Neuroscience Week 5: The midbrain Flashcards
THE MIDBRAIN
KEY FEATURES
- 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.
Identify
Identify
Identify
cerebral peduncle (aka crus cerebri)
The center of the crus divides into the corticonuclear tracts (aka corticobulbar tracts), medially, and the corticospinal tracts, laterally.
Weber’s Syndrome AKA
Midbrain Stroke
Midbrain Stroke: Weber’s Syndrome
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.
substantia nigra
- 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
red nucleus
- The red nucleus produces upper extremity flexion movements, which are observed in decorticate posturing – we discuss this in detail elsewhere.
Lateral midbrain
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).
Periaqueductal gray area
- 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.
Medial longitudinal fasciculus AKA
MLF
Medial longitudinal fasciculus
Plays an important role in conjugate horizontal eye movements.
Clinical Correlation - Internuclear ophthalmoplegia (MLF syndrome)
Internuclear ophthalmoplegia (MLF syndrome)
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.
Reticular formation
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.
The raphe nuclei affect
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.
The raphe nuclei lie along
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.
The caudal raphe group comprises
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.
Efferent projections from the rostral raphe group
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.
Posterior commissure
A white matter tract involved in the pupillary light reflex.
The nucleus of the posterior commissure helps control vertical eye movements.
Dorsal Midbrain Syndrome AKA
Parinaud Syndrome)
Dorsal Midbrain Syndrome (aka Parinaud Syndrome) Manifestations
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.
Dorsal Midbrain Syndrome (aka Parinaud Syndrome) Common Causes
Compressive:
- Pineal Tumors
- Hemorrhages with dorsal midbrain compression
Non-compressive:
- MS plaques
- Dorsal midbrain ischemic strokes
- Neuroinfectious causes
Superior colliculi
Involved in visual function.
Inferior colliculi
Auditory function
Cranial Nerve Nuclei: Motor nuclei
- 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.
Trochlear Palsy
- 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.
Cranial Nerve Nuclei: Sensory nuclei
Laterally, the mesencephalic trigeminal nucleus of CN 5 (which is a sensory nucleus).
Medial portion of the crus
lie the frontopontine tracts.
Lateral portion of the crus
lie the additional corticopontine tracts; they emanate from the occipital, parietal, and temporal cortices.
The red nuclei span the
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).
Injury in the vicinity of the red nucleus can produce
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
Midbrain Stroke: Benedikt’s Syndrome
This is a syndrome of ipsilateral 3rd nerve palsy and contralateral choreiform movements: it involves the red nucleus and neighboring third nerve.
Fibers from the superior cerebellar peduncle (the major outflow tract of the cerebellum) decussate in the
central midbrain tegmentum. Indicate that they lie below the level of the red nuclei in the lower midbrain.
Injury to Fibers from the superior cerebellar peduncle
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
Claude’s syndrome
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.
The central tegmental tract lies in the
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.
The tectospinal tract lies just anterior to the
medial longitudinal fasciculus
The tectospinal tract originates in the
uperior colliculus and decussates in the midbrain tegmentum and descends in front of the medial longitudinal fasciculus. It produces contralateral head turn.
Both the medial longitudinal fasciculus and the tectospinal tract maintain their ____________, ____________ throughout the height of the brainstem.
Posterior, Midline position
Regional stimulation of the superior colliculus stimulates
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.
