Chapter 48: Nervous System
-
Upload
stacey-lee -
Category
Documents
-
view
57 -
download
2
description
Transcript of Chapter 48: Nervous System
Chapter 48:Nervous System
Command and Control Center •The human brain contains an estimated 100 billion nerve cells, or neurons•Each neuron may communicate with thousands of other neurons
Figure 48.1
• The results of brain imaging and other research methods reveal that groups of neurons function in specialized circuits dedicated to different tasks
Nervous systems consist of circuits of neurons and supporting cells•All animals except sponges have some type of nervous system•What distinguishes the nervous systems of different animal groups is how the neurons are organized into circuits
Organization of Nervous Systems• The simplest animals with nervous systems, the
cnidarians have neurons arranged in nerve nets
Figure 48.2a
Nerve net
(a) Hydra (cnidarian)
• Sea stars have a nerve net in each arm connected by radial nerves to a central nerve ring
Figure 48.2b
Nervering
Radialnerve
(b) Sea star (echinoderm)
• In relatively simple cephalized animals, such as flatworms a central nervous system (CNS) is evident
Figure 48.2c
Eyespot
Brain
Nerve cord
Transversenerve
(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
Ventral nervecord
Segmentalganglion
Brain
Ventralnerve cord
Segmentalganglia
Figure 48.2d, e (d) Leech (annelid) (e) Insect (arthropod)
Anteriornerve ring
Longitudinalnerve cords
Ganglia
Brain
Ganglia
Figure 48.2f, g (f) Chiton (mollusc) (g) Squid (mollusc)
• Nervous systems in molluscs correlate with the animals’ lifestyles
• Sessile molluscs have simple systems while more complex molluscs have more sophisticated systems
• In vertebrates the central nervous system consists of a brain and dorsal spinal cord
• the PNS connects to the CNS
Figure 48.2h
Brain
Spinalcord(dorsalnervecord)
Sensoryganglion
(h) Salamander (chordate)
Information Processing• Nervous systems process information in three
stages– Sensory input, integration, and motor output
Figure 48.3
Sensor
Effector
Motor output
Integration
Sensory input
Peripheral nervoussystem (PNS)
Central nervoussystem (CNS)
• Sensory neurons transmit information from sensors that detect external stimuli and internal conditions
• Sensory information is sent to the CNS where interneurons integrate the information
• Motor output leaves the CNS via motor neurons which communicate with effector cells
• The three stages of information processing are illustrated in the knee-jerk reflex
Figure 48.4
Sensory neurons from the quadriceps also communicatewith interneurons in the spinal cord.
The interneurons inhibit motor neurons that supply the hamstring (flexor) muscle. This inhibition prevents the hamstring from contracting, which would resist the action of the quadriceps.
The sensory neurons communicate with motor neurons that supply the quadriceps. The motor neurons convey signals to the quadriceps, causing it to contract and jerking the lower leg forward.
4
5
6
The reflex is initiated by tapping
the tendon connected to the quadriceps
(extensor) muscle.
1
Sensors detecta sudden stretch in the quadriceps.
2 Sensory neuronsconvey the information to the spinal cord.
3
Quadricepsmuscle
Hamstringmuscle
Spinal cord(cross section)
Gray matter
White matter
Cell body of sensory neuronin dorsal root ganglion
Sensory neuron
Motor neuron
Interneuron
Neuron Structure• Most of a neuron’s organelles are located in the
cell body
Figure 48.5
Dendrites
Cell body
Nucleus
Axon hillock
AxonSignal direction
Synapse
Myelin sheath
Synapticterminals
Presynaptic cell Postsynaptic cell
• Most neurons have dendrites– Highly branched extensions that receive signals from
other neurons
• The axon is typically a much longer extension– That transmits signals to other cells at synapses– That may be covered with a myelin sheath
• Neurons have a wide variety of shapes that reflect their input and output interactions
Figure 48.6a–c
Axon
Cell body
Dendrites
(a) Sensory neuron (b) Interneurons (c) Motor neuron
Supporting Cells (Glia)• Glia are non-neuronal, supporting cells which
surround neurons and their axons to provide support, nutrition, and electrical insulation.
• In the CNS, astrocytes provide structural support for neurons and regulate the extracellular concentrations of ions and neurotransmitters
Figure 48.7 50 µ
m
• Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) are glia that form the myelin sheaths around the axons of many vertebrate neurons
Myelin sheathNodes of Ranvier
Schwanncell Schwann
cellNucleus of Schwann cell
Axon
Layers of myelin
Node of Ranvier
0.1 µm
Axon
Ion pumps and ion channels maintain the resting potential of a neuron•Across its plasma membrane, every cell has a voltage called a membrane potential•The inside of a cell is negative relative to the outside
• The membrane potential of a cell can be measured
Figure 48.9
APPLICATIONElectrophysiologists 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
Referenceelectrode
Voltage recorder
–70 mV
The Resting Potential• The resting potential is the membrane potential
of a neuron that is not transmitting signals• = -70mV
• In all neurons, the resting potential depends on the ionic gradients that exist across the plasma membrane
CYTOSOL EXTRACELLULARFLUID
[Na+]15 mM
[K+]150 mM
[Cl–]10 mM
[A–]100 mM
[Na+]150 mM
[K+]5 mM
[Cl–]120 mM
–
–
–
–
–
+
+
+
+
+
Plasmamembrane
Figure 48.10
• The concentration of Na+ is higher in the extracellular fluid than in the cytosol while the opposite is true for K+
• By modeling a mammalian neuron with an artificial membrane we can gain a better understanding of the resting potential of a neuron
Figure 48.11a, b
Inner chamber
Outer chamber Inner
chamberOuter chamber
–92 mV +62 mV
Artificialmembrane
Potassiumchannel
K+
Cl–
150 mMKCL
150 mMNaCl
15 mMNaCl
5 mMKCL
Cl–
Na+
Sodium channel
+ –
+ –
+ –
+ –
+ –
+ –
(a) Membrane selectively permeable to K+ (b) Membrane selectively permeable to Na+
• A neuron that is not transmitting signals contains has 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
Gated Ion Channels• Gated ion channels open or close in response to
a change in the membrane potential or binding of a ligand
Action potentials are the signals conducted by axons•If a cell has gated ion channels its membrane potential may change in response to stimuli that open or close those channels
• Some stimuli trigger a hyperpolarization an increase in the magnitude of the membrane potential
Figure 48.12a
+50
0
–50
–100
Time (msec)0 1 2 3 4 5
Threshold
Restingpotential Hyperpolarizations
Me
mb
ran
e p
ote
ntia
l (m
V)
Stimuli
(a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K+. The larger stimulus producesa larger hyperpolarization.
• Other stimuli trigger a depolarization– A reduction in the magnitude of the membrane
potential
Figure 48.12b
+50
0
–50
–100
Time (msec)0 1 2 3 4 5
Threshold
Restingpotential
Depolarizations
Me
mb
ran
e p
ote
ntia
l (m
V)
Stimuli
(b) Graded depolarizations produced by two stimuli that increase membrane permeability to Na+.The larger stimulus produces alarger depolarization.
• Hyperpolarization and depolarization are both called graded potentials because the magnitude of the change in membrane potential varies with the strength of the stimulus
Production of Action Potentials• In most neurons, depolarizations are graded
only up to a certain membrane voltage, called the threshold
• A stimulus strong enough to produce a depolarization that reaches the threshold triggers a different type of response, called an action potential
Figure 48.12c
+50
0
–50
–100
Time (msec)0 1 2 3 4 5 6
Threshold
Restingpotential
Me
mb
ran
e p
ote
ntia
l (m
V)
Stronger depolarizing stimulus
Actionpotential
(c) Action potential triggered by a depolarization that reaches the threshold.
• An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane– Is the type of signal that carries information along
axons
• Both voltage-gated Na+ channels and voltage-gated K+ channels are involved in the production of an action potential
• When a stimulus depolarizes the membrane Na+ channels open, allowing Na+ to diffuse into the cell
• As the action potential subsides– K+ channels open, and K+ flows out of the cell
• A refractory period follows the action potential– During which a second action potential cannot be
initiated
• The generation of an action potential
– – – – – – – –
+ + + + + + + + + + + ++ +
– – – – – –
+ +
– –
+ +
– –
+ +
– –
+ +
– –
+ +
– –
+ +
– –
+ +
– –
+ +
– –
+ +
– –
+ +
– –
+ +
– –
+ +
– –
– –
+ +
– –
+ +
– –
+ +
– –
+ +
Na+ Na+
K+
Na+ Na+
K+
Na+ Na+
K+
Na+
K+
K+
Na+ Na+
5
1 Resting state
2 Depolarization
3 Rising phase of the action potential
4 Falling phase of the action potential
Undershoot
1
2
3
4
5 1
Sodiumchannel
Actionpotential
Resting potential
Time
Plasma membrane
Extracellular fluid ActivationgatesPotassium
channel
Inactivationgate
Threshold
Mem
bran
e po
tent
ial
(mV
)
+50
0
–50
–100
Threshold
Cytosol
Figure 48.13
Depolarization opens the activation gates on most Na+ channels, while the K+ channels’ activation gates remain closed. Na+ influx makes the inside of the membrane positive with respectto the outside.
The inactivation gates on most Na+ channels close, blocking Na+ influx. The activation gates on mostK+ channels open, permitting K+ effluxwhich again makesthe inside of the cell negative.
A stimulus opens theactivation gates on some Na+ channels. Na+
influx through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential.
The activation gates on the Na+ and K+ channelsare closed, and the membrane’s resting potential is maintained.
Both gates of the Na+ channelsare closed, but the activation gates on some K+ channels are still open. As these gates close onmost K+ channels, and the inactivation gates open on Na+ channels, the membrane returns toits resting state.
Conduction of Action Potentials• An action potential can travel long distances
– By regenerating itself along the axon
Figure 48.14
– +– + + + + +
– +– + + + + +
+ –+ – + + + +
+ –+ – + + + +
+ –+ – – – – –+ –+ – – – – –
– – – –– – – –
– –– –
+ +
+ +
+ ++ + – – – –
+ ++ + – – – –
– –– – + + + +– –– – + + + +Na+
Na+
Na+
Actionpotential
Actionpotential
ActionpotentialK+
K+
K+
Axon
An action potential is generated as Na+ flows inward across the membrane at one location.
1
2 The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward.
3 The depolarization-repolarization process isrepeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.
K+
• At the site where the action potential is generated, usually the axon hillock– An electrical current depolarizes the neighboring
region of the axon membrane
Conduction Speed• The speed of an action potential increases with
the diameter of an axon• In vertebrates, axons are myelinated, also
causing the speed of an action potential to increase
• Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction
Cell body
Schwann cell
Myelin sheath
Axon
Depolarized region(node of Ranvier)
++ +
++ +
++ +
++
– –
– –
– –
–––
–
–
–
Figure 48.15
Neurons communicate with other cells at synapses•In an electrical synapse electrical current flows directly from one cell to another via a gap junction•The vast majority of synapses are chemical synapses
• In a chemical synapse, a presynaptic neuron releases chemical neurotransmitters, which are stored in the synaptic terminal
Figure 48.16
Postsynapticneuron
Synapticterminalof presynapticneurons
5 µ
m
• When an action potential reaches a terminal– The final result is the release of neurotransmitters
into the synaptic cleft
Figure 48.17
Presynapticcell
Postsynaptic cell
Synaptic vesiclescontainingneurotransmitter
Presynapticmembrane
Postsynaptic membrane
Voltage-gatedCa2+ channel
Synaptic cleft
Ligand-gatedion channels
Na+
K+
Ligand-gatedion channel
Postsynaptic membrane
Neuro-transmitter
1 Ca2+
2
3
4
5
6
Direct Synaptic Transmission• The process of direct synaptic transmission
involves the binding of neurotransmitters to ligand-gated ion channels
• Neurotransmitter binding causes the ion channels to open, generating a postsynaptic potential
• Postsynaptic potentials fall into two categories– Excitatory postsynaptic potentials (EPSPs)– Inhibitory postsynaptic potentials (IPSPs)
• After its release, the neurotransmitter diffuses out of the synaptic cleft– May be taken up by surrounding cells and degraded
by enzymes
Summation of Postsynaptic Potentials
• Unlike action potentials postsynaptic potentials are graded and do not regenerate themselves
• Since most neurons have many synapses on their dendrites and cell body– A single EPSP is usually too small to trigger an action
potential in a postsynaptic neuron
Figure 48.18a
E1 E1
Restingpotential
Threshold of axon ofpostsynaptic neuron
(a) Subthreshold, nosummation
Terminal branch of presynaptic neuron
Postsynaptic neuron E1
0
–70
Me
mb
ran
e p
ote
ntia
l (m
V)
• If two EPSPs are produced in rapid succession an effect called temporal summation occurs
Figure 48.18b
E1 E1
Actionpotential
(b) Temporal summation
E1
Axonhillock
• In spatial summation EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together
Figure 48.18c
E1 + E2
Actionpotential
(c) Spatial summation
E1
E2
• Through summation an IPSP can counter the effect of an EPSP
Figure 48.18d
E1 E1 + II
(d) Spatial summationof EPSP and IPSP
E1
I
Indirect Synaptic Transmission• In indirect synaptic transmission
– A neurotransmitter binds to a receptor that is not part of an ion channel
• This binding activates a signal transduction pathway– Involving a second messenger in the postsynaptic
cell, producing a slowly developing but long-lasting effect
Neurotransmitters• The same neurotransmitter can produce
different effects in different types of cells
• Major neurotransmitters
Table 48.1
Acetylcholine• Acetylcholine
– Is one of the most common neurotransmitters in both vertebrates and invertebrates
– Can be inhibitory or excitatory
Biogenic Amines• Biogenic amines
– Include epinephrine, norepinephrine, dopamine, and serotonin
– Are active in the CNS and PNS
Amino Acids and Peptides• Various amino acids and peptides are active in
the brain
Gases• Gases such as nitric oxide and carbon monoxide
are local regulators in the PNS
The vertebrate nervous system is regionally specialized•In all vertebrates, the nervous system
– Shows a high degree of cephalization and distinct CNS and PNS components
Figure 48.19
Central nervoussystem (CNS)
Peripheral nervoussystem (PNS)
Brain
Spinal cordCranialnerves
GangliaoutsideCNSSpinalnerves
• 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
Whitematter
Ventricles
Figure 48.20
The Peripheral Nervous System
• The PNS transmits information to and from the CNS– And plays a large role in regulating a vertebrate’s
movement and internal environment
• 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 systemPeripheral
nervous system
Somaticnervoussystem
Autonomicnervoussystem
Sympatheticdivision
Parasympatheticdivision
Entericdivision
Figure 48.21
• The somatic nervous system carries signals to skeletal muscles
• The autonomic nervous system regulates the internal environment, in an involuntary manner– Is divided into the sympathetic, parasympathetic,
and enteric divisions
• The sympathetic and parasympathetic divisions– Have antagonistic effects on target organs
Parasympathetic division Sympathetic division
Action on target organs: Action on target organs:
Location ofpreganglionic neurons:brainstem and sacralsegments of spinal cord
Neurotransmitterreleased bypreganglionic neurons:acetylcholine
Location ofpostganglionic neurons:in ganglia close to orwithin target organs
Neurotransmitterreleased bypostganglionic neurons:acetylcholine
Constricts pupilof eye
Stimulates salivarygland secretion
Constrictsbronchi in lungs
Slows heart
Stimulates activityof stomach and
intestines
Stimulates activityof pancreas
Stimulatesgallbladder
Promotes emptyingof bladder
Promotes erectionof genitalia
Cervical
Thoracic
Lumbar
Synapse
Sympatheticganglia
Dilates pupilof eye
Inhibits salivary gland secretion
Relaxes bronchiin lungs
Accelerates heart
Inhibits activity of stomach and intestines
Inhibits activityof pancreas
Stimulates glucoserelease from liver;inhibits gallbladder
Stimulatesadrenal medulla
Inhibits emptyingof bladder
Promotes ejaculation and vaginal contractionsSacral
Location ofpreganglionic neurons:thoracic and lumbarsegments of spinal cord
Neurotransmitterreleased bypreganglionic neurons:acetylcholine
Location ofpostganglionic neurons:some in ganglia close totarget organs; others ina chain of ganglia near spinal cord
Neurotransmitterreleased bypostganglionic neurons:norepinephrine
Figure 48.22
• The sympathetic division– Correlates with the “fight-or-flight” response
• The parasympathetic division– Promotes a return to self-maintenance functions
• The enteric division– Controls the activity of the digestive tract, pancreas,
and gallbladder
Embryonic Development of the Brain
• In all vertebrates the brain develops from three embryonic regions: the forebrain, the midbrain, and the hindbrain
Figure 48.23a
Forebrain
Midbrain
Hindbrain
Midbrain Hindbrain
Forebrain
(a) Embryo at one month
Embryonic brain regions
• By the fifth week of human embryonic development– Five brain regions have formed from the three
embryonic regions
Figure 48.23b
Telencephalon
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
(b) Embryo at five weeks
MesencephalonMetencephalon
Myelencephalon
Spinal cord
Diencephalon
Telencephalon
Embryonic brain regions
• As a human brain develops further the most profound change occurs in the forebrain, which gives rise to the cerebrum
Figure 48.23c
Brain structures present in adult
Cerebrum (cerebral hemispheres; includes cerebralcortex, white matter, basal nuclei)
Diencephalon (thalamus, hypothalamus, epithalamus)
Midbrain (part of brainstem)
Pons (part of brainstem), cerebellum
Medulla oblongata (part of brainstem)
(c) Adult
Cerebral hemisphereDiencephalon:
Hypothalamus
ThalamusPineal gland(part of epithalamus)
Brainstem:
Midbrain
Pons
Medullaoblongata
Cerebellum
Central canal
Spinal cord
Pituitarygland
The Brainstem• The brainstem consists of three parts
– The medulla oblongata, the pons, and the midbrain
• The medulla oblongata– Contains centers that control several visceral
functions
• The pons– Also participates in visceral functions
• The midbrain– Contains centers for the receipt and integration of
several types of sensory information
Arousal and Sleep• A diffuse network of neurons called the reticular
formation– Is present in the core of the brainstem
Figure 48.24
Eye
Reticular formation
Input from touch, pain, and temperature receptors
Input from ears
• A part of the reticular formation, the reticular activating system (RAS)– Regulates sleep and arousal
The Cerebellum• The cerebellum
– Is important for coordination and error checking during motor, perceptual, and cognitive functions
• The cerebellum is also involved in learning and remembering motor skills
The Diencephalon• The embryonic diencephalon develops into
three adult brain regions– The epithalamus, thalamus, and hypothalamus
• The epithalamus includes the pineal gland and the choroid plexus
• The thalamus is the main input center for sensory information going to the cerebrum and the main output center for motor information leaving the cerebrum
• The hypothalamus regulates – Homeostasis– Basic survival behaviors such as feeding, fighting,
fleeing, and reproducing
Circadian Rhythms• The hypothalamus also regulates circadian
rhythms– Such as the sleep/wake cycle
• Animals usually have a biological clock– Which is a pair of suprachiasmatic nuclei (SCN)
found in the hypothalamus
• Biological clocks usually require external cues to remain synchronized with environmental cycles
Figure 48.25
In the northern flying squirrel (Glaucomys sabrinus), activity normally begins with the onset of darkness and endsat dawn, which suggests that light is an important external cue for the squirrel. To test this idea, researchers monitored the activity of captivesquirrels 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 andwhen it was still.
EXPERIMENT
Light Dark Light
20
15
10
5
1
(a) 12 hr light-12 hr dark cycle (b) Constant darkness
12 16 20 24 4 8 12 12 16 20 24 4 8 12
Time of day (hr) Time of day (hr)
When the squirrelswere exposed to a regular light/darkcycle, their wheel-turning activity (indicated by the dark bars) occurredat roughly the same time every day.However, when they were kept inconstant darkness, their activity phasebegan about 21 minutes later each day.
RESULTS
The northern flying squirrel’s internal clock can run in constant darkness, but it does so onits own cycle, which lasts about 24 hours and 21 minutes. External (light) cues keep the clock running on a 24-hour cycle.
CONCLUSION
Dark
Day
s of
exp
erim
ent
The Cerebrum• The cerebrum develops from the embryonic
telencephalon
• The cerebrum has right and left cerebral hemispheres– That each consist of cerebral cortex overlying white
matter and basal nuclei
Left cerebralhemisphere
Corpuscallosum
Neocortex
Right cerebralhemisphere
Basalnuclei
Figure 48.26
• The basal nuclei are important centers for planning and learning movement sequences
• In mammals the cerebral cortex has a convoluted surface called the neocortex
• In humans, the largest and most complex part of the brain is the cerebral cortex, where sensory information is analyzed, motor commands are issued, and language is generated
• A thick band of axons, the corpus callosum provides communication between the right and left cerebral cortices
The cerebral cortex controls voluntary movement and cognitive functions•Each side of the cerebral cortex has four lobes
– Frontal, parietal, temporal, and occipital
Frontal lobe
Temporal lobe Occipital lobe
Parietal lobe
Frontalassociationarea
Speech
Smell
Hearing
Auditoryassociationarea
Vision
Visualassociationarea
Somatosensoryassociationarea
Reading
Speech
TasteS
omat
osen
sory
cor
tex
Mot
or c
orte
x
Figure 48.27
• Each of the lobes contains primary sensory areas and association areas
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
• In the somatosensory cortex and motor cortex neurons are distributed according to the part of the body that generates sensory input or receives motor input
Figure 48.28
TongueJawLips
Face
Eye
Brow
Neck
Thumb
Fingers
HandW
ristForearmE
lbowS
houlderT
runk
Hip
Knee
Primarymotor cortex Abdominal
organs
Pharynx
Tongue
TeethGumsJaw
Lips
Face
Nose
Eye
Fingers
HandForearm
Elbow
Upper arm
Trunk
Hip
Leg
Thumb
Neck
Head
Genitalia
Primarysomatosensory cortex
Toes
Parietal lobeFrontal lobe
Lateralization of Cortical Function• During brain development, in a process called
lateralization, competing functions segregate and displace each other in the cortex of the left and right cerebral hemispheres
• The left hemisphere becomes more adept at language, math, logical operations, and the processing of serial sequences
• The right hemisphere is stronger at pattern recognition, nonverbal thinking, and emotional processing
Language and Speech• Studies of brain activity
– Have mapped specific areas of the brain responsible for language and speech
Figure 48.29
Hearingwords
Seeingwords
Speakingwords
Generatingwords
Max
Min
• Portions of the frontal lobe, Broca’s area and Wernicke’s area are essential for the generation and understanding of language
Emotions• The limbic system
– Is a ring of structures around the brainstem
Figure 48.30
HypothalamusThalamus
Prefrontal cortex
Olfactorybulb
Amygdala Hippocampus
• This limbic system includes three parts of the cerebral cortex– The amygdala, hippocampus, and olfactory bulb
• These structures interact with the neocortex to mediate primary emotions – And attach emotional “feelings” to survival-related
functions
• Structures of the limbic system form in early development– And provide a foundation for emotional memory,
associating emotions with particular events or experiences
Memory and Learning• The frontal lobes
– Are a site of short-term memory– Interact with the hippocampus and amygdala to
consolidate long-term memory
• Many sensory and motor association areas of the cerebral cortex– Are involved in storing and retrieving words and
images
Cellular Mechanisms of Learning• Experiments on invertebrates have revealed the
cellular basis of some types of learning
Figure 48.31a, b
(a) Touching the siphon triggers a reflex thatcauses the gill to withdraw. If the tail isshocked just before the siphon is touched,the withdrawal reflex is stronger. Thisstrengthening of the reflex is a simple formof learning called sensitization.
(b) Sensitization involves interneurons thatmake synapses on the synaptic terminals ofthe siphon sensory neurons. When the tailis shocked, the interneurons releaseserotonin, which activates a signaltransduction pathway that closes K+
channels in the synaptic terminals ofthe siphon sensory neurons. As a result,action potentials in the siphon sensoryneurons produce a prolongeddepolarization of the terminals. That allowsmore 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 neuronsgenerate action potentials at a higher frequency,producing a more forceful gill withdrawal.
Siphon
Mantle
Gill
Tail
Head
Gill withdrawal pathway
Touchingthe siphon
Shockingthe tail Tail sensory
neuron
Interneuron
Sensitization pathway
Siphon sensoryneuron
Gill motorneuron
Gill
• In the vertebrate brain, a form of learning called long-term potentiation (LTP) involves an increase in the strength of synaptic transmission
Figure 48.32
PRESYNAPTIC NEURON
NO
Glutamate
NMDAreceptor
Signal transduction pathways
NO
Ca2+
AMPA receptor
POSTSYNAPTIC NEURON
Ca2+ initiates the phos-phorylation of AMPA receptors,making them more responsive.Ca2+ also causes more AMPAreceptors to appear in thepostsynaptic membrane.
5
Ca2+ stimulates thepostsynaptic neuron toproduce nitric oxide (NO).
6
The presynapticneuron releases glutamate.1
Glutamate binds to AMPAreceptors, opening the AMPA-receptor channel and depolarizingthe postsynaptic membrane.
2
Glutamate also binds to NMDAreceptors. If the postsynapticmembrane is simultaneouslydepolarized, the NMDA-receptorchannel opens.
3
Ca2+ diffuses into thepostsynaptic neuron.
4
NO diffuses into thepresynaptic neuron, causing it to release more glutamate.
7
P
Consciousness• Modern brain-imaging techniques suggest that
consciousness may be an emergent property of the brain that is based on activity in many areas of the cortex
CNS injuries and diseases are the focus of much research•Unlike the PNS, the mammalian CNS
– Cannot repair itself when damaged or assaulted by disease
•Current research on nerve cell development and stem cells
– May one day make it possible for physicians to repair or replace damaged neurons
Nerve Cell Development• Signal molecules direct an axon’s growth by
binding to receptors on the plasma membrane of the growth cone
• This receptor binding triggers a signal transduction pathway which may cause an axon to grow toward or away from the source of the signal
Figure 48.33a, b
Midline ofspinal cord
Developing axonof interneuron
Growthcone
Netrin-1receptor
Netrin-1
Floorplate
Celladhesionmolecules
SlitreceptorSlit
Developing axon of motor neuron
Netrin-1receptor
Slitreceptor
Slit
Netrin-1
1 Growth toward the floor plate.Cells in the floor plate of thespinal cord release Netrin-1, whichdiffuses away from the floor plateand binds to receptors on thegrowth cone of a developinginterneuron axon. Binding stimulatesaxon growth toward the floor plate.
2 Growth across the mid-line.Once the axon reaches thefloor plate, cell adhesion moleculeson the axon bind to complementarymolecules on floor plate cells,directing the growth of the axonacross the midline.
3 No turning back. Now the axon synthesizes receptors that bind to Slit,a repulsion protein re-leased by floor plate cells.This prevents the axonfrom growing back acrossthe midline.
Netrin-1 and Slit, produced by cellsof the floor plate, bind to receptorson the axons of motor neurons. Inthis case, both proteins act to repelthe axon, directing the motor neuronto grow away from the spinal cord.
(a) Growth of an interneuron axon toward and across the midline of the spinal cord(diagrammed here in cross section)
(b) Growth of a motor neuron axon awayfrom the midline of the spinal cord
• The genes and basic events involved in axon guidance are similar in invertebrates and vertebrates
• Knowledge of these events may be applied one day to stimulate axonal regrowth following CNS damage
Neural Stem Cells• The adult human brain contains stem cells that
can differentiate into mature neurons
Figure 48.34
10
m
• 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
Figure 48.35
Senile plaque Neurofibrillary tangle20 m
• 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