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Lecture 21
Major Senses
Sensory Perception
 The sensory nervous system tells the central nervous system
what’s happenin’!
 Sensory receptors
 Specialized sensory cells
that detect changes
inside and outside the
body
 Sensory organs
 Complex sensory
receptors
 Eyes, ears, taste buds
The path of sensory information
1. Stimulation
 Physical stimulus activates a sensory receptor
2. Transduction
 Converting the stimulus into an action potential


Stimulus-gated ion channels in sensory neuron are
opened or closed
An action potential is generated
3. Transmission
 Nerve impulse is conducted to the CNS

Two main types of sensory receptors
 Extroreceptors sense stimuli in external environment
 Introreceptors sense stimuli in internal environment
Sensing the Internal Environment
 Vertebrates use many different sensory receptors to
respond to changes in internal environment
 Temperature Change
 Two nerve endings in the skin
 One stimulated by cold, the other by warmth
 Blood chemistry
 Receptors in arteries sense blood CO2 levels
 Pain
 Special nerve endings within tissues near the surface
Sensing Pressure & Strectch
 Muscle contraction
 Sensory receptors called
proprioceptors
embedded within muscle
& tendons sense stretch
of muscle
 Touch
 Pressure receptors
buried below skin
 Blood pressure
 Neurons called
baroreceptors in major
arteries
Sensing Chemicals: Taste
 Taste
 Taste buds are located
in raised areas called
papillae
 Food chemicals dissolve
in saliva and contact the
taste cells
Sensing Chemicals: Smell
Smell
Olfactory receptor cells are
embedded in the epithelium of the
nasal passage
These are far more sensitive in
dogs than in humans
Evolution of Balance & Hearing
 Lateral Line and the fish’s sense of
hearing
 Fish are able to sense objects
that reflect pressure waves and
low-frequency vibrations
 The system consists of canals
running the length of the fish’s body
under the skin
 Canals have sensory structures
containing hair cells projecting
into a gelatinous cupula
 Vibrations produce movements
of the cupula
 Hair cells bend and depolarize
associated sensory neurons
Human Sensation of Gravity and Motion
 Receptors in the ear
inform the brain where
the body is in three
dimensions
 Balance
 Gravity is detected by shifting of
otolith sensory receptors
 These are located in a
gelatin-like matrix in the
utricle and saccule
chambers of the inner ear
 Motion
 Motion is detected by the deflection of hair cells by fluid in a
direction opposite to that of motion
 These hair cells are found in the cupula, tent-like assemblies in
the three semicircular canals
Properties of Sound
 Sound is:
 A pressure disturbance (alternating areas of high and low pressure)
originating from a vibrating object
 Composed of areas of rarefaction and compression
 Represented by a sine wave in wavelength, frequency, and amplitude
 Frequency – the number of waves that pass a given point in a given time
 Pitch – perception of different frequencies (we hear from 20–20,000 Hz)
 Amplitude – intensity of a sound measured in decibels (dB)
 Loudness – subjective interpretation of sound intensity
Sensing Sounds: Hearing
 When a sound is heard, air
vibration is detected
 Eardrum membrane is pushed
in and out by waves of air
pressure
 Three small bones (ossicles)
located on other side of
eardrum increase the vibration
force
 Amplified vibration is
transferred to fluid within the
inner ear
 Inner ear chamber is shaped
like a tightly coiled snail shell
and is called cochlea
Sensing Sounds: Hearing
 Cochlea are hair cells that rest on
a membrane running up and
down the chamber
 They are covered by another
membrane
 Sound waves entering the
cochlea cause this membrane
“sandwich” to vibrate
 Bent hair cells send nerve
impulses to brain
 Pitch is determined by different
frequencies causing different
parts of the membrane to vibrate
 Different sensory neurons are
fired
 Sound intensity is determined by
how often the neurons fire
PLAY
Transduction of Sound Waves
The Evolution of Vision
 Vision begins with the capture of light
energy by photoreceptors
 Many invertebrates have simple
visual systems
 Photoreceptors are clustered in an
eyespot
 Perceive light direction but not a
visual image
 Members of four phyla have evolved well-developed, image-forming eyes
 Annelids
 Mollusks
 Arthropods
 Vertebrates
 The eyes are strikingly similar in structure but are believed to have
evolved independently
Eyes in Three Phyla of Animals
Structure of the Vertebrate Eye
 The vertebrate eye works like a lens-focused camera
 Cornea – Transparent covering that
focuses light
 Lens – Completes the focusing
 Ciliary muscles – Change the shape of
the lens
 Iris – Shutter that controls amount of
light
 Pupil – Transparent zone
 Retina – The back surface of the eye
 Contains two types of
photoreceptors: rods and cones
 Fovea – Center of retina
 Produces the sharpest image
How Rods and Cones Work
 Rods are extremely sensitive
to dim light
 Cannot distinguish colors
 Do not detect edges
 Produce poorly defined
images
 Cones can detect color
 Detect edges well
 Produce sharp images
How Light is Converted to a Nerve Impulse
 Pigment in rods and cones are made from carotenoids
 cis-retinal is attached to a protein called opsin
 This light-gathering complex is called rhodopsin
 When light is absorbed by
cis-retinal, it changes shape
to trans-retinal
 This induces a change in the
shape of the opsin protein
 A signal-transduction
pathway is initiated leading
to generation of a nerve
impulse
Color Vision
 Three kinds of cone cells exist, each with its own opsin type
 Differences in opsin shape, affect the flexibility of the attached cisretinal
 This shifts the
wavelength at which it
absorbs light
420 nm – Blue
530 nm – Green
560 nm – Red
Colorblindness
 Colorblindness is a condition in which a person cannot see
all three colors
 Caused by a lack of
one or more types of
cones
 It is inherited as a
sex-linked trait and is
more likely to affect
males
Conveying the Light Information to the Brain
 Rods and cones are at
the rear of the retina,
not front!
 Light passes through
four types of cells
before it reaches them
 Photoreceptor
activation stimulates
bipolar cells, and then
ganglion cells
 Nerve impulse travels
through the optic nerve
to the cerebral cortex
Focusing the Eye
 Focusing for Distant Vision:
 Light from a distance needs little
adjustment for proper focusing
 Far point of vision – the distance
beyond which the lens does not
need to change shape to focus (20
ft.)
 Focusing for Close Vision:
 Accommodation – changing the
lens shape by ciliary muscles to
increase refractory power
 Constriction – the pupillary reflex
constricts the pupils to prevent
divergent light rays from entering
the eye
 Convergence – medial rotation of
the eyeballs toward the object
being viewed
Problems of Refraction
 Normal eye (Emmetropic) – with light focused properly
 Nearsighted (Myopic) – the focal point is in front of the retina
 Corrected with a concave lens
 Farsighted (Hyperopic) – the focal point is behind the retina
 Corrected with a convex lens
Muscles That Move the Eye
 Six strap-like extrinsic eye muscles
 Enable the eye to follow moving objects
 Maintain the shape of the eyeball
 Four rectus muscles originate from the annular ring
 Two oblique muscles move the eye in the vertical plane
Binocular Vision
 Primates and most predators
have eyes on front of the head
 The two fields of vision overlap
allowing the perception of 3-D
images and depth
 Prey animals generally have eyes
located on sides of the head
 This prevents binocular vision but
enlarges the perceptive field
Lacrimal Apparatus
 Consists of the lacrimal gland
and associated ducts
 Lacrimal glands secrete tears
 Tears
 Contain mucus, antibodies,
and lysozyme
 Enter the eye via lacrimal
excretory ducts
 Exit the eye medially via the
lacrimal punctum & lacrimal
canal
 Drain into the nasolacrimal
duct
Other Types of Sensory Reception
Heat
Pit vipers can locate warm
prey, using infrared radiation
Heat-detecting pit organs
Electricity
Used by aquatic
vertebrates to
locate prey and
mates
Magnetism
Eels, sharks and many birds
orient themselves in relation
to the Earth’s magnetic field