Cecie Starr Christine Evers Lisa Starr www.cengage.com/biology/starr

Chapter 30 Sensory Perception

(Sections 30.1 - 30.5) Albia Dugger • Miami Dade College

30.1 A Whale of a Dilemma

• Animals use sensory systems to receive signals from inside and outside the body, decode them, and become aware of touches, sounds, sights, odors, and other sensations • Whales rely heavily on their sense of hearing, and underwater sound pollution puts them at risk • 16 whales beached themselves when the US Navy tested a new sonar system – autopsies revealed blood in their ears and in acoustic fat

A Whale of a Dilemma

30.2 Detecting Stimuli and Forming Perceptions • The sensory portion of a vertebrate nervous system consists of sensory neurons that detect

stimuli

, nerves that carry information about the stimulus to the brain, and brain regions that process the information •

stimulus

• Form of energy that a sensory receptor detects

Sensory Neurons

• Different types of sensory neurons respond to different specific stimuli by producing action potentials • Types of sensory receptors include,

chemoreceptors, thermoreceptors, pain receptors, mechanoreceptors

, and

photoreceptors

Key Terms

•

chemoreceptor

• Sensory receptor that responds to a chemical •

thermoreceptor

• Temperature-sensitive sensory receptor •

pain receptor

• Sensory receptor that responds to tissue damage

Key Terms

•

mechanoreceptor

• Sensory receptor that responds to pressure, position, or acceleration •

photoreceptor

• Sensory receptor that responds to light

Sensory Receptors in the Skin

epidermis

Sensory Receptors in the Skin

detects touch, tissue damage detects heat detects cold detects touch dermis detects strong pressure detects hair movement Fig 30.2, p. 490

Sources of Information About a Stimulus • Three variables allow your brain to evaluate incoming action potentials and determine the location and intensity of a stimulus: 1. The nerve that delivers the action potentials 2. The frequency of action potentials 3. The number of sensory receptors firing

Frequency of Action Potentials

• Action potentials from a mechanoreceptor in skin; the stronger the pressure, the more action potentials per second

Frequency of Action Potentials

Light touch, low firing rate Increased pressure, higher firing rate 0 1 2 3 4 Time (seconds) Fig 30.3, p. 490

Sources of Information About a Stimulus • Stimulus duration also affects response • Continued stimulation of a receptor can lead to

sensory adaptation

, in which sensory neurons cease firing in spite of continued stimulation •

sensory adaptation

• Slowing or cessation of a sensory receptor’s response to an ongoing stimulus

Sensation and Perception

•

Sensation

is the detection of sensory signals;

perception

arises when the brain assigns meaning to those signals •

sensation

• Detection of a stimulus •

perception

• The meaning a brain derives from a sensation

Key Concepts

•

Sensory Pathways

• Sensory systems consist of sensory receptors, nerves that carry signals, and brain regions that receive and process sensory input • • Each type of sensory receptor reacts to a specific stimulus Information about stimuli is encoded in the number and frequency of action potentials

30.3 Somatic and Visceral Sensations • Our brain easily identifies the source of a

somatic sensation

such as touch on skin;

visceral sensations

that originate at receptors in walls of soft organs are less easily pinpointed •

somatic sensations

• Sensations such as touch and pain that arise when sensory neurons in skin, muscle, or joints are activated •

visceral sensations

• Sensations that arise when sensory neurons associated with organs inside body cavities are activated

The Somatosensory Cortex

• The human primary somatosensory cortex is a narrow strip of cerebral cortex that runs from the top of the head to just above the ear • Somatic sensory signals from receptors in skin and skeletal muscles reach sensory areas of the cerebral cortex, where interneurons are organized like maps of individual parts of the body surface

The Somatosensory Cortex

The Somatosensory Cortex

Fig 30.4, p. 491

Muscle Sense

• Sensory receptors report to the somatosensory cortex about touch, pain, and temperature • The fourth somatosensory sense is muscle sense, which relates to the positioning of body parts • Muscle spindles and mechanoreceptors near joints and tendons contribute to muscle sense; the more a muscle stretches, the more frequently receptors fire

Pain

• Injured or distressed body cells release local signaling molecules (histamine and prostaglandins) that stimulate neighboring

pain

receptors • Various neuromodulators (including endorphins and substance P) enhance or lessen pain signals •

pain

• Perception of tissue injury

Key Concepts

•

Somatic and Visceral Sensation

• Somatic sensations include touch, pressure, pain, temperature, and muscle sense • They start at mechanoreceptors in skin, muscles, and near joints • Visceral sensations arise from stimulation of receptors in walls of soft internal organs

Animation: Sensory Receptors in the Human Skin

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BBC Video: Protected by Our Sense of Touch

BBC Video: Using Vision to Replace the Sense of Touch

30.4 Do You See What I See?

• Most animals are sensitive to light, but only those with a camera eye form images as humans do • Eyes are sensory organs that contain a dense array of photoreceptors • Pigment molecules in photoreceptors absorb light energy, which is converted to action potentials and sent to the brain

Requirements for Vision

• Vision requires eyes and a brain with the capacity to interpret visual stimuli • Formation of an image requires an eye with a

lens

•

lens

• Transparent, disk-shaped structure that bends light rays so they fall on an eye’s photoreceptors

Types of Eyes

• Some invertebrates, such as earthworms, have photoreceptors but do not have eyes • Insects have

compound eyes

with many individual units, each with a lens • Cephalopods and vertebrates have

camera eyes

, with an adjustable opening that lets in light, and a single lens that focuses the light on a photoreceptor-rich

retina

Key Terms

•

compound eye

• Eye with many units each having its own lens •

camera eye

• Eye with an adjustable opening and a single lens that focuses light on a retina •

retina

• Layer of eye that contains photoreceptors

Compound Eye of a Deerfly

Compound Eye of a Deerfly

lens crystalline cone cells (usually four) screening pigment photo receptor cell sensory neuron ommatidium Fig 30.5a, p. 492

Camera Eye of Squid (Cephalopod)

Camera Eye of Squid (Cephalopod)

lens retina optic tract Fig 30.5b, p. 492

The Human Eye

• The eyeball is spherical with a three-layered structure: • Outer layer includes a transparent sclera

cornea

and white • Middle layer includes a dark choroid layer, ciliary body, two internal chambers, lens,

iris

and

pupil

• Innermost layer (the retina) contains the photoreceptors

Key Terms

•

cornea

• Clear, protective covering at front of vertebrate eye •

iris

• Circular muscle that adjusts the shape of the pupil to regulate how much light enters the eye •

pupil

• Adjustable opening that allows light into a camera eye

Structure of the Human Eye

sclera choroid

Structure of the Human Eye

retina iris lens pupil cornea aqueous humor ciliary muscle vitreous body fovea optic disk (blind spot) part of optic nerve Fig 30.6, p. 493

Animation: Eye Structure

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Focusing Mechanisms

•

Visual accommodation

is the altering of lens shape to focus on objects at different distances • Contraction of ciliary muscle focuses the lens for reading; relaxation allows the lens to flatten for distance vision •

visual accommodation

• Process of making adjustments to lens shape so light from an object falls on the retina

Visual Accommodation

Visual Accommodation

contracted ciliary muscle relaxed ciliary muscle fibers slack fibers taut A Near vision Contraction of ciliary muscle allows fibers to slacken and the lens becomes fatter and rounder.

B Distance vision Relaxation of ciliary muscle pulls fibers taut stretching the lens so it becomes thinner and flatter.

Fig 30.7, p. 493

Animation: Focusing of Distant and Near Sources of Light

Forming an Image

• The image on the retina is upside down and reversed left to right

–

the brain interprets the image

BBC Video: The Anatomy of Sight

30.5 The Human Retina

• The retina has two types of photoreceptors (rods and cones), each with stacks of membranous disks that contain pigment • Visual pigments (opsins) are derived from vitamin A, which is why vitamin A deficiency can impair vision

Rods Cells

•

Rod cells

detect dim light and are concentrated at the edges of the retina • All rods have the same pigment (rhodopsin), which is most excited by exposure to green-blue light •

rod cell

• • Photoreceptor that is active in dim light Provides coarse perception of image and detects motion

Cone Cells

• Three types of

cone cells

have different forms of the pigment photopsin that absorb red, blue, and green light – they are most abundant in the

fovea

•

cone cell

• Photoreceptor that provides sharp vision and allows detection of color •

fovea

• Retinal region where cone cells are most concentrated

Rods and Cones

Rods and Cones

cone cell stacked, pigmented membrane rod cell A The two types of photoreceptors in the retina Fig 30.9a, p. 494

Vision Processing in the Retina

• Rods and cones lie at the very rear of the retina, beneath layers of signal-processing neurons • When a rod or cone photoreceptor absorbs light, signals flow to neurons in the layer above, which process the signals and send them to ganglion cells • Bundled axons of ganglion cells make up the optic nerve

Structure of the Retina

Structure of the Retina

horizontal cell bipolar cell amacrine cell ganglion cell incoming rays of light rod cell cone cell B Multilayered structure of the retina Fig 30.9b, p. 494

Structure of the Retina

Structure of the Retina

blood vessel start of an optic nerve fovea (region with most cones) C Magnified view of the retina as seen through the pupil Fig 30.9c, p. 494

Animation: Organization of Cells in the Retina

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Vision Processing in the Brain

• Signals from the visual field of each eye travel along an optic nerve to the brain’s opposite hemisphere • Each optic nerve ends in a brain region (lateral geniculate nucleus) that processes the signals • Signals are conveyed to the visual cortex where the final integration process produces visual sensations

Key Concepts

•

Vision

• Vision requires eyes with a dense array of photoreceptors and a brain that integrates signals from them • • The vertebrate eye works like a film camera; a single adjustable opening lets in light A sensory pathway starts at the eye’s retina and ends in the visual cortex

Animation: Pathway to Visual Cortex

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