DESCENDING TRACTSDESCENDING TRACTS

Ashish Patil MD/PhDAshish Patil MD/PhDAll Saints School of Medicine All Saints School of Medicine

DominicaDominica

Generalizations: Motor PathsGeneralizations: Motor PathsTypical descending pathway consists of a Typical descending pathway consists of a

series of two motor neurons:series of two motor neurons:Upper motor neurons (UMNs)Upper motor neurons (UMNs)Lower motor neurons (LMNs)Lower motor neurons (LMNs)

Upper Motor NeuronsUpper Motor Neurons Are entirely within the CNS.Are entirely within the CNS. Originate in:Originate in:

Cerebral cortexCerebral cortexCerebellum Cerebellum Brainstem Brainstem

Form descending tracts mainly classified as:Form descending tracts mainly classified as: 1) Pyramidal Tracts: 1) Pyramidal Tracts:

a) Corticospinal tracta) Corticospinal tract b) Corticobulbar tractb) Corticobulbar tract 2) Extra Pyramidal Tracts2) Extra Pyramidal Tracts

Lower Motor NeuronsLower Motor NeuronsBegin in CNS.Begin in CNS.From anterior horns of spinal cord.From anterior horns of spinal cord.From brainstem cranial nerve nuclei.From brainstem cranial nerve nuclei.Made up of alpha motor neurons Made up of alpha motor neurons (A-(A-αα))..Make up spinal and cranial nerves.Make up spinal and cranial nerves.

Pyramidal SystemPyramidal SystemCharacteristics:Characteristics:

Upper motor neurons:Upper motor neurons:

Pyramidal SystemPyramidal SystemComponents:Components:

Corticospinal TractCorticospinal TractCorticobulbar TractCorticobulbar Tract

Corticospinal TractCorticospinal Tract Nerve fibers arise in Nerve fibers arise in

Pyramidal layer Layer Pyramidal layer Layer V of Cerebral cortexV of Cerebral cortex

CST origin Cerebral CortexCST origin Cerebral Cortex primary motor cortex primary motor cortex

(about 30%)(about 30%) supplementary motor supplementary motor

area and the premotor area and the premotor cortex (together also cortex (together also about 30%), about 30%),

somatosensory somatosensory cortex,parietal lobe, cortex,parietal lobe, and cingulate gyrus and cingulate gyrus supplies the restsupplies the rest

CST origin Cerebral CortexCST origin Cerebral Cortex primary motor cortex primary motor cortex

(about 30%)(about 30%) supplementary motor supplementary motor

area and the premotor area and the premotor cortex (together also cortex (together also about 30%), about 30%),

somatosensory somatosensory cortex,parietal lobe, cortex,parietal lobe, and cingulate gyrus and cingulate gyrus supplies the restsupplies the rest

CST course Cerebrum and CST course Cerebrum and MidbrainMidbrain

travel through the travel through the posterior limb of the posterior limb of the internal capsule in the internal capsule in the forebrain, to enter the forebrain, to enter the cerebellar peduncle cerebellar peduncle at the base of the at the base of the midbrainmidbrain

CST Pass through Mid Brain, CST Pass through Mid Brain, and Pons to Medullaand Pons to Medulla

In MedullaIn Medulla The corticospinal The corticospinal

tract, along with the tract, along with the corticobulbar tract, corticobulbar tract, form two pyramidal form two pyramidal bumps (Medullary bumps (Medullary Pyramids) on either Pyramids) on either side of the medulla of side of the medulla of the brain stemthe brain stem

CST MedullaCST Medulla At the level of medulla 90% At the level of medulla 90%

of the fibers cross over to of the fibers cross over to the midline and called as the midline and called as lateral corticospinal tractlateral corticospinal tract

8% of the fibres are 8% of the fibres are uncrossed called as uncrossed called as anterior corticospinal tractanterior corticospinal tract

2% of the fibres are 2% of the fibres are uncrossed and they travel uncrossed and they travel in lateral corticospinal tract in lateral corticospinal tract along with crossed fibersalong with crossed fibers

Corticospinal Tract DivisionsCorticospinal Tract DivisionsLateral corticospinal tract:Lateral corticospinal tract: most of these (about 90% of the total neurons) most of these (about 90% of the total neurons)

cross over in the medulla, while the rest (about cross over in the medulla, while the rest (about 2% of the total neurons) cross over at their 2% of the total neurons) cross over at their corresponding place in the spinal cordcorresponding place in the spinal cord

Supply all levels of spinal cord.Supply all levels of spinal cord. Controls the limbs and digitsControls the limbs and digits

Corticospinal Tract DivisionsCorticospinal Tract DivisionsAnterior corticospinal tract:Anterior corticospinal tract: Made up of uncrossed corticospinal fibers (8% Made up of uncrossed corticospinal fibers (8%

of the total) of the total) Cross near level of synapse with LMNsCross near level of synapse with LMNs Control Trunk MusclesControl Trunk Muscles

Corticospinal Tract FunctionsCorticospinal Tract FunctionsAdd speed and agility to conscious Add speed and agility to conscious

movements:movements:Especially movements of hand.Especially movements of hand.

Provide a high degree of motor control:Provide a high degree of motor control:(i.e., movement of individual fingers)(i.e., movement of individual fingers)

Corticospinal Tract LesionsCorticospinal Tract Lesions

Corticospinal Tract LesionsCorticospinal Tract LesionsNot complete paralysisNot complete paralysisNote: complete paralysis results if both Note: complete paralysis results if both

pyramidal and extrapyramidal systems are pyramidal and extrapyramidal systems are involved (as is often the case).involved (as is often the case).

Corticobulbar TractCorticobulbar Tract Innervates the headInnervates the head The corticobulbar tract originates in the primary The corticobulbar tract originates in the primary

motor cortex of the of the frontal lobe.. Upper end of face originate from Anterior Upper end of face originate from Anterior

Cingulate GyrusCingulate Gyrus

Motor Homonculus in actual Motor Homonculus in actual reallityreallity

Corticobulbar TractCorticobulbar Tract The tract descends The tract descends

through the through the corona radiata along along with corticospinal tractwith corticospinal tract

And then to And then to genu of of the the internal capsule with a few fibers in with a few fibers in the posterior limb of the posterior limb of the the internal capsule,,

Corticobulbar TractCorticobulbar Tract The tract descends The tract descends

through the through the corona radiata along along with corticospinal tractwith corticospinal tract

And then to And then to genu of of the the internal capsule with a few fibers in with a few fibers in the posterior limb of the posterior limb of the the internal capsule,,

Corticobulbar TractCorticobulbar Tract The tract descends The tract descends

through the through the corona radiata along along with corticospinal tractwith corticospinal tract

And then to genu of And then to genu of the internal capsule the internal capsule with a few fibers in with a few fibers in the posterior limb of the posterior limb of the internal capsule,the internal capsule,

Corticobulbar TractCorticobulbar Tract The tract descends The tract descends

through the corona through the corona radiata along with radiata along with corticospinal tractcorticospinal tract

And then to genu of And then to genu of the internal capsule the internal capsule with a few fibers in with a few fibers in the posterior limb of the posterior limb of the internal capsule,the internal capsule,

In MidbrainIn Midbrain The internal capsule The internal capsule

becomes the cerebral becomes the cerebral pedunclespeduncles

The white matter is The white matter is located in the ventral located in the ventral portion of the cerebral portion of the cerebral peduncles, called the crus peduncles, called the crus cerebri.cerebri.

The middle third of the The middle third of the crus cerebri contains the crus cerebri contains the corticobulbar and corticobulbar and corticospinal fiberscorticospinal fibers

In MidbrainIn Midbrain The internal capsule The internal capsule

becomes the cerebral becomes the cerebral pedunclespeduncles

The white matter is The white matter is located in the ventral located in the ventral portion of the cerebral portion of the cerebral peduncles, called the crus peduncles, called the crus cerebri.cerebri.

The middle third of the The middle third of the crus cerebri contains the crus cerebri contains the corticobulbar and corticobulbar and corticospinal fiberscorticospinal fibers

In MidbrainIn Midbrain The internal capsule The internal capsule

becomes the cerebral becomes the cerebral pedunclespeduncles

The white matter is located The white matter is located in the ventral portion of the in the ventral portion of the cerebral peduncles, called cerebral peduncles, called the crus cerebri.the crus cerebri.

The junction of medial and The junction of medial and midlle third of the crus midlle third of the crus cerebri contains the cerebri contains the corticobulbar and corticobulbar and corticospinal fiberscorticospinal fibers

Previously thought as followsPreviously thought as follows

Crainial Nerves with motor function Crainial Nerves with motor function controlled by corticobulbar fiberscontrolled by corticobulbar fibers

In actual RealityIn actual Reality

Corticobulbar FibersCorticobulbar Fibers End on Motor Nuclei of :End on Motor Nuclei of : 1) Trigeminal Nerve (V)1) Trigeminal Nerve (V) 2) Facial Nerve (VII)2) Facial Nerve (VII) 3) Glossopharyngeal 3) Glossopharyngeal

Nerve (IX)Nerve (IX) 4) Vagus Nerve (X)4) Vagus Nerve (X) 5) Hypoglossal Nerve 5) Hypoglossal Nerve

(XII)(XII) 6) Spinal Accessory 6) Spinal Accessory

Nerve (XI) ????Nerve (XI) ????

Corticobulbar FibersCorticobulbar Fibers End on Motor Nuclei of :End on Motor Nuclei of : 1) Trigeminal Motor 1) Trigeminal Motor

Nucleus (V)Nucleus (V) 2) Facial Motor Nucleus 2) Facial Motor Nucleus

(VII)(VII) 3) Nucleus Ambiguous 3) Nucleus Ambiguous

(IX and X CN)(IX and X CN) 4) Hypoglossal Nucleus 4) Hypoglossal Nucleus

(XII)(XII) 6) Spinal Accessory 6) Spinal Accessory

Nucleus (XI) ????Nucleus (XI) ????

Motor HomonculusMotor Homonculus

Motor Homonculus in actual Motor Homonculus in actual reallityreallity

Hypoglossal NerveHypoglossal Nerve

InnervatesInnervates (Unilateral innervation to opposite side )of (Unilateral innervation to opposite side )of

Genioglossus muscleGenioglossus muscle (Bilateral innervation of ) Hyoglossus, (Bilateral innervation of ) Hyoglossus,

styloglossus, geniohyoid, thyrohyoid, styloglossus, geniohyoid, thyrohyoid, intrinsic muscles of the tongueintrinsic muscles of the tongue

The neurons that innervate the genioglossus muscle The neurons that innervate the genioglossus muscle receive mainly crossed input from the opposite motor receive mainly crossed input from the opposite motor

cortex. cortex. Therefore, a stroke or other lesion involving the Therefore, a stroke or other lesion involving the leftleft

motor cortex or internal capsule causes weakness of the motor cortex or internal capsule causes weakness of the rightright genioglossus muscle. genioglossus muscle.

When the patient protrudes his tongue, the normal left When the patient protrudes his tongue, the normal left genioglossus muscle pushes the tongue to the right.genioglossus muscle pushes the tongue to the right.

However, this deficit is generally transient, lasting a few However, this deficit is generally transient, lasting a few days (Because other muscle of tongue innervated B/L by days (Because other muscle of tongue innervated B/L by hypoglossal nerve take over).hypoglossal nerve take over).

To Sum upTo Sum upUMN in cortispinal tract supply opposite UMN in cortispinal tract supply opposite

sidesideUMN in corticobulbar tract supply both UMN in corticobulbar tract supply both

side except for : a) Lower half of face and side except for : a) Lower half of face and b) Tongue Genioglosus muscle b) Tongue Genioglosus muscle

Ocular muscle why not by Ocular muscle why not by corticobulbar as previously thoughtcorticobulbar as previously thought

Voluntary control of eye movements is not Voluntary control of eye movements is not executed through the corticobulbar tractexecuted through the corticobulbar tractCrainial Nerve 3, 4 and 6 do not receive Crainial Nerve 3, 4 and 6 do not receive

innervation via corticobulbar tractinnervation via corticobulbar tractThere are discrete cortical areas There are discrete cortical areas

subserving the control of eye movementssubserving the control of eye movementsOne of these is called the frontal eye fieldOne of these is called the frontal eye field

(FEF).(FEF).

The FEF has recently been localized in humans to the The FEF has recently been localized in humans to the junction of the superior frontal sulcus with the precentral junction of the superior frontal sulcus with the precentral

sulcus in area 6, not in area 8 as in the monkey .sulcus in area 6, not in area 8 as in the monkey .

Area 6Area 6Area 8 as previously thought is Area 8 as previously thought is wrongwrong

Electrical stimulation of the FEF produces Electrical stimulation of the FEF produces conjugate horizontal gaze to the opposite side. conjugate horizontal gaze to the opposite side.

Thus, the Thus, the leftleft hemisphere controls hemisphere controls horizontal gaze to the horizontal gaze to the rightright. .

A A leftleft FEF lesion causes inability to FEF lesion causes inability to voluntarily look to thevoluntarily look to the right right (and the (and the patient looks toward the left ).patient looks toward the left ).

http://library.med.utah.edu/kw/hyperbrain/syllabus/syllabus10.html

In the case of In the case of horizontal gaze, these horizontal gaze, these axons decussate and axons decussate and terminate in an area of terminate in an area of the opposite reticular the opposite reticular formation near the formation near the abducens nucleus abducens nucleus called the paramedian called the paramedian pontine reticular pontine reticular formationformation (PPRF).(PPRF).

The abducens nucleus The abducens nucleus also has interneurons also has interneurons whose axons ascend in whose axons ascend in the opposite medial the opposite medial longitudinal fasciculus longitudinal fasciculus (MLF) to the oculomotor (MLF) to the oculomotor nucleus to cause nucleus to cause concurrent activation of concurrent activation of the medial rectus the medial rectus muscle of the opposite muscle of the opposite side. side.

There is a center for There is a center for vertical gaze located in vertical gaze located in the midbrain near the the midbrain near the posterior commissure posterior commissure and pretectal area.and pretectal area.

This center connects This center connects directly with the directly with the oculomotor and oculomotor and trochlear nucle via trochlear nucle via MLF MLF

What happened to Crainial What happened to Crainial Nerve XI (Spinal Acessory Nerve XI (Spinal Acessory

Nerve)Nerve)

Spinal acessory supplies trapezoid and Spinal acessory supplies trapezoid and sternocleidomastoid sternocleidomastoid

LMN bodies of CN XI located in posterolateral LMN bodies of CN XI located in posterolateral aspect of the anterior horn of the spinal cordaspect of the anterior horn of the spinal cord

For crainial part of SANFor crainial part of SANTraditionally, the accessory nerve is Traditionally, the accessory nerve is

described as having a small cranial described as having a small cranial component that descends from the component that descends from the medulla oblongata and briefly connects medulla oblongata and briefly connects with the spinal accessory component with the spinal accessory component before branching off of the nerve to join before branching off of the nerve to join the vagus nerve. the vagus nerve.

The spinal accessory nucleus extends from the The spinal accessory nucleus extends from the caudal medulla to about C5caudal medulla to about C5

Other C1 to C5 spinal roots are Other C1 to C5 spinal roots are controlled by corticospinal tractcontrolled by corticospinal tract

Supply shoulder and neck Supply shoulder and neck musclesmuscles

Supply shoulder and neck Supply shoulder and neck musclesmuscles

CN XI in Posterolateral aspect of CN XI in Posterolateral aspect of C1 to C5C1 to C5

Other Spinal LMN Cell Bodies C1 to Other Spinal LMN Cell Bodies C1 to C5 and BelowC5 and Below

CN XI in Posterolateral aspect of CN XI in Posterolateral aspect of C1 to C5C1 to C5

Other Spinal LMN Cell Bodies C1 to Other Spinal LMN Cell Bodies C1 to C5 and BelowC5 and Below

Supranuclear innervation of Spinal accesory Supranuclear innervation of Spinal accesory nerve is not well known but thought mostly nerve is not well known but thought mostly

corticobulbarcorticobulbar

Extrapyramidal SystemExtrapyramidal System Includes descending motor tracts that do Includes descending motor tracts that do

not pass through medullary pyramids or not pass through medullary pyramids or corticobulbar tracts.corticobulbar tracts.

Extrapyramidal SystemExtrapyramidal System

Rubrospinal TractRubrospinal TractOriginates In the midbrain Red NucleusOriginates In the midbrain Red NucleusCrosses to the other side of the midbrainCrosses to the other side of the midbrain In Spinal Cord Travels along Lateral In Spinal Cord Travels along Lateral

Corticospinal tractCorticospinal tractBut has fever axons than corticospinal tract But has fever axons than corticospinal tract

(less important)(less important)Terminates primarily in the cervical spinal Terminates primarily in the cervical spinal

cord, suggesting that it functions in upper limb cord, suggesting that it functions in upper limb but not in lower limb controlbut not in lower limb control

Rubrospinal TractRubrospinal Tract

Rubrospinal TractRubrospinal Tract Primarily facilitates flexion in the upper Primarily facilitates flexion in the upper

extremitiesextremities If cortex removed (Decorticate) only If cortex removed (Decorticate) only

corticospinal tract affected and Rubrospinal corticospinal tract affected and Rubrospinal activity normal and hence more flexion activity normal and hence more flexion (Decorticate Posture)(Decorticate Posture)

If Cerebral damage both Rubropinal and If Cerebral damage both Rubropinal and Corticospinal affected (Decerebrate Posture)Corticospinal affected (Decerebrate Posture)

Vestibulospinal TractVestibulospinal Tract Originates in vestibular nuclei:Originates in vestibular nuclei:

Receives major input from vestibular nerve: Receives major input from vestibular nerve: (CN VIII)(CN VIII)

Descends in anterior funiculus (column).Descends in anterior funiculus (column). Synapses with LMNs to extensor muscles Synapses with LMNs to extensor muscles

and to lesser extent flexors:and to lesser extent flexors:Primarily involved in maintenance of upright Primarily involved in maintenance of upright posture.posture.

Vestibulospinal tract damageVestibulospinal tract damage

The vestibulospinal reflex uses the vestibular organs as The vestibulospinal reflex uses the vestibular organs as well as skeletal muscle in order to maintain balance, well as skeletal muscle in order to maintain balance, posture, and stability in an environment with gravityposture, and stability in an environment with gravity

Damage person sways from side to side when the eyes Damage person sways from side to side when the eyes are closed. When an individual sways to the left side, the are closed. When an individual sways to the left side, the left lateral vestibulospinal tract is activated to bring the left lateral vestibulospinal tract is activated to bring the body back to midline (Rhomberg Sign)body back to midline (Rhomberg Sign)

The healthy side "over powers" the weak side in a way The healthy side "over powers" the weak side in a way that will cause the person to veer and fall towards the that will cause the person to veer and fall towards the injured sideinjured side

ThanksThanksTo be continuedTo be continued

Fiber TypesFiber TypesA Fibers:A Fibers:

Somatic, myelinated.Somatic, myelinated.Alpha (Alpha (αα):):

Largest, also referred to as Type I.Largest, also referred to as Type I.Beta (Beta (ββ):):

Also referred to as Type II.Also referred to as Type II.Gamma (Gamma (γγ):):Delta (Delta (δδ):):

Smallest, referred to as Type IV.Smallest, referred to as Type IV.

Fiber TypesFiber TypesB Fibers:B Fibers:

Lightly myelinated.Lightly myelinated.Preganglionic fibers of ANS.Preganglionic fibers of ANS.

C Fibers:C Fibers:Unmyelinated.Unmyelinated.Found in somatic and autonomic systems.Found in somatic and autonomic systems.Also referred to as Type IV fibers.Also referred to as Type IV fibers.

Fiber TypesFiber TypesNociceptors and thermoreceptors are Nociceptors and thermoreceptors are

related to C fibers or A-related to C fibers or A-δδ fibers. fibers.

Fiber TypesFiber TypesSensory fibers are either:Sensory fibers are either:

A-A-αα or A- or A-ββ fibers: fibers:Conduction rate = 30-120 m/sec.Conduction rate = 30-120 m/sec.

A-A-δδ fibers: fibers:Conduction rate = 4-30 m/sec.Conduction rate = 4-30 m/sec.

C fibers:C fibers:Conduction rate is less than Conduction rate is less than

2.5m/sec.2.5m/sec.