Download The Nervous System

Biology
A Guide to the Natural World
Chapter 27 • Lecture Outline
Communication and Control 1: The Nervous System
Fifth Edition
David Krogh
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27.1 Structure of the Nervous System
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The Nervous System
• The nervous system includes all the nervous
tissue in the body plus the body’s sensory
organs, such as the eyes and ears.
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The Nervous System
• Nervous tissue is composed of two kinds of
cells:
• Neurons: transmit nervous system messages.
• Glial cells: support neurons and modify their
signaling.
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The Nervous System
• The two major divisions of the human
nervous system are:
• The central nervous system (CNS), consisting
of the brain and spinal cord.
• The peripheral nervous system (PNS), which
includes all the neural tissue outside the CNS
plus the sensory organs.
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(b) How these two components interact
(a) The nervous system has two components
Central nervous system
Central nervous system (CNS)
brain
information processing
spinal cord
Peripheral nervous system (PNS)
sensory information
travels in
afferent division
motor information
travels in
efferent division,
which includes…
somatic
nervous
system
sensory receptors
in eyes nose, etc.
Peripheral
nervous system
autonomic
nervous
system
sympathetic
division
parasympathetic
division
cardiac muscle,
smooth muscle,
glands
skeletal
muscle
effectors
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Figure 27.1
Divisions of the Nervous System
• The PNS has an afferent division, which
brings sensory information to the CNS; and
an efferent division, which carries action
(motor) commands to the body’s
“effectors”—muscles and glands.
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Divisions of the Nervous System
• Within the PNS’s efferent division are two
subsystems:
• The somatic nervous system, which provides
voluntary control over skeletal muscles.
• The autonomic nervous system, which provides
involuntary regulation of smooth muscle,
cardiac muscle, and glands.
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Divisions of the Nervous System
• The autonomic system is further divided
into the sympathetic division, which
generally has stimulatory effects; and the
parasympathetic division, which generally
facilitates routine maintenance activities.
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27.2 Cells of the Nervous System
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Cells of the Nervous System
• There are three types of neurons:
• sensory neurons
• motor neurons
• interneurons
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Cells of the Nervous System
• Sensory neurons sense conditions inside and
outside the body and convey information about
these conditions to neurons inside the CNS.
• Motor neurons carry instructions from the
CNS to such structures as muscles or glands.
• Interneurons are located entirely within the
CNS and which interconnect other neurons.
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Cells of the Nervous System
(a) Three types of neurons
sensory neuron
afferent
neuron
interneuron
neuron within central
nervous system
motor neuron
efferent
neuron
effector
(muscle)
(b) Anatomy of a neuron
axon
synaptic
terminals
dendrites
cell body
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Figure 27.2
Cells of the Nervous System
• Each neuron multiple dendrites through
which signals travel to the neuron cell body
and a single axon that carries signals away
from the cell body.
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Cells of the Nervous System
• Glial cells produce fat-rich myelin, which
can surround neuronal axons and which
increases the speed of neural signals.
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(a) A myelinated axon
myelin nodes
glial cells
glial cell nucleus
myelin covering
axon
glial cell cytoplasm
(b) Anatomy of a nerve
nerve
blood
vessels
connective
tissue
axons
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Figure 27.3
Cells of the Nervous System
• A nerve is a bundle of axons in the PNS that
transmits information to or from the CNS.
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27.3 Nervous-System Signaling
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Nervous System Communication
• Nervous system communication can be
conceptualized as working through a twostep process:
1. signal movement down a neuron’s axon
2. signal movement from this axon to a second
cell across a structure known as a synapse
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Nervous System Communication
• An electrical charge difference, called a
membrane potential, exists across the
plasma membrane of neurons because the
inside of the neuron is negatively charged
relative to the outside.
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Nervous System Communication
• This form of potential energy is used when
special protein channels in the neuron’s
membrane open up on stimulation, thereby
allowing ions to flow into the neuron.
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Nervous System Communication
• This influx of ions at an initial point on the
axon triggers reactions that cause the
adjacent portion of the axonal membrane to
initiate the same influx of ions.
• Thus, a conducted nerve impulse, called an
action potential, moves down the entire
axon in a set of linked reactions.
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sodium ion
(a) Membrane potential
Electrical energy is stored across the plasma
membrane of a resting neuron. There are more negatively charged compounds just inside the membrane
than outside of it. As a result, the inside of the cell is
negatively charged relative to the outside. The charge
difference creates a form of stored energy called a
membrane potential. Protein channels (shown in green)
that can allow the movement of electrically charged
ions across the membrane remain closed in a resting
cell, thus maintaining the membrane potential.
Cell exterior is
positively charged
relative to interior
outside
cell
resting
membrane
potential
plasma
membrane
inside
cell
potassium ion
(b) Action potential
1. Nerve signal transmission begins
when, upon stimulation, some protein
channels open up, allowing a movement of positively charged sodium ions
(Na+) into the cell. For a brief time, the
interior of the cell becomes positively
charged at this location. On either side
of this location, however, the interior
of the cell remains negatively charged.
Attracted by this negative charge, the
Na+ ions move laterally in both
directions from their point of entry.
Cell interior becomes
positively charged
sodium ions
potassium ions
Cell interior becomes
negatively charged again
2. The Na+ gates close and the gates for
positively charged potassium ions (K+)
open up, allowing a movement of K+
out of the cell. With this, there is once
again a net positive charge outside the
membrane. Meanwhile, the arrival of
the charged Na+ ions “downstream”
from their original point of entry triggers an influx of Na+ ions at the next
Na+ channel.
3. Through repetition of this process, the
nerve signal is then propagated one
way along the axon. Given that Na+
ions move laterally in both directions
from their original point of entry, why
isn’t the signal propagated in both directions? Because once an Na+ channel has opened, it enters a brief period
in which it cannot respond to any additional stimulus. Thus, each “upstream”
Na+ channel remains briefly closed,
while each “downstream” channel is
opened in succession.
action potential
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Figure 27.4
How Neurons Work
Suggested Media Enhancement:
How Neurons Work
To access this animation go to folder C_Animations_and_Video_Files
and open the BioFlix folder.
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How Synapses Work
Suggested Media Enhancement:
How Synapses Work
To access this animation go to folder C_Animations_and_Video_Files
and open the BioFlix folder.
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Nervous System Communication
• A nerve signal moves from one neuron to
another across a synapse.
• This includes a “sending” neuron, a
“receiving” cell, and a tiny gap between the
two cells, called a synaptic cleft.
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Nervous System Communication
• A chemical called a neurotransmitter
diffuses across the synaptic cleft from the
sending neuron to the receiving neuron.
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Nervous System Communication
• It then binds with receptors on the receiving
neuron, thus keeping the signal going.
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sending neuron
receiving cell
synaptic
cleft
synaptic
terminal
arrival of
nerve impulse
initiation of
new impulse
mitochondrion
vesicles containing
neurotransmitter
molecules (such
as acetylcholine)
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neurotransmitter
receptors
Figure 27.5
Nervous System Communication
Animation 27.1: The Nervous System
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27.4 The Spinal Cord
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The Spinal Cord
• The spinal cord can receive input from
sensory neurons and direct motor neurons in
response, with no input from the brain.
• The spinal cord also channels sensory
impulses to the brain.
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The Spinal Cord
• In cross section, the spinal cord has a
darker, H-shaped central area, composed
mostly of the cell bodies of neurons; and a
lighter peripheral area, composed mostly of
axons.
• These two areas are the gray matter and
white matter of the spinal cord,
respectively.
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The Spinal Cord
• The central canal of the spinal cord is filled
with cerebrospinal fluid, which provides the
spinal cord with nutrients.
• Spinal nerves extend from the spinal cord to
most areas of the body.
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(a)
brain
cervical
spinal
nerves
thoracic
spinal
nerves
lumbar
spinal
nerves
(b)
white matter
gray
matter
sacral
spinal
nerves
ventral root
dorsal root
ganglion central canal
spinal
nerve
dorsal root
tip of
spinal cord
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Figure 27.6
The Spinal Cord
• Spinal cord motor neurons have cell bodies
that lie within the gray matter of the spinal
cord.
• The axons of these neurons leave the spinal
cord through its ventral roots.
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The Spinal Cord
• Sensory neurons, which transmit
information to the spinal cord, have their
cell bodies outside the spinal cord, in the
dorsal root ganglia.
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The Spinal Cord
• Dorsal and ventral roots come together, like
fibers being joined in a single cable, to form
a given spinal nerve.
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Reflexes
• Reflexes are automatic nervous system
responses, triggered by specific stimuli, that
help us avoid danger or preserve a stable
physical state.
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Reflexes
• The neural wiring of a single reflex, called a
reflex arc, begins with a sensory receptor,
runs through the spinal cord to a motor
neuron, and proceeds back out to an effector
such as a muscle or gland.
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1. Stimulus (tapping) arrives
and receptor is activated.
2. The signal from the receptor
reaches a sensory neuron
cell body in the dorsal root
ganglion.
spinal
cord
afferent
signal
receptor
reflex
arc
stimulus
effector
response
motor
efferent neuron
signal
3. The signal arrives at a sensory
neuron/motor neuron synapse
in the spinal cord. Information
processing takes place, prompting
a signal to be sent through the
motor neuron.
4. The motor neuron signal stimulates
the effector (the quadriceps muscles)
to contract. Note that CNS
processing for this reaction was
handled entirely in the spinal cord;
the brain was not involved.
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Figure 27.7
27.5 The Autonomic Nervous System
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The Autonomic Nervous System
• The sympathetic division of the autonomic
nervous system is often called the fight-orflight system because it generally prepares
the body to deal with emergencies.
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The Autonomic Nervous System
• The parasympathetic division is often called
the rest-and-digest system because it
conserves energy and promotes digestive
activities.
• Most organs receive input from both
systems.
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Parasympathetic division (rest and digest)
Sympathetic division (fight or flight)
constricts
pupil
dilates
pupil
stimulates
salivation
inhibits
salivation
cranial
nerves
slows
heart
accelerates
heart
cervical
nerves
facilitates
breathing
constricts
breathing
stimulates
digestion
thoracic
nerves
inhibits
digestion
stimulates
gallbladder
stimulates
release of
glucose
lumbar
nerves
contracts
bladder
sacral
nerves
secretes
adrenaline and
noradrenaline
relaxes
bladder
inhibits sex
organs
stimulates
sex organs
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Figure 27.8
27.6 The Human Brain
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The Human Brain
• There are six major regions in the adult
brain:
•
•
•
•
•
•
•
cerebrum
cerebellum
thalamus
hypothalamus
midbrain
pons
medulla oblongata
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The Human Brain
• The cerebrum also has a thin outer layer of
gray matter, the cerebral cortex, that
surrounds a much larger volume of cerebral
white matter.
• Differing portions of the cerebral cortex
play a central role in processing sensory
information and in carrying out nearly all of
our conscious mental activities.
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The Human Brain
• The cerebellum refines bodily movement
and balance based on sensory inputs.
• The thalamus receives much of the body’s
sensory information and then transfers it to
different regions of the cerebral cortex for
processing.
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(a)
cerebral
cortex
cerebrum
cerebellum
(b)
thalamus
hypothalamus
pituitary
gland
brainstem
midbrain
pons
medulla
oblongata
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Figure 27.9
The Human Brain
• The hypothalamus is critical to regulating
drives and maintaining homeostasis, in part
through its regulation of hormonal release.
• The brainstem is a collective term
containing:
• The midbrain
• Pons
• Medulla oblongata
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The Human Brain
• The midbrain helps maintain muscle tone
and posture.
• The pons serves primarily to relay messages
between the cerebrum and the cerebellum.
• The medulla oblongata helps regulate such
involuntary functions as breathing and
digestion.
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somatosensory cortex
thalamus
gustatory
cortex
auditory
cortex
olfactory
cortex
visual
cortex
vision
hippocampus
amygdala
smell
hypothalamus
brainstem
taste
taste
hearing
touch senses
touch senses
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Figure 27.10
27.7 Our Senses
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Our Senses
• All human senses operate through cells
called sensory receptors that respond to
stimuli.
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Our Senses
• The sensory receptors transform the
responses to stimuli into electrical signals
that travel through action potentials.
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Our Senses
• Signals from every sense except smell are
routed through the brain’s thalamus and
then to specific areas of the cerebral cortex.
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27.8 Touch
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Our Senses of Touch
• Touch works through a variety of sensory
receptors that distinguish among such
qualities as light or heavy pressure and new
or ongoing contact.
• In some sensory cells, the stretching of their
outer membrane prompts an influx of ions
that results in the initiation of a nerve
signal.
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(a) Touch receptors in the skin
hair
tactile receptor
free nerve endings
(pain, temperature)
epidermis
tactile receptor
dermis
hair follicle
receptor
tactile receptor
Pacinian
corpuscle
(b) How one touch receptor works
interior of nerve ending
pressure
stretch
stretch
stretch
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stretch
stretch
Figure 27.11
27.9 Smell
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Our Sense of Smell
• Our sense of smell, or olfaction, works
through a set of sensory receptors whose
dendrites extend into the nasal passages.
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Our Sense of Smell
• “Odorants,” which are molecules that have
identifiable smells, bind with hair-like
extensions of these dendrites, resulting in a
nerve signal to the brain.
• The higher processing centers of the brain
distinguish odorants sensing unique groups
of neurons that fire in connection with given
odorants.
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Our Sense of Smell
(b)
(a)
olfactory
bulb
to olfactory cortex, amygdala,
and hypothalamus
olfactory bulb
olfactory tract
supporting
cells
olfactory
receptor
cell
olfactory
epithelium
odorants
odorants
mucous
layer
cilia
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Figure 27.12
27.10 Taste
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Our Sense of Taste
• Our sense of taste works through a group of
taste cells, located in taste buds near the
surface of the tongue, which have receptors
that bind to “tastants,” or molecules of food
that elicit different tastes.
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Our Sense of Taste
• A given taste cell can respond through any
of four to six chemical signaling routes that
correspond to the basic tastes of sweet, sour,
salty, bitter, and the possible fifth and sixth
tastes of umami and calcium.
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papilla
taste buds
connective
tissue
salivary glands
muscle layer
papillae
taste bud
taste pore
microvilli taste connective dendrites
cell
tissue
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Figure 27.13
Our Sense of Taste
• The neurons that receive input from taste
cells vary in their response to different
tastants.
• The brain makes sense of the pattern of
input it gets from these neurons, thus
yielding the large number of tastes we
experience.
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27.11 Hearing
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Our Sense of Hearing
• Our sense of hearing is based on the fact
that vibrations result in “waves” of air
molecules that are, by turns, more and less
compressed than the ambient air around
them.
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Our Sense of Hearing
• These waves of compression bump up
against our eardrums (or tympanic
membranes), which in turn vibrate.
• This initiates a chain of vibration that ends
in the fluid-filled cochlea of the inner ear.
• “Hair cells” in the cochlea have ion
channels that open and close in response to
this vibration, resulting in nerve signals to
the brain.
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(a) Anatomy of the ear
incus
malleus
stapes
oval window
tympanic
membrane
cochlea
nerve
ear canal
outer ear
middle ear
inner ear
(b) From air vibration to nerve signal
3. The vibration of the stapes
focuses the sound-wave
vibration on the membrane
of the oval window.
2. The tympanic membrane
vibrates the three bones of
the middle ear; the malleus,
incus and stapes.
perception
of sound
2
1. Sound waves enter
through the ear
canal and vibrate
the tympanic
membrane.
5
3
sound
4. The oval window’s vibrations
cause fluid vibrations within the
coiled, tubular cochlea (shown
elongated here for illustrative
purposes).
4
5. These fluid vibrations cause
cells within the cochlea to
release a neurotransmitter,
which triggers a nerve signal
to the brain.
1
(c) How fluid triggers nerve signal
tectorial membrane
tectorial
membrane
vestibular
duct
hair
cells
cochlear
duct
tympanic
duct
nerve
basilar membrane
1. Seen in cross section,
the cochlea has vestibular
and tympanic ducts, in
which fluid is vibrating.
2. This vibration shakes the
basilar membrane, pushing
hair cells on it up against the
overlying tectorial membrane.
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nucleus
3. As the hair cells contact the
tectorial membrane, cilia on
them bend. This change in
position causes “trap door”
channels in the hair cells to
open, which allows
potassium ions (K+) to flow
into them.
4. This influx triggers an influx
of calcium ions (Ca2+) at the
base of the hair cells, which
in turn causes the cells to
release a neurotransmitter.
5. The neurotransmitter is
received by adjacent
dendrites and a nerve
signal is sent to the brain.
Figure 27.14
27.12 Vision
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Our Sense of Vision
• Light first enters the eye through the cornea
and then passes through the lens on its way
to the retina at the back of the eye.
• Light is bent or refracted by the cornea and
the lens in such a way that it ends up as a
tiny, sharply focused image on the retina.
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vitreous
body
retina
cornea
iris
pupil
lens
optic nerve
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Figure 27.15
Our Sense of Vision
• Light signals are converted to nervous system
signals by cells in the retina called
photoreceptors, which come in two varieties:
rods and cones.
• Rods function in dim light but are not sensitive
to color.
• Cones function best in bright light but are
sensitive to color.
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(a) Normal vision
light rays
converge on
the retina
(b) Farsighted
vision
light rays
converge
behind
the retina
(c) Nearsighted
vision
light rays
converge
in front of
the retina
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Figure 27.16
Our Sense of Vision
• These photoreceptors have pigments embedded
in membranes within them.
• When light strikes a pigment, it changes
pigment shape in a way that prompts a cascade
of chemical reactions that results in
neurotransmitter release being inhibited
between the rod or cone and its adjoining
connecting cell.
• This lack of release sends the signal,
“Photoreceptor stimulated here.”
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Our Sense of Vision
• Vision signals travel from photoreceptors
through two sets of adjoining cells, the
latter of which have axons that come
together to form the body’s optic nerves.
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Our Sense of Vision
• The brain does not passively record visual
information. Rather, it constructs images as
much as it records them.
• The visual perception operates through a
series of genetically based “rules” that
allow us to quickly make sense of what we
perceive.
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