Book Review And Study Questions For Chapter 6 - Part 2 Of 3

The tonsil of the cerebellum is located close to the midline and immediately above the medulla. Its position relative to the cerebellum is caudal, medial, and inferior. Tonsillarherniation may compress the medulla, and if sudden, may result in death. The other portions of the cerebellum usually do not herniate.(p.114–115)

The pars caudalis portion of the spinal trigeminal nucleus is located in the lateral medulla adjacent to the spinal trigeminal tract in cross-sectional levels between the obex and the C1 level of the spinal cord. This portion of the spinal trigeminal nucleus is responsible for relaying pain and thermal information originating from the face and oral cavity on one side to the ventral posteromedial nucleus on the contralateral side. The pars interpolaris is found at levels between the obex and the rostral end of the hypoglossal nucleus and the parsoralis between the interpolaris and the principal sensory nucleus. The principal sensory nucleus is in the pons and the mesencephalic nucleus is in the midbrain.(p.100–105)

The short-term loss of function, frequently involving a specific part of the body, is characteristic of a transient ischemic attack (commonly called a TIA).The follow-up MRI shows no lesion because there has been no permanent damage. TIAs are caused by a brief period of inadequate perfusion of a localized region of the nervous system; recovery is usually rapid and complete. However, TIAs, especially if repeated, maybe indicative of an impending stroke. Hemorrhagic strokes frequently result in some type of permanent deficit, and the central cord syndrome has bilateral deficits. A small embolic stroke would be visible on the follow-up MRI, and in this patient would have resulted in a persistent deficit. Syringobulbia may include long tract signs as well as cranial nerve signs.(p.160)

The substantia nigra contains a large population of melanin-containing cells, is located in the midbrain just internal to the crus cerebri, and the loss of these cells gives rise to the motor deficits characteristic of Parkinson disease. The neurotransmitter associated with these cells is dopamine. The reticular formation is in the core of the brainstem and the pontinenuclei are in the basilar pons; neither of these contains cells with melanin. The red nucleus is in the midbrain, but its red-dish tone is related to a rich vascular supply, not to cells containing a pigment.(p.133–135)

The pulvinar, geniculate nuclei, ventral posterome-dial and posterolateral nuclei, centromedian, and some other adjacent nuclei are served by the thalamogeniculate artery. The anterior and ventral anterior thalamic nuclei receive their blood supply from thalamoperforating arteries, the substantianigra via branches of P1 and P2 ,and globus pallidus from the lenticulostriate branches of M1.(p.160)

Penetrating branches of the posterior spinal artery serve the posterior columns (gracile and cuneate fasciculi) of the spinal cord at all levels. Branches of the posterior spinal artery also serve the gracile nucleus, but this structure is in the medulla, not in the spinal cord. The lateral corticospinal tract and the anterolateral system are served by the arterial vasocorona on the surface of the cord and the internal branches of the anterior spinal artery. The medial longitudinal fasciculus is in the territory of the anterior spinal artery.(p.90–94,96–97)

In this case the weakness of the upper and lower extremities on the right reflects damage to corticospinal fibers on the left side of the basilar pons. A lesion of these fibers on the right side of the pons would produce a left-sided weakness. Rubrospinal fibers are not located in the territory of paramedian branches of the basilar artery. Also, lesions of rubrospinal fibers and of the middle cerebellar peduncle do not cause weakness but may cause other types of motor deficits.(p.118–121,126)

The dorsomedial nucleus is located medial to the internal medullary lamina and, through its connections, one if its functions is to participate in arousal of the cerebral cortex. The other choices are in (anterior nucleus) or lateral to the internal medullary lamina, or, in the case of the globus pallidus, lateral to the internal capsule.(p.148–151,160)

The loss of pain and thermal sensations on the right side of the body correlates with a lesion involving the antero-lateral system on the left side of the spinal cord. A lesion of the right anterolateral system would result in a left-sided deficit. The gracile and cuneate fasciculi convey discriminative touch, vibratory sensation, and proprioception. The posterior spinocerebellar tract conveys similar information, but it is not perceived/recognized as such (consciously) by the brain.(p.90–93,96–97)

The putamen and the caudate nucleus originate from the same group of developing neurons, are collectively referred to as the neostriatum, and appear in the same shade of grey in a T1-weighted MRI. In general, the globus pallidus and pulvinar are distinctly lighter than the putamen and the dorsomedial nucleus frequently appears dark in a shade of grey distinctly different from that of the globus pallidus.(p.146–155)

The facial colliculus is an elevation in the floor of the fourth ventricle located medial to the sulcus limitans and formed by the underlying abducens nucleus and fibers (internal genu) originating from the facial nucleus. The vestibular
area, indicating the position of the vestibular nuclei, is lateral to the sulcus limitans and the hypoglossal nucleus is internal to the hypoglossal trigone in the medial floor of the ventricle in the medulla. The trigeminal and facial nuclei are located in the pontine tegmentum and do not border on the ventricular space.(p.118–119)

The exiting fibers of the abducens nerve (on the left)are in the territory of the paramedian branches of the basil artery and are laterally adjacent to corticospinal fibers in the basilar pons. Diplopia may result from lesions of the oculomotor and trochlear nerves, but these structures are not in the domain of the paramedian basilar branches. A lesion of the optic nerve results in blindness in that eye and damage to the facial root does not affect eye movement but may cause a loss of view of the external world if the palpebral fissure is closed due to facial muscle weakness.(p.50,118–121,126)

The solitary nucleus receives general visceral afferent(GVA) and special visceral afferent information (SVA, this input is taste) and is located in the region of the medulla served byposterior inferior cerebellar artery. All of the other choices are in the territory served by the anterior spinal artery.(p.112–113)

The prominent elevation formed on the caudal and medial wall of the third ventricle, at the general level of the posterior commissure, represents the location of the habenularnucleus. This is an excellent landmark to use in axial MRI when designating the separation between the third ventricle(rostral to this point on the midline) and the quadrigeminalcistern (caudal to this point).The pulvinar is lateral to the quadrigeminal cistern, the lamina terminalis forms the rostral wall of the third ventricle, and the massa intermedia bridges the space of the third ventricle. When present (in about 80% of patients) the Massa intermedia appears as a shadow in T2-weighted MRI bridging the third ventricle. The superior colliculus is a mesencephalic structure found in the quadrigeminal cistern.(p.30–31,136–137)

The red nucleus, exiting fibers of the oculomotor nerve, portions of the corticospinal fibers in the crus cerebri, and a number of other medially located structures are found in the territory of the penetrating branches of the basilar bifurcation. The paramedian branches of the basilar artery and the corticospinal fibers in the pyramid serve the abducens nerve by branches of the anterior spinal artery. The anterolateral system and the medial lemniscus are mainly, if not entirely, in the region of the midbrain served by branches of the quadrigeminal and posterior medial choroidal arteries.(p.49–50,132–135,138)

Weakness of the extremities accompanied by paralysis of the lateral rectus muscle (innervated by the abducens nerve) on the contralateral side indicates a lesion in the caudal and medial pons involving the abducens nerve root and corti-cospinal fibers. This is a middle alternating hemiplegia. Inferior alternating hemiplegia specifies involvement of the hypoglossal root and the pyramid, and superior alternating hemiplegia indicates damage to the oculomotor root and the crus cerebri. Alternating (or alternate) hemianesthesia and hemihypesthesia are sensory losses.(p.49–50,126)

In addition to other signs or symptoms, lesions in lateral areas of the brainstem may also interrupt hypothalamo-spinal fibers descending from the hypothalamus to the inter-mediolateral cell column in upper thoracic levels of the spinal cord. In this case the patient may present with a Horner syndrome, part of which is a small (constricted) pupil. In addition, the affected pupil may react slowly to reduced light. The antero-lateral system conveys somatosensory input and fibers of the medial longitudinal fasciculus (originating from the medulla)are primarily descending to spinal cord levels. Reticulospinal and vestibulospinal tracts influence spinal motor neurons.(p.49,112)

Damage to the gracile fasciculus on the left (at theT8 level this is the only part of the posterior columns present)accounts for the loss of vibratory sensation (and discriminative touch).Injury to the gracile fasciculus on the right would result in this type of deficit on the right side. The level of the cord damage is caudal to the cuneate fasciculi and the anterolateral system conveys pain and thermal sensations.(p.90–93,96–97)

The solitary nucleus is located immediately inferior(ventral) to the medial and spinal vestibular nuclei and is the only nucleus in the choices to receive a general visceral afferent(GVA) and special visceral afferent (SVA-taste) input. The inferior salivatory nucleus and the nucleus ambiguus are visceromotor (general visceral efferent [GVE] and special visceral efferent [SVE],respectively) and the spinal trigeminal and cochlear nuclei are sensory (general somatic afferent [GSA]and special somatic afferent [SSA],respectively).(p.104–109)

The superior salivatory nucleus lies adjacent to the exiting fibers of the facial nerve in a position just lateral to the abducens nucleus in caudal levels of the pons. The preganglionic axons originating from these cells distribute on peripheral branches of the facial nerve. The dorsal motor and inferior salivatory nuclei are in the medulla and associated, respectively, with the vagus and glossopharyngeal nerves. The Edinger-Westphal nucleus is related to the oculomotor nucleus and the intermediolateral cell column is located primarily in thoracic levels of the spinal cord.(p.188–119)

At caudal pontine levels most, if not all, of the medial lemniscus is located within the territory served by paramedian branches of the basilar artery. Penetrating branches of the anterior spinal artery serve the hypoglossal nucleus. The other choices are generally in the territories of short or long circumferential branches of the basilar artery.(p.50,118–121,126)