Cranial Nerves Lundy-Ekman –Chapter 13 –Chapter 14 –Chapter 15

Objectives 1.Identify the 12 cranial nerves by name and number, 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 the cranial nerves

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.

Same overall functions as spinal nerves, though a cranial nerve may not have all categories of functions –Motor –Sensory –Autonomic Some cranial nerves have special functions –Smell –Vision –Taste –Vestibular Functions of Cranial Nerves

Three main functions 1.Motor innervation to muscles of face, eyes, tongue, jaw and two neck muscles. 2.Somatosensory information from skin and muscles of face and TMJ, and special sensory information (olfactory, visual, auditory, vestibular, taste, and visceral sensations) 3.Parasympathetic regulation of heart rate, blood pressure, digestion, breathing, and some eye muscles.

Lesions of Cranial Nerves Damage to cranial nerve in periphery (trauma, inflammation) –Ipsilateral deficits Damage to cranial nerve nuclei in CNS (stroke) –Usually ipsilateral deficits –Can help localize lesion in the brain Damage to upper motor neurons that control cranial nerves (stroke) –Usually do not see significant weakness due to bilateral innervation of CN nuclei. (One significant exception)

Where Cranial Nerves Exit the Brain Telecephalon – 1 Diencephalon – 1 Midbrain – 2 Pons – 1 Border between pons and medulla - 3 Medulla – 4 Page 358: location of cell bodies in brainstem

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 (cell bodies in pons) 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 Enters at 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

Mesencephalic trigeminal nucleus – midbrain –Unconscious proprioception Chief sensory nucleus – mid pons Spinal trigeminal nucleus –Caudal pons to upper cervical cord

Trigeminal Reflexes Corneal Reflex – ophthalmic division. Facial nerve is efferent part of reflex. Masseter (Jaw Jerk) Reflex – mandibular division

VII- Facial Pontomedullary border Cell bodies in pons Motor –Muscles of facial expression –Glands of head and neck Sensory –Taste on anterior 2/3 rd 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 Cell bodies in pons 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 Cell bodies –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