Sensory Perception
Chapter 34
Impacts, Issues
A Whale of a Dilemma
Whales communicate and sense the world
around them using sound – a problem when
ships and defense-systems testing flood the
seas with noise
34.1 Overview of Sensory Pathways
Sensory receptors determine what stimuli an
animal can detect and respond to
Different kinds of sensory receptors produce
action potentials in response to different types of
stimuli
Sensory Receptor Diversity
Mechanoreceptors: mechanical energy
• Body position or acceleration
• Touch or stretching
• Pressure waves (hearing)
Pain receptors (nociceptors): tissue damage
• Some reflexes
Osmoreceptors: change in solute concentration
Sensory Receptor Diversity
Thermoreceptors: specific temperature or
temperature change
Chemoreceptors: specific solutes dissolved in
fluid (also function in smell)
Photoreceptors: light energy
• Including UV receptors in insects
Sensory Receptors
Mechanoreceptors in bat hearing,
thermoreceptors in snakes
Photoreceptors and UV Light
From Sensing to Sensation
In animals with a brain, input from sensory
neurons can give rise to sensation
The brain determines stimulus location and
strength by which axons respond, how many
respond, and frequency of action potentials
In sensory adaptation, sensory neurons cease
firing under continued stimulation
Sensory Information in Action
34.1 Key Concepts
How Sensory Pathways Work
Sensory receptors detect specific stimuli
Different animals have receptors for different
stimuli
Information from sensory receptors becomes
encoded in the number and frequency of action
potentials sent to the brain along particular
nerve pathways
34.2 Somatic and Visceral Sensations
Somatic sensations are signals from receptors in
the skin, joints, and skeletal muscles
• They travel along sensory neuron axons, to the
spinal cord, to the somatosensory cortex
Visceral sensations are signals from sensory
neurons in walls of internal organs
• Relayed to the spinal cord and the brain
The Somatosensory Cortex
Somatosensory cortex
• Part of the cerebral cortex
• Like the motor cortex, neurons are mapped to a
plan of the body
Example: Skin receptors
• Free nerve endings around roots of hairs,
Meissner’s corpuscles (touch), Pacinian capsules
(pressure), Ruffini endings, bulb of Krause
Body Regions in
the Somatosensory Cortex
Sensory Receptors in Human Skin
Pain
Pain
• Perception of a somatic or visceral tissue injury
• Injured cells release chemicals that stimulate pain
receptors, affected by neuromodulators
Referred pain
• Because pain signals usually originate with
somatic sources, the brain sometimes
misinterprets visceral pain as coming from the
skin or joints
Referred Pain
34.2 Key Concepts
Somatic and Visceral Senses
Somatic sensations such as touch are easily
localized and stem from receptors in the skin,
muscles, or near joints
Visceral sensations, such as a feeling of fullness
in your stomach, are less easily pinpointed; they
arise from receptors in the walls of internal
organs
34.3 Sampling the Chemical World
Both smell and taste begin when chemoreceptors
are stimulated by the binding of specific dissolved
molecules
Sense of Smell
Olfaction (sense of smell)
• Olfactory receptors detect water-soluble or
volatile chemicals, send signals to olfactory bulbs
• Olfactory nerves send signals to cerebral cortex
Pheromone
• A type of signaling molecule secreted by an
individual that affects others of the same species
• Detected by a vomeronasal organ
Sense of Smell
Sense of Taste
Taste receptors detect chemicals dissolved in
fluid, and have different structures and locations
in different animals
Humans have taste buds (in epithelial papillae
on the tongue) that detect five main sensations:
sweet, sour, salty, bitter, and umami
Sense of Taste
34.3 Key Concepts
Chemical Senses
The senses of smell and taste require
chemoreceptors, which bind molecules of
specific substances dissolved in the fluid bathing
them
34.4 Sense of Balance
Organs inside your inner ear are essential to
maintaining posture and a sense of balance
Somatic sensory receptors also contribute to
balance
Organs of Equilibrium
Organs of equilibrium
• Parts of sensory systems that monitor the body’s
positions and motions
Vestibular apparatus
• Contains organs of equilibrium in vertebrates
• Semicircular canals, sacs, saccule and utricle
Hair cells
• Mechanoreceptors with modified cilia
Organs of Equilibrium in the Inner Ear
34.5 Sense of Hearing
Hearing
• Perception of sound (mechanical energy)
Sound waves
• Human ears collect, amplify, and sort out sound
waves (pressure waves traveling through air)
• Wave amplitude determines loudness
• Wave frequency determines pitch
Wave Properties
The Vertebrate Ear
Outer ear gathers sound
Middle ear amplifies and transmits air waves
• Vibrations are transmitted from eardrum
(tympanic membrane), to hammer, anvil and
stirrup bones, to oval window
Inner ear (vestibular apparatus and cochlea)
• Cochlea contains organs of Corti with hairs cells
that generate action potentials
How Humans Hear
Fig. 34-12 (c-e), p. 585
34.6 Noise Pollution
Noise louder than 90 decibels (chainsaw, rock
concert, iPod earbuds at high volume) damages
hair cells in the cochlea
34.4-34.6 Key Concepts
Balance and Hearing
Organs in the ear function in balance and in hearing
The inner ear’s vestibular apparatus detects body position and motion
The outer and middle ear collect and amplify sound waves
Mechanoreceptors in the inner ear send signals about sound to the brain
34.7 Sense of Vision
Vision
• Detection of light in a way that provides a mental
image of objects in the environment
• Requires eyes and a brain with ability to interpret
visual stimuli
Eyes
• Sensory organs that hold photoreceptors
• Pigment molecules absorb light energy
Simple Vision
Some invertebrates (earthworms) detect light
with photoreceptors but do not form an image
Compound eyes (insects) produce a mosaic
image that is fuzzy but sensitive to motion
• Many individual units, each with its own lens
Lens: A transparent body that bends light rays
to converge on photoreceptors
Detailed Vision
Camera eye (cephalopods, vertebrates)
• Provides a richly detailed image
• Has an adjustable opening and a lens that
focuses light on a photoreceptor-rich retina
Retina
• A tissue densely packed with photoreceptors
• Signals travel from photoreceptors along optic
tracts to the brain
Camera Eye (Octopus)
Depth Perception: Forward-Facing Eyes
34.8 A Closer Look at the Human Eye
The human eye is a multilayered structure
surrounded by protective structures
• Bony orbit, eyelids, eyelashes, tears
• Mucous membrane (conjunctiva)
Outer layer of the eye
• Transparent cornea in front
• Elsewhere covered by white, fibrous sclera
A Closer Look at the Human Eye
Middle layer
• Choroid darkens the eye
• Iris controls the size of the pupil
• Ciliary body holds the lens in place
Two internal chambers
• Anterior: aqueous humor
• Inner eye: vitreous body and retina
Retinal Stimulation
Both the cornea and lens bend incoming light,
producing an upside-down image on the
photoreceptor-rich retina at the back of the eye
Focusing Mechanisms
Visual accommodation
• Changing shape or position of a lens so incoming
light falls on the retina, not in front or behind it
Ciliary muscle adjusts the shape of the lens
• Contracts or relaxes to focus on near or distant
objects
Focusing Mechanisms
34.9 From the Retina to the Visual Cortex
Processing of visual signals begins in the retina
Fovea
• Area of the retina dense with photoreceptors
• Normally, most light rays focus on the fovea
Examining the Retina
Cells of the Retina
Interneurons involved in vision processing:
• Amacrine cells, horizontal cells, bipolar cells
Photoreceptors:
• Rod cells detect dim light, coarse movement
• Cone cells detect sharp, color vision
Organization of the Retina
How Photoreceptors Work
Rod cells contain rhodopsin (opsin and retinal)
which responds to blue-green light
Humans have three types of cone cells– red,
green and blue – each with a different opsin
Photon absorption by opsins leads indirectly to
action potentials in other cells
Rod Cells and Cone Cells
Visual Processing
Interneurons that overlie photoreceptors receive
signals which converge on ganglion cells at the
start of the optic nerve (blind spot)
Signals cross over to opposite brain regions
(lateral geniculate nucleus) and are processed
Final integration process in the visual cortex
produces visual sensations
Experiment:
Response of Visual Cortex Cells
Flow of Information From Retina to Brain
34.10 Visual Disorders
Abnormalities in eye shape, in the lens, and in
cells of the retina can impair vision
Disorders may be caused by genetic conditions,
age-related changes, nutritional deficits, and
infectious agents
Some Vision Disorders
Color blindness
• X-linked recessive trait
Lack of focus
• Astigmatism, farsightedness, nearsightedness
Macular degeneration
• Loss of photoreceptors in center of visual field
Glaucoma
• Fluid pressure damages blood vessels, cells
Some Vision Disorders
Cataracts
• A clouding of the lens
Nutritional blindness
• Lack of vitamin A to make retinol
Infectious agents
• Bacteria, roundworms, syphilis, amoebas, fungi
Focusing Problems
Vision Disorders:
Macular Degeneration and Cataracts
34.7-34.10 Key Concepts
Vision
Most organisms have light-sensitive pigments,
but vision requires eyes
Vertebrates have an eye that operates like a film
camera; their retina, which has photoreceptors,
is analogous to the film
A sensory pathway starts at the retina and ends
in the visual cortex