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
• Nervous system links the two
– Consists of neurons and supporting cells
2
Nervous System Organization
• Vertebrates have three types of neurons
1. Sensory neurons (afferent neurons) carry
impulses to central nervous system (CNS)
2. Motor neurons (efferent neurons) carry
impulses from CNS to effectors (muscles
and glands)
3. Interneurons (association neurons) provide
more complex reflexes and associative
functions (learning and memory)
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Nervous System Organization
• Central nervous system (CNS )
– Brain and spinal cord
• Peripheral nervous system (PNS)
– 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
8
Nervous System Organization
• Neuroglia
– Support neurons both structurally and
functionally
– 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
10
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
11
Nerve Impulse Transmission
• The inside of the cell is more negatively
charged than the outside
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
• Two major forces act on ions in
establishing the resting membrane
potential
1. Electrical potential produced by unequal
distribution of charges
2. Concentration gradient produced by unequal
concentrations of molecules from one side of
the membrane to the other
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Nerve Impulse Transmission
• Sodium–potassium pump creates significant
concentration gradient
• Concentration of K+ is much higher inside the
cell
• Membrane not permeable to negative ions
• Leads to buildup of positive charges outside and
negative charges inside cell
• Attractive force to bring K+ back inside cell
• Equilibrium potential – balance between
diffusional force and electrical force
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Nerve Impulse Transmission
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Nerve Impulse Transmission
• Uniqueness of neurons compared with other
cells is not the production and maintenance of
the resting membrane potential
• Rather the sudden temporary disruptions to the
resting membrane potential that occur in
response to stimuli
• 2 types of changes
– Graded potentials
– Action potentials
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Nerve Impulse Transmission
• Graded potentials
– Small transient changes in membrane
potential due to activation of gated ion
channels
– Each gated channel is selective
– 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
• Hyperpolarization makes it more negative
• These small changes result in graded potentials
• Size depends on either the strength of the
stimulus or the amount of ligand available to
bind with their receptors
• Can reinforce or negate each other
• Summation is the ability of graded potentials to
combine
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Nerve Impulse Transmission
21
Nerve Impulse Transmission
• Action potentials
– Result when depolarization reaches the
threshold potential (–55 mV)
– Depolarizations bring a neuron closer to the
threshold
– Hyperpolarizations move the neuron further
from the threshold
– Caused by voltage-gated ion channels
• Voltage-gated Na+ channels
• Voltage-gated K+ channels
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Nerve Impulse Transmission
• Voltage-gated Na+ channels
– Activation gate and inactivation gate
– At rest, activation gate closed, inactivation gate open
– Transient influx of Na+ causes the membrane to
depolarize
• Voltage-gated K+ channels
– Single activation gate that is closed in the resting
state
– K+ 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, allor-none events with the same amplitude
• Do not add up or interfere with each other
• Intensity of a stimulus is coded by the
frequency, not amplitude, of action
potentials
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Nerve Impulse Transmission
• Propagation of action potentials
– 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
• Signal does not go back toward cell body
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Nerve Impulse Transmission
• Two ways to increase velocity of
conduction
– Axon has a large diameter
• Less resistance to current flow
• Found primarily in invertebrates
– 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
30
Synapses
• Intercellular junctions with the dendrites of
other neurons, with muscle cells, or with
gland cells
• Presynaptic cell transmits action potential
• Postsynaptic cell receives it
• Two basic types: electrical and chemical
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• Electrical synapses
– Involve direct cytoplasmic
connections between the
two cells formed by gap
junctions
– Relatively rare in
vertebrates
• 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
• Chemical 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
– Produces graded potentials in the
postsynaptic membrane
– 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 receptor in the postsynaptic
membrane
– Causes ligand-gated ion channels to open
– 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
• 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
• 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
40
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
• 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
1. Spatial summation
• Many different dendrites produce EPSPs
2. Temporal summation
• One dendrite produces repeated EPSPs
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Drug Addiction
• Habituation
– Prolonged exposure to a stimulus may cause
cells to lose the ability to respond to it
– Cell decreases the number of receptors
because there is an abundance of
neurotransmitters
– In long-term drug use, means that more of the
drug is needed to obtain the same effect
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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
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Drug Addiction
• Nicotine
– Binds directly to a specific receptor on
postsynaptic neurons of the brain
– Binds to a receptor for acetylcholine
– Brain adjusts to prolonged exposure by
“turning down the volume” by
• Making fewer receptors to which nicotine binds
• 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
• All of the subsequent evolutionary changes in nervous
systems can be viewed as a series of elaborations on
the characteristics already present in flatworms
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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
• 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 and
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 – gray matter
• Participate in the control of body movements
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Each of these regions of the cerebral cortex is
associated with a different region of the body
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Other Brain Structures
• Thalamus
– Integrates visual, auditory, and
somatosensory information
• Hypothalamus
– Integrates visceral activities
– Controls pituitary gland
• Limbic system
– Hypothalamus, hippocampus, and amygdala
– Responsible for emotional responses
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Complex Functions of the Brain
• Sleep and arousal
– One section of reticular formation is the
reticular-activating system
• Controls consciousness and alertness
– 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
• Different regions control various language activities
• Adept at sequential reasoning
– Right hemisphere is adept at spatial
reasoning
• Primarily involved in musical ability
• Nondominant hemisphere is also important for the
consolidation of memories of nonverbal
experiences
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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
– Two parts of the temporal lobes, the
hippocampus and the amygdala, are involved
in both short-term memory and its
consolidation into long-term memory
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Complex Functions of the Brain
• Alzheimer disease
– 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 ()
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Spinal Cord
• 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
• 2 zones
– Inner zone is gray matter
• Primarily consists of the cell bodies of
interneurons, motor neurons, and neuroglia
– Outer zone is white matter
• Contains cables of sensory axons in the dorsal
columns and motor axons in the ventral columns
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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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Knee-jerk reflex is monosynaptic
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Most reflexes in vertebrates involve a single interneuron
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The Peripheral Nervous System
• Consists of nerves and
ganglia
– Nerves are bundles of axons
bound by connective tissue
– Ganglia are aggregates of
neuron cell bodies
• Function is to receive info
from the environment,
convey it to the CNS, and to
carry responses to effectors
such as muscle cells
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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
– 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
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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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