Brainstem Principles

• each cranial nerve nucleus subserves a single sensory or motor task while nerve itself conveys many tasks

• nuclei often pool axons from multiple nerves:

• trigeminal nuclei and caudal solitary nuclei receive somatic sensory input from all cranial nerves

• nuclei form discontinuous rostral-caudal columns (Å to spinal cord columns)

> structures that appears in multiple brainstem areas are discussed multiple times below

• sulcus limitans principle: sensory columns LATERAL to motor columns

• look for sulcus limitans in pons and medulla

• occipital somites give rise to

1) oculomotor nucleus

2) trochlear nuclus

3) abducens nucleus

4) hypoglossal nucleus

• special motor equivalent to striated branchial = structures descended from branchial arches

• branchial arches give rise to cranial nerves and associated muscles, arterial supply and cartilage:

Arch 1: V3 and muscles of mastication & tensors

Arch 2: VII and muscles of facial expression & stapedius

Arch 3: IX and associated structures

Arch 4 & 6: X and XI and msucles of pharynx, larynx and soft palate

(arch 5 does not develop into anything in humans)

Note: Martin’s terminology is confusing, skeletal muscles of branchiomeric origin are just like any other striated skeletal muscle, despite being called "special visceral"

• brainstem notable for diffuse projection monoaminergic systems:

< 5HT projections from raphé

< NE projections from lateral reticular formation & locus ceruleus

< DA from basal midbrain, basal ganglia (esp. substantia nigra pars compacta and ventral tegmental area in midbrain)

< all send ascending fibers in median forebrain bundle

talk about reticular activating system

put in pics diagrams from misha1? Or straight from disk

make sure to mention the monkey difference in circuitry

 

 

 

 

 

 

 

 

 
 

MOTOR NUCLEI & targets

SENSORY NUCLEI & sources

NERVE

SOMATIC

BRANCHIOMERIC

VISCERAL

  

I

      

Olfactory bulb

II

      

LGN, superior colliculus

III

n. III

· extraocular m.

· levator palb.

  

Edinger-Westphal n.

· parasymp. pregang.

· lens accom

· pupl. const.

  

IV

‘pulley’

n. IV

· superior oblique

      

V

  

Motor n. V

· mastication m.

· tensor tympani

· tensor v. p.

  

Main (face epicritic), spinal (face protopathic) and mesencphalic n. V (jaw propioceptive)

VI

n. VI

· lateral rectus

      

VII

  

n. VII

· facial exp. m.

Salivatory

· submandb. gland

· submax. gland

· lacrimal gland

n. solitary tract (and trigeminal nuclei)

· ant 2/3 tongue taste

· ear skin sensory

VIII

      

Vestib. And cochlear n.

IX

  

n. ambiguus

· pharynx & larynx

· stylopharyngeus

Salivatory

· parotid gland

n. solitary tract (and trigeminal nuclei)

· post 1/3 tongue taste (& sensory)

· carotid sinus

X

‘wanderer’

  

n. ambiguus

· pharynx & larynx

Dorsal motor n. of vagus

n. solitary tract (and trigeminal nuclei)

· ear skin sensory

XI

Access n.

n. ambiguus

· cranial part to larynx

· spinal part C1-C5 alpha-MN axons to trapezius and SCM

    

XII

n. XII

      

 

 

 

 

 

 

MEDULLA (myelencephalon)

• the word ‘medulla’ derives from Latin medianus signifying middle, same root as for ‘medial’

• also termed ‘medulla oblangata’

Transition from Spinal Cord to Medulla

• 4th ventricle pushes structures posterior to lateral

• decussations:

• medullary pyramids: fills anterior medial fissure at medulla

• DCML

• zone of Lissauer becomes spinal trigeminal tract

• substantia gelatinosa becomes spinal trigeminal nucleus

• note: it is called the spinal trigeminal tract & nucleus because it does indeed exist in the spinal cord

• intermediate spinal gray becomes reticular formation (integration zone of sensory and motor)

• cranial nerve nuclei appear

• anterior horn cells into medulla called supraspinal nucleus

• spinal accessory emerges

accessory cuneate nucleus

• recall dorsal spinocerebellar tract: receives propioceptive afferents

• accessory cuneate nucleus is upper limb equivalent of Clarke’s column

• there are reciprocal uncrossed projections with the cerebellum

arcuate nucleus

• located anterior to the pyramids (shaped like an arc)

• part of the corticocerebellar system: cortical axons synapse in arcuate nucleus, arcuate nucleus neurons project as external arcuate fibers into the cerebellum

area postrema

• just rostral to obex, 4th ventricle side,

• consist of arterioles, sinusoids, and specialized ependyma

• outside BBB, a circumventricular organ (around-the-ventricle)

• area postrema is the only paired circumventricular organ

• inputs into area postrema:

> solitary nucleus and spinal cord

> incoming neurons secrete oxytocin, ADH, neurophysin

[ADH = antidiuretic hormone = vasopressin]

plays role in emesis: (vomiting)

• area postrema has emetic chemoreceptors, sensitive to such drugs as digitalis (drug used to promote cardiac contractility in heart failure), and apomorphine (opiate analgesic)

inferior olivary nucleus

• made of multiple nuclei: principle inferior olivary, medial inferior olivary, dosal accessory olivary

• olivocerebellar fibers project via inferior peduncle (most decussate before doing so)

• olivocerebellar fibers in the cerebellum are climbing fibers

• inputs to inferior olivary nuclei:

> cerebral cortex > red nucleus > PAG > inferior vestibular nucleus

> spinal trigeminal nucleus > contralateral deep cerebellar nuclei

> nuclei gracilis and cuneatus

medullary reticular formation

• phylogenetically the oldest part of the brain

• semiaggregated

• note medullary and pontine reticular formations continuous, share inputs and outputs

• site of sensorimotor integration (analogous to intermediate zone of spinal cord)

• single reticular neuron sends impulse both rostral and caudally, local and distant, rapid and slow

• spinoreticular projections travel in anterolateral fasciculus

• inputs to reticular formation:

> secondary sensory cranial nerve nuclei

> cerebral cortex, mainly sensory, but also motor and premotor

> deep cerebellar nuclei

> spinothalamic collaterals

> cranial nerve collaterals

• outputs from reticular formation go to:

< other areas of reticular formation at different brainstem levels

< cerebellum

< cranial nerve nuclei

< central tegmental tract uncrossed to intralaminar thalamic nucleus

uncrossed tract to intralaminar thalamic nuclei

< reticulospinal tract monosynaptic excitatory to alpha-motor nuerons of head and neck muscles AND di-synaptic inhibitory to alpha motor neurons at ALL levels

< rubrobulbar projections travel through central tegmental tract from red nucleus to inferior olive

•comparative neuroanatomy of the reticular formation:

• all mammals have the following circuit:

cortex –> reticular formation –> cranial nerve motor nuclei

• unique to primates is an additional direct corticobulbar tract:

cortex –> cranial nerve motor nuclei

• what may be the adaptive value of this circuit?

Trigeminal System: The Spinal Trigeminal Nucleus

• spinomedullary spinal trigeminal nucleus composed of three sections:

• rostral pars oralis,

stretches from pontomedullary junction to XII nucleus

• pars interpolaris, from XII to obex

• pars caudalis, from obex to C2-C3

• in the dorso-ventral orientation:

• mandibular V3 most dorsal

• maxillary V2 intermediate

• opthalmic V3 most ventral

• spinal trigeminal neurons

• cross through the reticular formation

• project to the thalamus with the contralateral medial lemniscus

• project to the reticular formation at multiple levels

• project to the cerebellum va the inferior cerebellar peduncle

 

 

 

 

 

inferior cerebellar peduncle

• most fibers are crossed olivocerebellar, lateral reticular nucleus of medulla, accessory cuneate, paramedian reticular, arcuate, perihypoglossal

sulcus limitans principle in the medulla

• dorsolateral in the medulla are

> alar afferents VIII, V, VII, IX, X descending in the spinal trigeminal

> taste fromVII, IX X travels in the solitary fasciculus inside of solitary nucleus

• ventromedial in the medulla are:

> dorsal motor nucleus of vagus X

> inferior salivatory nucleus of glossopharyngeal nerve IX

> nucleus ambiguus, posterior to inferior olive

> XI, X, IX (have outputs to pharynx and larynx)

XII nerve

• anterolateral to central canal: nucleus found at spinomedullary junction

• in rostral medulla XII nucleus replaced by prepositus nucleus

• XII nucleus receives monosynaptic projections from M1

• output to ipsilateral tongue muscles

• XII nerve exits ventrally between inferior olive and pyramid

XI nerve

• cranial nerve XI caudal n. ambiguus, in vagus to form inferior recurrent laryngeal nerve, innervates branchiomeric muscles

• spinal XI from anterior horn C1-C6, exit laterally bundle to denticular ligament, enters foramen magnum, exits skull via jugular foramen to sternocleidomastoid muscles and upper trapezius [since these muscle targets are somatic in origin they receive unilateral M1 input only]

• IX and X also exit through the jugular foramen (as do the jugular veins)

X Vagus

inputs to vagus

• cutaneous behind ear and external auditory meatus

• visceral pharynx, larynx, trachea, esophagus, abdominal/thorax

• taste buds on esophagus

outputs from vagus nerve to:

• striated muscle in pharynx, larynx

• parasympathetic ganglia in thorax and abdomen (GI tract until splenic flexure of colon- or mid-transverse, I’ve read both)

• vagus nerve lateral to central canal

• commissural nucleus of the vagus (found dorsal to the central canal), receives visceral inputs

• dorsal motor nucleus of vagus sends out preganglionic parasympathetic neurons

• to terminal ganglia in thoracic and abdominal cavities

• affects heart rate, gastric motility, bronchial musculature, bronchial secretion, colon up to splenic flexure

IX Glossopharyngeal

• somatic sensory afferents from cuteanous ear area and post pharynx, eustachian tube, tonsil

• inferior ganglion analogous to DRG

• tactile, thermal, pain, and taste for posterior one third of tongue

• innervates a single striated muscle: the stylopharyngeus

• visceral afferents from carotid body

• carotid sinus reflex

1) blood pressure up

2) increased pressure detected in baroreceptors in carotid body in carotid notch (where common carotid artery branches into external and internal branches)

3) baroreceptor axons in IX activate areas in caudal solitary nucleus

4) dorsal motor nucleus of vagus stimulated

5) parasympathetic preganglionic fibers from X emit ACh which in turn causes postganglionic fibers to emit ACh onto heart

6) cardiac muscarinic receptors activated: heart rate & contractility decreased

7) blood pressure decreases

• carotid sinus reflex is basis of ‘Vulcan neck pinch.’ If you stimulate carotid body in absence of actual increase in BP (by a Vulcan gripping your neck and squeezing the carotid body) , resulting decrease in BP from slowed heart will mean less blood to brain and hence syncope (you will faint) Note this is distinct from certain Judo neck holds which press on common carotid directly decreasing blood flow (temporarily) to brain.

• IX also involved in pharyngeal gag reflex

• autononmic output to otic ganglion, postganglionic fibers synapse on parotid gland (secretes saliva into mouth)

solitary nucleus

• location: dorsolateral to dorsal motor nucleus of the vagus

inputs to solitary tract

• rostral solitary nucleus receives taste from VII, IX, X

• anterior 2/3 tongue taste via VII

• posterior 1/3 tongue taste via IX

• epiglottis & area taste via X

• rostral solitary nucleus also termed gustatory nucleus

• caudal solitary nucleus receives visceral from X, VII, IX

• immunopositive for neuropeptide, substance P, somatostatin, enkephalin

outputs from solitary tract:

• nucleus ambiguus, reticular formation

• parabrachial nuclei, rostral pons

• VPm, parvicellular, taste area of thalamus

• involved in reflexes:

• along with XII and salivatory nuclei for salivatory reflex

• along with phrenic nerve and thoracic anterior horn for cough reflex and vomit reflex

nucleus ambiguus

• receives inputs from IX, X, and XI

• M1 has bilateral control over nucleus ambiguus

• participates in gag reflex:

> stimulation of pharynx causes contraction of pharyngeal muscles

> IX carries sensory afferents while X carries motor efferents

Outputs of Superior Salivatory Nucleus

• facial nerve sends preganglionic axons to synapse in ptergyopalatine ganglion, postganglionic fibers innervate lacrimal glands, nasal mucosal glands

• facial nerve also sends preganglionic axons to other ganglia, postganglionic to submandibular and sublingial salivary glands

• glossopharyngeal nerve preganglionic synapse in otic ganlgion postganglionic axons to parotid gland (saliva)

• vagus para pregangl to terminal ganlgia in thoracic and abdominal cavities, affect heart rate, gastric motility, bronchial msucle control, bronchial secretion (entire colon up to splenic flexure, then sacral nerves take over)

Medullary-Pontine Junction

• nodulus of cerebelllum as roof of 4th ventricle

• inferior cerebellar peduncle maximum size

• floor of 4th ventricle is nucleus prepositus medially to hypoglossal nucleus

• medial and inferior vestibular nuclei seen

• inferior vestibular nuclei has corarse myelinated fibers in it: descending primary vestibular fibers and cerebellar efferents

 

PONS (part of the metencephalon)

• ‘pons’ means bridge (note appearance of protuberance and peduncles as a bridge)

landmarks that you have entered the pons:

• inferior olive disappears

• inf cbll peduncle enters cerebellum

• corticospinal tract into ventral pons

• reticular formation englarges

• pontine CN nuclei

• gross ventral surface pontomedullary junction:

VI, VIII, VII nerves are present

phylogeny of pons

• older dorsal = tegmentum

• new ventral pons = pons proper

• think older part required for life (reticular system, repsiratory centers etc), while newer front more control from above, before was just local reflex

old dorsal pons

• old dorsal pontine tegmentum is continuation of medullar reticular formation

• V, VI, VII, VIII

• medial lemniscus slides horizontal

• trapezoid body

• MLF in floor of the 4th ventricle

• spinothalamic and anterior spinocerebellar tracts anterolaterally

• spina trigeminal medial/ventral to inferior cerebellar peduncle

vestibular nuclei floor of 4th ventricle

• pontine reticular formation

• projects to V motor nuclei rostrally

• is the main source of the reticulospinal tract

• reticular formation is posterior lateral to medial lemniscus, central tegmental tract

• inputs from midbrain to inferior olive; reticular formation outputs to thalamic projections

new ventral pons

• longitudinal: corticospinal & corticobulbar (into pontine tegmentum)

• corticopontine [from frontal, parietal, temporal and occipital cortex]

• pontine nuclei –> decussates –> enters middle cerebellar peduncle

VIII: vestibulocochlear

• axons projecting from spiral ganglion (cochlear afferents) and Scarpa’s ganglion (vestibular afferents, including of cristae ampullaris and maculae)

• afferents synapse into cochlear and vestibular nuclei

• dorsal cochlear nucleus is lateral to fourth ventricle (acoustic tubercle)

• ventral cochlear nucleus has tonotopy, high frequency dorsal and low frequency ventral

• cochlear nucleus is last unilateral auditory nucleus

• superior olive cholinergic projections travel in olivocochlear bundle in and through VIII to ultimately synapse on to outer hair cells (VIII efferents)

• the superior olive receives feedback from the inferior colliculus via the lateral lemniscus

acoustic reflex [relevant to cranial nerves b/c of role of V and VII]

• superior olive –> V –> tensor tympani: diminish sensistivity to sound

• superior olive –> VII –> stapedius–> dampens oscillation of stapes

vestibular nuclei

• floor of the fourth ventricle, rostral to nucleus XII up to level of nucleus VI

• primary vestibular fibers: most go to vestibular nuclei

• some primary vestibular fibers go through juxtarestiform body (the restiform body is the inferior cerebellar peduncle)

• vestibular nuclei receive reciprocal inputs from the cerebellum

• vestibular nucleus outputs:

• ascending projection MLF

• projection to extraocular III (substrate for VOR)

• crossed projection bilateral

• some outputs reach VPLc (mixed with others, no exclusive vestibular pathway)

VII: facial nerve

sends efferents to:

• somatic motor: muscles of facial expression

• autonomic: suprasalivatory nucleus –> cholinergic input to reticular formation –> preganglionic parasympathetic –> lacrimal, nasal mcuosa, submandibular glands

• stapedius

• lower face gets contralateral M1 only via corticobulbar tract

• upper face gets bilateral M1 via corticobulbar tract

receives afferents from:

• anterior 2/3 tongue taste

• external auditory meatus touch

Trigeminal

sensory afferents:

• face forhead, nose, mouth, teeth, cranial dura, deep pressure of teeth/hard-palate/periodontal/TMJ, stretch reflexes for muscles of mastication

• opthalmic V1, maxillary V2, mandibular V3

• semilunar ganglion for trigeminal sensory afferents

motor efferents:

• muscles of mastication

• 2 tensors (tensor veli palatini, tensor tympani)

 

trigeminal nerve related nuclei:

most rostral, midbrain: mesencephalic trigeminal nucleus

< cell bodies of jaw propioceptive neurons

< analogous to dorsal root ganglion

rostral, pons: principle (or main) sensory nucleus

< analogous to cuneate or gracile nuclei

< primary epicritic afferents synapse here

< output decussates and goes to VPm

< ascends in trigeminal lemniscus alongside medial lemniscus

< some V3 axons synapse onto neurons which project ipsilaterally to VPm

middle, pons and medulla: spinal trigeminal of V (also VII, IX, X)

< primary protopathic (and some epicritic) efferents synapse here

< pars caudalis neurons ascend to reticular formation

< some also continue in contralateral medial lemniscus, some to median raphe

< as with other protopathic paths, synapses on intralaminar thalamic nuclei (which has diffuse output, also specific output to anterior insular cortex)

< some spinal trigeminal output goes contralateral VPm (to localize pain)

caudal, spinal cord: substantia gelatinosa (C1, C2)

trigeminal related reflexes:

• tearing reflex

cornea –> V1 –> superior salivatory nucleus –> VII –> lacrimal secretion

• sneeze

trigeminal –> nucleus ambiguus –> reticular formation –> phrenic nerve –> intercostals [muscles between ribs] paroxysmal contraction

• vomiting

trigeminal –> vagal

• salivatory reflex

2° trigeminal –> inferior salivatory nucleus

• jaw jerk = myotatic stretch reflex

gently tap chin –> monosynaptic masseter reflex

pontine tegmentum

as you from caudal pons to level at trigeminal nerve root, you see

1) the 4th ventricle narrows,

2) fibers of the sup cbll peduncle form dorsolateral wall of 4th ventricle

3) fibers of inf and middle cbll pednuncles enter cbllm

4) cbllm dorsal to pons include deep cbll nuclei; ventral pons also bulges in size

• principal ascending sensory fibers form peripheral shell around pontine tegmentum

• medial lemniscus traversed by trapezoid body fibers

• central tegnemntal tract (dorsomedial to superior oliveary complex);

• spinothalamic and anterior spinocereballar tracts lateral to medial lemniscus

• MLF dorsally to each side of median raphe

VI: abducens nerve

• abducens go to lateral recti; abduct the eye

• abduct = move away from midline, i.e., abduct = to take away

• abducens exits at pontomedullary junction

• conjugate eye movements: internuclear neurons in abducens ascend MLF to contralateral III nucleus

• (vestibular and other inputs) –> paramedian pontine reticular formation –> interneuclear neurons of VI –> MLF –> contralateral nucleus of III

Pontine Reticular Formation

• reticular formation have cells which give rise to uncrossed reticulospinal fibers, descend as part of MLF

• other outputs ascend as part of central tegmental tract up to intralaminar thalamic nuclei

• many cells have axons that branch in both directions

Isthmus of the Hindbrain:

• the isthmus rhobencephali is the narrowest part of the hindbrain, is rostral to the cerebllum and caudal to the midbrain; there is a roof plate, a tegemntal region, and a ventral cortical part

• the roof is a thin superior medullary velum, which contains the decussating fibers of the trochlear nerve

• ventromedial to the mesencphalic nucleus of V are pigmented (with melanin) cells of the locus ceruleus (see below)

• lateral to central gray are superior cerebellar peduncles which are deucssating

• parabrachial nuclei along side superior cerebellar peduncles

• lateral parabrachial nuclei:

inputs from caudal nucleus solitarius (visceral input, such as glossopharyngeal blood pressure, baroceptors in carotid notch): output to hypothalamus and amygdala

• medial parabrachial nuclei

get afferents ofrom gustatory parts of nucleus solitarius (nucl solit afferents uncrossed in brainstem): outputs to thalamus, hypothalamus and amygdala

• Kölliker-Fuse nucleus

ventral to lateral parabrachial nucleus, projects to nucleus of solitary tract, involved in central control of respiration

• superior cerebellar peduncle is output tract from cerebellum carrying output of dentate and interposed deep cerebellar nuclei

locus ceruleus

• intermingled with mesencephalic nucleus of trigeminal

• full of norepinephrine, projects widely to all of telencephalon, diencephalon, all parts of brainstem, cbllm, and spinal cord!

• Output ascending: to/through lateral hypothalamus; at anteriror commisure splits into one for telencephalon (incl cerebral ctx) other for dinecephaln;

• Output local: superior and inferior colliculu; Purkinje cell bodies; brainstem to primary sensory and association nuclei, to pontine nuclei; down to spinal cord to innervate anterior and intermediate gray at all levels via anterior and lateral funiculi; lower brain stem noradenregic go to other targets than those of locus cerulues, including thoracic levels and intermediolateral cell columns

• roles of locus ceruleus: REM sleep, faciliation/inhibition of sensory neurons, cortical activation

raphe nuclei

• serotonergic fiber system; found in both pons and medulla, most conspicuous in pons, esp median nucleus of the raphe, and dorsal nucleus of raphe;

• also make substance P and enkephalin;

• ascending fibers of ROSTRAL raphe to ventral tegemental area joins medial forebrain bundle in lateral hypothalamus; enter substantia nigra, intralaminar thalamic nuclei, stria terminalis, septum, internal capsule; rostrally to frontal lobe

• DORSAL raphe project to substantia nigra, LGN, neostriatum, pyriform lobe, olfactory bulb, amygdaloid nuclar complex

• superior central nuc of raphe projects to pednuncular nucleus, mammlilary bodies, hippocampi

• related to slow wave sleep; circuits with locus ceruleus

MIDBRAIN (mesencephalon)

Midbrain has three parts:

1) tectum (roof, techo), also termed the quadrigeminal plate (quad for 4 colliculi) dorsal to aqueduct

2) central tegmentum, rostral continuation of pontine reticular formation

3) ventral massive crura cerebri (descending cortical projection, also termed cerebral peduncles or basis pedunculi)

• substantia nigra separates the midbrain tegmentum from the crus cerebri ventrally and the IV/III nuclei at the midline (themselves ventral to the PAG)

Pons-Midbrain Transition: Inferior Collicular Level

1) rostral 4th ventricle narrows to aqueduct

2) superior medullary velum replaced by inf. colliculi

3) sup cbll peducnle move ventromedially and decussate

4) tegmenutum smaller

5) lateral lemniscus (lateral tegmentum) move dorsally enter inf coll

• inf. colliculus itself responds to binaural stimulation

parabigeminal area

• small oval nucleus cholingeric neurons

• inputs from superificial sup coliculus (visuotopic organization)

• project bilaterally back to superior colliculi

• reposnd to visual stimuli

raphe nuclei of midbrain

• dorsal nucleus of raphe –> substantia nigra and putamen

median raphe nucleus ascending projections –> mesolimbic system –> midbrain reticular formation + hypothalamus + septal area + entorhinal cortex + hippocampal formation

• median raphe nucleus descending projections –> cerebellum, locus ceruleus, reticular formation of lower brain stem, raphe of pons and medulla

midbrain periaqueductal gray (PAG)

• many functions: analgesic, vocalization, reproduction, aggression, upward gaze

• inputs into PAG arise from:

hypothalamus

brain stem reticular formation

raphe nuclei

locus cerulues

spinal cord

• PAG neurons contain enkephalin, substance P, CCK, neurotensin, 5HT, dynorphin, somatostain; several neuropeptides per cell

• ventrolateral PAG stimulation causes analgesia as do microinjections of morphine into that area

interpeduncular nucleus

• dorsal to interpeduncualr fossa, receives fibers from habenular nucleus via the fasciculus retroflexus, a mainly cholinergic tract (implicated role in REM sleep)

• those that do not synapse onto the interpeduncular nucleus synapse on to the superior central nucleus, dorsal tegmental nucleus and PAG

dorsal tegmental nucleus

• receives fibers from mammilary bodies via the mammillotegemntal tract (closely related to DLF of Schütz)

• found in ventromedial PAG

• substrate for limbic signals dealing with visceral sensorimotor function and behavior

superior colliculus

•structurally, sup coll resemble cerebral cortex:

• alternate gray and white layers

• on surface: stratum zonale

• next deeper, stratum cinerum (outer gray)

• next deeper, stratum opticum (outer white)

• stratum lemnisci (deep gray and white)

• inputs to superior colliculus:

• retinotectal fibers from optic tract, bypass LGN direct to superior colliculus superficial layers (involved in motion detection)

• corticotectal from frontal, temporal, parietal, occipital (majority) enter stratum tectum and go deeper and superificial (deep layers multimodal)

• frontal eye field (Brodmann 8) project via trastegmental approahc to sup coliclus; conjugate eye movements; middl egray layer cells discharge prior to saccades; auditory cortex projects to deep sup coll

• outputs from superior colliculus

• tectothalamic superficial layers ipsilateral to pulvinar, dorsal LGN and pretectum; pulvinar gets most; sup. coll. to LGN and parabigeminal; tectogeniculate to intralaminar genciulate also; sp coll and LGN to ctex overalp…intermeditae deep coll to pretectum, PPRF, rostratl interstitial of MLF

• tectofugal descending, some are uncrossed to dosrolateral pontine nuclei, lateral reticuogemntal tnucleus, nuclears reticularis pontis oralis; pontinenuelcie thus input from supp coll, V1, A1, projects to VI and VII of cbll vernmis

• tectoreticular fiber to dorsal midbrain retic formation

• tectospinal/tectobulbar –> median raphe, MLF, laminae VII and VIII in spinal cord

• homologous areas are interconnected

• brainstem afferent to sup. coll. from inferior colliculus and auditory relay nuclei

III: oculomotor nerve

• somatic motor control of: medial rectus, inferior rectus, superior rectus, inferior oblique and levator palpebrae superioris muscles

• lateral somatic cell columns to extraocular, while central caudal nucleus controls levator

• parasympathetic preganglionic to ciliary ganglion, postganglionic to pupillary constrictors

• III nuclei ventral to periaqueductal gray in midbrain, the autonomic Edinger-Westphal nucleus most medially, and surrounded by the somatic motor nuclei of III

• involved in pupillary light reflex (described next) and accomodation (increase in curvature for near vision)

• convergence (eyes cross slightly when object approaches face) requires central nucleus of Perlia

 

 

 

 

 

 

 

pupillary light relfex

• constrict both pupils when light on one retina:

visual input –> retina –>brachium of superior colliculus —> pretectal nuclei –> parasympathetic preganglionic neurons –> some decussate in posterior commissure –> outputs to oculomotor nucleus then nerve –> synapse in ciliary ganglion –> postganglionic axons on to constrictor (sphincter) muscles of iris

• response is consensual if you illuinate one eye, constrict pupil on that side (ipsilateral or direct response), and contralateral eye (conssnsual response)

interstitial nucleus

• sup and medial vesibular nuclei, pretetcum, FEF, fasigial nucleus –> interstigial nucleus –> cross in ventral posterior commissure –> all somatic cell columns of oculmotor complex excet ventral; slow rotatory and vertical eye movements, posture, head movement

nucleaus Darkschewitsch

• found in ventrolateral gray

• projects to posterior commissure, though not into III

posterior commissure nuclei

• involved in vertical eye movements

prectectal region

• rostral to superior colliclus at levels of posterior commissure

• receives retinal, visual cortical, and LGN input

• pretectal olivary nucleus part of direct and consensual pupillary light reflex

posterior commissure

MARKS transistion from midbrain to diencephalon ependyma of cerebral aqueduct beneath post comm has tall columnar ciliated cells called the subcommissural organ, a secretory and circumventricular organ

 

IV: trochlear nerve

• goes to superior oblique which attaches to superior posterior aspect of the eye, when it shortens it pulls the rear of the eye up (depressing the eye down) and pulls the eye nasally

• trochlear because it loops through a notch of bone/cartilage called the trochlea (Greek = pulley)

• trochlear nucleus found at level of inferior colliculus, ventral to PAG

• trochlear axons decussate within CNS (decussating fiber move dorso-caudally, and make up superior medullary velum which acts as a roof over fourth ventricle)

• exits dorsal (only cranial nerve to do so)

• joins III entering cavernous sinus and through optic foramen to eyeball

 

 

 

 

 

 

 

 

 

 

red nucleus

• located in midbrain tegmentum

• caudal magnocellular, rostral parvocellular

• inputs from deep cerebellar nuclei and precentral/premotor/SMA cerebral cortex

• sup. cbll. peduncle decussates completely

• note that sup. cbll. peduncle decussates on way from cerebellum into red nucleus, and that rubrospinal projections decussate on way from red nucleus into spinal cord: why the double decussation? —> red nucleus alligned with cerebrum (controls controlateral muscles), while cerebellum is alligned with

• rubral efferents: cross in ventral tegmental decussation

1) to interposed nuclei of cerebellum

2) principal sensory and spinal trigeminal nuclei

3) facial nucleus

4) medullary relay nuclei

5) spinal cord

• stimulate red nucleus causes

1) EPSP in contralateral flexor alpha MN

2) IPSP in contra extension alpha MN

• red nucleus transmits flexor muscle tone

special note about oculomotor control:

• Primary motor cortex DOES NOT STEER eye movements

• rather areas in frontal lobe rostral to primary motor cortex (such as area 8, the frontal eye fields) and parietal areas play a role in reflexive and voluntary eye control)