Nervous System III - Special Senses

Dorsum of tongue

Location:

• Superior surface of tongue

Description:

• Mucous membrane contains taste buds

• Dense concentration of papillae gives dorsal surface "felt-like" appearance

• Divided by V-shaped sulcus terminalis into anterior (oral) part and posterior (pharyngeal)

• Anterior part has median furrow

• Posterior part is nodular due to presence of lingual tonsils

Foramen cecum of tongue

Location:

• Tongue (dorsum)

Description:

• Small pit located at the apex of the terminal sulcus

Comment:

• Remnant of embryonic thyroglossal duct

General sensory distribution of glossopharyngeal n. (CN IX)

Location:

• Middle ear

• Posterior 1/3 of tongue

• Pharynx

Comment:

• General sensation includes pain, touch, and temperature

Lingual tonsil

Location:

• Tongue (posterior part)

Description:

• Collection of lymphoid nodules in submucosal connective tissue

• Not surrounded by connective tissue capsule

Function:

• Traps foreign material and facilitates identification by lymphocytes

• Produces lymphocytes

Median sulcus of tongue

Location:

• Tongue (dorsum)

Description:

• Shallow median longitudinal groove on anterior part of tongue

Comment:

• Indicates position of underlying midline fibrous septum

Special sensory distribution of chorda tympani

Location:

• Anterior 2/3 of tongue (information from taste buds)

Comment:

• Special sensation includes smell, vision, taste, hearing, and balance

• Chorda tympani is branch of facial nerve (CN VII) that joins lingual nerve (a branch of CN V3) in infratemporal fossa

• Chorda tympani also contains preganglionic parasympathetic axons to submandibular ganglion

Special sensory distribution of glossopharyngeal n. (CN IX)

Location:

• Posterior 1/3 of tongue (information from taste buds)

Comment:

• Special sensation includes smell, vision, taste, hearing, and balance

Terminal sulcus of tongue

Location:

• Tongue (dorsum)

Description:

• "V"-shaped groove on posterior part of tongue

Comment:

• Separates anterior (oral) and posterior (pharyngeal) parts of tongue

Vallate papilla

Location:

• Tongue (dorsum)

Description:

• 8-12 large, flat-topped papillae located just anterior to terminal sulcus

• Contains numerous taste buds

Also known as:

• Circumvallate papilla

Comment:

• Glossopharyngeal nerve (CN IX) receives special sensory information (taste) from taste buds in these papillae

Olfactory bulb

Location:

• Lies on cribriform plate of ethmoid bone in anterior cranial fossa

• Ventral aspect of frontal lobe of brain

Description:

• Expanded anterior end of olfactory tract

• Site of synapse for olfactory neurons (CN I) after their axons pass through cribriform plate

Olfactory mucosa

Location:

• Nasal cavity (superior nasal septum and superior nasal concha and roof between)

Description:

• Contains olfactory neurons

Comment:

• Primary function: olfaction

Olfactory nn. (CN I)

Location:

• Mucosa of anterosuperior nasal cavity (olfactory epithelium)

• Anterior cranial fossa

Composition:

• Special sensation

Special sensation:

• Olfaction (smell)

CNS connection:

• Olfactory bulb

Cranial foramina:

• Cribriform plate of ethmoid bone

Comment:

• Special sensation includes smell, vision, taste, hearing, and balance

• Olfactory nerve formed by proximal process (axons) of olfactory neurons

• Olfactory axons project through cribriform plate to synapse in olfactory bulb

• Olfactory nerve also known as CN I

Olfactory tract

Location:

• Ventral aspect of frontal lobe

• Between olfactory bulb and medial aspect of temporal lobe

Description:

• Bundles of afferent and efferent axons

Respiratory nasal mucosa

Location:

• Nasal cavity (inferior two-thirds)

Description:

• Non-olfactory region of nasal mucosa

Comment:

• Primary function: warm and humidify inhaled air

Auricle of ear

Location:

• Head (lateral)

Description:

• Appendage of skin, cartilage, and connective tissue

• Contains part of external acoustic meatus

Also known as:

• External ear or pinna

Comment:

• Acts like a funnel to collect and modify sound waves

• Latin: auri = an ear

Cochlea

Location:

• Temporal bone (petrous part)

Description:

• Coiled membranous tube surrounded by bone

• Contains three fluid-filled chambers

Comment:

• Organ of hearing

• Contains spiral organ (of Corti) in cochlear duct

Semicircular canals

Location:

• Temporal bone (petrous part)

Description:

• The three semicircular canals are part of the bony labyrinth

• Each semicircular canal contains a membranous semicircular duct

• Orientation of ducts in perpendicular (X, Y, and Z) planes

Comment:

• The membranous semicircular ducts are part of the vestibular apparatus, i.e. organs of equilibrium

Temporal lobe

Location:

• Lateral and inferior portion of each cerebral hemisphere

• Inferior to lateral sulcus

Description:

• Lateral surface has three parallel gyri

Function:

• Primary hearing and smell areas

• Memory

• Speech perception and recognition (i.e., Wernicke's area - usually in left hemisphere)

Comment:

• Named for overlying bone

Tympanic membrane

Location:

• Temporal bone (petrous part)

Description:

• Thin, semi-transparent, oval membrane

• Separates external acoustic meatus (external ear) from tympanic cavity (middle ear)

Also known as:

• "Ear drum"

Comment:

• Attached to malleus (ossicle)

Vestibular of ear

Location:

• Temporal bone (petrous part)

Description:

• The vestibule is part of the bony labyrinth

• The vestibule contains the utricle and saccule

• The utricle and saccule are membranous sacs at base of semicircular ducts

Comment:

• The membranous utricle and saccule are part of the vestibular apparatus, i.e. organs of equilibrium

Vestibulocochlear n. (CN VIII)

Location:

• Posterior cranial fossa

• Petrous portion of temporal bone

Composition:

• Special sensation

Special sensation:

• Hearing (cochlea)

• Balance (semicircular canals and vestibule)

Sensory ganglion:

• Cochlear (spiral) ganglion (cochlear part of CN VIII)

• Vestibular ganglion (vestibular part of CN VIII)

CNS connection:

• Pons (vestibular nuclei)

• Medulla oblongata (cochlear and vestibular nuclei)

Cranial foramina:

• Internal acoustic meatus

Comment:

• Special sensation includes smell, vision, taste, hearing, and balance

• Vestibulocochlear nerve has two distinct functional components: vestibular (balance) and cochlear (hearing)

• Vestibulocochlear nerve also known as CN VIII

Anterior cavity

Location:

• Eye

• Between cornea and lens

Description:

• Cavity in eye

• Divided by iris into anterior and posterior chambers

• Filled with aqueous humor

Comment:

• Aqueous humor produced in posterior chamber; passes through pupil; enters anterior chamber; reabsorbed into venous system through scleral venous sinus (canal of Schlemm)

Anterior chamber

Location:

• Anterior cavity of eye

• Between cornea and iris

Description:

• Subdivision of anterior cavity

• Filled with aqueous humor

Comment:

• Aqueous humor produced in posterior chamber; passes through pupil; enters anterior chamber; reabsorbed into venous system through scleral venous sinus (canal of Schlemm)

Choroid

Location:

• Eye (vascular layer)

Description:

• Highly vascular layer

• Contains dark brown pigment - melanin

Function:

• Blood supply to outer part of neural retina

• Prevent light reflection within eye

Ciliary body

Location:

• Eye

• Part of vascular (middle) layer, between choroid and iris

Description:

• Composed of ciliary muscle and ciliary processes

Function:

• Anular ciliary smooth muscle controls tension of suspensory ligaments to adjust thickness of lens

• Ciliary processes produce aqueous humor

Comment:

• Aqueous humor produced by ciliary processes flows from posterior chamber to posterior chamber of anterior cavity where it is resorbed into venous system through the scleral venous sinus (canal of Schlemm)

Cornea

Location:

• Eye

Description:

• Transparent connective tissue layer of anterior 1/6th of eye

Function:

• Site for light refraction

• Protection of anterior eye

Iris

Location:

• Eye (posterior to cornea)

Description:

• Colored part of eye

• Diaphragm that regulates pupil size

• Contains pupillary constrictor and dilator muscles

Function:

• Controls size of pupil (pupil constricts and dilates in response to light)

Comment:

• Pupillary constrictor receives parasympathetic innervation, pupillary dilator receives sympathetic innervation

Lens

Location:

• Eye (posterior to iris)

Description:

• Biconvex lens

• Composed of cells called lens fibers

Function:

• Light refraction

• Focuses light onto neural retina

Optic disk

Location:

• Retina

• At junction with optic nerve

Description:

• Circular to oval area

• Composed of optic fibers (axons of retinal ganglion cells) that form optic nerve

• Central retinal artery enters eye through optic disk

Comment:

• Lack of photoreceptors in disk creates “blind spot” in visual field

Optic n. (CN II)

Location:

• Orbit

• Middle cranial fossa

Composition:

• Special sensation

Special sensation:

• Vision

CNS connection:

• Lateral geniculate nucleus of thalamus

Cranial foramina:

• Optic canal

Comment:

• Special sensation includes smell, vision, taste, hearing, and balance

• Optic nerve formed by axons of retinal ganglion cells

• Retinal ganglion cell axon pathway: optic nerve > optic chiasm > optic tract > brainstem nuclei (including lateral geniculate nucleus of thalamus)

• Optic nerve also known as CN II

Posterior cavity

Location:

• Eye

• Posterior to lens and its suspensory ligaments

Description:

• Cavity in eye

• Occupied by vitreous body (humor)

Function:

• Maintain normal intraocular pressure and shape of the eye

• Maintain lens and retina in place

• Refraction of light

Also known as:

• Vitreous chamber

Posterior chamber

Location:

• Anterior cavity of eye

• Between iris, ciliary body, and lens

Description:

• Subdivision of anterior cavity

• Filled with aqueous humor

Comment:

• Aqueous humor produced in posterior chamber; passes through pupil; enters anterior chamber; reabsorbed into venous system through scleral venous sinus (canal of Schlemm)

Pupil

Location:

• Iris

Description:

• Opening in center of iris

Function:

• Allows light to enter eye

Comment:

• Size of pupil controlled by pupillary constrictor and dilator muscles within iris

Retina

Location:

• Eye

Description:

• Inner tunic

• Composed of two layers

• Outer pigmented layer immediately inside choroid

• Inner neural layer contains photoreceptors and associated neurons

Comment:

• Photoreceptors are primary sensory neurons that respond to light

• Axons of retinal ganglion cells form optic nerve (CN I), which connects eye to brain

Sclera

Location:

• Eye (fibrous layer)

Description:

• Outer layer of posterior 5/6th of eye

• Dense collagenous connective tissue

Function:

• Protects eye

• Maintains shape of eye

Comment:

• "White" of the eye

Suspensory ligaments of lens

Location:

• Eye

• Between ciliary body and lens

Description:

• Transparent, elastic fibers

• Suspends lens from ciliary body

• Connects ciliary muscle to lens

Function:

• Permits thickness of lens to change with contraction/relaxation of ciliary muscle

• Contraction of ciliary muscle > relaxes suspensory ligaments > lens thickens

• Relaxation of ciliary muscle > tenses supsensory ligaments > lens gets thinner

Also known as:

• Zonular fibers or zonule of Zinn