Chapter 48/49
Nervous Systems
The human brain contains an estimated 100 billion nerve cells, or neurons Each neuron may communicate with thousands of
other neurons Complex information processing network is at work
Different neurons do different things
Functional magnetic resonance imaging (fMRI)can reconstruct a three-dimensional map of brain activity
The results of brain imaging and other research methods revealed 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 They have neurons arranged in nerve nets
Nerve net
(a) Hydra (cnidarian)
Sea Stars
Sea stars have a nerve net in each arm connected by radial nerves to a central nerve ring
Nervering
Radialnerve
(b) Sea star (echinoderm)
Flatworms In relatively simple cephalized animals, such
as flatworms a central nervous system (CNS) is evident
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
(d) Leech (annelid) (e) Insect (arthropod)
Anteriornerve ring
Longitudinalnerve cords
Ganglia
Brain
Ganglia
(f) Chiton (mollusc) (g) Squid (mollusc)
Molluscs
Nervous systems in molluscs correlate with the animals’ lifestyles
Sessile molluscs have simple systems More complex molluscs have more
sophisticated systems
Vertebrates
The central nervous system consists of a brain and dorsal spinal cord
The PNS connects to the CNS via nerves
Brain
Spinalcord(dorsalnervecord)
Sensoryganglion
(h) Salamander (chordate)
Information Processing
Nervous systems process information in three stages: Sensory input, integration, and motor output
Sensor
Effector
Motor output
Integration
Sensory input
Peripheral nervoussystem (PNS)
Central nervoussystem (CNS)
Sensory neurons transmit information from sensors (receptors) 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
Cells of the Nervous System
Neuron Structure Cell body Dendrites Axons Myelin Sheath
Dendrites
Cell body
Nucleus
Axon hillock
AxonSignal direction
Synapse
Myelin sheath
Synapticterminals
Presynaptic cell Postsynaptic cell
Schwann cells Synaptic Terminals Synapse Neurotransmitters
Types of neurons
Sensory Neurons Interneurons Motor Neurons
Supporting Cells (Glia)
Glia are supporting cells They are essential for the structural integrity of
the nervous system and for the normal functioning of 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 Resting Membrane Potential: membrane potential
of a neuron that is not transmitting signals The inside of a cell is negative relative to the
outside
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
Gated Ion Channels
Gated ion channels open or close In response to membrane stretch or the binding of
a specific ligand In response to a change in the membrane
potential
Action Potential
Action potentials are the signals conducted by nerve fibers
If a cell has gated ion channels Its membrane potential may change in response to
stimuli that open or close those channels
Hyperpolarization
Stimuli may trigger an increase in the magnitude of the membrane potential
+50
0
–50
–100
Time (msec)0 1 2 3 4 5
Threshold
Restingpotential Hyperpolarizations
Me
mb
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ote
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V)
Stimuli
(a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K+. The larger stimulus producesa larger hyperpolarization.
Depolarization
stimuli may trigger a reduction in the magnitude of the membrane potential
+50
0
–50
–100
Time (msec)0 1 2 3 4 5
Threshold
Restingpotential
Depolarizations
Me
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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
+50
0
–50
–100
Time (msec)0 1 2 3 4 5 6
Threshold
Restingpotential
Me
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Stronger depolarizing stimulus
Actionpotential
(c) Action potential triggered by a depolarization that reaches the threshold.
Action Potential
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
Step 1…Sodium gates
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
Step 2…Potassium gates
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
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
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.
– +– + + + + +
– +– + + + + +
+ –+ – + + + +
+ –+ – + + + +
+ –+ – – – – –
+ –+ – – – – –
– – – –
– – – –
– –
– –
+ +
+ +
+ ++ + – – – –
+ ++ + – – – –
– –– – + + + +
– –– – + + + +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+
Conduction of Action Potentials
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
Saltatory Conduction
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)
++ +
++ +
++ +
++
– –
– –
– –
–––
–
–
–
Synapse Electrical synapse
Electrical current flows directly from one cell to another via a gap junction
Chemical synapse A presynaptic neuron
releases chemical neurotransmitters, which are stored in the synaptic terminal
Postsynapticneuron
Synapticterminalof presynapticneurons
5 µ
m
Neurotransmitter release
When an action potential reaches a terminal the final result is the release of neurotransmitters into the synaptic cleft
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
Synaptic Transmission
Neurotransmitters bind to ligand-gated ion channels
Binding causes the ion channels to open, generating a postsynaptic potential and generation of action potential
Fate of neurotransmitter in the cleft Diffuses out of the synaptic cleft Taken up by surrounding cells and degraded by
enzymes Degraded in the cleft
Neurotransmitters
Different neurons may release different neurotransmitters
The same neurotransmitter can produce different effects in different types of cells
Neurotransmitters
Neurotransmitters
Acetylcholine Is one of the most common neurotransmitters in both
vertebrates and invertebrates Can be inhibitory or excitatory
Biogenic amines Include epinephrine, norepinephrine, dopamine, and
serotonin Are active in the CNS and PNS
Amino acids and peptides Are active in the brain
Gases such as nitric oxide and carbon monoxide Are local regulators in the PNS
Vertebrate Nervous System
Regionally specialized In all vertebrates, the
nervous system shows a high degree of cephalization and distinct CNS and PNS components
Central nervoussystem (CNS)
Peripheral nervoussystem (PNS)
Brain
Spinal cordCranialnerves
GangliaoutsideCNSSpinalnerves
Central Nervous System
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
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
PNS components
The somatic nervous system The autonomic nervous system
Peripheralnervous system
Somaticnervoussystem
Autonomicnervoussystem
Sympatheticdivision
Parasympatheticdivision
Entericdivision
Somatic and Autonomic
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
Sympathetic and parasympathetic divisions They 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
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
The Brainstem The brainstem consists of three parts: the medulla
oblongata, the pons, and the midbrain
Brainstem
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
All three areas are center of reflex actions
Arousal and Sleep A diffuse network of
neurons called the reticular formation is present in the core of the brainstem
A part of the reticular formation, the reticular activating system (RAS) regulates sleep and arousal
Eye
Reticular formation
Input from touch, pain, and temperature receptors
Input from ears
The Cerebellum Important for
coordination and error checking during motor, perceptual, and cognitive functions
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
The Cerebrum
Cerebral Hemispheres The cerebrum has right and left cerebral
hemispheres that each consist of cerebral cortex overlying white matter and basal nuclei
Left cerebralhemisphere
Corpuscallosum
Cortex
Right cerebralhemisphere
Basalnuclei
Basal Nuclei
The basal nuclei Are important centers for planning and learning
movement sequences
Cerebral Cortex
In mammals The cerebral cortex has a convoluted surface
called the neocortex In humans it’s the largest and most complex
part of the brain: 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
Cerebral Cortex
The cerebral cortex controls voluntary movement and cognitive functions
Frontal, parietal, temporal, and occipitalFrontal 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
Information Processing in the Cerebral Cortex
Each of the lobes contains primary sensory areas and association areas
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
Hearingwords
Seeingwords
Speakingwords
Generatingwords
Max
MinPET scans showing brain activity levels
Language and Speech
Portions of the frontal lobe, Broca’s area and Wernicke’s area are essential for the generation and understanding of language
Broca’s area damage: comprehend speech but have impaired ability to mechanically form speech
Wernicke’s area damage: no language comprehension, can’t understand either spoken or written language. Speak in rapid nonsensical manner
Emotions The limbic system is a ring of structures
around the brainstem
HypothalamusThalamus
Prefrontal cortex
Olfactorybulb
Amygdala Hippocampus
Limbic system
This limbic system includes three parts of the cerebral cortex: The amygdala, hippocampus, and olfactory bulb. Also some parts of the thalamus and hypothalamus
These structures interact with the neocortex to mediate primary emotions and attach emotional “feelings” to survival-related functions (reproduction, aggression, feeding)
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
CNS injuries and diseases
Unlike the PNS, the mammalian CNS Cannot repair itself when damaged or diseased
Research on nerve cell development and stem cells Hot area of research May one day make it possible for physicians to
repair or replace damaged neurons
Neural Stem Cells
The adult human brain contains stem cells that can differentiate into mature neurons
The induction of stem cell differentiation and the transplantation of cultured stem cells Are potential methods for
replacing neurons lost to trauma or disease
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Diseases and Disorders of the Nervous System
Mental illnesses and neurological disorders take a huge toll on society patient’s loss of a productive life high cost of long-term health care The direct costs of mental health services in the United States in 1996 totaled
$69.0 billion. This figure represents 7.3 percent of total health spending. An additional $17.7 billion was spent on Alzheimer’s disease The indirect costs of
mental illness were estimated in 1990 at $78.6 billion
Causes (genetic & environmental) are not often clear at present
Treatments that exist are not great and usually amount to control of symptoms and not cure for the underlying abnormality
Schizophrenia
About 1% of the world’s population suffers from schizophrenia
There are several forms each characterized by an inability to distinguish reality
Symptoms Hallucinations, delusions, blunted emotions, and
many other symptoms Treatments have focused on brain pathways
that use dopamine as a neurotransmitter
Depression
Two broad forms of depressive illness: 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 (amyloid protein) in the brain
Senile plaque Neurofibrillary tangle 20 m
Parkinson’s Disease
Parkinson’s disease is a motor disorder Caused by the death of dopamine-secreting
neurons in midbrain nucleus (substantia nigra) Characterized by difficulty in initiating movements,
slowness of movement, and rigidity
Multiple sclerosis
Caused by immune destruction of myelin sheaths
Leads to improper nerve impulse conduction and associated loss of function
Nerve toxins
Tetanus Blocks inhibitory synapses leading to spastic
paralysis Botulism
Prevents release of Acetylcholine leading to flaccid paralysis
Curare Prevents binding of Acetylcholine to receptors
leading to flaccid paralysis