bio mechanics of nerve 03

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  • 8/9/2019 Bio Mechanics of Nerve 03


  • 8/9/2019 Bio Mechanics of Nerve 03


    IntroductionyAnatomy And physiology peripheral


    yAnatomy And physiology spinal nerve

    yBiomechanics of peripheral nerve

    yBiomechanics of spinal nerve

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    Nervous systemInput




    Out put

    (motor/ glan ular)

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    Nervous Syste


    Brain, Spinal cord



    Spinal Nerve (31 pairs)


    ranial Nerve (12 pairs)


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    Anatomy and physiology of

    peripheral nerve

    Structure of peripheral nerveNerve fibers

    Connective tissues

    Blood vessels

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    Structure and function of nerve


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    AXONy This the anato ical connection between the cell body

    and its end organs.

    y The connection is aintained by the axonaltransport. This helps in the transporting the variousche ical synthesized (protein) in the cell body to theperiphery in both the directions.


    The speed of the axonal transport ay vary fro 1-400/ day

    Most of the axon of the PNS are yelinated one.

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    Features of myelinated nerve fibery The axon is surrounded by the yelin sheath for ed

    by the Schwann cells. The Schwann cell encircles theaxon in concentric layers of protein and lipids

    (cholesterol, lecithin, spingo yelin)alternatively. Itpushes the cytoplas and nucleus to periphery.

    y Node of Ranvier: this is the gap in the axon where theyelinated sheath is absent. The gap is about 1-2

    y This node of Ranvier helps in the saltatoryconduction of i pulse. Saltatory conduction isdirectly proportional to the dia eter of the fiber.

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    Sequence of the conduction of

    impulse in nerve fiber

    S all dia eter sensory nerve fiber

    Dull diffuse pain

    Sensory nerve fibers

    touch Pressure heat Cold Kinesthetic sensation

    Large dia eter nerve fibers

    skeletal uscles

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    Myelin sheath high insulating capacity. So it restrictsthe conduction of i pulse within the nerve fiber andprevent to sti ulate the neighbor fibers. e.g. like ainsulated copper wire winded in a otor.

    y The for ation of yelin sheath around the axon is

    calledmyelinogenesis. In the peripheral nerve itstarts at 4th onth of intrauterine life and co pletesonly in the 2ndyear after birth.

    y The covering of the yelin sheath is called


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    Properties of the nerve fiberExcitability


    Refractory period

    Su ation


    InfatigabilityAll or none law

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    Intraneural connective tissues


    Covering ofthe nerve orbundle


    Covering ofeach fascicle


    Covering of eachnerve fiber

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    Epineuriumy This is a connective layer.

    yActs as cushion during the ove ent of the fibers,

    protecting fascicles fro the external trau a andaintain the local oxygen supply syste via epineural

    blood vessels.

    yA ount of epineural connective tissue varies a ong

    the nerves and at different level within the sa e nerve.yWhere the nerve lies close to the bone or passes the

    joints, the epineuriu is ore abundant as the need ofprotection is greater in that locations.

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    Perineurium This a la ellar sheath that enco passes each fascicle.

    It has great echanical strength as well as

    bio echanical barrier. Strength: fascicle can be inflated with fluid up to

    pressure of about 1000 of Hg before the Perineuriuruptures.

    Biochemical barrier: che ically Perineuriu isolatesnerve fro their surrounding and preserves the ionicenviron ent of the interior of fascicle.

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    Endoneuriumy Co posed of fibroblast and collagen.

    y Interstitial pressure in the fascicle esp. endoneural

    fluid pressure (1.50.7 of Hg) is slightly elevatedco pared to subcutaneous (-4.70.8 of Hg) and

    uscle tissue (-22 of Hg).

    y This pressure is counteract by the perineuriu . When

    there is incision or injury to the perineuriu the nervefiber herniates affect icrocirculation & function

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    Microvascular system

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    y Peripheral nerve is well vascularized along with all

    three layers.y Both i pulse propagation and axonal transport

    depends up on the local oxygen supply. So theicroascular syste has a large reserve capacity.

    y The large blood vessels approach the nerve at differentseg ent along its course. Then they divide intoascending and descending branches.

    y These branches run longitudinally and frequentlyanasta ose with the vessels of perineuriu and

    endoneuriu .

    y This longitudinal syste consists of large arteriolesand venules of dia eter 50-100 .

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    yWithin each fascicle lies a longitudinally orientedcapillary plexus with loop for ation at various levels.The vessels penetrate the perineuriu obliquelythough the perineuriu . Due to the structural

    peculiarity, they are easily closed like valves.yWhen the endoneurial f luid pressure increases the

    intrafascicular blood flow decreases.

    y The built-in safety syste of longitudinal anasta oses

    provide wide range of safety.y E.g. if the scitica-tibial nerve co plex (15c ) is surgically separated

    fro the surrounding structure and regional nutrient vessels, thereis no detectable reduction in blood circulation. ( for 7-8c distal tocut end.)

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    In the early e ryological develop ental stages, the length ofthe spinal cord is sa e as the length of spinal colou n.

    In the fully grown individual the spinal cord ends at conusedullaris. (L1 vertebra level)

    The spinal nerve leaves the spinal canal through intervertebralfora en in the lu bar and sacral spine therefore has to pass frothe point where it leaves spinal cord.( lower thoracic to L1).

    At lu bosacral level the spinal nerve roots co e out as abundle, which rese bles the tail of horse. So its called as caudaequina.

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    y Two types of nerve roots are found in the spinalcord level. Ventral/anterior root and dorsal/posterior root.

    yAnterior nerve root carries otor fibers and

    posterior nerve root carries the sensory fibers.y The cell body of the otor axon lies in the anterior

    horn cells spinal cord.

    y The cell body of the sensory axon lies in the dorsal

    ganglion which lies close to the intervertebralfora en. Unlike the nerve roots, the dorsalganglion are not enclosed with the CSF and


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    y Both otor and sensory nerve roots co bine to for

    the spinal nerve which passes though intervertebralfora en.

    yAfter the spinal nerve leaves the intervertebralfora en, they divide in to two ain branches called

    dorsal ra i and ventral ra i.y The arachnoid space and CSF surrounding each nerve

    root pair reduces caudally. Co pression injury ofnerve root ay induce an increase in the per eability

    of the endoneurial capillaries, resulting in the ede a.y The location where the nerve roots are tightly enclosed

    by the connective tissues, ore prone for co pressioninjury: entrap ent syndro e.

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    Membranous covering of spinal nervey The axons in the endoneuriu are separated fro the CSF

    by a thin layer of connective tissues called root sheath.

    y Root sheathis the structural analogue to pia ater that

    covers spinal cord. It is 2-5 layers.y The cells of the proxi al part of the outer layers of root

    sheath are si ilar to that of pia ater and the distal partare si ilar to the dura ater.

    y The cells of inner layer of the root sheath is si ilar to the

    cells perineuriu of peripheral nerve.

    y The inner layers of the root sheath constitue a barrierbetween the endoneuriu and CSF. This is a weak barrier which prevent the acro olecule only.

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    Microvascular system of spinal nerve

    y The seg ental arteries divide into three brancheswhen they approaches intervertebral fora en.

    y Anterior branch: posterior abdo inal wall and lu ber plexus

    y Posterior branch: paraspinal uscles and facet joints.

    y Inter ediate branch: content of spinal canal

    y The inter ediate branch joins the nerve