Mechanism of Generation and Propagation of Nerve Impulse.docx

1. A PROJECT WRITE-UP
ON
Mechanism of Generation
&
Propagation of Nerve Impulse
For course B.Sc. (H) Biotechnology
Submitted By
Abhinav Baranwal
202110902120035
Supervised By
Dr. Gurminder Kaur
Assistant Professor
Faculty of Bioscience, Institute of Bioscience and Technology,
Shri Ramswaroop Memorial University, Lucknow- Deva Road,
Barabanki, UP, India, 225003
SHRI RAMSWAROOP MEMORIAL
UNIVERSITY

2. Acknowledgement
I would like to take this opportunity to express my profound gratitude
and deep regards to my teacher Dr. Gurminder Kaur for providing a
wonderful opportunity to do this project on the topic Mechanism of
Generation & Propagation of Nerve Impulse. The opportunity helped
me improve my research skills and helped me to learn new things about
Nervous Coordination. I would also like to show my utmost gratitude
towards my friends for their exemplary guidance, monitoring and
constant encouragement throughout the course of this project.
At last I would like to thank Dr. Gurminder Kaur for her blessing, help
and guidance given time to time that shall carry me a long way in the
journey of life.

3. Contents
Page No
1 INTRODUCTION 1
2 GENERATING NERVE IMPULSE 2
2.1 The Nerve Impulse 2
2.2 Resting Membrane Potential 3-4
3 INITIATION OF IMPULSE 5
3.1 Action Potential 5-6
3.1.1 All or None Phenomenon 6
3.2 Depolarization 6
4 CONDUCTION OF NERVE IMPULSE 7
5 REPOLARIZATION 8
6 REFRACTORY PERIOD 9
7 MYELIN AND PROPAGATION OF THE ACTION POTENTIAL 10
7.1 Special Faster Connections 12
BIBLIOGRAPHY 13

4. 1 | P a g e
Introduction
The form of the action electric-potential in nerve membranes in the
nerve cell-membranes are first described, it is, in essence, a nerve
impulse and can be formed by virtue of periodic and ruled changes of
non-uniform distribution of the sodium and potassium ions in the inner
and surface of nerve cell-membranes. The nerve impulse can be also
transported along the nerve fibre membranes under action of periodic
works of sodium pump and potassium pump arising from the bio-
energy released from the hydrolyses reaction of ATP molecules, which
is transported by Pam’s split on along the protein molecules. The
experiments verified that there is not the nerve impulse, or the action
electric- potential without the works of sodium pump and potassium
pump, or the bioenergy. We investigated further the properties of
transport of the nerve impulse along the Nerve Fibre membranes.
The unit of the nervous system in animals is the nerve cell or the
neuron. It is the unit of structure and function of nervous system, its
main function is to accept process and transfer the nerve information,
to complete the functions of the nervous system. It consists of nerve
cells having different sizes and forms.

5. 2 | P a g e
Generating Nerve Impulses
Fig: Lightning - This amazing cloud-to-surface lightning occurred whena difference in electrical charge
built up in a cloud relative to the ground. When the build-up of charge was great enough, a sudden
discharge of electricity occurred. A nerve impulse is similar to a lightning strike. Both a nerve impulse
and a lightning strike occur because of differences in electrical charge, and both result in an electric
current.
Neurons are excitable cells because their membranes are in a polarized
state.
The Nerve Impulse
A nerve impulse, like a lightning strike, is an electrical phenomenon. A
nerve impulse occurs because of a difference in electrical charge across
the plasma membrane of a neuron. How does this difference in
electrical charge come about? The answer involves ions, which are
electrically charged atoms or molecules.
We can also say that Nerve Cells have polarized membrane i.e., have
electrical potential difference or membrane potential. This is because
of variety of ion channels (pores formed by proteins) specific for
particular types of ions.

6. 3 | P a g e
Resting Membrane Potential
Fig: Establishment of resting membrane potentials in nerve fibres when the membrane potential is
caused by diffusion of both sodium and potassium ions plus pumping of both these ions by the Na+-K+
pump.
In a resting nerve fibre, the cytoplasm just beneath its membrane is
electronegative relative to the layer of extracellular fluid (ECF) just
outside the membrane. The inner side of membrane is seen to possess
a negative potential of about 70 mV relative to the outer side. This is
called resting membrane potential.
This result due to two factors:
1. The resting membrane has a poor permeability for Na+ although it
has a higher permeability for K+. Therefore, K+ can cross more easily
while Cl- and Na+ have more difficulty in crossing.

7. 4 | P a g e
2. Negatively charged protein molecule inside the neuron cannot cross
the plasma membrane.
Fig: Establishment of resting membrane potentials, whenthere is efflux of 3Na+ ions and influx of 2K+
ions.
The differential flow of the positively charged ions and the fact that the
negatively charged organic ions within the nerve fibre cannot pass out
cause an increasing positive charge on the outside of the membrane and
negative charge on the inside of the membrane. This makes the
membrane of the resting nerve fibre polarized (i.e., its outside being
positively charged with respect to the inside.)
Such electrochemical gradients are maintained by the active transport
of ions involving Na+ – K+ ion transmembrane pump. It pumps out
3Na+ for every 2K+ ions passed inwardly. K+ concentration is 30 times
more inside neuron than outside and Na+ concentration is 10 times
more in interstitial fluid as compared to inside of neuron.

8. 5 | P a g e
Initiation of Impulse
When stimulated voltage gated Na+ channel open which causes a rapid,
very localized, temporary inflow of Na+ into the cell which causes
development of net positive charge on the inner side of the membrane
in that area. This is called depolarization. It occurs at a particular
region of neuron called trigger zone. Voltage gated ion channels are
clustered in the area of triggers zone. Stimulus of threshold value
causes stoppage of Na+ – K+ ATPase pump.
Continued passage of Na+ ions into inside of neuron creates a reverse
potential of +20 millivolt to +30 millivolts, rarely to +60 millivolts. The
total change occurs in spike-like fashion which is also called spike
potential. It creates a potential that sets in a wave of depolarization
through the nerve fibre. The membrane potential which sets in a wave
of depolarization is called action potential.
 ACTION POTENTIAL
Fig: Diagram depicts opening of Na+ channel whensignal is received and marks start of Action
Potential.

9. 6 | P a g e
An action potential, also called a nerve impulse, is an electrical
charge that travels along the membrane of a neuron. It can be
generated when a neuron’s membrane potential is changed by
chemical signals from a nearby cell. In an action potential, the
cell membrane potential changes quickly from negative to
positive as sodium ions flow into the cell through ion channels,
while potassium ions flow out of the cell. The change in
membrane potential results in the cell becoming depolarized.
 All or None Phenomenon
An action potential works on an all-or-nothing basis.
That is, the membrane potential has to reach a certain
level of depolarization, called the threshold,
otherwise, an action potential will not start. This
threshold potential varies but is generally about 15
millivolts (mV) more positive than the cell's resting
membrane potential. If a membrane depolarization
does not reach the threshold level, an action potential
will not happen.
Depolarization
You can see in the figure that signal or stimulus is received and
the first channels to open are the sodium ion channels, which
allow sodium ions to enter the cell. The resulting increase in
positive charge inside the cell (up to about +40 mV) starts the
action potential. This is called the depolarization of the
membrane.
Fig: Stimulus received and Depolarization caused.

10. 7 | P a g e
Conduction of Nerve Impulse
Fig: Diagrammatic representation of impulse conduction through axon (at points A to B)
In the area of depolarization, the potential difference across the
membrane is small while its nearby region has large difference in
membrane potential. This produces a small local current in the area.
The local current becomes a stimulus and causes the voltage gated Na+
channels of next region to open and depolarized the area to produce
fresh action potential. This process will continue till the impulse
reaches the end of neuron.
A nerve impulse is defined as an electric signal that goes through the
dendrites to create an action potential or a nerve impulse. An action
potential results from the movement of ions in and out of a cell and it
particularly includes sodium and potassium ions. They are transferred
in and out of the cell via sodium and potassium channels and sodium-
potassium pumps.
Conduction of nerve impulses happens because of the presence of
active and electronic potentials with conductors. Internally, the
transmission of signals amidst the cells is done via a synapse. The
electrical synapse got its application in escape reflexes, the heart, and
the retina of vertebrates. Nerve conductors consist of relatively higher
membrane resistance and low axial resistance. They are significantly
utilized when there is a need for fast response and timing being
important. The ionic currents pass across the two cell membranes when
the action potential nears the stage of such synapse.

11. 8 | P a g e
Repolarization
Fig: An action potential graph of membrane potential over time. A neuron must reach a certain
threshold in order to begin the depolarization step of reaching the action potential. The figure
also shows the change in potential during the repolarization and refractory periods of the axon.
As the Na+ channels close, the membrane becomes extra permeable to
K+ ions which leads Potassium ion channels then to open, allowing
potassium ions to flow out of the cell, which ends the action potential.
The inside of the membrane becomes negative again. This is called
repolarization of the membrane.
Both of the ion channels then close, and the sodium-potassium pump
restores the resting potential of -70 mV. The action potential will move
down the axon toward the synapse like a wave would move along the
surface of the water. Above figure shows the change in potential of the
axon membrane during an action potential.
However, K+ ion channels remain open for a bit longer period so that
the membrane potential becomes more negative than -70mV. This is
called Hyperpolarization.

12. 9 | P a g e
Refractory Period
A new action potential cannot occur in an excitable fibre as long as the
membrane is still depolarized from the receding action potential. The
reason for this is that shortly after the action potential is initiated, the
sodium channels (or calcium channels, or both) become inactivated and
no amount of excitatory signal applied to these channels at his point
will open the inactivation gates. The only condition that will allow them
to reopen is for the membrane potential to return to or near the original
resting membrane potential level. Then, within another small fraction
of a second, the inactivation gates of the channels open and a new
action potential can be initiated.
So, the nerve now goes through a brief refractory period before
racing resting potential. During the refractory period, another action
potential cannot be generated in myelinated neurons, ion flows occur
only at the nodes of Ranvier. As a result, the action potential signal
"jumps" along the axon membrane from node to node rather than
spreading smoothly along the membrane, as they do in axons that do
not have a myelin sheath. This is due to a clustering of Na+ and K+
ion channels at the Nodes of Ranvier. Unmyelinated axons do not
have nodes of Ranvier, and ion channels in these axons are spread
over the entire membrane surface.
The period during which a second action potential cannot be elicited,
even with a strong stimulus, is called the absolute refractory period.
This period for large myelinated nerve fibres is about 1/2500 second.
Therefore, one can readily calculate that such a fibre can transmit a
maximum of about 2500 impulses per second.

13. 10 | P a g e
Myelin and Propagation of the Action
Potential
Fig: Propagation of action potentials in both directions along a conductive fibre.
For an action potential to communicate information to another neuron,
it must travel along the axon and reach the axon terminals where it can
initiate neurotransmitter release. The speed of conduction of an action
potential along an axon is influenced by both the diameter of the axon
and the axon’s resistance to current leak. Myelin acts as an insulator
that prevents current from leaving the axon, increasing the speed of
action potential conduction. Diseases like multiple sclerosis cause
degeneration of the myelin, which slows action potential conduction
because axon areas are no longer insulated so the current leaks.

14. 11 | P a g e
Fig: Action potential travel along a neuronal axon: The action potential is conducted down the axon
as the axon membrane depolarizes, then repolarizes.
A node of Ranvier is a natural gap in the myelin sheath along the axon.
These unmyelinated spaces are about one micrometre long and contain
voltage gated Na+ and K+ channels. The flow of ions through these
channels, particularly the Na+ channels, regenerates the action
potential over and over again along the axon. Action potential “jumps”
from one node to the next in saltatory conduction. If nodes of Ranvier
were not present along an axon, the action potential would propagate
very slowly; Na+ and K+ channels would have to continuously
regenerate action potentials at every point along the axon. Nodes of
Ranvier also save energy for the neuron since the channels only need
to be present at the nodes and not along the entire axon.

15. 12 | P a g e
Fig: Nodes ofRanvier: Nodes of Ranvier are gaps in myelin coverage along axons. Nodes contain
voltage-gated K+ and Na+ channels. Action potentials travel down the axon by jumping from one
node to the next.
Special faster connections
Electrical synapses that are fast are used in reflexes, the retinas of
different vertebrates, and also the heart. This is because they are
much faster as they do not require the slow diffusion process of
neurotransmitters through the synaptic gap. Hence, electrical
synapses are utilized when the fast response and coordination of
timing are important.
These synapses directly join the presynaptic and postsynaptic
cells directly. When an action potential reaches this kind of
synapse, the ionic currents cross the two cell membranes and
move inside the postsynaptic cell using the pores termed as
connexions. Hence, presynaptic action potential stimulates the
postsynaptic cell directly.

16. 13 | P a g e
Bibliography
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