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Transcript
Cranial Nerves • Lundy-Ekman – Chapter 13 – Chapter 14 – Chapter 15 Classification of Neural Systems • Located in spinal cord – – – – Somatic Sensory Visceral Sensory Visceral Motor Somatic Motor • Where are they located in the spinal cord Two additional categories in the brainstem • Sensory and motor which are not found in the spinal cord • Special Sensory – Senses not found in body • Branchial motor (special visceral afferents) – Derived from special embryonic tissues Objectives 1. Identify the 12 cranial nerves, their major functions, and the site of their connection with the brain 2. Describe the control of eye movements 3. Describe the control of the eye, including pupillary, consensual and accommodation reflexes 4. Describe the motor and/or sensory functions of cranial nerves V, VII, VIII, IX, X, XI, and XII 5. Identify the cranial nerves associated with eating, swallowing and speech 6. Describe the loss of function associated with damage to each of the cranial nerves 7. Describe the etiology and pathology of Trigeminal Neuralgia and Bell’s Palsy. Where Cranial Nerves Exit the Brain • • • • • • Telecephalon – 1 Diencephalon – 1 Midbrain – 2 Pons – 1 Border between pons and medulla - 3 Medulla - 4 Cranial Nerve I Olfactory Nerve • Sensory – Smell • Olfactory receptors in nasal mucosa – Axons travel through skull to the olfactory bulb – Information travels to medial temporal lobe of the cerebrum • Damage – anosmia – loss of sense of smell • How do we lose function in C.N. I – Head injury – injure axons as they pass through the skull – Smoking – Excessive nasal secretions – Chemicals • Loss of function – Anosmia – loss of sense of smell – Also decreases in taste • These neurons can regenerate, and the sense of smell may return Cranial Nerve II Optic Nerve • Sensory - sight – – – – – – Retina Optic Nerve - PNS Optic chiasm - Diencephalon Optic tract - CNS Lateral geniculate body – thalamus Primary Visual cortex Retina • Nasal retinal field • Temporal retinal field Optic Chiasm • Some fibers cross the midline – From nasal retinal field • The LGN and visual cortex receives information from the contralateral visual field Injuries to Visual System • Injury to optic nerve – • Injury to optic chiasm – Which fibers are being injured? – What visual field deficits will there be? • Bitemporal hemianopsia • Injury to Optic tract, LGN or visual cortex – Homonymous hemianopsia Cranial Nerves III, IV, and VI • • • • • • • Primarily motor III, IV: midbrain VI: pons-medulla border Extraocular muscles Raise upper eyelid Constrict pupil Adjust shape of lens Extraocular muscles • Medial rectus – III – Moves eye medially • Lateral rectus – VI • Medial and lateral rectus control horizontal eye movements • Superior rectus – III – Eye looks up • Inferior rectus – III – Eye looks down Oblique muscles • Superior oblique – IV – Eye abducted: Rotation – Eye adducted: eye looks down • Inferior oblique – III – Eye abducted: Rotation – Eye adducted: eye looks up • Levator palpebrae superioris – III – Elevates upper eyelid in part • Pupillary sphincter muscle - III – Constricts pupil • Ciliary muscle - III – Increases curvature of lens of eye C.N. III • Cell bodies – Oculomotor nucleus • Somatic motor – Edinger-Westphal nucleus • Parasympathetic fibers – Ciliary ganglia behind the eye in the orbit – Constrict pupil – Change curvature of lens for near vision » Accommodation C.N. IV • Cranial nerve IV – Cell bodies • In midbrain – Axons leave dorsal surface of brainstem (only one) • Cranial nerve VI – Cell bodies in pontine tegmentum Cranial Nerve VI Abducens nerve • Motor • Border between pons and medulla • Lateral rectus muscle – Action: Reflexes involving the eye • Pupillary reflex • Consensual reflex • Accommodation reflex Pupillary reflex • Stimulus: • Response: • Afferent nerve: • Efferent: Consensual Reflex • Stimulus: • Response: • Afferent nerve: • Efferent: Pathways for the pupillary reflex and the consensual reflex • Retina Pretectal nucleus in midbrain • parasympathetic nucleus of the oculomotor nerve • Ciliary ganglion • Pupillary sphincter Accommodation • Reflex when we look at near objects – Pupils constrict – Eyes converge • What muscle(s) – Lens becomes more convex • Requires visual cortex and frontal eye field in the frontal lobe Lesion of oculomotor nerve • • • • • • Ptosis – drooping of upper eyelid Ipsilateral eye looks down and out Difficulty moving eye medially Double vision Loss of reflexes Pupillary dilation Oculomotor nerve lesion • Space-occupying lesions in the cerebrum can compress the midbrain, interfering with the functioning of the oculomotor nerve. Trochlear Nerve • Located in midbrain • Superior oblique muscle – If eye adducted, pupil down and in – If eye abducted, rotates eye Lesion of Trochlear Nerve • Ipsilateral eye unable to look down and in. • Double vision – can be reduced by tilting of head. Abducens Nerve • Located between pons and medulla • Lateral rectus mm. – Abducts pupil Lesion of Abducens Nerve • Ipsilateral eye looks inward • Unable to abduct eye past midline • Double vision Abducens Nerve Lesion (Right) V - Trigeminal • Level of mid pons • Sensory to skin of face, mucous membranes of head, and meninges • Motor to muscles of mastication • Three main divisions – Ophthalmic – Maxillary – Mandibular – only division with motor fibers Trigeminal Reflexes • Corneal Reflex – ophthalmic division. Facial nerve is efferent part of reflex. • Masseter (Jaw Jerk) Reflex – mandibular division VII- Facial • Pontomedullary border • Motor – Muscles of facial expression – Glands of head and neck • Sensory – Taste on anterior 2/3rd of tongue Lesions • If damage to nerve or nucleus, will see deficits on whole ipsilateral face (Bell’s Palsy affects axons of facial nerve) • If damage to upper motor neurons (i.e. cerebral stroke), deficits on lower contralateral face. The forehead will be spared. VIII – Vestibulocochlear Nerve • Pontomedullary border • Two Divisions – Vestibular – posture and equilibrium – Auditory - hearing Deafness • Conduction deafness – Deficit in mechanism conveying vibrations from air to the cochlea • Sensorineural deafness – Deficit in cochlea – transduction of sound waves to action potentials – Deficit in nerves Vestibular Nerve • Vertigo – sensation of movement • Nystagmus – involuntary back and forth movements of eyes • Disequilibrium – loss of sense of balance IX – Glossopharyngeal Nerve • Medulla • Sensory – Posterior tongue – Skin of external ear – Mucosa of mouth – afferent limb of gag reflex • Motor – Muscles of throat – Parotid gland X - Vagus • Medulla • Main parasympathetic nucleus for head, neck, thoracic organs, and superior abdominal organs. • Motor for muscles of larynx and pharynx – Important in speech and swallowing Effects of Vagus Nerve Lesion • • • • Difficulty speaking Hoarseness Difficulty with swallowing Poor digestion – loss of parasympathetic innervation to GI tract XI – Accessory Nerve • Medulla – nucleus extends into upper cervical spinal cord • Motor to trapezius and sternocleidomastoid muscles. XII – Hypoglossal Nerve • Medulla • Motor to intrinsic and extrinsic muscles of ipsilateral tongue Conclusion Why are cranial nerves important? • Cranial nerves have important and unique functions, especially somatosensation and motor for the head and the special senses. • Cranial nerves are important in speaking (dysarthria) – – – – Larynx and soft palate – X Jaws – V Lips – VII Tongue – XII • Cranial nerves are important in swallowing. – Mouth and jaw muscles – V, VII, IX, X, XII – Pharynx, Larynx – IX, X – Esophagus and GI tract – X • Cranial nerves can indicate the level of a lesion to the nervous system. • Cranial nerve deficits can signal injury to other regions of the nervous system. – Pupilary reflex – increased intracranial pressure – Bitemporal hemianopsia – pituitary tumor