Transcript PowerPoint - Scranton Prep Biology

CAMPBELL BIOLOGY IN FOCUS
Urry • Cain • Wasserman • Minorsky • Jackson • Reece
38
Nervous and
Sensory Systems
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
© 2014 Pearson Education, Inc.
Overview: Sense and Sensibility
 Gathering, processing, and organizing
information are essential functions of all nervous
systems
© 2014 Pearson Education, Inc.
Figure 38.1
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Concept 38.1: Nervous systems consist of circuits
of neurons and supporting cells
 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
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Figure 38.2
Eyespot
Brain
Nerve
cords
Nerve net
Transverse
nerve
(a) Hydra (cnidarian)
(b) Planarian (flatworm)
Brain
Brain
Ventral
nerve cord
Spinal
cord
(dorsal
nerve
cord)
Sensory
ganglia
Segmental
ganglia
(c) Insect (arthropod)
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(d) Salamander (vertebrate)
 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
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 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
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 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
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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
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Figure 38.3
CNS
PNS
Neuron
VENTRICLE
Cilia
Oligodendrocyte
Schwann cell
Microglial cell
Capillary
Ependymal
cell
Astrocytes
50 m
Intermingling of
astrocytes with
neurons (blue)
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LM
Figure 38.3a
Astrocytes
50 m
Intermingling of
astrocytes with
neurons (blue)
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LM
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
© 2014 Pearson Education, Inc.
Figure 38.4
Central nervous
system (CNS)
Brain
Spinal cord
Peripheral nervous
system (PNS)
Cranial
nerves
Ganglia
outside
CNS
Spinal
nerves
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 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
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 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
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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
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Figure 38.5
Central Nervous
System
(information processing)
Peripheral Nervous
System
Afferent neurons
Efferent neurons
Sensory
receptors
Autonomic
nervous system
Motor
system
Control of
skeletal muscle
Internal
and external
stimuli
Sympathetic Parasympathetic
division
division
Enteric
division
Control of smooth muscles,
cardiac muscles, glands
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 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
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 The autonomic nervous system has sympathetic,
parasympathetic, and enteric divisions
 The enteric division controls activity of the
digestive tract, pancreas, and gallbladder
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 The sympathetic division regulates the “fight-orflight” response
 The parasympathetic division generates opposite
responses in target organs and promotes calming
and a return to “rest and digest” functions
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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
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Figure 38.6a
© 2014 Pearson Education, Inc.
Figure 38.6b
Brain structures in child and adult
Embryonic brain regions
Telencephalon
Cerebrum (includes cerebral cortex,
white matter, basal nuclei)
Diencephalon
Diencephalon (thalamus,
hypothalamus, epithalamus)
Mesencephalon
Midbrain (part of brainstem)
Metencephalon
Pons (part of brainstem), cerebellum
Myelencephalon
Medulla oblongata (part of brainstem)
Forebrain
Midbrain
Hindbrain
Midbrain
Hindbrain
Mesencephalon
Metencephalon
Diencephalon
Cerebrum
Diencephalon
Myelencephalon
Midbrain
Pons
Forebrain
Embryo at 1 month
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Telencephalon
Medulla
oblongata
Cerebellum
Spinal cord
Spinal
cord
Embryo at 5 weeks
Child
Figure 38.6ba
Embryonic brain regions
Telencephalon
Forebrain
Diencephalon
Midbrain
Mesencephalon
Metencephalon
Hindbrain
Myelencephalon
Brain structures in child and adult
Cerebrum (includes cerebral cortex, white matter,
basal nuclei)
Diencephalon (thalamus, hypothalamus, epithalamus)
Midbrain (part of brainstem)
Pons (part of brainstem), cerebellum
Medulla oblongata (part of brainstem)
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Figure 38.6bb
Mesencephalon
Midbrain
Hindbrain
Metencephalon
Diencephalon
Forebrain
Embryo at 1 month
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Telencephalon
Myelencephalon
Spinal
cord
Embryo at 5 weeks
Figure 38.6bc
Cerebrum
Diencephalon
Midbrain
Pons
Medulla
oblongata
Cerebellum
Spinal cord
Child
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Figure 38.6c
Left cerebral
hemisphere
Right cerebral
hemisphere
Cerebral
cortex
Corpus
callosum
Cerebrum
Basal
nuclei
Cerebellum
Adult brain viewed from the rear
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Figure 38.6d
Diencephalon
Thalamus
Pineal gland
Hypothalamus
Pituitary gland
Brainstem
Midbrain
Pons
Medulla
oblongata
Spinal cord
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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
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 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
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Figure 38.7
Key
Low-frequency waves characteristic of sleep
High-frequency waves characteristic of wakefulness
Location
Left
hemisphere
Right
hemisphere
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Time: 0 hours
Time: 1 hour
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
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 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
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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
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 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
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Figure 38.8
Thalamus
Hypothalamus
Olfactory
bulb
Amygdala
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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
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 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
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Figure 38.9
Nicotine
stimulates
dopaminereleasing
VTA neuron.
Inhibitory neuron
Dopaminereleasing
VTA neuron
Opium and heroin
decrease activity
of inhibitory
neuron.
Cocaine and
amphetamines
block removal
of dopamine
from synaptic
cleft.
Cerebral
neuron of
reward
pathway
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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
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 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
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Figure 38.10
Nucleus accumbens
Happy music
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Amygdala
Sad music
Figure 38.10a
Nucleus accumbens
Happy music
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Figure 38.10b
Amygdala
Sad music
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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
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Figure 38.11
Motor cortex (control
of skeletal muscles)
Frontal lobe
Somatosensory cortex
(sense of touch)
Parietal lobe
Prefrontal cortex
(decision
making,
planning)
Broca’s area
(forming speech)
Temporal lobe
Auditory cortex
(hearing)
Cerebellum
Wernicke’s area
(comprehending language)
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Sensory association
cortex (integration
of sensory
information)
Visual association
cortex (combining
images and object
recognition)
Occipital lobe
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
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Figure 38.12
Max
Hearing
words
Seeing
words
Min
Speaking
words
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Generating
words
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
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 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
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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
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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”
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Figure 38.UN02
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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)
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Figure 38.13
Cerebrum
(including pallium)
Cerebellum
Hindbrain
Thalamus
Midbrain
(a) Songbird brain
Cerebrum (including
cerebral cortex)
Thalamus
Midbrain
Hindbrain
(b) Human brain
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Cerebellum
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
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Figure 38.14
N1
N1
N2
N2
(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.
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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 longterm memory, which is stored in the cerebral cortex
 Some consolidation of memory is thought to occur
during sleep
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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
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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
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 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
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Figure 38.15
(a) Receptor is afferent neuron.
(b) Receptor regulates afferent neuron.
To CNS
To CNS
Afferent
neuron
Afferent
neuron
Receptor
protein
Neurotransmitter
Sensory
receptor
Stimulus
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Sensory
receptor
cell
Stimulus
leads to
neurotransmitter
release.
Stimulus
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
© 2014 Pearson Education, Inc.
 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
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Figure 38.16
Gentle pressure
Sensory
receptor
Low frequency of
action potentials
More pressure
High frequency of
action potentials
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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
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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
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Types of Sensory Receptors
 Based on energy transduced, sensory receptors fall
into five categories
 Mechanoreceptors
 Electromagnetic receptors
 Thermoreceptors
 Pain receptors
 Chemoreceptors
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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
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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
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Figure 38.17
Eye
Infrared
receptor
(a) Rattlesnake
(b) Beluga whales
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Figure 38.17a
Eye
Infrared
receptor
(a) Rattlesnake
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Figure 38.17b
(b) Beluga whales
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Thermoreceptors
 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
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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
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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
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 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
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Figure 38.18
Papilla
Tongue
Papillae Taste
buds
Taste bud
Key
Sweet
Salty
Sour
Bitter
Umami
Taste
pore
Sensory
neuron
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Food
molecules
Sensory
receptor cells
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
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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
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Figure 38.19
Ciliated
receptor
cells
Cilia
Statolith
Sensory
nerve fibers
(axons)
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Hearing and Equilibrium in Mammals
 In most terrestrial vertebrates, sensory organs for
hearing and equilibrium are closely associated in
the ear
© 2014 Pearson Education, Inc.
Figure 38.20
Middle
ear
Outer ear
Skull
bone
Inner ear
Stapes
Incus
Malleus
Semicircular
canals
Cochlear
duct
Auditory nerve
to brain
Bone
Auditory
nerve
Vestibular
canal
Tympanic
canal
Cochlea
Pinna
Oval
Auditory
window
canal
Tympanic
Round
membrane
window
Eustachian
tube
Organ
of Corti
1 m
Tectorial
membrane
Bundled hairs projecting from a hair cell
(SEM)
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Basilar
Hair
membrane cells
Axons of
sensory neurons
To
auditory
nerve
Figure 38.20a
Middle
ear
Outer ear
Skull
bone
Inner ear
Stapes
Incus
Malleus
Semicircular
canals
Auditory nerve
to brain
Cochlea
Pinna
© 2014 Pearson Education, Inc.
Oval
Auditory
window
canal
Round
Tympanic
window
membrane
Eustachian
tube
Figure 38.20b
Cochlear
duct
Bone
Auditory
nerve
Vestibular
canal
Tympanic
canal
Organ
of Corti
© 2014 Pearson Education, Inc.
Figure 38.20c
Tectorial
membrane
Basilar
membrane
© 2014 Pearson Education, Inc.
Hair
cells
Axons of
sensory neurons
To
auditory
nerve
1 m
Figure 38.20d
Bundled hairs projecting from a hair cell (SEM)
© 2014 Pearson Education, Inc.
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
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 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
© 2014 Pearson Education, Inc.
Figure 38.21
More
neurotransmitter
Less
neurotransmitter
0
−70
Time (sec)
(a) Bending of hairs in one direction
Membrane
potential (mV)
−70
0 1 2 3 4 5 6 7
© 2014 Pearson Education, Inc.
−50
Signal
Membrane
potential (mV)
Signal
Receptor
−50 potential
Receptor
potential
−70
0
−70
0 1 2 3 4 5 6 7
Time (sec)
(b) Bending of hairs in other direction
 The fluid waves dissipate when they strike the round
window at the end of the vestibular canal
© 2014 Pearson Education, Inc.
 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
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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
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Figure 38.22
Semicircular
canals
PERILYMPH
Cupula
Vestibular
nerve
Fluid
flow
Hairs
Hair
cell
Vestibule
Utricle
Saccule
© 2014 Pearson Education, Inc.
Nerve
fibers
Body movement
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
© 2014 Pearson Education, Inc.
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
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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
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Figure 38.23
LIGHT
DARK
Photoreceptor
Ocellus
Visual
pigment
Ocellus
© 2014 Pearson Education, Inc.
Nerve to
brain
Screening
pigment
Compound Eyes
 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
© 2014 Pearson Education, Inc.
Figure 38.24
2 mm
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Single-Lens Eyes
 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
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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
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Figure 38.25a
Sclera
Retina
Choroid
Retina
Suspensory
ligament
Photoreceptors
Fovea
Neurons
Rod
Cone
Cornea
Iris
Optic
nerve
Pupil
Aqueous
humor
Lens
Vitreous humor
Optic
disk
Central
artery and
vein of
the retina
Optic
nerve
fibers
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Ganglion Bipolar Horizontal
cell
cell
cell
Amacrine
cell
Pigmented
epithelium
Figure 38.25aa
Sclera
Suspensory
ligament
Choroid
Retina
Fovea
Cornea
Iris
Optic
nerve
Pupil
Aqueous
humor
Lens
Vitreous humor
© 2014 Pearson Education, Inc.
Optic
disk
Central
artery and
vein of
the retina
Figure 38.25ab
Retina
Photoreceptors
Neurons
Optic
nerve
fibers
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Rod
Ganglion Bipolar Horizontal
cell
cell
cell
Amacrine
cell
Cone
Pigmented
epithelium
Figure 38.25b
CYTOSOL
Rod
Synaptic
terminal
Cell
body
Retinal:
cis isomer
Outer Disks
segment
Light
Enzymes
Cone
Rod
Retinal:
trans isomer
Cone
Retinal
INSIDE OF DISK
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Opsin
Rhodopsin
Figure 38.25ba
Rod
Synaptic
terminal
Cone
Rod
Cone
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Cell
body
Outer Disks
segment
Figure 38.25bb
CYTOSOL
Retinal:
cis isomer
Light
Enzymes
Retinal:
trans isomer
Retinal
INSIDE OF DISK
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Opsin
Rhodopsin
Figure 38.25bc
Retinal:
cis isomer
Light
Enzymes
Retinal:
trans isomer
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Figure 38.25bd
Rod
Cone
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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
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 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
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Figure 38.26
Light
Active
rhodopsin
EXTRACELLULAR
FLUID
INSIDE OF DISK
Phospho- Disk
diesterase membrane
Plasma
membrane
Inactive
rhodopsin
CYTOSOL
Transducin
GMP
Membrane
potential (mV)
0
Dark
cGMP
Na
Light
−40
−70
Hyperpolarization
Time
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Na
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
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 In the light, rods and cones hyperpolarize, shutting
off release of glutamate
 The bipolar cells are then either depolarized or
hyperpolarized
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 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
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Processing of Visual Information in the Brain
 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”
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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
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 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
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 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
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The Visual Field
 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
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Figure 38.UN01
Circadian period (hours)
24
Wild-type
hamster
Wild-type
hamster with
SCN from
 hamster
23
22
21
 hamster
 hamster
with SCN
from wild-type
hamster
20
19
Before
procedures
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After surgery
and transplant
Figure 38.UN03
Cerebral
cortex
Cerebrum
Forebrain
Thalamus
Hypothalamus
Pituitary gland
Midbrain
Hindbrain
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Pons
Medulla
oblongata
Cerebellum
Spinal
cord