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