Ascending & Descending Tracts Of The Spinal Cord

Ascending & Descending Tracts Of The Spinal Cord

By

Dr.Faris Al-Haddad

M.B.Ch.B, PhD AnatomyCollege of Medicine,

Hawler Medical UniversityArbil, Iraq

e-mail: [email protected]

White matter of the spinal cord As in other regions of the CNS, white matter of

the spinal cord consists of a mixture of:

1. Nerve fibers, 2. Neuroglia, 3. Blood vessels.

It surrounds the gray matter, and its white color is due to the high proportion of myelinated nerve fibers.

The white matter, for purposes of description, may be divided into:

1. Anterior white column (or funiculus)

2. Lateral white column (or funiculi) ,

3. posterior white column (or funiculus),

4. Anterior white commisure.

1.1. Posterior columnPosterior column• Located between

the dorsal median sulcus & dorsal lateral sulcus

• Subdivided into two:

A. Fasciculi Gracilis:

B. Fasciculus Cunatus:

2. 2. Lateral Column:Lateral Column:• Located between

dorsal lateral & ventral lateral sulci

3. Anterior Column:3. Anterior Column:• Located between

the ventral median fissure & ventral lateral sulcus

4. Anterior White commissure is a bundle of nerve

fibers which cross the midline of the spinal cord just anterior to the gray commisure

Ascending and descending tracts of the Spinal Cord

Ascending tracts Descending tracts

Lat. spinothalamic corticospinal

Ant. spinothalamic reticulospinalFasciculi gracilis &

cuneatus tectospinalAnt. & Post.

Spinocerebellar Rubrospinal

Spinotectal Vestibulospinal

spinoreticular Descending autonomic

Spino-olivary -

Pain and temprature sensations:

Lateral spinothalamic tract (LST)

Destination Posterior central gyrus

3rd Order Neuron

Ventral posterolateral nucleus of Thalamus

PathwaysLateral spinothalamic,

Spinal lemniscus

2nd Order Neuron

Substantia gelatinosa

1st Order Neuron

Posterior root ganglion

Receptor Free nerve

endings

Notes on Lateral spinothalamic

tracts :• Posterolateral tract of Posterolateral tract of

Lissauer : Lissauer : 1st order neuron enters

posterior horn & divides into ascending and descending branches that travel for 1-2 segments, then terminate synapsing with 2nd order neurons in substantia gelatinosa.

• L. Spinoth.T. in cervical L. Spinoth.T. in cervical segment shows:segment shows:

1.1.Sacral fibersSacral fibers are lateral, while Cervical fibersCervical fibers are medial.

2.2.Pain fibersPain fibers are anterior, while Temperature fibersTemperature fibers are

posterior.

• As the LST ascends at medulla oblangata, is accompanied by :

a. anterior spinothalamic tract,

b. spinotectal tract.

Together they form

: Spinal Spinal

Lemniscus(SL).Lemniscus(SL).

Light touch and pressureAnterior spinothalamic tract

(AST)


Third-Order Neuron Ventral posterolateral nucleus of Thalamus

Pathways Anterior spinothalamic, Spinal lemniscus

Second-Order Neuron

Substantia gelatinosa of the contralateral dorsal

hornFirst-Order Neuron Posterior root ganglion

Receptor Free nerve endings

Notes on Anterior spinothalamic

tracts :• Posterolateral tract of Posterolateral tract of

Lissauer : Lissauer : 1st order neuron enters

posterior horn & divides into ascending & descend-ing branches that travel for 1-2 segments, then term-inate synapsing with 2nd order neurons in substan-tia gelatinosa.

• A. Spinoth.T. in cervical segment A. Spinoth.T. in cervical segment shows:shows:

Sacral fibersSacral fibers are lateral, while Cervical fibersCervical fibers are medial.

• As the AST ascends at medulla oblangata, is accompanied by :

a. lateral spinothalamic tract,

b. spinotectal tract.

Together they

form : Spinal Lemniscus Spinal Lemniscus

(SL).(SL).

Discriminative touch, vibratory sense, conscious muscle joint sense

Fasciculi gracilis and cuneatus (FG & FC)


Third-Order NeuronVentral posterolateral

nucleus of Thalamus of opposite side

Site of cross over Internal arcuate fibers

Second-Order Neuron

Nuclei gracilis and cuneatus in medulla

oblagata

PathwaysIpsilateral Fasciculi gracilis

& cuneatus & medial lemniscus

First-Order Neuron Posterior root ganglion

Receptor Meissner's corpuscles, pacinian corpuscles,

muscle spindles & tendon organs

Notes on FG and FC:• Fibers divide into : a. long ascending branches

(FG & FC) b. short descending

branches. • Both of (a & b) synapse with

cells in the : a. Posterior gray horn, b. Internuncial neurons, c. Anterior horn cells • These synapses are involved

with intersegmental reflexes.

Fasciculus Gracilis (FG)• present throughout the length of the spinal

cord• contains the long ascending fibers from the:1. Sacral spinal nerves2. Lumbar spinal nerves 3. lower 6 thoracic spinal nerves

Fasciculus Cuneatus (F.C) :

• contains the long ascending fibers from the :

1. upper six thoracic spinal nerves,2. all the cervical spinal nerves.

• The axons of the second-order neurons, called the internal arcuate fibersinternal arcuate fibers, cross the median plane, decussating with the corresponding fibers of the opposite side in the medulla as :

sensory decussation

• The fibers then ascend as a single compact bundle called medial lemniscus through the brainstem.

Cuneocerebellar tract:• Also called posterior external posterior external

arcuate fibers arcuate fibers

• 2nd order neuron axons of the nucleus cuneatus travel as to enter the cerebellum through the inferior cerebellar peduncle of the same side.

• The function of these fibers is to convey information of muscle joint sense to the cerebellum.

Unconcious Muscle Joint Sense Pathways to the Cerebellum

(Anterior Spinocerebellar Tract)

(Posterior Spinocerebellar Tract)

Destination Cerebellarcortex

Pathways

Through Superior & inferior cerebellar

peduncles respectivelyAnterior and posterior

spinocerebellarSecond-Order Neuron Nucleus dorsalisFirst-Order Neuron Posterior root ganglion

Receptor Muscle spindles, tendonorgans, joint receptors

AST

Other ascending tracts :

1. Spinotectal Tract 2. Spinoreticular Tract

3. Spino-olivary Tract

1.1.Spinotectal Tract : Spinotectal Tract : • This pathway provides afferent

information for spinovisual reflexes and brings about movements of the eyes & head in response to the source of the stimulation.

2. Spinoreticular Tract : The spinoreticular tract provides

an afferent pathway for the reticular formation, which plays an important role in influencing levels of consciousness.

3. Spino-olivary Tract: conveys information to cerebellum

from cutaneous proprioceptive organs.

The Descending Tracts of the Spinal Cord

(Corticospinal Tracts)

OriginPrimary motor cortex (area 4), secondary motor cortex (area 6),

parietal lobe (areas 3, 1, and 2)Pathway Corticospinal tracts

Pass through

Corona radiata, posterior limb of internal capsule & middle 3/5 of basis

pedunculi of midbrain

Site of crossover

in pyramids

of medulla

• 75-90% of fibers cross to contralateral side and descend in spinal cord as lateral corticospinal tracts . • 10-25% of fibers descend uncrossed in spinal cord as anterior corticospinal tracts and cross over at level of destination

Destination

Internuncial neurons or motor neurons

Notes about corticospinal tract:

• 2/3 of the fibers arise from the precentral gyrus ( area 4 & 6),

• 1/3 of the fibers arise from the postcentral gyrus ( areas 3, 1 and 2).

• In the medulla oblongata, In the medulla oblongata, the bundles become grouped together along the anterior border to form a swelling known as the pyramid (hence the alternative name, pyramidal tractpyramidal tract).

• The pyramidal tract controls rapid, skilled, voluntary movements, especially distal ends of limbs

• The pyramidal tract gives branches to cerebral cortex, basal nuclei, red nucleus, olivary nuclei, reticular formation. These branches keep the subcortical regions informed about the cortical motor activity.

Reticulospinal tracts

Origin Reticular formation

Pathway Reticulospinal tractsSite of

crossover Some cross at various levelsDestinatio

n Motor neurons

Function

Inhibit or facilitate voluntary move-ment, reflex activity, assist

hypothala-mus controls sympathetic, parasympa-thetic

outflows.

Notes about reticulospinal tract:

•From the pons, reticulospinal fibers are mostly descending uncrossed into the spinal cord and form the pontine reticulospinal tract.

•From the medulla, similar neurons send axons, which are crossed and uncrossed, to the spinal cord and form the medullary reticulospinal tract.

Tectospinal tract

Origin Superior colliculus of midbrain

Pathway Tectospinal tract

Site of crossover

Immediately decussates at posterior

Midbrain tegmentumDestinati

on Motor neurons

Function Reflex postural movements concerning sight

Notes about tectospinal tract:• Descends through spinal cord close to :

anterior median fissure.

• The majority of the fibers terminate in anterior gray column in the C1-C4 of the spinal cord by synapsing with internuncial neurons. These fibers are believed to be concerned with : turning of head to contralateral side in

response to visual stimulations

Rubrospinal tracts

Origin Red nucleus of midbrain

Pathway Rubrospinal tract

Site of crossover Immediately in midbrain

Destination Motor neurons in C1-C3

FunctionFacilitates activity of flexor

muscles and inhibits activity of extensor muscles in the upper limb

Notes about rubrospinal tract:• The neurons of the red nucleus receive

afferent impulses through connections with the :

1. cerebral cortex 2. cerebellum. This is believed to be an important

indirect pathway by which the cerebral cortex and the cerebellum can influence the activity of motor neurons of the spinal cord.

Vestibulospinal tract

Origin1. Medial vestibular nucleus

( medulla) 2. Lateral vestibular nucleus ( pons

)

Pathway1. Medial vestibulospinal tract from

MVN2. Lateral vestibulospinal tract from

LVNSite of

crossover1. Contain crossed & uncrossed

fibers2. Contains uncrossed fibers

Destination1.Dose not extend below T6 motor

neurons 2. Motor neurons of all spinal levels

Function Facilitates activity of extensor & inhibits flexor muscles

MLF= Medial Longitudenal Fasciculous

Notes about vestibulospinal tract:

• Lateral vestibulospinal tract is involved in:

1. maintenance of upright posture and balance;2. mediates head & neck movements in

response to vestibular sensory input

• Medial vestibulospinal tract mediates head movement while maintaining gaze fixation on an object

Descending autonomic fibers

OriginCerebral cortex, hypothalamus, amygdaloid complex, reticular

formation

PathwayDescending autonomic fibers

Probably through reticulospinal tract ?

Site of crossover

cross the midline in the brainstem ?

Destination

Sympathetic and parasympathetic outflows

Function Control sympathetic and parasympathetic systems

Notes about Descending autonomic tract:

• distinct tracts have not been recognized,

• investigation of spinal cord lesions has demonstr-ated that descending autonomic tracts do exist.

• probably form part of the reticulospinal tract.

Reflex Arc Its an involuntary response to a stimulus,

consists of the following anatomical structures:(1) a receptor organ, (2) an afferent neuron,(3) an efferent neuron, (4) an effector organ. A reflex arc involving only one synapse is referred to as a monosynaptic

reflex arc.

Lesion of spinal cord

Injury to the Ascending Tracts Within the Spinal Cord

• Lateral Spinothalamic Tract : injury results in :1. contralateral loss of pain sensations

below the level of the lesion, 2. contralateral loss of thermal sensations

below the level of the lesion, The patient will, therefore, not respond

to pinprick & cant recognize hot and cold objects placed in contact with the skin.

•Anterior Spinothalamic Tract injury of this tract produces • contralateral loss of light touch

sensations below the level of the lesion• contralateral loss of pressure sensations

below the level of the lesion• The patient will not feel the light touch of

a piece of cotton placed against the skin and cant feel pressure from a blunt object placed against the skin.

•Fasciculus Gracilis and Fasciculus Cuneatus

Injury results in:• Ipsilateral loss of position and movements

sensations of limbs below the level of the lesion.

• Ipsilateral loss of vibration sense below the level of the lesion

• Ipsilateral loss of two point discrimination below the level of the lesion.

•Tabes Dorsalis :• is caused by syphilis. • a selective destruction of nerve fibers at

the point of entrance of the posterior root into the spinal cord,

• specially in the lower thoracic and lumbosacral regions.

• Results in loss of some sensation & hypersensitivity others.

Descending tracts lesions

A. Upper Motor Neuron (UMN) Lesions: include:

1. Lesions of corticospinal tracts (Pyramidal Tracts)

2. Lesions of descending tracts other than the Corticospinal Tracts (Extrapyramidal Tracts)

1. Lesions of corticospinal tracts (Pyramidal Tracts)

signs include • The Babinski sign is present. .• The superficial abdominal reflexes are

absent. • The cremasteric reflex is absent. • There is loss of performance of fine-

skilled voluntary movements, especially at the distal end of the limbs.

2. Extrapyramidal Tracts lesions:

• Spastic paralysis,Spastic paralysis, (lower limb extended, and the upper limb

flexed),• Exaggerated deep muscle reflexes in Exaggerated deep muscle reflexes in

some flexors,some flexors,• Clasp-knife reactionClasp-knife reaction. The muscles, after

resistance on stretching, suddenly give way.

2. Lower Motor Neuron (LMN) Lesions :

• Caused by any lesion ( ex. Poliomyelitis) destroying neurons in the anterior gray column or its axon in the anterior root or spinal nerve.

• Has the following clinical signs:1. Flaccid paralysis of muscles supplied.2. Atrophy of muscles supplied.3. Loss of reflexes of muscles supplied.

4. Muscular fasciculation (visible useless muscle twitching )

5. Muscular contracture (shortening of the paralyzed muscles).

6. Reaction of degeneration, a muscle will no longer respond to interrupted electrical stimulation

Spinal Shock syndromeSpinal Shock syndrome Following a spinal cord injury there will be

:• a short term loss of all neurological

activity below the level of injury. This loss of neurological activity include loss of motor, sensory reflex & autonomic function.

• due to temporary physiologic disorganisation of spinal cord function, may last 30-60 minutes or up to 6 weeks.

Complete Cord Transection Syndrome

• It can be caused by fracture dislocation of the vertebral column,

• The following clinical features : 1. Bilateral lower motor neuron paralysis 2. Bilateral spastic paralysis below the level of

the lesion (due to extrapyramidal tract injury)3. Bilateral loss of all sensations below the level

of the lesion. 4. Bladder and bowel functions are no longer

under voluntary control (injury to descending autono-mic fibers have been destroyed).

Anterior cord syndromeResults in :1. Bilateral LMN paralysis in the segment

of lesion,2. Bilateral spastic paralysis below level of

the lesion,3. Bilateral loss of pain, temprature & light

touch below the level of the lesion,4. Two point discrimination & vibratory and

proprioception sensations are preserved.

Central cord syndromeResults in :1. Bilateral LMN paralysis in the segment

of lesion,2. Bilateral spastic paralysis below the

level of the lesion with characteristic sacral sparing,

3. Bilateral loss of pain, temprature & light touch and pressure sensations below the level of the lesion with characteristic sacral sparing.

Brown-sequard syndrome (hemisection of the

spinal cord)Results in :1. Ipsilateral LMN paralysis in segment of

lesion2. Ipsilateral spastic paralysis below the level

of the lesion,3. Ipsilateral band of cutaneous anasthesia in

the segment of the lesion4. Ipsilateral loss of two point didcrimination &

vibratory and proprioceptive sense below the level of lesion.

4. Contralteral loss of pain & temprature sense below the level of the lesion,

5. Contralateral partial loss of two point discrim-ination sense below the level of the lesion.

Funicli ( Columns)Funicli ( Columns)1.1. Dorsal FuniculusDorsal Funiculus• Located between the

dorsal median sulcus & dorsal lateral sulcus• Subdivided into two: A. Fasciculi Gracilis:• Located between dorsal median sulcus & dorsal

intermediate sulcus• Found at all levels B. Fasciculus Cunatus:• Between dorsal intermediate sulcus & septum & the

dorsal lateral sulcus• Found at C1-T6

2 1

2. Lateral Funiculus:2. Lateral Funiculus:• Located between

dorsolateral & ventrolateral sulci

3. Ventral Funiculus:3. Ventral Funiculus:• Located between

the ventral median fissure & ventral lateral sulcus

Relationship BetweenRelationship Between Vertebrae No.Vertebrae No. Sp.Cord Segment Sp.Cord Segment

C1-C7 Add 1T1-T6 Add 2T7-T9 Add 3T10 L1-L2T11 L3-L4T12 L5L1 Sacral & Coccygeal

Health & Medicine

Ascending & Descending Tracts Of The Spinal Cord