Midbrain and GSE Columns

Introduction to cranial nerves 3, 4, and 6.
These nerves are part of the GSE (General Somatic Efferent) column of the brainstem, excluding cranial nerve 12.
They control the movement of the eyes.
Spinal accessory will be regarded as SVE (Special Visceral Efferent) instead of GSE for functional reasons.

Cerebral Aqueduct and Ventricles

Cerebral aqueduct: A channel within the midbrain that allows for the passage of cerebrospinal fluid (CSF).
CSF is primarily produced in the two lateral ventricles.
Lateral ventricles contain telocoroidia, which are blood vessels that produce CSF.
Lateral ventricles empty into the third ventricle, which also contains telocoroidia on its roof.
The two lateral ventricles exit through the foramen of Monro into the third ventricle.
From the third ventricle, CSF flows through the cerebral aqueduct into the fourth ventricle.
Pressure gradient drives CSF flow from lateral ventricles to the third ventricle.

Periaqueductal Gray

Periaqueductal gray (PAG): Gray matter surrounding the cerebral aqueduct.
PAG is involved in processing pain.
Inputs to PAG include the PVN (paraventricular nucleus) via the hypothalamuspinal tract or the dorsal longitudinal fasciculus.
PAG also receives GVA (general visceral afferent) information.
PAG projects back down to the spinal cord and the spinal nucleus of the trigeminal system.
Function: ameliorate pain response and prevent pain signals from projecting up into the thalami.

Oculomotor Nucleus

Oculomotor nuclei: Located close to the periaqueductal gray; GSE nucleus for cranial nerve three.
The cell bodies of the GSE fibers are located in this nucleus.
Functions: lifting the eyelid, moving the eyes, controlling pupil diameter, and changing the shape of the lens.
Fibers projecting out are Lower Motor Neurons going to skeletal muscle.
Actual GSE Fiber type: Peripheral Nervous System fiber.

Edinger-Westphal Nucleus

Edinger-Westphal nucleus: Another nucleus around the periaqueductal gray; GVE (General Visceral Efferent) nucleus.
Fibers projecting out are GVE fibers.
GVE fibers combine with oculomotor fibers of the GSE to form a nerve.
GVE fibers are preganglionic and postganglionic.

GSE vs GVE

GSE (General Somatic Efferent): Targets skeletal muscle and consists of lower motor neurons.
GVE (General Visceral Efferent): PSNS (Parasympathetic Nervous System) for the eye, with preganglionic and postganglionic fibers.

Trochlear Nerve

Trochlear nucleus: Another GSE nucleus (cranial nerve 4).
Fibers from the trochlear nucleus go back posteriorly and decussate.
Trochlear nerve fibers are lower motor neurons and GSE fibers.
Innervates the superior oblique muscle.

Abducens Nerve

Abducens nucleus: Another GSE nucleus (cranial nerve 6).
The abducens nerve emerges anteriorly and goes to the lateral rectus muscle, causing abduction of the eye.
The abducens nucleus also has a connection that forms part of the medial longitudinal fasciculus (MILF).

Medial Longitudinal Fasciculus (MILF)

MILF: A prominent tract in the midbrain and brainstem.
Abducens projects to the oculomotor nucleus via the MILF.
The MILF is affected in multiple sclerosis, leading to eye movement problems.
Left abducens projects to the right oculomotor nucleus.
The fiber that goes to the lateral rectus is a lower motor neuron and GSE.

Cranial Nerves and GSE

Remember 3(oculomotor), 4(trochlear) , 6(abducens), and 12(hypoglossal) are the GSE cranial nerves.

Activation of Abducens Nucleus

If the right abducens nucleus is activated, the right lateral rectus muscle is activated, causing the right eye to move to the right.
Simultaneously, the left medial rectus muscle is activated, thanks to the connection of the medial longitudinal fasciculus, causing the left eye to move medially towards the right.
Pushing the abducens nucleus causes movement.
Activating left abducens moves the left eye laterally to the left, and the right eye medially to the left.
Lateral rectus moves the eye laterally, and the medial rectus moves the eye medially.

Transverse Section of Midbrain

Optic nerve (cranial nerve two) is located here, as well as the optic chiasm and optic tract.
Medial eminence: Gray matter structure extending down and forming the pituitary after the pituitary has been removed.
Mammillary bodies: Important for the Papez circuit (memory).
Crus cerebri: Contains upper motor neurons of the pyramidal system.
Substantia nigra pars compacta: Darkened area important for kick-starting motor movement.
Red nuclei: Contain upper motor neurons (extrapyramidal).
Cerebral aqueduct: Surrounded by cerebral aqueductal gray.

Upper vs Lower Motor Neurons

Oculomotor nucleus: Contains lower motor neurons and is located a little bit more rostral.
Edinger-Westphal nucleus: Also located here. This is PSNS output and apart of cranial nerve 3.
Lower motor neurons are GSE, targeting skeletal muscle of the eye.
Lower motor neurons are SVE, for the head and neck, targeting also skeletal muscle.

Nerves and Fiber Types

GSE fibers are general somatic efferent, targeting skeletal muscle.
GVE fibers are pre and postganglionic in the PSNS.
Oculomotor nerve (cranial nerve three) contains both GVE and GSE fibers targeting the eye. It is a mixed nerve.

Tegmentum & Tectum

Tegmentum: From the middle of cerebral aqueduct to the base.
Tectum: The roof of the midbrain.
Superior colliculi: Located on the Tectum that have upper motor neurons (extrapyramidal).
Posterior Commissure: Big fiber bundle and lots of connections for our two superior colliculi.
Note: They also have upper motor neurons, but extrapyramidal.

Cranial Nerve Targeting & Function

Trochlear(cranial nerve IV): Superior oblique
Abducens (cranial nerve VI): Lateral rectus
*Oculomotor (cranial nerve III): Everything else for GSE component (medial rectus, superior rectus, inferior recti, inferior oblique).
Inferior oblique causes extorsion of the eyes.
Superior oblique causes intorsion of the eyes. Innervated by trochlea.
Decussation happen because you end up requiring less less processing power if you decussate and then send the commissural bundles up in m one versus if you bring all the information up ipsilaterally and then decussate only through that commissural bundle.

Abducens (cont.)

*All located quite a bit relative to other structures, and are medially located (relative to other structures).
*Pons is located here.
*Deep cerebellar nuclei are located here.
*GSE and lower motor neurons
*Press left abducens to look to the left, press right to look to the right.
*Lateral rectus abducts, medial rectus adducts.

Eye Muscles Remembered

*Medial rectus, move the eye medially - CN III.
*Lateral rectus, abduct the eye - CN VI.
*Superior rectus, CN III
*Inferior rectus, CN III
*Inferior oblique, extorsion - CN III.
*Superior oblique, intorsion - CN IV.