Ascending& Descending tracts

Dr. Israa M. SulaimanDepartment of Anatomy

IMS/MSU

The ascending tracts

By the end of the lecture, students should be able to

• define the ascending tract • enumerate the tracts according to their functional

components • explain general outline of neuronal chain of ascending

tracts• illustrate and trace the neuronal chain of each tract• apply anatomical knowledge to correlate with the

clinical condition in case of injury to these tracts

contents– function of nervous system in general– sensory system overview – spinal cord and nerve tracts

• ascending tracts– organization in general– ascending tracts – functional components

nervous systemcommunication

receive informationtransform it into impulses ( transduction )

transmit impulses to the CNScorrelate / coordinate

transmit impulses to the effector organs response / action

CENTRAL NERVOUS SYSTEM

integration / processing / modulating

stimulus

receptor neurone

motor / descending tracts

effector organ / response

PNStransmission

lower motor neurone

sensory / ascending tracts

Sensory system

• sensory information– three basic information

Exteroceptive information Interoceptive information Proprioceptive information

sensory information

are received and carried by ascending tracts

• exteroceptive sensation origin:- outside the body

e.g. temp, touch, light, sound, chemicals, mechanicalreceptors:- surface layer of skin, mucosa

• proprioceptive sensation origin:- within the body

e.g. muscles, joints, tendonsreceptors – deeper layer of skin, tendons, joints, GTO, muscle

spindles, ligaments

sensory information

from the peripheral sensory endings

is conducted through the nervous system

by a series of neurones

information• conscious sensation

– reach the cerebral cortex

• unconscious sensation– reach to the areas other than cortex

spinal cord• Grey matter

– mostly made up of cell bodies of neurone

• White matter– composed of nerve fibres ( ascending and descending tracts )

embedded in neuroglial cells

nerve fibres• enter the spinal cord through posterior nerve root• after entering the spinal cord

sorted out and segregated into nerve bundles, tracts( origin, function, termination )

ascending tracts

bundles of nerve fibres

linking

spinal cord with higher centres of the brain

convey information

from soma / viscera to higher level of neuraxis

ascending sensory pathway are organized inthree neuronal chain

- First order neurone

- Second order neurone

- Third order neurone

First order neurone

• cell body in posterior root ganglion• peripheral process connects with sensory

receptor ending• central process enter the spinal cord through

the posterior root• synapse with second order neuron in spinal

gray matter

dorsal rootdorsal rootganglion

spinalnervedorsal

horn

FIRST ORDER NEURON

Second order neurone

• cell body in posterior gray column of spinal cord

• axon crosses the midline ( decussate )

• ascend & synapse with third order neuron in VPL nucleus of thalamus

SECOND ORDER NEURON

• cross the mid line

• in front of central canal

VPL

1st

2nd

Third order neurone

• cell body in the thalamus

• give rise to projection fibres to the cerebral cortex, postcentral gyrus ( sensory area )

ascending sensory pathway

( in general form )

from sensory endings

to

cerebral cortex

( note the three neurons chain )

ascending tracts in spinal cord

Tracts & their functional components

• lateral spinothalamic tract – pain, temperature

• anterior spinothalamic tract– touch, pressure

• posterior white column– conscious proprioceptive sense,

discriminative touch, vibratory sense

• spinocerebellar tract / cuneocerebellar tract – unconscious information from muscle,

joints, skin, subcutaneous tissues

sensation receptors pathways destination

Pain and temperature Free nerve endings Lateral STTSpinal lemniscus

Postcentral gyrus

Light touch and pressure Free nerve endings Anterior STTSpinal lemniscus

Postcentral gyrus

Discriminative touch, vibratory sense, conscious muscle joint sense

Meissner’s corpuscle, pacinian corpuscles, muscle spindles, tendon organs

Fasciculus gracilis and cuneatusMedial lemniscus

Postcentral gyrus

Main somatosensory pathways

Lateral spinothalamic tract pain and thermal impulses

( input from free nerve endings, thermal receptors )

• transmitted to spinal cord in delta A and C fibres

• central process enters the spinal cord through posterior nerve root, proceed to the tip of the dorsal gray column

• the central process of 1st order neuron

synapse with cell body of 2nd order neuron

in substantia gelatinosa of posterior gray column of the spinal cord

• the axon of 2nd order neuron

cross to the opposite side

in the anterior gray and white commissure and ascend in contralateral white column as lateral spinothalamic tract

• end by synapsing with 3rd order neuron in the ventral posterolateral nucleus of thalamus

• axon of the 3rd order neuron passes through the posterior limb of internal capsule and corona radiata to reach the postcentral gyrus of cerebral cortex ( area 3, 1 and 2 )

pain and temperature pathways

Clinical application

destruction of LSTT• loss of

– pain and thermal sensation – on the contralateral side– below the level of the lesion

patient will not

respond to pinprick

recognize hot and cold

Anterior spinothalamic tractlight touch and pressure impulses ( input from free nerve endings, Merkel’s tactile disks )

• First order neuron– dorsal root ganglion( all level )

• Second order neuron – in the dorsal horn, cross to the opposite side (decussate)– ascend in the contralateral ventral column as ASTT– end in VPL nucleus of thalamus

• Third order neuron – in the VPL nucleus of thalamus– project to cerebral cortex ( area 3, 1 and 2 )

touch and pressure pathways

Clinical application

destruction of ASTT

loss of touch and pressure sense – below the level of lesion – on the contralateral side of the body

Fasciculus gracilis and fasciculus cuneatusdiscriminative touch, vibratory sense and conscious muscle joint sense ( inputs from pacinian corpuscles, Messiner’s corpuscles, joint receptors, muscle spindles and Golgi tendon organs )

• axon of 1st order neuron enter the spinal cord

• passes directly to the posterior white column of the same side ( without synapsing )

• long ascending fibres travel upward in the posterior column of the same side as fasciculus gracilis and fasciculus cuneatus– ( FG – carrying fibres from lower thoracic, lumbar and sacral regions /

including lower limbs )– ( FC - only in thoracic and cervical segments / including upper limb

fibres )

• synapse on the 2nd order neuron in the nucleus gracilis and cuneatus of medulla oblongata of the same side.

lower 6 thoracic segments

lumbar segments

sacral segments

cervical segments

upper 6 thoracic segments

fasciculus gracilis

fasciculus cuneatus

[ nucleus G & C ]

in medulla

GC

• axons of 2nd order neuron

“ internal arcuate fibres ” cross the median plane

( sensory decussation )

• ascend as medial lemniscus

through medulla oblongata, pons, and midbrain

• synapse on the 3rd order neuron in ventral posteriolateral nucleus of thalamus

• axon of 3rd order neuron leaves and passes through the internal capsule, corona radiata to reach the postcentral gyrus of cerebral cortex area 3, 1 and 2 )

pathways for

conscious proprioception

discriminative touch

vibratory sense

Clinical application destruction of fasciculus gracilia and cuneatus

• loss of muscle joint sense, position sense, vibration sense and tactile discrimination

• on the same side • below the level of the lesion

(extremely rare to have a lesion of the spinal cord to be localized as to affect one sensory tract only )

Posterior & anterior spinocerebellar tract• transmit unconscious proprioceptive information to

the cerebellum

• receive input from muscle spindles, GTOs and pressure receptors

• involved in coordination of posture and movement of individual muscles of the lower limb

First order neuron• in dorsal root ganglion • axons end in nucleus dorsalis of Clarke

Second order neuron• cell body in nucleus dorsalis of Clarke • give rise to axons ascending to the cerebellum of the

same side ( anterior – crossed & uncrossed fibres / posterior – uncrossed fibres)

muscle joint sense pathways to cerebellum

• Spinotectal tract– passes pain, thermal, tactile information to superior

colliculus for spinovisual reflexes• cross the median plane• synapse in the superior colliculus • integrate visual and somatic sensory information

( it brings about the movement of eye and head towards the source of information )

• Spinoreticular tract– uncrossed fibres, synapse with neurones of reticular

formation (important role in influencing level of consciousness)

• Spino-olivary tract

spinotectal tract

spinoreticular tract

spino-oloivary tract

clinical application

• relief of pain

posterior rhizotomy (posterior root)

cordotomy (lateral STT)

• Injury

hemisection of spinal cord

• diseasestabes dorsalis / syringomyelia /

vascular

Hemisection of the spinal cord ( Brown Sequard’s syndrome )

• Dorsal column damage • Lateral column damage • Anterolateral column damage

• Damage to local cord segment and nerve roots

spinal cord hemisection

below the level of lesion

on the side of lesion

lateral column damage

• UMNL

dorsal column damage

• loss of position sense

• loss of vibratory sense

• loss of tactile discrimination

anterolateral system damage

• loss of sensation of pain and temperature on the side opposite the lesion

local segment side of lesion

Dorsal Root• irritate• destruction

Ventral root• flaccid paralysis

Lesions of central gray matter

• seen in syringomyelia ( progressive cavitation around or near the central canal of spinal cord especially in cervical segments )

• interrupt fibres of lateral spinothalamic tract that passes in front of the central canal

• loss of pain and temperature sensibility on both sides ( proprioception and light touch is spared )

sensory dissociation

Posterior root lesionsseen in tabes dorsalis ( neurosyphilis ) • bilateral degeneration of posterior root and

posterior funiculus ( particularly in lower segments of spinal cord )

ClinicallyInitial stage• Irritation - paraesthesia• Intermittant of attack of sharp painLater • decreased sensitivity to pain• loss of muscle stretch reflexes• loss of position sense, posture senses• positive Romberg sign ( visual compensation ) • walk with legs apart, high stepping gait

blood supply of spinal cord Anterior spinal artery

Posterior spinal arteries

Segmental spinal arteries

- radicular arteries

Feeder arteries

- Adamkiewicz

posterior 3rd of spinal cord

dorsal column

penetrating branches

• anterior and part of gray matter

circumferential branches

• anterior white matter

dorsal 1/3rd resulting from occlusion of the posterior spinal artery

ventral 2/3rd resulting from occlusion of the anterior spinal artery

Descending tractsBy the end of this lecture, students should be able to:define the tract enumerate the tracts according to their functional components illustrate and trace the neuronal chain of each tractapply their knowledge of anatomy to correlate with the clinical condition in relation to the injury to these tracts

Motor systemareas of the nervous system that are responsible for controlling movements

cerebellum

premotor cortex motor cortex

motor unit muscle spindle

pyramidal tract

sp cd internreurons & central pattern generator

extrapyramidal tracts

premotor cortex SMA PMC basal ganglia

cortical sensory area

MOTOR SYSTEM

I

II

III

IV

V

Level I• initiation, planning, programming of movements

– in response to desire to move

( probably originate in the limbic system and posterior parietal cortex ) – desire is translated into movements( basal ganglia and their cortical projections in the frontal lobe-SMA, PMC )

Level II• coordination of movements

– cerebellum( compare the intended movement / actual movement )

Level III• descending pathways

– pyramidal tract - CoSt• originates in the motor, premotor and somatosensory

corticies

• synapse direclty on MN , IN

– extrapyramidal tract – VeSt, ReSt, TeSt, RuSt• originate from subcortical structures• receive inputs from motor cortex• complex distribution, synapse on MN, IN

Level IV• motor organization in spinal cord

– alpha & gamma neurons– Renshaw’s cells– interneurons / CPGs – descending tracts

• CoSt, RuSt distal musculature – fine skilled movement• VeSt- ReSt- TeSt axial, proximal musculature – balance, posture

Level V• final common pathway

primary structure

responsible for translating

desire into a movement is

the basal ganglia

Introduction

• brain exerts powerful and subtle influences upon the activity of the voluntary musculature ( modulate, regulate, bias the activities of LMN )

– through the descending pathways

descending tracts• segregated bundles of nerve fibres in the white

matter of the spinal cord descending from the supraspinal centres

referred to as upper motor neurons ( UMN )• are concerned with somatic and visceral motor

activity• cells of origin lie in cerebral cortex and brain stem • regulate the LMN activity

motor homunculus

cerebral cortex

lower motor neurons ( LMN )

motor neurons that innervate the voluntary muscles• in anterior gray column of spinal cord / • motor nuclei of brainstem

– innervate skeletal muscles

form final common pathway

LMN

LMNconstantly bombarded by • nerve impulses( excitatory or inhibitory )

that descend from cerebral cortex, pons, midbrain and medulla

• sensory inputs from the posterior root

upper motor neurons ( UMN )

• the descending supraspinal pathways that influence the activity of the LMN e.g. CoSt, CoBt, RuSt, TeSt, ReSt, VeSt

UMN• control voluntary motor activity• maintenance of posture & equilibrium • control of muscle tone and reflex activity

generally exerts their effect • on groups of muscles ( not on one specific muscle )

• reciprocally on agonist and antagonist muscle group

cerebral cortex – midbrain - pons - medulla oblongata

descending tracts

LMN

sensory inputs

Corticospinal tract

• arises from the pyramidal cells of cerebral cortex– fibres travel through

• corona radiata• posterior limb of the internal capsule • cerebral peduncle ( middle 3/ 5th )

• pons • medulla oblongata ( passed through the pyramids )

• at the caudal part of medulla oblongata• most of the fibres 90 % cross the mid line

(motor decussation)

– descend in the lateral column as LCST– terminate on LMN of anterior gray column at all spinal

level

• remaining uncrossed fibres descend as ACST – eventually fibres cross the mid line and terminate on

LMN of anterior gray column of respective spinal cord segments

motor decussation

medulla oblongata

corticospinal tract

for fine skilled movements

Rubrospinal tract• nerve cells in red nucleus

( tegmentum of midbrain at the level of superior colliculus )

• nerve fibres / axons – cross the mid line – descend as rubrospinal tract

• through pons and medulla oblongata

• terminate anterior gray column of spinal cord

( facilitate the activity of flexor muscles )

Tectospinal tract• nerve cells in superior colliculus of

the midbrain

• nerve fibres/ axons– cross the mid line – descend close to medial longitudinal

fasciculus

• terminate in the anterior gray column of upper cervical segments of spinal cord

( responsible for reflex movement ofhead & neck in response to visual stimuli )

Vestibulospinal tract• nerve cells in vestibular nucleus

(in the pons and medulla oblongata– received afferents from inner ear

and cerebellum

• axons descend uncrossed – through medulla and through the

length of spinal cord

• synapse with neuron in the anterior gray column of the spinal cord

( balance by facilitate the activity of the extensor muscles )

Reticulospinal tract• nerve cells in reticular formation • fibres pass through

– midbrain, pons, and medulla oblongata

• end at the anterior gray column of spinal cord – control activity of motor neurons

(influence voluntary movement and reflex activity )

reticulospinal tract

clinical application

• pyramidal tract

refer to corticospinal tracts

• extrapyramidal tract

other than corticospinal tract

( VeSt, ReSt, TeSt, RuSt )

upper motor neuron lesion

• Babinski sign ( extensor plantar response )• Superficial abdominal reflexes ( absent )• Cremasteric reflex ( absent )• Loss of performance of fine skilled voluntary movement

lower motor neuron lesion

• flaccid paralysis• atrophy of muscles• loss of reflexes• muscular fasciculation• muscular contracture

extrapyramidal tract lesions

• severe paralysis with little or no atrophy• spasticity or hypertonicity• exaggeration of deep muscular reflexes and clonus• clasp-knife reaction

These motor pathways are complex and multisynaptic, and regulate:

• Axial muscles that maintain balance and posture

• Muscles controlling coarse movements of the proximal portions of limbs

• Head, neck, and eye movement

dorsal column

lateral STT

anterior STT

pyramidal tracts

extrapyramidal tracts

nerve roots

spinal cord hemisection

thank you