Neuron Structure and Nerve Impulse

1. Name: Chanaka Lakshan
Student ID: BS/15/10/15
Facilitator: Dr.Ariff
Subject: Human Physiology
Stream: Biomedical Science

2. Neuron
Structure &
Nerve
Impulse
Propagation

3. Neuron Structure
Ion Channels
Resting Membrane potential
Electrochemical force for Na & K
Positive feedback loop & interruption
Action Potential Graph & Explanation
Absolute Refractory Period
Relative Refractory Period
Conduction velocity
Summary

4. Neuron
Structure
Neuron is a
Specialized
type of cell
found in the
body.
A typical
neuron is
divided into ;
1.Cell body,
2.Dendrites,
3.Axon.

5. Cell Body
It Is main
nutritional and
metabolic
region of
neuron.
It receives
signals from
other cells and
sends them
towards the
axon.

6. Dendrites
Branched
dendrites
receive signals
coming in from
other cells and
send them
towards the
axon.

7. Axon
It generates
an action
potential, an
outgoing signal
also called a
nerve impulse,
and conducts it
to the next cell.
Axon is
transmitting or
conductive
region of the
neuron.

8. Ion Channels
Cell membrane is a lipid
bilayer which large integral
proteins embedded. Some
of these proteins have
watery pores called Ion
Channels through which
ion can pass. Ion Channels
control the movement of
ions through the neuronal
cell membrane. They are;
1.Selective
2.Passive or Active
3.Regionally
Located
4.Functionally
Unique
 Selective ion channels select ions for passage based
on charge, size ,attraction & holding of water by the
ion.
 Active Channels have Gates that can Open & Close It.
 Passive Channels are always open.
 Voltage gated Channels have Gates that are Controlled
by Voltage.
 Neuronal cell membrane have more +ve ions outside &
more –ve ions inside. This difference makes
membrane potential across cell membrane.
 Chemically gated channels have neurotransmitters
that bind to it.
 Chemically gated channels controlled by acetylcholine
& GABA bind to them causing it to open.
 When neuron is at rest, Voltage gated channels are
closed.
 In Action potential, it open & Close.
 Ions move through Open Channels.

9. Ion Channels
Location Function
• Cell membrane on
Dendrites.
• Cell body & Axon.
• Dendrites.
• Cell body.
• Axon Hillock, all along
unmyelinated axon.
• Nodes of Ranvier in
myelinated axons.
• Makes Resting
Membrane Potential.
• .
• Makes synaptic
Potentials.
•
• Generation &
Propagation of Action
Potentials.
• .

10. Resting Membrane
Potential (Resting
voltage)
1.It is the relatively static Membrane
Potential of quiescent cells.
2.It is opposed to the specific
dynamic electrochemical
phenomena called action
potential and graded membrane
potential.
3.It is a relatively stable, ground
value of transmembrane voltage in
animal and plant cells.
4.It is results from the membrane of
both Na+ & K+ ions. Activation gates
closed & inactivation gates open in
voltage gated Na+ channels, and
voltage gated k+ channels closed.
5.During resting potential, the cell’s
cytoplasm is negatively charged
relative to the outside of the cell. It
depends on distribution of both Na+
& K+ across cell membrane.
6.For many neurons, it is close to
-70mV.
7.Na+ & K+ Pump Essential to
Maintain It. ( When 3 Na+ go Out, 2
K+ comes In. )

11. Electrochemical
Force for
Na & K
1.Chemical Force & Electric
Force are either known as
Electrochemical gradient.
2.When they are equal, the
equilibrium potential occurs.
At that potential, no net
movement of K+ is occurred.
3.Forces from Concentration
gradient & electrical potential
combine to produce a large
Electrochemical gradient
drives Na+ into cell.
4.Positively charged Na+ ions
that have entered the neuron
make the membrane potential
more positive than
-90mV; which is the
equilibrium potential for K+.

12. Positive
Feedback Loop
& Interruption
*If the stimulus to the axon hillock is
great enough, neuron depolarizes by
about 15mV and reaches a point called
Threshold.
*At that point, action potential is
created.
*When & Only When a neuron reaches
Threshold, a Positive Feedback Loop
established.
*At Threshold, process of depolarization
becomes initiate & positive feedback
loop established and generate an action
potential.
*Action potential always have the same
amplitude & same duration & It is a all-
or-none event.
*Then, more Na flow into the cell,
therefore cell depolarize further and
opens still more voltage gated Na
Channels.
*Therefore, At threshold, depolarization
opens more voltage gated Na+
Channels.
*This positive feedback loop produces
the rising phase of action potential. This
ends when positive feedback loop is
interrupted. This breakdown is done by
2 processes;
1.Inactivation of voltage gated Na+
channels .
2.Opening of voltage gated k+
channels.
Depolarization
Open Voltage
gated Na+
Channels
Inward Flow
Of Na+

13. Action Potential
Graph &
Explanation
Rest
voltage gated Na+ & k+
channels closed.
Depolarization
voltage gated Na+ channels
open rapidly, resulting In
movement of Na+ into the cell.
Peak
Voltage gated Na+ channels
begin to inactivate & Voltage
gated K+ channels begin to
open. This initiates
repolarization.
Repolarization
With less Na+ moving to cell
and more K+ out to cell,
membrane potential becomes
more Negative, moving towards
resting value.
Hyperpolarization
Some voltage gated K+
channels remain open,
resulting in movement of K+
out of the cell.

14. Absolute
Refractory
Period
1.The absolute refractory
period is the interval from the
beginning of the action
potential until the fiber is able
to conduct another action
potential.
2.After the neuron has
generated an action potential, it
can not generate another one.
Many Na+ channels are inactive
& will not open, no matter what
voltage is applied to the
membrane. Most K+ channels
are open.
3.The neuron cannot generate
an action potential because Na+
cannot move in through
inactive channels & because K+
continues to move out through
open voltage gated channels.
4.A neuron cannot generate an
action potential during the
absolute refractory period.

15. Relative
Refractory
Period
1.Immediately after
the absolute
refractory period, the
cell can generate an
action potential, but
only if it is
depolarized to a value
more positive than
normal threshold.
2.This is true because
some Na+ channels
are still inactive &
some K+ channels are
still open.

16. Conduction
Velocity
*It is the speed with which
an action potential is
propagated.
It depends on;
1.The diameter of the axon.
2.How well the axon is
insulated with myelin.
Diameter-As it increases,
internal resistance to flow
of charge decreases &
Action Potential travel
faster.
Myelination-In it charge
flows only at nodes ,so
Action Potential generated
only at nodes.
More speed is gained by
Myelination than increasing
diameter.

17. Summary;
 A neuron is an electrically excitable specialized type of cell that processes and transmits
information through electrical and chemical signals with the presence of synapses, which are
complex membrane junctions that transmit signals to other cells with the aid of Cell body,
Dendrites & Axon.
 Ion Channels control the movement of ions through the neuronal cell membrane.
 Resting Membrane Potential is the relatively static Membrane Potential of quiescent cells.
 It is opposed to the specific dynamic electrochemical phenomena called action potential and
graded membrane potential.
 Chemical Force & Electric Force are either known as Electrochemical gradient.
 When they are equal, the equilibrium potential occurs. At that potential, no net movement of K+ is
occurred.
 If the stimulus to the axon hillock is great enough, neuron depolarizes by about 15mV and reaches
a point called Threshold.
 At Threshold, process of depolarization becomes initiate & positive feedback loop established and
generate an action potential.
 This positive feedback loop produces the rising phase of action potential. This ends when
positive feedback loop is interrupted.
 An action potential is a short-lasting event in which the electrical membrane potential of
a cell rapidly rises and falls, following a consistent trajectory.
 The absolute refractory period is the interval from the beginning of the action potential until the
fiber is able to conduct another action potential.
 Relative Refractory Period immediately after the absolute refractory period, the cell can generate
an action potential, but only if it is depolarized to a value more positive than normal threshold.
 Conduction Velocity depends on diameter and myelination of the action.
 It is the speed with which an action potential is propagated.

18. References:
 Bullock, TH; Horridge, GA (1965). Structure and Function in the Nervous Systems of Invertebrates. A series
of books in biology. San Francisco: W. H. Freeman.
 Bullock, TH; Orkand, R; Grinnell, A (1977). Introduction to Nervous Systems. A series of books in biology.
San Francisco: W. H. Freeman. ISBN 978-0-7167-0030-2.
 http://animatlab.com/
 Campbell & Reece (2005). Biology. Pearson Benjamin Cummings. ISBN 0-8053-7146-X.
 Hille, Bertil (2001) [1984]. Ion Channels of Excitable Membranes (3rd ed.). Sunderland, Mass: Sinauer
Associates, Inc. p. 5. ISBN 0-87893-321-2.
 Siegel, Allan; Sapru, Hreday (2005). Essential Neuroscience. p. 257. ISBN 978-0781750776
 Al, Martini, Frederic Et. Anatomy and Physiology' 2007 Ed.2007 Edition. Rex Bookstore, Inc.
p. 288. ISBN 978-971-23-4807-5.
 Bower, JM; Beeman, D (1995). The Book of GENESIS: Exploring Realistic Neural Models with the GEneral
NEural SImulation System. Santa Clara, Calif.: TELOS. ISBN 978-0-387-94019-9.
 Ntani, Georgia; Palmer, Keith T., Linaker, Cathy, Harris, E Clare, Van der Star, Richard, Cooper, Cyrus,
Coggon, David (15 August 2013). "Symptoms, signs and nerve conduction velocities in patients with
suspected carpal tunnel syndrome". BMC Musculoskeletal Disorders 14 (1): 1–10.
 Anderson, JA; Rosenfeld, E, eds. (1988). Neurocomputing: Foundations of Research. Cambridge, Mass.:
The MIT Press. ISBN 978-0-262-01097-9.

19. THANK YOU!