Draw It To Know It Notes.docx

THE MIDBRAIN KEY FEATURES  The cerebral peduncles lie anteriorly.  The posterior commissurelies posteriorly.  The bilateral, thinly-shaped substantia nigraare essential for motor activation.  The bilateral, circular-shaped, red nuclei.* The colliculilie posteriorly.  The cerebral aqueduct (of Sylvius) is the midbrain portion of the brainstem CSF.  The periaqueductal gray area surrounds the cerebral aqueduct. DETAILS Anteriorly, within the basis:  The cerebral peduncle (aka crus cerebri).  The center of the crus divides into the corticonuclear tracts (aka corticobulbar tracts), medially, and the corticospinal tracts, laterally. Clinical Correlation - 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.

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The 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.

Anterior aspect of the midbrain tegmentum:  The circular 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 (MLF)  Plays an important role in conjugate horizontal eye movements. Clinical Correlation - 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.

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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. Clinical Correlation - Dorsal Midbrain Syndrome (Parinaud Syndrome).

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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. Common Causes

Compressive:

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Pineal Tumors Hemorrhages with dorsal midbrain compression

Non-compressive:

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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

Cranial Neuropathy: 3rd Nerve Palsy

Remember: "DOWN and OUT" with a dilated pupil. Permission from Dr. Schwartzman adapt/reuse photograph within this image.

OCULOMOTOR PALSY Remember: "DOWN and OUT" with a dilated pupil.

OVERVIEW OF CAUSES: MEDICAL vs SURGICAL CAUSES CN 3 lesions are generally divided into medical or surgical (compressive) causes: classically, medical third nerve lesions spare the pupil. DIFFERENTIAL DIAGNOSIS

MEDICAL CAUSES Isolated third nerve lesions may occur with:

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Branch occlusion of the posterior cerebral artery. Most often these occur with contralateral weakness (fascicular third nerve) or with ataxia (Claude’s syndrome) or choreoathetosis (Benedict’s syndrome) from thalamoperforate branch occlusion of the P1 segment of the posterior cerebral artery. Rarely, an ascending interpeduncular branch occlusion from the top of the basilar artery.

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Myasthenia gravis frequently affects the third nerve with pupillary sparing and increased ptosis with fatigue. Rarely, demyelinating disease affects the third nerve.

Symmetrical ptosis and oculomotor weakness is characteristic of:  Mitochondrial disease (Kearns–Sayre syndrome, progressive external ophthalmoplegia)  Congenital myopathy (centronuclear or myotubular myopathy)  Oculopharyngeal dystrophy. Surgical (compressive) third nerve is seen with:  Aneurysms as follows:  Posterior communicating artery  Internal carotid artery  Superior cerebellar artery  Top of the basilar (often in conjunction with the fourth nerve)  Tumors:  Parasellar neoplasms  Medial sphenoid wing meningiomas  Skull-based tumors. 

The trochlear nucleus of CN 4.

Clinical Correlations - Trochlear Palsy

Cranial Neuropathy: 4th Nerve 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). ADVANCED ASPECTS OF MIDBRAIN ANATOMY Medial portion of the crus, lie the frontopontine tracts. Lateral portion, 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. Clinical Correlation - 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 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. Clinical Correlation - 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.

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 superior 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 posterior, midline position throughout the height of the brainstem. 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.

THE PONS From a clinician's perspective, the pons is, most notably, the neurobiological site of injury that produces locked-in syndrome.

The pontine basis comprises:  Pontine nuclei  Pontocerebellar fiber tracts cross the pons and pass into the middle cerebellar peduncle as an important step in the corticopontocerebellar pathway. Clinical Correlation: central pontine myelinolysis  Corticonuclear (aka corticobulbar) tracts  Corticospinal tracts (lateral to them)  The middle cerebellar peduncle (aka brachium pontis) in the dorsal pons.  The pontine nuclei project pontocerebellar fibers via the middle cerebellar peduncle as part of the corticopontocerebellar pathway.  The inferior cerebellar peduncle.  The superior cerebellar peduncle (aka brachium conjunctivum). -The middle and inferior cerebellar peduncles are the main inflow pathways into the cerebellum and that the superior cerebellar peduncle is the main outflow pathway from the cerebellum. * The inferior cerebellar peduncle comprises the restiform and juxtarestiform bodies. * Spinocerebellar, reticulocerebellar, and olivocerebellar fibers pass through the restiform body, whereas the juxtarestiform body primarily comprises fibers that pass between the vestibular nucleus and vestibulocerebellum. * The superior cerebellar peduncles attach to the upper pons and midbrain; the middle cerebellar peduncle attaches to the pons and extends to the pontomedullary junction; and the inferior cerebellar peduncle attaches to the lower pons and medulla.

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The medial lemniscus lies medially (unlike in the midbrain, where the red nuclei push the medial lemniscus out laterally).

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The spinothalamic tract lies lateral to it. The anterior trigeminothalamic tract (TTT) lies along the posterior wall of the medial lemniscus. Anterior spinocerebellar tract lies far lateral from it.

Different portions of the spinocerebellar and trigeminothalamic tracts are variably found at different brainstem levels due to their complex anatomy.

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The medial longitudinal fasciculus descends through the dorsal midline tegmentum (as it does throughout the brainstem).

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.  

The locus coeruleus lies in the posterior tegmentum. Locus coeruleus nuclei are most heavily concentrated in the pons and are concentrated most significantly with noradrenalin (aka norepinephrine). Given their noradrenergic make-up and widespread connections, the locus coeruleus most likely plays a role in attention and arousal.

Note that although most heavily concentrated in the pons, the locus coeruleus actually spans from the caudal end of the periaqueductal gray area in the lower midbrain to the facial nucleus in the mid-pons.

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The reticular formation most notably functions in wakefulness. It divides into lateral, medial, and median zones. The raphe nuclei populate the median zone. They are primarily serotinergic and are modulated by psychotropic medications.

CRANIAL NERVE NUCLEI Medially, the motor nuclei:  In midline, the abducens nucleus of CN 6.  Lateral to it, the motor trigeminal nucleus of CN 5.  The facial nucleus of CN 7.  The superior salivatory nucleus of CN 7. Laterally, the sensory nuclei:  The principal sensory nucleus of CN 5.  The vestibulocochlear nucleus of CN 8. Clinical Correlation - Cranial Neuropathies: 5, 6, 7, [8

](/term/neuroanatomy/cranial-neuropathy-8-th-nerve-palsy) ADVANCED ANATOMY The tectospinal tract continues to descend through the dorsal midline tegmentum; it produces contralateral head turn.  It originates in the superior colliculus of the midbrain, decussates in the dorsal midbrain tegmentum, and innervates the upper cervical spinal segments.  The central tegmental tract descends through the dorsal, central 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, which is a network involved in movement. Clinical Correlation - Oculopalatal Myoclonus

Oculopalatal Myoclonus Oculopalatal myoclonus is a segmental myoclonus with continuous, pendular oscillations most commonly in the vertical plane. Rhythmic movement of the platysma muscle in conjunction with the palate. It is distinguished from pendular nystagmus because of the presence of myoclonus of non-ocular structures, such as the palate. In the clinical syndrome of oculopalatal myoclonus, hypertrophy of the inferior olivary nucleus exists in association with dysfunction of the dentato-rubro-olivary pathway (Guillain Mollaret triangle). Hypertrophy of the inferior olivary nucleus exists in association with dysfunction of the dentato-rubro-olivary pathway (Guillain Mollaret triangle).

Guillain Mollaret Triangle The inferior olive contains climbing fibers; they project to the contralateral dentate nucleus of the cerebellum as part of the triangle of Guillain-Mollaret. The dentate nucleus of the cerebellum projects back to the contralateral red nucleus, and then down to the inferior olive of origin via the central tegmental tract.

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The rubrospinal tract lies just posterior to the anterolateral system. It produces upper extremity flexion. It originates from the red nucleus in the midbrain, crosses in the ventral tegmental area of the brainstem, and innervates the upper cervical spinal cord.

Pontine components of the auditory system  The lateral lemniscus tract in the lateral tegmentum.  The trapezoid body: a conglomeration of auditory fibers that lie between the lateral and medial leminscuses.  The superior olivary complex in the anterolateral tegmentum. It comprises three separate nuclei:  The nucleus of the trapezoid body.  The medial superior olivary nucleus.  The lateral superior olivary nucleus.