Brainstem (Week 2--Houser)

Question 1

Why is the brainstem so important?

Answer 1

1) Ascending and descending tracts of the spinal cord pass through
2) Cranial nerves located here
3) Centers for regulation of respiration, cardiovascular activity, consciousness, sleep-wake cycle all here

Question 2

Caudal to rostral organization of brainstem

Answer 2

Continuous with spinal cord caudally

Brainstem begins at foramen magnum

Medulla oblongata, pons, midbrain

Rostrally is diencephalon (thalamus)

Question 3

Dorsal to ventral organization of the brainstem

Answer 3

Dorsally is tectum, which is the location of the superior and inferior colliculi

Cerebral aqueduct and 4th ventricle

Tegmentum covers entire brainstem (region of cranial nerve nuclei, reticular formation and chemically-identified systems)

Basis is most ventral (region of many ascending and descending tracts)

Question 4

Caudal medulla

Answer 4

Gray matter surrounds central canal that is continuous with spinal cord

Dorsal columns of spinal cord extend to medulla and fasciculus gracilis continues to nucleus gracilis and fasciculus cuneatus continues to nucleus cuneatus

On ventral surface, have pyraminds (myelinated fibers) and anterior median fissure

Question 5

Rostral medulla

Answer 5

Central canal expands into 4th ventricle on dorsal surface; lower apex of 4th ventricle where it narrows into central canal is the obex

Area postrema is in walls of ventricle at the obex, and this region has no normal BBB but instead monitors blood for toxins and can trigger vomiting (“vomiting center”)

Hypoglossal nucleus and dorsal motor nucleus of the vagus are most medial on floor of 4th ventricle

On ventral side of rostral medulla, have pyramids and olives (more lateral) with inferior olivary nucleus (looks like mini-brain…) that provide major input to cerebellum

Inferior cerebellar peduncles (restiform body) are dorsolateral and extend toward cerebellum

Nucleus ambiguus

Question 6

Caudal Pons

Answer 6

Transversely oriented mass of fibers on ventral surface in basis pontis

Looks like “bridge” between cerebellar hemispheres but fibers do not connect cerebellar hemispheres, they connect pontine nuclei (within the basis pontis) to the cerebellum

Brachium pontis or middle cerebellar peduncle are large groups of fibers that enter cerebellum

Large 4th ventricle

Question 7

Superior, middle and inferior cerebellar peduncles

Answer 7

Superior cerebellar peduncle: rostral pons, roof of 4th ventricle

Middle cerebellar peduncle: pons

Inferior cerebellar peduncle: rostral medulla, dorsolaterally

Question 8

Rostral Pons

Answer 8

Brachium conjunctivum or superior cerebellar peduncle is at roof of 4th ventricle

4th ventricle is closing down

Question 9

Caudal midbrain

Answer 9

4th ventricle has turned into cerebral aqueduct (aqueduct of Sylvius)

Periaqueductal gray is gray matter around cerebral aqueduct

Cerebral peduncles on ventral surface, contain large groups of (myelinated) fibers descending from cortex to brainstem and spinal cord (corticopontine and corticospinal fibers)

Inferior colliculi are auditory

Superior cerebellar peduncle crossing

Question 10

Rostral midbrain

Answer 10

Mickey mouse with “o” for mouth!

Cerebral aqueduct

Superior colliculi are visual (not in main visual path but receive visual input in parallel with lateral geniculate nucleus of thalamus and participate in visuomotor control)

Cerebral peduncles on ventral surface, contain large groups of fibers descending from cortex to brainstem and spinal cord (corticopontine and corticospinal fibers)

Interpeduncular fossa between peduncles (oculomotor nerves emerge from interpeduncular fossa)

Substantia nigra is dorsal to axons of cerebral peduncle and divided into pars compacta (dorsal, contains DOPA neurons) and pars reticulata (ventral, contains GABA neurons); all part of basal ganglia system

Red nuclei?

Question 11

Superior and inferior colliculi

Answer 11

Superior colliculi: visual; dorsal surface of (rostral) midbrain

Inferior colliculi: auditory; dorsal surface of (caudal) midbrain

Question 12

Substantia nigra

Answer 12

In midbrain

Part of basal ganglia

Lots of cell bodies

Pars compacta: dorsal, contains DOPA neurons

Pars reticulata: ventral, contains GABA neurons (in ventral tegmental area (VTA))

Question 13

How are brain slices usually oriented?

Answer 13

Dorsal down

Ventral up

Question 14

Location of cranial nerve nuclei vs. nerves

Answer 14

Nuclei (cell bodies): located dorsally

Nerves exit ventrally or laterally

Question 15

Idealized (general) view of organization of brainstem nuclei at level of medulla

Answer 15

Motor nuclei medial, sensory nuclei lateral lined along dorsal surface of medulla (adjacent to 4th ventricle)

Separated by sulcus limitans (just as they are in the spinal cord)

This is general plan of cranial nerve nuclei but during development some nuclei shift ventrally and pattern is disrupted

Question 16

Order of cranial nerves rostrally to caudally

Answer 16

Ascending numerical order rostrally to caudally starting with CN III in midbrain and XII in medulla

Question 17

CN III (oculomotor)

Answer 17

Somatic motor nucleus and visceral motor (autonomic) nucleus (Edinger-Westphal nucleus)

Motor neurons innervate all extraocular eye muscles (incl LPS) except SO4 and LR6 to move eye up and in

Motor neurons except superior rectus project to ipsilateral eye

Emerges from interpeduncular fossa of midbrain

Question 18

Damage to CN III (oculumotor)

Answer 18

Damage to CN III causes down and out deviation of the eye due to unopposed action of superior oblique and lateral rectus

Lateral position of eye is called lateral strabismus

Patient will have diplopia (double vision)

Lose innervation of LPS which causes ptosis (drooping of eyelid)

Axons of fibers to extraocular muscles are on the inner surface of the nerve and are sensitive to vascular disease

Usually compressed between medial temporal lobe, cerebral peduncle and edge of tentorium

Question 19

Edinger Westphal nucleus

Answer 19

Provides autonomic component of CN III

Para pre neurons that form synapses in ciliary ganglia and then postganglionic neurons innervate pupillary constrictor and ciliary muscles

Axons of neurons in E-W nucleus travel with CN III

Normal function is to provide pupillary constriction

Question 20

Damage to E-W nucleus

Answer 20

Damage to E-W nucleus causes dilation of pupil (mydriasis) on ipsilateral side

Patient will have no pupillary light reflex (because can’t constrict pupil!)

Axons of these neurons are on the external surface of the nerve so are damaged first, sensitive to compression, so might see pupil dilation (blown pupils) as first symptom of CN III damage

Note: other eye WILL have reaction to light when light shined in contralateral eye because signal to both E-W nuclei from shining light into just one eye!

Question 21

Pathway of pupillary light reflex

Answer 21

1) Light into one eye
2) Signal travels bilaterally to pretectum
3) Signal to E-W nucleus
4) Signal to ciliary ganglion
5) Para post to pupillary constrictor to constrict pupils

Question 22

CN VI (abducens)

Answer 22

Innervates lateral rectus and abducts eye

Smallest and most medially located CN (out of VI, VII, VIII) that leaves the ventral surface of the brainstem along the groove between the basis pontis and medulla–just medial to medial longitudinal fasciculus (MLF)

Normally, CN VI innervates LR on same side and projects through MLF on opposite side to innervate CN III neurons for medial rectus (lateral gaze so both eyes look at the same thing)

Question 23

Damage to CN VI (abducens)

Answer 23

Damage to CN VI will cause eye on affected side to deviate medially (medial strabismus) and have difficulty moving laterally (abduction)

Also cannot move contralateral eye medially

Together, this is lateral gaze paralysis

Usually compressed over the temporal bone

Question 24

CN IV (trochlear)

Answer 24

Innervates superior oblique muscle on the contralateral side

Superior oblique moves eye downward and partially in (adducted)

Nerve fibers come out dorsally below inferior colliculus of midbrain and above pons??

Question 25

Damage to CN IV (trochlear)

Answer 25

**Damage** to CN IV causes **diplopia** (vertical) and affected (contralateral) eye appears slightly **elevated** and has trouble moving down and adducted Diplopia may be most noticeable when person looking down

Question 26

CN XII (hypoglossal)

Answer 26

Innervates muscles of the **tongue** Nucleus is strung out almost entire way down medulla, dorsal and near 4th ventricle? Nerve fibers emerge from a sulcus lateral to each pyramid, **between pyramid and olive**

Question 27

Damage to XII (hypoglossal)

Answer 27

Damage to CN XII causes tongue **deviation to affected/weak side** Possible **atrophy** of the tongue (so we know this is lower motor neuron damage)

Question 28

CN VII (facial)

Answer 28

Innervates **muscles of facial expression** Nucleus in **central pons**, ventral?

Question 29

Damage to CN VII (facial)

Answer 29

**Damage** to CN VII causes **facial paralysis** or **weakness** (generally of both upper and lower face) on **ipsilateral** side

Question 30

Ambiguus nucleus

Answer 30

Motor neurons in this nucleus innervate **pharynx** and **larynx** and **axons are distributed in CN IX and X** Damage to this nucleus causes **difficulty swalliwing** and **laryngeal function (hoarseness)** Located in **rostral medulla**

Question 31

Dorsal motor nucleus of the vagus nerve

Answer 31

Major **parasympathetic** nucleus of the brain Contains cel bodies of **para pre** fibers that influence **thoracic** and **abdominal viscera** Remember that vagus nerve has other motor and sensory fibers, but those axons have cell bodies in other nuclei (ambiguus and solitary nuclei)

Question 32

Trigeminal (V) sensory nuclear complex

Answer 32

Responsible for reception and initial processing of tactile, proprioceptive, pain and temp information from the head --\> transmission to cerebral cortex, cerebellum, reticular formation **Three sensory nuclei** associated with trigeminal afferents and span rostral midbrain all the way down to upper cervical spinal cord: 1) **Mesencephalic nucleus** (**proprioception**, **muscle spindle afferents**; most rostral) 2) **Principal sensory nucleus** (**fine touch**; near center of column in mid pons) 3) **Spinal nucleus** (**pain** and **temperature**; in caudal medulla looks like dorsal horn of spinal cord; most caudal)

Question 33

Spinal nucleus

Answer 33

**Pain** and **temperature** Primary afferents enter through 3 divisions of trigeminal nerve (ophthalmic, maxillary, mandibular) and cell bodies located in **trigeminal ganglion** Enter brainstem at level of pons then fibers enter s**pinal trigeminal tract** just **lateral to spinal trigeminal nucleus** Fibers **descend** and terminate on **spinal trigeminal nucleus** (divided into oral, interpolar and caudal regions with caudal nucleus particularly important in processing pain and temperature)

Question 34

Reflexes that depend on sensory nuclei associated with trigeminal afferents

Answer 34

**Corneal reflex**: touching cornea of one eye causes bilateral blinking and closing of the eyes; depends on **spinal nucleus of V pathway** **Jaw reflex**: tap chin and get masseter reflex; afferent limb of reflex is neuron from **mesencephalic trigeminal nucleus** which goes to **motor nucleus of CN V**, and motor neuron goes back to masseter (muscles of mastication by V3); depends on **mesencephalic nucleus pathway**

Question 35

Main (principal) sensory nucleus

Answer 35

Discriminative **tactile** and **proprioceptive** sensation Homologous to dorsal column nuclei of dorsal column-medial lemniscus tract Many fibers from main sensory nucleus cross midline and **JOIN medial lemniscus** to ascend to thalamus where info can get to cortex

Question 36

Mesencephalic trigeminal nucleus

Answer 36

Receives **Ia afferents** from **muscle spindles** in muscles of mastication and from **mechanoreceptors** in gums, teeth and hard palate Afferent fibers enter brainstem through **mandibular division of CN V** (just like other afferents), but their cell bodies are in **mesencephalic trigeminal nucleus within CNS** (not trigeminal ganglia) Some central processes end in **motor** and **sensory** **nuclei** of CN V but others enter **cerebellum**

Question 37

Nucleus of the Solitary Tract (solitary nucleus)

Answer 37

Group of **central fibers (tract)** surrounded by **cell bodies (nucleus)** Rostral part mediates **taste** sensations (VII, IX, X) Caudal parts receive information from visceral receptors that participate in **cardiovascular** and **respiratory** **reflexes** (IX, X)

Question 38

Auditory nuclei (VIII)--Central Pathway

Answer 38

Fibers of the auditory system take **different** **routes** on their way from cochlea to cerebral cortex, and cross at multiple locations 1) Primary auditory fibers in **cochlea** and their central processes form cochlear division of vestibulo-cochlear nerve (**CN VIII**) 2) Fibers enter brainstem near **ponto-medullary junction** and terminate in **dorsal cochlear** and **ventral cochlear nuclei** (located immediately lateral/dorsal to to inferior cerebellar peduncle in rostral medulla) 3) Fibers from cochlear nuclei **cross** midline to enter **lateral lemniscus** which projects to **inferior colliculus** in midbrain see steps 4-5) 4) Axons from **dorsal cochlear nucleus** cross midline and enter lateral lemniscus **without any synapses** but axons from **ventral cochlear nucleus take a little longer**--they project to superior olivary nuclei on either ipsilateral or contralateral side 5) Axons from **ventral cochlear nucleus** that went to **superior olivary complex** cross midline to form **trapezoid body** (located ventral to medial lemniscus) then join **lateral lemniscus** as it ascends to **inferior colliculus** 6) Neurons in **inferior colliculus** project to **medial** **geniculate**, a relay nucleus of the thalamus. The fibers form the **brachium of the inferior colliculus** 7) Neurons in medial geniculate project to p**rimary auditory cortex** (areas 41 and 42) which is located on superior surface of **temporal lobe**

Question 39

Damage where causes unilateral vs. bilateral hearing loss?

Answer 39

**Unilateral** hearing loss only if damage to **cochlear** **nuclei** (dorsal or ventral) As soon as you pass that point, will get **bilateral** hearing loss

Question 40

Corticobulbar pathway

Answer 40

From **cortex** to **brainstem** (**descending** fibers) Cerebral cortex gives info to **motor cranial nerve nuclei** Fibers **accompany corticospinal fibers** within **internal capsule and cerebral peduncle** (basis pedunculi) and continue to brainstem nucleus that they innervate, then synapse on interneurons or motor neurons directly

Question 41

Corticobulbar control

Answer 41

Supranuclear innervation of motor cranial nerve nuclei/corticobulbar control is primarily **bilateral** No clear area of decussation However, **contralateral is often stronger** AND **exception** is that neurons to **lower face** receive innervation primarily from contralateral cortex

Question 42

Damage to corticobulbar fibers to the face (within the brain) vs. damage to facial nerve or nucleus

Answer 42

Damage to **corticobulbar** **fibers** to the face (within the **brain**): weakness or paralysis of **lower face only**, on **opposite side** of lesion Damage to **facial nerve or nucleus:** weakness or paralysis of **entire face** on **same side** as lesion

Question 43

Symptoms if you get damage to right pyramid and medial lemniscus at the level of the rostral medulla

Answer 43

**Left side body weakness** because damaged right pyramid part of **corticospinal** tract **Left side loss of position sense** because damaged right medial lemniscus part of **dorsal column-medial lemniscus tract** **Tongue deviates to right** because damaged **hypoglossal nerve**

Question 44

Reticular formation

Answer 44

Occupies **central core** of brainstem, located within brainstem **tegmentum** (dorsal part of brainstem) Many aggregations of neurons within network of interlacing fibers traveling in many directions--**"net-like"** appearance Cells in reticular formation give rise to **long** ascending and descending fibers and these cells can influence each other via collaterals

Question 45

Connections in reticular formation

Answer 45

Convergence of **many types of afferents** Receives sensory information from **spinal cord, sensory cranial nerves, cerebellum, hypothalamus, basal ganglia** and **cerebral cortex** Efferents of reticular formation go to **spinal cord, midline** and **intralaminar nuclei of thalamus** and **hypothalamus**

Question 46

Functions of reticular formation

Answer 46

**Motor activity** **Respiratory** and **cardiovascular** functions Mechanisms of **sleep** and **consciousness**

Question 47

Eye movement control centers in reticular formation

Answer 47

**Lateral gaze center** generates rapid horizontal movements of the eye Near midline of pons, near abducens nucleus in a region called **paramedian pontine reticular formation (PPRF)** Receives input from cortical eye control regions in **contralateral cortex** (**eye fields**) then PPRF projects to neurons in **abducens nucleus** --\> **MLF** --\> **medial rectus motor neurons** in contralateral oculomotor nucleus; also projects to **lateral rectus on ipsilateral side** Each PPRF generates lateral gaze to **ipsilateral** side as PPRF

Question 48

Ascending reticular activating system

Answer 48

Maintenance of **consciousness**: normal cerebral cortex not capable of functioning in conscious manner unless it has input from reticular activating system of reticular formation in brainstem! **Widespread cortical activation** Areas of reticular formation that project to thalamic nuclei (midline and intralaminar nuclei) which in turn project to cerebral cortex **Midbrain-pontine reticular formation** (top of pons) is primary location of this system

Question 49

Motor pathways of the brainstem

Answer 49

These function in conjunction with corticospinal pathway to provide lots of **purposeful and automatic movement patterns** **Lateral pathways**: lateral corticospinal tract, rubrospinal tract **Intermediate**: lateral (medullary) reticulospinal **Medial pathways**: medial (pontine) reticulospinal, vestibulospinal, tectospinal

Question 50

General principle of what lateral vs. medial motor pathways do

Answer 50

**Lateral** pathways: **flexor** muscles and **fine** **motor** control, especially of upper limbs **Medial** pathways: **extensor** muscles and **postural** control

Question 51

Corticospinal and corticobulbar tracts

Answer 51

Provide most direct and specific projections from **cortex** to **motor neurons of spinal cord and brainstem** Some fibers from cortical neurons synapse **directly with motor neurons** Other neurons contact **interneurons** involved in various **reflex pathways**

Question 52

Rubrospinal tract

Answer 52

Very close to lateral corticospinal tract Originates in **magnocellular region of red nucleus**, crosses midline, **descends in lateral column** as rubrospinal tract (through **cervical levels only**) Neurons that give rise to this pathway receive afferents from cerebral cortex (corticorubral pathway) and cerebellum Facilitates **flexors** of upper limb and contributes to **skilled**, **goal-directed movements**

Question 53

Reticulospinal tracts

Answer 53

Major alternatives to (and cooperate with) the pyramidal tract for control of motor neurons **Lateral** (**medullary**) reticulospinal tract: facilitate voluntary motor activity and reduce strength of spinal reflexes **Medial** (**pontine**) reticulospinal tract: facilitates **extensor** muscles

Question 54

Lateral (medullary) reticulospinal tract

Answer 54

Originates in region **dorsal to inferior olivary complex** and descends in **anterior lateral columns** Originates from **bilateral** regions of reticular formation, but **ipsilateral** **projections** are most numerous May **reduce strength of some spinal cord reflexes** and facilitate more **voluntary** control of movement; when **intensely** activated, may produce **loss of muscle tone** and may be associated with decreased muscle tone that occurs during REM sleep

Question 55

Medial (pontine) reticulospinal tract

Answer 55

Originates from **pontine tegmentum** and descends ipsilaterally in **ventral column** **Medial** motor pathway Facilitates **proximal** and **extensor** muscles

Question 56

Lateral vestibulospinal tract

Answer 56

Originates from **lateral vestibular nucleus** and descends ipsilaterally in **ventral column** to all levels of spinal cord Receives input from **vestibular portion of CN VIII** and from **cerebellum** Facilitates **extensor** muscles of upper and lower limbs (is a **medial** pathway)

Question 57

What does it mean to be a medial pathway?

Answer 57

Medial pathways help with **extension**, **support**, adjustment in **posture**

Question 58

Cortex to brainstem to spinal cord pathways

Answer 58

All cortex to brainstem to spinal cord pathways have crossed by the time they reach the spinal cord even though decussation varies among pathways So obvi motor pathways that originate from right cortex influence primarily left side of body **Lateral vestibulospinal path is an exception** because receives little input directly from cortex and has ipsilateral influences

Question 59

Decorticate pattern of spasticity

Answer 59

An upper motor neuron syndrome Due to lesions of **motor** pathways **above red nucleus** Decorticate pattern of spasticity is **flexion** **of upper limbs** and extension of lower limbs (antigravity muscles) Common in lesions of **internal capsule, cerebral cortex, brainstem**

Question 60

Decerebrate pattern of spasticity

Answer 60

An upper motor neuron syndrome Massive brainstem lesion of **motor** pathway in brainstem **below red nucleus** Decerebrate pattern of spasticity is **extension** of body and all limbs

Question 61

Three neurotransmitter systems of the brainstem

Answer 61

**Serotonergic** **Noradrenergic** **Dopaminergic** Note: the brainstem is the location of many cell bodies of neurons in these systems but their axons are widely distributed throughout the CNS and many different types of receptors allow for many different effects

Question 62

Serotonergic pathways

Answer 62

Located in **raphe** nuclei that form continuous column near midline throughout brainstem tegmentum Logical organization with more rostrally located groups projecting to forebrain and cerebellum and more caudally located groups projecting to spinal cord Some fibers terminate in **substantia gelatinosa** to form part of **central pain control system** Ascending fibers involved in **sleep-wake cycle** (neurons in raphe increased activity during wakefulness and decreased activity with onset of sleep) Ascending fibers involved in **affective behavior** and may contribute to **mood elevation** (SSRIs used to treat depression)

Question 63

Noradrenergic pathways

Answer 63

Neurons located in cell groups of **pontine** and **medullary** **reticular** **formation**, including **locus ceruleus** in pons (just rostral to facial colliculus) Neurons have a **bluish** tint (locus ceruleus means blue place) Projections very **widespread**, branch extensively, and affect almost every region of brain and spinal cord Locus ceruleus plays important role in **sleep-wake cycle** Release of noradrenaline increased during periods of vigilance or increased **attention**, and this system may enhance ability of target neurons to respond to other inputs such as those from sensory systems

Question 64

Dopaminergic pathways

Answer 64

Neurons located in **midbrain** and concentrated in **substantia nigra** and **ventral tegmental area** Two major projection systems arise: 1) Substantia nigra, pars compacta to striatum (caudate and putamen) is called **nigrostriatal system**; loss of dopaminergic neurons in substantia nigra found in **Parkinson's disease** 2) Ventral tegmental region to septum, amygdala and frontal lobe are **mesolimbic** and **mesocortical** **dopaminergic systems**; overactivity in these systems hypothesized as cause of types of **schizophrenia**

Question 65

Corneal reflex

Answer 65

Touch the eye and both eyelids blink **Afferent** limb is the **trigeminal** nerve Trigeminal afferent fibers with **cell bodies in trigeminal ganglion** project directly to the **spinal nucleus of V** **Efferent** limb is **facial nerve** (which makes your orbicularis oculi close the eyelids on both sides)