Cranial Nerves I

1. Cranial Nerves I

2. Introduction • The head and neck are not innervated by spinal nerves; rather, they receive sensory information and send motor information via the 12 cranial nerves located in the brain. Although they are located within the skull, cranial nerves are considered part of the peripheral nervous system because they convey messages to and from the body's muscles and glands.

3. Introduction • The cranial nerves function as modified spinal nerves. While all the spinal nerves convey both sensory and motor information, cranial nerves may carry solely sensory information, solely motor information, or both. Cranial nerves that have only a sensory or motor component can be called pure nerves ; nerves that have both components can be called mixed nerves .

4. Introduction • The cranial nerve routes for sensory and motor circuits have different neuroanatomical connections. Sensory pathways are composed of 3 major neurons: the primary, the secondary, and the tertiary (see Figure 2). The cell bodies of primary neurons are usually located outside the CNS in sensory ganglia. They are homologous with the dorsal root ganglia of the spinal cord. The cell bodies of secondary neurons are in the dorsal gray matter of the brain stem, and their axons usually cross the midline and project to the thalamus. These are the actual sensory components of the cranial nerves. The cell bodies of the tertiary neurons are in the thalamus, and their axons project to the sensory cerebral cortex. There are two exceptions to this neuroanatomical scheme. Cranial nerves I and II (olfactory and optic) are special cases; the afferent fibers of their primary sensory neurons enter the brain stem and terminate on the secondary sensory neurons.

6. Introduction • Motor pathways are composed of only two major neurons: the upper motor neuron and the lower motor neuron. The upper motor neuron is usually located in the cerebral cortex. It's axon projects caudally to contact the lower motor neuron. Most, but not all, of the motor pathways that terminate in the brain stem project bilaterally to contact lower motor neurons on both sides of the midline. The lower motor neuron is located in the brain stem. These are the actual motor components of the cranial nerves.

8. Cranial Nerves • There are various mnemonics floating around to help students learn the cranial nerves. A few are shown below. Help for remembering the functions of each nerve are more mnemonics in the second table below.

10. Cranial Nerves - S=Sensory; M=Motor; B=Both

11. Cranial Nerve I: Olfactory Nerve • CN-I is a purely sensory nerve and serves the sense of smell. It originates in the olfactory cells of the nasal mucosa. Olfactory neurons are bipolar neurons that serve as the olfactory receptor. They are bathed by a layer of mucus where odorous substance dissolve into.

12. Olfactory Nerve • Bundles of fibers collect information and pass it to the olfactory bulb, which continues in a caudal direction to the olfactory tract. The olfactory tract extends into the olfactory trigone, where olfactory tract splits into lateral olfactory stria and medial olfactory stria. Most of the olfactory tract pass into the lateral olfactory stria, which run into lateral olfactory area.

14. Olfactory Nerve • Lateral olfactory area consists of the paleocortex of uncus, cortex of the entorhinal area (the anterior part of the parahippocampal gyrus) and the cortex in the region of the limen insulae. The uncus, cortex of the entorhinal area and cortex in the region of the limen insulae. The three parts are generally referred as pyriform cortex (pear shaped). Part of the amygdaloid body (amygdala) also is included in the lateral olfactory area. Lateral olfactory area is the principal region for the awareness of olfactory stimuli, so it is the primary olfactory area. • Because olfactory system shares entorhinal cortex with limbic system, which has extensive connections with the septal area and hypothalamus. The medial forebrain bundle contains fibers that connect septal area and hypothalamus with autonomic nuclei. Descending fibers from hypothalamus proceed to autonomic nuclei in the brain stem and spinal cord. So smell of food --> saliva secretion

15. Olfactory Nerve • Fracture of the floor of anterior fossa could damage the olfactory nerves and causing anosmia (loss of smell), leakage of CSF from nose (cerebrospinal fluid rhinorrhea). Tumor can also block the olfactory pathway, likely to be unilateral.

17. CN II: Optic Nerve • CN-II is also a purely sensory nerve serving the sense of vision. Photoreceptor, Rods, and cones receive light stimuli and transmitted as action potential to series layers of cells to ganglion cells. Rods is twenty times more than the number of cones. Rods is absent in the central fovea and gets more and more peripherally. It is more sensitive to dim light and responsible for peripheral vision. Cones is more sensitive to bright light and responsible for color vision. • Note that light and action potential travel at opposite directions. Action potentials generated from visual receptors pass via bipolar cells to ganglion cells, which axons bundle together to form the optic nerve.

19. Optic Nerve • Optic nerve originates in the ganglion cells of the retina and travels to the optic chiasm , where the fibers from the medial half of each retina cross to the opposite side while those from the lateral half of the retina remain on the same side.

23. Retinal projections • 1. Right half of two retinas terminate in the right lateral geniculate body and to right hemisphere. Left half of two retinas terminate in the left lateral geniculate body and to left hemisphere • 2. Upper quadrants peripheral to the macula (yellow spot) end in medial part of lateral geniculate, then project to anterior two thirds of visual cortex above calcrine sulcus • 3. lower quadrant peripheral to the macula end in lateral part of lateral geniculate, then project to anterior two thirds of visual cortex below calcrine sulcus. • 4. macula projects to a relatively large area on lateral geniculate body. Upper quadrant --> medial, lower quadrant --> lateral, posterior one third of the visual cortex

26. Visual Field Projection • Visual field: • Retinal image in the visual field is inverted and reversed from right to left (can be trained the other way around) • 1. Left visual field is represented in the right lateral geniculate body and in the visual cortex of the right brain. • 2. Upper half of the visual field is represented in the lateral portion of the lateral geniculate body, below calcrine sulcus. • 3. lower half of the visual field is represented in the medial portion of the lateral geniculate body, above the calcrine sulcus

28. Images projected from the visual field to the retina are inverted and reversed. Upper visual field goes to the lower retina, lower visual field goes to the upper retina Right visual field projects to the left hemiretina in each eye, left visual field projects to the right hemiretina Central fixation point for each eye falls on the fovea where highest visual acuity occurs

29. Optic nerve • surrounded by extension of meninges, so increased intracranial pressure can be presented by edema of optic disk (papilledema) • Partial crossing: • determines binocular vision. Left visual field is represented in the right hemisphere

30. Normal

31. Papilledema

32. Lateral geniculate • located in thalamus. Note that lateral geniculate body has six layers of cells and crossed and uncrossed optic tracts terminate into different layers of cells. Crossed fibers 1, 4, 6 and uncrossed fibers 2, 3, and 5.

33. Geniculocalcarine tract • originate from lateral geniculate , two branches for upper visual field and lower visual field, terminate into visual cortex, which is located on the upper and lower lips of calcrine sulcus. There is a detailed point to point projection of the retina on the lateral geniculate body and on the visual cortex. Therefore, a right temporal lesion in the brain will affect the right lower quadrants of both eyes (retina or visual fields ?), and left upper visual fields will be affected. What about a right parietal lesion?

34. Temporal lobe lesions can cause a contralateral homonymous superior quadrantopia visual field defect. Parietal lobe lesions can cause a contralateral homonymous inferior quadrantopia visual field defect

36. Visual defect • Macular sparing • This is due to the fact that input from the center of the retina (the macula) is spread over a large portion of the optic radiation and primary visual cortex.

40. Visual reflexes • Small bundle of fibers from optic nerve bypass lateral geniculate body and enter the superior brachium which connects with superior colliculus. Information can also be projected to pretectal area (group of small nuclei rostral to the superior colliculus.

41. Pupillary light reflex • Information collected from retina could be relayed to olivary pretectal nucleus (one of those nuclei in the pretectal area) then to Edinger-Westphal, nucleus of oculomotor complex finally to the sphincter pupillae in the orbit and contracts pupils. Be aware that both eyes should reflex to light entering into one eye, that is because each retina sends fibers to both sides and pretectal area projects fibers cross to the contralateral side of the Edinger-Westphal nucleus. • Some retinal ganglion cells have axons enter the retinohypothalamic tract, the visual stimuli can synchronize the intrinsic circadian rhythm of the firing pattern of the neurons in suprachiasmatic nucleus. Then influence the antigonadotrophic activity of pineal gland.

43. CN III, Oculomotor Nerve • CN-III is a purely motor nerve that serves to control all of the eye muscles except the lateral rectus and the superior oblique . It also contains parasympathetic fibers that control the ciliary muscle and pupillary constrictor muscles; when this part of the tract is damages, light shone in the affected eye does not cause constriction of the pupil.