anatomy of ascending and descending tracts
TRANSCRIPT
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