Neuroscience Week 5: The midbrain

Question 1

THE MIDBRAIN

KEY FEATURES

Answer 1

• The cerebral peduncles lie anteriorly.
• The posterior commissure lies posteriorly.
• The bilateral, thinly-shaped substantia nigra are essential for motor activation.
• The bilateral, circular-shaped, red nuclei.* The colliculi lie posteriorly.
• The cerebral aqueduct (of Sylvius) is the midbrain portion of the brainstem CSF.
• The periaqueductal gray area surrounds the cerebral aqueduct.

Question 2

Identify

Answer 2

Question 3

Identify

Answer 3

Question 4

Identify

Answer 4

Question 5

cerebral peduncle (aka crus cerebri)

Answer 5

The center of the crus divides into the corticonuclear tracts (aka corticobulbar tracts), medially, and the corticospinal tracts, laterally.

Question 6

Weber’s Syndrome AKA

Answer 6

Midbrain Stroke

Question 7

Midbrain Stroke: Weber’s Syndrome

Answer 7

A syndrome of ipsilateral third nerve palsy and contralateral face and body weakness from injury to the paramedian midbrain.

CLINICAL CASE
Patient presents with sudden onset of double vision and right-side weakness. Exam reveals left eye third nerve ophthalmoplegia with impaired pupillary constriction and also right face, arm, and leg weakness.

Question 8

substantia nigra

Answer 8

• lies just posterior to the white matter pathways in the base of the midbrain. It relies on dopamine, so its melanin-rich. The substantia nigra divides into:
• The pars compacta (posteriorly); loss of dopamine in the pars compacta results in Parkinson’s disease.
• The pars reticulata (anteriorly); this iron-rich division of the substantia nigra is fundamental to the direct and indirect pathways

Question 9

red nucleus

Answer 9

• The red nucleus produces upper extremity flexion movements, which are observed in decorticate posturing – we discuss this in detail elsewhere.

Question 10

Lateral midbrain

Answer 10

Cluster of major sensory tracts:

• Medial lemniscus.
• Anterior trigeminothalamic tract (we abbreviate TTT for trigeminothalamic trac).
• The spinothalamic tract (of the anterolateral system) (moving posteriorly).
• Then, the lateral lemniscus.
• The posterior trigeminothalamic tract (further posterior).

Question 11

Periaqueductal gray area

Answer 11

• Most notably contains opioids, which help in pain control.
• It is also packed with neuropeptides, monoamines, and amino acids.
• Electrical stimulation of the periaqueductal gray area to produce analgesia was first attempted in the 1970s but has had mixed results. Of note, the periaqueductal gray area receives ascending spinomesencephalic fibers via the anterolateral system, which play a role in the emotional aspect of pain, and it receives descending fibers from the hypothalamus via the dorsal longitudinal fasciculus.# Periaqueductal gray area functions include far-reaching modulation of sympathetic responses (ie, pupillary dilation and cardiovascular responses); parasympathetic-induced micturition; modulation of reproductive behavior; and even affect locomotion and vocalization. However, its most widely recognized function is in pain modulation.

Question 12

Medial longitudinal fasciculus AKA

Answer 12

MLF

Question 13

Medial longitudinal fasciculus

Answer 13

Plays an important role in conjugate horizontal eye movements.

Clinical Correlation - Internuclear ophthalmoplegia (MLF syndrome)

Question 14

Internuclear ophthalmoplegia (MLF syndrome)

Answer 14

In an internuclear ophthalmoplegia, the unaffected eye abducts but the ipsilateral eye is unable to adduct. The unaffected eye is not totally unaffected, it actually has horizontal nystagmus upon abduction, presumably because of the divergence that occurs from the left eye adduction failure. It commonly occurs from demyelinating plaques in multiple sclerosis.

Question 15

Reticular formation

Answer 15

Serves numerous functions; the most notable one is helping to produce wakefulness.

Initially, the indistinct histology of the reticular formation led people to believe it was simply a “diffuse arousal network,” but now the functional specialization of the reticular formation is well recognized.

The reticular formation divides into lateral, medial, and median zones. Indicate that the raphe nuclei populate the median zone.

They are primarily serotinergic and are modulated by psychotropic medications.

Question 16

The raphe nuclei affect

Answer 16

sleep–wake cycles, pain management, and motor activity but are most commonly referenced for their role in mood disorders and the hallucinatory effects of illicit drugs.

Question 17

The raphe nuclei lie along

Answer 17

much of the height of the midline brainstem as six separate subnuclei, which divide into rostral and caudal nuclear groups based on whether they lie above or below the mid-pons. The rostral raphe group (aka oral raphe group) comprises the upper pontine and midbrain raphe nuclei: the caudal linear, dorsal raphe, and median raphe nuclei.

Question 18

The caudal raphe group comprises

Answer 18

the lower pontine and medullary raphe nuclei: the raphe magnus, raphe obscurus, and raphe pallidus nuclei. Note that additional serotinergic reticular formation areas are also categorized as part of the raphe nuclei.

Question 19

Efferent projections from the rostral raphe group

Answer 19

mostly ascend into the upper brainstem and forebrain, whereas projections from the caudal raphe group primarily descend into the lower brainstem and spinal cord. Afferents to the raphe nuclei also exist, which generally originate from behavioral brain areas.

Question 20

Posterior commissure

Answer 20

A white matter tract involved in the pupillary light reflex.

The nucleus of the posterior commissure helps control vertical eye movements.

Question 21

Dorsal Midbrain Syndrome AKA

Answer 21

Parinaud Syndrome)

Question 22

Dorsal Midbrain Syndrome (aka Parinaud Syndrome) Manifestations

Answer 22

Manifestations of Dorsal Midbrain Syndrome

• Upgaze palsy with convergence-retraction nystagmus
• Meaning when the patient attempts to look up, there is convergent/retractory episodic jerking.
• Eyelid retraction
• Light-near dissociation
• Meaning, the pupils do NOT respond to light but do constrict to near response.

Question 23

Dorsal Midbrain Syndrome (aka Parinaud Syndrome) Common Causes

Answer 23

Compressive:

• Pineal Tumors
• Hemorrhages with dorsal midbrain compression

Non-compressive:

• MS plaques
• Dorsal midbrain ischemic strokes
• Neuroinfectious causes

Question 24

Superior colliculi

Answer 24

Involved in visual function.

Question 25

Inferior colliculi

Answer 25

Auditory function

Question 26

Cranial Nerve Nuclei: Motor nuclei

Answer 26

• The oculomotor complex of CN 3 in midline
• The Edinger-Westphal nucleus of CN 3, which is a key autonomic part of this complex.
• Clinical Correlations - Oculomotor Palsy
• The trochlear nucleus of CN 4.

Question 27

Trochlear Palsy

Answer 27

• The affected eye is elevated (aka hypertropic).
• In CN 4 palsy, the superior oblique fails to activate. Thus, the affected eye is elevated (aka hypertropic).
• CN 4 is long and thin and is the only cranial nerve to make a decussation; so its innervation originates from the opposite side of the brainstem.
• Patients are often unaware of this deficit, because they produce a head tilt to counter it. They tilt their head to the side opposite the affected eye in order to bring their eyes into alignment.

Question 28

Cranial Nerve Nuclei: Sensory nuclei

Answer 28

Laterally, the mesencephalic trigeminal nucleus of CN 5 (which is a sensory nucleus).

Question 29

Medial portion of the crus

Answer 29

lie the frontopontine tracts.

Question 30

Lateral portion of the crus

Answer 30

lie the additional corticopontine tracts; they emanate from the occipital, parietal, and temporal cortices.

Question 31

The red nuclei span the

Answer 31

mid and upper midbrain.

The red nuclei receive fibers from both the motor cortex and cerebellum. Each red nucleus connects with the ipsilateral inferior olive as part of the triangle of Guillain-Mollaret (via the central tegmental tract), and each red nucleus also sends rubrospinal tract fibers down the brainstem and spinal cord to produce flexion movements of the upper extremities (see Drawing 7-3).

Question 32

Injury in the vicinity of the red nucleus can produce

Answer 32

a low-frequency, coarse postural and action tremor on the contralateral side of the body, called a rubral tremor. Despite its name, which suggests a close relationship to the red nucleus, rubral tremor can occur from injury to other brainstem areas, as well, and also from injury to the cerebellum and thalamus.

Midbrain Stroke: Benedikt’s Syndrome

Question 33

Midbrain Stroke: Benedikt’s Syndrome

Answer 33

This is a syndrome of ipsilateral 3rd nerve palsy and contralateral choreiform movements: it involves the red nucleus and neighboring third nerve.

Question 34

Fibers from the superior cerebellar peduncle (the major outflow tract of the cerebellum) decussate in the

Answer 34

central midbrain tegmentum. Indicate that they lie below the level of the red nuclei in the lower midbrain.

Question 35

Injury to Fibers from the superior cerebellar peduncle

Answer 35

Injury to these crossing fibers produces cerebellar ataxia on the side of the body that the fibers originated from (regardless of where they are injured along their path). For instance, whether it happens pre- or post-decussation, injury to superior cerebellar fibers from the right cerebellum produces ataxia on the right side of the body.

Claude’s syndrome

Question 36

Claude’s syndrome

Answer 36

This is a syndrome of ipsilateral 3rd nerve palsy and contralateral ataxia from injury to post-decussation superior cerebellar fibers and the neighboring 3rd nerve.

Question 37

The central tegmental tract lies in the

Answer 37

central, dorsal tegmentum.

It carries ascending reticular fibers to the rostral intralaminar nuclei of the thalamus as part of the ascending arousal system and descending fibers from the red nucleus to the inferior olive as part of the triangle of Guillain-Mollaret.

Question 38

The tectospinal tract lies just anterior to the

Answer 38

medial longitudinal fasciculus

Question 39

The tectospinal tract originates in the

Answer 39

uperior colliculus and decussates in the midbrain tegmentum and descends in front of the medial longitudinal fasciculus. It produces contralateral head turn.

Question 40

Both the medial longitudinal fasciculus and the tectospinal tract maintain their ____________, ____________ throughout the height of the brainstem.

Answer 40

Posterior, Midline position

Question 41

Regional stimulation of the superior colliculus stimulates

Answer 41

efferent impulses through the tectobulbar tract to the brainstem for eye movements and through the tectospinal tract to the upper cervical nuclei for visually directed neck and head movements.