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 spinal 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 primary 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)