Sensory Systems
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Categories of Sensory
Receptors and Their
Actions
• Sensory information is conveyed to
the central nervous system (CNS) in
a 4-step process
• Stimulation
• Transduction - stimulus is transformed
into receptor potential in sensory
receptor
• Transmission - axon of sensory neuron
conducts action potentials
• Interpretation - sensory perception
created by brain.
• 3 classes of environmental
stimuli
• Mechanical forces (stimulate
mechanoreceptors)
• Chemicals (stimulate
chemoreceptors)
• Electromagnetic/thermal energy
(stimulate photoreceptors and
others)
• Free nerve endings - the
simplest sensory receptors,
respond to the bending or
stretching of sensory neuron
membrane, change in temp, or
chemicals like oxygen in the
extracellular fluid.
• Exteroceptors - sense stimuli in
external environment
• Exterior senses evolved before
vertebrates came on land
• Some function in water and on land
• Mammalian hearing, uses similar receptors
originally used in water
• Some can’t function in air
• Electrical organs of fish
• Some can’t function in water
• Infrared receptors
• Sensory systems provide levels
of information about the
external environment
• Determine an object is present
• Where the object is located
• Make a 3-D image of the object
and its surroundings
• Interoceptors - sense stimuli
from within body
• Muscle length/tension, limb
position, pain, blood chemistry,
blood volume/pressure and body
temp
• Simpler than exteroceptors
• Close to primitive sensory
receptors
• Sensory cells respond to simuli due
to stimulus-gated ion channels in
membranes
• Stimulus causes channels to open and
close depending on the system
• Depolarization of receptor cell (receptor
potential)
• Larger stimulus, greater degree of
polarization
• Decrease in size with distance from source
− Prevents irrelevant stimuli reception
• If great enough, production of action
Temperature and
Pressure
• Cutaneous receptors - receptors
of the skin (interoceptors)
• Respond to stimuli at border
between external and internal
environments
• Thermoreceptors - naked,
dendritic endings of sensory
neurons sensitive to temp
changes.
• Found within the hypothalamus
and monitor temp of circulating
blood which provides the CNS with
info on the body’s core temp.
• Skin has two populations of
thermoreceptors
• Cold receptors and warm
receptors - stimulated by
corresponding temps and
inhibited by opposite temps
• Cold receptors are right below
the epidermis, warm receptors
are slightly deeper in the dermis
• Nociceptors - transmit impulses
perceived by the brain as pain
• Most consist of free nerve endings
throughout the body near surfaces
where damage is likely to occur
• Some sensitive to actual tissue
damage, others respond before
damage
• Mechanoreceptors - contain sensory
cells with ion channels sensitive to
mechanical force applied to the
membrane
• Present in skin, dermis and
subcutaneous tissue
• Concentrated on fingertips and face
• Phasic - intermittently activated. Tonic continuously activated.
− Monitor duration of a touch and extent it’s
applied
• Pacinian corpuscles- monitor onset and
Muscle Contraction
• Within skeletal muscles of all
vertebrates except bony fish
are muscle spindles
• Sensory stretch receptors
• Consist of thin fibers wrapped
together, innervated by sensory
neuron activated when muscle
stretches
• Proprioceptors - muscle
spindles and receptors in
tendons and joints providing
info about position/movement of
body parts
• Muscle contracts, tension on
attached tendon
• Monitored by Golgi tension organs
− If high, elicits reflex to inhibit active
muscle
− Ensures muscles don’t damage tendons by
contracting too strongly
Blood pressure
• Monitored at 2 main sites
• Carotid sinus and aortic arch
• Walls of blood vessels have baroceptors
− Network of afferent neurons
− Detect tension/stretch in walls
• Blood pressure decreases, frequency of
impulses decrease.
• CNS overcompensates for homeostasis
and blood pressure increases, so does
impulse frequency.
Taste/Smell/Body
Position
• Taste buds - chemosensitive
epithelial cells
• Fish have taste buds over their
body, these are the most sensitive
vertebrate chemoreceptors
• Important to bottom-feeders so
they can sense the presence of
food in murky environments
• Terrestrial vertebrates have
taste buds on their tongue and
oral cavity
• Papillae are raised areas bearing
taste buds
• Taste buds are between 50-100
taste cells
• Microvilli on them called taste pore
• Salty/sour- directly through ion
channels
• Sweet/bitter - bind to surface
receptor proteins, trigger G
proteins which changes interior of
cell and open/closes ion channels
• Interact with other neurons carrying
smell info
• Fly taste receptors are in
sensory hairs on their feet
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• Smell chemoreceptor is in upper
nasal passages
• Since we’re surrounded by air,
our sense of smell (olfaction)
has specialized to detect
airborne particles
• Extremely acute sense in
mammals
• Humans can discern 1000s of
smells in contrast to the 4 tastes.
• Internal chemoreceptors
• Peripheral - sensitive to plasma pH
in aortic/carotid bodies
• Central - sensitive to cerebrospinal
fluid pH in medulla oblongata
• Stimulation indirectly affects
respiratory control center, increases
breathing rate
Lateral Line System
• Provides fish with sense of
“distant touch”
• Sense objects reflecting pressure
waves and low vibrations
• Supplements hearing
• Longitudinal canal in skin along sides
and in the head contain sensory
structures
− Hair cells - hair-like processes project into
capula (gelatinous membrane)
– Same length, stereocilia. Longer,
kinocilium
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• Vibrations move capula and
hairs bend
• Stereocilia bent in direction of
kinocilium, sensory neurons
stimulated
• Opposite direction, activity
inhibited
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Gravity/Angular
Acceleration
• Statocyst - helps invertebrates
orient selves with gravity
• Ciliated hair cells in membane
with crystals of calcium cabronate
are statoliths (“stones”) that
increase mass of membrane so
cilia bend when position changes
• Similar structure in inner ear of
vertebrates
• Membranous labyrinth of fluidfilled chambers and tubes
• Size of a pea
• Receptors of gravity in 2 chambers
• Utricle and saccule
• Similar to lateral line of fish
• Utricle - sensitive to horizontal
acceleration
• Saccule - to vertical acceleration
• Continuous with 3 semicircular canals, which
detect angular acceleration (head rotates)
− Together, all form vestibular apparatus
• Brain uses info to maintain balance and
equilibrium
Hearing/Ears
• Works better in water because
it transmits pressure waves
more efficiently
• Auditory stimuli travel farther
and quicker than chemical ones
and provide better directional
info
Structure of ear
• Vibration goes through ear
canal to eardrum (tympanic
membrane)
• Movement of 3 bones (ossicles)
in middle ear
• Malleus (hammer)
• Ineus (anvil)
• Stapes (stirrups)
• Similar to Weberian ossicles of fish,
which are small bones that vibrate to
a fish’s saccule
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• Middle ear connected to throat
by Eustachian tube
• Equalizes air pressure btw middle
ear and external environment
• “ear popping”
• Inner ear consists of cochlea, a bony
structure containing cochlear duct
of membranous labyrinth
• Above is vestibular canal, below is
tympanic canal
• Fluid-filled
• Stapes vibrate against oval window
into inner ear, pressure waves to
tympanic canal, pushing round
window that transmits back to the
middle ear
Transduction in Cochlea
• Bottom of duct, basilar
membrane.
• Hair cells project into tectorial
membrane
• Organ of Corti
• Basilar vibrates, cilia bend
depolarizing hair cells which
stimulate action potentials in
neurons that project to the brain
and are interpreted as sound
Frequency Localization
in Cochlea
• Elastic fibers in basilar
membrane
• Base of cochlea have short, stiff
fibers
• At apex, 5x longer and 100x more
flexible
• Resonant frequency higher at base
(high pitches)
• Lower at apex
• Frequency range in humans
between 20-20,000 cycles per
second (hertz) in kids
• High pitch detection ability
down with age
• Dogs can hear 40,000 hertz
Sonar
• Our ears help determine
direction of the source of
sound, but not a reliable
measure of distance
• Bats, shrews, dolphins, and
whales perceive distance by sonar
• Echolation - emit sounds and
determine time it takes sound to
reach object and come back
Evolution of Eye
• Invertebrates have eyespots,
sensitive to direction of light
source but can’t construct a
visual image
• Annelids, mollusks, arthropod
and chordates evolved imageforming eyes
• Believed to have evolved
independently
Structure of Vertebrate
Eye
• Sclera (“white of eye”) - tough
connective tissue
• Transparent cornea - light enters
and focus begins
• Iris- colored part of eye
• Pupil - opening in iris that contracts
in bright light
• Lens - light passes through pupil to
here where light is focused onto
retina at back of the eye.
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• Lens attached by suspensory
ligament to ciliary muscles
• Shape influenced by amount of tension
• Ciliary muscle contracts, ligament slack, lens
rounded and powerful
− Close vision
• Opposite reactions
− Far vision
• Near/far-sighted people don’t properly
focus image on retina
• Lens of fish/amphibians move in and out
like camera to focus.
Vertebrate
photoreceptors
• Retina has 2 photoreceptors
(rods and cones)
• Rods - black and white vision
when illumination is dim
• Cones - high visual acuity and
color vision
• 100 million rods, 3 million cones in
each retina
• Their light capturing molecules
(or photopigments) are
• Rhodopsin in rods, photopsin in
cones
• 3 kinds of cones in humans
• Different AA sequences causes
shift in absorption maximum
• Region of electromagnetic spectrum
best absorbed by pigment
• 500 max in rhodopsin
• In photopsin, 420 (blue), 530 (green)
and 560 (red)
• Cones also referred to as blue, green
and red cones
• Most vertebrates have color vision
• Fish/turtles/birds, 4-5 cones see near
ultra-violet light
• Many mammals have only 2 (squirrels)
• Retina is made of 3 layers
• 1. Rods and cones
• 2. Bipolar cells
• 3. Ganglion cells
• Flows of sensory info opposite
to path of light through retina
• A rod or cone contains sodium
channels in the plasma
membrane, may of which are
open in the dark
• Dark current
• Cyclic guanosine monophosphate
(cGMP) required to keep channels
open
Visual Processing in
Vertebrate Retina
• Action potentials gathered along
ganglion cells are relayed through
the lateral geniculate nuclei of the
thalamus and projected to the
occipital lobe
• Binocular vision
• When both eyes are fixed on one object,
each eye views object from different
angle
• Useful for predators
• Prey have an enlarged overall
receptive field rather than binocular
Diversity of Sensory
Experiences
• Heat
• Electromagnetic radiation is too
low in energy to be detected by
photoreceptors
• Infrared radiation falls on
membrane and warms it, thermal
receptors are then stimulated
• In snakes this info is used as the
visual center is used in other
vertebrates
• Electricity
• All aquatic animals generate
electric currents from muscle
contractions
• Weak electrical discharges help
construct a 3-D image of their
environment
• Electroreceptors called ampullae
of Lorenzini are used by sharks to
detect muscle contractions of
prey
• Magnetism
• There has been speculation of
magnetic receptors in vertebrates,
but it is still poorly understood
• However eels, sharks, bees, birds and
some bacteria seem to navigate along
magnetic field lines of the earth
Disease/Disorders
• Color blindness - due to an
inherent lack of one or more
types of cones
• Men are more likely than women
to be color blind because the trait
is on the X chromosome.
• Vertigo - an inner ear
disturbance that causes one to
feel extremely dizzy
• CIPA - congenital insensitivity to
pain with anhidrosis
• Lack of sensory perception,
inability to feel pain.
• Caused by a mutation on an
autosomal chromosome which
appears to control nerve growth
• Pain messages aren’t lost, but
aren’t being sent to the brain.