tract. It controls the diameter of the pupil of the eyes, and is also involved in many other functions of various body systems.
Muscle layers
This type of tissue forms muscle layers in the walls of lymphatic and blood vessels, as well as all hollow organs. Usually this is two or three layers. The thick circular one is the outer layer, the middle one is not necessarily present, the thin longitudinal one is the inner layer. The blood vessels supplying the muscle tissue, as well as the nerves, run parallel to the axis of the muscle cells between their bundles. Smooth muscle cells can be divided into 2 types: unitary (united, grouped) and autonomous myocytes.
Autonomous myocytes
Autonomous cells function quite independently of each other, since each such cell is innervated by a nerve ending. They were found in the muscle layers of large blood vessels, as well as in the ciliary muscle of the eye. Also of this type are the cells that make up the muscles that lift the hair.
Unitary myocytes
Unitary muscle cells, on the contrary, are closely intertwined with each other, so that their membranes can not only adhere tightly to each other, forming desmosomes, but also merge, forming nexuses (gap junctions). Bundles are formed as a result of this combination. Their diameter is about 100 microns, and their length reaches several mm. They form a network and are woven into its cells. Fibers of autonomic neurons are innervated by bundles, and they become functional units of smooth muscle tissue. Depolarization upon excitation of one cell of the beam spreads very quickly to neighboring ones, since the resistance of the gap junctions is low. Tissues consisting of unitary cells are found in most organs. These include the ureters, uterus, and digestive tract.
Myocyte contraction
The contraction of myocytes is caused in smooth tissue, as in striated tissue, by the interaction of myosin and actin filaments. This is similar to the different types of muscle tissue in humans. These threads are distributed less orderly within the myoplasm than in the striated muscle. This is due to the lack of transverse striations in smooth muscle tissue. Intracellular calcium is the final executive link that controls the interaction of myosin and actin filaments (that is, the contraction of myocytes). The same applies to the striated muscle. However, the details of the control mechanism differ significantly from the latter.
The vegetative axons passing through the very thickness of the smooth muscle tissue do not form synapses, which is typical for striated tissue, but numerous thickenings along the entire length, which play the role of synapses. The thickenings release a transmitter that diffuses to nearby myocytes. Receptor molecules are located on the surface of these myocytes. The mediator interacts with them. It causes depolarization of the outer membrane of the myocyte.
Features of smooth muscle tissue
The nervous system, its autonomic department, is controlled without the participation of consciousness by the work of smooth muscles. The bladder muscles are the only exception. Control signals are either directly implemented or indirectly through hormonal (chemical, humoral) influences.
The energetic and mechanical properties of this type of muscle tissue ensure the maintenance of (controlled) tone of the walls of hollow organs and blood vessels. This is due to the fact that smooth tissue functions efficiently and does not require large amounts of ATP. It has a lower speed of action than striated muscle tissue, but it is capable of contracting for a longer time, in addition, it can develop significant tension and change its length over a wide range.
So, we looked at the types of muscle tissue and the features of their structural organization. Of course, this is just basic information. You can describe the types of muscle tissue for a long time. The pictures will help you visualize them.
STRUCTURAL-FUNCTIONALSKELETAL CHARACTERISTICS
MUSCLES AND ITS MECHANISM
ABBREVIATIONS Structural unit of skeletal muscle
is a muscle fiber - highly elongated
multinucleate cell.
The length of the muscle fiber depends on the size
muscles and ranges from a few millimeters
up to several centimeters. Fiber thickness
varies from (10-100 µm).
Muscle types
There are three types in the human body
muscles:
skeletal, cardiac (myocardium) and smooth.
On microscopic examination in
skeletal and cardiac muscles
striations are detected, so they
called striated muscles. Skeletal muscles are attached primarily to
bones, which is what gave them their name.
Skeletal muscle contraction is initiated
nervous
impulses
And
obeys
conscious
control,
those.
carried out arbitrarily.
Smooth muscle contraction is initiated
impulses, some hormones and not
depends on the will of a person. The muscle fiber is surrounded by a two-layer
lipoprotein electrically excitable membrane sarcolemma,
which
covered
network
collagen fibers, which give it strength and
elasticity.
There are several types of skeletal muscles
muscle fibers: slow twitch
(MS) or red and fast-twitch
(BS) or white.
Molecular mechanism of contraction.
Skeletal muscles contain contractile muscles
proteins:
actin
And
myosin.
Mechanism
their
interactions during an elementary act
muscular
reductions
explains
theory
sliding threads developed by Hasley and
Hanson.
The structure of muscle fiber
Sarcolemma - plasma membrane coveringmuscle fiber (connects to the tendon, which
attaches muscle to bone; tendon transmits force
produced by muscle fibers of bone and such
way
carried out
movement).
Sarcolemma
has selective permeability for various
substances and has transport systems using
which maintain different concentrations of ions
Na+, K+, as well as Cl- inside the cell and in the intercellular
liquid, which leads to the appearance of
surface membrane potential - required
conditions for the occurrence of muscle fiber excitation.
Sarcoplasma
–
gelatinous
liquid,
filling
gaps
between
myofibrils
(contains
dissolved
proteins,
microelements,
glycogen, myoglobin, fats, organelles). About 80%
fiber volume is occupied by long contractile filaments
- myofibrils. System of transverse tubes. This is the T network
tubes (transverse), is a continuation
sarcolemma; they interconnect while passing
among myofibrils. Provide fast
transmission of nerve impulses (propagation
excitation) inside the cell to individual
myofibrils.
Sarcoplasmic reticulum (SR) – network
longitudinal tubes, located parallel
myofibrils; this is the site of Ca2+ deposition,
which is necessary to ensure the process
muscle contraction.
The contractile proteins actin and myosin form
in myofibrils thin and
thick
myofilaments.
They
are located
parallel to each other inside the muscle cell
Myofibrils
present
yourself
contractile elements of muscle fiber - bundles of “threads” (filaments). Myofibril structure:
1. Partitions - called Z - plates,
they are divided into sarcomeres.
Sarcomere structure:
They show a regular sequence
alternating transverse light and dark
stripes,
which
due to
special
mutual position
actin
And
myosin
filaments
(transverse
striping).
The middle of the sarcomere is occupied by “thick” filaments
myosin. (A – dark disk)
On
both ends of the sarcomere are
"thin" actin filaments. (I-disc light) Actin filaments attach to Z –
plates, Z plates themselves
limit the sarcomere.
In a resting muscle, the ends of the thin and
fat
filaments
only
weak
overlap at the border between A and I disks.
N – zone (lighter) in which there is no
overlap
threads
(Here
only myosin filaments are located),
is located in drive A.
M - line is located in the center of the sarcomere
– place to hold thick threads
(built from supporting proteins.)
The theory of sliding threads.
Sarcomere shortening:The muscle contracts as a result of shortening many
sarcomeres connected in series
myofibrils.
During contraction, thin actin filaments
slide along thick myosin fibers, moving between them
to the middle of their bundle and sarcomere.
The main provisions of the theory of sliding threads:
During muscle contraction, the actin and
myosin filaments do not shorten (width of A-disc
always remains constant, while I-disks and H-zones
when contracting they become narrower).
The length of the threads does not change when the muscle is stretched (thin
filaments are pulled out from the spaces between thick
threads, so that the degree of overlap of their bundles
decreases).
10. Work of cross bridges.
The movement of the heads creates a combined force,like a “comb” that moves actin filaments towards
middle of the sarcomere. Only due to rhythmic
separation and reattachment of myosin
heads, the actin filament can be pulled towards
middle of the sarcomere.
When the muscle relaxes, the myosin heads
separated from actin filaments.
Since actin and myosin filaments can easily
slide relative to each other, resistance
relaxed muscles stretch very low.
Muscle lengthening during relaxation wears
passive character.
11. Conversion of chemical energy into mechanical energy.
ATP is a direct source of energy forabbreviations.
When a muscle contracts, ATP is broken down into
ADP and phosphate.
Rhythmic activity of transverse bridges, i.e.
e. cycles of their attachment to actin and detachment
from it, providing muscle contraction,
are possible only through the hydrolysis of ATP, and
accordingly, upon activation of ATPase, which
directly involved in the breakdown of ATP into
ADP and phosphate.
12. Molecular mechanism of muscle contraction.
The contraction is triggered by a nerve impulse. At the same time, insynapse - the point of contact of the nerve ending with
the sarcolemma releases the mediator (neurotransmitter) acetylcholine.
Acetylcholine (Ach) causes changes in permeability
membranes for some ions, which in turn
leads to the emergence of ionic currents and is accompanied by
membrane depolarization. As a result, on her
action potential appears on the surface or
gets excited.
Potential
actions
(excitation)
spreads deep into the fiber through T-systems.
A nerve impulse causes a change in permeability
membranes of the sarcoplasmic reticulum and leads to
liberation
ions
Ca2+
from
bubbles
sarcoplasmic reticulum.
13. Electromechanical interface
Sending a command to abbreviate fromexcited cell membrane to
myofibrils
V
depth
cells
(electromechanical
pairing)
includes
V
myself
some
sequential processes, key
the role in which Ca2+ ions play.
14.
1. Electromechanical coupling occursthrough capacity dissemination
actions on membranes of the transverse system
inside the cell, then the excitation passes to
longitudinal system (EPR) and causes
release of what is deposited in the muscle
cell Ca2+ into the intracellular space,
which surrounds the myofibrils. This leads to
reduction
2. Ca2+ is removed from the intracellular space
in the depot (ER channels) due to the work of calcium
pumps on EPR membranes.
3. Only due to electrical transmission
transverse system, fast
mobilization of calcium reserves deep in the fiber, and
only this can explain the very short
latent period between stimulus and
abbreviation.
15.
Functional role of ATP:- in a resting muscle - prevents connection
actin filaments with myosin filaments;
- during muscle contraction - supplies
the necessary energy for the movement of thin threads
relatively thick, which leads to shortening
muscles or developing tension;
- in the process of relaxation - provides energy
active transport of Ca2+ into the reticulum.
16. Types of muscle contractions. Optimum and pessimum of muscle contraction
Depending on changes in muscle fiber lengthThere are two types of its contraction - isometric and
isotonic.
Muscle contraction in which the length of the muscle
decreases as the force it develops is called
auxotonic.
Maximum force during auxotonic experimental
conditions (with a tensile elastic connection between the muscle and
force sensor) is called the auxotonic maximum
abbreviations. It is much less than the force it develops
muscle at constant length, i.e. with isometric
abbreviation.
Contraction of a muscle in which its fibers are shortened
at constant voltage is called isotonic.
Contraction of a muscle that increases tension
and the length of the muscle fibers remains unchanged,
called isometric
17.
Muscular work is equal to the productdistance (muscle shortening) to the weight of the load,
which lifts the muscle.
With isotonic tetanic activation
muscles, the amount of shortening depends on the load and
rate of muscle shortening.
The lower the load, the more shortening in
unit of time. Unused muscle
shortens at maximum speed, which
depends on the type of muscle fiber.
Muscle power is equal to the product
the force it develops on the speed of shortening
18.
A relaxed muscle maintaining its “resting length” due tofixation of both its ends, does not develop the force that
would be transmitted to the sensor. But if you pull one of it
end so that the fibers stretch, a
passive tension. Thus, the muscle is able
rest elastic. Modulus of elasticity of resting muscle
stretching increases. This elasticity is mainly due to
manner by tensile structures that are located
parallel
relatively
tensile
myofibrils
(“parallel
elasticity")
.
Myofibrils
V
in a relaxed state there is practically no effect
tensile resistance; actin and myosin filaments
related
transverse
bridges,
easily
slide
relative to each other. Preliminary degree
stretching determines the magnitude of passive stress
resting muscle and the amount of additional force,
which a muscle can develop if activated at a given
length
19.
The peak force under such conditions is calledmaximum isometric contraction.
When a muscle is strongly stretched, the force of contraction
decreases because actin filaments are extended from
myosin bundles and, accordingly, a smaller zone
overlapping of these threads and the possibility
formation of cross bridges.
With a very strong muscle strain, when
actin and myosin filaments stop
overlap, myofibrils are not capable of
develop strength. This proves that muscle strength
is the result of interaction
actin and myosin filaments (i.e.
formation of cross bridges between them).
Under natural conditions of muscle contraction
are mixed - the muscle is usually not only
shortens, but its tension also changes.
20.
Depending on the duration there aresingle and tetanic muscle contractions.
Single muscle contraction in an experiment
caused by a single electrical stimulation
electric shock In isotonic mode, single
contraction begins through a short hidden
(latent) period, followed by a rise phase
(shortening phase), then a decline phase (phase
relaxation) (Fig. 1). Usually muscle
shortened by 5-10% of the original length.
The duration of the action potential of muscle fibers is also
varies and is 5-10 ms taking into account slowdown
repolarization phases.
Muscle fiber obeys the “all or
nothing”, i.e. responds to threshold and
suprathreshold stimulation identical in
size with a single contraction.
21.
The contraction of a whole muscle depends on:1. on the strength of the stimulus with direct irritation
muscles
2. on the number of nerve impulses entering the muscle during
nerve irritation.
An increase in the strength of the stimulus leads to an increase in the number
contracting muscle fibers.
A similar effect is observed in natural conditions - with
an increase in the number of excited nerve fibers and frequency
impulses (more PD nerve impulses arrive to the muscle), the number of contracting muscle fibers increases.
With single contractions, the muscle gets tired
insignificant.
Tetanic contraction is a continuous continuous
contraction of skeletal muscle. It is based on the phenomenon
summation of single muscle contractions.
Single curve
gastrocnemius contractions
frog muscles:
1-latent period,
2- phase of shortening,
22.
When applied to muscle fiber ordirectly
on
muscle
two
fast
successive irritations,
emerging
reduction
It has
big
amplitude and duration. At the same time, actin filaments and
myosin additionally slide relative to each other
friend. Reductions may not involve previously
contracted muscle fibers, if the first
the stimulus caused a subthreshold depolarization in them,
and the second increases it to a critical value.
Summation of contractions during repeated stimulation
muscles or the supply of PD to it occurs only
when the refractory period is over
(after the disappearance of the muscle fiber PD).
23.
When impulses arrive to the muscle during itsrelaxation, serrated tetanus occurs, during
shortening time - smooth tetanus (Fig.).
Tetanus amplitude greater than
maximum single muscle contraction.
Tension developed by muscle fibers
with smooth tetanus, usually 2-4 times more,
than with a single contraction, however the muscle
gets tired faster. Muscle fibers are not
manage to restore energy resources,
used up during contraction.
The amplitude of smooth tetanus increases with
increasing frequency of nerve stimulation. At
some (optimal) stimulation frequency
the amplitude of smooth tetanus is greatest (optimum frequency of stimulation)
24.
Rice. Contractions of the frog gastrocnemius muscle duringincreased frequency of irritation of the sciatic nerve
(s/s - stimuli per second): a - single contraction;
b-d - superimposing contraction waves on top of each other and
formation of different types of tetanic contraction.
At a frequency of 120 st/s - pessimal effect
(muscle relaxation during stimulation) – e
25.
With excessively frequent nerve stimulation (more than 100imp/c) muscle relaxes due to block
conduction of excitation in neuromuscular
synapses - Vvedensky pessimum (pessimum
stimulation frequency). Vvedensky's pessimum can be
get also with direct, but more frequent irritation
muscles (more than 200 impulses/s). Vvedensky's pessimum is not
is the result of muscle fatigue or depletion of the transmitter in the synapse, which is proven by the fact
resumption of muscle contraction immediately after
reducing the frequency of irritation. Braking
develops at the neuromuscular junction when
nerve irritation.
In vivo muscle fibers
contract in the dentate tetanus mode or
even single consecutive contractions.
26.
However, the form of muscle contraction as a wholeresembles smooth tetanus.
Causes
this
asynchrony
ranks
motor neurons and contractile asynchrony
reactions of individual muscle fibers, involvement
in reducing their large number, due to
why the muscle contracts smoothly and smoothly
relaxes, can remain in a state for a long time
reduced state due to alternation
contractions of many muscle fibers. At
this muscle fibers of each motor
units contract synchronously.
27.
Functional unit of muscle -motor unit
Concepts. Innervation of skeletal muscle fibers
carried out by motor neurons of the spinal cord or
brain stem. One motor neuron with its branches
the axon innervates several muscle fibers.
The set of motor neurons and those innervated by them
muscle fibers are called motor
(neuromotor) unit. Number of muscle
motor unit fibers vary widely
within different muscles. Motor units
small in muscles adapted for fast
movements, from several muscle fibers to
several dozen of them (finger muscles, eyes,
language). On the contrary, in the muscles that carry out
slow movements (maintaining posture with muscles
trunk), motor units are large and include
hundreds and thousands of muscle fibers
28.
Atreduction
muscles
V
natural
(natural) conditions can be registered
its electrical activity (EMG electromyogram) using needle or skin electrodes. In a completely relaxed muscle
There is almost no electrical activity. At
small
tension,
For example
at
maintaining
poses,
motor
units
discharged at a low frequency (5-10 pulses/s),
at high voltage pulse frequency
increases on average to 20-30 pulses/s. EMG allows us to judge functional ability
neuromotor units. From a functional point
motor units are divided into
slow and fast.
29.
motor neurons and slow muscle fibers (red).Slow motor neurons are generally low-threshold, so
as usual, these are small motor neurons. Sustainable level
impulses in slow motor neurons are already observed
with very weak static muscle contractions, with
maintaining the pose. Slow motor neurons are capable of
maintain long-term discharge without noticeable reduction
pulse frequency over a long period of time.
That's why they are called low-fatigue or
tireless motor neurons. Surrounded by slow
muscle fibers have a rich capillary network, allowing
obtain large amounts of oxygen from the blood.
Increased myoglobin content facilitates transport
oxygen in muscle cells to mitochondria. Myoglobin
causes the red color of these fibers. Besides,
fibers contain a large number of mitochondria and
oxidation substrates - fats. All this determines the use of slow muscle fibers more
efficient aerobic oxidative pathway
30.
Fast motor units are made up offast motor neurons and fast muscle neurons
fibers Fast high-threshold motor neurons
are included in the activity only to ensure
relatively large static and
dynamic muscle contractions, as well as at the beginning
any cuts to increase speed
increase in muscle tension or report
required acceleration for a moving part of the body. How
the greater the speed and strength of movements, i.e. the more
the power of the contractile act, the greater the participation
fast motor units. Fast
motor neurons are classified as fatigueable - they do not
capable of long-term maintenance
high frequency discharge
31.
Fast twitch muscle fibers (white muscle fibers)fibers) are thicker, contain more
myofibrils have greater strength than
slow fibers. These fibers are surrounded by less
capillaries, cells have fewer mitochondria,
myoglobin and fats. Oxidative activity
enzymes in fast fibers are lower than in
slow, but the activity of glycolytic
enzymes, glycogen reserves are higher. These fibers are not
have great endurance and more
adapted for powerful, but relatively
short-term cuts. Fast activity
fiber matters for performance
short-term high-intensity work,
such as sprinting
32.
The rate of contraction of muscle fibers isdirectly dependent on the activity of myosin-ATPase
- an enzyme that breaks down ATP and thereby
promoting the formation of cross bridges
and the interaction of actin and myosin
myofilaments. Higher activity of this
enzyme in fast muscle fibers
provides higher speed
contractions compared to slow fibers
Tone – weak overall muscle tension
(develops at very low stimulation frequencies).
The strength and speed of muscle contraction depends on
the number of motor muscles involved in the reduction
units (the more motor units
activated – the stronger the contraction).
Reflex tone - (observed in some
groups of postural muscles) a state of involuntary
sustained muscle tension
33.
Muscle efficiencyDuring muscle activation, an increase
intracellular Ca 2+ concentration leads to
reduction and increased breakdown of ATP; at
this increases the metabolic rate of the muscle
100-1000 times. According to the first law
thermodynamics (law of conservation of energy),
chemical energy released in the muscle
must be equal to the sum of mechanical energy
(muscle work) and heat generation
34.
Efficiency.Hydrolysis of one mole of ATP provides 48 kJ of energy,
40–50% - turns into mechanical work, and
50-60% dissipated as heat at startup
(initial heat) and during contraction
muscles, the temperature of which is
rises. However, under natural conditions
mechanical efficiency of muscles is about 20-30% since in
reduction time and processes after it
requiring energy expenditure, go outside
myofibrils (work of ion pumps,
oxidative regeneration of ATP - heat
recovery)
35.
Energymetabolism
.
In
time
long-term
uniform
muscular
activity, aerobic regeneration of ATP occurs during
check
oxidative
phosphorylation.
The energy required for this is released in
as a result of the oxidation of carbohydrates and fats. System
is in a state of dynamic equilibrium -
the rates of ATP formation and breakdown are equal.
(intracellular
concentrations
ATP
And
creatine phosphate are relatively constant) With
long-term sports loads speed
ATP breakdown in muscles increases by 100 or
1000 times. Continuous loading is possible if
speed
recovery
ATP
increases
according to consumption. Oxygen consumption
muscle tissue increases 50-100 times;
increases the rate of breakdown of glycogen in
muscles.
36.
Anaerobic breakdown - glycolysis: ATP is formed in 2-3times faster, and the mechanical energy of the muscle is 2-3 times
higher than with long-term operation provided
aerobic mechanisms. But resources for anaerobic
metabolism is quickly exhausted, metabolic products
(lactic acid) cause metabolic acidosis.,
which limits performance and causes
fatigue. Anaerobic processes are necessary for
providing energy for short-term extreme
effort, as well as at the beginning of prolonged muscle
work because adaptation of the oxidation rate (and
glycolysis) to the increased load requires some time.
The oxygen debt approximately corresponds to
the amount of energy obtained anaerobically is not yet
compensated by aerobic ATP synthesis.
Oxygen debt is caused by (anaerobic)
hydrolysis of creatine phosphate, can reach 4 l and can
increase to 20 l. Part of the lactate is oxidized in the myocardium
and part (mainly in the liver) is used for synthesis
glycogen.
37.
The ratio of fast and slow fibers. HowThe more fast fibers a muscle contains, the more
its possible contraction force.
Cross section of a muscle.
The terms “absolute” and “relative” muscle strength:
"total muscle strength" (determined by maximum
voltage in kg that it can develop) and “specific
muscle strength" - the ratio of this tension in kg to
physiological cross-section of the muscle (kg/cm2).
The larger the physiological cross-section of the muscle,
the more load she is able to lift. For this reason
muscle strength with obliquely arranged fibers is greater
force developed by a muscle of the same thickness, but with
longitudinal arrangement of fibers. To compare strength
different muscles the maximum load they are able to
raise, divide by the area of their physiological transverse
sections (specific muscle strength). Calculated this way
force (kg/cm2) for the human triceps brachii muscle - 16.8,
biceps brachii - 11.4, shoulder flexor - 8.1,
gastrocnemius muscle - 5.9, smooth muscle - 1 kg/cm2.
38.
In various muscles of the body the relationship betweennumber of slow and fast muscle fibers
not the same, therefore the strength of their contraction, and
the degree of shortening is variable.
With a decrease in physical activity - especially
high intensity, which requires
active participation of fast muscle fibers, the latter thin out (hypotrophy) faster,
than slow fibers, their decreases faster
number
Factors influencing the strength of muscle contraction.
The number of contracting fibers in a given muscle. WITH
increases in contractile fibers
the strength of muscle contractions as a whole. In natural
conditions, the force of muscle contraction increases with
an increase in nerve impulses reaching the
muscle,
in the experiment - with increasing strength of irritation.
39.
Moderate stretching of the muscle also leads toincreasing its contractile effect. However
in case of excessive stretching, contraction force
decreases. This is demonstrated in the experiment with
dosed muscle stretching: muscle
overstretched so that the actin and myosin filaments do not
overlap, then the total muscle strength is zero.
As you approach your natural resting length,
in which all myosin filament heads are capable of
contact with actin filaments, force
muscle contraction increases to a maximum.
However, with a further decrease in length
muscle fibers due to the overlap of actin filaments and
myosin force of muscle contraction again
decreases due to a decrease in possible
zones of contact between actin and myosin filaments.
40.
Functional state of the muscle.When a muscle gets tired, the magnitude of its contraction
decreases.
Muscle work is measured by the product
lifted load by the amount of its shortening.
Dependence of muscle work on load
obeys the law of average loads. If the muscle
contracts without load, its external work is equal to
zero. As the load increases, the work
increases, reaching a maximum at medium
loads Then it gradually decreases with
increasing load. Work becomes equal
zero with a very large load, which the muscle
its contraction is not able to increase tension
100-200 mg.
41.
SMOOTH MUSCLE.Smooth muscle does not have a transverse
striation. Spindle-shaped cells connected
special intercellular contacts (desmosomes).
Rate of myofibril sliding and ATP breakdown
100-1000 times lower. Well suited for
long-term sustainable reduction, which is not
leads to fatigue and significant energy consumption.
Capable of spontaneous thetan contractions,
which are of myogenic origin and not
neurogenic like skeletal muscles.
Myogenic excitation.
Myogenic excitation occurs in cells
pacemakers (pacemakers), who have
electrophysiological properties.
Pacemaker potentials depolarize their membrane
to a threshold level, causing an action potential. Ca
2+ enters the cell - the membrane depolarizes, then
42.
Spontaneous activity of pacemakers can be modulatedautonomic nervous system and its mediators
(acetylcholine enhances activity leading to more frequent and
strong contractions, and norepinephrine has
opposite action).
Excitation propagates through “gap junctions”
(nexuses) between plasma membranes
adjacent muscle cells. The muscle behaves like
a single functional unit, synchronously reproducing
activity of your pacemaker. Smooth muscle can be
completely relaxed in both shortened and extended
condition. Strong stretching activates contraction.
Electromechanical interface. Excitation
smooth muscle cells cause either an increase in Ca entry
through voltage-gated calcium channels, or
releases from calcium depots, which in any case
leads to an increase in intracellular concentration
calcium and causes activation of contractile structures.
Relaxation is slow because... ion absorption rate
Ca is very low.
Skeletal muscle tissue
Sectional diagram of a skeletal muscle.
Structure of skeletal muscle
Skeletal (striated) muscle tissue- elastic, elastic tissue capable of contracting under the influence of nerve impulses: one of the types of muscle tissue. Forms the skeletal muscles of humans and animals, designed to perform various actions: body movement, contraction of the vocal cords, breathing. Muscles consist of 70-75% water.
Histogenesis
The source of development of skeletal muscles are myotome cells - myoblasts. Some of them differentiate in places where so-called autochthonous muscles are formed. Others migrate from myotomes to mesenchyme; at the same time, they are already determined, although outwardly they do not differ from other mesenchymal cells. Their differentiation continues in places where other muscles of the body are formed. During differentiation, 2 cell lines arise. The cells of the first merge, forming symplasts - muscle tubes (myotubes). The cells of the second group remain independent and differentiate into myosatellites (myosatellite cells).
In the first group, differentiation of specific organelles of myofibrils occurs; gradually they occupy most of the lumen of the myotube, pushing the cell nuclei to the periphery.
The cells of the second group remain independent and are located on the surface of the myotubes.
Structure
The structural unit of muscle tissue is muscle fiber. It consists of myosimplast and myosatellitocytes (companion cells), covered with a common basement membrane.
The length of the muscle fiber can reach several centimeters with a thickness of 50-100 micrometers.
The structure of myosymplast
The structure of myosatellites
Myosatellites are mononuclear cells adjacent to the surface of the myosymplast. These cells are poorly differentiated and serve as adult stem cells of muscle tissue. In case of fiber damage or prolonged increase in load, cells begin to divide, ensuring the growth of myosymplast.
Mechanism of action
The functional unit of skeletal muscle is the motor unit (MU). ME includes a group of muscle fibers and the motor neuron that innervates them. The number of muscle fibers that make up one IU varies in different muscles. For example, where fine control of movements is required (in the fingers or in the muscles of the eye), the motor units are small, they contain no more than 30 fibers. And in the gastrocnemius muscle, where fine control is not needed, there are more than 1000 muscle fibers in the ME.
Motor units of the same muscle can be different. Depending on the speed of contraction, motor units are divided into slow (S-ME) and fast (F-ME). And F-ME, in turn, is divided according to its resistance to fatigue into fatigue-resistant (FR-ME) and fast-fatigable (FF-ME).
The motor neurons innervating these MEs are divided accordingly. There are S-motoneurons (S-MN), FF-motoneurons (F-MN) and FR-motoneurons (FR-MN). S-ME are characterized by a high content of myoglobin protein, which is capable of binding oxygen (O2). Muscles predominantly composed of this type of ME are called red muscles due to their dark red color. Red muscles perform the function of maintaining human posture. Extreme fatigue of such muscles occurs very slowly, and restoration of functions occurs, on the contrary, very quickly.
This ability is determined by the presence of myoglobin and a large number of mitochondria. Red muscle MEs typically contain a large number of muscle fibers. FR-ME constitute muscles that are capable of performing rapid contractions without noticeable fatigue. FR-ME fibers contain a large number of mitochondria and are capable of generating ATP through oxidative phosphorylation.
Typically, the number of fibers in FR-ME is less than in S-ME. FF-ME fibers are characterized by a lower mitochondrial content than FR-ME, as well as the fact that ATP is produced in them through glycolysis. They lack myoglobin, so muscles consisting of this type of ME are called white. The white muscles develop a strong and rapid contraction, but tire quite quickly.
Function
This type of muscle tissue provides the ability to perform voluntary movements. The contracting muscle acts on the bones or skin to which it is attached. In this case, one of the attachment points remains motionless - the so-called fixation point(lat. punctum fixum), which in most cases is considered as the initial section of the muscle. A moving muscle fragment is called moving point, (lat. punctum mobile), which is the place of its attachment. However, depending on the function performed, punctum fixum can act as punctum mobile, and vice versa.
Notes
see also
Literature
- Yu.I. Afanasyev, N.A. Yurina, E.F. Kotovsky Histology. - 5th ed., revised. and additional.. - Moscow: Medicine, 2002. - 744 p. - ISBN 5-225-04523-5
Links
- - Mechanisms of muscle tissue development (English)
| Muscular system | |
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Wikimedia Foundation. 2010.
Muscle tissue(textus musculares) represent a group of animal and human tissues of different origins that have a common property - contractility. This property is achieved by these tissues due to the presence of special contractile structures in them - myofilaments. The following main types of muscle tissue are distinguished:
smooth (non-striated) muscle tissue and striated (striated) muscle tissue. The latter, in turn, are divided into skeletal muscle tissue and cardiac muscle tissue. Some specialized varieties of other tissues also have the property of contractility. These include the so-called epithelial muscle tissue (in the sweat and salivary glands) and neuroglial muscle tissue (in the iris) (Table 9).
Smooth (non-striated) muscle tissue
Smooth muscle tissue(textus muscularis nonstriatus) develops from mesenchyme. It makes up the motor apparatus of internal organs, blood and lymphatic vessels. Its contractions are slow, tonic in nature. The structural unit of smooth muscle tissue is an elongated spindle-shaped cell - smooth myocyte. It is covered with a plasmalemma, to which the basement membrane and connective tissue fibers adjoin the outside. Inside the cell, in its center, in the myoplasm, there is an elongated nucleus, around which mitochondria and other organelles are located.
Contractile protein filaments were discovered in the myoplasm of myocytes under an electron microscope - myofilaments. Distinguish actin, myosin and intermediate myofilaments. Actin and myosin myofilaments ensure the act of contraction itself, and intermediate ones protect smooth myocytes from excessive expansion during shortening. Myofilaments of smooth myocytes do not form discs, therefore these cells do not have transverse striations, and are called smooth, non-striated. Smooth myocytes regenerate well. They divide by mitosis, can develop from poorly differentiated connective tissue cells, and are capable of hypertrophy. Between the cells there is a supporting stroma of smooth muscle tissue - collagen and elastic fibers that form dense networks around each cell. Smooth muscle cells synthesize the fibers of this stroma themselves.
Striated (striated) muscle tissue
As already mentioned, this group of striated muscle tissues includes skeletal and cardiac muscle tissue. These tissues are united primarily on the basis of the cross-striations of their special organelles - myofibrils. However, in terms of their origin, general structural plan and functional features, these two types of striated muscle tissue differ significantly.
Striated skeletal muscle tissue
Skeletal muscle tissue(textus muscularis striatus sceletalis) develops from segmented mesoderm, more precisely from its central sections, called myotomes. The structural and functional unit of this tissue is multinuclear myosymplasts - striated muscle fibers. From the surface they are covered sarcolemma - a complex formation consisting of a three-layer muscle fiber plasmalemma, a basement membrane and an externally adjacent network of connective tissue fibers. Under the basement membrane, adjacent to the plasmalemma of the muscle fiber, there are special muscle cells - satellites. Inside the muscle fiber, in its sarcoplasm, along the periphery, there are numerous nuclei, and in the center, along the fiber, there are special organelles - myofibrils. Mitochondria and other common organelles in muscle fiber are located around the nuclei and along the myofibrils. Under an electron microscope, myofibrils consist of threads - myofilaments - actnioid, thinner (about 5-7 nm in diameter) and thicker - myosin (about 10-20 nm in diameter).
Actin myofilaments containing the protein actin form isotropic disks (I). These are light-colored, non-birefringent discs. In the center of the disks I passes Z-line -telophragm. This line divides the disk I on two half-discs. In the Z-line area there are so-called triads. Triads consist of tubular elements - T-tubules, formed by pressing the plasma membrane into the muscle fiber. Through these tubes the nerve impulse travels to the myofibrils. In each triad, one T-tubule contacts two terminal cisterns of the sarcoplasmic reticulum, which ensures the release of calcium ions necessary for the contractile act. In the area of the Z-lines of the disk I The ends of actin myofilaments converge. Myosin myofilaments, containing the protein myosin, form anisotropic (A) dark disks that are birefringent. In the center of disk A passes M-line - mesophragm. In M-linny the ends of myosin myofibrils converge and a network of tubules of the sarcoplasmic reticulum is discovered. The alternation of dark and light discs in the myofibrils gives the muscle fiber a transverse striation. The structural unit of myofibrils is the myomer (sarcomere) - this is the section of the myofibril between two Z-lines. Its formula is A+2 1/2 I.
According to modern concepts, each muscle fiber is divided into: contractile apparatus, consisting of multifibrils, including actin and myosin myofilaments; trophic apparatus, which includes sarcoplasm with nuclei and organelles; special membrane apparatus of triads; support apparatus, including the sarcolemma with endomysium and membrane structures of lines Z and M; and finally nervous apparatus, represented by motor neuromuscular endings - motor plaques and sensory nerve endings - neuromuscular spindles.
In skeletal muscle tissue there are whiteand red muscle fibers. White muscle fibers contain little sarcoplasm and myoglobin and many multifibrils. On a cross section, densely located myofibrils are clearly visible in white muscle fibers. They provide a strong but short-lasting contraction. Red muscle fibers contain a lot of sarcoplasm and therefore a lot of myoglobin and little myofibrils. On a cross section, in such muscle fibers the myofibrils are arranged loosely in the form of groups, forming polygons called Conheim fields. These fields are separated from each other by layers of sarcoplasm. Red muscle fibers contain many mitochondria and are capable of long-term contraction. Each skeletal muscle, as an organ, contains both white and red muscle fibers. However, their ratio in different muscle groups is not the same.
Each muscle fiber is surrounded on the outside by a layer of loose fibrous connective tissue called endomysium(endomysium). Groups of muscle fibers are surrounded perimysium(perimysium), and the muscle itself is a dense connective tissue membrane - epimysium(epimysium).
Striated skeletal muscle tissue is capable of regeneration. Contraction of muscle tissue is interpreted from the perspective of the sliding theory: actin myofilaments move in and slide between myosin ones.
Cardiac muscle tissue
Cardiac muscle tissue (textus muscularis cardiacus) is striated (striated) muscle tissue. However, it has a number of significant differences in its structure from skeletal muscle tissue. This tissue develops from the visceral layer of mesoderm, more precisely, from the so-called myoepicardial plate. The structural unit of cardiac muscle tissue is striated cells - cardiac myocytes or cardiomyocytes(miocyti cardiaci) with one or two nuclei located in the center. Along the periphery of the cytoplasm in cardiomyocytes there are myofibrils, which have the same structure as in skeletal muscle fiber. There are a large number of mitochondria (sarcosomes) located around the nucleus and along the myofibrils. Cardiomyocytes are separated from each other insert discs(disci intercalati), educated desmosomes and gap junctions. Through these discs, cardiomyocytes are united end to end into cardiac muscle fibers, anastomosing with each other and contracting as a single unit. In cardiac muscle tissue, cardiomyocytes are distinguished, - contractileor typical and conductive or atypical, components of the conduction system of the heart. Conducting cardiomyocytes are larger and contain fewer myofibrils and mitochondria. Their nuclei are often eccentrically located.Internal organs, skin, blood vessels.
Skeletal muscles together with the skeleton they form the musculoskeletal system of the body, which ensures the maintenance of posture and movement of the body in space. In addition, they perform a protective function, protecting internal organs from damage.
Skeletal muscles are an active part of the musculoskeletal system, which also includes bones and their joints, ligaments, and tendons. Muscle mass can reach 50% of total body weight.
From a functional point of view, the motor system also includes motor neurons that send nerve impulses to muscle fibers. The bodies of motor neurons that innervate skeletal muscles with axons are located in the anterior horns of the spinal cord, and those innervating the muscles of the maxillofacial region are located in the motor nuclei of the brain stem. The axon of a motor neuron branches at the entrance to the skeletal muscle, and each branch participates in the formation of the neuromuscular synapse on a separate muscle fiber (Fig. 1).
Rice. 1. Branching of the motor neuron axon into axon terminals. Electron diffraction pattern
Rice. The structure of human skeletal muscle
Skeletal muscles are made up of muscle fibers that are organized into muscle bundles. The set of muscle fibers innervated by the axon branches of one motor neuron is called a motor (or motor) unit. In the eye muscles, 1 motor unit can contain 3-5 muscle fibers, in the trunk muscles - hundreds of fibers, in the soleus muscle - 1500-2500 fibers. The muscle fibers of the 1st motor unit have the same morphofunctional properties.
Functions of skeletal muscles are:
- movement of the body in space;
- movement of body parts relative to each other, including the implementation of respiratory movements that provide ventilation of the lungs;
- maintaining body position and posture.
Skeletal muscles, together with the skeleton, make up the musculoskeletal system of the body, which ensures the maintenance of posture and movement of the body in space. Along with this, skeletal muscles and the skeleton perform a protective function, protecting internal organs from damage.
In addition, striated muscles are important in the production of heat, which maintains temperature homeostasis, and in the storage of certain nutrients.
Rice. 2. Functions of skeletal muscles
Physiological properties of skeletal muscles
Skeletal muscles have the following physiological properties.
Excitability. It is ensured by the property of the plasma membrane (sarcolemma) to respond with excitation to the arrival of a nerve impulse. Due to the greater difference in the resting potential of the membrane of striated muscle fibers (E 0 about 90 mV), their excitability is lower than that of nerve fibers (E 0 about 70 mV). Their action potential amplitude is greater (about 120 mV) than that of other excitable cells.
This makes it possible in practice to quite easily record the bioelectrical activity of skeletal mice. The duration of the action potential is 3-5 ms, which determines the short duration of the absolute refractoriness phase of the excited muscle fiber membrane.
Conductivity. It is ensured by the property of the plasma membrane to form local circular currents, generate and conduct action potentials. As a result, the action potential propagates along the membrane along the muscle fiber and inward along the transverse tubes formed by the membrane. The speed of action potential is 3-5 m/s.
Contractility. It is a specific property of muscle fibers to change their length and tension following the excitation of the membrane. Contractility is provided by specialized contractile proteins of the muscle fiber.
Skeletal muscles also have viscoelastic properties that are important for muscle relaxation.
Rice. Human skeletal muscles
Physical properties of skeletal muscles
Skeletal muscles are characterized by extensibility, elasticity, strength and the ability to perform work.
Extensibility - the ability of a muscle to change length under the influence of a tensile force.
Elasticity - the ability of a muscle to restore its original shape after the cessation of tensile or deforming force.
- the ability of a muscle to lift a load. To compare the strength of different muscles, their specific strength is determined by dividing the maximum mass by the number of square centimeters of its physiological cross-section. Skeletal muscle strength depends on many factors. For example, on the number of motor units excited at a given time. It also depends on the synchronicity of the motor units. The strength of the muscle also depends on the initial length. There is a certain average length at which a muscle develops maximum contraction.
The strength of smooth muscles also depends on the initial length, the synchronicity of excitation of the muscle complex, as well as on the concentration of calcium ions inside the cell.
Muscle ability do work. Muscle work is determined by the product of the mass of the lifted load and the height of the lift.
Muscle work increases by increasing the mass of the load being lifted, but up to a certain limit, after which an increase in load leads to a decrease in work, i.e. the lift height decreases. Maximum work is performed by the muscle at medium loads. This is called the law of average loads. The amount of muscle work depends on the number of muscle fibers. The thicker the muscle, the more load it can lift. Prolonged muscle tension leads to fatigue. This is due to the depletion of energy reserves in the muscle (ATP, glycogen, glucose), the accumulation of lactic acid and other metabolites.
Auxiliary properties of skeletal muscles
Extensibility is the ability of a muscle to change its length under the influence of a tensile force. Elasticity is the ability of a muscle to return to its original length after the cessation of the tensile or deforming force. Living muscle has small but perfect elasticity: even a small force can cause a relatively large lengthening of the muscle, and its return to its original size is complete. This property is very important for the normal functions of skeletal muscles.
The strength of a muscle is determined by the maximum load that the muscle is able to lift. To compare the strength of different muscles, their specific strength is determined, i.e. the maximum load that a muscle is able to lift is divided by the number of square centimeters of its physiological cross-section.
The ability of a muscle to do work. The work of a muscle is determined by the product of the magnitude of the lifted load and the height of the lift. The work of the muscle gradually increases with increasing load, but up to a certain limit, after which an increase in load leads to a decrease in work, since the height of lifting the load decreases. Consequently, maximum muscle work is performed at average loads.
Muscle fatigue. Muscles cannot work continuously. Long-term work leads to a decrease in their performance. A temporary decrease in muscle performance that occurs during prolonged work and disappears after rest is called muscle fatigue. It is customary to distinguish between two types of muscle fatigue: false and true. With false fatigue, it is not the muscle that becomes tired, but a special mechanism for transmitting impulses from nerve to muscle, called a synapse. The reserves of mediators in the synapse are depleted. With true fatigue, the following processes occur in the muscle: accumulation of under-oxidized breakdown products of nutrients due to insufficient oxygen supply, depletion of energy sources necessary for muscle contraction. Fatigue is manifested by a decrease in the force of muscle contraction and the degree of muscle relaxation. If the muscle stops working for a while and is at rest, then the work of the synapse is restored, and metabolic products are removed with the blood and nutrients are delivered. Thus, the muscle regains the ability to contract and produce work.
Single cut
Stimulation of a muscle or the motor nerve innervating it with a single stimulus causes a single contraction of the muscle. There are three main phases of such a contraction: the latent phase, the shortening phase and the relaxation phase.
The amplitude of a single contraction of an isolated muscle fiber does not depend on the strength of stimulation, i.e. obeys the “all or nothing” law. However, the contraction of an entire muscle, consisting of many fibers, when directly stimulated depends on the strength of the stimulation. At threshold current, only a small number of fibers are involved in the reaction, so the muscle contraction is barely noticeable. With increasing strength of irritation, the number of fibers covered by excitation increases; the contraction increases until all fibers are contracted (“maximal contraction”)—this effect is called Bowditch’s ladder. Further intensification of the irritating current does not affect muscle contraction.
Rice. 3. Single muscle contraction: A - moment of muscle irritation; a-6 - latent period; 6-в - reduction (shortening); v-g - relaxation; d-d - successive elastic vibrations.
Tetanus muscle
Under natural conditions, the skeletal muscle receives from the central nervous system not single excitation impulses, which serve as adequate stimuli for it, but a series of impulses, to which the muscle responds with a prolonged contraction. Prolonged muscle contraction that occurs in response to rhythmic stimulation is called tetanic contraction, or tetanus. There are two types of tetanus: serrated and smooth (Fig. 4).
Smooth tetanus occurs when each subsequent excitation impulse enters the shortening phase, and toothed - into the relaxation phase.
The amplitude of the tetanic contraction exceeds the amplitude of a single contraction. Academician N.E. Vvedensky substantiated the variability of the tetanus amplitude by the unequal value of muscle excitability and introduced the concepts of optimum and pessimum of stimulation frequency into physiology.
Optimal This is the frequency of stimulation at which each subsequent stimulation enters the phase of increased excitability of the muscle. In this case, tetanus of maximum magnitude (optimal) develops.
Pessimal This is the frequency of stimulation at which each subsequent stimulation occurs in a phase of reduced excitability of the muscle. The magnitude of tetanus will be minimal (pessimal).
Rice. 4. Contraction of skeletal muscle at different frequencies of stimulation: I - muscle contraction; II — mark of irritation frequency; a - single contractions; b- serrated tetanus; c - smooth tetanus
Muscle contraction modes
Skeletal muscles are characterized by isotonic, isometric and mixed modes of contraction.
At isotonic When a muscle contracts, its length changes, but the tension remains constant. This contraction occurs when the muscle does not overcome resistance (for example, does not move a load). Under natural conditions, contractions of the tongue muscles are close to the isotonic type.
At isometric contraction in the muscle during its activity, tension increases, but due to the fact that both ends of the muscle are fixed (for example, the muscle is trying to lift a large load), it does not shorten. The length of the muscle fibers remains constant, only the degree of their tension changes.
They are reduced by similar mechanisms.
In the body, muscle contractions are never purely isotonic or isometric. They always have a mixed character, i.e. There is a simultaneous change in both the length and tension of the muscle. This reduction mode is called auxotonic, if muscle tension predominates, or auxometric, if shortening predominates.
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