Nerve Pathways - Functions, Lesions and Adhesions

8/7/2019 Nerve Pathways - Functions, Lesions and Adhesions

1/33

Nerve Pathways:Nerve Pathways:

Functions, Lesions and AdhesionsFunctions, Lesions and Adhesions

D.Robbins


2/33

Spinal cordSpinal cord

The spinal cord is a cylinder of CNS. The spinal cord exhibits subtle cervicaland lumbar (lumbosacral) enlargements produced by extra neurons insegments that innervate limbs. The region of spinal cord caudal to thelumbar enlargement is conus medullaris. Caudal to this, a terminal filamentof glial tissue extends into the tail.

A spinal cord segment=

a portion of spinal cord that gives rise to a pair(right & left) of spinal nerves. Each spinal nerve is attached to the spinalcord by means of dorsal and ventral roots composed of rootlets. Spinalsegments, spinal roots, and spinal nerves are all identified numerically byregion, e.g., 6th cervical (C6) spinal segment.


3/33

Nerve rootsNerve roots

Both the spinal cord (CNS) and spinal roots

(PNS) are enveloped by meningeswithin

the vertebral canal. Spinal nerves (which are

formed in intervertebral foramina) are

covered by connective tissue (epineurium,perineurium, & endoneurium) rather than

meninges.

Sacral and caudal spinal roots (surrounding

the conus medullaris and terminal filamentand streaming caudally to reach

corresponding intervertebral foramina)

collectively constitute the cauda equina.

MORE SUITABLE IMAGE NEEDED

Image taken from

wikipedia


4/33

Afferent NervesAfferent Nerves

Primary afferent neurons have their unipolarcell bodies in spinal ganglia.

Their axons traverse dorsal roots, penetratethe spinal cord (at the dorsolateral sulcus)and bifurcate into cranial and caudal

branches which extend over severalsegments within white matter of the dorsalfuniculus.

PrimaryAfferent Neuron = the first neuron in a spinal reflex or ascending spinal pathway.

Collateral branches from the cranial andcaudal branches enter the gray matter tosynapse on interneurons and projectionneurons (or directly on efferent neuronsfor the myotatic reflex).

In some cases (discriminative touch), thecranial branches of incoming axonsascend directly to the brainstem wherethey synapse on projection neurons ofthe pathway.


5/33

Spinal Cord Cross SectionSpinal Cord Cross Section

Image taken from: http://cas.bellarmine.edu/tietjen/HumanBioogy/SpinalCord01.gif


6/33

Ascending Pathways:Ascending Pathways:

In general, pathways may be categorised into three broadfunctional types:

1) Conscious discrimination/localisation (e.g., pricking pain,warmth, cold,discriminative touch, kinesthesia) requires a specific ascending spinal pathway to

the contralateral thalamus which, in turn, sends an axonal projection to the cerebralcortex. Generally there are three neurons in the conscious pathway and the axon ofthe projection neuron decussates and joins a contralateral tract.

2)Affective related (emotional & alerting behavior) information involves ascendingspinal pathways to the brainstem. Projection neurons are non-specific. They receivesynaptic input of different modalities and signal an ongoing magnitude of sensory

activity, but they cannot signal where orwhat activity.

3) Subconscious sensory feedback for posture/movement control involves ascendingspinal pathways principally to the cerebellum or brainstem nuclei that project to thecerebellum. Generally there are only two neurons in a subconscious pathway andthe axon of the projection neuron joins an ipsilateral tract.


7/33

Nerve pathwaysNerve pathways

Ascending Tracts

Tract Signal function

Dorsal columnsVibration, tactile sensation, conscious

proprioception

Spinocerebeller Proprioception

Spinothalamic (lateral andanterior)

Pain, temperature, itch (lateral), crude

touch (anterior)

Spinoreticular PainSpinomesencephalic Pain

Spino-cervico-thalamic Pain (touch?)

Spinohypothalamic Pain


8/33

Dorsal Column and SpinocerebellarDorsal Column and Spinocerebellar

PathwaysPathways Dorsal columnpathway carries

infoon tactile sensation,pressureand proprioception.

In the dorsal tract, the sensoryneuronssynapse in an areaknown asClarke's nucleus or

"Clarke's column".

Thisis a columnof relay neuroncell bodies within the medial graymatter within the spinal cord inlayer VII (just beneath the dorsalhorn),specifically betweenT1-L1.These neurons thensend axons

up the spinal cord and formsynapsesin the accessory (lateral)cuneate nucleus, lateral to thecuneate nucleusin the medulla.

Spinocerebellarpathway carriesinfoonproprioception

Clarkes

Column(L1-T1)

C

G

Z

Thalamus

N.B. cerebellar feedback actually occursposteriorly not laterally,howeverin a 2D diagram its easier to represent it this way.


9/33

Spinoreticular and SpinothalimicSpinoreticular and Spinothalimic

pathwayspathways The Spinothalamic Tract, like theDorsal Column-Medial LemniscusTract, use three neurons to conveysensory information from theperiphery to conscious level at thecerebral cortex.

The Spinothalamic tract carriesinformationonpain, temperature andcrude touch.

The Spinoreticularpathway carriesinfoonpain, temperature and crudetouch.

Thalamus Thalamus

P

M

N.B. cerebellar feedback actually occursposteriorly not laterally,howeverin a 2D diagram its easier to represent it this way.


10/33

Descending Spinal Pathways:Descending Spinal Pathways:

Axons of brain projection neurons travel in descending tracts in spinal whitematter. They arise from various locations in the brain and synapse primarilyon interneurons within the spinal cord.

By synapsing on interneurons, descending tracts regulate:

1) spinal reflexes;

2) excitability of efferent neurons (for posture and movement); and

3) excitability of spinal projection neurons, i.e., the brain is able to regulatesensory input to itself. In some cases, descending tracts affect axonterminals of primary afferent neurons, blocking release of neurotransmitter(presynaptic inhibition).


11/33

Descending Tracts

Tract Signal function

Corticospinal (pyramidal) Fine voluntary motor control of the limbs.Thepathway also controls voluntary body posture

adjustments.

Rubrospinal Involved ininvoluntary adjustment of arm positioninresponse tobalance information;support of the body.

Reticulospinal (1) Pontine Regulates variousinvoluntary motor activities andassistsinbalance (leg extensors). Some patternmovements e.g.stepping

(2) Medullary Inhibits firing ofspinal and cranial motorneurons,control of antigravity muscles.

Vestibulospinal (1) Medial It is responsible for adjusting posture to maintainbalance (neck muscles).(2) Lateral It is responsible for adjusting posture to maintain

balance (body/lower limb).

Tectospinal Controls head and eye movements, Involved ininvoluntary adjustment of head positionin response to

visual information.

Nerve pathwaysNerve pathways


12/33

Corticospinal tractCorticospinal tract

Travels from the cerebral cortex down to

the spinal cord.

CST

actually consistsof twoseparatetractsin the spinal cord: the lateral

corticospinal tract and the anterior

corticospinal tract. Contains mostly

motor axons.

Referred to as apyramidal tractas

when the tract passes the medulla,it

forms a dense bundle ofnerve fibres

that isshaped somewhat like a pyramid

Lateral

CST

Anterior

CST


13/33

Rubrospinal tractRubrospinal tract

Travels from the cerebral cortex down to

the spinal cord via the red nucleus.An

extra-pyramidal motor tract.

Its main role is the mediationof voluntary

movement. It is responsible for large

muscle movement such as the arms andthe legs as well as for fine motor control. It

facilitates the flexion and inhibits the

extensionin the upper extremities


14/33

Reticulospinal TractReticulospinal Tract

An extra-pyramidal motor tractwhich travels from the reticularformation.

The tract is divided into twoparts, themedial (orpontine) and lateral (or

medullary) reticulospinal tracts (MRSTand LRST).

1. Integratesinformation from themotorsystems to coordinate automatic

movementsof locomotion and posture.

2.Facilitates and inhibits voluntarymovement,influences muscle tone.

P

M


15/33

Vestibulospinal TractVestibulospinal Tract

Inputsoriginate from the labyrinthinesystem via the vestibularnerve andfrom the cerebellum.

The medial part of thevestibulospinal tract projectbilaterally down the spinal cord andtriggers the cervical spinal circuits,controlling a correct positionof thehead and neck.

The lateral part of the

vestibulospinal tract projectsipsilateral down to the lumbarregion.There it helps to maintain anupright and balanced posture bystimulating extensor motorneuronsin the legs.

V


16/33

Descending PathwaysDescending Pathways

Pathway Upper limb Lower limb

Cortico/-pyramidalThisTract functions to modulate the activity ofAlpha

or Gamma MotorNeurons as directed by the Motor

Cortex.

Rubro-spinal Stimulates flexors

Reticulo-spinalMedullary inhibits extensors and excites flexors

Pontine excites extensors and inhibits flexors

(Generally upper limb)

Vestibulo-spinal

Doesnt affect upper limbs

but helpsposition head and

neck in response tobody

tilting (medial)

Stimulates extensors

(lateral)

Tecto-spinal Control of head,neck and eye movements.


17/33

Spinal Cord Cross SectionSpinal Cord Cross Section

Image taken from; http://img.medscape.com/pi/emed/ckb/clinical_procedures/1134815-1148570-1177.jpg


18/33

Gray matterorganisationGray matterorganisation

Two schemes have evolved for organizing neuroncell bodies within gray matter. Either may be usedaccording to which works best for a particularcircumstance.

1) Spinal Laminaespinal gray matter is dividedinto ten laminae (originally based on observations ofthick sections in a neonatal cat). The advantage is thatall neurons are included. The disadvantage is thatlaminae are difficult to distinguish.

I-VI: Posterior/Dorsal horn

Lamina I: Posterormarginal nucleus

Laminae II/III: Substansia gelatinosa

Laminae III/IV/V: Nucleuspropius

Lamina VI: Nucleus dorsalis

VII-IX: Anterior/Ventral hornLamina VII: Intermediolateral nucleus

Lamina VIII: Motorinterneurons

Lamina IX: Motorneurons which also contain the Onufsnucleusin

the sacral region

Lamina X: Neurons bordering central canal


19/33

Spinal NucleiSpinal Nuclei

2) Spinal Nucleirecognizable clusters of cells are identified as nuclei [anucleus is a profile of a cell column]. The advantage is that distinct nuclei aregenerally detectable; the disadvantage is that the numerous neurons outside ofdistinct nuclei are not included


20/33

Image taken from: http://images3.wikia.nocookie.net/psychology/images/thumb/c/c0/Medulla_spinalis_-_Substantia_grisea_-_English.svg/400px-Medulla_spinalis_-_Substantia_grisea_-_English.svg.png


21/33

Motor NeuronsMotor Neurons

Motor neurons are split into two groups: Upper and Lowermotor neurons.

Upper motor neurons originate in the motor regionof the cerebral

cortex of the brainstem and carry motorinformation down to thefinal commonpathway, that is, any motorneurons that are notdirectly responsible forstimulating the target muscle.

The cell bodiesof these neurons are some of the largest in thebrain, approaching nearly 100m in diameter.

These neurons connect the brain to the appropriate level in thespinal cord, from which point nerve signals continue to the musclesby meansof the lower motor neurons.lower motor neurons.


22/33

Motor neuronsMotor neurons

Lower motor neurons (LMNs) are the motorneurons connecting

the brainstem and spinal cord to muscle fibers, transmitting nerve

impulses from the upper motorneurons to the muscles.A lower

motorneuron's axon terminateson an effector (muscle).

Lower motorneurons are classified based on the type of muscle

fibre they innervate:

Alpha motorneuronsAlpha motorneurons (-MNs)innervate extrafusal muscle fibers, the most

numerous type of muscle fibre and the one involved in muscle contraction.

Gamma m otorneuronsGamma motorneurons (-MNs)innervate intrafusal muscle fibers, which

together with sensory afferents compose muscle spindles.These are part of

the system forsensing body position (proprioception).


23/33

Descending Pathway LesionsDescending Pathway Lesions

Anupper motor neuron lesion is a lesionof the neural pathwayabove the anterior horn cell or motornucleiof the cranial nerves.

Thisisin contrast to a lower motor neuron lesion, which affects

nerve fibers travelling from the anterior hornof the spinal cord to therelevant muscle(s).

Upper motor neuron lesions are indicated by:

Spasticity,increase in tone in the extensor muscles (lower limbs)or flexor muscles (upperlimbs)

Clasp-knife response where initial resistance to movement is followed by relaxation

Weakness in the flexors (lower limbs)or extensors (upper limbs),but no muscle wasting

Increase Deep tendon reflex (DTR)

Presence ofBabinski sign


24/33

Descending Lesions cont.Descending Lesions cont.

Damage to lower motor neurons, lower motor neurone lesionslower motor neurone lesions (LMNL)

causes:

Decreased tone

Decreased strength

And:

Decreased reflexes in affected areas.

These findings are in contrast to findings in upper motor neurone lesions.

LMNL is indicated by:

Abnormal EMG potentials, fasciculations, paralysis, weakening of muscles, and

neurogenic atrophy of skeletal muscle.


25/33

Ascending Pathway LesionsAscending Pathway Lesions

Lossofsensory input from relevant pathway E.g. Spinothalamic tract

Unilateral lesion usually causes contralateral anaesthesia (lossofsensation (pain andtemperature)).Anaesthesia will normally begin 1-2 segmentsbelow the level of lesion,affecting all caudal body areas.Thisis clinically tested by using pinpricks.

If le sionis hemisection (halfway across the spinal cord) (causing hemiplegia))itis known asBrown-Squard syndrome.

Brown-Squard syndrome may be caused by a spinal cord tumour, trauma (such as agunshot wound orpuncture wound to the neck orback),ischemia (obstructionof ablood vessel),orinfectiousorinflammatory diseasessuch as tuberculosis,or multiplesclerosis.

Any presentationofspinal injury which is anincomplete lesion canbe called apartial Brown-Squard orincomplete Brown-Squard syndrome,so long asit hascharacterized by featuresof a motor losson the same side of the spinal injuryand lossofsensationon the opposite side.


26/33

Lesion signsLesion signs

Lesions have positive ornegative signs.

Positive (also called release phenomena) = abnormal and stereotyped

responses that are explained are explained by the withdrawal of tonic

inhibition (e.g. decerebrate rigidity).

Negative signs reflect the lossofparticular capacitiesnormally

controlled by the damaged systems.


27/33

Difference between positive andDifference between positive and

negative signs of lesionnegative signs of lesion

1.)Diseases affecting the descending pathways give rise to

spasticity whereas diseasesof motorneurons donot.

2.)Diseases affecting motorneurons directly result in denervationatrophy and reduced muscle volume, whereas this doesnot occur

with damage to the descending pathway.

3.)Damage to the descending systems tend tobe distributed more

diffusely in limbor face muscles and often affects large groupsofmuscles e.g. the flexors. In contrast, degenerationin the local

groupsof motorneurons tends to affect musclesin a patchy way

and may evenbe limited tosingle muscles.


28/33

AdhesionsAdhesions

A.)Anteriorly locatedforamnum magnum

tumour.

B.) Spondylotic

protrusionsinto thecervical canal.

C.) Intramedullary

glial tissue scaror

circumscribedoedema, asin

multiple sclerosis

and spinal cord

injury.

D.)Fracture of theodontoid process.

E.) Compression

fracture of thoracic

process, withkyphtoic

angulation.

F & G.) Pedicles

deformed byosteophytic spurs.

The following information and images were all taken from: Biomechanicsof the Nervous System: Breig Revisited(http://www.neurodynamicsolutions.com/breig-revisited.php)


29/33

Fissure FormationFissure Formation

Sitesof tearing in thecervical cord resulting

from compressionby a

body impinging onit

from (A) anterior and

(B)posterior directions.

A.)A transverse tearin

the posteriorside results

from an anterior

compression combined

with cervical extension.

B.)A transverse tearin

the anteriorside of the

cord occurs from a

posterior compression

irrespective of whether

the cervical canal is

flexed or extended.


30/33

Effects of scar tissueEffects of scar tissue

Scar tissue occursinnormal

tissue after damage and

forms with higher

collagenous content than thatof the original tissues.

This resultsin a stiffer

structure that adapts

differently topressure in

either tensionor

compression that the originaltissues.


31/33

Formation of vortices in cord pulpFormation of vortices in cord pulp

Extrusionof cord substance by

fractured or displaced bone usually

continues forsome time after a

transverse fissure has appeared.

Viscous tissue elements are therefore

forced into the pial sheath and flow in

cranial and caudal directions.

The flow is augmented by the elastic

retractionof the membranesof the

severed nerve fibres.The resistance to

flow canset up vortices.


32/33

Influence ofposture on adhesionsInfluence ofposture on adhesions

Impingement e.g.

marginofpetrous

bone, calcified tissue,

tumour.

Clivus tumour,or

anterior locatedforamen magnum

tumour.

Intramedullary firm

body setting up

bending tensile

stresses.

Herniated lumbar disc

creating stressin

nerve roots.

Flexion

exacerbates all

stressesin the

spinal cord no

matter what the

level!!


33/33

Questions?Questions?

Thanks for listeningThanks for listening

Documents

Nerve Pathways - Functions, Lesions and Adhesions