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VT 106
Comparative Anatomy and Physiology
Sense Organs
SENSE ORGANS – organs that detect conscious sensations (esthesia)
Sensory Receptors – specialized cells or dendrites that detect stimuli in the internal
or external environment
thermoreceptors – dendrites that detect temperature changes
mechanoreceptors – dendrites or cells that detect mechanical distortions of cell
membranes caused by touch or pressure, stretch or bend, vibrations
photoreceptors – cells that detect electromagnetic stimuli (light)
chemoreceptors – dendrites or cells that detect concentration of specific chemicals
pain receptors (nociceptors) – dendrites stimulated mainly by chemicals released
by tissue damage or inflammation
may also be stimulated by intense mechanical or thermal stimuli
anesthesia – loss of conscious sensation
general anesthesia – complete loss of sensation and consciousness
blocks stimulation of cerebral cortex
local (regional) anesthesia – loss of sensation in a localized area
blocks conduction of impulses by sensory nerves
GENERAL SENSES – distributed throughout skin, connective tissues, and organs
of the body
pain sensations
pain receptors (nociceptors)
superficial pain – skin and subcutaneous tissues
deep pain – muscles, joints, bones
visceral pain – internal organs (no pain receptors in brain)
analgesics – drugs that decrease pain perception
temperature sensations
thermoreceptors that detect heat and cold
superficial thermoreceptors – found in skin
consciously perceived – mainly cause behavioral changes
central thermoreceptors – hypothalamus
detect core temperature (temperature of blood)
control reflexes for sweating, shivering, piloerection (hair
elevation), and thyroid function
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proprioception
mechanoreceptors in muscles, tendons, and joints that monitor location
of body parts
monitored by cerebellum to coordinate movements
muscle spindle – detects stretching of skeletal muscle
brain regulates sensitivity to control muscle tone
tactile sensations
mechanoreceptors in skin and connective tissues that detect touch and
pressure
Meissner's corpuscles – encapsulated dendrites in dermal papillae of skin
detect fine touch
hair root plexuses – dendrites wrapped around hair follicles
detect movements of hairs
Pacininan corpuscles – encapsulated dendrites found in many tissues
detect deep pressure
visceral sensations – related to organ functions
chemoreceptors – detect levels of chemicals (eg. nutrients, water)
mechanoreceptors – detect stretch of organs (intense stimulation causes pain)
stimulate conscious sensations such as hunger, thirst, nausea
SPECIAL SENSES – sensory receptors in complex sensory organs
OLFACTION – sense of smell
chemoreceptors in nasal cavity
Olfactory Epithelium – sense organ for smell
3 types of cells:
1) olfactory receptors – first-order neurons
bipolar neurons with 1 knob-shaped dendrite with olfactory cilia
cilia have receptors for odorant molecules
olfactory nerve – bundled axons extending through cribriform plate
2) supporting cells – columnar epithelium
support and protect olfactory receptors
3) basal stem cells – divide and differentiate to produce new receptors
olfactory glands – in underlying connective tissue
secrete mucus on surface – dissolves odorant molecules
Vomeronasal Organ – specialized olfactory organ within incisive bones of some
species, may communicate with nasal cavity and/or mouth
detects pheromones (odorant molecules used for communication purposes)
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Physiology of Olfaction
odorants dissolve in mucus
odorants bind to receptors on olfactory cilia, causing stimulation
threshold stimulation produces an action potential, which propagates along
the olfactory nerve, through cribriform plate
(olfactory threshold is low – as little as 4 odorant molecules)
olfactory nerves (cranial nerve I) synapse in olfactory bulbs with
second-order neurons
olfactory tract – second-order neurons running to olfactory cortex
(temporal lobe)
GUSTATION – sense of taste
chemoreceptors on papillae of tongue and in mouth and pharynx
Primary Tastes:
sour, sweet, salty, bitter
umami – meaty or savory
(watery)
taste is augmented by olfaction and tactile sensations
Taste Buds – sense organs for taste on papillae of tongue
2 types of cells:
gustatory cells – sensory receptors
taste hairs – long microvilli with receptors for certain molecules
taste pore – opening in papilla; tastant molecules pass through to
bind to taste hairs
supporting cells – support and protect gustatory cells
differentiate to form new gustatory cells
Physiology of Gustation
tastants dissolve in saliva
tastants bind to and stimulate taste hairs
threshold stimulation triggers action potential in first-order neuron
first-order neurons run via cranial nerves VII, IX, X to brainstem
second-order neurons – brainstem to thalamus
third-order neurons – thalamus to gustatory cortex (parietal lobe)
VISION – sense of sight
photoreceptors in eyes
Accessory Structures of the Eye
eyelids (palpebrae) – skin flaps that protect eye
nictitating membrane – third eyelid; movement is passive
tarsal (Meibomian) glands – modified sebaceous glands of eyelids
oily secretions coat margin of eyelid
prevents eyelids sticking together and tears spilling out of eye
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conjunctiva – protective mucous membrane on surface of eye and inner
eyelids; mucus contains lysozyme – bacteriocide
bulbar conjunctiva – lines eye
palpebral conjunctiva – lines inner eyelid
conjunctival sac – space between; site for giving eye medications
lacrimal apparatus – moistens and protects eye
lacrimal glands – produce tears (watery secretion)
lacrimal puncta and lacrimal sac – drain tears from eye
nasolacrimal duct – drains tears into nasal cavity
extrinsic eye muscles – move eyeball
innervated by cranial nerves III, IV, VI
dorsal rectus – upward rotation
ventral rectus – downward rotation
lateral rectus – lateral rotation
medial rectus – medial rotation
dorsal and ventral obliques – roll or stabilize eyeball
retractor bulbi – retracts eye into orbit
ANATOMY OF THE EYE
3 layers of eye:
1) Fibrous Layer – outer layer
cornea – anterior transparent portion, avascular
3 layers of cornea:
outer – stratified squamous epithelium
middle – orderly layers of collagen
inner - simple squamous epithelium
sclera – “white” of eye
dense fibrous connective tissue
gives rigid shape to eyeball
attachment site for extrinsic muscles
2) Vascular Layer (uvea) – middle layer
choroid – posterior portion
highly vascular - provides nutrients to retina
pigmented
melanin – absorbs stray light
tapetum lucidum – reflective region in some species
aids vision in low light conditions
iris – anterior colored portion
regulates light entering eye
pupil – opening in iris where light enters eye; shape varies
circular smooth muscle – decreases pupil diameter
parasympathetic response (ANS)
radial smooth muscle – increases pupil diameter
sympathetic response (ANS)
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ciliary body - between choroid and iris
secretes aqueous humor
ciliary muscle – focuses lens
suspensory ligaments – attach lens to ciliary body
3) Nervous Layer (retina) – inner layer on posterior eyeball
location of visual receptors
ophthalmoscope – instrument for viewing retina
optic disc – optic nerve (cranial nerve II) and blood vessels pass
through eye wall
blind spot – no visual receptors
2 layers of retina:
1) pigmented layer – outer layer
simple cuboidal epithelial cells containing melanin
2) neural layer – inner layer
contains photoreceptors and neurons which process visual
impulses before passing them to the brain
3 cell layers of neural layer:
1) photoreceptor layer – outermost layer
rods – very sensitive to low light (night vision)
black and white vision only
many rods in nocturnal animals
cones – sensitive to different light wavelengths(colors)
need more light for stimulation
give sharper vision in color
central fovea – depression in retina with high
concentration of photoreceptors
area of sharpest vision, only found in some
species (eg. primates, birds)
2) bipolar cell layer – middle layer
adjusts contrast of visual image
3) ganglion cell layer – inner layer
cell bodies of first-order neurons (CN II)
axons cross retinal surface to optic disc
Lens – transparent, avascular structure
composed of layers of transparent proteins (crystallins)
normally round – elasticity allows it to change shape and recoil
cataract – opacity of lens which obscures vision
Lens Divides Eye Into 2 Compartments:
1) aqueous compartment – anterior to lens
anterior chamber – anterior to iris
posterior chamber – posterior to iris
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aqueous humor – watery fluid filling aqueous compartment
produced by ciliary body capillaries filtering blood
circulates to anterior chamber
returns to blood at the canal of Schlemm found at
limbus –junction of cornea and sclera
2) vitreous compartment – posterior to lens
vitreous humor – jelly-like substance filling compartment
holds retina in place
intraocular pressure – pressure in eye regulated by aqueous humor
glaucoma – eye disease characterized by increased intraocular pressure
usually due to decreased drainage of aqueous humor
PHYSIOLOGY OF VISION
Photoreceptors – (rods and cones)
photopigments – organic molecules that absorb light
retinal – vitamin A derivative that absorbs light
activates a chain of reactions that stimulates
receptor cell
opsins (4 types) – molecules that promote absorption of
different wavelengths of light
rhodopsin (in rods) – absorbs any visible light
cones – different opsins absorb different wavelengths
of light
stimulation of photoreceptors releases neurotransmitters
bipolar cell layer integrates photoreceptor signals
ganglion cells (first-order neurons) integrate bipolar layer signals
action potentials are triggered and conducted to the optic disc
optic nerves travel to optic chiasm
optic chiasm – optic nerves converge and half of axons cross over
optic tracts – first-order axons run to thalamus
second-order neurons – thalamus to visual cortex (occipital lobe)
cerebral hemispheres process information from opposite visual
field
binocular vision – depth perception due to overlap of visual fields of
right and left eyes; placement of eyes varies depending on lifestyle
predators – eyes placed forward
prey – eyes placed laterally
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Image Formation
Refraction – bending of light rays as they pass through substances of
differing densities
cornea – accounts for most refraction
lens – refraction varies due to changing shape of lens
produces inverted image on retina (upside-down and left-right
reversal); brain interprets correct orientation
Accommodation – focusing light rays on retina by changing curvature of
the lens = changing refraction
ciliary muscle – circular muscle
contraction reduces tension on lens = lens more round
relaxation increases tension on lens = flattens lens
distant vision (20 ft or more) – parallel light rays
focus on retina with relaxed ciliary muscle = flattened lens
(no work required)
near vision (less than 20 ft) – divergent light rays
require more refraction to focus image on retina
contracted ciliary muscle = rounder lens (work required)
HEARING AND EQUILIBRIUM
mechanoreceptors in inner ear
ANATOMY OF THE EAR – 3 main regions
1) External ear – collects and funnels sound waves into ear
pinna – funnel of elastic cartilage covered by skin
skeletal muscles can direct pinna towards sounds
external auditory canal – leads from pinna to eardrum
L-shaped in most animals
vertical canal
horizontal canal passes into temporal bone
ceruminous glands and hairs protect ear from foreign material
otitis externa – excess secretions, hair, and narrowed canals can trap
moisture and promote growth of bacteria and yeast
tympanic membrane (eardrum) – between external and middle ear
outer layer – epidermis
middle layer – connective tissue with collagen & elastic fibers
inner layer – simple cuboidal epithelium
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2) Middle ear – air-filled cavity in temporal bone; lined by cuboidal epithelium
auditory ossicles – 3 smallest bones in body
suspended across middle ear by ligaments
malleus (hammer) – attached to inner tympanic membrane
articulates with incus
incus (anvil) – middle bone; articulates with malleus and stapes
stapes (stirrup) – articulates with incus
periosteum attaches to oval window (opening to inner ear)
2 muscles protect ear from loud noises by limiting vibrations
tensor tympani muscle – attached to malleus
stapedius muscle – attached to stapes
round window – membrane-covered opening to inner ear
Eustachian (auditory) tube – connects middle ear to nasopharynx
opens to equalize air pressure in middle ear
otitis media – middle ear infection
bacteria from pharynx can enter through auditory tube
guttural pouch – sac-like extension of the auditory tube in horses
3) Inner ear – location of organs for hearing and equilibrium
bony labyrinth – series of cavities in temporal bone that enclose organs
of hearing and equilibrium
perilymph – fluid similar to CSF within bony labyrinth
3 regions of bony labyrinth:
1) vestibule – central portion
saccule and utricle – 2 membranous sacs which function in
static equilibrium; filled with endolymph (fluid)
2) 3 semicircular canals – at 90 degrees to each other
semicircular ducts – 3 membranous tubes which function in
dynamic equilibrium; filled with endolymph
3) cochlea – snail shell-shaped structure
cochelar duct – coiled, membranous duct which functions in
hearing; filled with endolymph
organ of Corti – sensory organ for hearing
hair cells – mechanoreceptors; hairs which
produce graded potentials when bent
supporting cells – support and protect hair cells
tectorial membrane – stiff membrane in center of
cochlear duct
cochlear duct divides cochlea into 2 channels
vestibular duct – begins at oval window
tympanic duct – ends at round window
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PHYSIOLOGY OF HEARING
sound waves – alternating ripples of high and low pressure air originating
from a vibrating object
frequency – number of waves/second (Hz)
high frequency = high pitch
Pathway for Hearing:
sound waves funneled into external ear and strike tympanic membrane
malleus transmits vibrations to incus, incus to stapes, and stapes to oval
window
vibrations at oval window membrane produce pressure waves in
perilymph of cochlea
perilymph pressure waves travel from vestibular duct to tympanic duct
to round window (round window prevents echo)
pressure waves produce vibrations of cochlear duct
hair cells bend against tectorial membrane
each region of the cochlear duct vibrates at a different frequency
brain interprets as pitch
stimulated hair cells release of neurotransmitter
neurotransmitter triggers action potential in first-order neurons of
cochlear branch of c.n.VIII
c.n. VIII runs to brainstem
second-order neurons – brainstem to thalamus
third-order neurons – thalamus to auditory cortex (temporal lobe)
EQUILIBRIUM – sense of balance
vestibular apparatus – organs of equilibrium (saccule, utricle,
semicircular ducts)
equilibrium is also affected by visual and proprioceptive sensations
Static Equilibrium – detects body position relative to gravity
saccule and utricle (otolithic organs)
macula – thickened region in wall of each organ
hair cells – mechanoreceptors
otolithic membrane – gel-like layer resting on hair cells
otoliths – calcium carbonate crystals in gel
as head tilts, otoliths (due to gravity) pull otolithic
membrane downhill, bending sensory hairs
stimulated hair cells release neurotransmitter
neurotransmitter triggers action potential in first-order
neurons of vestibular branch of c.n.VIII
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Dynamic Equilibrium – detects head movements
semicircular ducts – lie at right angles to each other in 3 planes
(sagittal, dorsal, and transverse)
ampulla - dilated portion at base of each duct
crista – elevation with hair cells
cupula – gelatinous material coating hair cells
as head moves, ducts and their structures move, but endolymph &
cupula inside lag (due to inertia) and bend hair cells
bent hair cells release neurotransmitter
triggers action potentials in vestibular branch of c.n.VIII
vestibular branch of c.n. VIII runs to brain stem
second-order neurons run to thalamus and cerebellum
complex integration pathways monitor position and movements and
use input to coordinate movements of eyes, head & neck, and
adjust muscle tone to maintain balance
AVIAN SENSES
VISION – highly developed sense in birds
diurnal (active in day) birds have color vision and may detect UV light
eyes are large – sclera is supported by sclerotic ring
shape of eye is variable – round, flattened, tubular
nictitating membrane has voluntary muscle – birds can use it to protect eye
iris contains voluntary muscle – birds can regulate amount of light entering eye
retina has more photoreceptors and fewer blood vessels than mammals
pecten – vascular structure on retina
commonly hemorrhages due to head trauma
HEARING
no pinna
external auditory canal is covered by auricular feathers
columella – single bone in middle ear
nocturnal birds (eg. owls) have extremely sensitive hearing
ears are large with more hairs cells
ear placement is asymmetrical to help localize sounds
TOUCH
feather follicles are supplied with sensitive touch receptors
many birds have touch sensitive bills and tongues used to locate food
taste and smell are usually not well-developed in birds
main sense used for finding food in most species is vision
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