Block 5 Neuro Atlas Ch 6 W Expl Part 1

Alternating sensory losses accompanied by a motor deficit on the same side as the loss of vibratory sensation are characteristics of the Brown-Séquard syndrome (also commonly called a spinal cord hemi section).The Wallenberg syndrome is seen in lesions of the medulla, and the Benedikt, Claude, and Weber syndromes are seen in lesions of the midbrain. In these brainstem syndromes there are usually characteristic cranial nerve and long tract signs and symptoms.(p.50,88–93,96–97)

A medullary lesion that results in weakness of the extremities on one side indicates involvement of the corti-cospinal fibers located in the pyramid on the contralateral side; these fibers largely cross in the pyramidal (motor) decussation. Rubrospinal and vestibulospinal fibers influence the activity of spinal motor neurons, but isolated lesions of these fibers would not result in a unilateral weakness of upper and lower extremities. The anterolateral system and the medial lemniscus are sensory tracts.(p.103–109,112)

Wild flailing movements of the extremities, especially the upper, are hemiballistic movements (hemiballismus);these are characteristic of a lesion in the subthalamic nucleus. Damage to the cerebellar cortex and nuclei and the lenticularnucleus will result in motor deficits, but these are usually described as involving tremor, ataxia, and related motor problems. The ventral lateral nucleus is a thalamic relay center for motor information and the ventral posterolateral nucleus is a sensory relay nucleus. Lesions of these nuclei will result in motor (but not hemiballismus) and sensory deficits.(p.145–149,160)

The claustrum is the thin layer of grey matter that is located between the extreme and external capsules. It is generally regarded as being functionally related to the insular cortex. The external medullary lamina is found at the interface of the lateral portions of the thalamus with the internal capsule and the lamina terminalis is the thin structure forming the rostral wall of the third ventricle. The putamen is located medial to the external capsule and lateral to the globus pallidus and the stria terminalis is a fiber bundle in the groove between the body of the caudate nucleus and the dorsal thalamus.(p.146–154)

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)

Fibers conveying pain and thermal sensations cross the midline in the anterior white commissure. Consequently, a lesion of this structure, as in syringomyelia, would result in a bilateral loss of these sensations, reflecting the levels of the syrinx. Damage to fibers of the anterolateral system results in a loss of these sensations on the contralateral side and the posterior columns convey proprioception, discriminative touch, and vibratory sense. The medial longitudinal fasciculus does not contain fibers conveying sensory input.(p.92–93,96–97)

The masticatory muscles receive their motor innervation via the motor neurons located in the trigeminal motornucleus on the ipsilateral side; this excludes the right trigeminal nucleus. Facial motor neurons innervate the muscles offacial expression on the ipsilateral side and the hypoglossal nucleus innervates the ipsilateral side of the tongue.(p.122–123,126)

The ventral posterolateral nucleus of the thalamus receives the pathways (medial lemniscus and anterolateral sys-tem) that relay the information lost as a result of the lesion in this woman. The ventral posteromedial nucleus relays comparable information from the face and the medial geniculate nucleus is related to the auditory system. Lesions in the subthalamic nucleus result in hemiballismus. The anterolateral system relays pain and thermal sense; this is only part of the sensory deficits experienced by this woman.(p.144–145,160)

In this patient the weakness of the right lower extremity is related to a lesion of lateral corticospinal tract fibers on the right side of the spinal cord. The left corticospinal tract serves the left side of the spinal cord and the left lower extremity. Rubrospinal, reticulospinal, and vestibulospinal fibers influence the activity of spinal motor neurons; however, the deficits related to corticospinal tract damage (significant weakness) will dominate over the lack of excitation to flexor or extensor motor neurons in the spinal cord via these tracts.(p.90–93,96–97)

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)

Although this patient initially presented with complete motor and sensory losses, some function had returned by36 hours; in this case the lesion is classified as an incomplete lesion of the spinal cord. Patients with no return of function at 24hours and no sacral sparing have suffered a lesion classified as complete and it is unlikely that they will recover useful neurological function. In a central cord and a large syringomyelia there is sparing of posterior column sensations and in a hemi section the loss of motor function is unilateral.(p.96–97)

Weakness of the muscles of the face, particularly when upper and lower portions of the face are involved, indicate a lesion of either the facial motor nucleus or the exiting fibers of the facial nerve; both are located in the lateral pontinetegmentum at caudal levels. The hypoglossal nucleus innervates muscles of the tongue, the trigeminal nucleus innervates masticatory muscles, and the abducens nucleus innervates the lateral rectus muscle, all on the ipsilateral side. The arcuatenucleus is a group of cells located on the surface of the pyramid.(p.118–121,126)

A pineal tumor impinging on the superior colliculus may result in a paralysis of upward gaze (Parinaud syndrome).The inferior colliculus is related to the auditory system, trochlear fibers innervate the ipsilateral superior oblique muscle, and the posterior commissure contains fibers related to the pupillary light pathway. Occlusion of the great cerebral vein may cause serious neurological deficits but not specifically a paralysis of upward gaze.(p.138,144–145)

The loss of pain and thermal sensations beginning at the level of the umbilicus (T10 dermatome) on the left side results from damage to fibers of the anterolateral system at about the T8 level on the right. These fibers ascend one to two levels as they cross the midline. Damage at the T6 level would result in a loss beginning at the T8 level on the contralateral side and damage at the T10 level would result in a loss beginning at about the T12 level.(p.90–93,96–97)

The rubrospinal tract lies immediately anterior (ventral) to, and partially overlaps with, the lateral corticospinal tract. The anterolateral system is in the anterolateral area of the spinal cord and is spatially separated from the lateral corti-cospinal tract. The gracile fasciculus is in the posterior columns, the medial longitudinal fasciculus is in the ventral funiculus, and the anterior spinocerebellar tract is located on the antero-lateral surface of the spinal cord.(p.88–93)

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)

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 fibers of the anterolateral system are located in the lateral portion of the pontine tegmentum anterior (ventral) to the facial motor nucleus; these fibers convey pain and thermal inputs. The spinal trigeminal tract and the anteriortrigeminothalamic tract also convey pain and thermal input but from the ipsilateral and contralateral sides of the face, respectively. The lateral lemniscus is auditory in function and the medial lemniscus conveys proprioception, vibratory sense, and discriminative touch.(p.50,118–121,126)

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 anterior spinal artery serves the medial portion of the medulla, an area that encompasses the medial lemniscus, exiting roots of the hypoglossal nerve, and the corticospinal fibers in the pyramid. The anterolateral system, spinal trigeminal tract, and rubrospinal tract are in the territory of the posterior inferior cerebellar artery (commonly called PICA by clinicians).The posterior spinal artery in the caudal medulla and spinal cord serves the gracile fasciculus.(p.104–109,112)

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)

Reticulospinal fibers (medial and lateral) and later-al vestibulospinal fibers are found predominantly in the anterolateral area of the spinal cord; medial vestibulospinal fibers are located in the medial longitudinal fasciculus. In the decerebrate patient, the descending influence of rubrospinal fibers on spinal flexor motor neurons is removed, and descending influence on extensor motor neurons is predominant. The posterior columns, posterolateral area of the cord, and the posterolateral tract do not contain vestibulospinal or reticulospinalfibers. The intermediate zone, a part of the spinal cord grey matter, contains some of the terminals of these fibers but not the descending tracts in toto.(p.88–93,210–211)

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 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 anterolateral system is located internal to the position of the anterior spinocerebellar tract; damage to this area of the spinal cord would most likely result in a loss of pain and thermal sensations on the contralateral side of the body below the lesion. The lateral corticospinal tract is located internal to the posterior spinocerebellar tract, the anterior white commissure and the anterior corticospinal tract are located in the anterior funiculus of the cord, and the cuneate fasciculus is in the posterior column medial to the posterior horn at upper thoracic and cervical levels.(p.88–93)

The artery of Adamkiewicz is usually located at theT12–L1 spinal cord levels and is more frequently (about 65% of the time) seen on the left side. The other cord levels listed may have small spinal medullary arteries but not the large diameter vessel characteristic of Adamkiewicz.(p.88–89,96–97)

The head of the caudate nucleus is located in the lateral wall of the anterior horn of the lateral ventricle and receives its blood supply from the medial striate artery (also called the artery of Heubner).This vessel also serves much of the anterior limb of the internal capsule. The lenticulostriate arteries serve a large part of the lenticular nucleus and portions of the surrounding internal capsule, and thalamo perforating arteries serve anterior portions of the dorsal thalamus. The anterior choroidal artery provides blood supply to inferior portions of the internal capsule, optic tract, and structures in the medial portions of the temporal lobe. The medial posteriorchoroidal artery serves choroid plexus in the lateral and third ventricles and adjacent areas of the lateral midbrain and caudomedial thalamus.(p.154–159,160)

The anterior choroidal artery serves the optic tract(homonymous hemianopsia) and the inferior portions of the posterior limb of the internal capsule (weakness of the extremities).The ophthalmic artery, via its central retinal branch, serves the retina; the anterior cerebral artery serves the lower extremity areas of the motor and sensory cortices; and distal segments of the posterior cerebral artery serve the medial temporal cortex and the visual cortex. The lateral posteriorchoroidal artery serves the choroid plexus in the lateral ventricle and some adjacent structures.(p.148–152,160)

The thalamoperforating arteries serve the more rostral portions of the dorsal thalamus. These vessels may originate as a single trunk or as several vessels from the P1 segment of the posterior cerebral artery. The anterior choroidal artery serves the optic tract, inferior portions of the internal capsule, choroidplexus in the temporal horn, and structures in the medial region of the temporal lobe. The thalamogeniculate artery sup-plies blood to the caudal thalamus, the medial striate arteries to the head of the caudate nucleus, and the lateral striate arteries to much of the lenticular nucleus.(p.146–150,160)

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)