7.nervous tissue

7/26/2019 7.Nervous Tissue

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The most complex system in the body histologicallyand physiologically

Network of billion neurons and many more supporting

glial cells Process information and generate various responses


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Derived from the ectoderm

In the embryo, it arises from Neural tube: where brain and spinal cord are

derived (glial cells, ependymal cells, andepithelial cells of the choroid plexus)

Neural crest: nervous tissue outside of CNS likethe ganglia, capsular cells, Schwann cells


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Irritability Capacity to react in a gradual manner to physical

or chemical stimuli

Conductivity Transmit excitation rapidly from one place to

another

Integrates and coordinates functions ofthe organs

Synthesize chemical compound Neurotransmitters and neuromodulators


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Neurons or Nerve Cells Structural and functional unit of the nervous

tissue

Non-neural or Supporting Cells


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Parts of a Neuron


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Cell body (perikaryon) trophic or synthetic center

Covered by cell membrane

Vary in size and shape and themode of branching of its processesWell developed RER and free

ribosomesNissl bodies Neurofilaments


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Cell body Generally in the PNS, the body is rounded In the CNSthe cell body is typically

angular or polygonal with slightlyconcave surfaces between the processes

Axon hillockis the conical extension inthe cell body where the axon arises


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Nucleus Large, spherical or ovoid with conspicuous

nucleolus, pale staining or vesicular

Cell processesDendrites

Principal signal reception and processing sites

Dendritic spines: first locale for synaptic signalsneuronal plasticity

Axons

Long processes extending from axon hillock Covered by axolemma

Initial segment :site for excitatory and inhibitorystimuli summation

Collateral branches : axon branches


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AxonsAnterograde transport : kinesinRetrograde transport : dynein

Membrane potential

Potential difference (-65mV)

Action potential or impulse

-65mV+30mV


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Basis Axon Dendrites

Number of Processes 1 to a neuron Several or none

Length Generally longer Shorter, terminate inthe vicinity of neuron

Size and shape Small, cylindrical &

uniform in diameter

Broad-based & taper

off as it branches

Nissl bodies Absent Present

Contour or outline Smooth Rough due to

presence of fine

spines which are

sites of synapticcontact

Presence of myelin

sheath

Present Absent

Direction of impulses Away from neurons Toward neurons


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Types of Neurons: (Based on the

size and shape of their processes)

Most neurons


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Retina, olfactorymucosa, cochlear and

vestibular ganglia


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Spinal and cranial

ganglia


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Motor (efferent) Neurons

Control effector organs

Sensory (afferent) Neurons

Reception of sensory stimuli


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Sites of functional contact betweenneurons or between neurons and othereffector cells (muscle and gland cells)

Responsible for the unidirectionaltransmission of nerve impulses


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Converts an electrical signal (impulse)from the pre-synaptic cell into a chemicalsignal that acts on the post-synaptic cell

Transmits information by releasingneurotransmitters during signaling

process


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Types of Synapse


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Glial cells Astrocytes

Oligodendrocytes

Microglial cells Ependymal cells

Neuroloemmocyte

Neuropil: dense network of fibers fromprocesses of both neurons and glial cells thatfills the interneuronal space of the CNS


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Cell body is small and nucleus smaller thanastrocytes

Fewer cell processes and seldom branch

Responsible for synthesis of myelin in theCNS

Predominant glial cell in the CNS white

matter


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Star-shaped; largest of the glial cells Most numerous of glial cells

Have bundles of intermediate filaments madeof glial fibrillary acid protein (GFAP)


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Fibrous astrocytes Located in the white matter

With few long processes

Protoplasmic astrocytes Found in the gray matter

With many short-branched processes


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Maintain homeostasis in the extracellularfluid

Bind neurons to capillaries and to the piamater

Structural support

Repair processes


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Small elongated cells with short irregularprocesses

Phagocytic cells of the Mononuclear-phagocytic system


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A layer of cuboidal or low columnar cells liningthe ventricles of the brain and the central canal ofthe spinal cord


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Central Nervous System (CNS)

Brain

Spinal cord


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Cerebrum Cerebellum

Spinal cord

White matter

Gray matter


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Six layers with most neurons arrangedvertically

Efferent pyramidal neurons mostabundant

Integration of sensory info andinitiation of voluntary motor

responses


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Coordinates muscular activity

3 layers: Outer molecular layer

Central layer Purkinje cells Inner granular layer

Formed by smallest neurons


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Surfaces of cerebrum and cerebellum(forming cerebral cortex)

Contain neuronal bodies, dendrites andthe initial unmyelinated portions of axonsand glial cells This is the region at which synapses occur

Nuclei are aggregates of neuronal bodiesforming islands of gray matter embeddedin the white matter


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Central portion

Main component is myelinated axons and themyelin-producing oligodendrocytes

Does not contain neuronal cell bodies


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White matter is peripheral while graymatter is central

Assuming the shape of an H Where central canal (lined by ependymal

cells) is located

Anterior horn (motor)

Posterior horn (sensory)


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Gray matter Contains abundant astrocytes and large

neuronal bodies

White matter Surrounds gray matter

Contains oligodendrocytes and tracts of myelinatedaxons


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Main components are Nerves

Ganglia

Nerve endings


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Consist of axons enveloped by a specialmyelin sheath

Groups of nerve fibers constitute the tracts of

the brain, spinal cord and peripheral nerves Myelin sheath form gaps along the path called

the nodes of Ranvier Represent the spaces between adjacent

Schwann cells (neurolemmocytes) alongthe length of the axon


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Cranial Nerves

Spinal nerves


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I Olfactory VII Facial

II Optic VIII Auditory

III Oculomotor IX Glossopharyngeal

IV Trochlear X Vagus

V Trigeminal XI Spinal Accessory

VI Abducens XII Hypoglossal


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31 pairs of spinal nerves

Ganglia Sensory ganglia

Autonomic ganglia


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Afferent pathways (ascending pathways) Carry sensory impulses toward the CNS

Efferent pathways (descending pathways) Innervate skeletal muscle or effector organs by

transmitting motor impulses away from theCNS


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Relay information from the CNS to musclesand effector organs through Cranial and spinal nerve tracts that are

arranged in fascicles Multiple fascicles bound together to form the

peripheral nerve


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Somatic Nervous System Consists of pathways that regulate voluntary

motor control Ex. Skeletal muscles

Autonomic Nervous System Involuntary control of organ system (viscera)

Sympathetic Nervous System: mobilize energy intimes of need (fight or flight)

Parasympathetic Nervous System: conserve andrestore energy


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Cranium

Meninges

CSF Vertebral Column


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Composed of 8 bones that fuse early inchildhood

Cranial vault encloses and protects the brainand its associated structures

Floor of the cranial vault is irregular andcontains many foramina for cranial nerves,blood vessels and the spinal cord to exit


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Dura mater

Arachnoid membrane

Pia mater


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Thick external dense, fibroelatic CTcontinous w/ the periosteum

Composed of 2 layers with venous sinuses

between them

Epidural space Space between the cranium and dura mater


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Characterized by its spongy web-likestructure

Arachnoid villi protrudes into venous

sinuses Space between the dura and arachnoid is

the subdural space which contain many

veins disruption of veins may cause subdural

hematoma


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Follows the contour of the brain and spinalcord very closely

Provides support for the blood vesselssupplying the brain tissue

Subarachnoid space lies between thearachnoid and pia mater Contains cerebrospinal f luid Disruption of the intracranial vessels can lead

to subarachnoid hemorrhage

The choroid plexus arise from pial membrane


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Clear, colorless fluid which cushions theCNS by protecting soft tissue from joltsand blows

Produced by choroid plexuses made up ofependymal cells

Approximately 600 ml is produced daily

Circulate within ventricles of brain,

subarachnoid space and spinal cord


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Choroid plexuses of the lateral, third, andfourth ventricles produce the major portionof the CSF

Normal CSF pressure is 120

180 mm ofH 2O or approximately 9 14 mm Hgpressure

CSF from lateral ventricles


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CSF from lateral ventricles

Interventricular foramen(foramen of Monro)

Third ventricle

Cerebral aqueduct

(aqueduct of Sylvius)

Fourth ventricle

Pass either foramen of Luschka or

foramen of Magendie

Subarachnoid space ofthe brain and spinal cord


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33 vertebrae 7 cervical

12 thoracic

5 lumbar 5 fused sacral

4-5 fused coccygeal

Intervertebral disks


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Highly selective interface between the bloodand the CNS

Depends on special properties of the capillary

endothelium(main structural component) Have tight junctions that are highly resistant to

passage of ions or small molecules

Perivascular feet of astrocytes


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Protects the CNS from abrupt changes in theconcentration of ions in the extra-cellularfluid and from molecules that may interfere

with normal neural function Disadvantage

It excludes some drugs particularly antibiotics


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Epineurium Dense ,irregular fibrous coat

Perineurium Sleeves of specialized CT formed by layers of

flattened epithelial-like cells

Surrounds each fascicle

Endoneurium Sparse layer of loose CT that merges w/ type IV

collagen, laminin and proteins produced by theSchwann cells


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Contain both afferent and efferent fibers Nerves possessing sensory fibers are called sensory

nerves; those with motor fibers are motor nerves

Most nerves have mixed nerves


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Ovoid structures containing neuronal cellbodies and glial cells supported by connectivetissues

Serve as relay stations to transmit nerveimpulses Sensory gangliareceive afferent impulses that

go to the CNS

Autonomic gangliaare especially found in thewalls of the digestive tract


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Satellite cells Flattened cells which form a continuous

capsule around nerve cell bodies in the

ganglion


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Schwann cells or Neurolemmocytes Elongated cells arranged end to end along

the course of the nerve fibers in the PNS

Form a long thin tube which surround thenerve fiber called the sheath of Schwannor neurilemma

Responsible for the formation of myelinsheath in the PNS


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Myelination of PNS axons


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Neuronal Plasticity and Regeneration

Neuronal stem cells are present in adult CNS

the potential of neural stem cells to allow

regeneration of CNS components is a subject

of intense investigation

peripheral nerves have a good capacity forregeneration and return of function


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Thank you!


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7.nervous tissue

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