Brainstem I

General Information – Brainstem

  • Corridor for all sensory, motor, cerebellar, and cranial nerve information
  • Contains nuclei for the cranial nerves and cerebellum, consciousness, tone, posture, cardiac and respiratory functions, etc.
  • Both Grand Central Station and Central Power Supply


Surface Features of the Brainstem

  • Located within the posterior fossa
  • Cranial nerves emerge roughly in numerical sequence from rostral to caudal
  • Rostral limit of the brainstem occurs at the midbrain-diencephalic junction; Caudal limit of the brainstem occurs at the cervicomedullary junction


Brainstem Anatomy – Overview



Dorsal surface

  • Superior/inferior colliculi (form tectum -“roof”- of midbrain)

Ventral surface

  • Cerebral peduncles




Dorsal surface

  • Limited by 4th ventricle

Dorsolateral surface

  • Attached to cerebellum via the cerebellar peduncles




Ventral surface

  • Pyramids descending to pontomedullary junction to pyramidal decussation

Rostral surface

  • Inferior olivary nuclei

Caudal surface

  • Posterior columns and posterior column nuclei


Sensory and Motor Organization of the Cranial Nerves

  • 3 motor columns and 3 sensory columns that run in an interrupted fashion thru brainstem
  • Cranial nerves analogous to spinal nerves (Motor nuclei is located ventrally while sensory nuclei is located dorsally)



Somatic Motor Nuclei

  • Innervate extraocular and intrinsic tongue muscles

Branchial Motor Nuclei

  • CN V, VII, IX, X, XI
  • Innervate muscles derived from branchial arches, including muscles of mastication, facial expression, middle ear, pharynx, larynx, sternomastoid, and upper portion of trapezius

Parasympathetic Nuclei

  • CN III, VII, IX, X
  • Provide preganglionic parasympathetic fibers innervating glands, smooth muscle, and cardiac muscle of the heart, lungs, and digestive tract




Visceral Sensory Column

  • CN VII, IX, X, IX
  • Receives taste input, as well as inputs for control of cardiac, respiratory, GI, sleep regulation

General Somatosensory Nuclei OR Trigeminal Nuclei

  • CN V, VII, IX, X
  • Mediate touch, pain, temperature, position, vibration sense for face, sinuses, and meninges

Special Somatic Sensory

  • Special senses are olfaction, vision, hearing, vestibular, sense, and taste
    • Olfaction (I) and vision (II) do NOT have primary sensory nuclei in brainstem
    • Brainstem special somatic sensory nuclei mediate hearing and positional equilibrium


Cranial Nerves

Assessed with a neurologic exam

CN I Olfactory Nerve


  • Functional Category: Special somatic sensory

Function: Olfaction

  • Pathway: chemoreceptors in nasal cavities —> olfactory nerves (exit out the cribriform plate) —> olfactory bulbs (orbitofrontal) —> olfactory tracts —> olfactory processing areas
  • Lesion Information
  • Causes: head trauma, neoplasms (typically meningiomas), basal meningitis, etc.
  • Results in: Anosmia (olfactory loss)
  • Unilateral: rarely aware because contralateral can compensate; should test each nostril separately
  • Bilateral: usually aware; will affect taste


CN II Optic Nerve


  • Functional Category: Special somatic sensory

Function: Vision

  • Pathway: retinal ganglion cells —> optic nerve (out optic canal) —> optic chiasm —> optic tract —>
  • General Information
  • Two functions in transmitting light: 1) visual information to cortex and 2) light intensity to brainstem
  • Unlike all other nerves (except bit of acoustic) is coated with myelin so susceptible to CNS illnesses like MS
  • Lesion Information
  • Causes: glaucoma, optic neuritis, elevated ICP, optic glioma, schwannoma, meningioma, trauma
  • Results in: monocular visual loss or monocular scotomas; can be partial


CN III Oculomotor


  • Functional Category: Somatic motor

Function: All extraocular muscles, except superior oblique and lateral rectus

  • Functional Category: Parasympathetic

Function: Pupil constriction and accommodation reflex

  • Location: Nuclei in midbrain; traverses the cavernous sinus, exits skull via superior orbital fissure
  • Lesion Information
  • Leads to distinctive constellation: dilated pupil, ptosis, and outward deviation (abduction)


CN IV Trochlear


  • Functional Category: Somatic Motor

Function: Superior oblique muscle; causes depression and intorsion of eye

  • Location: Nuclei in midbrain; traverses the cavernous sinus, exits skull via superior orbital fissure


CN VI Abducens Nerve


  • Functional Category: Somatic motor

Function: Lateral rectus muscle, causes abduction of eye (turns eye out)

  • Location: Nuclei in pons; traverses the cavernous sinus, exits skull via superior orbital fissure
  • Lesion Information
  • Results in inward deviation, but NO ptosis or pupil changes


CN V Trigeminal Nerve


  • Functional Category: General Somatic Sensory

Function: General sensation for face, mouth, anterior 2/3rds of tongue, nasal sinuses, meninges

  • Functional Category: Motor (small motor root)

Function: Muscles of mastication and tensor tympani muscle

  • General Information

3 major branches: ophthalmic (V1), maxillary (V2), and mandibular (V3)

  • Location: all divisions enter pons; sensory nucleus extends from midbrain to spinal cord
  • Lesion Information
  • Disorders of trigeminal rare except for trigeminal neuralgia (tic douloureux): Patients experience recurrent episodes of brief severe pain; Usually begin after age 35; Often provoked by chewing, shaving, or touching trigger point on face; Cause mostly unknown although can occur in MS
  • Sensory loss in distribution of trigeminal nerve can be caused by trauma, metastatic disease, herpes zoster, aneurysms
  • Lesions of trigeminal brainstem nuclei cause ipsilateral loss of facial sensation


CN VII Facial Nerve


  • Functional Category: Branchial Motor (main function)

Function: Muscles of facial expression, stapedius muscle, and part of digastric muscle

  • Functional Category: Parasympathetic

Function: Parasympathetics to lacrimanal glands and salivary glands (except parotid)

  • Functional Category: Visceral Sensory

Function: Taste from anterior 2/3rds of tongue

  • Functional Category: General somatic sensory

Function: Sensation from a small region near the external auditory meatus

  • Location: nuclei and nerve entry points located in both pons and medulla
  • Lesion Information
  • Important to distinguish between facial weakness caused by UMN and LMN lesions
  • Unilateral UMN lesions: Spares forehead (mainly affects lower portion of contralateral side); also see “neighbor” effects such as arm weakness, sensory changes, aphasia, dysarthria
  • LMN lesions: Affect entire half of face (does NOT spare forehead); do NOT see neighbor effects
  • Bell’s Palsy (unilateral facial weakness): most common facial nerve disorder
  • All divisions of facial nerve impaired within few hours or days; Cause unknown, although perhaps viral or inflammatory; Also see retroauricular pain, hyperacusis, dry eye, ipsilateral loss of taste


CN VIII Vestibulocochlear Nerve


  • Functional Category: Special somatic sensory

Function: Hearing and vestibular sensation

  • Location: nuclei primarily in pons, but also medulla
  • Lesion Information
  • Unilateral hearing loss can result from disorders of external auditory canal, middle ear, cochlea, 8th nerve, or cochlear nuclei
  • Because info crosses bilaterally at multiple levels once enter brainstem, unilateral hearing loss is NOT caused by lesions in the CNS proximal to the cochlear nuclei
  • Impaired hearing divided into:
    • Conductive hearing loss: Caused by abnormalities of external auditory canal or middle ear
    • Sensorineural hearing loss: Caused by disorders of cochlea or 8th nerve
  • Most common tumor in region is acoustic neuroma (almost always unilateral except in NF2, where tumors can be bilateral); Early symptoms include hearing loss, tinnitus, and unsteadiness
  • True Vertigo: Spinning sensation of movement (most indicative of vestibular disease)
  • “Dizziness” vague term to describe many different sensations
  • Vertigo caused by lesions anywhere in vestibular pathway (most are peripheral involving inner ear)
  • In posterior fossa disease, vertigo will accompany other symptoms (diplopia, visual changes, somatosensory changes, weakness, incoordination)
  • Other causes of vertigo:
  • Benign paroxysmal positional vertigo (most common): Vertigo lasting for few seconds
  • Vestibular neuritis: Several days of intense vertigo
  • Meniere’s disease: Recurrent episodes of vertigo
  • Vertebrobasilar ischemia or infarct, encephalitis, tumors, demyelination in posterior fossa, drugs and toxins


CN IX Glossopharyngeal Nerve


  • Functional Category: Branchial Motor

Function: Stylopharyngeus muscle (elevates pharynx during talking/swallowing & contributes to gag reflex)

  • Functional Category: Parasympathetic

Function: Parasympathetics to parotid gland (for salivation)

  • Functional Category: General somatic sensory

Function: Sensation from middle ear, region near external auditory meatus, pharynx, and posterior 1/3 of tongue

  • Functional Category: Visceral Sensory

Function: Taste from posterior 1/3rd of tongue

  • Functional Category: Visceral Sensory

Function: Chemo- and baroreceptors of carotid body

  • Location: nuclei in medulla


CN X Vagus Nerve


  • Means “wandering” in Latin; from wandering course it takes with parasympathetic innervation
  • Functional Category: Branchial Motor

Function: Pharyngeal muscles (swallowing) and laryngeal muscles (voice box)

  • Functional Category: Parasympathetic

Function: Parasympathetics to heart, lungs, and digestive tract

  • Functional Category: General somatic sensory

Function: Sensation from pharynx, meninges, and small region near external auditory meatus

  • Functional Category: Visceral Sensory

Function: Taste from epiglottis and pharynx

  • Functional Category: Visceral Sensory

Function: Chemo- and baroreceptors of aortic arch

  • Location: nuclei in medulla


CN XI Spinal Accessory Nerve


  • Functional Category: Branchial Motor

Function: Sternomastoid and upper part of trapezius muscle

  • Location: Arises not from brainstem, but from upper cervical spinal cord
  • Lesion Information:
  • LMN lesions of CN XI may cause ipsilateral weakness of shoulder shrug or arm elevation AND weakness of head turning away from the lesion
  • UMN lesions can also cause deficits in head turning, toward opposite side of lesion


CN XII Hypoglossal Nerve


  • Functional Category: Somatic Motor

Function: Intrinsic muscles of tongue

  • Location: nuclei in medulla
  • Lesion Information:
  • UMN lesions will cause contralateral tongue weakness
  • LMN lesions cause ipsilateral tongue weakness (toward side of lesion when protruded)


Clinical Information Regarding Cranial Nerves

Disorders of CN IX, X, XI, and XII


  • Most disorders arise from central lesions, but occasionally affected by diabetic neuropathy, demyelination, motor neuron disease, and traumatic, inflammatory, neoplastic, toxic, etc.
  • Glossopharyngeal neuralgia: clinically similar to trigeminal neuralgia but involves sensory distribution of CN IX, causing episodes of severe throat and ear pain
  • Glomus tumors: rare disorder can affect lower cranial nerves

Hoarseness, Dysarthria, and Dysphagia


  • Causes can range from UMN lesions (corticobulbar pathways) to LMN lesions to disorders of the neuromuscular junction or muscles themselves
  • Voice disorders: occur when larynx or vocal cords impacted, which can result from mechanical, neural, or muscle disorders; can also occur from lesions of CN X
  • Hoarseness: disorders of vocal cords causing asynchronous vibratory patterns; often caused by mechanical factors such as swelling, nodules, polyps, or neoplasms of the cords
  • Breathiness: caused by paralysis/paresis of the vocal cord(s), resulting from air leak at glottis; often mistakenly called hoarseness
  • Dysarthria: abnormal articulation of speech, which should be distinguished from aphasia
  • Can occur from muscles of articulation (jaw, lips, palate, pharynx, tongue), the neuromuscular junction, or damage to CN V, VII, IX, X, or XII. Can also occur because of damage to motor cortex, cerebellum, basal ganglia, or corticobulbar pathways
  • Dysphagia: impaired swallowing
  • Can be caused by dysfunction of muscles of tongue, palate, pharynx, epiglottis, larynx, or esophagus; by lesions of CN IX, X, XII, or by dysfunction at neuromuscular junction or corticobulbar tracts; Often occurs with dysarthria.
  • Swallowing includes:
    • oral prep phase: prep of food bolus by mastication
    • oral phase: movement of bolus in anterior-posterior direction by tongue
    • pharyngeal phase: propulsion of bolus through pharynx
    • esophageal phase: opening of upper esophageal sphincter, peristalsis, and into stomach

Laughing and Crying


  • (Brainstem nuclei: CN VII, IX, X, and XII)
  • Pseudobulbar affect: uncontrollable bouts of laughter or crying without feeling the usual associated emotions; emotional incontinence. Caused by: abnormal reflex activation of laughter and crying circuits in brainstem
  • Pseudobulbar palsy: used to describe dysarthria and dysphagia caused by UMN lesions in corticobulbar pathway (e.g., frontal lobe) NOT brainstem (“bulb”) – thus, pseudo
  • Gelastic epilepsy: rare seizure disorder causing episodes of inappropriate laughter usually associated with lesions of hypothalamus (occasionally in temporal lobe seizures)

Bulbar Cranial Nerves IX, X, XI


  • Bulbar injury = Bulbar palsy, which includes: Dysarthria, dysphagia, and hypoactive jaw/gag reflex NOT associated with cognitive changes (whereas pseudobulbar palsy is)


Cranial Nerve Summary Sheet

Cranial Nerve Sensory Motor Chief Functions Examination Symptoms of Dysfunction
I – Olfactory Sensory Smell Odors applied to each nostril Anosmia
II – Optic Sensory Vision Visual acuity; visual fields Anopsia
III – Oculomotor Motor Moves eyes in all directions but those served by IV and VI Reaction to light, eyelid movement; Medial and vertical eye movements Dilated pupil; ptosis; outward deviation; diplopia; uneven dilation of pupils
  Parasympathetic Pupillary constriction and accommodation    
IV – Trochlear Motor Moves eye down and in Down and in eye movements Diplopia
V – Trigeminal Sensory General senses for head Light touch, pain by pinprick; hot/cold; corneal reflex; jaw reflex; jaw movements Decreased sensation in face; attacks of severe pain (trigeminal neuralgia); jaw weakness; asymmetric chewing
  Motor Chewing    
VI – Abducens Motor Moves eye out Lateral movements of the eye Diplopia; deviation of eye inward
VII – Facial Sensory Taste for anterior 2/3rds of tongue Facial movements/expression; taste Unilateral facial paralysis (Bell’s palsy); loss of taste on anterior 2/3rds of tongue
  Motor Moves face    
  Parasympathetic Salivation and lacrimation (tearing)    
VIII – Vestibulocochlear Sensory Hearing; position and movement of head Audiogram tests hearing; stimulate by rotating patient or by irrigating ear (caloric test) Deafness; tinnitus; dysequilibrium; feelings of disorientation in space
IX – Glossopharyngeal Sensory Posterior 1/3 of tongue, tonsil, pharynx, middle ear Taste; Test pharyngeal or gag reflex by touching walls of pharynx Spasms of pain in posterior pharynx
  Motor Swallowing    
  Parasympathetic Salivation    
X – Vagus Sensory General sensation for pharynx, larynx, esophagus, external ear; chemo/baroreception for heart; visceral sensation for thoracic/abdominal Observe palate in phonation; partial reflex by touching walls of pharynx Hoarseness, poor swallowing, and loss of gag reflex
  Motor Speech, swallowing    
  Parasympathetic Cardiovascular, respiratory, gastrointestinal    
XI – Spinal Accessory Motor Movement of head and shoulder Movement, strength, and bulk of neck and shoulder muscles Wasting of neck with weakened rotation; Inability to shrug
XII – Hypoglossal Motor Movement of tongue Tongue movements; tremor, wasting or wrinkling of tongue Wasting of tongue with deviation to the side of lesion on protrusion

Sensory: 1, 2, 8; Midbrain: 3, 4, (5); Motor: 3, 4, 6, 11, 12; Pons: 5, 6, 7, 8; Mixed: 5, 7, 9, 10; Medulla: (5) (7) (8) 9, 10, 11, 12