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The Nervous System
Chapter 44
Nervous System Organization
All animals must be able to respond to
environmental stimuli
-Sensory receptors = Detect stimulus
-Motor effectors = Respond to it
-The nervous system links the two
-Consists of neurons and supporting cells
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Nervous System Organization
Vertebrates have three types of neurons
-Sensory neurons (afferent neurons) carry
impulses to central nervous system (CNS)
-Motor neurons (efferent neurons) carry
impulses from CNS to effectors (muscles
and glands)
-Interneurons (association neurons)
provide more complex reflexes and
associative functions (learning and memory)
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Nervous System Organization
The CNS consists of the brain and spinal cord
The peripheral nervous system (PNS)
consists of sensory and motor neurons
-Somatic NS stimulates skeletal muscles
-Autonomic NS stimulates smooth and
cardiac muscles, as well as glands
-Sympathetic and parasympathetic NS
-Counterbalance each other
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CNS
Brain and Spinal Cord
Motor Pathways
PNS
Sensory Pathways
Sensory neurons
registering external
stimuli
Sensory neurons
registering external
stimuli
Somatic nervous
system
(voluntary)
Sympathetic nervous
system
"fight or flight"
Autonomic nervous
system
(involuntary)
Parasympathetic nervous
system
"rest and repose"
central nervous system (CNS)
peripheral nervous system (PNS)
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Nervous System Organization
Neurons have the same basic structure
-Cell body = Enlarged part containing
nucleus
-Dendrites = Short, cytoplasmic extensions
that receive stimuli
-Axon = Single, long extension that
conducts impulses away from cell body
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Nervous System Organization
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Nervous System Organization
Neurons are supported both structurally and
functionally by cells called neuroglia
-Schwann cells and oligodendrocytes
produce myelin sheaths surrounding axons
-In the CNS, myelinated axons form white
matter
-Dendrites/cell bodies form gray matter
-In the PNS, myelinated axons are bundled
to form nerves
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Nervous System Organization
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Nerve Impulse Transmission
A potential difference exists across every
cell’s plasma membrane
-Negative pole = Cytoplasmic side
-Positive pole = Extracellular fluid side
When a neuron is not being stimulated, it
maintains a resting potential
-Ranges from -40 to -90 millivolts (mV)
-Average about -70 mV
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Nerve Impulse Transmission
The inside of the cell is more negatively
charged than the outside because of:
1. Sodium-potassium pump = Brings two
K+ into cell for every three Na+ it pumps out
2. Ion leakage channels = Allow more K+ to
diffuse out than Na+ to diffuse in
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Nerve Impulse Transmission
There is a buildup of positive charge outside
and negative charge inside the membrane
-This electrical potential is an attractive force
to bring K+ ions back into the cell
-Balance between diffusional and electrical
forces leads to the equilibrium potential
The resting membrane potential can be
viewed using a voltmeter and two electrodes
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Nerve Impulse Transmission
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Nerve Impulse Transmission
There are two types of potentials:
-Graded potentials and action potentials
Graded potentials are small transient
changes in membrane potential due to
activation of gated ion channels
-Most are closed in the normal resting cell
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Nerve Impulse Transmission
Chemically-gated or ligand-gated channels
-Ligands are hormones or neurotransmitters
-Induce opening
and cause changes
in cell membrane
permeability
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Nerve Impulse Transmission
Depolarization makes the membrane
potential more positive, whereas a
hyperpolarization makes it more negative
-These small changes result in graded
potentials
-Can reinforce or negate each other
Summation is the ability of graded potentials
to combine
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Nerve Impulse Transmission
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Nerve Impulse Transmission
Action potentials result when depolarization
reaches the threshold potential
The action potential is caused by voltagegated ion channels
-Two different channels are used:
-Voltage-gated Na+ channels
-Voltage-gated K+ channels
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Nerve Impulse Transmission
When the threshold voltage is reached,
sodium channels open rapidly
-Transient influx of Na+ causes the
membrane to depolarize
In contrast, potassium channel opens slowly
-Efflux of K+ repolarizes the membrane
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Nerve Impulse Transmission
The action potential has three phases:
-Rising, falling and undershoot
Action potentials are always separate, all-ornone events with the same amplitude
-Do not add up or interfere with each other
The intensity of a stimulus is coded by the
frequency, not amplitude, of action
potentials
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Nerve Impulse Transmission
Each action potential, in its rising phase,
reflects a reversal in membrane polarity
-Positive charges due to influx of Na+ can
depolarize the adjacent region to threshold
-And so the next region produces its own
action potential
-Meanwhile, the previous region repolarizes
back to the resting membrane potential
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Nerve Impulse Transmission
Two ways to increase velocity of conduction:
1. Axon has a large diameter
-Less resistance to current flow
-Found primarily in invertebrates
2. Axon is myelinated
-Action potential is only produced at the
nodes of Ranvier
-Impulse jumps from node to node
-Saltatory conduction
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Nerve Impulse Transmission
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Synapses
Synapses are intercellular junctions
-Presynaptic cell transmits action potential
-Postsynaptic cell receives it
Two basic types: electrical and chemical
Electrical synapses involve direct
cytoplasmic connections between the two
cells formed by gap junctions
-Relatively rare in vertebrates
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Synapses
Chemical synapses have a synaptic cleft
between the two cells
-End of presynaptic
cell contains
synaptic vesicles
packed with
neurotransmitters
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Synapses
Action potential triggers influx of Ca2+
-Synaptic vesicles fuse with cell membrane
-Neurotransmitter is released by exocytosis
-Diffuses to other side of cleft and binds
to chemical- or ligand-gated receptor
proteins
-Neurotransmitter action is terminated by
enzymatic cleavage or cellular uptake
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Synapses
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Neurotransmitters
Acetylcholine (ACh)
-Crosses the synapse
between a motor
neuron and a muscle
fiber
-Neuromuscular
junction
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Neurotransmitters
Acetylcholine (ACh)
-Binds to ligand-gated receptor in the
postsynaptic membrane
-Produces a depolarization called an
excitatory postsynaptic potential (EPSP)
-Stimulates muscle contraction
-Acetylcholinesterase (AChE) degrades
ACh
-Causes muscle relaxation
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Neurotransmitters
Amino acids
-Glutamate is the major excitatory
neurotransmitter in the vertebrate CNS
-Glycine and GABA (g-aminobutyric acid)
are inhibitory neurotransmitters
-Open ligand-gated channels for Cl–
-Produce a hyperpolarization called an
inhibitory postsynaptic potential
(IPSP)
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Neurotransmitters
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Neurotransmitters (Cont.)
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Neurotransmitters
Biogenic amines
-Epinephrine (adrenaline) and
norepinephrine are responsible for the
“fight or flight” response
-Dopamine is used in some areas of the
brain that control body movements
-Serotonin is involved in the regulation of
sleep
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Neurotransmitters
Neuropeptides
-Substance P is released from sensory
neurons activated by painful stimuli
-Intensity of pain perception depends on
enkephalins and endorphins
Nitric oxide (NO)
-A gas ; produced as needed from arginine
-Causes smooth muscle relaxation
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Synaptic Integration
Integration of EPSPs (depolarization) and
ISPSs (hyperpolarization) occurs on the
neuronal cell body
-Small EPSPs add together to bring the
membrane potential closer to the threshold
-IPSPs subtract from the depolarizing effect
of EPSPs
-And will therefore deter the membrane
potential from reaching threshold
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Synaptic Integration
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Synaptic Integration
There are two ways that the membrane can
reach the threshold voltage
-Spatial summation
-Many different dendrites produce EPSPs
-Temporal summation
-One dendrite produces repeated EPSPs
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Drug Addiction
Prolonged exposure to a stimulus may cause
cells to lose the ability to respond to it
-This process is called habituation
-The cell decreases the number of
receptors because there is an
abundance of neurotransmitters
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Drug Addiction
Cocaine affects neurons in the brain’s
“pleasure pathways” (limbic system)
-Binds dopamine transporters and prevents
the reuptake of dopamine
-Dopamine survives longer in the synapse
and fires pleasure pathways more and more
-Prolonged exposure triggers the limbic
system neurons to reduce receptor numbers
-The cocaine user is now addicted
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Drug Addiction
Nicotine binds directly to a specific receptor
on postsynaptic neurons of the brain
-Brain adjusts to prolonged exposure by
“turning down the volume” in two ways:
1. Making fewer nicotine receptors
2. Altering the pattern of activation of
the nicotine receptors
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The Central Nervous System
Sponges are only major phylum without nerves
Cnidarians have the simplest nervous system
-Neurons linked to each other in a nerve net
-No associative activity
Free-living flatworms (phylum Platyhelminthes)
are simplest animals with associative activity
-Two nerve cords run down the body
-Permit complex muscle control
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Vertebrate Brains
All vertebrate brains have three basic divisions:
-Hindbrain or rhombencephalon
-Midbrain or mesencephalon
-Forebrain or prosencephalon
In fishes,
-Hindbrain = Largest portion
-Midbrain = Processes visual information
-Forebrain = Processes olfactory information
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Vertebrate Brains
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Vertebrate Brains
The relative sizes of different brain regions
have changed as vertebrates evolved
-Forebrain became the dominant feature
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Vertebrate Brains
Forebrain is composed of two elements:
-Diencephalon
-Thalamus: Integration and relay center
-Hypothalamus: Participates in basic
drives & emotions; controls pituitary gland
-Telencephalon (“end brain”)
-Devoted largely to associative activity
-Called the cerebrum in mammals
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Cerebrum
The increase in brain size in mammals reflects
the great enlargement of the cerebrum
-Split into right and left cerebral
hemispheres, which are connected by a
tract called the corpus callosum
-Each hemisphere receives sensory input
from the opposite side
-Hemispheres are divided into: frontal,
parietal, temporal and occipital lobes
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Cerebrum
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Cerebrum
Cerebral cortex
-Outer layer of the cerebrum
-Contains about 10% of all neurons in brain
-Highly convoluted surface
-Increases threefold the surface area of
the human brain
-Divided into three regions, each with a
specific function
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Cerebrum
Cerebral cortex
-Primary motor cortex: Movement control
-Primary somatosensory cortex: Sensory
control
-Association cortex: Higher mental functions
Basal ganglia
-Aggregates of neuron cell bodies
-Form islands of grey matter within the
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cerebrum’s white matter
Cerebrum
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Cerebrum
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Other Brain Structures
Thalamus
-Integrates visual, auditory and
somatosensory information
Hypothalamus
-Integrates visceral activities
-Controls pituitary gland
-Forms limbic system, with hippocampus
and amygdala
-Responsible for emotional responses 58
Complex Functions of the Brain
Sleep and arousal
-One section of reticular formation controls
consciousness and alertness
-Reticular-activating system controls
both sleep and the waking state
-Brain state can be monitored by means of
an electroencephalogram (EEG)
-Records electrical activity
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Complex Functions of the Brain
Language
-Left hemisphere is “dominant” hemisphere
-Adept at sequential reasoning
Spatial recognition
-Right hemisphere is adept at spatial
reasoning
-Primarily involved in musical ability
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Complex Functions of the Brain
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Complex Functions of the Brain
Memory
-Appears dispersed across the brain
-Short-term memory is stored in the form of
transient neural excitations
-Long-term memory appears to involve
structural changes in neural connections
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Complex Functions of the Brain
Alzheimer disease is a condition where
memory and thought become dysfunctional
-Two causes have been proposed
1. Nerve cells are killed from the outside in
-External protein: b-amyloid
2. Nerve cells are killed from the inside out
-Internal proteins: tau (t)
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Spinal Cord
The spinal cord is a cable of neurons
extending from the brain down through the
backbone
-Enclosed and
protected by
the vertebral
column and
the meninges
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Spinal Cord
It serves as the body’s “information highway”
-Relays messages between the body and
the brain
It also functions in reflexes
-The knee-jerk reflex is monosynaptic
-However, most reflexes in vertebrates
involve a single interneuron
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The Peripheral Nervous System
The PNS consists of nerves and ganglia
-Nerves are bundles
of axons bound by
connective tissue
-Ganglia are
aggregates of
neuron cell bodies
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The Peripheral Nervous System
Sensory neurons:
-Axons enter the dorsal surface of the spinal
cord and form dorsal root of spinal nerve
-Cell bodies are grouped outside the spinal
cord in dorsal root ganglia
Motor neurons:
-Axons leave from the ventral surface and
form ventral root of spinal nerve
-Cell bodies are located in the spinal cord
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The Somatic Nervous System
Somatic motor neurons stimulate the skeletal
muscles to contract
-In response to conscious command or
reflex actions
The antagonist of the muscle is inhibited by
hyperpolarization (IPSPs) of spinal motor
neurons
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The Autonomic Nervous System
Composed of the sympathetic and
parasympathetic divisions, plus the
medulla oblongata
In both, efferent motor pathway has 2 neurons
-Preganglionic neuron: exits the CNS and
synapses at an autonomic ganglion
-Postganglionic neuron: exits the ganglion
and regulates visceral effectors
-Smooth or cardiac muscle or glands 71
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The Autonomic Nervous System
Sympathetic division
-Preganglionic neurons originate in the
thoracic and lumbar regions of spinal cord
-Most axons synapse in two parallel chains
of ganglia right outside the spinal cord
Parasympathetic division
-Preganglionic neurons originate in the brain
and sacral regions of spinal cord
-Axons terminate in ganglia near or even
within internal organs
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The Autonomic Nervous System
Autonomic effects are mediated by the action
of G protein-coupled receptors
-The receptor is activated by binding to its
ligand (Ach, for example)
-The G protein is activated
-It activates the effector protein
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The Autonomic Nervous System
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