2. What is an excitable tissue?
âą Tissues which are capable of
generation and transmission of
electrochemical impulses along the
membrane.
âą An ability of specialized cells to
respond to certain stimuli by
producing electrical signals.
3. Excitable tissues in the
human body
ï” Nerves
ï” Muscles
Cardiac Muscle
Smooth Muscle
Skeletal Muscle
4. Understand excitable tissue
ï” Suppose you have a dead frog. (Yes, that's kind of
gross, but let's just imagine it for a second.)
ï” What would happen if you applied an electrical
stimulus to the nerve in the frog's leg? Creepily
enough, the dead leg would kick!
ï” The Italian scientist Luigi Galvani discovered this fun
fact back in the 1700s, somewhat by accident during
a frog dissection.
ï” Today, we know that the frog's leg kicks
because neurons (nerve cells) carry information via
electrical signals.
5. Resting Membrane Potential
âą A potential difference exists across all cell
membranes
âą This is called
â Resting Membrane Potential (RMP)
6. Resting membrane potential
explained
ï” Imagine taking two electrodes
and placing one on the outside
and the other on the inside of the
plasma membrane of a living cell.
ï” If you did this, you would
measure an electrical potential
difference, or voltage, between
the electrodes. This electrical
potential difference is called
the membrane potential.
7. Resting Membrane potential
ï” For a cellâs membrane potential, the reference point
is the outside of the cell. In most resting neurons, the
potential difference across the membrane is
about 70 to 90 mV (1 mV is 1/1000 of avolt) with the
inside of the cell more negative than the outside.
ï” That is, neurons have a resting membrane
potential (or simply, resting potential) of about -
70 to -90 mV.
8. Resting Membrane Potential
ï” Because there is a potential difference across the cell
membrane, the membrane is said to be polarized.
ï” If the membrane potential becomes more positive than it is at
the resting potential, the membrane is said to be depolarized.
ï” If the membrane potential becomes more negative than it is at
the resting potential, the membrane is said to
be hyperpolarized.
9. Where does the resting membrane
potential come from?
ï” The resting membrane potential is determined
by the
ï¶ uneven distribution of ions (charged particles)
between the inside and the outside of the cell, and
ï¶ by the different permeability of the membrane to
different types of ions.
10. Distribution of ions inside
and outside the cell
K+ and organic anions (such as those found in proteins
and amino acids) are present at higher concentrations
inside the cell than outside. In contrast, Na+
plus, Clâ
usually present at higher concentrations outside the cell.
11. How ions move across the cell
membrane
ï” Because they are charged, ions can't pass
directly through the hydrophobic ("water-
fearing") lipid regions of the membrane.
Instead, they have to use specialized channel
proteins that provide a hydrophilic ("water-
loving") tunnel across the membrane.
ï” Some channels, known as leak channels, are
open in resting neurons. Others are closed in
resting neurons and only open in response to
a signal.
12. âą Ion channels that mainly allow K+
to pass are
called potassium channels, and ion channels that mainly
allow Na+ to pass are called sodium channels.
âą The resting membrane potential depends mainly on movement
of K+ through potassium leak channels. Howevr both Na+ and K+
contribute to resting potential.
13. âą Potassium concentration intracellular is
more
âą Membrane is freely permeable to K+
âą There is an efflux of K+
Flow of Potassium
K+ K+
K+
KK
+
+
K+
K
+
K
+
K+
K+
14. Entry of positive ions in to the extracellular fluid
creates positivity outside and negativity inside
Flow of Potassium
K+ K+
K+
KK
+
+
K+ K+ K+
K+
K+
15. âą Outside positivity resists efflux of K+
âą (since K+ is a positive ion)
âą At a certain voltage an equilibrium is reached
and K+ efflux stops
Flow of Potassium
K+ K+
K+
KK
+
+
K+ K+ K+
K+
K+
16. Nernst potential (Equilibr ium potential)
The potential across the cell membrane at which the
net diffusion of ions across the cell membrane due to
concentration gradient stops.
âą Nernst equation determines this potential
Where R= Universal Gas constt
T = Absolute Temp,
z = ion Valence
F = Faraday, an electrical Const
17. ï”Nernst potential for K+ ions
Nernst Equation:
EMF = (RT/zF) x log (Cin / Cout)
RT/zF = -61
Conc of K+ ions inside the cell=140 mEq/l
Conc of K+ ions outside the cell= 4 mEq/l
EMF(mv)= - 61 log 140/4
= -61 log 35
= - 94mv
18. ï”Nernst potential for Na+ ions
Nernst Equation:
EMF = (RT/zF) x log (Cin / Cout)
RT/zF = -61
Conc of K+ ions inside the cell=14 mEq/l
Conc of K+ ions outside the cell= 142 mEq/l
EMF(mv)= - 61 log 14/142
= +61 mv
19. The Goldman Equation
âą When the membrane is permeable to several
ions the equilibrium potential that develops
depends on
â Polarity of each ion
â Membrane permeability
â Ionic concentration
âą This is calculated using Goldman Equation
(or GHK Equation)
20. ï” In the normal nerve fiber, the permeability of the
membrane to potassium is about 100 times as great
as its permeability to sodium.
ï” Goldman equation gives a potential inside the
membrane of â86 millivolts, which is near the
potassium potential
21. Contribution of Na/K PUMP:-
- This is a powerful electrogenic pump on the cell
membrane.
- It Pump 3 Na to outside & 2 K to inside, causing â
loss of +ve ions ,loss of + ve charge from inside ,
negativity about - 4mV inside
-4mv
22. ï” Nernst potential for Potassium -94mv
ï” Nernst potential for Sodium +61mv
ï” Putting these values in Gold man equation, gives a value of -
86mv
Which is nearer to K+ diffusing potential
ï” Na- K pump provides - 4mv
ï” i.e adding -86 and -4mv= -90mv
ï” Resting membrane potential in nerves is -90 mv
23. Resting Membrane Potential in
Various Excitable Tissues
ï” Large Myelinated Nerve fibers
ï” Skeletal Muscle Fibers = - 90mv
ï” Ventricular Muscle fibers
ï” Smooth Muscle fiber & } = -55 to -60 mv
ï” Self Excitatory Tissues
24. Action potential
ï” Definition:
ï” Abrupt / sudden Change (reversal) in resting
membrane potential in response to a threshold
stimulus.
ï” Stimulus:
âAny Change in the environmentâ
ï” TYPES: a. Electrical
b. Mechanical
c. Chemical
25. Action Potential (A.P.)
âą When an impulse is generated
â Inside becomes positive
â Causes depolarisation
â Nerve impulses are transmitted as AP
27. Inside of the membrane is
âą Negative
â During RMP
âą Positive
â When an AP is generated
-90
+30
28. âą Initially membrane is slowly depolarised
âą Until the threshold level is reached
â (This may be caused by the stimulus)
Threshold level
-90
+30
29. âą Then a sudden
change in polarisation
causes sharp
upstroke
(depolarisation) which
goes beyond the zero
level up to +30 mV
-90
+30
30. âą Then a sudden
decrease in
polarisation causes
initial sharp down
stroke (repolarisation)
-90
+30
31. âą When reaching the
Resting level rate
slows down
âą Can go beyond the
resting level
â hyperpolarisation
-90
+30
32. âą Spike potential
â Sharp upstroke and
downstroke
âą Time duration of AP
â 1 msec
-90
+30
33. ï” Ion channels called volted gated
channels responsible for action potential
ï” Two types of channel:
ï” Na+ channel
ï” K+ channel
Physiological basis of AP
35. âą When the threshold level is reached
â Voltage-gated Na+ channels open up
â Since Na conc outside is more than the inside
â Na influx will occur
â Positive ion coming inside increases the
positivity of the membrane potential and
causes depolarisation
â When it reaches +30, Na+ channels closes
â Then Voltage-gated K+ channels open up
â K+ efflux occurs
â Positive ion leaving the inside causes more
negativity inside the membrane
â Repolarisation occurs
36. âą Since Na+ has come in and K+ has
gone out
âą Membrane has become negative
âą But ionic distribution has become
unequal
âą Na+/K+ pump restores Na+ and K+
conc slowly
ï”By pumping 3 Na+ ions outward and
2 K+ ions inward