Download Lecture 1_General physiology of excitative tissue. Physiology of

General physiology of
excitative tissue.
Physiology of muscles
and nerves. Features of
functioning of muscles
cranial facial area.
DETERMINATION OF “PHYSIOLOGY”
NOTION. PHYSIOLOGICAL
SUBJECTS
Physiology is the science about the regularities of organisms‘
vital activity in connection with the external environment
PHYSIOLOGICAL SUBJECTS
1. Aged physiology
2. Clinical physiology.
3. Physiology of labor.
4. Psychophysiology.
5. Ecological physiology.
6. Physiology of sport.
7. Space physiology.
8. Pathologic physiology.
Methods of physiology
• a) Observation (This is the method in which the scientists
don‘t mix in course of vital processes. They only make use
of vision and description of all changes. On the base of this
changes they make conclusions.)
• b) Experiment (There are two kinds of experiments: acute
and chronic. Acute experiment was doing with the helps of
anesthesia. It may be accompanied by cut off the nerves,
introduction the different substances. The chronic
experiment was doing in vital animals, for example, after
the acute experiment scientists can used the observation.)
• c) Examination (This is the method of examine the patient
with different diseases, for example, with using the
different apparatuses.)
• d) Modeling
Rest membrane potential
There is a potential difference across the membranes of most if not all cells,
with the inside of the cells negative to the exterior. By convention, this
resting membrane potential (steady potential) is written with a minus
sign, signifying that the inside is negative relative to the exterior. Its
magnitude vanes considerably from tissue to tissue, ranging from -9 to –
100 mV. When 2 electrodes are connected through a suitable amplifier to a
CRO and placed on the surface of a single axon, no potential difference is
observed. However, if one electrode is inserted into the interior of the cell,
a constant potential difference is observed, with the inside negative
relative to the outside of the cell at rest. This resting membrane potential
is found in almost all cells. In neurons, it is usually about –70 mV. .
voltmeter
I-electrods
cell
cell
Active transport of ions
• There are two kind of ion’s transport: active and passive.
Active transport is doing due to the energy of ATP. The
sodium-potassium pump responsible for the coupled active
transport of Na+ out of cells and K+ into cells is a unique
protein in the cell membrane. This protein is also an
adenosine triphosphatase, ie, an enzyme that catalyzes the
hydrolysis of ATP to adenosine diphosphate (ADP), and it is
activated by Na+ and K+. Consequently, it is known as
sodium-potassium-activated adenosine triphosphatase
(Na+-K+ ATPase). The ATP provides the energy for
transport. The pump extrudes three Na+ from the cell for
each two K+ it takes into the cell, ie, it has a coupling ratio
of 3/2.
•The origin of excitation
•a) Characteristic of experimental
stimulus (According to the force
its divided on the under threshold,
threshold and upper threshold.)
•b) Characteristic of experimental
stimulus (According to the nature
its divided on chemical,
mechanical, temperature,
electrical)
Local
answer,
critical
level
of
depolarization Local answer is arised
only on under threshold stimulus.
Critical level of depolarization is the
point
from
which
the
action
membrane potential can developed.
ACTION POTENTIAL
1 – rest membrane potential; 2 – local response; 3 –
Critical level of depolarization; 4 – depolarization; 5 –
repolarization; 6 – negative step potential; 7 – positive step
potential
mV
Outer
Membrane
Inner
Active potential (А) and excitability (В)
Depolarization
Repolarization
Negative step potential
Positive step potential
Latent addition
Absolute refractivity
3B - Relative refractivity
4B - Exaltation
5B - Supernormal period
• a) Condition of carrying (1. Anatomic
integrity of nerve‘s filament. 2.
Physiological full value.)
• b) Laws of carrying (1. Double-sided
conduction. 2. Isolated of conducting. 3.
Conducting of excitation without
attenuation.)
• c) Carrying in myelinated nerves (In
myelin filaments conducting of excitation
is doing from node of Ranvier to node of
Ranvier.)
• d) Carrying in nonmyelinated nerves (In
nonmyelin filaments conducting of
excitation is doing uninterrupted.)
Common characteristic of chemical
synapses
• Chemical synapses is the junctions in which
the transmission of information do through
the direct passage with chemical
substances from cell to cell. These
substances named mediators.
• Classification of chemical synapses
• These synapses named for the type of
mediator – cholinergic (mediator –
acetylcholine), adrenergic (mediator –
epinephrine, nor epinephrine), serotonin
(mediator – serotonin), dopaminergic
(mediator – dopamine), GABA-ergic
(mediator – gamma-amino butyric acid).
synaptic activity
• Active membrane potential go along the
nerve to presynaptic end – presynaptic
membrane have depolarilazed – the
Ca2+-cannals activated – Ca2+-go to the
presynaptic
end
–
Ca2+-activated
transport of vesiccles with the mediator
along the neurofilaments to presynaptic
membrane – the mediator pick out from
presynaptic ends to the synaptic split –
molecules of mediator diffuse through the
synaptic split to postsynaptic membrane –
molecules of mediator interact with the
receptors on the postsynaptic membrane
– this interaction lead to the conformation
of
receptors
and
activation
of
corresponding substances.
Common characteristic of electrical
synapses
• Electrical synapses is the junctions in which the
transmission of information do through the direct
passage of bioelectrical signal from cell to cell.
This synapses has small synaptic split (to 5 nm),
low specific resistance between the presynaptic
and postsynaptic membranes. There are the
transverse canals in both membranes with the
diameter of 1 nm.
• a) Excitatory transmitter (Excitatory impulses go
to the synapse and increase permeability of
postsynaptic cell membrane to Na+.)
• b) Inhibitory transmitter (Inhibitory impulses go
to the synapse and increase permeability of
postsynaptic cell membrane to Cl-, not to Na+.)
Electromyography
Activation of motor units can be studied by
electromyography, the process of recording
the electrical activity of muscle on a cathoderay oscilloscope. This may be done in humans
by using small metal disks on the skin
overlying the muscle as the pick-up electrodes
or in un anesthetized humans or animals by
using hypodermic needle electrodes. The
record obtained with such electrodes is the
electromyogramm
(EMG).
With
needle
electrodes, it is usually possible to pick up the
activity of single muscle fibers.
Characteristic of skeletal muscles
• Resting and active potentials of muscle fiber (The electrical events
in skeletal muscle and the ionic fluxes underlying them are similar
to those in nerve, although there are quantitative differences in
timing and magnitude. The resting membrane potential of skeletal
muscle is about -90 mV. The action potential lasts 2-4 ms and is
conducted along the muscle fiber at about 5 m/s. The absolute
refractory period is 1-3 ms long and the after-polarizations, with
their related changes in threshold to electrical stimulation, are
relatively prolonged. The chronaxie of skeletal muscle is generally
somewhat longer than that of nerve.
• Although the electrical properties of the individual fibers in a
muscle do not differ sufficiently to produce anything resembling a
compound action potential, there are slight differences in the
thresholds of the various fibers. Furthermore, in any stimulation
experiment, some fibers are farther from the stimulating
electrodes than others. Therefore, the size of the action potential
recorded from a whole muscle preparation is proportionate to the
intensity of the stimulating current between threshold and
maximal current intensities. The distribution of ions across the
muscle fiber membrane is similar to that across the nerve cell
membrane. As in nerve, depolarization is a manifestation of Na+
influx, and repolarization is a manifestation of K+ efflux.)
Solitary contraction
•
•
•
The process by which the shortening of the contractile elements in muscle
is brought about is a sliding of the thin filaments over the thick filaments.
The width of the A bands is constant, whereas the Z lines move closer
together when the muscle contracts and farther apart when it is stretched.
As the muscle shortens, the thin filaments from the opposite ends of the
sarcomere approach each other; when the shortening is marked, these
filaments apparently overlap.
The sliding during muscle contraction is produced by breaking and reforming of the crosslinkages between actin and myosin. The heads of the
myosin molecules link to actin at an angle, produce movement of myosin
on actin by swiveling, and then disconnect and reconnect at the next
linking site, repeating the process in serial fashion. Each single cycle of
attaching, swiveling, and detaching shortens the muscle 1 %.
The immediate source of energy for muscle contraction is ATP. Hydrolysis
of the bonds between the phosphate residues of this compound is
associated with the release of a large amount of energy, and the bonds
are therefore referred to as high-energy phosphate bonds. In muscle, the
hydrolysis of ATP to adenosine diphosphate (ADP) is catalyzed by the
contractile protein myosin; this adenosine triphosphatase (ATPase)
activity is found in the heads of the myosin molecules, where they are in
contact with actin.
•
•
The process by which depolarization of the muscle fiber initiates
contraction is called excitation-contraction coupling. The action
potential is transmitted to all the fibrils in the fiber via the T system. It
triggers the release of Ca2+ from the terminal cisterns, the lateral sacs
of the sarcoplasmic reticulum next to the T system. The Ca2+ initiates
contraction. Ca2+ initiates contraction by binding to troponin C. In resting
muscle, troponin I is tightly bound to actin, and tropomyosin covers the
sites where myosin heads bind to actin. Thus, the troponin-tropomyosin
complex constitutes a “relaxing-protein” that inhibits the interaction
between actin and myosin. When the Ca2+ released by the action
potential binds to troponin C, the binding of troponin I to actin is
presumably weakened, and this permits the tropomyosin to move laterally.
This movement uncovers binding sites for the myosin heads, so that ATP is
split and contraction occurs.
Shortly after releasing Ca2+, the sarcoplasmic reticulum begins to
reaccumulate Ca2+. The Ca2+ is actively pumped into longitudinal
portions of the reticulum and diffuses from there to the cisterns, where it
is stored. Once the Ca2+ concentration outside the reticulum has been
lowered sufficiently, chemical interaction between myosin and actin ceases
and the muscle relaxes. If the active transport of Ca2+ is inhibited,
relaxation does not occur even though there are no more action
potentials; the resulting sustained contraction is called a contracture. It
should be noted that ATP provides the energy for the active transport of
Ca2+ into the sarcoplasmic reticulum. Thus, both contraction and
relaxation of muscle require ATP.
Connection between excitation and contraction
• It is important to distinguish between the electrical
and mechanical events in muscle. Although
oneresponse does not normally occur without the
other, their physiologic basis and characteristics are
different. Muscle fiber membrane depolarization
normally starts at the motor end-plate, the specialized
structure under the motor nerve ending.
• A single action potential causes a brief contraction
followed by relaxation. This response is called a
muscle twitch; the action potential and the twitch
are plotted on the same time scale. The twitch starts
about 2 ms after the start of depolarization of the
membrane, before repolanzation is complete. The
duration of the twitch varies with the type of muscle
being tested. “Fast” muscle fibers, primarily those
concerned with fine, rapid, precise movement, have
twitch durations as short as 7.5 ms. “Slow” muscle
fibers, principally those involved in strong, gross,
sustained movements, have twitch durations up to
100ms.
Summation of contraction and tetanus of muscles
•
•
The electrical response of a muscle fiber to repeated stimulation is like
that of nerve. The fiber is electrically refractory only during the rising and
part of the falling phase of the spike potential. At this time, the contraction
initiated by the first stimulus is just beginning. However, because the
contractile mechanism does not have a refractory period, repeated
stimulation before relaxation has occurred produces additional activation
of the contractile elements and a response that is added to the contraction
already present. This phenomenon is known as summation of
contractions. The tension developed during summation is considerably
greater than that during the single muscle twitch. With rapidly repeated
stimulation, activation of the contractile mechanism occurs repeatedly
before any relaxation has occurred, and the individual responses fuse into
one continuous contraction. Such a response is called a tetanus (tetanic
contraction). It is a complete tetanus when there is no relaxation
between stimuli, and an incomplete tetanus when there are periods of
incomplete relaxation between the summated stimuli. During a complete
tetanus, the tension developed is about 4 times that developed by the
individual twitch contractions.
The stimulation frequency at which summation of contractions occurs is
determined by the twitch duration of the particular muscle being studied.
For example, if the twitch duration is 10 ms, frequencies less than 1/10
ms (100/s) cause discrete responses interrupted by complete relaxation,
and frequencies greater than 100/s cause summation.
Peculiarities of smooth muscles
•
•
•
•
Resting membrane potential may be from –50 mV to –60 mV. In this
process take place K+, Na+, Cl-. There are a large concentration of Na+,
Cl- in the cells.
Active potential (Prolongation of it may be from 20-50 ms to 1 second;
amplitude is less that in skeletal muscles. Active potential end by afterhyperpolarization. The main role in the beginning of it have Ca+.)
Elasticity, plasticity and tensility (Another special characteristic of smooth
muscle is the variability of the tension it exerts at any given length. If a
piece of visceral smooth muscle is stretched, it first exerts increased
tension. However, if the muscle is held at the greater length after
stretching, the tension gradually decreases. Sometimes the tension falls to
or below the level exerted before the muscle was stretched. It is
consequently impossible to correlate length and developed tension
accurately, and no resting length can be assigned. In some ways,
therefore, smooth muscle behaves more like a viscous mass than a rigidly
structured tissue, and it is this property that is referred to as the
plasticity of smooth muscle.
The consequences of plasticity can be demonstrated in the intact animal.
For example, the tension exerted by the smooth muscle walls of the
bladder can be measured at varying degrees of distention. After each
addition of fluid, the tension was measured for a period of time.
Immediately after each increment of fluid, the tension was higher; but
after a short period of time, it decreased.)
Characteristic of cardiac muscle
•
•
Resting membrane and action potential of cardiac muscle cells (The
resting membrane potential of individual mammalian cardiac muscle cells
is about -80 mV (interior negative to exterior). Stimulation produces a
propagated action potential that is responsible for initiating contraction.
Depolarization proceeds rapidly and an overshoot is present, as in skeletal
muscle and nerve, but this is followed by a plateau before the membrane
potential returns to the baseline. In mammalian hearts, depolarization
lasts about 2 ms, but the plateau phase and repolarization last 200 ms or
more. Repolarization is therefore not complete until the contraction is half
over.
As in other excitable tissues, changes in the external K+ concentration
affect the resting membrane potential of cardiac muscle, whereas changes
in the external Na+ concentration affect the magnitude of the action
potential. The initial rapid depolarization and the overshoot are due to a
rapid increase in Na+ permeability similar to that occurring in nerve and
skeletal muscle, whereas the second plateau phase is due to a slower
starting, less intense, and more prolonged increase in Ca2+ permeability.
The third phase is the manifestation of a delayed increase in K+
permeability. This increase produces the K+ efflux that completes the
repolarization process. The Na+ channel in cardiac muscle is often called
the fast channel. It probably has 2 gates, an outer gate that opens at the
start of depolarization, at a membrane potential of -60 to -70 mV, and a
second inner gate that then closes and precludes further influx until the
action potential is over (Na+ channel inactivation). The Ca2+ channel is
called the slow channel. It is activated at a membrane potential of -30 to 40 mV and inactivates much more slowly than the fast channel.
Mechanic properties
• The contractile response of cardiac muscle begins just after
the start of depotanzation and lasts about 1,5 times as long
as the action potential. The role of Ca2+ in excitationcontraction coupling is similar to its role in skeletal muscle,
except that Ca2+ entering from the ECF as well as Ca2+
from the sarcoplasmic reticulum contributes to contraction.
Responses of the muscle are all or none in character, ie, the
muscle fibers contract fully if they respond at all. Since
cardiac muscle is absolutely refractory during most of the
action potential, the contractile response is more than half
over by the time a second response can be initiated.
Therefore, tetanus of the type seen in skeletal muscle
cannot occur. Of course, tetanizalion of cardiac muscle for
any length of time would have lethal consequences, and in
this sense the fact that cardiac muscle cannot be tetanized
is a safety feature. Ventricular muscle is said to be in the
“vulnerable period” just at the end of the action potential,
because stimulation at this time will sometimes initiate
ventricular fibrillation.
Electromyography
Activation of motor units can be studied by
electromyography, the process of recording
the electrical activity of muscle on a cathoderay oscilloscope. This may be done in humans
by using small metal disks on the skin
overlying the muscle as the pick-up electrodes
or in un anesthetized humans or animals by
using hypodermic needle electrodes. The
record obtained with such electrodes is the
electromyogramm
(EMG).
With
needle
electrodes, it is usually possible to pick up the
activity of single muscle fibers.
Types of Contraction
• Muscular contraction involves shortening
of the contractile elements, but because
muscles have elastic and viscous elements
in series with the contractile mechanism,
it is possible for contraction to occur
without an appreciable decrease in the
length of the whole muscle. Such a
contraction is called isometric (“same
measure” or length). Contraction against a
constant load, with approximation of the
ends of the muscle, is isotonic (“same
tension”).
Summation of contraction
and tetanus of muscles
Active potential of cardiomyocytes
Phase 0 –depolarization;
Phase 1 – rapid initial repolarization;
Phase 2 – plateau;
Phase 3 – rapid ending repolarization;
Phase 4 – rest.