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The Nervous System
Chapter 26
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Outline
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Invertebrate Nervous System
Neurons
Nerve Impulses
Synapse
Drug Addiction
Evolution of Vertebrate Brain
Vertebrate Nervous System
Sensory Receptors
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Evolution of Animal Nervous System
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In most invertebrates, and all vertebrates,
sensory receptors and motor effectors are
linked by the nervous system.
 Association neurons are located in the
brain and spinal cord, and together are
know as central nervous system.
 Motor neurons and Sensory neurons
together comprise peripheral nervous
system.
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Three Types of Neurons
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Invertebrate Nervous Systems
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Sponges are the only major phylum of
multicellular animals lacking nerves.
Reflexes - Cnidarian neurons are linked
together through nerve net.
 No associative activity.
Associative Activities - Free-living flatworms
have two nerve cords running down bodies.
 Permits complex control of muscles.
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Invertebrate Nervous Systems
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Evolutionary Path to Vertebrates
 More sophisticated sensory mechanisms.
 Differentiation into central and peripheral
nervous systems.
 Differentiation of sensory and motor
nerves.
 Increased complexity of association.
 Elaboration of the brain.
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Evolution of Nervous System
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Neurons
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All neurons have same basic structure.
 Cell Body is enlarged region containing
nucleus.
 Dendrites are short slender input channels
extending from end of cell body.
 Axon is single output channel extending
out from other end of cell body.
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Neurons
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Most neurons require nutritional support
provided by companion neuroglial cells.
 Schwann cells and oligodendrocytes
envelop axon with sheath of myelin acting
as electrical insulator.
- During development, cells wrap
themselves around each axon several
times to form myelin sheath.
 Uninsulated gaps (nodes of Ranvier).
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Neuron Structure and Myelin Sheath Formation
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The Nerve Impulse
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When a neuron is at rest, active transport
channels in plasma membrane transport
sodium ions out of the cell and potassium
ions into the cell.
 Plasma membrane is polarized.
- Resting Potential is charge separation
between outside and inside of cell.
- Sodium channels flood into neuron from
outside, momentarily depolarizing inside
of membrane.
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The Nerve Impulse
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Electrical current moves in and out of neuron
through voltage-gated channels.
 Sodium channels flood into neuron from
outside, momentarily depolarizing inside of
membrane.
- Moving reversal known as action
potential.
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The Synapse
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Synapse is junction of axon with another cell.
 Presynaptic Membrane is located on near
side of synapse.
 Postsynaptic Membrane is located on far
side of synapse.
Neurotransmitters are chemical messengers
that carry nerve impulses across synapse.
 Control switches of nervous system.
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Synapse Events
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The Synapse
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Excitatory Synapse - Sodium channel
through membrane is opened by chemical
neurotransmitter.
Inhibitory Synapse - Binding with
neurotransmitter opens potassium channel
and inhibits start of an action potential.
 Integration - Various excitatory and
inhibitory electrical effects cancel or
reinforce one another.
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Drug Addiction
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When a cell is exposed to chemical signal for
a prolonged period, it tends to lose ability to
respond with its original intensity.
 If receptor proteins within synapses are
exposed to high levels of neurotransmitter
molecules for prolonged periods, the nerve
cell often responds by inserting fewer
receptor proteins into the membrane.
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Drug Addiction
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Cocaine
 Neuromodulator (prolongs transmission of
signal across synapse) that causes large
amounts of neurotransmitters to remain in
synapses for long periods of time.
- Transmit pleasure messages using the
neurotransmitter dopamine.
 Nerve cells may eventually lower
number of receptor proteins on
surface.
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Evolution of the Vertebrate Brain
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Hindbrain was major component of early
brains, and is still found in fishes today.
 May be considered extension of spinal
cord devoted primarily to coordinating
motor reflexes.
- Much of coordination carried on within
cerebellum.
 Remainder of brain devoted to
reception and processing sensory
information.
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Evolution of the Vertebrate Brain
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Midbrain is composed primarily of optic lobes
which receive and process visual information.
Forebrain is devoted to processing olfactory
information.
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Evolution of the Vertebrate Brain
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Dominant Forebrain
 Sensory information increasingly centered in
forebrain starting with amphibians.
- Diencephalon
 Thalamus - Relay center between
incoming information and cerebrum.
 Hypothalamus - Controls pituitary
secretions.
- Telencephalon - Associative activity
(Cerebrum)
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Evolution of Vertebrate Brain
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How the Brain Works
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Cerebrum is large rounded area of brain
divided by groove into right and left halves.
 Functions in language, conscious thought,
memory, personality, vision, and a host of
other activities.
- Much of activity occurs in thin gray outer
layer (cerebral cortex).
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Human Brain
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How The Brain Works
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Thalamus is major site of sensory
processing, and controls balance.
Hypothalamus integrates all internal
activities.
 Controls centers in brain stem regulating
body temperature, blood pressure,
respiration, and heartbeat.
 Linked to areas of cerebral cortex by limbic
system.
- Deep-seated drives
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How The Brain Works
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Language and Other Higher Functions
 Left hemisphere is dominant hemisphere
for language.
- Adept at sequential reasoning.
 Non-dominant hemisphere is adept at
spatial reasoning, musical ability, and
important for consolidation of memories of
nonverbal experiences.
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The Spinal Cord
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Cable of neurons extending from the brain
down through the backbone.
 Surrounded and protected by vertebrae,
through which spinal nerves pass out to
the body.
- Motor nerves from spine control most
muscles below the head.
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Vertebrate Nervous System
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Voluntary Nervous System
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Voluntary Nervous System relays commands
to skeletal muscles.
 Can be controlled by conscious thought.
Reflex - Produces rapid motor response to
stimulus because sensory neuron passes
information directly to a motor neuron.
 Most involve single connecting interneuron
between sensory neuron and motor
neuron.
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Autonomic Nervous System
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Autonomic Nervous System stimulates glands
and relays commands to smooth muscles.
 Cannot be controlled by conscious thought.
 Composed of:
- Sympathetic nervous system - Takes
command in time of stress.
- Parasympathetic nervous system Conserves energy by slowing processes
down.
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Sensory Receptors
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Sensory receptors are specialized sensory
cells that detect changes inside and outside
the body.
 Sensory organs - relatively complex
receptors.
Path of sensory information
 Stimulation
 Transduction
 Transmission
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Sensory Receptors
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Receptors can initiate nerve impulses by
opening or closing stimulus-gated channels.
 Exteroceptors - Sense stimuli in external
environment.
 Interoceptors - Sense stimuli within the
body.
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Sensing the Internal Environment
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Vertebrate body uses variety of sensory
receptors to respond to internal environment.
 Temperature Change
 Blood Chemistry
 Pain
 Muscle Contraction
 Blood Pressure
 Touch
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Sensing Gravity: Balance
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Brain needs point of reference to maintain
balance.
 Otolith sensory receptors are located in
hollow chambers within inner ear and
detect gravity.
Brain senses motion by employing a receptor
in which fluid deflects cilia in a direction
opposite that of motion.
 Semicircular canals
- Cilia arranges in cupula assembly.
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Inner Ear and Motion Sensing
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Sensing Chemicals: Taste and Smell
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Taste
 Taste buds
- Papillae
 Chemicals dissolve in saliva and
contact taste cells.
Smell
 Chemically sensitive neurons in nose have
cell bodies embedded within epithelium of
the nasal passage.
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Taste and Smell
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Sensing Sounds: Hearing
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Eardrum membrane pushed in and out by
waves of air pressure.
 Three small bones located on other side of
eardrum act as lever system to increase
vibration force.
- Inner ear chamber (cochlea) connected
to throat by eustachian tube so that
pressure is equalized.
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Sensing Sounds: Hearing
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Sound receptors within cochlea are hair cells
resting on a membrane running up and down
middle of chamber.
 When sounds enter cochlea, sound waves
cause membrane to vibrate, causing hairs
to send nerve impulses to sensory neurons
that travel to the brain.
- Sound frequency and intensity.
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Structure and Function of Human Ear
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Sensing Body Position
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Lateral Line System
 Sense objects that reflect pressure waves
and low frequency vibrations.
- Sensory structures within longitudinal
canal in fish’s skin that extends along
each side of the body and within several
canals in the head.
Sonar
 Echolocation
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Lateral Line System
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Sensing Light: Vision
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Evolution of the Eye
 Annelids, mollusks, arthropods, and
vertebrates have evolved well-developed,
image-forming eyes.
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Sensing Light: Vision
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Structure of the Eye
 Cornea - Focuses light on covering
 Lens - Completes focusing
 Ciliary Muscles - Changes shape of lens
 Iris - Regulates amount of light
 Pupil - Transparent zone
 Retina - Light sensitive receptor cells
 Rods and Cones - Photoreceptors
- Fovea - Center of retina
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Human Eye
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Sensing Light: Vision
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Pigments in rods and cones are made from
carotenoids.
 When it receives photon of light, pigment
undergoes change in shape.
- Must alter shape of attached opsin
protein.
 Cis-retinal pigment absorbs light,
rotates sharply upward, forming transretinal form of pigment.
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Sensing Light: Vision
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Color Vision
 Each of three kinds of cone cells possess
a different version of the opsin protein.
- Differences in shape affect flexibility of
attached retinal pigment, shifting
wavelength at which it absorbs light.
 455 nanometers - blue
 530 nanometers - green
 625 nanometers - red
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Sensing Light: Vision
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Conveying Light Information to the Brain
 Action potentials propagated along ganglion
cells are relayed through lateral geniculate
nuclei of thalamus and projected to occipital
lobe of the cerebral cortex.
- Pattern of activity among ganglion cells
across retina encodes a point-to-point
map of receptive field, allowing retina and
brain to image objects in visual space.
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Sensing Light: Vision
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Binocular Vision
 When both eyes are trained on an object,
each eye views the object from a slightly
different view.
- Slight displacement provides ability to
perceive three-dimensional images and
sense depth.
 Prey generally have eyes at side of
head, preventing binocular vision, but
enlarging overall perceptive fields.
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Binocular Vision
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Other Types of Sensory Reception
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Heat
Electricity
Magnetism
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Review
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Invertebrate Nervous System
Neurons
Nerve Impulses
Synapse
Drug Addiction
Evolution of Vertebrate Brain
Vertebrate Nervous System
Sensory Receptors
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Copyright © McGraw-Hill Companies
Permission required for reproduction or display
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies