CHAPTER 8

Neurologic

Anatomy and Physiology

EMBRYOLOGY Migrating axons from retinal ganglion cells converge to the optic stalk, which later becomes the optic disk, and reach the ON by the seventh week. They continue centrally, guided by CNS pigment, to synapse at the lateral geniculate nucleus (LGN). Myelination starts proximally at the LGN in the seventh month. Neuroglial cells of the optic stalk support the axons and gradually obliterate the optic stalk lumen.

CRANIAL NERVES (Figs. 8–1 and 8–2) CN I and II are found in the diencephalon, III and IV in the midbrain, V, VI, VII, and VIII in the pons, and IX, X, XI, and XII in the medulla.

I. Olfactory nerve: may be affected by meningioma in association with Foster Kennedy syndrome.

II. Optic nerve: length is 1 mm intraocular, 25 mm intraorbital, 10 mm intracanalicular, and 17 mm intracranial to the chiasm (mnemonic: telephone number 125–1017).

Disk vasculature: laminar and prelaminar portions of the disk are supplied by the posterior ciliary artery (watershed area and site of AION); the intraorbital ON is supplied by the central retinal artery and pial vessels. The ophthalmic artery is nasal to the vein at the disk.

Nerve vasculature: peripheral, larger ON axons are metabolically supported by CSF and pial vessels (thus, they are relatively protected from toxins because of the blood–brain barrier). Central, smaller macular axons are supplied by the central retinal artery, which lies central within the ON within 12 mm of the globe.

The subdural space of the ON does not communicate with the subdural space of the cranium, as it fuses with the periosteum of the optic canal and is only a potential space. The dura is then contiguous with the sclera.

III. Oculomotor nerve: nucleus lies in the dorsal midbrain at the level of the superior colliculus. The levator nucleus is central and thus gives bilateral innervation; the SR fibers cross to innervate the contralateral muscle. All other subnuclei give ipsilateral fibers. CN III also supplies parasympathetic fibers from the Edinger-Westphal nucleus (afferent

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Figure 8–1 The right brainstem and cranial nerve anatomy, right posterolateral view.

input to the Edinger-Westphal nucleus is from the retina). CN III fasciculus exits the brainstem in the subarachnoid space, travels to the cavernous sinus, then through the SOF as a superior division (supplying SR and levator muscles) and inferior division (innervating IR, IO, and MR muscles and carrying parasympathetics to the pupil sphincter and ciliary body).

IV. Trochlear nerve: longest intracranial course innervating the SO. The nucleus lies in the midbrain tegmentum at the level of the inferior colliculus, near the midline but ventral to the cerebral aqueduct. Axons exit dorsally and cross (thus contralateral innervation), curving around the cerebral peduncle, then passing between the posterior cerebral and superior cerebral arteries along with CN III. They course anterior, piercing the dura at the tentorium cerebelli, then into the cavernous sinus and through the SOF, crossing medially to innervate the SO.

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Figure 8–2 Left cavernous sinus and associated structures, coronal section.

V. Trigeminal nerve: provides facial and ocular sensation and also innervates the muscles of mastication (temporalis, masseter, and pterygoids), the tensor veli palatini, the tensor tympani, the anterior belly of digastric, and the mylohyoid muscle.

Three divisions: CN V1 (ophthalmic), CN V2 (maxillary), and CN V3 (mandibular).

Lesions of CN V1 are often neoplasm (especially acoustic neuroma); CN V2, usually orbital trauma, or maxillary sinus disease (often the first sign of nasopharyngeal CA); and CN V3, often involved with nasopharyngeal tumor or middle fossa disease. Also trigeminal neuralgia or Raeder’s paratrigeminal neuralgia (CN V palsy plus Horner’s syndrome).

VI. Abducens nerve: most commonly injured by increased intracranial pressure or hydrocephalus (also known as the ‘‘tumor’’ nerve). The nucleus lies in the pontine tegmentum, close to the midline, just ventral to the fourth ventricle. Axons exit ventrally, then pass through the subarachnoid space, up the clivus through Dorello’s canal, through the petrous space, and into the cavernous sinus. In the cavernous sinus, CN VI floats in the lumen, joined briefly by sympathetics, and enters the orbit through the inferior division SOF, to innervate the LR muscle.

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VII. Facial nerve: originates in the pontine nucleus to supply the muscles of facial expression and provide parasympathetic input to secretory salivary and lacrimal glands. The parasympathetics begin in the superior salivatory nucleus in the pons, exit as the nervus intermedius, and join CN VII before they branch off as the greater superficial petrosal nerve, synapsing in the pterygopalatine ganglion. They then pass through the inferior orbital fissure, joining the zygomaticotemporal nerve, which sends a branch to the lacrimal gland for reflex tearing.

VIII. Auditory nerve: lesions often cause tinnitus, decreased hearing, vertigo, and a peripheral nystagmus.

IX. Glossopharyngeal nerve: innervates the stylopharyngeus muscle and the pharynx constrictor muscle and provides sensation to the upper pharynx and distal third of the tongue.

X. Vagus nerve: innervates the pharynx, larynx, heart, lungs, esophagus, and stomach.

XI. Accessory nerve: innervates the sternocleidomastoid (SCM) and trapezius muscles and may be involved in myotonic dystrophy or Meigs’ blepharospasm.

XII. Hypoglossal nerve: innervates the tongue muscle.

RETINAL ORGANIZATION Main retinal function is to convert information about brightness (luminance) into information about contrast. Retinal organization is only concerned about relative quantities, except the pupillary control cells, which travel to the pretectum and need direct luminance information.

Simplified, the retina functions as five main layers:

Photoreceptors (rods and cones): respond to light or color by hyperpolarization (in the dark, normally have a graded depolarizing response with the release of neurotransmitter glutamate) and project to the horizontal and bipolar cells.

Horizontal cells: located in the outer plexiform layer and respond to glutamate neurotransmitter from the photoreceptors with a graded release of inhibitory gamma-aminobutyric acid (GABA) neurotransmitter; function as a lateral connector of the photoreceptors and bipolar cells to detect center-surround contrast.

Bipolar cells: located in the inner nuclear layer and respond to glutamate neurotransmitter from the photoreceptors; project to amacrine and ganglion cells.

Amacrine cells: lateral connector of bipolar and ganglion cells with various types of cells, neurotransmitters, and functions, including sensing movement and direction; located in the inner plexiform layer.

Ganglion cells: carry the visual information to the brain and generate an action potential (not a graded response) when depolarized by glutamate released from bipolar cells.

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Bipolar receptive field: all parts of the retina that affect activity in a particular bipolar cell depolarize that cell if directly connected (comprise the ‘‘center’’ of receptive field) or hyperpolarize and oppose it if indirectly connected to it by a horizontal cell (‘‘surround’’ of receptive field). Each bipolar cell may be a depolarizing cell as described above or a hyperpolarizing cell, which responds to objects that are darker than the background, and thus are hyperpolarized by light in the center and depolarized by light falling on the surround.

‘‘Off ’’ bipolar cells: depolarized by glutamate released by photoreceptors in the dark

‘‘On’’ bipolar cells: hyperpolarized by glutamate, and thus are depolarized by the decreased photoreceptor release of glutamate in light

Color: certain horizontal cells are excited by green-absorbing cones and inhibited by red cones (and vice versa); other horizontal cells are excited by blue cones and inhibited by red and green cones, known as red/green and yellow/blue opponent color cells.

Ganglion cell functional division: ganglion cells have a receptive field concentric center/surround organization like bipolar cells: ‘‘on’’ center ganglion cells are excited by ‘‘on’’ center bipolar cells, and ‘‘off ’’ center ganglion cells are excited by ‘‘off’’ center bipolar cells.

Magnocellular (M) cell axons: large diameter; carry information about dim illumination and motion to the LGN layers 1 and 2.

Parvocellular (P) cells: small axons; are responsible for color and fine detail (because they receive input from a small number of bipolar cells and thus have small receptive fields) and travel to the LGN layers 3, 4, 5, and 6.

OPTIC NERVE Optic disk is a vertical oval (average dimensions 1.78 mm 1.90 mm) with an optic cup that is normally a horizontal oval. Axons receive glial support from astrocytes in the NFL and laminar and prelaminar layers. Oligodendrocytes in the retrolaminar ON produce myelin. The lamina cribosa is composed of stacked, fenestrated collagen and laminin plates.

Axons: 1.2 million axons arise from 5 million to 100 million ganglion cells and respect the horizontal raphae (vs. optic tract fiber organization that respects the vertical meridian).

Macular axons travel in the papulomacular bundle (PMB); temporal axons curve above and below the PMB (thus, the optic disk is thickest at the superior and inferior poles as more fibers enter from the periphery of the retina), and nasal fibers enter the disk more directly.

Peripheral axons travel deep in the NFL and enter the disk peripherally. Macular central axons lie superficial in the retina and pass into the center of the disk and nerve.

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Macular axons occupy the temporal wedge of the distal ON, then are diffusely oriented in the chiasm. Inferior ON fibers pass into

the chiasm and continue in the medial optic tract. Superior ON fibers pass into the lateral optic tract.

Vascular (Fig. 8–3): the NFL is supplied from the central retinal artery, prelaminar and laminar layers from short posterior ciliary arteries, and the retrolaminar layer from the central retinal artery. No normal disk vessels leak on FA because of the blood–brain barrier (formed by cellular tight junctions, pericytes, and nonfenestrated endothelium).

RETINA PROJECTS TO FOUR SUBNUCLEI The majority of fibers synapse at the LGN; however, a small number of fibers do not synapse at the LGN

Figure 8–3 Cross section of the anterior optic nerve, showing branches from the central retinal artery supplying the retrolaminar portion of the nerve and the NFL, and the prelaminar and laminar areas of the nerve supplied by the short posterior ciliary arteries.

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but instead travel to the superior colliculus, pretectum, and suprachiasmatic nucleus.

Lateral geniculate nucleus (LGN): responsible for object perception but is mainly a gateway and does little processing. It does have ‘‘on’’ and ‘‘off ’’ center cells similar to the retina and and also yellow/blue and red/green coded cells. The LGN projects to the primary visual cortex (also known as the striate pathway because on gross specimen it is seen as a stripe):

Primary visual cortex has six layers: tract fibers enter at layer IV and project to layers II and III, which project to layer V, which projects to the superior colliculus and layers II, III, and V, which project to layer VI, which projects to secondary visual cortical centers and feedback loop to the LGN.

Secondary visual cortical centers:

Temporal cortex: concerned with shape and what information

Parietal cortex: concerned with color and movement and where information

Superior colliculus: involved with eye movement control. Projects to the pulvinar, then to the cortex (extrastriate pathway).

Pretectum: concerned with pupil control and near synkinesis. Projects to incalated neurons, which are excitatory to the Edinger-Westphal nucleus (unlike cortical and spinal-reticular input and sleep and coma states that are inhibitory to the Edinger-Westphal nucleus).

Suprachiasmatic nucleus: involved with diurnal rhythms and hormonal control.

BRODMANN’S AREAS Number 17: primary occipital cortex (the macula is inside on the banks of the calcarine fissure in a watershed zone between the posterior and middle cerebral arteries). Number 18: splenium, which connects the right and left occiputs. Number 19: occipitoparietal area.

Fig. 8–4 shows the superior view of right cerebral blood vessels and the anterior/posterior visual pathways.

EYE MOVEMENT CONTROL Purpose is to stabilize gaze and put the object of interest on the fovea. Retinal slip velocity describes the smooth pursuit eye movement system that uses retinal information about the image- slip-velocity of the target to match the gaze-velocity to the actual target velocity (need a gain of 1 to keep image on the retina).

Gaze control: horizontal gaze includes pursuits (ipsilateral parietal origin) and saccades (fast movements from the contralateral frontal lobe).

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Figure 8–4 Superior view of selected right cerebral blood vessels and their relationship to the anterior and posterior visual pathways.

Horizontal gaze control: cortical input from the contralateral frontal eye fields, ipsilateral parietal lobe, and vestibular apparatus project to the ipsilateral paramedian pontine reticular formation (PPRF) adjacent to the CN VI nucleus. This input stimulates the ipsilateral LR muscle, which then projects interneurons, crossing into the medial longitudinal fasciculus (MLF) to the CN III nucleus (stimulating the contralateral MR muscle), with the result that the eyes look toward the ipsilateral side (Fig. 8–5).

Vertical gaze control: originates in the frontal eye fields or superior colliculus that projects to the rostral interstitial nucleus

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Figure 8–5 Schema of nuclei and neural pathways involved with horizontal eye movement control. The rostral interstitial nucleus of the MLF (riMLF), interstitial nucleus of Cajal (INC), and the trochlear nucleus (CN IV) are involved with vertical eye movment control.

of the MLF (riMLF) and then to CN III and IV nuclei. The interstitial nucleus of Cajal (INC) is involved with vertical pursuits.

Gaze acquisition mechanisms:

Saccades: voluntary eye movements with about 200 msec of latency. Initated from the PPRF, saccades have two components: initial pulse (burst of neuronal firing from burst cells), followed by step (to maintain the smooth movement, the firing neuron must overcome the viscous drag of muscle and the elastic restraining forces of orbit). The neural integrator keeps the burst and step in the same frequency and is located in the nucleus prepositus hypoglossi (NPH) and the medial vestibular nucleus (MVN) for horizontal movement and the interstitial nucleus of Cajal for vertical saccades.

Quick phases of nystagmus: fast 500 degrees/second.

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TABLE 8–1

Comparison of the Sympathetic and Parasympathetic Autonomic Nervous System

COM-T, catechol-o-methyltransferase; dopa, dihydroxyphenylalanine; IOP, intraocular pressure; MAO, monoamine oxidase.

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