Brain Stem Anatomy

Question 1

Which cranial nerves are contained in the brainstem? Where do they exit?

Answer 1

• Midbrain: CN III and IV exit medially
• Pons: CN VI exits medially; CN V, VII, and VIII exit laterally
• Medulla: CN XII exits medially; CN IX and X exit laterally
• CN I, II, and XI are not contained in the brainstem
• The ones that exit laterally are mixed cranial nerves
• The ones that exit medially are purely motor cranial nerves

Question 2

Which tracts are contained in the cerebral peduncles?

Answer 2

• Corticospinal: synapses with LMN of spinal nerves
• Corticopontine: synapses with pontine nuclei
• Corticobulbar: synapses with LMN of cranial nerves

Question 3

Where is the cerebral aqueduct and fourth ventricle relative to the brainstem?

Answer 3

The cerebral aqueduct is within the midbrain and is surrounded by periaqueductal gray matter.

The fourth ventricle overlies the pons and medulla–the inferior part of the fourth ventricle is dorsal to the rostral part of the medulla.

Question 4

Where are the superior and inferior colliculi found? What processes do they function in? What structure is found just above them and which cranial nerve exits just below them?

Answer 4

The superior and inferior colliculi are on the dorsal surface of the midbrain. The superior colliculus functions in conjugate gaze while the inferior colliculus is a central auditory structure.

The pinal gland is located above the superior colliculi. CN IV is the only dorsally exiting and completely crossed cranial nerve (crossed in the anterior medullary velum) and it exits beneath the inferior colliculi.

Question 5

What separates the pons from the cerebellum?

Answer 5

Fourth ventricle

Question 6

Where does the corticospinal tract course in the brainstem? What will a lesion to the corticospinal tract in the brainstem result in?

Answer 6

The corticospinal tract originates in the primary motor cortex, passes through the posterior limb of the internal capsule, courses ventrally and medially through the brainstem, and decussates at the pyramical decussation (rostral medulla) just before entering the spinal cord.

A lesion to the corticospinal tract in the brainstem results in contralateral spastic hemiparalysis.

Corticobulbar axons course with corticospinal axons to innervate LMN of cranial nerves in the brainstem.

Question 7

Where does the dorsal column-medial lemniscus system course in the brainstem? What does a lesion of this system in the brainstem result in?

Answer 7

The dorsal column-medial lemniscus system originates in the periphery and carries sensations of vibration, proprioception, and light touch. It travels up the dorsal column and synapses in the nucleus cuneatus/gracilis which decussates in the caudal medulla as the internal arcuate fibers. These project up as the medial lemniscus which courses medially through the medulla (dorsal to the corticospinal tract) and moves dorsally and laterally as it passes through the pons and midbrain. The orientation of the arm-trunk-leg fibers rotates such that the leg fibers go from being most ventral in the medulla to the most dorsal/lateral in the midbrain. It synapses in the ventral posterolateral nucleus of the thalamus and projects then to the somatosensory cortex.

A lesion in the brainstem above the internal arcuate fibers results in contralateral loss of these types of sensations.

Question 8

Where does the spinothalamic system course in the brainstem? What symptoms result from its lesion in the brainstem?

Answer 8

The spinothalamic tract carries peripheral pain and temperature sensations and fibers decussate in the spinal cord and enter the brainstem on the contralateral side. They course laterally through the medulla, pons, and midbrain and synapse in the ventral posterolateral nucleus of the thalamus.

A lesion results in a loss of pain and temperature on the contralateral side.

Question 9

With what tract do the descending fibers of the hypothalamic tract course? What does a lesion of this system result in?

Answer 9

Descending hypothalamic fibers course with the spinothalamic tract in the lateral brainstem. They arise from the hypothalamus and descend without crossing. They synapse with preganglionic sympathetic and parasympathetic neurons in the brainstem and spinal cord.

A lesion is always ipsilateral and produces a central Horner’s syndrome–miosis, ptosis, and anhydrosis. It may also be associated with spinothalamic symptoms.

Question 10

What is the role of the medial longitudinal fasciculus? What diseases affect it?

Answer 10

The MLF courses through the brainstem in the floor of the fourth ventricle adjacent to the central canal in the medulla and the cerebral aqueduct in the midbrain. It links the vestibular nuclei and the centers for conjugate gaze with the abducens, trochlear, and oculomotor nuclei.

The MLF is susceptible to diseases like multiple sclerosis and neurosyphilis. Lesions result in internuclear ophthalmoplegia which disrupts horizontal conjugate gaze.

Question 11

What four things does every brainstem section include?

Answer 11

• A long tract
• A cranial nerve nucleus or nerve
• A trigeminal nucleus
• A cerebellar peduncle or fibers forming one

Question 12

How are lower motor neurons of cranial nerves innervated? Why does this system of innervation work? Exceptions?

Answer 12

Lower motor neurons of cranial nerves receive bilateral simultaneous input from upper motor neurons beause most structures innervated by cranial nerves span the midline.

If you lesion all of the corticobulbar fibers on one side of the brain, there is generally no weakness because the opposite side of the brain provides innervation.

Exception: the only LMN fibers of the facial nerve that are bilaterally innervated are those to muscles that shut the eye and wrinkle the forehead–UMN to LMN to muscles of nose and mouth are contralateral only.

CN VII LMN lesion: Bell’s palsy (contralateral)

CN VII UMN lesion: maintains forehead wrinkling and eye closing (contralateral) but nose wrinkling and mouth are weak.

Question 13

How do lesions to the hypoglossal and vagus nerve differ from lesions to the UMN that go to the hypoglossal nucleus and nucleus ambiguus?

Answer 13

Lesions to the hypoglossal nerve result in tongue deviation toward the side of the lesion while lesions to the UMN to the hypoglossal nucleus result in deviation away from the side of the lesion.

Lesions to the vagus result in deviation of the uvula away from the lesioned nerve while lesions to the UMN to the nucleus ambiguus results in deviation of the uvula toward the lesioned corticobulbar fibers.

Question 14

What is the sulcus limitans?

Answer 14

Marks the division between the sensory and motor neurons in the brainstem. Motor neurons are located medially, sensory neurons are located laterally.

Question 15

Which motor nuclei are located most medially in the brainstem? Do their LMN fibers cross or remain ipsilateral?

Answer 15

The nuclei of nerves that innervate generic skeletal muscle are located most medially. This is why their LMN exit the brainstem medially.

Midbrain

• Oculomotor nucleus (superior rectus fibers project contralaterally, the rest ipsilaterally)
• Trochlear nucleus (crosses to innervate contralateral superior oblique)

Pons

• Abducens nucleus (nerve remains ipsilateral)

Medulla

• Hypoglossal nucleus (nerve remains ipsilateral)

Question 16

What nuclei are contained in the intermediate column of the medial half of the brainstem? What do they innervate?

Answer 16

The intermediate column contains preganglionic parasympathetic neurons which exit the brainstem with CN III, VII, IX, and X:

Midbrain

• Edinger-Westphal nucleus: rostral mid-brain, lateral to the oculomotor nucleus. Project to the ciliary ganglion to innervate the sphincter pupillae and ciliary muscles

Pons:

• Superior Salivatory Nuclus (VII): caudal pons, gives rise to preganglionic para-sympathetics of the facial nerve which synpase in the submandibular ganglion and the pterygopalatine ganglion.

Medulla:

• Inferior salivatory nucleus (IX): rostral medulla, gives rise to preganglionic parasympathetics of IX which synapse in the otic ganglion to innervate the parotid
• Dorsal motor nucleus of X: lateral to the hypoglossal nucleus in the floor of the fourth ventricle. Its fibers exit with the vagus and terminate in ganglia in thoracic and abdominal viscera.

Question 17

What nuclei are contained in the lateral column of the motor nuclei? What do they innervate?

Answer 17

The lateral column of motor nuclei contain LMN that innervate skeletal muscles derived from pharyngeal arches–CN V, VII, IX, and X:

Pons:

• Motor nucleus of V: rostral pons, innervate muscles of mastication among others. Lesion may result in jaw protursion toward side of the lesion.
• Facial motor nucleus: caudal pons, axons loop around the abducens nucleus forming the facial colliculus which joins sensory and parasympathetic fibers to exit at the pontomedullary junction. Lesions result in Bell’s palsy.

Medulla:

• Nucleus ambiguus: contains LMN that exit in CN IX and X and innervate most of the muscles of the palate, pharynx, and larynx, as well as some parasympathetics that innervate the heart. Lesions result in palate weakness (ipsilateral), dysphagia, and hoarseness.

Question 18

What is the solitary nucleus?

Answer 18

The solitary nucleus is located in the medulla just lateral to the sulcus limitans. The rostral portion receives taste from VII, IX, and X and relays it to the ventral postero-medial nucleus of the thalamus.

The caudal part of the solitary nucleus is the cardio-respiratory center which responds to CO2 and O2 levels in the blood, blood pressure, and stretch receptors in the lungs. This information is conveyed by CN IX and X.

The solitary nucleus also receives sensations from the pharynx and larynx from CN IX and X.

Lesions may result in respiratory failure and loss of gag reflex or cough reflex

Question 19

What are the cochlear nuclei? Where do they project?

Answer 19

The cochlear nerve fibers synapse in ventral and dorsal cochlear nuclei. The ventral coclear nuclei project to both superior olivary nuclei in the bons (binaural inputs to localize sound), the lateral lemniscus to convey auditory input to the inferior colliculus, and the inferior colliculus projects to the medial geniculate body of the thalamus.

Sensorineural hearing loss results from a loss of hair cells in the cochlea or from a lesion in this pathway.

Question 20

What is the vestibular nucleus? Where does it project?

Answer 20

The vestibular nerve innervates hair cells in the ampullary crests in the semicircular ducts and in the utricular and saccular maculae which respond to angular and linear acceleration. The vestibular nerve projects to the vestibular nuclei and the flocculonodular lobe of the cerebellum. Axons in the MLF interconnect the vestibular nuclei with centers for cnojugate gaze.

Vestibular lesions may cause vertigo

Question 21

What are the three trigeminal sensory nuclei? What do they respond to?

Answer 21

Principle sensory nucleus of V:

• Located in rostral pons where the nerve fibers enter.
• Receives sensory input that conveys disciminative touch sensations
• Project to the ventral posteromedial nucleus of the thalamus which sends fiber to the somatosensory cortex.

Spinal nucleus of V:

• Nucleus oralis: receives information regarding face touch, oral, and nasal cavities
• Interpolar nucleus: responds to pain and temperature from the tooth pulp
• Nucleus caudalis: senses face pain and temperature
• Forms the trigeminothalamic tract and projects to the brainstem with the medial lemniscus and synapses in the ventral posteromedial nucleus of the thalamus

Mesencephalic nucleus of V:

• Extends into the midbrain and contains cells of neural crest origin
• Innervate stretch receptors of muscles of mastication
• Projects to the motor nucleus of V as part of the jaw reflex.

Question 22

How are the dermatomes on the face distributed?

Answer 22

The face has an onion topography for pain and temperature sensation based on the nucleus caudalis. V1 covers the forehead and bridge of nose, V2 covers lips and nose up to temple, V3 contains the lower jaw

Question 23

What do the cerebellar peduncles contain?

Answer 23

The superior cerebellar peduncle mainly conveys axons from the cerebellum to deep cerebellar nuclei.

The middle cerebellar peduncle conveys axons from the pontine nuclei into the cerebellum.

The inferior cerebellar peduncle conveys axons both into the cerebellum from the spinal cord and brainstem and out of the cerebellum from deep cerebellar nuclei.

The deep cerebellar nuclei are the fastigial nucleus, the interposed nuclei, and the dentate nucleus.

Question 24

Which arteries supply the regions below?

Answer 24

Question 25

Which arteries supply the region below?

Answer 25

Question 26

Which arteries supply the region below?

Answer 26