brain stem & cranial nerves

acetylcholine (ACh)

• regulates excitability of thalamus

• memory and learning

• prominent role in PNS

  • neurotransmitter at NMJ

• production: pons & midbrain, basal forebrain

norepinephrine (NE)

• increases brain responsiveness and speeds information processing

  • attention, sleep, wake, learning, memory, anxiety

• production: pons (locus coeruleus)

serotonin

• regulation of cortical arousal

• sleep/wake

• control of mood

• agression

• pain modulation

• production: raphe nuclei (medulla), pons, midbrain

dopamine

• reward and pleasure via sccumbens nucleus

• cognition

• motivation

• initiation of movement

• production: substantia nigra (in midbrain), ventral tegmental area in midbrain

motor pathways

• pyramidal

  • voluntary motor control

  • originate in M1

  • cross in caudal medulla

    • LCST

    • medial CST

    • corticobulbar

• extrpyramidal

  • automatic control

    • postural adjustments

  • originate in brain stem

  • rubrospinal

  • reticulospinal

  • tectospinal

  • vestibulospinal (vestibular nucleus)

endogenous pain control

*reduces perception of pain

• the brain changes/modifies neural information

• descending control

• areas involved:

  • periaqueductal gray

    • midbrain —> enkephalins

  • locus coerelus

    • pons —> NE

  • raphe nucleus

    • medulla —> serotonin

extrapyramidal descending pathways

*know start/stop and general function

key functions of brain stem

• vital functions

  • respiration, HR, BP, vomiting, micturition

  • more caudal (medulla, pons)

• consciousness

  • more rostral (midbrain)

  • arousal

  • sleep/wake

  • alertness

• conduction of pathways

  • skeletal muscle

  • facial

• integration

  • sensory to motor processes

midbrain structures

• cerebral peduncle

• rostral border

  • crus cerebri (covering)

  • cerebral peduncle (white matter)

• 3 regions

  • tectum (dorsal aspect of midbrain)

    • superior colliculi

      • reflex orienting of eyes

      • receives info from visual, auditory and somatosensory systems

      • start of tectospinal pathway

    • inferior colliculi

      • processing and localization of sound

  • tegmentum (ventral aspect of midbrain)

    • CN III and IV nuclei

      • control head, face and neck

    • periaqueductal gray —> dopamine (substantia nigra, VTA)

    • red nucleus (rubrospinal tract)

pons structures

• dorsal border —> 4th ventricle

• ventral protrusion contains motor info

  • pontine nuclei

  • CST and corticobulbar fibers

  • major input into Cb

• dorsal contain sensory info

  • STT and DCML fibers

  • trigeminal tract

• nuclei for CN V-VIII

  • vestibular nucleus (vestibulospinal tract)

• locus coereulus

  • NE

  • dorsal aspect

medulla structures

• dorsal border —> 4th ventricle

• caudal border —> end of 4th ventricle

• ventral surface

  • pyramids

    • CST and corticobulbar

  • olives

    • lateral to pyramids

    • motor learning

• dorsal surface

  • dorsal columns (nucleus gracile and cuneate)

• nuclei CN IX-XII

blood supply

• medulla

  • PICA

  • vertebral

• pons

  • basilar

  • AICA

• midbrain

  • PCA

  • SCA

reticular formation

• extends entire brain stem

• network of nuclei and axonal connections

• integrating, coordinating and influencing wide areas of CNS and PNS

• extends from midbrain and projects through cervical SC

reticular formation (rostral vs. caudal)

• rostral

  • consciousness

  • arousal

  • sleep/wake

• caudal

  • vital function

    • cardiovascular

    • autonomic rhythms of respiration

  • patterned behaviors

reticular formation medial vs. lateral

• medial

  • coordination of head/eye movements

  • somatosensory, vestibular and proprioceptive units

• lateral

  • interneurons

    • influence CN nuclei and limbic system

  • premotor neurons that project to SC

    • autonomic functions

      • respiration

      • BP

      • abdominal pressure

      • micturition

reticular activating system (RAS)

• projections from reticular formation to subcortical, cortical region and SC

  • major role as integration and relay center

• significant contributor to

  • attention

  • arousal

  • pain modulation

  • modulation of muscle tone

ascending RAS

• project to thalamus and cortices

  • dopaminergenic (VTA)

  • noradrenergic (NE, locus coerelus)

  • serotonergic

  • histaminergic

  • ACh

  • glutamate

descending RAS

• motor

  • reticulospinal tract (starts in RAS)

    • modulate tone, balance, posture

    • coordinate body movements influenced by sensory info

  • vestibulospinal —> posture

  • pain modulation

corticobulbar tract

• function

  • voluntary motor control for head and eyes (not reflexive)

• pathway

  • M1 (descending)

  • internal capsule (white matter)

  • crus cerebri/cerebral peduncles

  • descends to brain stem

    • crosses at level of CN nuclei

    • mostly bilateral projections

      • upper face in bilateral

      • lower face is unilateral

  • innervates CN nuclei

    • CN are LMN

CN VII (facial expression)

trigeminal pathway

function:

• light touch, proprioception, pain & temperature

• all conscious sensation in face

• analogous to DCML and anterolateral systems for body

*trigeminal nerve = CN V —> provides sensory info to nuclei in BS

—> trigeminal nuclear complex:

• 3 nuclei:

  • mesencephalic (midbrain)

    • proprioception

  • principal (chief/main)

    • pons

    • light/fine touch

  • spinal nucleus

    • pons

    • pain and temperature

1st order:

• trigeminal ganglia

• CN V nuclei —> synapse on 1 of 3 types

2nd order:

• cross and ascend via trigeminal mensicus

• synapse in ventroposteromedial nuclei (VPM) of thalamus

3rd order:

• ascends through posterior limb of internal capsule to S1

cranial nerves

• nerves of eyes, head, and neck

  • sensory and motor

• nuclei are mostly located in BS

• motor neurons are LMN —> injury to CN = LMN injury

• voluntary conscious motor output driven by corticobulbar tract

• conscious sensation communicated through trigeminal pathway

CN I

*olfactory nerve (only sensory info that does not connect with thalamus)

function: smell

pathway:

• chemoreceptors in nose communicate with olfactory bulb

• direct ipsilateral connection to cortex

  • not thalamus

• processed in temporal lobe

  • limbic region

CN II

*optic nerve

function: vision

pathway:

• emerge in retina

not a true cranial nerve (better described as a tract)

CN III

*oculomotor nerve (midbrain)

function:

• extra ocular muscles of eye

  • up, down, ADDuction

  • raising eyelid

• autonomic function

  • accommodation reflex (near focus)

    • control ciliary muscles that change shape of lens for near viewing

  • pupillary constriction

    • sphincter muscles around the iris

    • parasympathetic

CN IV

*trochlear nerve (midbrain)

function:

• extraocular muscles of eye

  • eye movement: down and in (adduction)

unique pathway:

• only CN to exit on dorsal aspect of brain (has to wrap around) —> very vulnerable to injury

• contralateral eye muscles

CN V

*trigeminal nerve (pons)

function:

• sensory: somatosensory nerve for face

• motor: muscle of mastication

3 divisions:

• ophthalamic

• maxillary

• mandibular

CN VI

*abducens nerve (pons)

function:

• ABduction

CN VII

*facial nerve (pons)

function:

• motor: muscles of facial expression

• sensory:

  • sensation around ear

  • taste —> anterior 2/3 of tongue

• autonomic

  • lacrimation, salivation

*as CN VII exits BS it wraps around CN VI —> frequently injured together

• driven by corticobulbar tract

bell’s palsy

CN VII (pons)

• most common cause of facial paralysis

• thought to be caused by herpes virus (HPV)

  • via retrograde transport

  • one-sided facial droop

    • LMN injury

CN VIII

*vestibulocochlear (pons)

function:

• vestibular

• hearing

CN IX

*glossopharyngeal nerve (medulla)

function:

• taste (bitter)

• salivation

• gag reflex

• contributes to swallowing and phonation

CN X

*vagus nerve (medulla)

• longest cranial nerve

• descends through neck, exits out of CNS

function:

• muscles of phonation and swallowing

• parasympathetic

  • heart, lung, digestive tract

• sensory:

  • pharynx, meninges, small region around ear

  • taste from epiglottis and pharynx

CN XI

*spinal accessory nerve (medulla)

function:

• sternocleidomastoid —> turns head

• trapezius —> elevates shoulder

CN XII

*hypoglossal nerve (medulla)

function:

• muscles of tongue

• clinically significant

  • deviation of tongue towards the side of weakness

UMN vs LMN

• damage CN XII - LMN

• damage CBB - UMN

  • damage L CBB —> cannot provide R hypoglossal nerve

posturing in brain stem injury

*rubrospinal (UE flexors) and reticulospinal (trunk and proximal muscle extension/flexion) are involved

• decorticate posturing

  • flexor posturing

  • UE flexion/LE extension

  • rubrospinal intact

• decerebrate posturing

  • extensor posturing

  • UE and LE extension

  • rubrospinal no longer intact

  • reticulospinal intact