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Brainstem and Cranial Nerves: Part 2

Notes

Brainstem and Cranial Nerves: Part 2

Sections


Brainstem & Cranial Nerves: Part 2

Summary

Midbrain

  • The periaqueductal gray area surrounds the cerebral aqueduct (of Sylvius).
  • The cerebral peduncles (aka crus cerebri) carry the motor tracts, specifically, they carry 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 (whereas sensory pathways mostly localize posteriorly).
    • They are down, here, because we are using an anatomical section – in radiographic view (the way in which we look at the midbrain, clinically), the ears will be up.
  • The bilateral, thinly-shaped, substantia nigra are essential for motor activation.
  • The substantia nigra comprises both a melanin-rich pars compacta, which contains dopaminergic cells that are essential for movement and a pars reticulata, which is an essential component of the direct and indirect pathways.
    • As a clinical correlation, pars compacta degneration results in Parkinson's disease (which causes motor stiffness and freezing).
    • Although it's an over-simplification, we can imagine pars compacta as a key generator of movement (it puts the car in drive!) and pars reticulata as a key modulator of movement.
  • Bilateral, circular-shaped, red nuclei activate upper extremity flexion movements, which are observed in decorticate posturing – a sign of central nervous system injury.
    • The red nucleus is not actually red but pink (due to its iron-containing pigment).

Pons

  • The pontine nuclei within the anterior pons; they receive input from cerebral cortical neurons.
  • The middle cerebellar peduncle (aka brachium pontis) lies in the dorsal pons, which we abbreviate MCP; it's the major inflow pathway from the brainstem to the cerebellum.
  • The pontine nuclei project pontocerebellar fibers into the middle cerebellar peduncle as an important step in the corticopontocerebellar pathway, which modulates movement.
  • The corticopontocerebellar fibers descend to the pontine nuclei, synapse, and then project across midline through the MCP into the contralateral cerebellar cortex.
    • As a clinical correlation, widespread injury to the anterior pons causes locked-in syndrome.
    • Nearly 20 million fibers are dedicated to the modulation of movement via the corticopontocerebellar pathway whereas only 1 million fibers are dedicated to the initiation of movement via the corticospinal tract.

medulla

Cerebellar Modules

  • Vestibulocerebellum (aka archicerebellum), which comprises midline vestibulo- and olivocerebellar fibers that are important for equilibrium and eye movements.

It is phylogenetically the oldest portion of the cerebellum and is derived, most notably, from the flocculonodular lobe.

  • The spinocerebellum (aka paleocerebellum), which receives the spinocerebellar tracts and plays a major role in postural stability.

It is derived from the anterior lobe and the majority of the vermian and paravermian posterior lobe.

  • The pontocerebellum (aka neocerebellum), which acts through the corticopontocerebellar pathway for fine motor movements.
    • It is phylogenetically the newest portion of the cerebellum and is derived from the remainder of the posterior lobe.

Full-Length Text

  • Here, we'll continue our learning of the brainstem, cerebellum, and cranial nerves.
    • We've erased our table of the cranial nerves, so we can take notes on some key features of the brainstem and cerebellum.

We'll begin with the brainstem.

Show that we'll use axial sections.

  • Include the posterior-anterior orientational plane of our individual sections.
    • Notice that's its different than the plane of our neighboring diagram.
  • First, draw the midbrain.
  • Draw the periaqueductal gray area, which surrounds the cerebral aqueduct (of Sylvius).
  • Then, anteriorly, draw the cerebral peduncles (aka crus cerebri), which are down, here, because we are using an anatomical section – in radiographic view (the way in which we look at the midbrain, clinically), the ears will be up.
  • Now, denote that the cerebral peduncles carry the motor tracts, specifically, they carry 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 (whereas sensory pathways mostly localize posteriorly).

Then, from anterior to posterior, draw the:

  • Bilateral, thinly-shaped, substantia nigra, which are essential for motor activation.
    • Denote that the substantia nigra comprises both a melanin-rich pars compacta, which contains dopaminergic cells that are essential for movement and a pars reticulata, which is an essential component of the direct and indirect pathways.
    • As a clinical correlation, denote that pars compacta degneration results in Parkinson's disease (which causes motor stiffness and freezing).
    • Although it's an over-simplification, we can imagine pars compacta as a key generator of movement (it puts the car in drive!) and pars reticulata as a key modulator of movement.
  • Bilateral, circular-shaped, red nuclei.
    • Denote that they upper extremity flexion movements, which are observed in decorticate posturing – a sign of central nervous system injury.
    • The red nucleus is not actually red but pink (due to its iron-containing pigment).

Next, drop down a level and draw the pons: the mid-brainstem level.

  • Draw representative pontine nuclei within the anterior pons; they receive input from cerebral cortical neurons.
  • Then, draw the middle cerebellar peduncle (aka brachium pontis) in the dorsal pons, which we abbreviate MCP; it's the major inflow pathway from the brainstem to the cerebellum.
  • Show that the pontine nuclei project pontocerebellar fibers into the middle cerebellar peduncle as an important step in the corticopontocerebellar pathway, which modulates movement.
  • Denote that corticopontocerebellar fibers descend to the pontine nuclei, synapse, and then project across midline through the MCP into the contralateral cerebellar cortex.
  • As a clinical correlation, widespread injury to the anterior pons causes locked-in syndrome.
    • Consider that nearly 20 million fibers are dedicated to the modulation of movement via the corticopontocerebellar pathway whereas only 1 million fibers are dedicated to the initiation of movement via the corticospinal tract.

Now, draw the medulla.

  • Cap off its most anterior region with the medullary pyramids: the medullary correlate of the cerebral peduncles.
  • Denote that they carry the descending corticospinal tract fibers, which decussate at the cervico-medullary junction.
  • Now, draw the inferior olive just behind the medullary pyramids (in what is called the anterior tegmentum of the medulla).

Turn to our table and

  • Next, denote that the inferior olive contains climbing fibers.
  • Denote that they project to the contralateral cerebellum as part of the clinically important triangle of Guillain-Mollaret.

Lastly, let's turn our attention to the 3 modules of the cerebellum, which are the most helpful classification for a simple understanding of the clinical-functional anatomy of the cerebellum. Denote that they are the:

  • Vestibulocerebellum (aka archicerebellum), which comprises midline vestibulo- and olivocerebellar fibers that are important for equilibrium and eye movements.
    • It is phylogenetically the oldest portion of the cerebellum and is derived, most notably, from the flocculonodular lobe (and the anterior tip of the vermis (the lingula).
  • The spinocerebellum (aka paleocerebellum), which receives the spinocerebellar tracts and plays a major role in postural stability.
    • It is derived from the anterior lobe and the majority of the vermian and paravermian posterior lobe.
  • The pontocerebellum (aka neocerebellum), which acts through the corticopontocerebellar pathway for fine motor movements.
    • It is phylogenetically the newest portion of the cerebellum and is derived from the remainder of the posterior lobe.