Physiology of Excitable tissue

1. Physiology of Excitable
Tissues

2. Basic Physiological Properties of
Tissues. Concept of Excitable Tissues
 A property is understood as a permanent
characteristics of an object.
 Physiological properties include irritability,
excitability, conductibility, lability,
contractility and the ability to secretion.

3. Irritability
 is an ability of a tissue to change
metabolism under stimulation. The
property of irritability is what
distinguishes objects of organic
nature from those of inorganic nature

4. Excitability
 The property of excitability is inherent
only to three kinds of body tissues:
muscular, nervous and glandular tissues
 Excitability is understood as an ability of
excitable tissues to respond to stimuli by
development of excitation in the form of a
specific response and a bioelectrical
process. Excitability is characterized by
two parameters: threshold of
excitability and latent period of
excitation.

5. Threshold of excitability
 is the minimum force of stimulus
required to produce excitation in an
excitable structure (cell or tissue)
 The lower the threshold of
excitability, the higher the
excitability,
 that is, threshold of excitability
and excitability are inversely
proportional!

6. Latent period of excitation
 is a time interval from the moment
of application of a stimulus to
manifestation of the first signs of
excitation.
 The shorter the latent period, the
higher the excitability!

7. Variation in excitability at excitation
 1. period of latent
addition
 2. absolute refractory
phase characterized by
absolute
irresponsiveness of
tissue (due to complete
absence of excitability)
 3. relative refractory
phase
 4. supranormal period
 5. subnormal period

8.  In the period of latent addition the
excitability of excitable tissue rises above
the initial level which means decrease in
the excitability threshold.
 During the absolute refractory phase
excitability drops to zero. This means that
in this period an excitable tissue is
incapable to respond to any stimulus
however strong it is.

9.  In the relative refractory phase
excitability starts to gradually
increase, but reaches the initial
level only at the final stage of
excitation. In this period the tissue
will respond to a stimulus only if its
force exceeds the excitability
threshold

10.  In the supranormal period
excitability again rises above the
normal level which is reflected by
reduction in the excitability
threshold.
 And in the subnormal period
excitability slightly decreases with a
corresponding increase in the
excitability threshold.

11. Conductibility
 is an ability of tissues to propagate
excitation
 The property of conductibility is
most expressed in nervous tissue,
less expressed – in muscle tissue,
and is lowest in glandular tissue.
 Conductibility is measured in m/sec.

12. Conductibility
 Conductibility of skeletal muscle
tissue is from 3 to 5 m/s;
 of smooth muscle tissue – from
0.02 to 0.1 m/s and
 of nervous tissue – from 0.5 to 120
m/s depending on the type of nerve
fibers innervating these tissues

13. Lability of Tissues
 Lability (from Latin “labilis”
meaning unstable) is a property
that determines functional mobility
of excitable tissue.
 This term was introduced into
science by a notable Russian
physiologist N. Wedensky.

14. Measure of Lability
 According to Wedensky, a measure
of lability is the largest possible
number of elementary excitation
cycles which can be reproduced by
an excitable structure per unit time
in accordance with the frequency of
stimulation.

15. States of Cells and Tissues
 Relative physiological rest is the minimal
level of vital activity of tissue in the absence
of stimuli
 Active state is determined by different
relationships between two basic physiological
processes: excitation and inhibition
 Fatigue is a temporary reduction in the
working capacity of tissue that results from its
prolonged or intensive activity, and is
associated with depletion of plastic and
energetic resources and with accumulation of
metabolites in tissue

16. “Animal Electricity”
 Discovery of “animal
electricity” in frogs was the
result of the so-called
“balcony experiment”
conducted by Galvani in
1786 when he studied
effects of electric discharges
of different nature on nerve-
muscle preparations and
preparations of hind legs of a
frog.

17. “Balcony Experiment”
 when a preparation of hind
legs of a frog hanging on
copper hooks, occasionally
came into contact with
iron balcony rails, muscles
contracted
 A. Volta was interpreted
this phenomenon as a
result of initiation of direct
current in a circuit
between different metals

18.  in the second
experiment without any
contacts between
different kinds of
metals;
 and in the experiment
of Matteuci with two
nerve-muscle
preparations, one of
which being excited by
electric current served
as a source of excitation
for the other

19. “Action Current”
 German physiologist Emil Du
Bois-Reymond in 1841
demonstrated initiation of
electric current in electric circuit
between a damaged and
undamaged portions of a
muscle; this current was called
by him “resting current”. At
excitation of a muscle the
magnitude of the resting
current decreased. He called
this variation in the resting
current “action current”

20. Resting Membrane Potential
 Modern electrophysiological
equipment permits to record
potential difference in a resting
excitable cell (3).
 For an experiment there are
needed: microelectrodes 0.5 μm
in diameter (made of metal or
glass), DC amplifier (2) and
oscilloscope (1).
 One microelectrode is inserted
into the cell (4), and the other
is placed on its surface (5).

21. Resting Membrane Potential
 Potential difference between the
surface of the cell membrane and
its protoplasm is displayed on the
oscilloscope screen as a deflection
of the beam from the isoelectric
line.
 The more the beam is deflected, the
greater the potential difference.

22. Membrane Theory
 According to Bernstein, the main factors in
formation of potential difference in a
resting cell were semi-permeability of the
membrane and asymmetry of ions
 In general, J. Bernstein’s theory was in
agreement with the mechanism of
electrogenesis, but in some cases it failed
to provide adequate explanation of the
origin of the overshoot phenomenon (a
portion of a high-voltage peak), neither
could it explain the origin of initial ionic
gradients.

23. Modern Membrane-Ionic Theory
 Special energy
consuming
mechanisms (“a
potassium-sodium
pump”) create ionic
asymmetry in excitable
cells: Na+ ions
accumulate among
cells, and K+ ions
accumulate inside
cells;

24. Modern Membrane-Ionic Theory
 According to the theory of
electrolytic dissociation of
Arrhenius, cations in the protoplasm
and the extracellular fluid are in the
electrostatic interaction with their
complementary anions;

25. Modern Membrane-Ionic Theory
 A cell membrane plays the leading
role in formation of resting
membrane potential: in a resting
state the membrane is selectively
permeable to K+ ions and less
permeable to Cl- ions.

26. Modern Membrane-Ionic Theory
 Due to the above conditions, K+ ions flow
out of the cell carrying anions with which
they are in the electrostatic interaction.
However, the anions cannot flow out of
the cell, since, being large in mass, they
cannot pass through the cell membrane.
 Thus, the cell membrane functions as a
sort of a filter, on one side of which K+
ions accumulate forming positive charge
on the cell surface, and on the other side
anions accumulate forming negative
charge in the protoplasm.

27. Variation of Membrane Potential at
Excitation
 Activation of a cell with a stimulus
induces excitation in it. Depending on the
force of the stimulus, excitation may be
local or propagating. A local response is
one of manifestations of local excitation.
A local response is a variation in the
membrane potential of a cell induced by a
subthreshold stimulus.
 If a cell is stimulated by a threshold or
suprathreshold stimulus, it fires an action
potential

28. Action Potential
 Action potential is a short-term
variation in the membrane
potential of an excitable cell
induced by application of a
threshold or suprathreshold
stimulus.
 Action potential includes the
following components :
 resting membrane potential –
1; local response (LR) – 2;
high-voltage peak (HVP) – 3+4;
afterpotential processes:
afterpotential depolarization –
5; afterpotential
hyperpolarization – 6.

29. Critical depolarization level (CDL)
 is the level of
membrane potential of
a cell at which it fires
action potential. A
process of decrease in
the initial potential
difference is termed
“depolarization”, and a
process of the
recovery of the initial
potential difference is
termed
“repolarization”

30. Comparative Characteristics of
Local Response and Action Potential
Characteristics Local Response Action Potential
 1. Response to stimulus induced by induced by application
application of subthreshold stimulus of threshold or
suprathreshold
stimulus
 2. Realization “law of force” “all-or-none” law
 3. Ability to summation + -
 4. Excitability increases decreases
 5. Conductibility increases decreases
 6. Propagation with decay with no decay
 7. Permeability to Na+ ions increases at first increases,
then decreases
 8. Permeability to K+ ions does not change increases during
repolarization

31. Correlation between variation of
membrane potential at excitation
and variation of excitability of a cell

32. Selective Membrane Channels
 It has been shown recently that in
membranes of excitable cells there
exist specific (selective) sodium,
potassium, chlorine and calcium
channels, which selectively pass
only Na+, K+, Cl-, Ca++ ions.
These channels work by the gate
mechanisms (active and inactive)
and are potential-dependent

33. Structure of selective membrane
channels
 A – relative resting state; B – activation;
 C – inactivation

34. Thank you for attention