Transcript Special Senses

Special Senses
Chapter 8
Special Senses
• Smell, taste, sight, and
hearing
• Equilibrium
• Special sense receptors:
– large, complex sensory organs
(eyes, ears)
– localized clusters of receptors
(taste buds and olfactory
epithelium)
• Senses blend to give us our
sensations
The Eye & Vision
External Anatomy of the Eye
• Only anterior 1/6 of eye’s surface can
normally be seen
• Accessory structures:
–
–
–
–
Extrinsic eye muscles
Eyelids
Conjunctiva
Lacrimal apparatus
Accessory Structures
• Extrinsic eye muscles
– Six muscles are attached to outer
surface of each eye
– Produce gross eye movements
Accessory Structures
• Eyelids
– 4 layers: skin, muscle,
connective tissue,
conjunctiva (mucous
membrane)
– Moved by orbicularis oculi
muscles (close) and
levatator palpebrae
superioris (open)
Accessory Structures
• Lacrimal apparatus
– Consists of lacrimal gland &
ducts that drain lacrimal
secretions into nasal cavity
– Lacrimal glands: continuously
secrete dilute salt solution
(tears) onto anterior surface
of eyeball
• Secretion contains mucus,
antibodies, and lysozyme which
is an enzyme that destroys
bacteria
• Cleans and protects eye surface
as it moistens & lubricates it
Internal Structures
• Eyeball is a hollow sphere
• Wall composed of three layers:
– Fibrous layer
• Sclera
• Cornea
– Vascular layer
• Choroid
• Ciliary body
• Iris
– Sensory layer
• Retina
– Pigmented layer (prevent light from scattering)
– Neural layer (photoreceptors)
Fibrous Layer
• Sclera: thick, white connective tissue (“white of
the eye”)
– Opaque due to large, disorganized collagenous and
elastic fibers
– For protection & attachment
• Cornea: clear “window” through which light
enters eye
– anterior portion of eye
– many nerve endings (pain fibers), easily repairs itself
– avascular, so only tissue in the body that can be
transplanted without fear of rejection
– helps focus entering light rays
– Connective tissue with thin layer of epithelium;
unusually regular fiber pattern
Fibrous Layer: Sclera & Cornea
Vascular Layer
•
Choroid: blood-rich nutritive tunic that contains a dark
pigment
–
–
–
–
•
Prevents
Modified
Produces
Contains
light from scattering inside the eye
anteriorly to form ciliary body & iris
melanin to absorb light
blood vessels
Ciliary body: two smooth muscle structures to which
the lens is attached with the ciliary zonule (ligament)
– Forms internal ring around front of eye (muscle fibers &
ligaments)
•
Iris: pigmented, has an opening called the pupil through
which light passes
– Thin diaphragm composed of muscle tissue and connective
tissue
– Circular and radial muscle fibers control size of pupil
(stimulated by photons of light)
–
Regulates the amount of light which enters the eye
Choroid
Ciliary Body
Sensory Layer: Retina
• Retina – two layers which extends
anteriorly only to the ciliary body
– Pigmented layer
• composed of pigment cells that absorb light
and prevent light from scattering inside the
light
• act as phagocytes to remove dead or damaged
receptor cells
• store vitamin A needed for vision
Sensory Layer: Retina
• Neural layer – contains millions of receptor
cells
– Photoreceptors (rods & cones): respond to
light, bipolar neurons
• 70% of sensory receptors are in the eyes
• Rods: more sensitive in low light, gives general
outline, seen as black and white
• Cones: less sensitive in low light, sharp picture,
color
– Electrical impulse leaves the retina via the
optic nerve & nerve impulses are transmitted
to the optic cortex which results in vision
– Fovea: all cones, sharpest vision (visual acuity)
– Optic disc (“blind spot”) – where optic nerve
leaves eyeball
Retina
Sensory Layer: Retina
• Night blindness: fewer working rods
(lack of vitamin A)
• Day blindness: lack of working cones
LINK
Sensory Layer: Retina
•
Rods: rhodopsin breaks down into opsin & retinal
when struck by photons; initiates chemical reaction
(action potential) which is sent to visual cortex
(occipital lobe)
– In bright light, nearly all rhodopsin I broken down, reducing
rod sensitivity
•
Cones: three different light sensitive proteins
connected to retinal – each most sensitive to a
particular wavelength of visible light
– Depending on which is stimulated, brain interprets that
color
– Erythrolabe: red
Read pg. 286
– Chlorolabe: green
– Cyanolabe: blue
– Mixing & interpretation of color occurs in the brain, not
the retina! (i.e. red light in one light & green light in
another eye will be seen as yellow)
Visual Pigments
If all are stimulated
at once, see white!
Color Blindness
• Total color blindness: lack of all
three cones
• Partial color blindness: lack of one
cone type (lack of red or green
receptor is most common)
• Sex-linked condition (carried on X
chromosome)
Color Blindness
4 Sex-Linked Traits:
1. Normal Color Vision:
A: 29, B: 45, C: --, D:
26
2. Red-Green Color-Blind:
A: 70, B: --, C: 5, D: -3. Red Color-blind:
A: 70, B: --, C: 5, D: 6
4. Green Color-Blind:
A: 70, B: --, C: 5, D: 2
Internal Structures: Lens
• Lies directly behind iris & pupil
• Focuses light entering the eye on the retina
(changes shape to focus)
• Held upright in the eye by a suspensory
ligament (ciliary zonule) attached to ciliary
body
• Epithelial cells (cytoplasm
is transparent part)
Cataracts
• With age, lens becomes increasingly
hard and opaque
• Cataracts result from this process
and cause vision to become hazy and
distorted
– Can eventually cause blindness in
affected eye
• Treatment: surgical removal of lens
and replacement with lens implant
or special cataract glasses
Internal Structures: Humors
• Aqueous humor
– Anterior to the lens, clear
watery fluid
– Fills space between cornea and
lens
– Nourishes, helps hold shape
• Vitreous humor
– Reinforces
eyeball internally
– Posterior to lens
Glaucoma
• aqueous humor made more quickly than
can be removed or drainage is blocked
• pressure builds, damage results from
compression of retina and optic nerve
Physiology of Vision
• Light rays are bent (refracted) as light
encounters the cornea, aqueous humor,
lens, and vitreous humor
• Refractory power of the lens can be
changed by changing its shape (ciliary body
controls shape of lens)
• Accommodation: ability of eye to focus for
close objects (less than 20 ft. away)
– Image formed on retina is a real image
(inverted)
Vision
Visual Pathways
•
•
Axons carrying impulses from retina are bundled
together at posterior aspect of eyeball and issue
from back of eye as optic nerve
Optic chiasma: fibers from medial side of eye cross
over to opposite side of brain
– Fiber tracts that result are optic tracts
– Each optic tract contains fibers from the lateral side of
the eye on the same side and the medial side of the
opposite eye
– Optic tract fibers synapse with neurons in the
thalamus, whose axons form the optic radiation which
runs to the occipital lobe of brain (visual interpretation
occurs)
•
•
Each side of brain receives visual input from both
eyes
Visual fields overlap to give humans binocular vision
The Ear: Hearing & Balance
Anatomy of the Ear
• Divided into three major areas
– External (outer) ear
– Middle ear
– Internal (inner) ear
External (Outer) Ear
• Auricle (pinna) – “ear”
– Collects and directs sound waves into the
auditory canal (function largely lost in
humans)
• External acoustic meatus (auditory canal)
– Short, narrow chamber carved into temporal
bone of skull
– Ceruminous glands – secrete cerumen
(earwax)
– Tympanic membrane (eardrum) – vibrate
when sound waves strike it; separates
external & middle ear
Middle Ear
• Small, air-filled, mucosa-lined cavity
within temporal bone
• Transfers vibrations via the ossicles
(smallest bones in body)
– malleus (hammer), incus (anvil), and
stapes (stirrup)
– Stapes passes vibration to the oval
window of the inner ear
• Pharyngotympanic tube (auditory
tube): pressure needs to be equalized
to enable eardrum to vibrate
Middle Ear
Otitis Media
• Inflammation of the middle ear
• Pharyngotympanic tubes run more
horizontally in children
• Ear tubes – implanted in ear drum to
allow pus to drain into external ear
canal
• Infants with bottles “propped” or fed
lying flat can get fluid in their ears
through the pharyngotympanic tube
Internal (Inner) Ear
• Bony (osseous) cavity located behind the eye
socket
– Filled with perilymph (fluid)
– Membranous labyrinth suspended in perilymph,
contains endolymph
• Three subdivisions
– Cochlea
– Vestibule
– Semicircular canals
• Lined with hair cells
(mechanoreceptors)
Equilibrium
• Vestibular apparatus: equilibrium receptors of
ear
– Static equilibrium: maculae receptors
• Report on changes in position of head when body not
moving (keep head erect)
• Otoliths: tiny stones that roll in response to changes
in pull of gravity
– Dynamic equilibrium: bending of cupula indicates
rotation (gelatinous cap)
• Report on changes when body moving (i.e. spinning)
• Receptors stimulate hair cells, which send
impulses via the vestibular nerve to the
cerebellum
• Work together with proprioceptors for
control & balance
Hearing
• Spiral organ of corti – contains
hair cells (hearing receptors)
• Vibrations set cochlear fluids in
motion, pressure waves cause
vibrations
• impulses are sent via cochlear
nerve to temporal lobe
(auditory cortex)
Chemical Senses:
Smell & Taste
Olfaction
• Olfactory receptors (chemoreceptors): receptors
for sense of smell, occupy a postage stamp-size
area in roof of each nasal cavity
– 10-100,000,000 receptors in nose
• Olfactory filaments (axons) make up olfactory
nerve (cranial nerve I) which conducts impulses
to olfactory cortex of brain
– Olfactory lobes of brain (gray matter) – situated
over nose (bottom of frontal lobe)
• Olfactory impulses closely tied to limbic system
• Sensitive receptors, just a few molecule can
activate them (thousands of smells)
• Adapt rather quickly when exposed to
unchanging stimulus
Gustation: Taste
• Taste buds: specific receptors for taste widely
scattered in the oral cavity
– Live 7-10 days!
• protection (low threshold – bitter)
• 10,000 taste buds (mostly on tongue)
• Five tastes: sweet, salty, sour, bitter, and
umami (savory)
– Flavors: combination of 5 tastes and olfactory
and touch sensations
• 1st order neuron  medulla  hypothalamus or
thalamus (limbic)  parietal lobe (conscious
perception of taste)