2. Objectives
1. Nerve structure and types.
2. Current and definition of capacitance and resistance and their equations .
3. Ions concentration in nerve impulse and resting potential.
4. Action potential.
5. Equations of action potential.
6. Nernst law.
7. Myelinated and unmyelinated axon.
8. Terms associated with action potential.
9. Mechanism of action potential in the body and its curve.
10. Causes and Affects.
11. The relationship between action potential and equilibrium.
3. NERVE
A bundle of fibers that uses
electrical and chemical signals
to transmit sensory and motor
information from one body part
to another. The fibrous
portions of a nerve are
covered by a sheath called
myelin and/or a membrane
called neurilemma.
6. ELECTRICAL CURRENTWhen a continuous path is connected between the terminals of a battery,
we have an electric circuit. When an electric circuit is formed, a charge
can flow through the wires of the circuit, from one terminal of the battery
to the other, as long as the path is continuous. Any flow of charge such as
this is called an electric current.
More precisely, the electric current in a wire is defined as the net amount
of charge that passes through the wire’s full cross section at any point per
unit time. Thus, current, I, is defined as:
I =ΔQ
Δt
Where ∆Q is the amount of charge that passes through the conductor at
any location during the time interval Δt.
Electric current is measured in coulombs per second; this is given a
special name, ampere (which is abbreviated as A).
7. ELECTRICAL RESISTANCE
● Electrical resistance is the resistance to the flow of current or
electrical charges.
● We define electrical resistance as the proportionality factor between
the voltage V (between the ends of the wire) and the current I (passing
through the wire): V=IR
● The unit for resistance is called the ohm and is abbreviated as Ω
● All electronic devices, from heaters to to lightbulbs to stereo
amplifiers, offer resistance to the flow of current.
● In many circuits, particularly in electronic devices, resistors are used
to control the amount of current.
8. CAPACITANCE
● A capacitor is a device used to store electrical charges and normally
consists of conducting objects (usually plates or sheets) placed near
each other but not touching.
● For a given capacitor, it is found that the amount of charge Q acquired
by each plate is proportional to the magnitude of the potential
difference V between the plates: Q=CV.
● The constant of proportionality, C, is called the capacitance of the
capacitor.
● The unit of capacitance is coulombs per volt, and this unit is called a
farad (F).
9. EXAMPLES
● Electrical Current
A steady current of 2.5A exists in a wire for 4 minutes. How
much total charge passes by a given point in the circuit during
those 4 minutes?
● Capacitance
A capacitor of charge, 150C has a potential difference of
100V. What is the capacitance of the capacitor?
● Electrical Resistance
A small flashlight bulb draws 300mA from its 1.5-V battery.
What is the resistance of the bulb?
11. The Action Potential
● Neurons communicate over long distances by generating and sending
an electrical signal called a nerve impulse, or action potential.
● The action potential is a large change in membrane potential from a
resting value of about -70 millivolts to a peak of about +30 millivolts,
and back to -70 millivolts again.
● The action potential results from a rapid change in the permeability of
the neuronal membrane to sodium and potassium.
● The weak stimuli do not produce an action potential. Thus we say that
the action potential is an all-or-none event.
12. Nernst Equation.
Resting Channels in Glial Cells Are Selective for
Potassium Only.
● K+ ions are present at a high concentration inside the cell glial
cells, and they are selectively permeable to them, K+ ions
tend to diffuse from inside to outside the cell, down their
chemical concentration gradient.
● Since opposite charges attract each other, the positive
charges on the outside and the negative charges on the inside
collect locally on either surface of the membrane.
13. ● The separation of charge resulting from the diffusion of K+
gives rise to an electrical potential difference: positive outside,
negative inside.
● The more K+ continues to flow, the more charge will be
separated and the greater will be the potential difference.
● Since K+ is positively charged, this potential difference tends to
oppose the further efflux of K+.
14. Nernst Potential.
K+
ions are subject of two forces:
● The chemical force.
● The electric force.
At a certain point K+ ion diffusion reaches a potential level where the
chemical force driving K+ ions outside balances the electric force driving
K+ into the cell.This potential is called Equilibrium potential or 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.”
15. Nernst Equation.
The Nemst equation can be used to find the equilibrium potential of any ion that
is present on both sides of a membrane permeable to that ion(also called the
Nernst potential)
The equilibrium potential for any ion X can be calculated from an equation
derived in 1888 from basic thermodynamic principles by the German physical
chemist Walter Nernst:
16. Where,
Ex
is the Nernst potential for the give ion X.
R is the gas constant (8.314 J.K−1.mol−1)
T the temperature (in degrees Kelvin),
z the valence of the ion, For example, z is +1 for Na+.
F the Faraday constant, (96,485 C.mol−1)
and [Xo
] and [X1
] are the concentrations of the ion outside and
inside of the cell.
Nernst equation can also be written as:
17. Goldman-Hodgkin and Katz (GHK)
Equation
•When a membrane is permeable to two different ions,
the Nernst equation can no longer be used to precisely
determine the membrane potential. It is possible,
however, to apply the GHK equation. This equation
describes the potential across a membrane that is
permeable to both Na+
and K+
.
18. ● R (Gas Constant) = 8.314472 (J/K·mol)
● T (Absolute Temperature) = °C + 273.15 (°K)
● F (Faraday's Constant) = 9.6485309×104
(C/mol)
●
19. •potassium (K+
) and sodium (Na+
). Chloride is
assumed to be in equilibrium.
When chloride (Cl−
) is taken into account, its part is flipped
to account for the negative charge.
20. ● Vm
is the membrane potential. This equation is used to
determine the resting membrane potential in real cells, in
which K+
, Na+
, and Cl-
are the major contributors to the
membrane potential.
● the unit of Vm
is the Volt. However, the membrane potential
is typically reported in millivolts (mV).
● If the channels for a given ion (Na+
, K+
, or Cl-
) are closed, then
the corresponding relative permeability values can be set to
zero. For example, if all Na+
channels are closed, pNa
= 0.
21. Myelinated/Medullated neuron: The neuron whose axon is
covered by myelin sheath (myelin means white) is called
myelinated neuron. The conduction of nerve impulse is faster
in this neuron than non-myelinated neuron due to presence of
myelin sheath over the axon. Myelin sheath avoids the loss of
impulse during conduction.
22. Non-myelinated/non-Medullated neuron: The neuron whose axon is not
covered by myelin sheath is called non-myelinated neuron. The conduction
of nerve impulse in this neuron is slow than myelinated neuron due to
absence of myelin sheath so there is more chances of loss of impulse
during conduction.
24. 1- When a neuron is at rest it is in a state of polarization.(-70
mV)
● There is an excess of sodium (Na+) ions outside of the
cell membrane that create a positive charge.
● there is an excess of potassium (K+) ions inside the cell
along with negatively charged molecules that produce a
negative charge inside the cell membrane.
25. 2- A stimulus is received by the dendrites of a nerve
cell,This causes the Na+
channels to open.
● For an electrical stimulate smaller than a critical threshold
value which equals to -55 mv
No significant axon potential changes occur .
● A stimulus above the threshold level :
If the opening is sufficient to drive the interior potential from -70 mV up to -55
mV, the process continues.
26. 3-The Na+ influx drives the interior of the cell
membrane up to about +30 mV. The process to this
point is called depolarization.
27. 4- The Na+
channels close and the K+
channels open
the membrane begins to repolarize back toward its
rest potential.*
28. 5-The repolarization typically overshoots the rest
potential to about -90 mV. This is called
hyperpolarization.
Hyperpolarization prevents the neuron from receiving another stimulus
during this time, or at least raises the threshold for any new stimulus.
Part of the importance of hyperpolarization is in preventing any stimulus
already sent up an axon from triggering another action potential in the
opposite direction. In other words, hyperpolarization assures that the
signal is proceeding in one direction.
29. 6-After hyperpolarization, the Na+/K+ pump eventually
brings the membrane back to its resting state of -70 mV
Due to the changes in the k permeability and The pump does gradually
reestablish the resting Na and K concentration.
30.
31.
32. • The equilibrium potential is the membrane potential where the net flow through
any open channels is 0. In other words,the chemical and electrical forces are in
balance. The equilibrium potential for Na+
is ~+60 mV and for K+
is ~-88 mV.
• The Na+
-K+
pumps in nerve cells provide for the long-term maintenance of
these concentration gradients. They keep the intracellular concentrations of K+
high and the Na+
low, and thereby maintain the Na+
equilibrium potential and the
K+
equilibrium potential. The pumps are necessary for the long-term
maintenance of the "batteries" so that resting potentials and action potentials
can be supported.
33. • There is a "gush" of Na+
that comes into the cell with each
action potential. Although, there is some influx of Na+
, it is
minute compared to the intracellular concentration of Na+
. The
influx is insufficient to make any noticeable change in the
intracellular concentration of Na+
. Therefore, the Na+
equilibrium
potential does not change during or after an action potential.
34.
35.
36.
37. ● http://www.ucl.ac.uk/~sjjgsca/NerveRestingPot.html
● https://www.youtube.com/watch?v=_Lj_F9GADa4&t=124s
● http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/actpot.html
● https://www.youtube.com/watch?v=Jyh8HhK80X8
● physics 3rd edition , 1998 ,author : KANE JOSEPH and STERNHEIM.MORTON,publishers :
Jhon Wiley and Sons page : 437 - 441 .
● Source: Boundless. “Nerve Impulse Transmission within a Neuron: Action Potential.”
Boundless Biology Boundless, 08 Aug. 2016. Retrieved 20 Mar. 2017 from
https://www.boundless.com/biology/textbooks/boundless-biology-textbook/the-nervous
-system-35/how-neurons-communicate-200/nerve-impulse-transmission-within-a-neuro
n-action-potential-762-11995/
