Lecture 13: Cranial Nerves and Special Senses

What are the cranial Nerves (CNs)?

12 peripheral nervous structures

• Originate within cranium

• Enter/exit brain stem directly

CNs primarily

innervate the face and its internal/external structures

One of cranial nerve

innervates thoracic and abdominal organs

CNs are numbered

from anterior to posterior

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

Mnemonics for cranial nerves

On, On, On, They Traveled And Found Voldemort Guarding Very Ancient Horcruxes

I. Olfactory Nerve

Smell, small nerves in nasal cavity roof

• Pass through cribriform plate of ethmoid

CN I: How do we smell?

Bipolar neurons in mucous membrane, odorants (Scent molecules) bind to olfactory nerves, and can arrive at CN I in 2 ways

Two ways smell can arrive at CN I

1. Nostrils during breathing

2. Oropharynx during chewing

CN I fibers pass through

Cribriform plate of ethmoid

    • synapse with two long olfactory tracts

Projections from olfactory bulb have multiple destinations

Olfaction → MANY areas of the brain

   • Affects memory, emotion, behavior, salivation, taste, etc

II. Optic Nerve : Vision

Formed by axons of the ganglionic cells in the retina of the eye

Eye structures:

Sclera

Iris

Pupil

Conjunctive

Eyelids

Sclera

White outer layer of eye

• Dense CT

Iris

Pigmented ring of smooth muscle

• Pigment = melanin

Pupil

Central hole in iris

• Allows light into eye

Conjunctive

Membrane protecting eye and inner eyelid

    • Present, but not visible

Eyelids

Dense connective tissue core

• Skin on external surfaces

• Conjunctiva on inner surface

• Protect and spread tears

Lacrimal gland

Secretes tears and are spread across the eyes by the eyelids

Drained into the nasal cavity thru a duct

Conjunctivitis: AKA “Pink eye”

Infection and inflammation of conjunctiva

3 major layers (tunics) of Eye

1.Fibrous Tunic

2.Vascular Tunic

3.Nervous Tunic

1.Fibrous Tunic

Sclera, cornea, and lens

2.Vascular Tunic

Major blood supply to/from eye

3.Nervous Tunic

Sensory and autonomics

  • Retinal cells

Light must pass through several structures on its way to the retina

1.Cornea → 2. Aqueous humor → 3. Iris (pupil) → 4.Lens → 5.Vitreous humor

1.Cornea

Dome shaped protective outer layer

2. Aqueous humor

Watery fluid between cornea and lens

3. Iris ( + pupil)

Muscular ring that controls amount of light entering eye

    • Pupil: Hole in center of iris

4.Lens

Curved, transparent structure

      -Directs light towards retina

5.Vitreous humor

Gel -like substance. Fills most of the eye

Essentials of optic pathway

Light passes through pupil, lens, humors, and strikes retina

               • Optic nerve sends impulses to thalamus, Brodman areas 17, 18,& 19, and other regions of brain

III. Oculomotor Nerve

IV. Trochlear Nerve

VI. Abducens Nerve (6)

Control skeletal muscles that move the eyeball

Viewing distant objects

Lens under tension (smooth muscle)

Flatter lens → see farther away

Viewing nearby objects:

Accommodation reflex:  Relaxation of lens

          • Now more curved → See closer objects in focus

V. Trigeminal Nerve: Three nerve bundles

Opthalmic, Maxillary, Mandibular

Sensation for the bones & skin of the face, teeth & cavities of head

Trigeminal nerve also innervates

The muscles of mastication

-Masseter, temporalis, medial pterygoid, lateral pterygoid

VII. Facial Nerve (7)

Motor to muscles of facial expression

        • Taste from anterior 2/3 of tongue

Facial Nerve (7) Parasympathetics Fibers

Lacrimal glands (tears) and Salivary glands (not the parotid)

VIII. Vestibulocochlear Nerve

Made of 2 nerves: Vestibular and Cochlear

Balance and hearing

How do we hear ?

The ear (AKA pinna or auricle) receives vibrations in the air (sound waves)

Vibrations pass thru outer ear f

Vibration route

First enter outer ear via ear canal, then strike tympanic membrane (eardrum), and then the vribrations are carried thru 3 tiny ossicles in the middle ear

Malleus

Incus

Stapes

Temporal bone

Contains tiny bony structures that generates sense of hearing and balance

Cochlea

Snail shell shaped spiral filled with fluid called endolymph

Vibration in fluid causes cilia to bend and create an action potential

The vestibular system

a sensory system in the inner ear that balance and proprioception

Made of 3 horseshoe-shaped semicircular canals, all oriented differently, and filled with endolymph

Dynamic Equilibrium: Sensing angular movement and velocity

1. Head moves

2. endolymph flows

3. hair cells bend

4. Action potential generated along vestibular nerve to pons

Utricle and Saccule

bulging chambers in vestibule contain fluid & hair cells with suspended crystals - OTOLITHS

Otoliths role

Motion causes crystals to move the hair cells creating a electrical impulse → Important for static equilibrium

IX. Glossopharyngeal Nerve

9, Taste/touch Posterior 1/3 of Tongue

Flavor 5 tastes:

Salty, sweet, bitter, sour, and umiam (savory)

Taste actually comes from

Direct chemical stimulation of tastebuds, stimulated by olfactory receptors by vapors from food

mechanoreceptors, nociceptors and chemoreceptors of oral cavity: Texture, Spice, heat

3 Receptors for taste

Stimulation of mechanoreceptors, nociceptors and chemoreceptors of oral cavity: Texture, Spice, heat

Taste buds

Onion shaped cells on tongue, soft palate, and epiglottis

Have receptors that tastants (molecules from food/drink)

Buds generate action potentials that ride along:

• Facial (CN VII)

• Glossopharyngeal (CNIX)

• Vagus (CN X)

X. Vagus Nerve: Somatic Motor/Sensory

10, Sensory and motor to epithelium and muscles of pharynx and larynx

X. Vagus Nerve: Parasympathetics to heart and airway

parasympathetic "rest and digest" nervous system, connecting the brain to organs like the heart and lungs

X. Vagus Nerve: Autonomics control

control of smooth muscle and gland secretion in digestive system

XI. Spinal Accessory Nerve Motor to 2 muscles:

11, Sternocleidomastoid (head movement) and Trapezius (shoulder abd)

XII. Hypoglossal nerve:

Muscles of the tongue