Biology in Focus - Chapter 38

CAMPBELL BIOLOGY IN FOCUS
© 2014 Pearson Education, Inc.
Urry • Cain • Wasserman • Minorsky • Jackson • Reece
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
38
Nervous and
Sensory Systems

Overview: Sense and Sensibility
 Gathering, processing, and organizing
information are essential functions of all nervous
systems

Figure 38.1

 The ability to sense and react originated billions of
years ago in prokaryotes
 Hydras, jellies, and cnidarians are the simplest
animals with nervous systems
 In most cnidarians, interconnected nerve cells form
a nerve net, which controls contraction and
expansion of the gastrovascular cavity
Concept 38.1: Nervous systems consist of circuits
of neurons and supporting cells

Figure 38.2
(a) Hydra (cnidarian)
Spinal
cord
(dorsal
nerve
cord)
Brain
(b) Planarian (flatworm)
(c) Insect (arthropod) (d) Salamander (vertebrate)
Sensory
ganglia
Brain
Nerve
cords
Eyespot
Transverse
nerve
Segmental
ganglia
Brain
Nerve net
Ventral
nerve cord

 In more complex animals, the axons of multiple
nerve cells are often bundled together to form
nerves
 These fibrous structures channel and organize
information flow through the nervous system
 Animals with elongated, bilaterally symmetrical
bodies have even more specialized systems

 Cephalization is an evolutionary trend toward a
clustering of sensory neurons and interneurons at
the anterior
 Nonsegmented worms have the simplest clearly
defined central nervous system (CNS), consisting
of a small brain and longitudinal nerve cords

 Annelids and arthropods have segmentally arranged
clusters of neurons called ganglia
 In vertebrates
 The CNS is composed of the brain and spinal cord
 The peripheral nervous system (PNS) is composed
of nerves and ganglia

Glia
 Glia have numerous functions to nourish, support,
and regulate neurons
 Embryonic radial glia form tracks along which newly
formed neurons migrate
 Astrocytes (star-shaped glial cells) induce cells
lining capillaries in the CNS to form tight junctions,
resulting in a blood-brain barrier

Figure 38.3
Ependymal
cell
Neuron
CNS PNS
Oligodendrocyte
Schwann cell
Microglial cell
VENTRICLE
Cilia
Capillary
Astrocytes
Intermingling of
astrocytes with
neurons (blue)
LM
50µm

Figure 38.3a
Astrocytes
Intermingling of
astrocytes with
neurons (blue)
LM
50µm

Organization of the Vertebrate Nervous System
 The spinal cord runs lengthwise inside the vertebral
column (the spine)
 The spinal cord conveys information to and from the
brain
 It can also act independently of the brain as part of
simple nerve circuits that produce reflexes, the
body’s automatic responses to certain stimuli

Figure 38.4
Spinal cord
Central nervous
system (CNS)
Peripheral nervous
system (PNS)
Cranial
nerves
Spinal
nerves
Ganglia
outside
CNS
Brain

 The brain and spinal cord contain
 Gray matter, which consists mainly of neuron cell
bodies and glia
 White matter, which consists of bundles of
myelinated axons

 The CNS contains fluid-filled spaces called ventricles
in the brain and the central canal in the spinal cord
 Cerebrospinal fluid is formed in the brain and
circulates through the ventricles and central canal
and drains into the veins
 It supplies the CNS with nutrients and hormones and
carries away wastes

The Peripheral Nervous System
 The PNS transmits information to and from the CNS
and regulates movement and the internal
environment
 In the PNS, afferent neurons transmit information to
the CNS and efferent neurons transmit information
away from the CNS

Figure 38.5
Afferent neurons
Sensory
receptors
Internal
and external
stimuli
Autonomic
nervous system
Motor
system
Control of
skeletal muscle
Sympathetic
division
Enteric
division
Control of smooth muscles,
cardiac muscles, glands
Parasympathetic
division
Efferent neurons
Peripheral Nervous
System
Central Nervous
System
(information processing)

 The PNS has two efferent components: the motor
system and the autonomic nervous system
 The motor system carries signals to skeletal
muscles and can be voluntary or involuntary
 The autonomic nervous system regulates smooth
and cardiac muscles and is generally involuntary

 The autonomic nervous system has sympathetic,
parasympathetic, and enteric divisions
 The enteric division controls activity of the
digestive tract, pancreas, and gallbladder

 The sympathetic division regulates the “fight-or-
flight” response
 The parasympathetic division generates opposite
responses in target organs and promotes calming
and a return to “rest and digest” functions

Concept 38.2: The vertebrate brain is
regionally specialized
 The human brain contains 100 billion neurons
 These cells are organized into circuits that can
perform highly sophisticated information processing,
storage, and retrieval

Figure 38.6a

Figure 38.6b
Medulla
oblongata
Embryonic brain regions Brain structures in child and adult
Forebrain
Hindbrain
Midbrain
Telencephalon
Myelencephalon
Metencephalon
Forebrain
Hindbrain
Midbrain
Diencephalon
Cerebrum (includes cerebral cortex,
white matter, basal nuclei)
Medulla oblongata (part of brainstem)
Pons (part of brainstem), cerebellum
Midbrain (part of brainstem)
Diencephalon (thalamus,
hypothalamus, epithalamus)
Telencephalon
Myelencephalon
Metencephalon
Diencephalon
Mesencephalon
Mesencephalon
Embryo at 1 month Embryo at 5 weeks
Spinal
cord
Child
Diencephalon
Midbrain
Cerebellum
Spinal cord
Pons
Cerebrum

Figure 38.6ba
Embryonic brain regions
Brain structures in child and adult
Forebrain
Hindbrain
Midbrain
Telencephalon
Myelencephalon
Metencephalon
Diencephalon
Cerebrum (includes cerebral cortex, white matter,
basal nuclei)
Medulla oblongata (part of brainstem)
Pons (part of brainstem), cerebellum
Midbrain (part of brainstem)
Diencephalon (thalamus, hypothalamus, epithalamus)
Mesencephalon

Figure 38.6bb
Forebrain
Hindbrain
Midbrain
Telencephalon
Myelencephalon
Metencephalon
Diencephalon
Mesencephalon
Embryo at 1 month Embryo at 5 weeks
Spinal
cord

Figure 38.6bc
Medulla
oblongata
Child
Diencephalon
Midbrain
Cerebellum
Spinal cord
Pons
Cerebrum

Figure 38.6c
Basal
nuclei
Cerebellum
Cerebrum
Corpus
callosum
Cerebral
cortex
Left cerebral
hemisphere
Right cerebral
hemisphere
Adult brain viewed from the rear

Figure 38.6d
Diencephalon
Thalamus
Pineal gland
Hypothalamus
Pituitary gland
Spinal cord
Brainstem
Midbrain
Medulla
oblongata
Pons

Arousal and Sleep
 Arousal is a state of awareness of the external world
 Sleep is a state in which external stimuli are
received but not consciously perceived
 Arousal and sleep are controlled in part by clusters
of neurons in the midbrain and pons

 Sleep is an active state for the brain and is regulated
by the biological clock and regions of the forebrain,
which regulate the intensity and duration of sleep
 Some animals have evolutionary adaptations that
allow for substantial activity during sleep
 For example, in dolphins, only one side of the brain
is asleep at a time

Figure 38.7
Location
Left
hemisphere
Right
hemisphere
Time: 1 hour
Key
Time: 0 hours
Low-frequency waves characteristic of sleep
High-frequency waves characteristic of wakefulness

Biological Clock Regulation
 Cycles of sleep and wakefulness are examples of
circadian rhythms, daily cycles of biological activity
 Mammalian circadian rhythms rely on a biological
clock, a molecular mechanism that directs periodic
gene expression
 Biological clocks are typically synchronized to light
and dark cycles and maintain a roughly 24-hour
cycle

 In mammals, circadian rhythms are coordinated by
a group of neurons in the hypothalamus called the
suprachiasmatic nucleus (SCN)
 The SCN acts as a pacemaker, synchronizing the
biological clock

Emotions
 Generation and experience of emotions involve
many brain structures including the amygdala,
hippocampus, and parts of the thalamus
 These structures are grouped as the limbic system

 Generation and experience of emotion also require
interaction between the limbic system and sensory
areas of the cerebrum
 The brain structure that is most important for
emotional memory is the amygdala

Figure 38.8
Thalamus
Hypothalamus
Amygdala
Olfactory
bulb
Hippocampus

The Brain’s Reward System and Drug Addiction
 The brain’s reward system provides motivation for
activities that enhance survival and reproduction
 The brain’s reward system is dramatically affected
by drug addiction
 Drug addiction is characterized by compulsive
consumption and an inability to control intake

 Addictive drugs such as cocaine, amphetamine,
heroin, alcohol, and tobacco enhance the activity of
the dopamine pathway
 Drug addiction leads to long-lasting changes in the
reward circuitry that cause craving for the drug

Figure 38.9
Inhibitory neuronNicotine
stimulates
dopamine-
releasing
VTA neuron.
Cerebral
neuron of
reward
pathway
Dopamine-
releasing
VTA neuron
Opium and heroin
decrease activity
of inhibitory
neuron.
Cocaine and
amphetamines
block removal
of dopamine
from synaptic
cleft.
Reward
system
response

Functional Imaging of the Brain
 Functional imaging methods are transforming our
understanding of normal and diseased brains
 In positron-emission tomography (PET) an injection
of radioactive glucose enables a display of metabolic
activity

 In functional magnetic resonance imaging, fMRI, the
subject lies with his or her head in the center of a
large, doughnut-shaped magnet
 Brain activity is detected by changes in local oxygen
concentration
 Applications of fMRI include monitoring recovery
from stroke, mapping abnormalities in migraine
headaches, and increasing the effectiveness of brain
surgery

Figure 38.10
Nucleus accumbens Amygdala
Happy music Sad music

Figure 38.10a
Nucleus accumbens
Happy music

Figure 38.10b
Amygdala
Sad music

Concept 38.3: The cerebral cortex controls
voluntary movement and cognitive functions
 The cerebrum is essential for language, cognition,
memory, consciousness, and awareness of our
surroundings
 The cognitive functions reside mainly in the cortex,
the outer layer
 Four regions, or lobes (frontal, temporal, occipital,
and parietal), are landmarks for particular functions

Figure 38.11
Frontal lobe
Temporal lobe
Occipital lobe
Parietal lobe
Cerebellum
Motor cortex (control
of skeletal muscles)
Somatosensory cortex
(sense of touch)
Wernicke’s area
(comprehending language)
Auditory cortex
(hearing)
Broca’s area
(forming speech)
Prefrontal cortex
(decision
making,
planning)
Sensory association
cortex (integration
of sensory
information)
Visual association
cortex (combining
images and object
recognition)
Visual cortex
(processing visual
stimuli and pattern
recognition)

Language and Speech
 The mapping of cognitive functions within the cortex
began in the 1800s
 Broca’s area, in the left frontal lobe, is active when
speech is generated
 Wernicke’s area, in the posterior of the left frontal
lobe, is active when speech is heard

Figure 38.12
Hearing
words
Seeing
words
Speaking
words
Generating
words
Max
Min

Lateralization of Cortical Function
 The left side of the cerebrum is dominant regarding
language, math, and logical operations
 The right hemisphere is dominant in recognition of
faces and patterns, spatial relations, and nonverbal
thinking
 The establishment of differences in hemisphere
function is called lateralization

 The two hemispheres exchange information through
the fibers of the corpus callosum
 Severing this connection results in a “split brain”
effect, in which the two hemispheres operate
independently

Information Processing
 The cerebral cortex receives input from sensory
organs and somatosensory receptors
 Somatosensory receptors provide information
about touch, pain, pressure, temperature, and the
position of muscles and limbs
 The thalamus directs different types of input to
distinct locations

Frontal Lobe Function
 Frontal lobe damage may impair decision making
and emotional responses but leave intellect and
memory intact
 The frontal lobes have a substantial effect on
“executive functions”

Figure 38.UN02

Evolution of Cognition in Vertebrates
 In nearly all vertebrates, the brain has the same
number of divisions
 The hypothesis that higher order reasoning requires
a highly convoluted cerebral cortex has been
experimentally refuted
 The anatomical basis for sophisticated information
processing in birds (without a highly convoluted
neocortex) appears to be a cluster of nuclei in the
top or outer portion of the brain (pallium)

Figure 38.13
Cerebrum
(including pallium)
Thalamus
Midbrain
Hindbrain
Cerebellum
Cerebrum (including
cerebral cortex)
Thalamus
Midbrain
Hindbrain Cerebellum
(a) Songbird brain
(b) Human brain

Neural Plasticity
 Neural plasticity is the capacity of the nervous
system to be modified after birth
 Changes can strengthen or weaken signaling at a
synapse
 Autism, a developmental disorder, involves a
disruption of activity-dependent remodeling at
synapses
 Children with autism display impaired communication
and social interaction, as well as stereotyped and
repetitive behaviors

Figure 38.14
(a) Synapses are strengthened or weakened in response to
activity.
(b) If two synapses are often active at the same time, the strength
of the postsynaptic response may increase at both synapses.
N1
N2
N1
N2

Memory and Learning
 Neural plasticity is essential to formation of memories
 Short-term memory is accessed via the
hippocampus
 The hippocampus also plays a role in forming long-
term memory, which is stored in the cerebral cortex
 Some consolidation of memory is thought to occur
during sleep

Concept 38.4: Sensory receptors transduce
stimulus energy and transmit signals to the
central nervous system
 Much brain activity begins with sensory input
 A sensory receptor detects a stimulus, which alters
the transmission of action potentials to the CNS
 The information is decoded in the CNS, resulting in
a sensation

Sensory Reception and Transduction
 A sensory pathway begins with sensory reception,
detection of stimuli by sensory receptors
 Sensory receptors, which detect stimuli, interact
directly with stimuli, both inside and outside the body

 Sensory transduction is the conversion of stimulus
energy into a change in the membrane potential of a
sensory receptor
 This change in membrane potential is called a
receptor potential
 Receptor potentials are graded; their magnitude
varies with the strength of the stimulus

Figure 38.15
(a) Receptor is afferent neuron.
Afferent
neuron
(b) Receptor regulates afferent neuron.
Sensory
receptor
To CNS
Stimulus
Afferent
neuron
Receptor
protein
To CNS
Sensory
receptor
cell Stimulus
Neurotransmitter
Stimulus
leads to
neuro-
transmitter
release.

Transmission
 Sensory information is transmitted as nerve impulses
or action potentials
 Neurons that act directly as sensory receptors
produce action potentials and have an axon that
extends into the CNS
 Non-neuronal sensory receptors form chemical
synapses with sensory neurons
 They typically respond to stimuli by increasing the
rate at which the sensory neurons produce action
potentials

 The response of a sensory receptor varies with
intensity of stimuli
 If the receptor is a neuron, a larger receptor potential
results in more frequent action potentials
 If the receptor is not a neuron, a larger receptor
potential causes more neurotransmitter to be
released

Figure 38.16
Gentle pressure
Sensory
receptor
More pressure
Low frequency of
action potentials
High frequency of
action potentials

Perception
 Perception is the brain’s construction of stimuli
 Action potentials from sensory receptors travel
along neurons that are dedicated to a particular
stimulus
 The brain thus distinguishes stimuli, such as light or
sound, solely by the path along which the action
potentials have arrived

Amplification and Adaptation
 Amplification is the strengthening of stimulus energy
by cells in sensory pathways
 Sensory adaptation is a decrease in
responsiveness to continued stimulation

Types of Sensory Receptors
 Based on energy transduced, sensory receptors fall
into five categories
 Mechanoreceptors
 Electromagnetic receptors
 Thermoreceptors
 Pain receptors
 Chemoreceptors

Mechanoreceptors
 Mechanoreceptors sense physical deformation
caused by stimuli such as pressure, touch, stretch,
motion, and sound
 Some animals use mechanoreceptors to get a feel
for their environment
 For example, cats and many rodents have sensitive
whiskers that provide detailed information about
nearby objects

Electromagnetic Receptors
 Electromagnetic receptors detect electromagnetic
energy such as light, electricity, and magnetism
 Some snakes have very sensitive infrared receptors
that detect body heat of prey against a colder
background
 Many animals apparently migrate using Earth’s
magnetic field to orient themselves

Figure 38.17
(b) Beluga whales
(a) Rattlesnake
Infrared
receptor
Eye

Figure 38.17a
(a) Rattlesnake
Infrared
receptor
Eye

Figure 38.17b
(b) Beluga whales

 Thermoreceptors detect heat and cold
 In humans, thermoreceptors in the skin and anterior
hypothalamus send information to the body’s
thermostat in the posterior hypothalamus
Thermoreceptors

Pain Receptors
 In humans, pain receptors, or nociceptors, detect
stimuli that reflect conditions that could damage
animal tissues
 By triggering defensive reactions, such as withdrawal
from danger, pain perception serves an important
function
 Chemicals such as prostaglandins worsen pain by
increasing receptor sensitivity to noxious stimuli;
aspirin and ibuprofen reduce pain by inhibiting
synthesis of prostaglandins

Chemoreceptors
 General chemoreceptors transmit information about
the total solute concentration of a solution
 Specific chemoreceptors respond to individual kinds
of molecules
 Olfaction (smell) and gustation (taste) both depend
on chemoreceptors
 Smell is the detection of odorants carried in the air,
and taste is detection of tastants present in solution

 Humans can distinguish thousands of different odors
 Humans and other mammals recognize just five
types of tastants: sweet, sour, salty, bitter, and
umami
 Taste receptors are organized into taste buds,
mostly found in projections called papillae
 Any region of the tongue can detect any of the five
types of taste

Figure 38.18
Tongue
Taste
buds
Sensory
receptor cells
Key
Sensory
neuron
Taste
pore
Food
molecules
Taste bud
Papillae
Papilla
Umami
Bitter
Sour
Salty
Sweet

Concept 38.5: The mechanoreceptors responsible
for hearing and equilibrium detect moving fluid
or settling particles
 Hearing and perception of body equilibrium are
related in most animals
 For both senses, settling particles or moving fluid is
detected by mechanoreceptors

Sensing of Gravity and Sound in Invertebrates
 Most invertebrates maintain equilibrium using
mechanoreceptors located in organs called
statocysts
 Statocysts contain mechanoreceptors that detect the
movement of granules called statoliths
 Most insects sense sounds with body hairs that
vibrate or with localized vibration-sensitive organs
consisting of a tympanic membrane stretched over
an internal chamber

Figure 38.19
Statolith
Ciliated
receptor
cells
Sensory
nerve fibers
(axons)
Cilia

Hearing and Equilibrium in Mammals
 In most terrestrial vertebrates, sensory organs for
hearing and equilibrium are closely associated in
the ear

Figure 38.20
Outer ear Inner ear
Middle
ear
Malleus
Skull
bone
Incus
Stapes Semicircular
canals
Auditory nerve
to brain
Auditory
canal
Tympanic
membrane
Oval
window
Round
window
Eustachian
tube
Cochlea
Pinna
Auditory
nerve
Cochlear
duct
Organ
of Corti
Vestibular
canal
Tympanic
canal
Bone
To
auditory
nerve
Tectorial
membrane
Basilar
membrane
Axons of
sensory neurons
Hair
cells
Bundled hairs projecting from a hair cell
(SEM)
1µm

Figure 38.20a
Outer ear Inner ear
Middle
ear
Malleus
Skull
bone
Incus
Stapes Semicircular
canals
Auditory nerve
to brain
Auditory
canal
Tympanic
membrane
Oval
window
Round
window
Eustachian
tube
Cochlea
Pinna

Figure 38.20b
Auditory
nerve
Cochlear
duct
Organ
of Corti
Vestibular
canal
Tympanic
canal
Bone

Figure 38.20c
To
auditory
nerve
Tectorial
membrane
Basilar
membrane
Axons of
sensory neurons
Hair
cells

Figure 38.20d
Bundled hairs projecting from a hair cell (SEM)
1µm

Hearing
 Vibrating objects create pressure waves in the air,
which are transduced by the ear into nerve impulses,
perceived as sound in the brain
 The tympanic membrane vibrates in response to
vibrations in air
 The three bones of the middle ear transmit the
vibrations of moving air to the oval window on the
cochlea

 The vibrations of the bones in the middle ear create
pressure waves in the fluid in the cochlea that travel
through the vestibular canal
 Pressure waves in the canal cause the basilar
membrane to vibrate and attached hair cells to
vibrate
 Bending of hair cells causes ion channels in the
hair cells to open or close, resulting in a change in
auditory nerve sensations that the brain interprets
as sound

Figure 38.21
Receptor
potential
More
neuro-
trans-
mitter
Time (sec)
Membrane
potential(mV)
Signal
0 1 2 3 4 5 6 7
(a) Bending of hairs in one direction
−50
−70
0
−70
Receptor
potential
Less
neuro-
trans-
mitter
Time (sec)
Membrane
potential(mV)
Signal
0 1 2 3 4 5 6 7
(b) Bending of hairs in other direction
−50
−70
0
−70

 The fluid waves dissipate when they strike the round
window at the end of the vestibular canal

 The ear conveys information about
 Volume, the amplitude of the sound wave
 Pitch, the frequency of the sound wave
 The cochlea can distinguish pitch because the
basilar membrane is not uniform along its length
 Each region of the basilar membrane is tuned to a
particular vibration frequency

Equilibrium
 Several organs of the inner ear detect body
movement, position, and balance
 The utricle and saccule contain granules called
otoliths that allow us to perceive position relative to
gravity or linear movement
 Three semicircular canals contain fluid and can
detect angular movement in any direction

Figure 38.22
Semicircular
canals
Vestibular
nerve
Vestibule
Saccule
Utricle
Fluid
flow
Nerve
fibers
Hair
cell
Hairs
Cupula
Body movement
PERILYMPH

Concept 38.6: The diverse visual receptors of
animals depend on light-absorbing pigments
 The organs used for vision vary considerably among
animals, but the underlying mechanism for capturing
light is the same

Evolution of Visual Perception
 Light detectors in animals range from simple
clusters of cells that detect direction and intensity of
light to complex organs that form images
 Light detectors all contain photoreceptors, cells
that contain light-absorbing pigment molecules

Light-Detecting Organs
 Most invertebrates have a light-detecting organ
 One of the simplest light-detecting organs is that of
planarians
 A pair of ocelli called eyespots are located near the
head
 These allow planarians to move away from light and
seek shaded locations

Figure 38.23
LIGHT
DARK
Ocellus
Ocellus
Visual
pigment
Photoreceptor
Nerve to
brain
Screening
pigment

 Insects and crustaceans have compound eyes,
which consist of up to several thousand light
detectors called ommatidia
 Compound eyes are very effective at detecting
movement
Compound Eyes

Figure 38.24
2 mm

 Single-lens eyes are found in some jellies,
polychaetes, spiders, and many molluscs
 They work on a camera-like principle: the iris
changes the diameter of the pupil to control how
much light enters
 The eyes of all vertebrates have a single lens
Single-Lens Eyes

The Vertebrate Visual System
 Vision begins when photons of light enter the eye
and strike the rods and cones
 However, it is the brain that “sees”
Animation: Near and Distance Vision

Figure 38.25a
Sclera Choroid
Retina
Fovea
Cornea
Suspensory
ligament
Iris
Optic
nerve
Pupil
Aqueous
humor
Lens
Optic
disk
Vitreous humor
Central
artery and
vein of
the retina
Retina
Optic
nerve
fibers
Rod Cone
Neurons
Photoreceptors
Ganglion
cell
Amacrine
cell
Bipolar
cell
Horizontal
cell Pigmented
epithelium

Figure 38.25aa
Sclera Choroid
Retina
Fovea
Cornea
Suspensory
ligament
Iris
Optic
nerve
Pupil
Aqueous
humor
Lens
Optic
disk
Vitreous humor
Central
artery and
vein of
the retina

Figure 38.25ab
Retina
Optic
nerve
fibers
Rod Cone
Neurons
Photoreceptors
Ganglion
cell
Amacrine
cell
Bipolar
cell
Horizontal
cell Pigmented
epithelium

Figure 38.25b
DisksSynaptic
terminal
Cell
body
Outer
segment
Cone
Cone
Rod
Rod
CYTOSOL
Retinal:
cis isomer
Retinal:
trans isomer
EnzymesLight
Retinal
Opsin
Rhodopsin
INSIDE OF DISK

Figure 38.25ba
DisksSynaptic
terminal
Cell
body
Outer
segment
Cone
Cone
Rod
Rod

Figure 38.25bb
CYTOSOL
Retinal:
cis isomer
Retinal:
trans isomer
EnzymesLight
Retinal
Opsin
Rhodopsin
INSIDE OF DISK

Figure 38.25bc
Retinal:
cis isomer
Retinal:
trans isomer
EnzymesLight

Figure 38.25bd
Cone
Rod

Sensory Transduction in the Eye
 Transduction of visual information to the nervous
system begins when light induces the conversion
of cis-retinal to trans-retinal
 Trans-retinal activates rhodopsin, which activates
a G protein, eventually leading to hydrolysis of cyclic
GMP

 When cyclic GMP breaks down, Na+
channels close
 This hyperpolarizes the cell
 The signal transduction pathway usually shuts off
again as enzymes convert retinal back to the cis form

Figure 38.26
Active
rhodopsin
Transducin
Light
Inactive
rhodopsin
Phospho-
diesterase
Disk
membrane
Membrane
potential (mV)
Plasma
membrane
Hyper-
polarization
EXTRA-
CELLULAR
FLUID
INSIDE OF DISK
CYTOSOL
Dark Light
Time
GMP
cGMP
Na+
Na+
−40
−70
0

Processing of Visual Information in the Retina
 Processing of visual information begins in the retina
 In the dark, rods and cones release the
neurotransmitter glutamate into synapses with
neurons called bipolar cells
 Bipolar cells are either hyperpolarized or depolarized
in response to glutamate

 In the light, rods and cones hyperpolarize, shutting
off release of glutamate
 The bipolar cells are then either depolarized or
hyperpolarized

 Signals from rods and cones can follow several
pathways in the retina
 A single ganglion cell receives information from an
array of rods and cones, each of which responds to
light coming from a particular location
 The rods and cones that feed information to one
ganglion cell define a receptive field, the part of the
visual field to which the ganglion cell can respond
 A smaller receptive field typically results in a sharper
image

 The optic nerves meet at the optic chiasm near the
cerebral cortex
 Sensations from the left visual field of both eyes are
transmitted to the right side of the brain
 Sensations from the right visual field are transmitted
to the left side of the brain
 It is estimated that at least 30% of the cerebral cortex
takes part in formulating what we actually “see”
Processing of Visual Information in the Brain

Color Vision
 Among vertebrates, most fish, amphibians, and
reptiles, including birds, have very good color vision
 Humans and other primates are among the minority
of mammals with the ability to see color well
 Mammals that are nocturnal usually have a high
proportion of rods in the retina

 In humans, perception of color is based on three
types of cones, each with a different visual pigment:
red, green, or blue
 These pigments are called photopsins and are
formed when retinal binds to three distinct opsin
proteins

 Abnormal color vision results from alterations in the
genes for one or more photopsin proteins
 The genes for the red and green pigments are
located on the X chromosome
 A mutation in one copy of either gene can disrupt
color vision in males

 The brain processes visual information and controls
what information is captured
 Focusing occurs by changing the shape of the lens
 The fovea is the center of the visual field and
contains no rods but a high density of cones
The Visual Field

Figure 38.UN01
Wild-type
hamster
Wild-type
hamster with
SCN from
τ hamster
τ hamster
τ hamster
with SCN
from wild-type
hamster
After surgery
and transplant
Before
procedures
Circadianperiod(hours)
24
23
22
21
20
19

Figure 38.UN03
Cerebral
cortex
Forebrain
Hindbrain
Midbrain
Thalamus
Pituitary gland
Hypothalamus
Spinal
cord
Cerebellum
Pons
Cerebrum
Medulla
oblongata

Biology in Focus - Chapter 38

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Biology in Focus - Chapter 38

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