Book Review And Study Questions For Chapter 6 - Part 3 Of 3

The deviation of the tongue to the left on attempted protrusion is the best localizing sign in this woman. This is especially the case when the deviation of the tongue is seen in concert with the motor and sensory losses described for this patient. This clearly indicates a lesion in the medial medulla encompassing the corticospinal fibers, medial lemniscus, and exiting fibers on the hypoglossal nerve. Motor and sensory losses, without the cranial nerve sign, could suggest a lesion at several different levels of the neuraxis.(p.112)

The uncus is at the rostral and medial aspect of the parahippocampal gyrus, and, in this position, is directly adjacent to the anterolateral aspect of the midbrain. The diencephalon is rostral to this point and the medulla, the most caudal part of the brainstem, is located in the posterior fossa. Late stages of uncal herniation may, but not always, result in damage to the rostral pons; this is especially so if the patient becomes decerebrate. The cerebellum is not involved in uncal herniation.(p.20,22,24,152–153)

Fibers comprising the anterolateral system convey pain and thermal sensations from the body, excluding the face. These fibers are located in lateral portions of the medulla adjacent to the spinal trigeminal tract; this latter tract relays pain and thermal sensations from the face. The gracile and cuneatefasciculi convey proprioception, discriminative touch, and vibratory sense in the spinal cord and the medial lemniscus conveys this same information from the medulla to the dorsal thalamus.(p.104–109,112)

All of the sensory deficits seen in this woman reflecta lesion in the medial lemniscus, which is located in the medial medulla in the territory of the anterior spinal artery. The anterolateral system and the spinal trigeminal tract convey pain and thermal sensations from the body (sans face) and face, respectively. The solitary tract is made up of the central processes of viscerosensory fibers and the medial longitudinal fasciculus at this level contains descending fibers that influence spinal motor neurons.(p.106–109,112)

The vocalis muscle (this muscle is actually the medial portion of the thyroarytenoid muscle) is innervated, via the vagus nerve, by motor neurons located in the nucleus ambiguus. The gracile nucleus conveys sensory input from the body and the spinal trigeminal nucleus relays sensory input from the face. The hypoglossal nucleus is motor to the tongue and the facial nucleus is motor to the muscles of facial expression.(p.104–109,112)

The loss of pain and thermal sensations on one side of the face correlates with damage to the spinal trigeminal tract; in this case the loss is ipsilateral to the lesion. The anterolateral system relays pain and thermal sensations from the contralateral side of the body, the solitary tract conveys visceral sensory input (especially taste),and the medial lemniscus contains fibers relaying information related to position sense and dis-criminative touch. The medial longitudinal fasciculus does not contain sensory fibers.(p.104–109,112)

The root of the oculomotor nerve conveys GSE fibers to four of the six major extra ocular muscles and GVE parasympathetic preganglionic fibers to the ciliary ganglion from which postganglionic fibers travel to the sphincter muscle of the iris. Pressure on the oculomotor root, as in uncal herniation, will usually compress the smaller diameter, and more superficially located GVE fibers first. Optic nerve damage results in blindness in that eye, injury to sympathetic fibers to the eye results in constriction of the pupil, and an abducens root injury results in an inability to abduct that eye. A lesion of corticonuclear fibers in the crus results primarily in motor deficits related to the facial, hypoglossal, and accessory nerves.(p.138)

The dentate nucleus appears as a long thin undulating line within the white matter core of the cerebellar hemi-sphere. It is frequently described as having the three-dimensional shape of a crumpled bag with its hilus (the opening of the bag) directed rostromedially. The other cerebellar nuclei(fastigial, globose, emboli form) are small clumps of cells, and the red nucleus is found in the midbrain, not in the cerebellum.(p.114–115)

The posterior inferior cerebellar artery (commonly called PICA by clinicians) serves the posterolateral portion of the medulla, which encompasses the anterolateral system, spinal trigeminal tract, and nucleus ambiguus. The anterior and medial areas of the medulla (containing the pyramid, medial lemniscus, and hypoglossal nucleus/nerve) are served by the anterior spinal artery and the anterolateral area of the medulla (the region of the olivary nuclei) is served by penetrating branches of the vertebral artery. The posterior spinal artery serves the posterior column nuclei in the medulla and the anterior inferior cerebellar artery (commonly called AICA) serves caudal portions of the pons and cerebellum.(p.104–109,112)

Motor neurons in the nucleus ambiguus innervate, primarily through the vagus nerve, the muscles of the throat that move a bolus of food from the oral cavity to the esophagus. The tongue, via the hypoglossal nucleus and nerve, may move food around in the mouth and toward the back of the oral cavity, but the actual act of swallowing is through the action of pharyngeal and laryngeal musculature. The dorsal motor vagal and inferior salivatory nuclei are both visceromotor (autonomic) nuclei, and the medial vestibular nucleus is involved in the regulation of eye movement and balance and equilibrium.(p.50,104–107,112)

The inferior salivatory nucleus is located in the rostral medulla, medial to the solitary tract and nuclei and inferior to the medial vestibular nucleus. Preganglionic axons that originate from these cells distribute on branches of the glossopharyngeal nerve. The dorsal motor nucleus is in the medulla, its axons travel on the vagus nerve. The superior salivatory nucleus is in the caudal pons and is associated with the facial nerve. Cells of the Edinger-Westphal nucleus are associated with the oculomotor nucleus of the midbrain and the intermediolateral cell column is located primarily in thoracic levels of the spinal cord.(p.104–107)

The restiform body is a large fiber bundle located in the posterolateral area of the medulla in the region served byposterior inferior cerebellar artery (PICA).This structure contains a variety of cerebellar afferent fibers including those of the posterior spinocerebellar tract. Damage to the vestibular nuclei will result in a tendency to fall to the ipsilateral side but will also produce diplopia (double vision) and nausea; symptoms not experienced by this patient. The anterolateral system is sensory, the nucleus ambiguus is motor to muscles of the throat (including the vocalis),and the corticospinal tract is not in the PICA territory.(p.104–109,112)

The oculomotor nucleus (containing general somatic efferent [GSE] cell bodies),along with the Edinger-Westphal(containing general visceral efferent [GVE] cell bodies) nucleus, is found in the most anterior and medial portion of the periaqueductal grey at the superior colliculus level. The trochlear nucleus is found at a comparable position, but at the cross-sectional level of the inferior colliculus. The mesencephalic nucleus is found in the lateral area of the periaqueductal grey, and the trigeminal and abducens nuclei are located in the pons.(p.132–133)

A superior alternating (or alternate) hemiplegia is characterized by a loss of most eye movement (damage to oculomotor nerve fibers) on the ipsilateral side and weakness of the upper and lower extremities (damage to corticospinal fibers in the crus cerebri) on the contralateral side. The abducens nerve is the cranial nerve involved in a middle alternating hemiplegia and the hypoglossal is that nerve involved in an inferior alternating hemiplegia. The trigeminal nerve innervates the muscles of mastication and the trochlear nerve innervates the superior oblique muscle.(p.132–133,138)

The decussation of the superior cerebellar peduncle is a prominent fiber bundle located in the tegmentum of the midbrain directly on the midline at the level of the inferior colliculus. This bundle is made up of cerebellar efferent fibers. Thered nucleus is located in the midbrain tegmentum, but not on the midline. Decussating trigeminothalamic fibers are found in the medulla and do not form a visible structure on the midline. The motor decussation is a compact bundle on the midline, but it is in the medulla, not the midbrain. The main parts of the substantia nigra are in the midbrain, are seen in sagittal MRI, but they are definitely not on the midline.(p.130–1

Fibers conveying discriminative touch, vibratory sensations, and proprioception are located in the lateral lemniscus; those from the contralateral upper extremity are medial, whereas those from the contralateral lower extremity are lateral. This man has difficulty walking due to a lesion of fibers conveying position sense from the lower extremity, not due to a lesion influencing descending fibers passing to spinal motorneurons. Fibers of the anterolateral system convey pain and thermal sensation. Rubrospinal and corticospinal are motor infunction; however this man has no weakness.(p.130–133)

Syringobulbia is a cavitation within the medulla. Cavitation in this location may communicate with a cavity in cervical levels of the spinal cord (syringomyelia).Hydromyelia refers to a cavity of the spinal cord that is lined with ependymal cells. The central cord and Brown-Séquard syndromes are lesions of the spinal cord that give rise to characteristic motor and sensory losses.(p.112)

Weakness of the extremities accompanied by paralysis of muscles on the contralateral side of the tongue (seen as a deviation of the tongue to that side on protrusion) indicates a lesion in the medulla involving the corticospinal fibers in the pyramid and the exiting hypoglossal roots. This is an inferior alternating hemiplegia. Middle alternating hemiplegia refers to a lesion of the pontine corticospinal fibers and the root of the abducens nerve, and superior alternating hemiplegia specifies damage to the oculomotor root and crus cerebri. Alternating(alternate) hemianesthesia and hemihypesthesia are sensory losses.(p.112)

Uncal herniation compresses the lateral portion of the brainstem, eventually resulting in compression of the corti-cospinal fibers in the crus cerebri. Weakness on the patient’s left side indicates damage to corticospinal fibers in the right crus. In situations of significant shift of the midbrain due to the herniation, the contralateral crus also may be damaged, resulting in bilateral weakness. Although all other choices contain corticospinal fibers, none of these areas are directly involved inuncal herniation.(p.138)

The anterolateral system is located just internal to the brachium of the inferior colliculus in the lateral portions of the midbrain tegmentum. This tract conveys pain and thermal sensations from the contralateral side of the body excluding the face. Corticospinal fibers are located in the crus cerebri, the mesencephalic tract at the lateral edge of the periaqueductal(central) grey, and the central tegmental tract is, as its name indicates, in the central part of the tegmentum. Oculomotorfibers within the midbrain leave the nucleus, arch through the tegmentum, and exit on the medial surface of the basis pedunculi into the interpeduncular cistern.(p.130–133)