HISTOLOGY OF THE HISTOLOGY OF THE NERVOUS SYSTEMNERVOUS SYSTEM

(AN OVERVIEW)(AN OVERVIEW)

Dr. SAHAR HAFEEZDr. SAHAR HAFEEZ

drsaharhafeez@yahoo.comdrsaharhafeez@yahoo.com

Learning ObjectivesLearning Objectives

The students should be able to;The students should be able to;

Enlist the names & functions of different Enlist the names & functions of different types of Neuronstypes of Neurons

Enlist the names & functions of different Enlist the names & functions of different types of Neurogliatypes of Neuroglia

Demonstrate an understanding towards the Demonstrate an understanding towards the structural differences structural differences of of neurons and neuroglianeurons and neuroglia

Define the terms Define the terms GangliaGanglia & & NucleiNuclei and and Gray & White MattersGray & White Matters..

Demonstrate an understanding towards the arrangement of different Demonstrate an understanding towards the arrangement of different layers of a typical nervelayers of a typical nerve..

ANATOMICAL ORGANIZATION of the ANATOMICAL ORGANIZATION of the NERVOUS SYSTEMNERVOUS SYSTEM

NervousSystem

CNS PNS ANS

BRAIN

SPINALCORD

CRANIALNERVES

SPINALNERVES

SYMPATHETIC

PARA-SYMPATHETIC

Cell types present in the Nervous SystemCell types present in the Nervous System

Nervous tissue is made up of 2 types of cells;Nervous tissue is made up of 2 types of cells;Neurons Neurons (structural & functional unit)(structural & functional unit)NeurogliaNeuroglia (Supporting cells) (Supporting cells)

Parts of a NeuronParts of a Neuron

Cell body:Cell body:

Trophic unitTrophic unit

Dendrites:Dendrites:Receptive unitReceptive unit

Axon:Axon:Conductive unitConductive unit

Perikaryon/Cell Body /SomaPerikaryon/Cell Body /Soma

““part of a neuron that encloses the nucleus and other organelles part of a neuron that encloses the nucleus and other organelles necessary to maintain and repair the neuron”.necessary to maintain and repair the neuron”.

Cell Body OrganellesCell Body OrganellesNucleusNucleusGolgi apparatusGolgi apparatusRERRER

Ribosomes (=Nissl substance)Ribosomes (=Nissl substance)

Perikaryon/Soma/Nerve cell bodyPerikaryon/Soma/Nerve cell body

Most neurons have their cell bodies located in different parts of Most neurons have their cell bodies located in different parts of CNSCNS

Clusters of cell bodies in the CNS are known as Clusters of cell bodies in the CNS are known as NucleiNucleiClusters of cell bodies in PNS are known as Clusters of cell bodies in PNS are known as GangliaGanglia

Axon CharacteristicsAxon Characteristics Carries information to another neuron or muscle cell.Carries information to another neuron or muscle cell.

Often relatively long.Often relatively long.

Single (one per neuron).Single (one per neuron).

Ends in short branched processes called Ends in short branched processes called telodendria.telodendria.

May have collateral branches.May have collateral branches.

Covered by neurolemma mCovered by neurolemma made up of Schwann cells.ade up of Schwann cells.

Note: axon is the only part of a neuron that Note: axon is the only part of a neuron that is ever myelinated.is ever myelinated.

Myelin Sheath

Telodendria

Dendrites (Characteristics)Dendrites (Characteristics)

Carry information to the cell body.Carry information to the cell body.

Usually multiple.Usually multiple.

Relatively short.Relatively short.

Often branched.Often branched.

Have receptors for Have receptors for neurotransmitters.neurotransmitters.

Conduct local potentials.Conduct local potentials.

Relationship B/W Axon & DendritesRelationship B/W Axon & Dendrites

Classification of NeuronsClassification of Neurons

Neurons are classified according to;Neurons are classified according to;

StructureStructure Unipolar/Pseudo unipolar neuronsUnipolar/Pseudo unipolar neurons Bipolar neuronsBipolar neurons Multipolar neuronsMultipolar neurons

FunctionFunction Sensory neuronsSensory neurons Motor neuronsMotor neurons Association neuronsAssociation neurons

Structural Classification of NeuronsStructural Classification of Neurons

Pseudo unipolar Neurons: Pseudo unipolar Neurons: Present in the dorsal root Present in the dorsal root ganglia of spinal cordganglia of spinal cord

Bipolar NeuronsBipolar Neurons: : Present in Present in the Olfactory epithelium of the Olfactory epithelium of nose as well as in the Retina nose as well as in the Retina of eyeof eye

Multipolar NeuronsMultipolar Neurons: : Present Present everywhere else.everywhere else.

Functional Classification of NeuronsFunctional Classification of Neurons Sensory (afferent) Neurons:Sensory (afferent) Neurons:

Transmit impulses from the Transmit impulses from the sensory peripheral receptors to sensory peripheral receptors to CNSCNS

Motor (efferent) Neurons: Motor (efferent) Neurons: Carry impulses away from Carry impulses away from

CNS to peripheral organsCNS to peripheral organs

Association/Inter-Neurons:Association/Inter-Neurons: Present only in the CNS and Present only in the CNS and

transmit impulses across the transmit impulses across the neurons of CNSneurons of CNS

Neuroglia = Neuroglia = nerve gluenerve glue

Distinguished by their much smaller size as compare to Distinguished by their much smaller size as compare to neurons.neurons.

Outnumber neurons in CNS by 9:1 ratio.Outnumber neurons in CNS by 9:1 ratio. Are mainly of six types:Are mainly of six types:

4 of them are found in CNS4 of them are found in CNS AstrocytesAstrocytes MicrogliaMicroglia OligodendrocytesOligodendrocytes Ependymal cellsEpendymal cells

2 of them are found in PNS2 of them are found in PNS Satellite cellsSatellite cells Schwann cellsSchwann cells

Neuroglia of CNSNeuroglia of CNS

Astrocytes= Astrocytes= star-shapedstar-shaped Derived from neural crest cells.Derived from neural crest cells. Most abundant type of glial cellsMost abundant type of glial cells Function to physically support neurons.Function to physically support neurons. Brace the neurons & anchor them to Brace the neurons & anchor them to

the near by capillaries (create BBB)the near by capillaries (create BBB)

Neuroglia of CNSNeuroglia of CNS Oligodendrocytes:Oligodendrocytes:Derived from neural crest cells.Derived from neural crest cells.Function to myelinate axons within CNS.Function to myelinate axons within CNS.

Microglia:Microglia:Derived from embryonic mesenchyme.Derived from embryonic mesenchyme.May transform into phagocytes within CNSMay transform into phagocytes within CNS

Ependymal Cells:Ependymal Cells:

Derived from neural crest cells.Derived from neural crest cells.Line ventricles of brain & central canal of Spinal cord.Line ventricles of brain & central canal of Spinal cord.

(The protective role of Microglia is very important as cells (The protective role of Microglia is very important as cells of the immune system are denied across the CNS)of the immune system are denied across the CNS)

Neuroglia in the PNSNeuroglia in the PNS

Schwann Cells:Schwann Cells:Derived from neural crest Derived from neural crest cells.cells.Myelinate axons in the PNS.Myelinate axons in the PNS.

Satellite Cells:Satellite Cells:Surround nerve cell bodySurround nerve cell bodyMay aid in controlling May aid in controlling neuronal chemical neuronal chemical environmentenvironment

Myelination of AxonsMyelination of Axons• Myelin protects & electrically

insulates the fibers

• In CNS, Axons are myelinated by ‘oligodendrocytes

• In PNS, Axons are myelinated by ‘Schwann cells’.

• Schwann cells myelinate the fibers like a jelly roll.

• One Axon in PNS is myelinated by many Schwann cells.

• The gap b/w individual Schwann cells is k/a ‘Node of Ranvier’

Epineurium:Epineurium: Outermost Outermost connective tissue sheet connective tissue sheet covering all the bundles of covering all the bundles of fibers in a nerve.fibers in a nerve.

Perineurium:Perineurium: Middle Middle connective tissue sheet connective tissue sheet covering individual bundle of covering individual bundle of fibers.fibers.

Endoneurium:Endoneurium: Innermost Innermost sheet covering the individual sheet covering the individual Axon of each fiber in a Axon of each fiber in a bundle. bundle.

Coverings of a Typical Peripheral Nerve

GangliaGanglia

Button-shaped enlargements with in the PNS produced by a collection of neuron cell bodies.

Functionally speaking, Ganglia are the relaying station outside CNS.

Unipolar cell bodies are found in spinal/Dorsal root ganglia

Multipolar cell bodies are found in autonomic ganglia

White & Grey Matter of CNSWhite & Grey Matter of CNSWhite Matter: Areas of CNS mostly made up of Myelinated axons (as the myelin is white colored)

Grey Matter: Areas of the CNS mostly made up of somas of neurons.

Arrangement of white & grey matter;

In the Spinal cord:White matter outside & Grey matter inside

In the Brain: Grey matter outside & white matter inside. Some isolated clusters of grey matter embedded within the inner white matter (Nuclei)

THANK YOUTHANK YOU

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