BRAINSTEM

JOEL P. CARREON M.D.

Brainstem

• It is made up of midbrain, pons and medulla oblongata

• Occupies the posterior cranial fossa of the skull• Stalklike in shape

Functions:1. serves as conduit for ascending tracts and descending tracts2. has important reflex centers associated with respiration, cardiovascular system, conciousness3. contains nuclei of CN III to XII

MIDBRAIN

Gross Appearance

• 2cm (0.8 inch) in length• Connects the pons &

cerebellum with the forebrain

• Traversed by a narrow channel, filled with CSF– cerebral aqueduct

Posterior Surface

• Rounded eminences • 4 Colliculi – (corpora quadrigemina)

• divided into superior & inferior pairs by a vertical & transverse groove

Posterior Surface

• Superior colliculi – centers for visual

reflexes• Inferior colliculi – lower auditory centers

Posterior Surface

• Trochlear nerves – emerge in the midline

below the inferior colliculi;

– small diameter nerves that wind around the lateral aspect of midbrain & enters lateral wall of cavernous sinus

Lateral Aspect of Midbrain

• Superior & Inferior brachia ascend anterolaterally

• Superior brachium – passes from superior colliculus to LGB & optic

tract• Inferior brachium – connects inferior colliculus to MGB

Anterior Aspect of Midbrain

• Interpeduncular fossa – deep depression in

the midline• Posterior perforated

substance – region where several

small blood vessels perforate the floor of interpeduncular fossa

• Crus cerebri – on either side of

interpeuncular fossa

Anterior Aspect of Midbrain

• Oculomotor nerve – emerges from a groove on medial side of crus

cerebri, passes forward in lateral wall of cavernous sinus

INTERNAL STRUCTURE OF MIDBRAIN

• Cerebral Peduncles – two lateral halves of the midbrain,– Divided into A & P part– Crus Cerebri – anterior part– Tegmentum – posterior part

INTERNAL STRUCTURE OF MIDBRAIN

• Substantia Nigra – pigmented band of gray matter that divides the

peduncles

INTERNAL STRUCTURE OF MIDBRAIN

• Cerebral aqueduct – narrow cavity of the

midbrain that connects 3rd & 4th ventricle

• Central gray matter – surrounds cerebral

aqueduct• Tectum – part of midbrain posterior

to the cerebral aqueduct, contains 4 swellings (corpora quadrigemina)

TRANSVERSE SECTION: LEVEL OF INFERIOR COLLICULI

• Inferior Colliculus – large nucleus of gray

matter that forms part of auditory pathway

– receives terminal fibers of lateral lemniscus

– continues through inferior brachium to medial geniculate body

TRANSVERSE SECTION: LEVEL OF INFERIOR COLLICULI

• Trochlear Nucleus– situated in central gray matter

close to median plane just posterior to medial longitudinal fasciculus

– fibers pass laterally & posteriorly around central gray matter, leaves the midbrain below inferior colliculi

– fibers of trochlear nerve decussate in superior medullary velum

TRANSVERSE SECTION: LEVEL OF INFERIOR COLLICULI

• Mesencephalic nuclei of trigeminal nerve– lateral to cerebral

aqueduct• Decussation of superior

cerebellar peduncles– occupies central part of

tegmentum anterior to cerebral aqueduct

TRANSVERSE SECTION: LEVEL OF INFERIOR COLLICULI

• Medial Lemniscus– posterior to substantia

nigra• Spinal & trigeminal

nuclei– lateral to medial

lemniscus• Lateral Lemniscus– posterior to trigeminal

lemniscus

TRANSVERSE SECTION: LEVEL OF INFERIOR COLLICULI

• Substantia Nigra– large motor nucleus between

the tegmentum & crus cerebri

– composed of medium-sized multipolar neurons

– contains inclusion granules of pigment melanin in cytoplasm

– concerned with muscle tone– connected to cerebral cortex,

spinal cord, hypothalamus, basal nuclei

TRANSVERSE SECTION: LEVEL OF INFERIOR COLLICULI

Crus Cerebri• separated from tegmentum by

substantia nigra• contains impt descending tracts• corticospinal & corticonuclear

fibers occupy the middle 2/3 of crus

• frontopontine fibers occupy medial part of crus

• temporopontine fibers occupy lateral part of crus

• these descending tracts connect cerebral cortex to anterior gray column of cells of spinal cord, cranial nerve nuclei, pons & cerebellum

TRANSVERSE SECTION: LEVEL OF THE SUPERIOR COLLICULI

• Superior Colliculus• large nucleus of gray matter that

forms part of visual reflexes• connected to lateral geniculate

body by superior brachium• receives afferent fibers from

optic nerve, visual cortex, & the spinotectal tract

• efferent fibers form the tectospinal & tectobulbar tracts (probably responsible for the movements of eye, head & neck in response to visual stimuli

TRANSVERSE SECTION: LEVEL OF THE SUPERIOR COLLICULI

• Oculomotor nucleus– situated in central gray matter close to median

plane just posterior to MLF– fibers pass anteriorly to red nucleus to emerge on

medial side of crus cerebri in interpeduncular fossa

TRANSVERSE SECTION: LEVEL OF THE SUPERIOR COLLICULI

• Medial, Spinal & Trigeminal Lemnisci– form a curved band posterior to substantia nigra– Lateral lemniscus does not extend to this level

TRANSVERSE SECTION: LEVEL OF THE SUPERIOR COLLICULI

• Red Nucleus– rounded mass of gray matter between cerebral

aqueduct & substantia nigra– reddish hue is due to its vascularity & the

presence of an iron-containing pigment in the cytoplasm of its neurons

TRANSVERSE SECTION: LEVEL OF THE SUPERIOR COLLICULI

• Red Nucleus• Afferent nucleus from:– Cerebral cortex from corticospinal fibers– Cerebellum through the superior cerebellar

peduncle– Lentiform nucleus, subthalamic & hypothalamic

nuclei, substantia nigra, spinal cord

TRANSVERSE SECTION: LEVEL OF THE SUPERIOR COLLICULI

• Red Nucleus• Efferent fibers pass to: – Spinal cord through rubrospinal tract– Reticular formation through rubroreticular tract– Thalamus– Subatantia Nigra

TRANSVERSE SECTION: LEVEL OF THE SUPERIOR COLLICULI

• Reticular formation– situated in tegmentum

lateral & posterior to red nucleus

• Crus Cerebri– contains corticospinal,

corticonuclear & corticopontine fibers (same at the level of inferior colliculus)

Weber’s SyndromeBasal Midbrain Infarct

• caused by occlusion of a branch of posterior cerebral artery that supplies the midbrain

• results in necrosis of brain tissue involving oculomotor nerve & crus cerebri

• S/Sx:– Ipsilateral ophthalmoplegia– Contralateral paralysis of lower

part of face, tongue, & arm & leg– Lateral deviation of eyeball

(paralysis of medial rectus)– Ptosis– Dilated pupil unresponsive to light

& accommodation

Benedikt’s Syndrome

• Paramedian Midbrain Infarct

• Necrosis involves medial lemniscus & red nucleus

• Contralateral hemianesthesia & involuntary movements of limbs to opposite side

PONS

GENERAL FEATURES OF PONS

• Situated in the posterior cranial fossa beneath the tentorium cerebelli

• Possesses cranial nerve nuclei (CN 5, 6, 7 & 8)• Conduit for ascending & descending tracts

(corticonuclear, corticopontine, corticospinal, MLF, spinal, lateral & medial lemniscus)

GROSS APPEARANCE

• Anterior to the cerebellum

• Connects the medulla oblongata to the midbrain

• it is one inch (2.5cm) long

• “bridge” connecting the right & left cerebellar hemispheres

GROSS APPEARANCEAnterior Surface

• convex from side to side• shows transverse fibers that converge on each

side (middle cerebellar peduncle)• Basilar groove: shallow groove in the midline

which lodges the basilar artery

GROSS APPEARANCEAnterolateral surface

• trigeminal nerve emerges on each side• medial part: smaller, motor root• lateral part: larger, sensory root

GROSS APPEARANCE

• Groove between pons & medulla (medial to lateral)– Abducent, facial, vestibulocochlear

nerves

GROSS APPEARANCE Posterior Surface

• hidden from view by cerebellum

• forms upper half of floor of 4th ventricle

• triangular in shape• Limited laterally by

superior cerebellar peduncles, divided into symmetrical halves by median sulcus

GROSS APPEARANCE Posterior Surface

• Medial eminence– elongated elevation lateral to the sulcus,

bounded laterally by sulcus limitans

GROSS APPEARANCE Posterior Surface

• Facial colliculus– expansion at inferior end of medial

eminence produced by the root of facial nerve winding around nucleus of abducent nerve

GROSS APPEARANCE Posterior Surface

• Substantia furruginea– bluish-gray floor of

the superior part of sulcus limitans

• Area Vestibuli – lateral to sulcus

limitans, produced by underlying vestibular nuclei

INTERNAL STRUCTURE OF THE PONS

• Divided into A & P part by transversely running fibers of trapezoid body

• Tegmentum –posterior part

• Basal – anterior part

Transverse section through the Caudal Part

• Passes through the facial collicullus

Transverse section through the Caudal Part

• Medial Lemniscus– rotates as it passes

from the medulla into the pons– situated in most

anterior part of tegmentum– accompanied by the

spinal & lateral lemnisci

Transverse section through the Caudal Part

• Facial Nucleus– posterior to lateral part of medial lemniscus

• Facial colliculus – produced by the fibers of facial nerve

winding around the nucleus of abducent nerve

Transverse section through the Caudal Part

• Medial Longitudinal Fasciculus– situated beneath the

floor of 4th ventricle on either side of midline

– main pathway that connects vestibular & cochlear nuclei with the nuclei controlling EOM (oculomotor, trochlear & abducent nuclei)

Transverse section through the Caudal Part

Medial Vestibular Nucleus• lateral to the abducent

nucleus, in close relation to inferior cerebellar peduncle

• Found at this level:• Superior of lateral & inferior

part of superior vestibular nucleus

• Posterior & Anterior Cochlear nuclei

Transverse section through the Caudal Part

• Spinal nucleus of trigeminal nerve & its tract– anteromedial to inferior cerebellar peuncle

Transverse section through the Caudal Part

• Trapezoid body–made up of fibers derived from cochlear

nuclei & nuclei of trapezoid body– lie transversely on anterior part of

tegmentum

Transverse section through the Caudal PartBasilar part of Pons contains:

• Pontine nuclei – small masses of nerve

cells– where corticopontine

fibers of crus cerebri of midbrain terminate

– Axons of pontine nuclei give origin to Transverse fibers of pons

Transverse section through the Caudal PartBasilar part of Pons contains:

• Transverse Fibers of Pons– cross the midline &

intersect the corticospinal & corticonuclear tract, breaking them into small bundles

– enter the middle cerebellar peduncle & are distributed to cerebellar hemisphere

Transverse section through the Caudal PartBasilar part of Pons contains:

• Transverse Fibers Middle Cerebellar Peduncle Cerebellum– forms the main

pathway linking the cerebral cortex to the cerebellum

Transverse section through the Cranial Part

• Passes through the trigeminal nuclei

• Internal structure is similar to that seen at the caudal level except it now contains motor & principal sensory nuclei of trigeminal nerve

Transverse section through the Cranial Part

• Motor Nucleus of Trigeminal Nerve– beneath the lateral part

of 4th ventricle within reticular formation

– motor fibers travel anteriorly thru the substance of the pons and exit on anterior surface

Transverse section through the Cranial Part

• Principal Sensory Nucleus of Trigeminal Nerve– situated on lateral

side of motor nucleus

– continuous inferiorly with nucleus of spinal tract

Transverse section through the Cranial Part

• Trapezoid Body & Medial lemniscus– same position

(anterior part of tegmentum)

• Lateral & Spinal Lemnisci– Lateral to medial

lemniscus

TUMORS OF THE PONSAstrocytoma of the Pons

• the most common tumor of the brainstem occurring in childhood

• S/Sx (ipsilateral CN paralysis & contralateral hemiparesis):

• Weakness of facial muscle on same side (facial nerve nucleus)

• Weakness of lateral rectus msn on 1 or both sides (abducent N nucleus)

• Nystagmus (vestibular nucleus)• Weakness of jaw muscles (trigeminal N nucleus)

TUMORS OF THE PONSAstrocytoma of the Pons

• Impairment of hearing (cochlear nuclei)• Contralateral hemi/quadriparesis (corticospinal fibers)• Anesthesia to light tough, preserved pain over face

(principal sensory nucleus of CN5; spinal nucleus & tract of CN5 intact)

• Contralateral sensory defects of trunk & limbs (medial & spinal lemnisci)

• Ipsilateral cerebellar s/sx (corticopontocerebellar tracts)• Impairment of conjugate eye deviation (MLF)

Pontine Hemorrhage

• Blood supply of pons: basilar artery, & anterior, inferior, superior cerebellar arteries

• S/Sx: Ipsilateral facial paralysis & contralateral paralysis of limbs – Paralysis of conjugate eye deviation (abducent

nucleus & MLF)

Gross Appearance of Medulla Oblongata

• Connects the pons superiorly with the spinal cord inferiorly

• Junction of the medulla and spinal cord is at the origin of the anterior and posterior roots of the first cervical spinal nerve, which corresponds approximately to the level of the foramen magnum

• Conical in shape • Central canal – continues upward to the lower half

of medulla • Cavity of 4th ventricle – expansion of the upper

half of medulla

anterior median fissure of the medulla • anterior surface of medulla • Continuous inferiorly with the anterior median fissure of

the spinal cord • on each side are swelling called the PYRAMID

pyramid • composed of bundles of nerve fibers -> corticospinal

fibers (from large nerve cells in precentral gyrus of the cerebral cortex

• tapers inferiorly • where nerve fibers cross the opposite side forming the

decussation of the pyramids

anterior external arcuate fibers • few nerve fibers that emerge from the anterior median fissure

above the decussation and pass laterally over the surface of the medulla to enter the cerebellum

olives • posterolateral to the pyramids • oval elevations by inferior olivary nuclei • rootlets of Hypoglossal nerve- groove between pyramid and olive • roots of Glossopharyngeal and vagus nerves and cranial roots of

the accessory nerve- groove between olive and inferior cerebellar peduncle

• posterior to the olives are the INFERIOR CEREBELLAR PEDUNCLES-> connect medulla to cerebellum

floor of the fourth ventricle• forms by the posterior surface of inferior half of the

medulla oblongata • continuous with the posterior spinal cord and possesses a

posterior median sulcus

gracile nucleus • produce an elongated swelling-> gracile tubercle • situated on each side of the median sulcus

• cuneate nucleus • Produce a swelling-> cuneate tubercle • lateral to the gracile nucleus

INTERNAL STRUCTUREexpansion of neural tube-> form hindbrain vesicle-> 4th ventricle-> extensive lateral spread-> alteration if position of derivatives of alar and basal plates of the embryo

Four levels1. level of decussation of pyramids 2. level of decussation of lemnisci 3. level of the olives 4. level just inferior to the pons

1. LEVEL of DECUSSATION of PYRAMIDS

– the great motor decussation – superior – corticospinal occupy and form pyramid – inferior ->cross median plane->continue down the SC

in lateral white column as lateral corticospinal tract – fasciculus gracilis and cuneatus – ascend superiorly

posterior to the central gray matter – nucleus gracilis and cuneatus – appear as posterior

extensions of the central gray – substantia gelatinosa –in posterior gray column;

continuous with the inferior end of the nucleus of the spinal tract of the trigeminal nerve

2. LEVEL of the DECUSSATION of LEMNISCI • inferior half of the medulla oblongata• great sensory decussation • anterior to the central gray matter and posterior to the pyramids • formed from the internal arcuate fibers- • emerged from the anterior aspects of the nucleus gracilis and cuneatus • travel anteriorly and laterally around central gray matter-> curve

medially toward midline -> decussate with fibers of opposite side • nucleus of the spinal tract of the trigeminal nerve- lateral to the

internal arcuate fibers • the spinal tract of the trigeminal nerve lies lateral to the nucleus • the lateral and anterior spinothalamic tracts and the spinotectal tracts

occupy an area lateral to the decussation of the lemnisci – SPINAL LEMNISCUS

• spinocerebellar,vestibulospinal and rubrospinal tracts – anterolateral of medulla oblongata

3. LEVEL of the Olives• passes across inferior part of fourth ventricle• amount of gray matter increase at this level• found are nuclei of CN VIII, IX, X, XI, XII

A. olivary nuclear complex• largest nucleus is inferior olivary nucleus • gray matter shaped like a crumpled bag • responsible for the elevation on the surface of the

medulla called the OLIVE • smaller dorsal and medial accessory olivary nuclei also

are present

Olivary (cont’d)

• Inferior cerebellar peduncle- where fibers of inferior olivary nucleus cross to enter cerebellum

• Afferent fibers: reach inferior olivary nuclei from spinal cord (spino-olivary tracts) and from the cerebellum and cerebral cortex

• FUNCTION: voluntary muscle movement

B. vestibulocochlear nuclei

• Made up of ff nuclei: • medial vestibular nucleus • inferior vestibular nucleus • lateral vestibular nucleus • superior vestibular nucleus • two cochlear nuclei : anterior and posterior cochlear

nucleus • anterior cochlear nucleus on anterolateral aspect of the

inferior cerebellar peduncle • posterior cochlear nucleus on posterior aspect of the

peduncle lateral to floor of the fourth ventricle

C. nucleus ambiguus • consists of large motor neuron • situated deep within the reticular formation • emerging nerve fibers join the glossopharyngeal, vagus

and cranial part of the accessory n • FUNCTION: contribute to voluntary skeletal mucle

D. central gray matter • Passing from medial to lateral : • hypoglossal nucleus • dorsal nucleus of vagus • nucleus of tractus solitarius • medial and inferior vestibular nuclei

pontine nuclei • arcuate nuclei (inferiorly displaced) • situated on anterior of pyramids • receives nerve fibers from the cerebral cortex and send

efferent fibers to the cerebellum through the ANTERIOR EXTERNAL ARCUATE FIBERS

pyramids • situated in anterior part of the medulla separated by

anterior median fissure • contain the corticospinal and some corticonuclear fibers • corticospinal fibers descend to spinal cord • corticonuclear fibers are distributed to motor nuclei of

cranial nerves w/in medulla

reticular formation• Has diffuse mixture of nerve fibers and cells• deeply placed posterior to the olivary nucleus • also present in pons and medulla

CN IX,X and cranial part of CN XI • run forwad and laterally through the reticular formation • Emerge from between the olives and inferior cerebellar peduncles

CN XII • run anteriorly and laterally through the reticular formation and

emerge between the pyramids and the olives

4. LEVEL just INFERIOR to the PONS• The lateral vestibular nucleus has replaced the inferior vestibular

nucleus cochlear nuclei are visible on the anterior and posterior surfaces of the inferior cerebellar peduncle

CLINICAL NOTES Medulla Oblongata

• contains many cranial nerve nuclei that are concerned with vital functions (e.g., regulation of heart rate and respiration)

• serves as a conduit for the passage of ascending and descending tracts connecting the spinal cord to the higher centers of the nervous system.

• Involved in demyelinating diseases, neoplasms, and vascular disorders. Raised Pressure in the Posterior Cranial Fossa and Its Effect on the

Medulla Oblongata • tumors of the posterior cranial fossa-> ↑ICP-> tends to be pushed

toward the area of least resistance ->downward herniation of the medulla and cerebellar tonsils through the foramen magnum.

• Symptoms of headache, neck stiffness, and paralysis of the glossopharyngeal, vagus, accessory, and hypoglossal nerves owing to traction

• extremely dangerous to perform a lumbar puncture

Arnold-Chiari Phenomenon

• congenital anomaly in which there is a herniation of the tonsils of the cerebellum and the medulla oblongata through the foramen magnum into the vertebral canal.

• blockage of the exits in the roof of the fourth ventricle to the cerebrospinal fluid->internal hydrocephalus.

• S/Sx – involvement of last four cranial nerves

Vascular Disorders of the Medulla Oblongata 1. Lateral Medullary Syndrome of Wallenberg • The lateral part of the medulla oblongata supplied by: the

posterior inferior cerebellar artery, which is usually a branch of the vertebral artery

• Thrombosis of the arteries produces signs and symptoms: dysphagia and dysarthria due to paralysis of the ipsilateral palatal and laryngeal muscles (innervated by the nucleus ambiguus); analgesia and thermoanesthesia on the ipsilateral side of the face (nucleus and spinal tract of the trigeminal nerve);

• vertigo, nausea, vomiting, and nystagmus (vestibular nuclei); ipsilateral Horner syndrome (descending sympathetic fibers); ipsilateral cerebellar signs—gait and limb ataxia (cerebellum or inferior cerebellar peduncle); and contralateral loss of sensations of pain and temperature (spinal lemniscus—spinothalamic tract)

2. Medial Medullary Syndrome • The medial part of the medulla oblongata is supplied by:

vertebral artery • Thrombosis produces

signs and symptoms: contralateral hemiparesis (pyramidal tract), contralateral impaired sensations of position and movement and tactile discrimination (medial lemniscus), and ipsilateral paralysis of tongue muscles with deviation to the paralyzed side when the tongue is protruded (hypoglossal nerve).