Transcript Chapter 39 Neural Signaling

Biology, Seventh Edition
Solomon • Berg • Martin
Chapter 39
Neural Signaling
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 39 Neural Signalling
• Neural signaling process
• Reception of information by a
sensory receptor
• Transmission by afferent neuron
to the central nervous system
• Integration by CNS interneurons
• Efferent neuron transmission
• Action by effectors
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Biology, Seventh Edition
CHAPTER 39 Neural Signalling
Stimulus response
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CHAPTER 39 Neural Signalling
• Glial cells
• Support and nourish neurons
• Microglia are phagocytic cells
• Astrocytes
–Some are phagocytic
–Others help regulate composition of
the CNS extracellular fluid
–May induce and stabilize synapses
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CHAPTER 39 Neural Signalling
• Oligodendrocytes
• Glial cells that form myelin
sheaths around axons in the CNS
• Schwann cells
• Form sheaths around axons in
the peripheral nervous system
(PNS)
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CHAPTER 39 Neural Signalling
• Structure of a typical neuron
• A cell body contains the nucleus
and most of the organelles
• Many branched dendrites extend
from the cell body
• Single long axon extends from
the cell body and forms branches
called axon collaterals
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CHAPTER 39 Neural Signalling
Structure of a multipolar neuron
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CHAPTER 39 Neural Signalling
• Dendrites receive stimuli and
send signals to the cell body
• Axon transmits signals into its
terminal branches that end in
synaptic terminals
• Many axons are surrounded by
an insulating myelin sheath
formed of Schwann cells
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CHAPTER 39 Neural Signalling
• Nodes of Ranvier
• Gaps in the sheath between
successive Schwann cells
• Nerve
• Several hundred axons wrapped
in connective tissue
• Ganglion
• Mass of neuron cell bodies
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CHAPTER 39 Neural Signalling
Nerve structure
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CHAPTER 39 Neural Signalling
• Neuron resting potential
• In a resting neuron, the inner
surface of the plasma membrane
is negatively charged compared
with the outside
• Potential difference of about -70
millivolts (mV) across the
membrane
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CHAPTER 39 Neural Signalling
• Differences in concentrations of
specific ions—Na+ (sodium) and
K+ (potassium)—inside the cell
relative to the extracellular fluid
• Selective permeability of the
plasma membrane to these ions
• Ions pass through specific
passive ion channels
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CHAPTER 39 Neural Signalling
• K+ leaks out more readily than
Na+ can leak in
• Cl- (chlorine) ions accumulate
along the inner surface of the
plasma membrane
• Gradients that determine the
resting potential are maintained
by ATP
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CHAPTER 39 Neural Signalling
Resting potential
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CHAPTER 39 Neural Signalling
• Sodium-potassium pumps
• Continuously transport three
sodium ions out of the neuron for
every two potassium ions
transported in
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CHAPTER 39 Neural Signalling
Voltage-activated ion channels
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CHAPTER 39 Neural Signalling
• Depolarized membrane
• Stimulus caused the membrane
potential to become less negative
• Hyperpolarized membrane
• Membrane potential becomes
more negative than the resting
potential
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CHAPTER 39 Neural Signalling
• Graded potential
• Local response that varies in
magnitude depending on the
strength of the applied stimulus
• Fades out within a few mm of its
point of origin
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CHAPTER 39 Neural Signalling
• Action potential
• Wave of depolarization that
moves down the axon
–Voltage across the membrane
declines to a critical point
–Voltage-activated ion channels
open
–Na+ flows into the neuron
–Action potential is generated
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Action potential
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CHAPTER 39 Neural Signalling
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CHAPTER 39 Neural Signalling
• Action potential is an all-ornone response
• No variation exists in the strength
of a single impulse
• Membrane potential either
exceeds threshold level, leading
to transmission of an action
potential, or it does not
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CHAPTER 39 Neural Signalling
• Repolarization
• As the action potential moves
down the axon, repolarization
occurs behind it
• During depolarization, the axon
enters a refractory period
–Time when it cannot transmit
another action potential
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CHAPTER 39 Neural Signalling
Resting state
Depolarization
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CHAPTER 39 Neural Signalling
Repolarization
Return to resting state
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CHAPTER 39 Neural Signalling
• Continuous conduction
• Takes place in unmyelinated
neurons
• Involves the entire axon plasma
membrane
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CHAPTER 39 Neural Signalling
• Saltatory conduction
• More rapid than continuous
conduction
• Takes place in myelinated
neurons
• Depolarization skips along the
axon from one node of Ranvier to
the next
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CHAPTER 39 Neural Signalling
Saltatory
conduction
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CHAPTER 39 Neural Signalling
• Synapses
• Junction between two neurons or
between a neuron and effector
• Most synapses are chemical
• Transmission depends on release
of neurotransmitter from synaptic
vesicles in the synaptic terminals
of a presynaptic neuron
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CHAPTER 39 Neural Signalling
• Neurotransmitters
• Acetylcholine
–Triggers contraction of skeletal
muscle
• Glutamate
–Main excitatory neurotransmitter in
the brain
• GABA
–Inhibitory neurotransmitter
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CHAPTER 39 Neural Signalling
• Biogenic amines
–Norepinephrine
–Serotonin
–Dopamine
–Play important roles in regulating
mood
–Dopamine is important in motor
function
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CHAPTER 39 Neural Signalling
• Neuropeptides
–Endorphinsm
–Enkephalins
• Nitric oxide (NO)
–Gaseous neurotransmitter that
transmits signals from the
postsynaptic neuron to the
presynaptic neuron
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CHAPTER 39 Neural Signalling
• Synaptic transmission
• Calcium ions cause synaptic
vesicles to fuse with the
presynaptic membrane and
release neurotransmitter into the
synaptic cleft
• Neurotransmitter combines with
specific receptors on a
postsynaptic neuron
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Synaptic transmission
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CHAPTER 39 Neural Signalling
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CHAPTER 39 Neural Signalling
• Neurotransmitter receptors
• Many are proteins that form
ligand-gated ion channels
• Others work through a second
messenger such as cAMP
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CHAPTER 39 Neural Signalling
• Excitatory and inhibitory signals
• Excitatory postsynaptic potential
(EPSP)
–Bring the neuron closer to firing
• Inhibitory postsynaptic potential
(IPSP)
–Move the neuron farther away from
its firing level
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CHAPTER 39 Neural Signalling
• A postsynaptic neuron
integrates incoming stimuli and
“decides” whether or not to fire
• Each EPSP or IPSP is a graded
potential
• Varies in magnitude depending
on the strength of the stimulus
applied
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CHAPTER 39 Neural Signalling
• The mechanism of neural
integration is summation
• Process of adding and
subtracting incoming signals
• By summation of several EPSPs,
the neuron may be brought to
critical firing level
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CHAPTER 39 Neural Signalling
• Temporal summation
• Repeated stimuli cause new
EPSPs to develop before
previous EPSPs have decayed
• Spatial summation
• Postsynaptic neuron stimulated
at several different places
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CHAPTER 39 Neural Signalling
• Convergence
• Single neuron is controlled by
converging signals from two or
more presynaptic neurons
• Permits the CNS to integrate
incoming information from
various sources
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CHAPTER 39 Neural Signalling
• Divergence
• Single presynaptic neuron
stimulates many postsynaptic
neurons
• Allows widespread effect
• Reverberation
• Axon collateral synapses with an
interneuron
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Reverberation
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CHAPTER 39 Neural Signalling