Neuro

• The cranial nerves are commonly damaged by trauma or disease, and testing for their integrity forms part of every physical examination.
• The 12 pairs of cranial nerves (CNs)

  • I. Olfactory
  • II. Optic
  • III. Oculomotor
  • IV. Trochlear
  • V. Trigeminal
  • VI. Abducens
  • VII. Facial
  • VIII. Vestibulocochlear
  • IX. Glossopharyngeal
  • X. Vagus
  • XI. Accessory
  • XII. Hypoglossal
• Cranial Nerve Organization


  • The olfactory, optic, and vestibulocochlear nerves are entirely sensory
  • The oculomotor, trochlear, abducens, accessory, and hypoglossal nerves are entirely motor
  • The trigeminal, facial, glossopharyngeal, and vagus nerves are both sensory and motor nerves
• Functional components of cranial nerves

  • Afferent Fibers: General somatic afferent (GSA), Special somatic afferent (SSA), General visceral afferent (GVA), Special visceral afferent (SVA)
  • Efferent Fibers: General somatic efferent (GSE), General visceral efferent (GVE), Special visceral efferent (SVE)
• Cranial Nerves

  • I. Olfactory - Sensory (SVA), Smell, Openings in cribriform plate of ethmoid
  • II. Optic - Sensory (SSA), Vision, Optic canal
  • III. Oculomotor - Motor (GSE, GVE), Raises upper eyelid, turns eyeball upward, downward, and medially; constricts pupil; accommodates eye, Superior orbital fissure
  • IV. Trochlear - Motor (GSE), Assists in turning eyeball downward and laterally, Superior orbital fissure
  • V. Trigeminal - Ophthalmic division: Sensory (GSA), Cornea, skin of forehead, scalp, eyelids, and nose; also mucous membrane of paranasal sinuses and nasal cavity, Superior orbital fissure
  • Maxillary division: Sensory (GSA), Skin of face over maxilla; teeth of upper jaw; mucous membrane of nose, the maxillary sinus, and palate, Foramen rotundum
  • Mandibular division: Motor (SVE), Muscles of mastication, mylohyoid, anterior belly of digastric, tensor veli palatini, and tensor tympani, Sensory (GSA), Skin of cheek, skin over mandible and side of head, teeth of lower jaw and temporomandibular joint; mucous membrane of mouth and anterior part of tongue, Foramen ovale
  • VI. Abducent - Motor (GSE), Lateral rectus muscle turns eyeball laterally, Superior orbital fissure
  • VII. Facial - Motor (SVE), Muscles of face and scalp, stapedius muscle, posterior belly of digastric, and stylohyoid muscles, Sensory (SVA), Taste from anterior two-thirds of tongue, from floor of mouth and palate, Secretomotor (GVE), Submandibular and sublingual salivary glands, the lacrimal gland, and glands of nose and palate, Internal acoustic meatus, facial canal, stylomastoid foramen
  • VIII. Vestibulocochlear - Vestibular: Sensory (SSA), From utricle and saccule and semicircular canals-position and movement of head, Internal acoustic meatus
  • Cochlear: Sensory (SSA), Organ of Corti-hearing, Internal acoustic meatus
  • IX. Glossopharyngeal - Motor (SVE), Stylopharyngeus muscle-assists swallowing, Secretomotor (GVE), Parotid gland parasympathetic, Sensory (GVA, SVA, GSA), General sensation and taste from posterior third of tongue and pharynx; carotid sinus (baroreceptor); and carotid body (chemoreceptor), Jugular foramen
  • X. Vagus - Motor (GVE, SVE), Heart, esophagus, stomach, and great thoracic blood vessels; larynx, trachea, bronchi, and lungs; alimentary tract from pharynx to splenic flexure of colon; liver, kidneys, and pancreas, Sensory (GVA, SVA, GSA), Jugular foramen
  • XI. Accessory - Cranial root: Motor (SVE), Muscles of soft palate (except tensor veli palatini), pharynx (except stylopharyngeus), and larynx (except cricothyroid) in branches of vagus, Jugular foramen
  • Spinal root: Motor (SVE), Sternocleidomastoid and trapezius muscles, Jugular foramen
  • XII. Hypoglossal - Motor (GSE), Muscles of tongue (except palatoglossus) controlling its shape and movement, Hypoglossal canal

  
• Corticonuclear fibers


  1. Originate from pyramidal cells in inferior part of precentral gyrus (area 4) and adjacent part of postcentral gyrus

  2. Descend through corona radiata and genu of internal capsule

  3. Pass through midbrain just medial to corticospinal fibers in basis pedunculi

  4. End by synapsing either directly with lower motor neurons within cranial nerve nuclei or indirectly through internuncial neurons
• Majority of corticonuclear fibers to motor cranial nerve nuclei cross the median plane before reaching the nuclei. Bilateral connections are present for all the cranial motor nuclei except for part of the facial nucleus that supplies the muscles of the lower part of the face and a part of the hypoglossal nucleus that supplies the genioglossus muscle.
• Somatic Motor and Branchiomotor Nuclei

  • Axons of nerve cells situated within the brain
  • Form motor nuclei and innervate striated muscle
  • Each nerve cell with its processes is referred to as a lower motor neuron
• General Visceral Motor Nuclei

  • Form the cranial outflow of the parasympathetic portion of the autonomic nervous system
  • Include Edinger-Westphal nucleus of oculomotor nerve, superior salivatory and lacrimal nuclei of facial nerve, inferior salivatory nucleus of glossopharyngeal nerve, and dorsal motor nucleus of vagus
  • Receive numerous afferent fibers, including descending pathways from hypothalamus
• Cranial Nerve Sensory Nuclei

  • Include somatic and visceral afferent nuclei
  • Axons of nerve cells outside the brain situated in ganglia on the nerve trunks or in sensory organs
  • Central processes of these cells enter the brain and terminate by synapsing with cells forming the sensory nuclei
  • Axons from these nuclear cells cross the midline and ascend to other sensory nuclei, such as the thalamus, where they synapse
• Olfactory Nerves (Cranial Nerve I)


  1. Arise from olfactory receptor nerve cells in olfactory mucous membrane in upper part of nasal cavity

  2. Olfactory receptor cells have a small bipolar nerve cell with a coarse peripheral process that projects short cilia (olfactory hairs) into the mucus, and a fine central process that forms the olfactory nerve fibers

  3. Olfactory nerve fibers pass through openings in cribriform plate of ethmoid bone to enter olfactory bulb
• Olfactory Bulb


  • Possesses several types of nerve cells, including largest mitral cells
  • Incoming olfactory nerve fibers synapse with dendrites of mitral cells to form synaptic glomeruli
  • Also receives axons from contralateral olfactory bulb through olfactory tract
• Olfactory Tract


  1. Narrow band of white matter running from posterior end of olfactory bulb beneath inferior surface of frontal lobe

  2. Consists of axons of mitral and tufted cells of bulb and some centrifugal fibers from opposite olfactory bulb

  3. Divides into medial and lateral olfactory striae as it reaches anterior perforated substance

  4. Lateral stria carries axons to olfactory area of cerebral cortex (periamygdaloid and prepiriform areas)

  5. Medial stria carries fibers that cross median plane in anterior commissure to pass to opposite olfactory bulb
• Primary and Secondary Olfactory Cortex


  • Periamygdaloid and prepiriform areas are primary olfactory cortex
  • Entorhinal area (area 28) of parahippocampal gyrus, which receives connections from primary olfactory cortex, is secondary olfactory cortex
  • Responsible for appreciation of olfactory sensations
  • In contrast to other sensory pathways, olfactory afferent pathway has only two neurons and reaches cerebral cortex without synapsing in thalamic nuclei
• Primary olfactory cortex sends nerve fibers to many other centers within the brain to establish connections for emotional and autonomic responses to olfactory sensations.
• Optic Nerve (Cranial Nerve II)

  1. Fibers are axons of cells in ganglionic layer of retina

  2. Converge on optic disc and exit eye about 3-4 mm to nasal side of posterior pole
• Periamygdaloid and prepiriform areas of the cerebral cortex


  Primary olfactory cortex
• Entorhinal area (area 28) of the parahippocampal gyrus


  Secondary olfactory cortex
• The olfactory afferent pathway has only two neurons and reaches the cerebral cortex without synapsing in one of the thalamic nuclei
• The primary olfactory cortex sends nerve fibers to many other centers within the brain to establish connections for emotional and autonomic responses to olfactory sensations
• Optic nerve


  Axons of the cells in the ganglionic layer of the retina
• The fibers of the optic nerve are myelinated, but the sheaths are formed from oligodendrocytes rather than Schwann cells, since the optic nerve is comparable to a tract within the central nervous system
• Optic nerve


  1. Leaves the orbital cavity through the optic canal

  2. Unites with the optic nerve of the opposite side to form the optic chiasma
• Optic chiasma


  Situated at the junction of the anterior wall and floor of the third ventricle
• Optic chiasma


  1. Fibers from the nasal (medial) half of each retina cross the midline and enter the optic tract of the opposite side

  2. Fibers from the temporal (lateral) half of each retina pass posteriorly in the optic tract of the same side
• Optic tract


  1. Emerges from the optic chiasma

  2. Passes posterolaterally around the cerebral peduncle

  3. Most of the fibers terminate by synapsing with nerve cells in the lateral geniculate body

  4. A few of the fibers pass to the pretectal nucleus and the superior colliculus of the midbrain and are concerned with light reflexes
• Lateral geniculate body


  • A small, oval swelling projecting from the pulvinar of the thalamus
  • Consists of six layers of cells, on which synapse the axons from the optic tract
• Optic radiation


  1. The fibers are the axons of the nerve cells of the lateral geniculate body

  2. The tract passes posteriorly through the retrolenticular part of the internal capsule

  3. Terminates in the visual cortex (area 17), which occupies the upper and lower lips of the calcarine sulcus on the medial surface of the cerebral hemisphere
• Visual association cortex (areas 18 and 19)


  Responsible for recognition of objects and perception of color
• Neurons conducting visual impulses to the visual cortex

  • Rods and cones (specialized receptor neurons in the retina)
  • Bipolar neurons (connect the rods and cones to the ganglion cells)
  • Ganglion cells (whose axons pass to the lateral geniculate body)
  • Neurons of the lateral geniculate body (whose axons pass to the cerebral cortex)
• In binocular vision, the right and left fields of vision are projected on portions of both retinae

  
• Binocular vision


  1. The image of an object in the right field of vision is projected on the nasal half of the right retina and the temporal half of the left retina

  2. In the optic chiasma, the axons from these two retinal halves are combined to form the left optic tract

  3. The lateral geniculate body neurons project the complete right field of vision on the visual cortex of the left hemisphere and the left visual field on the visual cortex of the right hemisphere

  4. The lower retinal quadrants (upper field of vision) project on the lower wall of the calcarine sulcus, while the upper retinal quadrants (lower field of vision) project on the upper wall of the sulcus

  5. The macula lutea is represented on the posterior part of area 17, and the periphery of the retina is represented anteriorly

  
• Visual reflexes


  • Direct light reflex
  • Consensual light reflex
  • Accommodation reflex
  • Corneal reflex
  • Visual body reflexes
  • Pupillary skin reflex
• Direct and consensual light reflexes

  1. Afferent impulses travel through the optic nerve, optic chiasma, and optic tract

  2. A small number of fibers leave the optic tract and synapse on nerve cells in the pretectal nucleus

  3. Impulses are passed by axons of the pretectal nerve cells to the parasympathetic nuclei (Edinger-Westphal nuclei) of the third cranial nerve on both sides

  4. Parasympathetic nerves travel through the third cranial nerve to the ciliary ganglion in the orbit

  5. Postganglionic parasympathetic fibers pass through the short ciliary nerves to the eyeball and the constrictor pupillae muscle of the iris
• Accommodation reflex


  1. Afferent impulses travel through the optic nerve, the optic chiasma, the optic tract, the lateral geniculate body, and the optic radiation to the visual cortex

  2. Visual cortex is connected to the eye field of the frontal cortex

  3. Cortical fibers descend through the internal capsule to the oculomotor nuclei in the midbrain

  4. Oculomotor nerve travels to the medial recti muscles

  5. Some descending cortical fibers synapse with the parasympathetic nuclei (Edinger-Westphal nuclei) of the third cranial nerve on both sides

  6. Parasympathetic nerves travel through the third cranial nerve to the ciliary ganglion in the orbit

  7. Postganglionic parasympathetic fibers pass through the short ciliary nerves to the ciliary muscle and the constrictor pupillae muscle of the iris
• Corneal reflex


  1. Afferent impulses from the cornea or conjunctiva travel through the ophthalmic division of the trigeminal nerve to the sensory nucleus of the trigeminal nerve

  2. Internuncial neurons connect with the motor nucleus of the facial nerve on both sides through the medial longitudinal fasciculus

  3. Facial nerve and its branches supply the orbicularis oculi muscle, which causes closure of the eyelids
• Visual body reflexes


  1. Visual impulses follow the optic nerves, optic chiasma, and optic tracts to the superior colliculi

  2. Impulses are relayed to the tectospinal and tectobulbar (tectonuclear) tracts and to the neurons of the anterior gray columns of the spinal cord and cranial motor nuclei
• Pupillary skin reflex


  1. Afferent sensory fibers are believed to have connections with the efferent preganglionic sympathetic neurons in the lateral gray columns of the first and second thoracic segments of the spinal cord

  2. White rami communicantes of these segments pass to the sympathetic trunk, and the preganglionic fibers ascend to the superior cervical sympathetic ganglion

  3. Postganglionic fibers pass through the internal carotid plexus, long ciliary nerves, and short ciliary nerves to the dilator pupillae muscle of the iris
• Oculomotor nerve


  Entirely motor in function
• Main oculomotor nucleus


  • Situated in the anterior part of the gray matter that surrounds the cerebral aqueduct of the midbrain
  • Supplies all the extrinsic muscles of the eye except the superior oblique and the lateral rectus