DR. ZAHOOR ALI SHAIKH
LECTURE --9
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Neuron (nerve cell) consists of Three Basic
Parts :
1– Cell Body
2– Dendrites
3– Axon
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 Cell
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Body
The nucleus and organelles are present in the
cell body. From the cell body many
extensions known as DENDRITES project to
increase the surface area for receiving the
signals from other neurons.
Cell Body and Dendrites are Input zone.
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Some neuron have up to 400,000 dendrites.
In cell body Graded Potentials (Local
potentials) are produced in response to
incoming signals.
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 AXON
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or NERVE FIBER
Axon is a single, elongated, tubular extension
from the cell body.
Axon conducts Action Potential from the cell
body and terminates on other cells.
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 AXON
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Axon Hillock
It is the first portion of Axon + Cell body
from which axon leaves, it is called Axon
Hillock
AXON HILLOCK is the place where Action
Potential are generated.
Action potential are then conducted from
Axon Hillock along the axon to its branched
endings called Axon Terminals.
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 AXON
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Axon Terminals – release chemical
messengers to other cells with which they
come in contact
Therefore Axon is the conducting zone of
neuron.
And Axon Terminals are Output zone.
Axon may be less than 1mm or longer than
1meter depending on the area they innervate.
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REMEMBER
Dendrites & cell body – Input zone ( Receive)
Axon -- Conducting zone
Axon Terminals – Output zone

Action Potential are initiated at Axon Hillock
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In neuron, graded potentials ( local
potentials) are generated in the Dendrites and
Cell body due to chemical signals.
If the potential is strong that can initiate
Action potential at the Axon Hillock than
impulse (AP) is conducted throughout the
axon.
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
There are TWO types of conduction:
1– Continuous conduction or Contiguous
conduction.
2– Saltatory conduction or Jumping
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Continuous or Contiguous conduction occurs
in UNMYELINATED nerve fibers.
Saltatory Conduction occurs in MYELINATED
nerve fibers.
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Conduction depends on Two factors:
1) Diameter of the nerve fiber
2) Axon (Nerve fiber) is Myelinated or
Unmyelinated
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 Diameter
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of Nerve fiber
Nerve fiber with small diameter – conducts
slowly
Nerve fiber with large diameter – conducts
fast
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 Myelinated
Nerve Fibers
 Myelinated nerve fibers conduct FAST then
Unmyelinated nerve fibers.
 Why?
 Myelinated nerve fibers have saltatory
conduction.
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SUMMARY
Small diameter fiber conduct SLOW
Large diameter fiber conduct FAST
Myelinated nerve fiber conduct VERY FAST.
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Why Myelinated Nerve Fibers have increased
speed of conduction or increase Nerve
Conduction Velocity [NCV] ?
Because Myelinated fibers are axons covered
with myelin sheath, a thick layer composed of
lipids.
Myelin Sheath is present at regular intervals
along the length of nerve fiber.
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Myelin Sheath acts as insulator.
Why ?
Because water soluble ions which carry
current across the membrane, can not pass.
Between the myelinated regions – there is
NODE OF RANVIER, where axonal membrane
is not covered with myelin sheath and
exposed to Extra-Cellular Fluid [ECF].
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In Myelinated Fiber, current can flow at NODE
OF RANVIER to produce Action Potential.
Therefore impulse jumps from node to node,
skipping over the myelinated portion of axon.
This type of conduction in Myelinated Fiber is
called ‘SALTATORY CONDUCTION’ [saltare
means to jump].
Saltatory Conduction [in myelinated fibers] is
fast than continuous conduction in
unmyelinated fibers.
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What is Myelin Sheath ?
 Myelin Sheath is formed by myelin forming
cells in Peripheral Nervous System [PNS],
SCHWANN CELLS.
 Myelin Sheath is not a part of neuron but
formed by Schwann Cells that cover the axon.
 Please Note – In Central Nervous System
(CNS) i.e Brain and Spinal cord myelin is
formed by OLIGODENDROCYTES.
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In case of a cut axon in peripheral nerve,
there is REGENERATION (Re-growth) of axon
due to Schwann cell.
In case of CNS ( Brain and Spinal cord), axons
which are myelinated by Oligodendrocytes,
do not regenerate.
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In Unmyelinated Fibers, nerve impulse passes
along the entire length of nerve fiber. It is
slow conduction as compared to myelinated
fibers.
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Structure of neuron – Cell body, Dendrites, Axon.
 Axon – Action Potential [AP] is generated at axon hillock and AP is
conducted by axon.
 Axon is the conducting zone of neuron.
 Input Zone – Dendrites and Cell Body.
 Output Zone – Axon terminals
 Graded Potential [Local Potential] is generated at dendrites and cell body
due to chemical signals.
 Types of Nerve Conduction
- Contiguous Conduction [continuous] occurs in unmyelinated nerve fiber
- Saltatory Conduction – occurs in myelinated nerve fiber
 Factors affecting conduction
- Diameter of Nerve Fiber
- Myelinated or Unmyelinated Nerve Fiber
 What is myelin sheath?
 What is Node of Ranvier?
 Difference between Contiguous Conduction and Saltatory Conduction?

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