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Neural Signaling
Chapter 40
Learning Objective 1
•
Describe the processes involved in neural
signaling: reception, transmission,
integration, and action by effectors
Neural Signaling 1
(1) Reception of information
•
by a sensory receptor
(2) Transmission by an afferent neuron
•
to the central nervous system (CNS)
(3) Integration by interneurons
•
in the central nervous system (CNS)
Neural Signaling 2
(4) Transmission by an efferent neuron
•
to other neurons or effector
(5) Action by effectors
•
the muscles and glands
Peripheral Nervous System (PNS)
•
Made up of
•
•
sensory receptors
neurons outside the CNS
Response to
Stimulus
External stimulus
(e.g., vibration, movement,
light, odor)
Internal stimulus
(e.g., change in blood
pH or blood pressure)
RECEPTION
Detection by
external
sense organs
Detection by
internal
sense organs
TRANSMISSION
Sensory (afferent) neurons
transmit information
Fig. 40-1a, p. 846
Central Nervous System
(brain and spinal cord)
INTEGRATION
Interneurons sort
and interpret
information
TRANSMISSION
Motor (efferent) neurons
transmit impulses
ACTION BY EFFECTORS
(muscles and glands)
e.g., animal
runs away
e.g., espiration
rate increases;
blood pressure
rises
Fig. 40-1b, p. 846
External stimulus
(e.g., vibration, movement,
light, odor)
Internal stimulus
(e.g., change in blood
pH or blood pressure)
RECEPTION
Detection by
internal
sense organs
Detection by external
sense organs
TRANSMISSION
Sensory (afferent) neurons transmit information
Central Nervous System
(brain and spinal cord)
INTEGRATION
Interneurons sort
and interpret
information
TRANSMISSION
Motor (efferent) neurons
transmit impulses
ACTION BY
EFFECTORS
(muscles and
glands)
e.g., animal
runs away
e.g., espiration
rate increases;
blood pressure
rises
Stepped Art
Fig. 40-1, p. 846
KEY CONCEPTS
•
Neural signaling involves reception,
transmission, integration, and action by
effectors
Learning Objective 2
•
What is the structure of a typical neuron?
•
Give the function of each of its parts
Neurons
•
Specialized to
•
•
•
receive stimuli
transmit electrical and chemical signals
Cell body
•
contains nucleus and organelles
Dendrites
•
Many branched dendrites
•
•
extend from cell body of neuron
specialized to receive stimuli and send signals
to the cell body
Axons 1
•
A single long axon
•
•
•
extends from neuron cell body
forms branches (axon collaterals)
Transmits signals into terminal branches
•
which end in synaptic terminals
Axons 2
•
Myelin sheath
•
•
•
surrounds many axons
insulates
Schwann cells
•
form the myelin sheath in the PNS
Axons 3
•
In the CNS
•
•
sheath is formed by other glial cells
Nodes of Ranvier
•
gaps in sheath between successive Schwann
cells
Neuron Structure
Dendrites covered
with dendritic spines
Cell body
Axon
collateral
Cytoplasm of
Schwann cell
Synaptic terminals
Axon
Nucleus
Myelin
sheath
Nucleus
Axon
Nodes of
Ranvier
Schwann Terminal
cell
branches
Fig. 40-2, p. 847
Nerves and Ganglia
•
Nerve
•
•
•
several hundred axons
wrapped in connective tissue
Ganglion
•
mass of neuron cell bodies in the PNS
Nerve Structure
Ganglion
Cell bodies
Myelin sheath
Artery
Vein
Axon
(a)
Fig. 40-3a, p. 848
100 µm
(b)
Fig. 40-3b, p. 848
Learn more about the structure
of neurons and nerves by
clicking on the figures in
ThomsonNOW.
Learning Objective 3
•
Name the main types of glial cells
•
Describe the functions of each
Glial Cells
•
Support and nourish neurons
•
Are important in neural communication
Glial Cell Types 1
•
Astrocytes
•
•
•
•
physically support neurons
regulate extracellular fluid in CNS (by taking
up excess potassium ions)
communicate with one another (and with
neurons)
induce and stabilize synapses
Glial Cell Types 2
•
Oligodendrocytes
•
•
form myelin sheaths around axons in CNS
Schwann cells
•
form sheaths around axons in PNS
Glial Cell Types 3
•
Microglia
•
•
Phagocytic cells
Ependymal Cells
•
•
•
line cavities in the CNS
contribute to formation of cerebrospinal fluid
serve as neural stem cells
KEY CONCEPTS
•
Neurons are specialized to receive stimuli
and transmit signals; glial cells are
supporting cells that protect and nourish
neurons and that can modify neural
signals
Learning Objective 4
•
How does the neuron develop and
maintain a resting potential?
Neural Signals
•
Electrical signals transmit information
•
along axons
•
Plasma membrane of resting neuron (not
transmitting an impulse) is polarized
•
Inner surface of plasma membrane is
negatively charged
•
relative to extracellular fluid
Resting Potential
•
Potential difference of about -70 mV
•
•
across the membrane
Magnitude of resting potential
(1) differences in ion concentrations (Na+, K+)
inside cell relative to extracellular fluid
(2) selective permeability of plasma membrane
to these ions
Axon
40
20
0
–20
–40
–60
–80
–70 mV
Time
Amplifier
Plasma
membrane
+
–
–
+
Electrode placed
inside the cell
+
–
–
+
+
–
–
+
Electrode placed
outside the cell
+
–
+
–
–
+
+
–
–
+
–
+
(a) Measuring the resting potential of a neuron.
Fig. 40-4a, p. 850
Ions
•
Pass through specific passive ion channels
•
•
•
K+ leak out faster than Na+ leak in
Cl- accumulate at inner surface of plasma
membrane
Large anions (proteins)
•
•
cannot cross plasma membrane
contribute negative charges
Sodium–Potassium Pumps
•
Maintain gradients that determine resting
potential
•
•
transport 3 Na+ out for each 2 K+ in
Require ATP
Extracellular fluid
3 Na+
Na+
K+
Diffusion out
K+
K+
CI
Na+
–
+
+
+
+
+
–
–
–
–
–
–
K+
K+
CI–
Diffusion in
A
Na /K
pump
K+
K+
Na+
_
CI–
Plasma
membrane
Na+
K+
Na+
Na+
Na+
+
K+
2 K+
_
A
_
CI
CI–
A
–
Cytoplasm
(b) Permeability of the neuron membrane.
CI–
Fig. 40-4b, p. 850
KEY CONCEPTS
•
The resting potential of a neuron is
maintained by differences in
concentrations of specific ions inside the
cell relative to the extracellular fluid and by
selective permeability of the plasma
membrane to these ions
Learning Objective 5
•
Compare a graded potential with an action
potential
•
Describe the production and transmission
of each
Membrane Potential
•
Membrane is depolarized
•
•
if stimulus causes membrane potential to
become less negative
Membrane is hyperpolarized
•
if membrane potential becomes more
negative than resting potential
Graded Potential
•
A local response
•
Varies in magnitude
•
•
depending on strength of applied stimulus
Fades out
•
within a few millimeters of point of origin
Action Potential 1
•
Action potential is a wave of depolarization
•
•
that moves down the axon
Generated when
•
•
•
voltage across the membrane declines to a
critical point (threshold level)
voltage-activated ion channels open
Na+ ions flow into the neuron
Voltage-Activated Ion Channels
Extracellular
fluid
Activation
gate
Cytoplasm
Inactivation
gate
(a) Sodium channels.
(b) Potassium channels.
Fig. 40-6, p. 852
Voltage-Activated Ion Channels
During an Action Potential
Membrane potential (mV)
Spike
Depolarization
Repolarization
Threshold
level
Resting
state
Time (milliseconds)
(a) Action potential.
Fig. 40-7a, p. 853
Axon
Extracellular
fluid
Sodium
channel
Potassium
channel
Cytoplasm
1
Resting state.
2
Depolarization.
3
Repolarization.
4
Return to resting
state.
(b) The action of the ion channels in the plasma membrane determines the
state of the neuron.
Fig. 40-7b, p. 853
Action Potential 2
•
An all-or-none response
•
•
•
no variation in strength of a single impulse
either membrane potential exceeds threshold
level or it does not
Once begun, an action potential is selfpropagating
Repolarization
•
As an action potential moves down an
axon, repolarization occurs behind it
Transmission of
an Action
Potential
Stimulus
Axon
Area of depolarization
Potassium
channel
Action potential
Sodium
channel
(1) Action potential is transmitted as wave of depolarization that travels down
axon. At region of depolarization, Na+ diffuse into cell.
Fig. 40-8a, p. 854
Area of repolarization
Area of depolarization
Action potential
(2) As action potential progresses along axon, repolarization occurs quickly
behind it.
Fig. 40-8b, p. 854
Refractory Periods
•
During depolarization, the axon enters an
absolute refractory period
•
•
when it can’t transmit another action potential
When enough gates controlling Na+
channels have been reset, the neuron
enters a relative refractory period
•
when the threshold is higher
Learn more about ion channels
and action potentials by clicking
on the figures in ThomsonNOW.
KEY CONCEPTS
•
Depolarization of the neuron plasma
membrane to threshold level generates an
action potential, an electrical signal that
travels as a wave of depolarization along
the axon
Learning Objective 6
•
Contrast continuous conduction with
saltatory conduction
Continuous Conduction
•
Involves entire axon plasma membrane
•
Takes place in unmyelinated neurons
Saltatory Conduction
•
Depolarization skips along axon from one
node of Ranvier to the next
•
•
•
more rapid than continuous conduction
takes place in myelinated neurons
Nodes of Ranvier
•
•
sites where axon is not covered by myelin
Na+ channels are concentrated
Saltatory
Conduction
Area of
action potential
1
Saltatory
conduction
Nodes of
Ranvier
Axon
Schwann cell
2
Fig. 40-9a, p. 855
3
4
Direction of
depolarization
Fig. 40-9b, p. 855
Learning Objective 7
•
Describe the actions of the
neurotransmitters identified in the chapter
Synapses
•
Junctions between two neurons
•
•
or between a neuron and effector
Most synapses are chemical
•
some are electrical synapses
Synaptic Transmission
•
A presynaptic neuron releases
neurotransmitter (chemical messenger)
from its synaptic vesicles
Neurotransmitters 1
•
Acetylcholine
•
•
triggers contraction of skeletal muscle
Biogenic amines
•
•
•
norepinephrine, serotonin, dopamine
important in regulating mood
dopamine is also important in motor function
Neurotransmitters 2
•
Some amino acids
•
•
•
glutamate (excitatory neurotransmitter in brain)
GABA (widespread inhibitory neurotransmitter)
Neuropeptides (opioids)
•
•
endorphins (e.g. beta-endorphin)
enkephalins
Neurotransmitters 3
•
Nitric oxide (NO)
•
•
gaseous neurotransmitter
transmits signals from postsynaptic neuron to
presynaptic neuron (opposite direction from
other neurotransmitters)
Learning Objective 8
•
Trace the events that take place in
synaptic transmission
•
Draw diagrams to support your description
Synaptic Transmission
•
Calcium ions cause synaptic vesicles to
fuse with presynaptic membrane
•
•
releases neurotransmitter into synaptic cleft
Neurotransmitter diffuses across the
synaptic cleft
•
combines with specific receptors on a
postsynaptic neuron
Synaptic Transmission
Synaptic
vesicles
Plasma
membrane of
postsynaptic
neuron
0.25 µm
(a) The TEM shows synaptic terminals
filled with synaptic vesicles.
Fig. 40-10a, p. 858
Fig. 40-10bc, p. 858
Axon of
presynaptic
neuron
Voltage-gated
Ca2+ channel
Synaptic
terminal
1
Synaptic
vesicle
Ca2+
2
Neurotransmitter
molecule
3
4
Ligand-gated
channels
Postsynaptic
neuron
5
Receptor for
neurotransmitter
Postsynaptic
membrane
(b) How a neural
impulse is transmitted
across a synapse.
Fig. 40-10b, p. 858
Ca2+
Synaptic
terminal
Presynaptic
membrane
Synaptic
cleft
Na+
Postsynaptic
membrane
(c) Neurotransmitter binds with receptor.
Ligand-gated channel opens, resulting in
depolarization.
Fig. 40-10c, p. 858
Neurotransmitter Receptors
•
Many are proteins that form ligand-gated
ion channels
•
Others work through a second messenger
such as cAMP
Learn more about synaptic
transmission by clicking on the
figure in ThomsonNOW.
KEY CONCEPTS
•
Neurons signal other cells by releasing
neurotransmitters at synapses
Learning Objective 9
•
Compare excitatory and inhibitory signals
and their effects
Binding of Neurotransmitter
to a Receptor
•
Binding causes either
•
•
•
excitatory postsynaptic potential (EPSP)
or inhibitory postsynaptic potential (IPSP)
Depending on the type of receptor
EPSPs and IPSPs
•
EPSPs
•
•
bring neuron closer to firing
IPSPs
•
move neuron farther away from its firing level
Learning Objective 10
•
Define neural integration
•
Describe how a postsynaptic neuron
integrates incoming stimuli and “decides”
whether or not to fire
Neural Integration
•
Process of summing (integrating) incoming
signals
•
Summation
•
process of adding and subtracting incoming
signals
Summation
•
Each EPSP or IPSP is a graded potential
•
•
•
vary in magnitude
depending on strength of stimulus applied
Summation of several EPSPs
•
brings neuron to critical firing level
Temporal Summation
•
Occurs when repeated stimuli cause new
EPSPs to develop before previous EPSPs
have decayed
Spatial Summation
•
Occurs when several closely spaced
synaptic terminals release
neurotransmitter simultaneously
•
stimulating postsynaptic neuron at several
different places
Neural Integration
Postsynaptic membrane potential (mV)
Threshold
level
Resting
potential
Time (msec)
(a) Subthreshold
(no summation).
(b) Temporal
summation.
(c) Spatial
summation.
(d) Spatial
summation of
EPSPs and IPSPs.
Fig. 40-11, p. 860
KEY CONCEPTS
•
During integration, incoming neural signals
are summed; temporal and spatial
summation can bring a neuron to
threshold level
Learning Objective 11
•
Distinguish among convergence,
divergence, and reverberation
•
Explain why each is important
Neural Circuits
•
Complex neural circuits are possible
because of associations such as
convergence and divergence
Convergence
•
A single neuron is affected by converging
signals from two or more presynaptic
neurons
•
Allows CNS to integrate incoming
information from various sources
Divergence
•
A single presynaptic neuron stimulates
many postsynaptic neurons
•
allowing widespread effect
Neural Circuits
(a) Convergence of neural input.
Several presynaptic neurons
synapse with one postsynaptic
neuron.
(b) Divergence of neural output. A
single presynaptic neuron synapses
with many postsynaptic neurons.
Fig. 40-12, p. 861
Reverberating Circuits
•
Important in
•
•
•
•
rhythmic breathing
mental alertness
short-term memory
Depend on positive feedback
•
new impulses generated again and again until
synapses fatigue
Reverberating Circuits
1
2
(a) Simple reverberating circuit. An axon collateral of the second
neuron turns back on its own dendrites, so the neuron continues to
stimulate itself.
Fig. 40-13a, p. 861
Interneuron
Axon collateral
1
2
3
(b) Reverberating circuit with interneuron. An axon collateral of the
second neuron synapses with an interneuron. The interneuron synapses
with the first neuron in the sequence. New impulses are triggered again
and again in the first neuron, causing reverberation.
Fig. 40-13b, p. 861