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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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Types of Neurons
Vertebrates have
3 types of neurons
-Sensory neurons
to CNS(afferent neurons)
-Motor neurons
(efferent neurons)
to effectors
(muscles and glands)
-Interneurons
(association neurons)
provide associative
functions
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A Neuron
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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A Neuron’s Charge
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
When a neuron is
not being stimulated,
it maintains a resting
Potential @ -70mv
2. Ion leakage channels = Allow more K+ to
diffuse out than Na+ to diffuse in
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Sodium-potassium pump
Helps create a Neuron’s charge
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Synapses
Synapses are intercellular junctions
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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Neurons Begin to Communicate
@ Synapses
Synapses are intercellular junctions
-Presynaptic cell transmits action potential
-Postsynaptic cell receives it
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More Facts About the Synapse
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 10
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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Excitatory and Inhibitory
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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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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Synaptic Integration
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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
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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Nerve Impulse Transmission
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Nerve Impulse Transmission
Action potentials result when depolarization
reaches the threshold potential about
-55mV
Remember resting potential is -70mV so
Positive charge has to come in to get it
to be -55mV!!
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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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The intensity of a stimulus is coded by the
frequency, not amplitude, of action potentials
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Propagation of an Action Potential
-The action potential is caused by
voltage-gated ion channels
-Two different channels are used:
-Voltage-gated Na+ channels
-Voltage-gated K+ channels
-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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Nervous System Organization
Neurons are supported both structurally and
functionally by cells called neuroglia
-Schwann cells (PNS) and oligodendrocytes
(CNS) produce myelin sheaths surrounding
axons
-In the CNS, myelinated axons form white matter
-In the CNS dendrites/cell bodies form gray matter
-In the PNS, myelinated axons are bundled to form
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nerves
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
Action potential is only
produced at the
nodes of Ranvier
Insulation allows for charge,
accumulated at a distance, to be
detected by neighbor-Na+channels
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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 sensitivity to stimulation by
neurotransmitters
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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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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
The relative sizes of different brain regions have
changed as vertebrates evolved
Forebrain became the dominant
feature...Neuronal correlate to: learning,
association, emotions.
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Compare Brain Diagrams
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Vertebrate Brains
Forebrain is composed of 2 elements:
-Diencephalon
-Thalamus: Integration and relay center
-Hypothalamus: Participates in basic drives &
emotions; controls pituitary gland
-Telencephalon (“end brain”)
-Devoted largely to associative activity
-ALSO Called the CEREBRUM in mammals
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Cerebral Cortex is the outer
layer of the Cerebrum
Cerebral cortex
-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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Cerebral Cortex 3 Regions
Increase in brain size in mammals reflects
the great enlargement of the cerebrum
Cerebral cortex
1Primary motor cortex: Movement control
2Primary somatosensory cortex: Sensory
control
3Association cortex: Higher mental functions
Basal ganglia
-Aggregates of neuron cell bodies
-Form islands of grey matter within the cerebrum’s
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white matter
Sensory/Motor Homunculus
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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
-It serves as the body’s
“information highway”
-Relays messages between
the body and the brain
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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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Cutaneous Spinal Reflex (in Somatic Nervous
System)
Use “SAME DAVE” Acronym
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The Peripheral Nervous System
PNS is divided into the:
Somatic System
&
Autonomic System
.
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Somatic System
In GENERAL, The Somatic System is
responsible for skeletal muscle voluntary
movements & reflexes.
Stretch receptor
Nerve fiber (muscle spindle)
Sensory
Stimulus
neuro
Dorsal root
ganglion
Monosynaptic
synapse
White
matter
Motor neuron
Gray
matter
Skeletal
muscle
Spinal cord
Quadriceps
muscle
(effector)
Response
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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 50
The Autonomic Nervous System
Efferent Ganglia outside of spinal cord
General Model Seen In
Both Sympathetic &
Parasympathetic
Division
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