NEURON
OBJECTIVES
By the end of this lecture you should be able to
• Describe the functional structure of neuron – Cell body,
Dendrites, Axon.
• List the types of Nerve Conduction
• List the factors that affecting nerve conduction
• Illustrate the functional importance of is myelin sheath
& Node of Ranvier
• Discreminate between Contiguous Conduction and
Saltatory Conduction
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NEURON
• Neuron (nerve cell) consists of Three Basic
Parts :
1– Cell Body
2– Dendrites
3– Axon
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Neuron
• Cell body
– Houses the nucleus and organelles
• Dendrites
– Project from cell body and increase surface area
available for receiving signals from other nerve cells
– Signal toward the cell body
Dendrite and cell body serve as the neurons input zone.
Neuron
• Axon
– Nerve fiber
– Single, elongated tubular extension that
conducts action potentials away from the
cell body
– Conducting zone of the neuron
Axon
– Axon hillock
• First portion of the axon plus the region of the
cell body from which the axon leaves
• Neuron’s trigger zone(where Action Potential
are generated).
– Axon terminals
• Release chemical messengers that
simultaneously influence other cells with which
they come into close association
• Output zone of the neuron
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Neuron
NEURON (cont)
REMEMBER
• Dendrites & cell body – Input zone ( Receive)
• Axon -- Conducting zone
• Axon Terminals – Output zone
• Action Potential are initiated at Axon Hillock
(trigger zone)
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Action Potentials are propagated from the Axon
Hillock to the Axon terminal
In cell body Graded Potentials (Local potentials) are produced in
response to incoming signals.
Action potentials are Initiated at the Axon Hillock, and
conducted throughout a nerve fiber
Conduction of Action Potentials
• Two types of propagation
– Contiguous conduction
• Conduction in unmyelinated fibers
• Action potential spreads along every portion of the
membrane
– Saltatory conduction
• Rapid conduction in myelinated fibers
• Impulse jumps over sections of the fiber covered with
insulating myelin
–
Contiguous Conduction
Myelinated Fibers
Saltatory Conduction
Saltatory Conduction: Action Potential Propagation in a
Myelinated Axon
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Saltatory Conduction
• Propagates action potential faster than
contiguous conduction because action potential
does not have to be regenerated at myelinated
section
• Myelinated fibers conduct impulses about 50
times faster than unmyelinated fibers of
comparable size
• Myelin
– Primarily composed of lipids
– Formed by oligodendrocytes in CNS
– Formed by Schwann cells in PNS
CONDUCTION IN NERVE FIBERS
• Conduction depends on Two factors:
1) Diameter of the nerve fiber
2) Axon (Nerve fiber) is Myelinated or
Unmyelinated
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CODUCTION IN NERVE FIBERS (cont)
Diameter of Nerve fiber
• Nerve fiber with small diameter – conducts
slowly
• Nerve fiber with large diameter – conducts
fast
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CONDUCTION IN NERVE FIBER (cont)
Myelinated Nerve Fibers
• Myelinated nerve fibers conduct FAST then
Unmyelinated nerve fibers.
Why?
• Myelinated nerve fibers have saltatory
conduction.
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CONDUCTION IN NERVE FIBER (cont)
SUMMARY
• Small diameter fiber conduct SLOW
• Large diameter fiber conduct FAST
• Myelinated nerve fiber conduct VERY FAST.
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Regeneration of Nerve Fibers
• Regeneration of nerve fibers depends on its
location
• Schwann cells in PNS guide the regeneration
of cut axons
• Fibers in CNS myelinated by oligodendrocytes
do not have regenerative ability
– Oligodendrocytes inhibit regeneration of cut
central axons
References
• Human physiology by Lauralee Sherwood,
seventh edition
• Text book physiology by Guyton &Hall,11th
edition
• Text book of physiology by Linda .s
contanzo,third edition