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An Introduction to the Nervous System
• Organs of the Nervous System
• Brain and spinal cord (Processing incoming info and
creating motor commands)
• Sensory receptors of sense organs (eyes, ears,
pressure, pain, chemicals, etc.)
• Nerves connect nervous system with other systems( a
nerve is a bundle of axons)
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Anatomical Divisions of the Nervous System
• The Central Nervous System (CNS)
• Consists of the spinal cord and brain
• Contains neural tissue, connective tissues, and blood
vessels
• Functions of the CNS are to process and coordinate:
• Sensory data from inside and outside body
• Motor commands control activities of peripheral organs (e.g.,
skeletal muscles)
• Higher functions of brain intelligence, memory, learning,
emotion
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Anatomical Divisions of the Nervous System
• The Peripheral Nervous System (PNS)
• Includes all neural tissue outside the CNS
• Functions of the PNS
• Deliver sensory information to the CNS
• Carry motor commands to peripheral tissues and systems
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12-1 Divisions of the Nervous System
• The Peripheral Nervous System (PNS)
• Nerves (also called peripheral nerves)
• Bundles of axons with connective tissues and blood
vessels
• Carry sensory information and motor commands in PNS
• Cranial nerves — connect to brain
• Spinal nerves — attach to spinal cord
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12-1 Divisions of the Nervous System
• Functional Divisions of the PNS
• Afferent division (means “entrance”)
• Carries sensory information
• From PNS sensory receptors to CNS
• Efferent division (means “exit”)
• Carries motor commands
• From CNS to PNS muscles and glands
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12-1 Divisions of the Nervous System
• Functional Divisions of the PNS
• Receptors and effectors of afferent division
• Receptors
• Detect changes or respond to stimuli
• Neurons and specialized cells
• Complex sensory organs (e.g., eyes, ears)
• Effectors
• Respond to efferent signals
• Cells and organs
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Functional Divisions of the PNS
• Somatic nervous system (SNS)
• Controls voluntary and involuntary (reflexes)
muscle skeletal contractions
• Autonomic nervous system (ANS)
• Controls subconscious actions, contractions of
smooth muscle and cardiac muscle, and
glandular secretions
• Sympathetic division has a stimulating effect
• Parasympathetic division has a relaxing effect
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Figure 12-1a The Anatomy of a Multipolar Neuron
Dendrites
Perikaryon
Nucleus
Axon
Cell body
Telodendria
This color-coded figure
shows the four general
regions of a neuron.
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Figure 12-1b The Anatomy of a Multipolar Neuron
Dendritic branches
Nissl bodies (RER
and free ribosomes)
Mitochondrion
Axon hillock
Initial segment
of axon
Axolemma
Telodendria
Golgi apparatus
Neurofilament
Nucleus
Axon
Synaptic
terminals
Nucleolus
Dendrite
PRESYNAPTIC CELL
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See Figure 12–2
An understanding of neuron
function requires knowing its
structural components.
POSTSYNAPTIC
CELL
12-2 Neurons
• The Structure of Neurons
• The synapse
• Presynaptic cell
• Neuron that sends message
• Postsynaptic cell
• Cell that receives message
• The synaptic cleft
• The small gap that separates the presynaptic
membrane and the postsynaptic membrane
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• Neurotransmitters
• Are chemical messengers
• Are released at presynaptic membrane
• Affect receptors of postsynaptic membrane
• Are broken down by enzymes (like AChE)
• Are reassembled at synaptic terminal
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12-2 Neurons
• Types of Synapses
• Neuromuscular junction
• Synapse between neuron and muscle
• Neuroglandular junction
• Synapse between neuron and gland
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Figure 12-2 The Structure of a Typical Synapse
Telodendrion
Synaptic terminal
Endoplasmic
reticulum
Mitochondrion
Synaptic
vesicles
Presynaptic
membrane
Postsynaptic
membrane
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Synaptic
cleft
12-2 Neurons
• Functions of Sensory Neurons
• Monitor internal environment (visceral sensory neurons)
• Monitor effects of external environment (somatic sensory
neurons)
• Structures of Sensory Neurons
• Unipolar
• Cell bodies grouped in sensory ganglia
• Processes (afferent fibers) extend from sensory
receptors to CNS
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12-2 Neurons
• Three Types of Sensory Receptors
1. Interoceptors
•
Monitor internal systems (digestive, respiratory,
cardiovascular, urinary, reproductive)
•
Internal senses (taste, deep pressure, pain)
2. Exteroceptors
•
External senses (touch, temperature, pressure)
•
Distance senses (sight, smell, hearing)
3. Proprioceptors
•
Monitor position and movement (skeletal muscles and
joints)
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12-2 Neurons
• Motor Neurons
• Carry instructions from CNS to peripheral effectors
• Via efferent fibers (axons)
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12-2 Neurons
• Motor Neurons
•
Two major efferent systems
1. Somatic nervous system (SNS)
•
Includes all somatic motor neurons that innervate skeletal
muscles
2. Autonomic (visceral) nervous system (ANS)
•
Visceral motor neurons innervate all other peripheral (like
internal organs) effectors
•
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Smooth muscle, cardiac muscle, glands, adipose tissue
12-2 Neurons
• Interneurons
• Most are located in brain, spinal cord, and autonomic
ganglia
• Between sensory and motor neurons
• Are responsible for:
• Distribution of sensory information
• Coordination of motor activity
• Are involved in higher functions
• Memory, planning, learning
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12-6 Axon Diameter and Speed
• Information
• “Information” travels within the nervous system
• As propagated electrical signals (action potentials)
• The most important information (vision, balance,
motor commands)
• Is carried by large-diameter, myelinated axons
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12-4 Transmembrane Potential
• Ion Movements and Electrical Signals
• All plasma (cell) membranes produce electrical
signals by ion movements
• Transmembrane potential is particularly important to
neurons
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12-4 Transmembrane Potential
• Five Main Membrane Processes in Neural
Activities
1. Resting potential
•
The transmembrane potential of resting cell
2. Graded potential
•
Temporary, localized change in resting potential
•
Caused by stimulus
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12-4 Transmembrane Potential
• Five Main Membrane Processes in Neural
Activities
3. Action potential
•
Is an electrical impulse
•
Produced by graded potential
•
Propagates along surface of axon to synapse
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12-4 Transmembrane Potential
• Five Main Membrane Processes in Neural
Activities
4. Synaptic activity
•
Releases neurotransmitters at presynaptic membrane
•
Produces graded potentials in postsynaptic
membrane
5. Information processing
•
Response (integration of stimuli) of postsynaptic cell
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Figure 12-8 An Overview of Neural Activities
Graded
potential
Action potential
may
produce
Resting
potential
stimulus
produces
Presynaptic neuron
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Figure 12-8 An Overview of Neural Activities
triggers
Information
processing
Postsynaptic cell
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12-4 Transmembrane Potential
• The Transmembrane Potential
•
Three important concepts
1. The extracellular fluid (ECF) and intracellular fluid
(cytosol) differ greatly in ionic composition
• Concentration gradient of ions (Na+, K+)
2. Cells have selectively permeable membranes
3. Membrane permeability varies by ion
A&P FLIX Resting Membrane Potential
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Figure 12-9 The Resting Potential is the Transmembrane Potential of an Undisturbed Cell
EXTRACELLULAR FLUID
–70
–30
Cl–
0
+30
mV
3 Na+
Na+ leak
channel
K+
leak
channel
Sodium–
potassium
exchange
pump
Plasma
membrane
CYTOSOL
Protein
2 K+
Protein
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Protein
12-4 Transmembrane Potential
• Changes in the Transmembrane Potential
• Transmembrane potential rises or falls
• In response to temporary changes in membrane
permeability
• Resulting from opening or closing specific membrane
channels
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12-4 Transmembrane Potential
• Sodium and Potassium Channels
• Membrane permeability to Na+ and K+ determines
transmembrane potential
• They are either passive or active
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12-4 Transmembrane Potential
• Passive Channels (Leak Channels)
• Are always open
• Permeability changes with conditions
• Active Channels (Gated Channels)
• Open and close in response to stimuli
• At resting potential, most gated channels are closed
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12-4 Transmembrane Potential
• Three Classes of Gated Channels
1. Chemically gated channels
2. Voltage-gated channels
3. Mechanically gated channels
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12-4 Transmembrane Potential
•
•
•
Chemically Gated Channels
•
Open in presence of specific chemicals (e.g., ACh) at a binding site
•
Found on neuron cell body and dendrites
Voltage-gated Channels
•
Respond to changes in transmembrane potential
•
Have activation gates (open) and inactivation gates (close)
•
Characteristic of excitable membrane
•
Found in neural axons, skeletal muscle sarcolemma, cardiac muscle
Mechanically Gated Channels
•
Respond to membrane distortion
•
Found in sensory receptors (touch, pressure, vibration)
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Figure 12-11a Gated Channels
Chemically gated channel
Resting state
Presence of ACh
ACh
Channel closed
Binding
site
Gated
channel
Channel open
A chemically gated Na+ channel that
opens in response to the presence of
ACh at a binding site.
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Figure 12-11b Gated Channels
Voltage-gated channel
–70 mV
Channel closed
–60 mV
Channel open
+30 mV
Channel inactivated
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Inactivation
gate
Figure 12-11c Gated Channels
Mechanically gated channel
Channel closed
Applied
pressure
Channel open
Pressure
removed
Channel closed
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12-4 Transmembrane Potential
• Transmembrane Potential Exists Across Plasma
Membrane
• Because:
• Cytosol and extracellular fluid have different chemical/ionic
balance
• The plasma membrane is selectively permeable
• Transmembrane Potential
• Changes with plasma membrane permeability
• In response to chemical or physical stimuli
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12-4 Transmembrane Potential
• Graded Potentials
• Also called local potentials
• Changes in transmembrane potential
• That cannot spread far from site of stimulation
• Any stimulus that opens a gated channel
• Produces a graded potential
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12-4 Transmembrane Potential
• Graded Potentials
• The resting state
• Opening sodium channel produces graded potential
• Resting membrane exposed to chemical
• Sodium channel opens
• Sodium ions enter the cell
• Transmembrane potential rises
• Depolarization occurs
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Figure 12-12 Graded Potentials
Resting State
Initial
segment
Resting membrane with closed chemically gated sodium ion channels
–70 mV
EXTRACELLULAR
FLUID
CYTOSOL
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12-4 Transmembrane Potential
• Graded Potentials
• Depolarization
• A shift in transmembrane potential toward 0 mV
• Movement of Na+ through channel
• Produces local current
• Depolarizes nearby plasma membrane (graded potential)
• Change in potential is proportional to stimulus
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Figure 12-12 Graded Potentials
Stimulus
applied
here
Stimulation
Membrane exposed to chemical that opens the sodium ion channels
–65 mV
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Figure 12-12 Graded Potentials
Graded Potential
Spread of sodium ions inside plasma membrane produces a local current
that depolarizes adjacent portions of the plasma membrane
Local
current
–60 mV
–65 mV
Local current
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–70 mV
12-4 Transmembrane Potential
• Graded Potentials
• Repolarization
• When the stimulus is removed, transmembrane
potential returns to normal
• Hyperpolarization
• Increasing the negativity of the resting potential
• Result of opening a potassium channel
• Opposite effect of opening a sodium channel
• Positive ions move out, not into cell
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12-5 Action Potential
• Action Potentials
• Propagated changes in transmembrane potential
• Affect an entire excitable membrane
• Link graded potentials at cell body with motor end
plate actions
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12-5 Action Potential
• Initiating Action Potential
• All-or-none principle
• If a stimulus exceeds threshold amount
• The action potential is the same
• No matter how large the stimulus
• Action potential is either triggered, or not
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Figure 12-14 Generation of an Action Potential
Resting
Potential
–70 mV
The axolemma contains both voltagegated sodium channels and voltagegated potassium channels that are
closed when the membrane is at the
resting potential.
KEY
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=
Sodium ion
=
Potassium ion
12-5 Action Potential
• Four Steps in the Generation of Action Potentials
• Step 1: Depolarization to threshold
• Step 2: Activation of Na channels
• Step 3: Inactivation of Na channels and activation
of K channels
• Step 4: Return to normal permeability
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12-5 Action Potential
• Step 1: Depolarization to threshold
• Step 2: Activation of Na channels
• Rapid depolarization
• Na+ ions rush into cytoplasm
• Inner membrane changes from negative to positive
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Figure 12-14 Generation of an Action Potential
Depolarization to Threshold
–60 mV
Local
current
KEY
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=
Sodium ion
=
Potassium ion
Figure 12-14 Generation of an Action Potential
Activation of Sodium
Channels and Rapid
Depolarization
+10 mV
KEY
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=
Sodium ion
=
Potassium ion
12-5 Action Potential
• Step 3: Inactivation of Na channels and
activation of K channels
• At +30 mV
• Inactivation gates close (Na channel inactivation)
• K channels open
• Repolarization begins
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Figure 12-14 Generation of an Action Potential
Inactivation of Sodium
Channels and Activation
of Potassium Channels
+30 mV
KEY
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=
Sodium ion
=
Potassium ion
12-5 Action Potential
• Step 4: Return to normal permeability
• K+ channels begin to close
• When membrane reaches normal resting potential (–70
mV)
• K+ channels finish closing
• Membrane is hyperpolarized to –90 mV
• Transmembrane potential returns to resting level
• Action potential is over
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Figure 12-14 Generation of an Action Potential
Closing of Potassium
Channels
–90 mV
KEY
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=
Sodium ion
=
Potassium ion
Figure 12-14 Generation of an Action Potential
Sodium channels close, voltagegated potassium channels open
Transmembrane potential (mV)
DEPOLARIZATION
REPOLARIZATION
Resting
potential
Voltage-gated sodium
ion channels open
Threshold
All channels closed
Graded potential
causes threshold
ABSOLUTE REFRACTORY PERIOD
Cannot respond
Time (msec)
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RELATIVE REFRACTORY PERIOD
Responds only to a larger
than normal stimulus
12-5 Action Potential
• Propagation of Action Potentials
•
•
Propagation
•
Moves action potentials generated in axon hillock
•
Along entire length of axon
Two methods of propagating action potentials
1. Continuous propagation (unmyelinated axons)
2. Saltatory propagation (myelinated axons)
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12-5 Action Potential
• Continuous Propagation
• Of action potentials along an unmyelinated axon
• Affects one segment of axon at a time
• Steps in propagation
• Step 1: Action potential in segment 1
• Depolarizes membrane to +30 mV
• Local current
• Step 2: Depolarizes second segment to threshold
• Second segment develops action potential
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Figure 12-15 Continuous Propagation of an Action Potential along an Unmyelinated Axon
As an action potential develops at the initial segment , the
transmembrane potential at this site depolarizes to +30 mV.
Action
potential
Extracellular Fluid
+30 mV
–70 mV
–70 mV
Na+
Cell membrane
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Cytosol
Figure 12-15 Continuous Propagation of an Action Potential along an Unmyelinated Axon
As the sodium ions entering at  spread away from the
open voltage-gated channels, a graded depolarization
quickly brings the membrane in segment  to threshold.
Graded depolarization
–60 mV
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–70 mV
12-5 Action Potential
• Continuous Propagation
• Steps in propagation
• Step 3: First segment enters refractory period
• Step 4: Local current depolarizes next segment
• Cycle repeats
• Action potential travels in one direction (1 m/sec)
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Figure 12-15 Continuous Propagation of an Action Potential along an Unmyelinated Axon
An action potential now occurs in segment  while
segment  beings repolarization.
Repolarization
(refractory)
+30 mV
Na+
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–70 mV
Figure 12-15 Continuous Propagation of an Action Potential along an Unmyelinated Axon
As the sodium ions entering at segment  spread laterally,
a graded depolarization quickly brings the membrane in
segment  to threshold, and the cycle is repeated.
–60 mV
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12-5 Action Potential
• Saltatory Propagation
• Action potential along myelinated axon
• Faster and uses less energy than continuous
propagation
• Myelin insulates axon, prevents continuous
propagation
• Local current “jumps” from node to node
• Depolarization occurs only at nodes
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Figure 12-16 Saltatory Propagation along a Myelinated Axon
An action potential
has occurred at the
initial segment .
Extracellular Fluid
–70 mV
–70 mV
+30 mV
Na+
Myelinated
internode
Myelinated
internode
Plasma membrane
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Myelinated
internode
Cytosol
Figure 12-16 Saltatory Propagation along a Myelinated Axon
A local current
produces a graded
depolarization that
brings the axolemma
at the next node to
threshold.
–60 mV
Local
current
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–70 mV
Figure 12-16 Saltatory Propagation along a Myelinated Axon
An action potential
develops at node . Repolarization
(refractory)
+30 mV
Na+
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–70 mV
Figure 12-16 Saltatory Propagation along a Myelinated Axon
A local current
produces a graded
depolarization that
brings the axolemma
at node  to
threshold.
–60 mV
Local
current
A&P FLIX Propagation of an Action Potential
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12-6 Axon Diameter and Speed
• Axon Diameter and Propagation Speed
• Ion movement is related to cytoplasm concentration
• Axon diameter affects action potential speed
• The larger the diameter, the lower the resistance
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12-7 Synapses
• Synaptic Activity
• Action potentials (nerve impulses)
• Are transmitted from presynaptic neuron
• To postsynaptic neuron (or other postsynaptic cell)
• Across a synapse
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12-7 Synapses
• Two Types of Synapses
1. Electrical synapses
•
Direct physical contact between cells
2. Chemical synapses
•
Signal transmitted across a gap by chemical
neurotransmitters
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12-7 Synapses
• Electrical Synapses
• Are locked together at gap junctions (connexons)
• Allow ions to pass between cells
• Produce continuous local current and action potential
propagation
• Are found in areas of brain, eye, ciliary ganglia
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12-7 Synapses
• Chemical Synapses
• Are found in most synapses between neurons and all
synapses between neurons and other cells
• Cells not in direct contact
• Action potential may or may not be propagated to
postsynaptic cell, depending on:
• Amount of neurotransmitter released
• Sensitivity of postsynaptic cell
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12-7 Synapses
• Two Classes of Neurotransmitters
1. Excitatory neurotransmitters
•
Cause depolarization of postsynaptic membranes
•
Promote action potentials
2. Inhibitory neurotransmitters
•
Cause hyperpolarization of postsynaptic membranes
•
Suppress action potentials
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12-7 Synapses
• The Effect of a Neurotransmitter
• On a postsynaptic membrane
• Depends on the receptor
• Not on the neurotransmitter
• For example, acetylcholine (ACh)
• Usually promotes action potentials
• But inhibits cardiac neuromuscular junctions
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12-7 Synapses
• Cholinergic Synapses
•
Any synapse that releases ACh at:
1. All neuromuscular junctions with skeletal muscle
fibers
2. Many synapses in CNS
3. All neuron-to-neuron synapses in PNS
4. All neuromuscular and neuroglandular junctions of
ANS parasympathetic division
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12-7 Synapses
• Events at a Cholinergic Synapse
1. Action potential arrives, depolarizes synaptic terminal
2. Calcium ions enter synaptic terminal, trigger
exocytosis of ACh
3. ACh binds to receptors, depolarizes postsynaptic
membrane
4. ACh removed by AChE
•
AChE breaks ACh into acetate and choline
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Figure 12-17 Events in the Functioning of a Cholinergic Synapse
An action potential arrives and
depolarizes the synaptic terminal
Presynaptic neuron
Synaptic vesicles
ER
Action potential
EXTRACELLULAR
FLUID
Synaptic
terminal
Initial
segment
AChE
POSTSYNAPTIC
NEURON
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Figure 12-17 Events in the Functioning of a Cholinergic Synapse
Extracellular Ca2+ enters the synaptic
terminal, triggering the exocytosis of ACh
ACh
Ca2+
Ca2+
Synaptic cleft
Chemically gated
sodium ion channels
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Figure 12-17 Events in the Functioning of a Cholinergic Synapse
ACh binds to receptors and depolarizes
the postsynaptic membrane
Initiation of
action potential
if threshold is
reached at the
initial segment
Na+ Na+
Na+
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Na+
Na+
Figure 12-17 Events in the Functioning of a Cholinergic Synapse
ACh is removed by AChE
Propagation of
action potential
(if generated)
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Table 12-4 Synaptic Activity
Mitochondrion
Acetylcholine
Synaptic
vesicle
SYNAPTIC
TERMINAL
Choline
Acetylcholinesterase
Acetate
POSTSYNAPTIC
MEMBRANE
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SYNAPTIC
CLEFT
(AChE)
ACh
receptor
12-8 Neurotransmitters and Neuromodulators
• Other Neurotransmitters
• At least 50 neurotransmitters other than ACh,
including:
• Biogenic amines
• Amino acids
• Neuropeptides
• Dissolved gases
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12-8 Neurotransmitters and Neuromodulators
• Important Neurotransmitters
• Other than acetylcholine
• Norepinephrine (NE)
• Dopamine
• Serotonin
• Gamma aminobutyric acid (GABA)
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12-8 Neurotransmitters and Neuromodulators
• Norepinephrine (NE)
• Released by adrenergic synapses
• Excitatory and depolarizing effect
• Widely distributed in brain and portions of ANS
• Dopamine
• A CNS neurotransmitter
• May be excitatory or inhibitory
• Involved in Parkinson’s disease and cocaine use
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12-8 Neurotransmitters and Neuromodulators
• Serotonin
• A CNS neurotransmitter
• Affects attention and emotional states
• Gamma Aminobutyric Acid (GABA)
• Inhibitory effect
• Functions in CNS
• Not well understood
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12-8 Neurotransmitters and Neuromodulators
• Chemical Synapse
• The synaptic terminal releases a neurotransmitter that
binds to the postsynaptic plasma membrane
• Produces temporary, localized change in permeability
or function of postsynaptic cell
• Changes affect cell, depending on nature and number
of stimulated receptors
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12-8 Neurotransmitters and Neuromodulators
• Many Drugs
• Affect nervous system by stimulating receptors that
respond to neurotransmitters
• Can have complex effects on perception, motor
control, and emotional states
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12-8 Neurotransmitters and Neuromodulators
• Neuromodulators
• Other chemicals released by synaptic terminals
• Similar in function to neurotransmitters
• Characteristics of neuromodulators
• Effects are long term, slow to appear
• Responses involve multiple steps, intermediary compounds
• Affect presynaptic membrane, postsynaptic membrane, or
both
• Released alone or with a neurotransmitter
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12-8 Neurotransmitters and Neuromodulators
• Neuropeptides
• Neuromodulators that bind to receptors and activate
enzymes
• Opioids
• Neuromodulators in the CNS
• Bind to the same receptors as opium or morphine
• Relieve pain
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12-9 Information Processing
• Information Processing
• At the simplest level (individual neurons)
• Many dendrites receive neurotransmitter messages
simultaneously
• Some excitatory, some inhibitory
• Net effect on axon hillock determines if action potential
is produced
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12-9 Information Processing
• Postsynaptic Potentials
•
Graded potentials developed in a postsynaptic cell
•
In response to neurotransmitters
• Two Types of Postsynaptic Potentials
1. Excitatory postsynaptic potential (EPSP)
•
Graded depolarization of postsynaptic membrane
2. Inhibitory postsynaptic potential (IPSP)
•
Graded hyperpolarization of postsynaptic membrane
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12-9 Information Processing
• Inhibition
•
A neuron that receives many IPSPs
•
Is inhibited from producing an action potential
•
Because the stimulation needed to reach threshold is
increased
• Summation
•
To trigger an action potential
•
One EPSP is not enough
•
EPSPs (and IPSPs) combine through summation
1. Temporal summation
2. Spatial summation
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12-9 Information Processing
• Temporal Summation
• Multiple times
• Rapid, repeated stimuli at one synapse
• Spatial Summation
• Multiple locations
• Many stimuli, arrive at multiple synapses
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Figure 12-19a Temporal and Spatial Summation
First stimulus arrives
FIRST
STIMULUS
Initial
segment
Second stimulus arrives and is
added to the first stimulus
SECOND
STIMULUS
Action potential is generated
ACTION
POTENTIAL
PROPAGATION
Threshold
reached
Temporal Summation. Temporal summation occurs on a membrane that receives two depolarizing stimuli from the
same source in rapid succession. The effects of the second stimulus are added to those of the first.
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Figure 12-19b Temporal and Spatial Summation
Two stimuli arrive simultaneously
TWO
SIMULTANEOUS
STIMULI
Action potential is generated
ACTION
POTENTIAL
PROPAGATION
Threshold
reached
Spatial Summation. Spatial summation occurs when sources of stimulation
arrive simultaneously, but at different locations. Local currents spread the
depolarizing effects, and areas of overlap experience the combined effects.
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Figure 12-21a Presynaptic Inhibition and Presynaptic Facilitation
Action
potential
arrives
GABA
release
Inactivation of
calcium channels
Ca2+
2. Less calcium
enters
1. Action
potential
arrives
3. Less
neurotransmitter
released
Presynaptic inhibition
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4. Reduced
effect on
postsynaptic
membrane
Figure 12-21b Presynaptic Inhibition and Presynaptic Facilitation
Action
potential
arrives
Serotonin
release
Activation of
calcium channels
Ca2+
Ca2+
2. More calcium
enters
1. Action
potential
arrives
3. More
neurotransmitter
released
Presynaptic facilitation
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4. Increased
effect on
postsynaptic
membrane