Special Senses

Chapter 15

Anatomy of an Eyeball

• Accessory structures• 3 tunics (layers)– Fibrous (cornea & sclera)– Vascular (choroid)– Sensory (retina)

• Segments – Anterior divided into chambers– Posterior – Filled with humors (fluid)

• Lens

Accessory Eye Structures• Eye muscles

– Rectus as named; oblique opposite and lateral

– Diplopia: muscle weakness/alcohol– Strabismus: uncontrolled rotation

• Eyebrows• Eyelids

– Blink to spread secretions– Eyelashes trigger blinking

• Conjunctiva– Mucus prevents drying out– Conjunctivitis

• Lacrimal apparatus– Tears clean, protect, and moisten– Excess secretions

• Emotional tears unique to humans• Stuffy/runny nose when cry• Watery eyes with cold

• Avascular CT• Sclera (white of the eye)– Protects and shapes– Muscle attachment– Continuous with dura mater

• Cornea (transparent)– Outer stratified squamous, why?– Inner simple squamous maintain clarity– Innervated– Transplants not rejected between people

Fibrous Tunic

Vascular Tunic• Choroid– Vascularized to supply nutrients– Melanocytes to absorb light

• Ciliary body– Smooth muscle ring ciliary muscles control lens shape– Ciliary processes secrete aqueous humor– Suspensory ligaments hold lens in place

• Iris– Colored portion of ciliary body

• Brown pigment only (varies)• Less scatters light = blues/greens/grays (babies)

– Encircles the pupil (2 smooth muscle layers)

Sensory Tunic• Pigmented layer (outer)– Prevents light scattering– Phagocytize damaged photoreceptors

• Neural layer (inner)– Photoreceptors, bipolar cells, ganglion cell

• Rods and cones• Blind spot (optic disc) filled• Macula lutea and fovea centralis

– Rapid eye movement for rapid scene changes

– Vascular supply from choroid and central vein/artery• Opthalmologist examines

• Retinal detachment when layers separate– Vitreous humor seeps in– Photoreceptors lose nutrients = blindness

Humors

• Anterior segment with aqueous humor– Similar to CSF– Continual development– Nutrients & O2 to lens, cornea, & retina– Blocked drainage = up pressure = glaucoma

• Posterior segment with vitreous humor– Transmits light, support lens, & intraocular

pressure– Unchanged from embryonic development

The Functioning Eye

• Light enters the pupil,regulated by the iris

• Passes through a convex lens – Avascular– Lens fibers added through life

• Cataracts = clouding of lens due to loss of nutrients

• Lens is shaped by the ciliary body to focus light on the retina (accommodation)– Refraction of light converges to a focal point– Real image forms upside down and reversed

Visual Pathway• Visual field

– Overlap to provide depth perception = 3D vision

• Ganglion cells • Optic nerve• Optic chiasm

– Nasal and temporal visual field• Optic tract• Thalamus

– LGN• Primary visual cortex

– Conscious perception of images

Olfactory Receptors• Ciliated bipolar cells

– Located in olfactory epithelium (pseudostratified ciliated)

– Mucus captures and dissolves odorants• Pass through cribriform plates• Synapse in olfactory bulbs• Odorant detection

– Humans can distinguish 10,000 odors– Some is pain (ammonia, chili, methanol)– Combinations of different

odorant/receptor binding– Replaceable, but responsiveness

declines with age

Olfactory Neural Pathway• Olfactory receptors synapse with

mitral cells– Contained in glomeruli– Receptor type specific– Refines smell

• Mitral cells signal via olfactory tracts

• 2 pathways– Olfactory cortex– Hypothalamus, limbic system =

emotional connection

Gustation• Taste buds detect molecules in solution

– About 10,000• Four familiar and 1 other found in papillae

– Sweet: organic substances• Alcohol, sugar, amino acids

– Sour : acids, H+ in solution– Salty: inorganic salts– Bitter: alkaloids

• Aspirin, nicotine, caffeine– Umami: glutamate & aspartate

• Meats, cheeses, and protein-rich foods (MSG)• Each receptor responsive to a particular type of substance

– Often mixes– Many ‘tastes’ (80%) are really smell (head colds)

Papillae• Fungiform– Mushroom shaped– Tops of, all over tongue

• Foliate– Fold in side walls

• Circumvallate– Largest, fewest, back of

tongue• Filiform– Hair like projections all over tongue– Do not have taste buds– Roughness

Gustatory Neural Pathway

• Cranial nerves (VII and IX) carry sensations to medulla

• Relay through the thalamus into primary gustatory cortex

• Pathway initiates digestive process too

Regions of the Ear• Outer ear

– Pinna, external auditory canal, and tympanic membrane (separates)

• Middle ear– Pharyngotympanic tube equalizes pressure

b/w middle ear and atmosphere (‘pop’)– Function of tympanic membrane– Ossicles (malleus, incus, & stapes) amplify signal

• Inner ear– Membranous labyrinths w/i bony labryinth

• Cochlea houses the hearing organ• Vestibule report on changes of head position

– Saccule and utricle for gravity and acceleration– Semicircular canals for rotation of head

The Cochlea• Scala vestibuli

– Perilymph: like CSF– Oval window

• Scala Tympani– Perilymph – Round window

• Scala media (Cochlear duct)– Endolymph: K+ rich intracellular

fluid– Organ of Corti

– Contains hair cells embedded in a basilar membrane– Vestibular membrane– Tectorial membrane bends cells as basilar membrane moves

• Signal to auditory nerve

Frequency and Amplitude• Sounds detected as changes in AP’s

– Pitch depends on frequency• High pitch = higher frequency

– Basilar membrane responsive to certain frequencies• 20 to 20,000 Hz; 1500 – 4000 most

sensitive– Loudness depends on amplitude

• Louder sounds = higher amplitude

• Vigorous vibrations in cochlea = more bending = more AP’s

• Hair cells easily damaged due to prolonged exposure to certain frequencies

Physiology of Hearing• Pinna collects sound waves

– Travel down auditory canal to tympanic membrane– Moves ossicles with vibrations

• Stapes pushes on oval window, in and out– Creates fluid pressure waves in scala vestibuli perilymph

• Pressure waves deform scala tympani to push round window in and out– Pressure changes move endolymph– Highest frequency at base (oval window), lowest at apex

• Pressure changes in endolymph, from perilymph changes, moves the basilar membrane

• Hair cells on Organ of Corti bend as they move against the tectorial membrane– Generates nerve impulses that leave via the cochlear nerve

Auditory Pathway• AP signals from cochlea to medulla

– Cochlear nuclei

• Some fibers cross to olives (collection of nuclei in the medulla) , all ascend into MGN(medial geniculate nucleus) in the thalamus– Pass through inferior colliculi (reflex

area)– Interactions with superior colliculi to

turn toward sound

• Synapse in primary auditory cortex

• Localization utilizes relative intensity and timing

http://openlearn.open.ac.uk/file.php/3373/SD329_1_027i.jpg

Dynamic Equilibrium• Maintain body position

after initiation of mov’t• Within semicircular

canals– Rotation within 1 of 3

planes– Endolymph moves

opposite direction of mov’t

– Reverse to signal stop• Dizzy feeling

Static Equilibrium

• Linear changes only– E.g. elevator changes or car

acceleration/deceleration• Vestibule– Saccule: vertical, hairs

horizontal– Utricle: horizontal, hairs

vertical• Maculae overlaid by otoliths• Mov’t displaces in opposite

direction

Motion Sickness

• Results from conflict between eyes and equilibrium sensors in the inner ear– Feeling motion, but not seeing it (inside structure)– One system is hallucinating, implying toxins in

system = vomiting

• Dramamine inhibits input from equilibrium sensors

• Astronauts learn to control