• When muscle is stimulated it contracts.
Contraction is a mechanical or physical
event.
Changes During Muscle
Contraction:
• Electrical Changes
• Physical Changes.
• Histological or molecular changes.
• The cell membranes of all body cells in
the resting condition are, polarized
which means that they show an
electrical potential difference,
commonly used term for potential
difference is only potential.
• Membrane potential refers to a
separation of charges across the
membrane or a difference in the relative
number of cations and anions in the
ICF and ECF.
• Potential is measured is unit of volts
but because the membrane potential is
relatively low, the unit used is the
millivolt (mV) (1 mV = 1/1,000 volt).
Excitable Tissues
• Tissues of the body can generate and
propagate nerve impulses which are
waves of electro-chemical activity
along their membranes. These include
nerve cells and muscle cells.
Membrane Potentials Caused by
Diffusion
• Diffusion Potential caused by an ion
concentration difference on the two
sides of the membrane.
• Potassium Ion
• Potassium concentration is great inside
a nerve fiber membrane but very low
outside the membrane.
There is a strong tendency for extra
numbers of potassium ions to diffuse
outward through the membrane as:
• The membrane at this instant is freely
permeable to the potassium ions but
not to any other ion.
• Because of the large potassium
concentration gradient between inside
and outside the membrane.
• K-ions move outside i.e positive
electrical charges go to the outside,
thus creating electropositivity on the
outside the membrane and
electronegativity is created inside
because of negative anions that remain
behind that do not diffuse outward with
the potassium.
• Within a millisecond the potential
difference between the inside and outside,
called the diffusion potential, becomes
great enough to block further net outward
potassium diffusion .
• The potential difference is about 94
millivolts, with negativity inside the fiber
membrane (normal mammalian nerve
fiber).
Sodium Ion :
At this time
• There is high concentration of
positively charged sodium ions on the
outside as compared to inside the
membrane.
• The membrane is highly permeable to
the sodium ions but impermeable to all
other ions.
• Diffusion of the positively charged
sodium ions to the inside creates a
membrane potential that is negativity
outside and positivity inside.
• Again, the membrane potential rises
high enough within milliseconds to
block further net diffusion of sodium
ions to the inside.
• The potential is about 61 millivolts
positive inside the fiber (in the
mammalian nerve fiber).
• The excitable tissues have the ability to
produce rapid, transient changes in
their membrane potential when excited.
• These brief fluctuations in potential
serve as electric signals.
Resting Membrane Potential
• The constant membrane potential
present in the cells of excitable tissue
when they are at rest i.e when they are
not producing electric signals is termed
as resting membrane potential.
Ions Involved In Memb. Potential
• The unequal disturbution of a few key
ions between the ICF and ECF and their
selective movement through the
plasma membrane are responsible for
the electrical properties of the
membrane.
• In the body, electric charge is carried
by ions. The ions primarily responsible
for the generation of the memb.
potential are K-ion, Na-ion, Cl-ion.
• Other ions ( calcium, magnesium,
phosphate , bicarbonate) do not make a
direct contribution to the memb.
potential.
• Potassium ions are present in much
higher concentration in the ICF
whereas, Sodium ions are present in
great conc. in the ECF. unit used for
these ions is mmol/L.
Transport Of Na And K-ions
Various ions tend to diffuse from one side
of the membrane to the other depending
upon their:
• Electrochemical gradients.
• The permeability of cell membrane to
these ions which varies greatly.
• Diffusion of K+ and Na+ ions can take
place through the channels present in
the cell membrane which are called K+
and Na+ ion leak channels.
• The membrane at rest is about 100
times more permeable to K+ than to
Na+ ions.
• Na and K-ion inaddition, to the active
carrier mechanism can passively cross
the memb. through protein channels
that are specific for them.
• K-ion can cross more easily as the
memb. has more channels open for it.
Also at resting potential in a nerve cell
membrane is 50-70% more permeable
to K-ion than Na-ion.
There are still other processes by
which these ions can move across the
cell membrane these are:
• The voltage-gated Na+ and K+
channels.
• The electrogenic Na+ K+ ATPase pump.
• The effects of an unequal concentration of
ions on the two sides of semipermeable
membrane can be analyzed mathematically.
• Suppose the membrane is only permeable
to K+ ions. K+ ions are present in higher
concentration inside interior, therefore,
they tend to diffuse along their
concentration gradient into the
extracellular fluid.
• But soon an equilibrium is reached at
which the efflux of K+ ions out of the cell
stops. The membrane potential at which
this equilibrium will exist is called the
Equilibrium potential for K+.
• Other names for equilibrium potential
are Nernst or reversal or diffusion
potentials.
• It is obvious that at the equilibrium
potential for K+ ions, the cell
membrane will become impermeable to
these ions.
• The magnitude of the net forces
tending to move each ion across the
membrane is determined by the resting
potential of the cell membrane and the
equilibrium potential for that ion
actually it is the difference between
these two values.
•12 /5/09
Relation of the Diffusion
Potential to the Concentration
Difference
The Nernst Potential.
• The diffusion potential level across a
membrane that exactly opposes the net
diffusion of a particular ion through the
membrane is called the Nernst potential
for that ion.
• The magnitude of this Nernst potential
is determined by the ratio of the
concentrations of that specific ion on
the two sides of the membrane.
• Greater this ratio, greater the tendency
for the ion to diffuse in one direction
and therefore greater Nernst potential
required to prevent additional net
diffusion.
• Nernst equation, used to calculate the
Nernst potential for any ion at normal
body temperature of 98.6°F (37°C):
Concentration inside
EMF (millivolts = ± 61 log
Concentration outside
•Where EMF is electromotive force.
• When using this formula, it assumed
that ECF outside the membrane has
zero potential, and the Nernst potential
is the potential inside the membrane.
Sign Used For The Potential
Positive: when negative ion diffuses
from inside to outside.
Negative: when positive ion diffuses
from inside to outside.
Factors Effecting Diffusion
Potential
• The polarity of the electrical charge of
each ion.
• The permeability of the membrane (P)
to each ion.
• The concentrations (C) of the
respective ions on the inside (i) and
outside (0) of the membrane.
• Thus, the following formula, called the
Goldman equation, or the Goldman-
Hodgkin-Katz equation gives the
calculated membrane potential on the
inside of the membrane.
Measuring the Membrane
Potential
BOOK!
Resting Membrane Potential
of Nerves
• The resting membrane potential of
large nerve fibers when not
transmitting nerve signals is about -90
millivolts.
• That is, the potential inside the fiber is
90 millivolts more negative than the
potential in the extracellular fluid on the
outside of the fiber.
Sodium-Potassium (Na+-K+) Pump.
• All cell membranes of the body have a
powerful Na+ K+-pump.
• It is an electrogenic pump that pumps
more positive charges outside than to
the inside.
• It continuously pumps 3 Na-ions to the
outside of the cell and 2 K-ions to the
inside of the cell.
• Leaving a net deficit of positive ions on
the inside this causes a negative
potential inside the cell membrane.
• The Na-K- pump cause large
concentration gradients for sodium and
potassium across the resting nerve
membrane:
• Na+ (outside):
142 mEq/L
• Na+ (inside):
14 mEq/L
• K+ (outside):
4 mEq/L
• K+ (inside):
140 mEq/L
Potassium-Sodium Leak
Channel.
• These are protein channels in the nerve
membrane through which potassium
and sodium ions can leak.
• These channels are 100 times more
permeable to potassium than to
sodium.
Origin of the Normal Resting
Membrane Potential
• Contribution of the K-ion diffusion
potential.
• Contribution of the Na-ion diffusion
through the nerve membrane.
• Contribution of the Na-K pump.
•Contribution of the K-ion
diffusion potential.
•Contribution of the Na-ion diffusion
through the nerve membrane.
•
Na-K-Pump
• Diffusion of potassium and sodium
alone would give a membrane potential
of about -86 millivolts (mainly due to
potassium diffusion).
• An additional - 4 millivolts is
contributed to the membrane potential
by the electrogenic Na+-K+ pump,
giving a net membrane potential of -90
millivolts.
• End Of Todays Lecture!!
4.
• The membrane is electrically neutral.
An equal number of positive (+) and
negative (-) charges are on each side of
the membrane, so no membrane
potential exists.