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NEURONDR. ZAHOOR ALI SHAIKH

LECTURE --9

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Neuron (nerve cell) consists of Three Basic Parts :

1– Cell Body 2– Dendrites 3– Axon

NEURON

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Cell 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.

NEURON

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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.

NEURON (cont)

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AXON 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.

NEURON

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AXON 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.

NEURON(cont)

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AXON 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.

NEURON (cont)

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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

NEURON (cont)

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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.

NEURON (cont)

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There are TWO types of conduction: 1– Continuous conduction or Contiguous

conduction. 2– Saltatory conduction or Jumping

TYPES OF CONDUCTION IN NEURON

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Continuous or Contiguous conduction occurs in UNMYELINATED nerve fibers.

Saltatory Conduction occurs in MYELINATED

nerve fibers.

Continuous Conduction

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Conduction depends on Two factors: 1) Diameter of the nerve fiber 2) Axon (Nerve fiber) is Myelinated or

Unmyelinated

CONDUCTION IN NERVE FIBERS

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Diameter of Nerve fiber Nerve fiber with small diameter – conducts

slowly Nerve fiber with large diameter – conducts

fast

CODUCTION IN NERVE FIBERS (cont)

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Myelinated Nerve Fibers Myelinated nerve fibers conduct FAST then

Unmyelinated nerve fibers. Why? Myelinated nerve fibers have saltatory

conduction.

CONDUCTION IN NERVE FIBER (cont)

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SUMMARY

Small diameter fiber conduct SLOW Large diameter fiber conduct FAST Myelinated nerve fiber conduct VERY FAST.

CONDUCTION IN NERVE FIBER (cont)

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Myelinated & Unmyelinated Fibers

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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.

Myelinated Fibers

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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].

Myelin Sheath

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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.

Myelinated 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.

Myelinated Fibers

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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.

CLINICAL APPLICATION

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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.

Unmyelinated 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?

What You Should Know From This Lecture

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Thank you