Excitable tissues are capable of generating and transmitting electrochemical impulses along cell membranes. The resting membrane potential in most neurons is around -70mV due to uneven distribution of ions like potassium and sodium across the cell membrane. When a threshold stimulus is reached, voltage-gated ion channels allow rapid sodium influx and potassium efflux, causing a brief reversal of the potential known as an action potential. This propagates the electrochemical signal along the membrane.

1. Excitable Tissues,
Resting Membrane
Potential & Action
Potential
DR ANWAR H. SIDDIQUI
PHYSIOLOGY, JNMC, AMU

2. What is an excitable tissue?
• Tissues which are capable of
generation and transmission of
electrochemical impulses along the
membrane.
• An ability of specialized cells to
respond to certain stimuli by
producing electrical signals.

3. Excitable tissues in the
human body
 Nerves
 Muscles
Cardiac Muscle
Smooth Muscle
Skeletal Muscle

4. Understand excitable tissue
 Suppose you have a dead frog. (Yes, that's kind of
gross, but let's just imagine it for a second.)
 What would happen if you applied an electrical
stimulus to the nerve in the frog's leg? Creepily
enough, the dead leg would kick!
 The Italian scientist Luigi Galvani discovered this fun
fact back in the 1700s, somewhat by accident during
a frog dissection.
 Today, we know that the frog's leg kicks
because neurons (nerve cells) carry information via
electrical signals.

5. Resting Membrane Potential
• A potential difference exists across all cell
membranes
• This is called
– Resting Membrane Potential (RMP)

6. Resting membrane potential
explained
 Imagine taking two electrodes
and placing one on the outside
and the other on the inside of the
plasma membrane of a living cell.
 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
 For a cell’s 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.
 That is, neurons have a resting membrane
potential (or simply, resting potential) of about -
70 to -90 mV.

8. Resting Membrane Potential
 Because there is a potential difference across the cell
membrane, the membrane is said to be polarized.
 If the membrane potential becomes more positive than it is at
the resting potential, the membrane is said to be depolarized.
 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?
 The resting membrane potential is determined
by the
 uneven distribution of ions (charged particles)
between the inside and the outside of the cell, and
 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, Cl−
usually present at higher concentrations outside the cell.

11. How ions move across the cell
membrane
 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.
 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. • 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.
• The resting membrane potential depends mainly on movement
of K+ through potassium leak channels. Howevr both Na+ and K+
contribute to resting potential.

13. • Potassium concentration intracellular is
more
• Membrane is freely permeable to K+
• 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. • Outside positivity resists efflux of K+
• (since K+ is a positive ion)
• 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.
• Nernst equation determines this potential
Where R= Universal Gas constt
T = Absolute Temp,
z = ion Valence
F = Faraday, an electrical Const

17. 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. 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
• When the membrane is permeable to several
ions the equilibrium potential that develops
depends on
– Polarity of each ion
– Membrane permeability
– Ionic concentration
• This is calculated using Goldman Equation
(or GHK Equation)

20.  In the normal nerve fiber, the permeability of the
membrane to potassium is about 100 times as great
as its permeability to sodium.
 Goldman equation gives a potential inside the
membrane of −86 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 →
loss of +ve ions ,loss of + ve charge from inside ,
negativity about - 4mV inside
-4mv

22.  Nernst potential for Potassium -94mv
 Nernst potential for Sodium +61mv
 Putting these values in Gold man equation, gives a value of -
86mv
Which is nearer to K+ diffusing potential
 Na- K pump provides - 4mv
 i.e adding -86 and -4mv= -90mv
 Resting membrane potential in nerves is -90 mv

23. Resting Membrane Potential in
Various Excitable Tissues
 Large Myelinated Nerve fibers
 Skeletal Muscle Fibers = - 90mv
 Ventricular Muscle fibers
 Smooth Muscle fiber & } = -55 to -60 mv
 Self Excitatory Tissues

24. Action potential
 Definition:
 Abrupt / sudden Change (reversal) in resting
membrane potential in response to a threshold
stimulus.
 Stimulus:
“Any Change in the environment”
 TYPES: a. Electrical
b. Mechanical
c. Chemical

25. Action Potential (A.P.)
• When an impulse is generated
– Inside becomes positive
– Causes depolarisation
– Nerve impulses are transmitted as AP

26. RMP -90
+30
Hyperpolarisation
0
Threshold potential
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Time (millisecond)

27. Inside of the membrane is
• Negative
– During RMP
• Positive
– When an AP is generated
-90
+30

28. • Initially membrane is slowly depolarised
• Until the threshold level is reached
– (This may be caused by the stimulus)
Threshold level
-90
+30

29. • Then a sudden
change in polarisation
causes sharp
upstroke
(depolarisation) which
goes beyond the zero
level up to +30 mV
-90
+30

30. • Then a sudden
decrease in
polarisation causes
initial sharp down
stroke (repolarisation)
-90
+30

31. • When reaching the
Resting level rate
slows down
• Can go beyond the
resting level
– hyperpolarisation
-90
+30

32. • Spike potential
– Sharp upstroke and
downstroke
• Time duration of AP
– 1 msec
-90
+30

33.  Ion channels called volted gated
channels responsible for action potential
 Two types of channel:
 Na+ channel
 K+ channel
Physiological basis of AP

34.  N+ channel
 K+ channel

35. • When the threshold level is reached
– Voltage-gated Na+ channels open up
– Since Na conc outside is more than the inside
– Na influx will occur
– Positive ion coming inside increases the
positivity of the membrane potential and
causes depolarisation
– When it reaches +30, Na+ channels closes
– Then Voltage-gated K+ channels open up
– K+ efflux occurs
– Positive ion leaving the inside causes more
negativity inside the membrane
– Repolarisation occurs

36. • Since Na+ has come in and K+ has
gone out
• Membrane has become negative
• But ionic distribution has become
unequal
• Na+/K+ pump restores Na+ and K+
conc slowly
By pumping 3 Na+ ions outward and
2 K+ ions inward