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Chapter 48
Nervous System
1. Nervous systems perform the three
overlapping functions of sensory input,
integration, and motor output
Networks of neurons either intricate
connections form nervous systems
• Neuron Structure and Synapses.
– The neuron is the structural and functional unit of the
nervous system.
• Nerve impulses are conducted along a
neuron.
– Dentrite  cell body  axon hillock  axon
– Some axons are insulated by a myelin sheath.
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• Axon endings are called synaptic terminals.
– They contain neurotransmitters which
conduct a signal across a synapse.
• A synapse is the junction between a presynaptic
and postsynaptic neuron.
• Neurons differ in terms of both function and
shape.
Fig. 48.4
• Types of Nerve Circuits.
– Single presynaptic neuron  several postsynaptic
neurons.
– Several presynaptic neurons  single postsynaptic
neuron.
– Circular paths.
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Supporting Cells (Glia)
• Glia are supporting cells
– That are essential for the structural integrity of
the nervous system and for the normal
functioning of neurons
• In the CNS, astrocytes
Figure 48.7
50 µm
– Provide structural support for neurons and
regulate the extracellular concentrations of ions
and neurotransmitters
• Oligodendrocytes (in the CNS) and
Schwann cells (in the PNS)
– Are glia that form the myelin sheaths around
the axons of many vertebrate neurons
Node of Ranvier
Layers of myelin
Axon
Schwann
cell
Axon
Figure 48.8
Myelin sheath
Nodes of
Ranvier
Schwann
cell
Nucleus of
Schwann cell
0.1 µm
• A Simple
Nerve
Circuit –
the Reflex
Arc.
– A reflex is
an
autonomic
response.
ANIMATION
• A ganglion is a cluster of nerve cell bodies
within the PNS.
• A nucleus is a cluster of nerve cell bodies
within the CNS.
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• The membrane potential of a cell can be
measured
APPLICATION Electrophysiologists use intracellular recording to measure the membrane potential of
neurons and other cells.
TECHNIQUE
A microelectrode is made from a glass capillary tube filled with an electrically conductive
salt solution. One end of the tube tapers to an extremely fine tip (diameter < 1 µm). While looking through a
microscope, the experimenter uses a micropositioner to insert the tip of the microelectrode into a cell. A
voltage recorder (usually an oscilloscope or a computer-based system) measures the voltage between the
microelectrode tip inside the cell and a reference electrode placed in the solution outside the cell.
Microelectrode
–70 mV
Voltage
recorder
Figure 48.9
Reference
electrode
• How a Cell Maintains a
Membrane Potential.
– Cations.
• Na+ is the principal
extracellular cation.
• K+ the principal
intracellular cation.
– Anions.
• Cl– is principal
extracellular anion.
• Proteins, amino acids,
sulfate, and phosphate
are the principal
intracellular anions.
-70 mV
The Resting Potential
• The resting potential
– Is the membrane potential of a neuron that is
not transmitting signals
• In all neurons, the resting potential
– Depends on the ionic gradients that exist across
the plasma membrane
EXTRACELLULAR
FLUID
CYTOSOL
[Na+]
15 mM
–
+
[Na+]
150 mM
[K+]
150 mM
–
+
[K+]
5 mM
–
+
[Cl–]
10 mM
–
[Cl–]
+ 120 mM
[A–]
100 mM
–
+
Plasma
membrane
Figure 48.10
• The concentration of Na+ is higher in the
extracellular fluid than in the cytosol
– While the opposite is true for K+
• A neuron that is not transmitting signals
– Contains many open K+ channels and fewer open
Na+ channels in its plasma membrane
• The diffusion of K+ and Na+ through these
channels
– Leads to a separation of charges across the
membrane, producing the resting potential
• Ungated ion channels allow ions to diffuse
across the plasma membrane.
– These channels are always open.
• This diffusion does not achieve an equilibrium
since sodium-potassium pump transports these
ions against their concentration gradients.
Fig. 48.7
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Changes in the membrane potential of
a neuron give rise to nerve impulses
• Excitable cells have the ability to generate large
changes in their membrane potentials.
– Gated ion channels open or close in response to
stimuli.
• The subsequent diffusion of ions leads to a change in the
membrane potential.
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• Hyperpolarization.
– Gated K+ channels open
 K+ diffuses out of the
cell  the membrane
potential becomes more
negative.
Fig. 48.8a
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• Depolarization.
– Gated Na+ channels open
 Na+ diffuses into the
cell  the membrane
potential becomes less
negative.
Fig. 48.8b
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• The Action Potential:
All or Nothing
Depolarization.
– If graded potentials sum
to -55mV a threshold
potential is achieved.
• This triggers an action
potential.
– Axons only.
Fig. 48.8c
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• In the resting state closed voltage-gated K+
channels open slowly in response to
depolarization.
• Voltage-gated Na+ channels have two gates.
– Closed activation gates open rapidly in response to
depolarization.
– Open inactivation gates close slowly in response to
depolarization.
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• Step 1: Resting State.
Fig. 48.9
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• Step 2: Threshold.
Fig. 48.9
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• Step 3: Depolarization phase of the action
potential.
Fig. 48.9
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• Step 4: Repolarizing phase of the action
potential.
Fig. 48.9
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Animation 2
• Schwann cells are found within the PNS.
– Form a myelin sheath by insulating axons.
Fig. 48.5
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• Saltatory conduction.
– In myelinated neurons only unmyelinated regions of
the axon depolarize.
• Thus, the impulse moves faster than in unmyelinated
neurons.
Fig. 48.11
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Chemical or electrical communication
between cells occurs at synapses
• Electrical Synapses.
– Action potentials travels directly from the presynaptic
to the postsynaptic cells via gap junctions.
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• Chemical Synapses.
– More common than electrical synapses.
– Postsynaptic chemically-gated channels exist for ions
such as Na+, K+, and Cl-.
• Depending on which gates open the postsynaptic neuron
can depolarize or hyperpolarize.
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• In a chemical synapse, a presynaptic neuron
– Releases chemical neurotransmitters, which are
stored in the synaptic terminal
Postsynaptic
neuron
5 µm
Synaptic
terminal
of presynaptic
neurons
Figure 48.16
Animation
Animation 2
5. Neural integration occurs at the
cellular level
• Excitatory postsynaptic potentials (EPSP)
depolarize the postsynaptic neuron.
– The binding of neurotransmitter to postsynaptic
receptors open gated channels that allow Na+ to
diffuse into and K+ to diffuse out of the cell.
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• Inhibitory postsynaptic potential (IPSP)
hyperpolarize the postsynaptic neuron.
– The binding of neurotransmitter to postsynaptic
receptors open gated channels that allow K+ to diffuse
out of the cell and/or Cl- to diffuse into the cell.
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• Summation: graded potentials (EPSPs and
IPSPs) are summed to either depolarize or
hyperpolarize a postsynaptic neuron.
Fig. 48.14
Inhibits pain
Organization of Nervous Systems
• The simplest animals with nervous systems,
the cnidarians
– Have neurons arranged in nerve nets
Nerve net
Figure 48.2a
(a) Hydra (cnidarian)
• Sea stars have a nerve net in each arm
– Connected by radial nerves to a central nerve
ring
Radial
nerve
Nerve
ring
Figure 48.2b
(b) Sea star (echinoderm)
• In relatively simple cephalized animals,
such as flatworms
– A central nervous system (CNS) is evident
Eyespot
Brain
Nerve
cord
Transverse
nerve
Figure 48.2c
(c) Planarian (flatworm)
• Annelids and arthropods
– Have segmentally arranged clusters of neurons called
ganglia
• These ganglia connect to the CNS
– And make up a peripheral nervous system (PNS)
Brain
Brain
Ventral
nerve cord
Ventral
nerve
cord
Segmental
ganglia
Segmental
ganglion
Figure 48.2d, e
(d) Leech (annelid)
(e) Insect (arthropod)
• Nervous systems in molluscs
– Correlate with the animals’ lifestyles
• Sessile molluscs have simple systems
– While more complex molluscs have more
sophisticated systems
Anterior
nerve ring
Ganglia
Brain
Longitudinal
nerve cords
Figure 48.2f, g
Ganglia
(f) Chiton (mollusc)
(g) Squid (mollusc)
• In vertebrates
– The central nervous system consists of a brain
and dorsal spinal cord
– The PNS connects to the CNS
Brain
Spinal
cord
(dorsal
nerve
cord)
Figure 48.2h
Sensory
ganglion
(h) Salamander (chordate)
Evolutionary Trends
• Nervous systems become centralized - formation
of longitudinal cords
• Conduction along pathway becomes one way afferent and efferent fibers
• pathways within CNS become more complex
(interneurons) - more flexible behavior
• More segregation and specialization
• Formation of a Brain - Cephalization
• More and complex organs
1. Vertebrate nervous systems have
central and peripheral components
• Central nervous system (CNS).
– Brain and spinal cord.
• Both contain fluid-filled spaces which contain cerebrospinal
fluid (CSF).
– The central canal of the spinal cord is continuous with the ventricles of the
brain.
– White matter is composed of bundles of myelinated
axons
– Gray matter consists of unmyelinated axons, nuclei, and
dendrites.
• Peripheral nervous system.
– Everything outside the CNS.
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• The brain provides the integrative power
– That underlies the complex behavior of vertebrates
• The spinal cord integrates simple responses to
certain kinds of stimuli
– And conveys information to and from the brain
• The central canal of the spinal cord and the
four ventricles of the brain
– Are hollow, since they are derived from the
dorsal embryonic nerve cord
Gray matter
White
matter
Ventricles
Figure 48.20
The divisions of the peripheral nervous
system interact in maintaining
homeostasis
• Structural composition of the PNS.
– Paired cranial nerves that originate in the
brain and innervate the head and upper body.
– Paired spinal nerves that originate in the spinal
cord and innervate the entire body.
– Ganglia associated with the cranial and spinal
nerves.
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• The cranial nerves originate in the brain
– And terminate mostly in organs of the head and
upper body
• The spinal nerves originate in the spinal cord
– And extend to parts of the body below the head
• The PNS can be divided into two functional
components
– The somatic nervous system and the autonomic
nervous system
Peripheral
nervous system
Somatic
nervous
system
Autonomic
nervous
system
Sympathetic
division
Figure 48.21
Parasympathetic
division
Enteric
division
• A closer look
at the (often
antagonistic)
divisions of
the
autonomic
nervous
system
(ANS).
Fig. 48.18
Embryonic development of the vertebrate
brain reflects its evolution from three
anterior bulges of the neural tube
Fig. 48.19
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• As a human brain develops further
– The most profound change occurs in the
forebrain, which gives rise to the cerebrum
Brain structures present in adult
Cerebrum (cerebral hemispheres; 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)
Diencephalon:
Cerebral hemisphere
Hypothalamus
Thalamus
Pineal gland
(part of epithalamus)
Brainstem:
Midbrain
Pons
Pituitary
gland
Medulla
oblongata
Spinal cord
Cerebellum
Central canal
Figure 48.23c
(c) Adult
4. Evolutionary older structures of the
vertebrate brain regulate essential
autonomic and integrative functions
• The Brainstem.
– The “lower brain.”
– Consists of the medulla oblongata, pons, and
midbrain.
– Derived from the embryonic hindbrain and
midbrain.
– Functions in homeostasis, coordination of
movement, conduction of impulses to higher brain
centers.
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• The Medulla and Pons.
– Medulla oblongata.
• Contains nuclei that control visceral (autonomic
homeostatic) functions.
–
–
–
–
–
Breathing.
Heart and blood vessel activity.
Swallowing.
Vomiting.
Digestion.
• Relays information to and from higher brain centers.
• Sleep
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• Pons.
– Contains nuclei involved in the regulation of
visceral activities such as breathing.
– Relays information to and from higher brain
centers.
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• The Midbrain.
– Contains nuclei involved in the integration of
sensory information.
• Superior colliculi are involved in the regulation of
visual reflexes.
• Inferior colliculi are involved in the regulation of
auditory reflexes.
– Relays information to and from higher brain
centers.
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• The Reticular System, Arousal, and Sleep.
– The reticular activating system (RAS) of the
reticular formation.
• Regulates sleep
and arousal.
• Acts as a
sensory filter.
Fig. 48.21
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– Sleep and wakefulness produces patterns of
electrical activity in the brain that can be
recorded as an electroencephalogram (EEG).
• Most dreaming
occurs during
REM (rapid
eye movement)
sleep.
Fig. 48.22b-d
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The Cerebellum
• The cerebellum
– Is important for coordination and error checking
during motor, perceptual, and cognitive functions
– The Cerebellum.
• Develops from part of the metencephalon.
• Functions to error-check and coordinate motor
activities, and perceptual and cognitive factors.
• Relays sensory information about joints, muscles,
sight, and sound to the cerebrum.
• Coordinates motor commands issued by the
cerebrum.
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The Diencephalon
• The embryonic diencephalon develops into
three adult brain regions
– The epithalamus, thalamus, and hypothalamus
– Epithalamus.
• Includes a choroid plexus and the pineal gland.
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– Thalamus.
• Relays all sensory information to the cerebrum.
– Contains one nucleus for each type of sensory
information.
• Relays motor information from the cerebrum.
• Receives input from the cerebrum.
• Receives input from brain centers involved in the
regulation of emotion and arousal.
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– Hypothalamus.
• Regulates autonomic activity.
– Contains nuclei involved in thermoregulation, hunger,
thirst, sexual and mating behavior, etc.
– Regulates the pituitary gland.
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– The Hypothalamus and Circadian Rhythms.
• The biological clock is the internal timekeeper.
– The clock’s rhythm usually does not exactly match environmental events.
– Experiments in which humans have been deprived of external cues have
shown that biological clock has a period of about 25 hours.
• In mammals, the hypothalamic suprachiasmatic nuclei
(SCN) function as a biological clock.
– Produce proteins in response to light/dark cycles.
• This, and other biological clocks, may be
responsive to hormonal release, hunger, and
various external stimuli.
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• Biological clocks usually require external
cues
– To remain synchronized with environmental cycles
EXPERIMENT
In the northern flying squirrel (Glaucomys sabrinus), activity normally begins with the onset of darkness and ends
at dawn, which suggests that light is an important external cue for the squirrel. To test this idea, researchers monitored the activity of captive
squirrels for 23 days under two sets of conditions: (a) a regular cycle of 12 hours of light and 12 hours of darkness and (b) constant darkness.
The squirrels were given free access to an exercise wheel and a rest cage. A recorder automatically noted when the wheel was rotating and
when it was still.
(a) 12 hr light-12 hr dark cycle
Light
Dark
Light
Dark
1
Days of experiment
RESULTS
When the squirrels
were exposed to a regular light/dark
cycle, their wheel-turning activity
(indicated by the dark bars) occurred
at roughly the same time every day.
However, when they were kept in
constant darkness, their activity phase
began about 21 minutes later each day.
(b) Constant darkness
5
10
15
20
Figure 48.25
12
16
20
24
4
Time of day (hr)
8
12
12
16
20
24
4
Time of day (hr)
CONCLUSION
The northern flying squirrel’s internal clock can run in constant darkness, but it does so on
its own cycle, which lasts about 24 hours and 21 minutes. External (light) cues keep the clock running on a 24-hour cycle.
8
12
The cerebrum is the most highly
evolved structure of the mammalian
brain
• The cerebrum is
derived from the
embryonic
telencephalon.
Fig. 48.24a
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• The cerebrum is divided into left and right
cerebrum hemispheres.
– The corpus callosum is the major connection
between the two hemispheres.
– The left hemisphere is primarily responsible for the
right side of the body.
– The right hemisphere is primarily responsible for the
left side of the body.
• Cerebral cortex: outer covering of gray matter.
– Neocortex: region unique to mammals.
• The more convoluted the surface of the neocortex the more
surface area the more neurons.
• Basal nuclei: internal clusters of nuclei.
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6. Regions of the cerebrum are
specialized for different functions
• The
cerebrum is
divided into
frontal,
temporal,
occipital, and
parietal
lobes.
Fig. 48.24b
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• Frontal lobe.
– Contains the primary motor cortex.
• Parietal lobe.
– Contains the primary somatosensory cortex.
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Information Processing in the
Cerebral Cortex
• Specific types of sensory input
– Enter the primary sensory areas
• Adjacent association areas
– Process particular features in the sensory input and
integrate information from different sensory areas
Connects right and
left hemispheres
Major input center for sensory
info going to cerebrum also
main output center for motor
info leaving cerebrum
Coordination of movement
and balance
Homeostasis regulation: thermostat, hunger,
thirst, sex, fight or flight
Superchiasmatic nuclei acts as a biological clock
Controls visceral functions:
breathing, heart, blood vessel,
swallowing, vomiting, digestion
Link to Probe da Brain
• Integrative Function of the Association Areas.
– Much of the cerebrum is given over to
association areas.
• Areas where sensory information is integrated and
assessed and motor responses are planned.
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• The brain exhibits plasticity of function.
– For example, infants with intractable epilepsy
may have an entire cerebral hemisphere
removed.
• The remaining hemisphere can provide the function
normally provided by both hemispheres.
• Lateralization of Brain Function.
– The left hemisphere.
• Specializes in language, math, logic operations, and the
processing of serial sequences of information, and visual
and auditory details.
• Specializes in detailed activities required for motor control.
– The right hemisphere.
• Specializes in pattern recognition, spatial relationships,
nonverbal ideation, emotional processing, and the parallel
processing of information.
• Language and Speech.
– Broca’s area.
• Usually located in the left hemisphere’s frontal lobe
• Responsible for speech production.
– Wernicke’s area.
• Usually located in the right hemisphere’s temporal lobe
• Responsible for the comprehension of speech.
– Other speech areas are involved generating
verbs to match nouns, grouping together
related words, etc.
Grammatical
refinement of
words – speech
production
Linguistic meaning
determined on left
side comprehension
Higher frequency
sounds sent to right
area of brain for
emotional overtones
Written words
translated into
sounds
• Emotions.
– In mammals, the limbic system is composed of
the hippocampus, olfactory cortex, inner portions
of the cortex’s lobes, and parts of the thalamus
and hypothalamus.
• Mediates basic emotions (fear, anger), involved in
emotional bonding, establishes emotional memory
– For example,
the amygdala
is involved in
recognizing
the emotional
content of
facial expression.
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Fig. 48.27
Connects higher
brain
involved in
complex
learning,
reasoning and
personality and
emotion
Center of convergence for sensory data and a major
organizer of emotional information-may act as a
memory filter - tying info to an event or emotion
• Memory and Learning.
– Short-term memory stored in the frontal
lobes.
– The establishment of long-term memory
involves the hippocampus.
• The transfer of information from short-term to longterm memory.
– Is enhanced by repetition (remember that when you are
preparing for an exam).
– Influenced by emotional states mediated by the amygdala.
– Influenced by association with previously stored information.
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– Different types of long-term memories are
stored in different regions of the brain.
– Memorization-type memory can be rapid.
• Primarily involves changes in the strength of
existing nerve connections.
– Learning of skills and procedures is slower.
• Appears to involves cellular mechanisms similar to
those involved in brain growth and development.
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Cellular Mechanisms of Learning
• Experiments on invertebrates
– Have revealed the cellular basis of some types
of learning
Siphon
(a) Touching the siphon triggers a reflex that
causes the gill to withdraw. If the tail is
shocked just before the siphon is touched,
the withdrawal reflex is stronger. This
strengthening of the reflex is a simple form
of learning called sensitization.
Mantle
Gill
Tail
Head
Figure 48.31a, b
(b) Sensitization involves interneurons that
make synapses on the synaptic terminals of
the siphon sensory neurons. When the tail
is shocked, the interneurons release
serotonin, which activates a signal
transduction pathway that closes K+
channels in the synaptic terminals of
the siphon sensory neurons. As a result,
action potentials in the siphon sensory
neurons produce a prolonged
depolarization of the terminals. That allows
more Ca2+ to diffuse into the terminals,
which causes the terminals to release more
of their excitatory neurotransmitter onto the gill
motor neurons. In response, the motor neurons
generate action potentials at a higher frequency,
producing a more forceful gill withdrawal.
Gill withdrawal pathway
Touching
the siphon
Siphon sensory
neuron
Gill motor
neuron
Gill
Sensitization pathway
Interneuron
Shocking
the tail
Tail sensory
neuron
EPSPs
• In the vertebrate brain, a form of learning
called long-term potentiation (LTP)
– Involves an increase in the strength of synaptic
transmission
1 The presynaptic
neuron releases glutamate.
Glutamate binds to AMPA
receptors, opening the AMPAreceptor channel and depolarizing
the postsynaptic membrane.
2
PRESYNAPTIC NEURON
7
NO diffuses into the
presynaptic neuron, causing
it to release more glutamate.
NO
NMDA
receptor
6
Ca2+ stimulates the
postsynaptic neuron to
produce nitric oxide (NO).
5
Ca2+ initiates the phosphorylation of AMPA receptors,
making them more responsive.
Ca2+ also causes more AMPA
receptors to appear in the
postsynaptic membrane.
Figure 48.32
Glutamate
AMPA receptor
NO
P
Glutamate also binds to NMDA
receptors. If the postsynaptic
membrane is simultaneously
depolarized, the NMDA-receptor
channel opens.
3
Ca2+
Signal transduction pathways
POSTSYNAPTIC NEURON
Ca2+ diffuses into the
postsynaptic neuron.
4
• Functional changes in synapses in synapses of the
hippocampus and amygdala are related to
memory storage and emotional conditioning.
– Long-term depression (LTD) occurs when a
postsynaptic neuron displays decreased
responsiveness to action potentials.
• Induced by repeated, weak stimulation.
– Long-term potentiation (LTP) occurs when a
postsynaptic neuron displays increased responsiveness
to stimuli.
• Induced by brief, repeated action potentials that strongly
depolarize the postsynaptic membrane.
• May be associated with memory storage and learning.
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• Human Consciousness.
– Brain imaging can show neural activity
associated with:
• Conscious perceptual choice
• Unconscious processing
• Memory retrieval
• Working memory.
– Consciousness appears to be a whole-brain
phenomenon.
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7. Research on neuron development
and neural stem cells may lead to
new approaches for treating CNS
injuries and diseases
• The mammalian PNS has the ability to repair
itself, the CNS does not.
– Research on nerve cell development and neural
stem cells may be the future of treatment for
damage to the CNS.
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Nerve Cell Development
• Signal molecules direct an axon’s growth
– By binding to receptors on the plasma
membrane of the growth cone
• Nerve Cell Development.
Fig. 48.28
• Neural Stem Cells.
– The adult human brain does produce new nerve
cells.
• New nerve cells have been found in the
hippocampus.
• Since mature human brain cells cannot undergo cell
division the new cells must have arisen from stem
cells.
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Neural Stem Cells
• The adult human brain
– Contains stem cells that can differentiate into
mature neurons
Figure 48.34
• The induction of stem cell differentiation
and the transplantation of cultured stem
cells
– Are potential methods for replacing neurons
lost to trauma or disease
Diseases and Disorders of the
Nervous System
• Mental illnesses and neurological disorders
– Take an enormous toll on society, in both the
patient’s loss of a productive life and the high
cost of long-term health care
Schizophrenia
• About 1% of the world’s population
– Suffers from schizophrenia
• Schizophrenia is characterized by
– Hallucinations, delusions, blunted emotions, and
many other symptoms
• Available treatments have focused on
– Brain pathways that use dopamine as a
neurotransmitter
Depression
• Two broad forms of depressive illness are
known
– Bipolar disorder and major depression
• Bipolar disorder is characterized by
– Manic (high-mood) and depressive (low-mood)
phases
• In major depression
– Patients have a persistent low mood
• Treatments for these types of depression
include
– A variety of drugs such as Prozac and lithium
Alzheimer’s Disease
• Alzheimer’s disease (AD)
– Is a mental deterioration characterized by
confusion, memory loss, and other symptoms
• AD is caused by the formation of
– Neurofibrillary tangles and senile plaques in the
brain
Senile plaque
Neurofibrillary tangle
20 m
Figure 48.35
• A successful treatment for AD in humans
– May hinge on early detection of senile plaques
Parkinson’s Disease
• Parkinson’s disease is a motor disorder
– Caused by the death of dopamine-secreting
neurons in the substantia nigra
– Characterized by difficulty in initiating
movements, slowness of movement, and
rigidity
• There is no cure for Parkinson’s disease
– Although various approaches are used to
manage the symptoms