Download Sensory input: Sensory structures, classification by function

Chapter 9 Lecture Notes
page 1
Chapter 9 - General and Special Senses
I. receptor structure and physiology
A. receptors are specialized cells or nerve endings that detect changes in
the external or internal environment (stimuli)

they are able to convert stimuli into action potentials
B. not all sensory information reaches the cerebral cortex
stimulus 
receptor is activated 
action potential in sensory/afferent neuron 
subconscious regions of brain and spinal cord 
cerebral cortex
1. adaptation is decreases sensitivity of the receptor to a constant
stimulus
2. sensory input is screened by the reticular formation in the brain
stem
3. input from some receptors goes to subcortical regions of the
brain
C. receptor classification
1. nocireceptors detect tissue damage and warn us about the
damage by making us feel pain
nocireceptors can be activated by pressure, extreme
temperatures, or chemicals released by damaged cells
2. thermoreceptors detect changes in temperature
located in skin, muscles, liver and hypothalamus
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3. mechanoreceptors detect stretch, pressure and bending
used for hearing and equilibrium, touch in skin, blood pressure
monitoring in arteries, body position in joints, and muscle
length in skeletal muscles
4. chemoreceptors detect chemicals
used for taste, smell, and monitoring of oxygen and pH of the
blood
5. photoreceptors detect light
used for vision
II. general senses
A. pain - nocireceptors
1. located in the skin, joint capsules, periosteum, and around blood
vessels
2. do not adapt
3. analgesia prevents the perception of pain
a. intrinsic analgesic system blocks transmission from the
sensory neuron to interneurons
b. extrinsic analgesia can be applied at the level of the receptor
(aspirin), the afferent pathway (benzocaine blocks the
transmission of action potentials in the afferent neuron) or
the CNS (morphine blocks transmission from the sensory
neuron to interneurons)
4. referred pain is the perception of pain coming from uninjured body
parts due to convergence in the afferent pathway
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Chapter 9 Lecture Notes
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B. touch and pressure: free and encapsulated nerve endings
C. temperature
D. proprioception:
1. free nerve endings in joint capsules detect stretch on the capsule
2. Golgi tendon organs detect tension in tendons
3. muscle spindles detect skeletal muscle length
III. special senses
A. olfaction
1. olfactory receptors are located in the olfactory epithelium in the
superior nasal cavity just inferior to the cribriform plate of the
ethmoid bone
2. olfactory receptors are neurons that have dendrites called cilia that
project from the surface of the olfactory epithelium
3. molecules from inhaled air dissolve in mucus covering the
olfactory epithelium and bind with membrane receptors in
olfactory cilia, initiating action potentials in olfactory receptor
cells
4. the receptor cell axon (cranial nerve I) projects upward through the
olfactory foramina of the cribriform plate and synapses with
other neurons in the olfactory bulb
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5. the olfactory tract goes from the olfactory bulb to the olfactory
cortex, the hypothalamus, and the limbic system
B. gustation
1. gustatory receptors are located in taste buds in the epithelium of
the tongue papillae, mouth and pharynx
2. gustatory receptors have microvilli that project to the surface of the
epithelium
3. molecules dissolved in saliva bind with membrane receptors in the
gustatory microvilli, changing its membrane potential and
causing the release of a neurotransmitter
4. there are at least 5 types of gustatory membrane receptors:
sweet, sour, bitter, salty, and umami (detects glutamate, which
is present in high protein foods such as meat)
5. the neurotransmitter initiates action potentials in the sensory
neuron innervating the cell; these neurons are located in cranial
nerves VII, IX, X
6. information from these neurons is routed via the thalamus to the
gustatory cortex, where information from smell and pain
receptors is integrated with gustatory input
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C. vision
1. accessory structures:
a. eyelids and glands - protect eyes, keep surface of eye clean
and moist
b. conjunctiva - a layer of epithelium lining the eyelids and
covering the exposed surface of the eye except the
cornea
c. lacrimal apparatus - lacrimal gland is in orbit (eye socket in
skull) superior and lateral to eyeball; tears contain
lysozyme; tears drain through lacrimal pores at medial
canthus into nasolacrimal duct which opens into the nasal
cavity
d. extrinsic muscles - 6 skeletal muscles attached to the
outside of the eyeball that move it
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2. structure of eyeball
a. fibrous tunic:
 sclera (white)
 cornea (anterior, clear, curved)
b. vascular tunic:
 iris - muscle that controls the amount of light entering the
eye by constricting or dilating; controlled by oculomotor
nerve
 ciliary body - contains a muscle that controls the lens for
focusing and holds the lens in place via suspensory
ligaments
 choroid - distributes blood vessels to retina
c. neural tunic
 pigmented layer absorbs light
 neural layer contains photoreceptors (rods and cones),
bipolar cells and ganglion cells
rods
no color discrimination
more sensitive - used in dim light
lower acuity (sharpness of image)
peripheral
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cones
color discrimination
less sensitive - used in bright light
higher acuity
central (fovea centralis)
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 optic disc where optic nerve leaves eye has no
photoreceptors - blind spot
 fovea centralis - indentation in center of retina where
images are focused
d. lens - used to focus light on the retina
 held in place by suspensory ligaments attached to ciliary
body
 elastic
 clear
 shape adjusted by ciliary muscle ; controlled by
oculomotor nerve
e. chambers


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anterior cavity anterior to lens - contains aqueous humor
 anterior chamber between cornea and iris
 posterior chamber between iris and lens
aqueous humor:
produced by ciliary body
reabsorbed into canal of Schlemm
produces intraocular pressure
posterior cavity posterior to lens - contains vitreous body
(gelatinous)
 holds retina in place
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3. vision
a. refraction - light entering the eye is bent inwards by the
curved surfaces of the cornea and lens
b. accommodation - the lens changes shape to focus the
image on the retina

near vision

far vision
c. convergence - the eyes move medially to focus on close
objects
d. photoreception -
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
rods and cones contain visual pigment molecules that
have two components: retinal and opsin

retinal is sensitive to light and changes shape when
exposed to light
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Chapter 9 Lecture Notes

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when retinal changes shape it separates from opsin,
activating photoreceptor cell
4. afferent pathway
a. photoreceptor activates bipolar cell, which activates ganglion
cell
b. axons of the ganglion cells leave eyeball at optic disc to form
optic nerve
c. left and right optic nerves meet at the optic chiasm, where
axons from half of each eye cross to opposite sides of the
brain
d. optic tract contains axons from both eyes and projects to
thalamus and midbrain (origin of the oculomotor nerve)
e. from thalamus, other neurons carry visual information to
visual cortex
D. hearing
1. structure of ear
a. outer or external ear
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
pinna directs sound waves into external auditory canal which
ends at tympanic membrane

sound waves make tympanic membrane vibrate
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b. middle ear

ossicles (malleus, incus, stapes) are attached to tympanic
membrane and oval window of inner ear

ossicles amplify vibrations from tympanic membrane and
transmit them to the inner ear

two small skeletal muscles can adjust the level of
amplification to somewhat protect the ear from loud noises

the auditory tube runs from the middle ear to the pharynx
and equalizes air pressure across the tympanic membrane
c. inner ear

3-part cavity in temporal bone; contains a fluid-filled
membrane complex suspended in fluid
vestibule - contains two membrane compartments: utricle and
saccule
cochlea - contains one coiled membrane compartment: cochlear
duct
in the cochlear duct is the organ of Corti
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basilar membrane
hair cells - stereocilia
tectorial membrane
fibers of cochlear branch of VIII
semicircular canals (3) - each contains a horseshoe-shaped
membrane compartment: semicircular ducts
2. hearing

the stapes causes vibrations in the perilymph that spread into
the cochlea and vibrate the basilar membrane

this vibrates the hair cells up and down; as they go up the
tectorial membrane stops them and bends the “hairs”

bending the hairs changes their rate of neurotransmitter release
and stimulates the afferent neuron
a. pitch - each frequency of sound stimulates a different area of
the organ of Corti
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b. loudness - the louder the sound, the more hair cells in a given
pitch area are activated
3. afferent pathway
cochlear branch of VIII
medulla oblongata
inferior colliculus - motor response
thalamus
primary auditory cortex
E. equilibrium
1. static - utricle and saccule contain maculae that monitor:
a. body position with respect to gravity
“hairs” are bent downwards by the pull of gravity
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b. linear acceleration
inertia bends the “hairs”
small crystals of calcium carbonate act like gravity and inertia
“detectors”
2. dynamic - semicircular ducts contain cristae that monitor rotational
movement
when the head or body turns, the fluid in the duct lags behind
and bends the crista, bending the “hairs” of the embedded hair
cells
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