Notes on Brain Stem

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• Contains: corticospinal and dorsal column-medial lemniscal pathways.

Cranial nerve nuclei in medulla:


• Position analogous to dorsal horn of spinal cord; extends to upper cervical cord to point of entry of the fifth cranial nerve in the pons; pain and temperature sensations travel through trigeminal ganglion.

Solitary nucleus:

• Taste and visceral sensations.

Nucleus ambiguus:

• Located dorsal to inferior olive; lesion causes deviation of uvula away from lesion; nasal regurgitation of fluids, hoarseness, dysphagia.

Dorsal motor nucleus of CN X:

• Located lateral to hypoglossal nucleus in floor of 4th ventricle; contains fibers that innervate thorax, foregut, midgut, GI tract.

Hypoglossal nucleus:

• Located near midline beneath central canal and 4th ventricle; contains fibers that innervate tongue muscles.

Accessory nucleus:

• Located in cervical spinal cord; lesions don't affect spinal accessory nerve.


• Contains: corticospinal tracts, medial lemniscal, spinothalamic tracts, lateral lemniscus (auditory pathway), and medial longitudinal fasciculus.

• Pons also contains spinal trigeminal nucleus and mesencephalic nucleus.

Cranial nerve nuclei in pons:

Abducens nucleus:

• Located near midline in floor of 4th ventricle.

Facial motor nucleus:

• Located ventrolateral to abducens nucleus.

Superior olivary nucleus:

• Located ventral to nucleus of CN VII; receives auditory impulses.

Vestibular nuclei:

• Located near posterior surface of pons lateral to abducens nucleus.

Trigeminal nuclei in pons:

Motor nucleus:

• Located medial to main sensory nucleus.

Sensory nucleus:

• Lateral to motor nucleus.


• Located between pons and diencephalon.

• Receives fibers that carry auditory information, and fibers that move the eyes.

• Contains substantia nigra: this uses dopamine and GABA.

Cranial nerve nuclei in midbrain:

Trochlear nucleus:

• located beneath periaqueductal gray; close by are oculomotor nucleus and nucleus of Edinger-Westphal.

Lesion of (Facial n.) CN VII:

• Ipsilateral paralysis of muscles of facial expression.

• Inability to frown, shut eyes, and causes drooping corner of mouth.

Lesion of facial motor nucleus:

• Drooping of corner of mouth on contralateral side.

Lesions that cause hearing loss:

• Lesion in middle ear or inner ear.

• Lesion on CN VIII.

• Lesion on cochlear nuclei inside brain stem at pontomedullary juncion.

• Lesions involving the cochlear duct, spiral ganglion, or inferior colliculus result in an inability to localize sound.

Ear, Auditory, and Vestibular System Components:

Vestibular nuclei:

• Receive fibers from afferent vestibular n., which innervates semicircular ducts, utricle and saccule.

Utricle and saccule:

• Sense linear acceleration and positional change.

Semicircular ducts:

• Sense rotational movement.

Secondary vestibular fibers:

Involved in production of conjugate eye movements - that is - a moving head stays focused on a stationary object.


• Perception of rotation.

• Lesion: peripheral or central vestibular structures.

• Also caused by drugs.


• Unilateral vestibular nerve or vestibular nucleus lesions.

• Vestibule-ocular reflex is indicator of brain stem function in comatose patients.

Caloric nystagmus test:

• Used to test integrity of vestibulo-ocular reflex in comatose patients.

Warm water introduced:

Step 1: horizontal semicircular ducts slowly move eyes to opposite direction.

Step 2: cerebral cortex quickly moves eyes toward warm water ear side.

Cool water introduced:

Step 1: opposite vestibular complex slowly moves eyes toward cool water ear.

Step 2: corrective or fast phase of nystagmus quickly moves eyes opposite from cool water ear.

COWS: Cool Opposite Warm Same.

Horizontal gaze center:

Frontal eye field - Brodmann area 8:

• Center for contralateral gaze.

Pons - pontine gaze center (PPRF):

• Center for ipsilateral gaze.

Medial longitudinal fasciculus (MLF) lesion:

• inability to adduct one eye on attempted gaze to the opposite side.


• CN 3: medial rectus (adduction).

• CN 6: lateral rectus (abduction).

Ask patient to look to right:

• MLF lesion on left: only right eye looks to right; left eye cannot look right.

• CN 6 nerve lesion: only left eye looks right; right eye cannot look right.

• CN 6 nucleus lesion: neither eye can look to right.

• Left cerebral cortex lesion: neither eye can look to right, but slowly drift to left.

• Same for looking left.

Medial medullary syndrome:

• Vertebral artery or anterior spinal artery occlusion.

• CN 12 lesion.

• Corticospinal tract lesion.

• Medial lemniscus lesions.

• Tongue licks side of lesion.

Lateral Medullary (Wallenberg) Syndrome:

• PICA occlusion.

• CNs affected: 8, 12, 5, and 10.

• Spinothalamic tract lesions.

• Horner Syndrome.

• Vertigo, nausea, vomiting, dysphagia, absent gag reflex.

Medial pontine syndrome:

• Occlusion of paramedian branches of basilar arteries.

• Affects: CN 6, corticospinal tract, and medial lemniscus tract.

Lateral pontine syndrome:

• AICA or SCA occlusion.

• Spinothalamic tract lesions.

• Horner Syndrome.

• Vestibular nuclei and pathway lesions.

• Spinal tract lesions.

• CN 7 lesions.

• CN 5 lesions.

Pontocerebellar angle syndrome:

• CN 8 acoustic neuroma (schwannoma) lesion.

• Compresses CN VII and V.

Medial midbrain (Weber) Syndrome:

• PCA occlusion.

• CN III affected.

• Ptosis, mydriasis, external strabismus.

• Patient unable to frown and shut eye - blink reflex is intact.

Parinaud syndrome:

• Pineal tumor compressing superior colcliculi.

• Upward or vertical gaze is paralyzed + pupillary defects.

Additional Notes:

• Dorsal respiratory neurons in medulla: nucleus of tractus solitarius.

• Mixed cranial nerves: CN 3, 5, 7, 9, 10.

• Mesonephric trigeminal nucleus involved in: jaw jerk.

• Intorsion of eye: CN IV.

Additional Readings:

Basic Neurology

1. Peripheral Nervous System
2. Central Nervous System
3. The Ventricular System
4. The Spinal Cord
5. The Brain Stem
6. The Cerebellum
7. Visual Pathways
8. Diencephalon
9. Basal Ganglia
10. Cerebral Cortex
11. Sleep Disorders
12. Autonomic Nervous System
13. Cranial Nerves and Parasympathetic Ganglia
14. Cells of the Nervous System
15. Cerebrospinal fluid
16. Additional short notes on Cerebrum
17. Functions and Diseases of Cerebrum
18. Subcortical Grey Matter
19. Notes on The Spinal Cord
20. Regulation of Heart Rate by Autonomic Nervous System
21. Action Potentials, Axon Conduction, and Neuromuscular Junction
22. Types of Seizures
23. What is a Cough Reflex?
24. Notes on Congenital Prosopagnosia
25. Findings in Parkinson's Disease
26. Types of Heat Strokes
27. Types of Strokes
28. What is Benign Intracranial Hypertension?
29. What is Cauda Equina Syndrome?
30. Cranial Nerve Locations in Brain Stem

Neurology Videos

1. Video of Neurology Examination in a Clinical Setting

Medical Images

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1. Nervous System Disorders
2. Histology of Nervous Tissue
3. Cranial Nerve Reflexes
4. Motor System Examination

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