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Brainstem, Cerebellum, Cranial Nerves

Notes

Brainstem, Cerebellum, Cranial Nerves

Sections




Brainstem & Cranial Nerves

Brainstem

See: Brainstem
The brainstem is organized from superior to inferior:

Midbrain

See: Midbrain

  • Lies below the thalamic structures.
  • Contains cerebral peduncles, which look like Mickey Mouse ears!
  • Clinical Correlation - Weber's Syndrome

Key Structures

Cerebral Peduncles

The cerebral peduncles carry the motor tracts: the corticonuclear tracts (aka corticobulbar tracts), medially, and the corticospinal tracts, laterally. The anterior localization of the cerebral peduncles helps us remember that throughout the brainstem, motor tracts localize anteriorly.

Periaqueductal gray area

Surrounds the cerebral aqueduct (of Sylvius). It is a key site of opioid action for nociception (pain modulation).

Substantia nigra

The bilateral, thinly-shaped, substantia nigra, which are essential for motor activation and are degenerated in Parkinson's disease.

Red nuclei

They provide upper extremity flexion movements, which are observed in decorticate posturing – a sign of central nervous system injury.

Pons

See: Pons

  • A key site of motor synapses.
  • Its large basis comes from its large motor nuclei – like a bag of marbles.
  • Clinical Correlation - Locked-In Syndrome

Key Structures

Pontine nuclei project pontocerebellar fibers into the middle cerebellar peduncle as an important step in the corticopontocerebellar pathway, which modulates movement.

Medulla

See: Medulla

Key Structures

Medullary pyramids

The medullary pyramids cap off the anterior aspect of the medulla (the medullary correlate of the cerebral peduncles). They carry the descending corticospinal tract fibers, which decussate at the cervico-medullary junction.

Inferior olive

The inferior olive contains climbing fibers, which project to the contralateral cerebellum as part of the clinically important triangle of Guillain-Mollaret.

Cerebellum

See: Cerebellum

  • Attaches to the posterior brainstem.

Vestibulocerebellum

The oldest portion of the cerebellum. It comprises midline vestibulo- and olivocerebellar fibers that are important for equilibrium and eye movements.

Spinocerebellum

The spinocerebellum developed after the vestibulocerebellum. It receives the spinocerebellar tracts and plays a major role in postural stability.

Pontocerebellum

The newest portion. It acts through the corticopontocerebellar pathway for fine motor movements.

Cranial Nerves

Functional groups

Somatomotor set

  • CNs 3, 4, 6, 12, and 11 are part of the somatomotor set.
  • They all innervate midline musculature and have midline-lying nuclei.

Solely special sensory set

  • CNs 1, 2, 8 are the solely special sensory set.
  • These CNs purely serve sensory function.

Pharyngeal arch set

  • CNs 5, 7, 9, 10 are the pharyngeal arch set.
  • These nerves all contain numerous functions.

Somatomotor Set

CN 3, oculomotor nerve:

  • Exits the midbrain near midline; midline brainstem strokes often injure this nerve.
  • It innervates several ocular muscles, which are the following extraocular recti muscles: medial, superior, and inferior; it innervates the inferior oblique and levator palpebrae muscles; and it produces pupillary constriction; thus, it serves 4 of the 6 primary eye movements and constricts the pupils.
  • Clinical Correlation: 3rd Nerve Palsy

CN 4, trochlear nerve:

  • Wraps around the outside of the pons (even though it originates from midline of the lower midbrain).
  • CN 4 decussates (crosses midline) as it exits the brainstem contralateral (on the side opposite) to its side of origin – it's the only CN to make this decussation.
  • It innervates the superior oblique, which provides intorsion and depression. If someone has a head tilt, it may be to bring their eyes into alignment from a CN 4 injury.
  • Clinical Correlation: 4th Nerve Palsy

CN 6, abducens nerve:

  • Exits near midline from the pontomedullary junction.
  • It innervates the lateral rectus, which rotates the eye outward.
  • Clinical Correlation: 6th Nerve Palsy

CN 12, hypoglossal nerve:

  • Emerges paracentrally from the mid-medullary level.
  • Just like CNs 3 and 6, CN 12 has a central (midline) course.
  • It provides tongue protrusion.

CN 11, spinal accessory nerve:

  • Emerges laterally from the cervical spinal cord.
    • CN 11 is no longer thought to have additional brainstem origins.
  • It provides head turn and shoulder shrug.
    • Raise your shoulder and feel the action of trapezius, which it innervates.

Solely Special Sensory Set

CN 1, olfactory nerve

  • Serves our sense of smell.
  • The olfactory nerve is the short course of olfactory wiring from olfactory epithelial surface to the olfactory bulb.

CN 2, optic nerve

  • Functions in vision.
    • It's thick because of how much wiring is devoted to vision – most of the posterior brain solely deals with visual processing!
  • The pituitary gland can grow into and injure the optic chiasm.

CN 8, vestibulocochlear nerve

  • Carries hearing (auditory) and balance (sensory) – two separate sensory functions.

Pharyngeal Arch Set

CN 5, trigeminal nerve

  • Provides motor innervation for mastication (chewing).
  • Receives facial sensation.
    • The sensory portion of CN 5 is thick due to the sensitivity of facial sensation.

CN 7, facial nerve

  • Provides facial movement
  • Provides taste sensation from the anterior two-thirds of the tongue.
  • Facial palsy (droop), which is flattening of one side of the face is a common presenting neurologic complaint.

CN 9, glossopharyngeal nerve
&
CN 10, vagus nerve

  • Learn them together because their actions are similar and most easily remembered when grouped.

As a simplification, both CNs are involved:

  • Swallow
  • Cardio-pulmonary function
  • CN 9 is involved in salivation
  • CN 10 is involved in gut motility.
  • CNs 9 and 10 are carry key autonomic functions.

Mneomnic

On Old Olympus's Towering Top, A Finn And German Viewed Some Hops.

CN 1 – Olfactory (On)
CN 2 – Optic (Old)
CN 3 – Oculomotor (Olympus)
CN 4 – Trochlear (Towering)
CN 5 – Trigeminal (Tops)
CN 6 – Abducens (A)
CN 7 – Facial (Finn)
CN 8 – Auditory or Vestibular (aka vestibulocochlear nerve).
CN 9 – Glossopharyngeal (German)
CN 10 – Vagus (Viewed)
CN 11 – Spinal accessory (Some)
CN 12 – Hypoglossal (Hops)

Clinical Challenge Questions

General Localization Question

Use the following clinical case to further reinforce your general understanding of nervous system localization.

Cranial Nerves

See if you can answer this neurological physical exam question.