Download 12-4 Membrane Potential

Chapter 12
Neural Tissue
Lecture Presentation by
Lee Ann Frederick
University of Texas at Arlington
© 2015 Pearson Education, Inc.
An Introduction to the Nervous System
•  The Nervous System
•  Includes all neural tissue in the body
•  Neural tissue contains two kinds of cells
1.  Neurons
•  Cells that send and receive signals
2.  Neuroglia (glial cells)
•  Cells that support and protect neurons
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An Introduction to the Nervous System
•  Organs of the Nervous System
•  Brain and spinal cord
•  Sensory receptors of sense organs (eyes, ears,
etc.)
•  Nerves connect nervous system with other
systems
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12-1 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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12-1 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
•  Carries sensory information
•  From PNS sensory receptors to CNS
•  Efferent division
•  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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12-1 Divisions of the Nervous System
•  Functional Divisions of the PNS
•  The efferent division
•  Somatic nervous system (SNS)
•  Controls voluntary and involuntary (reflexes) skeletal
muscle contractions
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12-1 Divisions of the Nervous System
•  Functional Divisions of the PNS
•  The efferent division
•  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
© 2015 Pearson Education, Inc.
Figure 12-1 A Functional Overview of the Nervous System.
Organization of the Nervous System
Central Nervous System (CNS)
(brain and spinal cord)
Peripheral Nervous
System (PNS)
(neural tissue
outside the CNS)
Integrate, process, and coordinate
sensory data and motor commands
Sensory information
within
afferent division
Motor commands
within
efferent division
includes
Somatic nervous
system (SNS)
Autonomic
nervous system (ANS)
Parasympathetic
division
Effectors
Receptors
Special sensory
receptors
monitor smell, taste,
vision, balance, and
hearing
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Visceral sensory
receptors
monitor internal
organs
Somatic sensory
receptors
monitor skeletal
muscles, joints,
and skin surface
Skeletal
muscle
Sympathetic
division
•  Smooth
muscle
•  Cardiac
muscle
•  Glands
•  Adipose
tissue
12-2 Neurons
•  Neurons
•  The basic functional units of the nervous system
•  The structure of neurons
•  The multipolar neuron
•  Common in the CNS
•  Cell body (soma)
•  Short, branched dendrites
•  Long, single axon
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Figure 12-2a The Anatomy of a Multipolar Neuron.
Dendrites
Perikaryon
Cell body
Nucleus
Telodendria
Axon
a This color-coded figure shows the
four general regions of a neuron.
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Figure 12-2b The Anatomy of a Multipolar Neuron.
Nissl bodies (RER
and free ribosomes)
Dendritic branches
Mitochondrion
Axon hillock
Initial segment
of axon
Axolemma
Axon
Telodendria
Direction of action potential
Golgi apparatus
Neurofilament
Nucleolus
Axon
terminals
Nucleus
Dendrite
See Figure 12–3
Presynaptic cell
b An understanding of neuron function requires
knowing its structural components.
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Postsynaptic cell
12-2 Neurons
•  The Structure of Neurons
•  The synapse
•  Area where a neuron communicates with another
cell
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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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12-2 Neurons
•  The Synapse
•  The synaptic terminal
•  Is expanded area of axon of presynaptic neuron
•  Contains synaptic vesicles of neurotransmitters
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12-2 Neurons
•  Neurotransmitters
•  Are chemical messengers
•  Are released at presynaptic membrane
•  Affect receptors of postsynaptic membrane
•  Are broken down by enzymes
•  Are reassembled at axon 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 12-3 The Structure of a Typical Synapse.
Telodendrion
Axon
terminal
Mitochondrion
Synaptic
vesicles
Presynaptic
membrane
Postsynaptic Synaptic
membrane
cleft
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Figure 12-4 Structural Classifications of Neurons.
Anaxonic neuron
Bipolar neuron
Dendritic
branches
Unipolar neuron
Multipolar neuron
Dendrites
Dendrites
Initial
segment
Cell body
Axon
Dendrite
Cell body
Cell body
Axon
Cell
body
Axon
terminals
Axon
Axon
Axon
terminals
a Anaxonic neurons have
more than two processes,
and they are all dendrites.
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b Bipolar neurons have
two processes
separated by the
cell body.
c Unipolar neurons have a
single elongated process,
with the cell body located
off to the side.
Axon
terminals
d Multipolar neurons have
more than two processes;
there is a single axon and
multiple dendrites.
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
effectors
•  Smooth muscle, cardiac muscle, glands,
adipose tissue
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12-2 Neurons
•  Motor Neurons
•  Two groups of efferent axons
•  Signals from CNS motor neurons to visceral
effectors pass synapses at autonomic ganglia
dividing axons into:
•  Preganglionic fibers
•  Postganglionic fibers
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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
© 2015 Pearson Education, Inc.
12-3 Neuroglia
•  Neuroglia
•  Half the volume of the nervous system
•  Many types of neuroglia in CNS and PNS
© 2015 Pearson Education, Inc.
12-3 Neuroglia
•  Four Types of Neuroglia in the CNS
1.  Ependymal cells
•  Cells with highly branched processes; contact
neuroglia directly
2.  Astrocytes
•  Large cell bodies with many processes
3.  Oligodendrocytes
•  Smaller cell bodies with fewer processes
4.  Microglia
•  Smallest and least numerous neuroglia with many
fine-branched processes
© 2015 Pearson Education, Inc.
Figure 12-5 An Introduction to Neuroglia (Part 1 of 2).
Neuroglia
are found in
Central Nervous System
contains
Ependymal cells
Astrocytes
Line ventricles
(brain) and central
canal (spinal cord);
assist in producing,
circulating, and
monitoring
cerebrospinal fluid
Maintain blood–brain
barrier; provide
structural support;
regulate ion, nutrient,
and dissolved gas
concentrations;
absorb and recycle
neurotransmitters;
form scar tissue after
injury
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Oligodendrocytes
Myelinate CNS
axons; provide
structural
framework
Microglia
Remove cell
debris,
wastes, and
pathogens by
phagocytosis
12-3 Neuroglia
•  Ependymal Cells
•  Form epithelium called ependyma
•  Line central canal of spinal cord and ventricles of
brain
•  Secrete cerebrospinal fluid (CSF)
•  Have cilia or microvilli that circulate CSF
•  Monitor CSF
•  Contain stem cells for repair
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12-3 Neuroglia
•  Astrocytes
•  Maintain blood–brain barrier (isolates CNS)
•  Create three-dimensional framework for CNS
•  Repair damaged neural tissue
•  Guide neuron development
•  Control interstitial environment
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12-3 Neuroglia
•  Oligodendrocytes
•  Myelination
•  Increases speed of action potentials
•  Myelin insulates myelinated axons
•  Makes nerves appear white
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12-3 Neuroglia
•  Oligodendrocytes
•  Nodes and internodes
•  Internodes – myelinated segments of axon
•  Nodes (also called nodes of Ranvier)
•  Gaps between internodes
•  Where axons may branch
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12-3 Neuroglia
•  Myelination
•  White matter
•  Regions of CNS with many myelinated nerves
•  Gray matter
•  Unmyelinated areas of CNS
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12-3 Neuroglia
•  Microglia
•  Migrate through neural tissue
•  Clean up cellular debris, waste products, and
pathogens
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Figure 12-6b Neuroglia in the CNS (Part 1 of 2).
Gray matter
White matter
CENTRAL CANAL
Ependymal
cells
Gray
matter
Neurons
Microglial
cell
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Figure 12-6b Neuroglia in the CNS (Part 2 of 2).
Gray matter
White matter
Myelinated
axons
Internode
Myelin
(cut)
Axon
White
matter
Oligodendrocyte
Astrocyte
Axolemma
Node
Unmyelinated
axon
Basement
membrane
Capillary
b A diagrammatic view of neural tissue in the CNS, showing relationships between neuroglia
and neurons
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12-3 Neuroglia
•  Neuroglia of the Peripheral Nervous System
•  Ganglia
•  Masses of neuron cell bodies
•  Surrounded by neuroglia
•  Found in the PNS
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Figure 12-5 An Introduction to Neuroglia (Part 2 of 2).
Neuroglia
are found in
Peripheral Nervous System
contains
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Satellite cells
Schwann cells
Surround neuron
cell bodies in
ganglia; regulate O2,
CO2, nutrient, and
neurotransmitter
levels around
neurons in ganglia
Surround all axons in
PNS; responsible for
myelination of
peripheral axons;
participate in repair
process after injury
12-3 Neuroglia
•  Neuroglia of the Peripheral Nervous System
•  Satellite cells
•  Also called amphicytes
•  Surround ganglia
•  Regulate environment around neuron
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12-3 Neuroglia
•  Neuroglia of the Peripheral Nervous System
•  Schwann cells
•  Also called neurilemma cells
•  Form myelin sheath (neurilemma) around
peripheral axons
•  One Schwann cell sheaths one segment of axon
•  Many Schwann cells sheath entire axon
© 2015 Pearson Education, Inc.
Figure 12-7a Schwann Cells, Peripheral Axons, and Formation of the Myelin Sheath (Part 1 of 2).
Axon hillock
Nucleus
Myelinated
internode
Initial
segment
(unmyelinated)
Nodes
Axon
Axolemma
Myelin covering
internode
a A myelinated axon, showing the organization
of Schwann cells along the length of the axon.
© 2015 Pearson Education, Inc.
Dendrite
Figure 12-7c Schwann Cells, Peripheral Axons, and Formation of the Myelin Sheath.
1
A Schwann cell first surrounds a
portion of the axon within a
groove of its cytoplasm.
Schwann
cell
2
The Schwann cell then begins to
rotate around the axon.
3
As the Schwann cell rotates, myelin is
wound around the axon in
multiple layers, forming
a tightly packed
membrane
Myelin
Axon
Schwann cell cytoplasm
c Stages in the formation of a myelin sheath by a single Schwann cell along a portion of a single axon.
© 2015 Pearson Education, Inc.
12-3 Neuroglia
•  Neurons and Neuroglia
•  Neurons perform:
•  All communication, information processing, and
control functions of the nervous system
•  Neuroglia preserve:
•  Physical and biochemical structure of neural tissue
•  Neuroglia are essential to:
•  Survival and function of neurons
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12-3 Neuroglia
•  Neural Responses to Injuries
•  Wallerian degeneration
•  Axon distal to injury degenerates
•  Schwann cells
•  Form path for new growth
•  Wrap new axon in myelin
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12-3 Neuroglia
•  Nerve Regeneration in CNS
•  Limited by chemicals released by astrocytes that:
•  Block growth
•  Produce scar tissue
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Figure 12-8 Peripheral Nerve Regeneration after Injury (Part 1 of 4).
1
Fragmentation of
axon and myelin
occurs in distal
stump.
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Axon
Myelin Proximal stump
Distal stump
Figure 12-8 Peripheral Nerve Regeneration after Injury (Part 2 of 4).
2
Schwann cells
form cord, grow
into cut, and
unite stumps.
Macrophages
engulf degenerating axon and
myelin.
Macrophage
Cord of proliferating Schwann cells
© 2015 Pearson Education, Inc.
Figure 12-8 Peripheral Nerve Regeneration after Injury (Part 3 of 4).
3
Axon sends buds
into network of
Schwann cells
and then starts
growing along
cord of Schwann
cells.
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Figure 12-8 Peripheral Nerve Regeneration after Injury (Part 4 of 4).
4
Axon continues to
grow into distal
stump and is
enclosed by
Schwann cells.
© 2015 Pearson Education, Inc.
12-4 Membrane Potential
•  Ion Movements and Electrical Signals
•  All plasma (cell) membranes produce electrical
signals by ion movements
•  Membrane potential is particularly important to
neurons
© 2015 Pearson Education, Inc.
12-4 Membrane Potential
•  Five Main Membrane Processes in Neural
Activities
1.  Resting potential
•  The membrane potential of resting cell
2.  Graded potential
•  Temporary, localized change in resting potential
•  Caused by stimulus
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12-4 Membrane 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 Membrane 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
© 2015 Pearson Education, Inc.
12-4 Membrane Potential
•  The Membrane 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
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A&P FLIX Resting Membrane Potential
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12-4 Membrane Potential
•  Passive Forces Acting across the Plasma
Membrane
•  Chemical gradients
•  Concentration gradients (chemical gradient) of ions
(Na+, K+)
•  Electrical gradients
•  Separate charges of positive and negative ions
•  Result in potential difference
© 2015 Pearson Education, Inc.
Figure 12-9 Resting Membrane Potential.
© 2015 Pearson Education, Inc.
Figure 12-9 Resting Membrane Potential (Part 1 of 2).
Passive Chemical Gradients
Active Na+/K+ Pumps
The intracellular concentration
of potassium ions (K+) is
relatively high, so these ions
tend to move out of the cell
through potassium leak
channels. Similarly, the
extracellular concentration of
sodium ions (Na+) is relatively
high, so sodium ions move
into the cell through sodium
leak channels. Both of these
movements are driven by a
concentration gradient, or
chemical gradient.
Sodium–potassium (Na
+/K+) exchange pumps
maintain the concentration
of sodium and potassium
ions across the plasma
membrane.
Cl–
+
–
+
+
+
Sodium–
potassium
exchange
pump
K+ leak
channel
–
+
–
+
–
KEY
Sodium ion (Na+)
+ Potassium ion (K )
– Chloride ion (Cl )
+
-
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–
+
+
+
+
+
Na+ leak
channel
–
–
–
+
ADP
ATP
2 K+
+
+
–
+ 3 Na+
+ +
+
+
+
+
+
+
+
–
–
Protein
–
+
–
+
–
+
– –
Protein
–
+
Figure 12-9 Resting Membrane Potential (Part 2 of 2).
Passive Electrical Gradients
Potassium ions leave the
cytosol more rapidly than
sodium ions enter because
the plasma membrane is much
more permeable to potassium
than to sodium. As a result,
there are more positive
charges outside the plasma
membrane. Negatively charged
protein molecules within the
cytosol cannot cross the
plasma membrane, so there
are more negative charges on
the cytosol side of the plasma
membrane. This results in an
electrical gradient across
the plasma membrane.
–
+
Resting Membrane
Potential
Whenever positive and
negative ions are held apart,
a potential difference arises.
We measure the size of that
potential difference in
millivolts (mV). The resting
membrane potential for
most neurons is about –70
mV. The minus sign shows
that the inner surface of the
plasma membrane is
negatively charged with
respect to the exterior.
–30
–70
0
EXTRACELLULAR
FLUID
+30
+
mV
–
+
+
+
+
+
+
+
–
–
–
+
Plasma
membrane
–
+
+
+
+
–
–
–
+
–
– Protein
–
–
Protein
+
© 2015 Pearson Education, Inc.
–
–
–
+
–
CYTOSOL
+
KEY
+
+
–
Sodium ion (Na+)
Potassium ion (K+)
Chloride ion (Cl-)
12-4 Membrane Potential
•  Electrical Currents and Resistance
•  Electrical current
•  Movement of charges to eliminate potential
difference
•  Resistance
•  The amount of current a membrane restricts
© 2015 Pearson Education, Inc.
12-4 Membrane Potential
•  The Electrochemical Gradient
•  For a particular ion (Na+, K+) is:
•  The sum of chemical and electrical forces
•  Acting on the ion across a plasma membrane
•  A form of potential energy
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12-4 Membrane Potential
•  Equilibrium Potential
•  The membrane potential at which there is no net
movement of a particular ion across the cell
membrane
•  Examples:
•  K+ = -90 mV
•  Na+ = +66 mV
© 2015 Pearson Education, Inc.
Figure 12-10a Electrochemical Gradients for Potassium and Sodium Ions.
Potassium Ion Gradients
a At normal resting membrane potential, an electrical gradient
opposes the chemical gradient for potassium ions (K+). The
net electrochemical gradient tends to force potassium ions
out of the cell.
Potassium
chemical
gradient
Resting
membrane
potential
–70 mV
+
Potassium
electrical
gradient
+
+
+
+
+
+
–
–
–
– +
+ + –+
+ + + K + –
–
Cytosol
+
© 2015 Pearson Education, Inc.
Net potassium
electrochemical
gradient
+
+
Plasma
membrane
+
+
–
– –
+ – +
+
–
+
Protein –
–
Figure 12-10b Electrochemical Gradients for Potassium and Sodium Ions.
Potassium Ion Gradients
b If the plasma membrane were freely permeable to potassium
ions, the outflow of K+ would continue until the equilibrium
potential (–90 mV) was reached. Note how similar it is to the
resting membrane potential.
Potassium
chemical
gradient
Equilibrium
potential
–90 mV
+
+
Potassium
electrical
gradient
+
+
+
+
+
+
+
Plasma
membrane
+
+
+
+
+
+
+
+
– –
–
– –
+
+
+– + –+
+ – +
K
–
+ + +
+ – Protein
–
–
–
Cytosol
–
–
+
© 2015 Pearson Education, Inc.
Figure 12-10c Electrochemical Gradients for Potassium and Sodium Ions.
Sodium Ion Gradients
c At the normal resting membrane potential, chemical and
electrical gradients combine to drive sodium ions (Na+)
into the cell.
Sodium chemical Sodium
gradient
electrical
gradient
Resting
membrane
potential
–70 mV
+
+
–
–
–
+
–
© 2015 Pearson Education, Inc.
+
+
+
+
+ +
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Plasma
membrane
–
–
–
Net sodium
electrochemical
gradient
– –
– –
Cytosol
–
+
–
Protein
+
–
Figure 12-10d Electrochemical Gradients for Potassium and Sodium Ions.
Sodium Ion Gradients
d If the plasma membrane were freely permeable to sodium ions,
the influx of Na+ would continue until the equilibrium potential
(+66 mV) was reached. Note how different it is from the resting
membrane potential.
Sodium
electrical
gradient
Sodium
chemical
gradient
Equilibrium
potential
+66 mV
–
–
+
+
+
+
© 2015 Pearson Education, Inc.
+
+
+
–
+
+
+
+
+
+
Cytosol
–
+
+
–
+
+
+
+
+
+
–
–
–
–
+
+
+
+
+
Plasma
membrane
+
+
–
+
+
+
+
+
+
+
–
+
–
Protein
+
–
+
–
+
12-4 Membrane Potential
•  Active Forces across the Membrane
•  Sodium–potassium ATPase (exchange pump)
•  Is powered by ATP
•  Carries 3 Na+ out and 2 K+ in
•  Balances passive forces of diffusion
•  Maintains resting potential (-70 mV)
© 2015 Pearson Education, Inc.
12-4 Membrane Potential
•  The Resting Potential
•  Because the plasma membrane is highly
permeable to potassium ions:
•  The resting potential of approximately -70 mV is
fairly close to -90 mV, the equilibrium potential for K
+
•  The electrochemical gradient for sodium ions is
very large, but the membrane’s permeability to
these ions is very low
•  Na+ has only a small effect on the normal resting
potential, making it just slightly less negative than
the equilibrium potential for K+
© 2015 Pearson Education, Inc.
12-4 Membrane Potential
•  The Resting Potential
•  The sodium–potassium exchange pump ejects
3 Na+ ions for every 2 K+ ions that it brings into the
cell
•  It serves to stabilize the resting potential when the
ratio of Na+ entry to K+ loss through passive channels
is 3:2
•  At the normal resting potential, these passive and
active mechanisms are in balance
•  The resting potential varies widely with the type of cell
•  A typical neuron has a resting potential of
approximately -70 mV
© 2015 Pearson Education, Inc.
12-4 Membrane Potential
•  Changes in the Membrane Potential
•  Membrane potential rises or falls
•  In response to temporary changes in membrane
permeability
•  Resulting from opening or closing specific
membrane channels
© 2015 Pearson Education, Inc.
12-4 Membrane Potential
•  Sodium and Potassium Channels
•  Membrane permeability to Na+ and K+ determines
membrane potential
•  They are either passive or active
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12-4 Membrane 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
© 2015 Pearson Education, Inc.
12-4 Membrane Potential
•  Three States of Gated Channels
1.  Closed, but capable of opening
2.  Open (activated)
3.  Closed, not capable of opening (inactivated)
© 2015 Pearson Education, Inc.
12-4 Membrane Potential
•  Three Classes of Gated Channels
1.  Chemically gated channels
2.  Voltage-gated channels
3.  Mechanically gated channels
© 2015 Pearson Education, Inc.
Figure 12-11a Gated Channels.
a Chemically gated channel
+
Resting state
Presence of ACh
+
ACh
Binding
site
Gated
channel
Channel closed
+
+
+
+
+
Channel open
A chemically gated (ligand-gated)
Na+ channel that opens in response
to the presence of ACh (ligand) at a
binding site.
© 2015 Pearson Education, Inc.
Figure 12-11b Gated Channels.
b Voltage-gated channel
–70 mV
Activation
gate
+
+
Channel closed
–60 mV
Channel open
+30 mV
Inactivation
gate
+
+
+
+
+
+
Channel inactivated
A voltage-gated Na+ channel that
responds to changes in the membrane
potential. At its resting membrane
potential of –70 mV, the channel is
closed; at –60 mV, the channel opens;
at +30 mV, the channel is inactivated.
© 2015 Pearson Education, Inc.
Figure 12-11c Gated Channels.
c Mechanically gated channel
+
+
+
+
+
Channel closed
+
Applied
+
+
+ pressure
+
+
Channel open
+
+
+
+
Pressure
removed
+
Channel closed
A mechanically gated channel, which
opens in response to distortion of
the membrane.
© 2015 Pearson Education, Inc.
Figure 12-12 Graded Potentials (Part 1 of 3).
Resting State
Resting membrane with closed chemically gated sodium ion channels
Initial
segment
+ Extracellular
+
+
Fluid
–70 mV
+
+
+
+
+
+ + +
+
+
+
+
+ + +
+
+
+
–
© 2015 Pearson Education, Inc.
–
–
–
–
–
–
–
–
–
– –
–
Cytosol
–
Figure 12-12 Graded Potentials (Part 2 of 3).
1
Stimulation
Membrane exposed to chemical that opens the sodium ion channels
Stimulus
applied
here
+ + –65 mV
+ + + + +
+
+
+
+ +
+ + +
+ +
+
+ +
+ + + +
+ +
+ +
+ + + +
+
–
–
–
–
–
+ + + + +
+ + +
+
© 2015 Pearson Education, Inc.
–
–
–
–
+ + + +
+
+
+ + + +
–
–
–
–
–
–
Figure 12-12 Graded Potentials (Part 3 of 3).
2
Graded Potential
Spread of sodium ions inside plasma membrane produces a local
current that depolarized adjacent portions of the plasma membrane
+
+ +
Local + + +
+
current
+
+ + +
+
+ +
–
–
© 2015 Pearson Education, Inc.
– –
–60 mV
+
+
+
–
–
+
+
–65 mV
+
+ + +
–
+ +
+ +
+ +
+ + + +
–
–
+
+ +
+
+
–
+
+
+ + +
+
+
+ +
+ +
+
+
+
Local current
+
+
+
–
–
–
–
–70 mV
+
–
–
–
–
–
12-4 Membrane Potential
•  Graded Potentials
•  Repolarization
•  When the stimulus is removed, membrane 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
© 2015 Pearson Education, Inc.
STOP: NEXT CLASS…….
•  READ UP ON ACTION POTENTIAL
© 2015 Pearson Education, Inc.
12-5 Action Potential
•  Initiating Action Potential
•  Initial stimulus
•  A graded depolarization of axon hillock large
enough (10 to 15 mV) to change resting potential
(-70 mV) to threshold level of voltage-gated
sodium channels (-60 to -55 mV)
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
Figure 12-14 Generation of an Action Potential (Part 3 of 9).
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
+
+
© 2015 Pearson Education, Inc.
= 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
© 2015 Pearson Education, Inc.
Figure 12-14 Generation of an Action Potential (Part 4 of 9).
1
Depolarization to
Threshold
–60 mV
Local
current
+
+
+
+
+
–
+
+
+
– +
+ + +
+
–
+
+
The stimulus that initiates an action
potential is a graded depolarization large
enough to open voltage-gated sodium
channels. The opening of the channels
occurs at the membrane potential known
as the threshold.
KEY
+
+
© 2015 Pearson Education, Inc.
= Sodium ion
= Potassium ion
Figure 12-14 Generation of an Action Potential (Part 5 of 9).
2
Activation of Sodium
Channels and Rapid
Depolarization
+10 mV
+
+
+
+
–
+
+ + –
+
+
+
+
+
+
+
+
When the sodium channel activation gates
open, the plasma membrane becomes
much more permeable to Na+. Driven by
the large electrochemical gradient, sodium
ions rush into the cytoplasm, and rapid
depolarization occurs. The inner membrane
surface now contains more positive ions
than negative ones, and the membrane
potential has changed from –60 mV to a
positive value.
KEY
© 2015 Pearson Education, Inc.
+
+
= Sodium ion
= Potassium ion
Figure 12-14 Generation of an Action Potential (Part 6 of 9).
3
Inactivation of Sodium
Channels and Activation
of Potassium Channels
+30 mV
+
+ +
+
+
+
+
+
+
+
+ +
+
+
+ +
As the membrane potential approaches
+30 mV, the inactivation gates of the voltagegated sodium channels close. This step is
known as sodium channel inactivation,
and it coincides with the opening of
voltage-gated potassium channels.
Positively charged potassium ions move out
of the cytosol, shifting the membrane
potential back toward resting levels.
Repolarization now begins.
KEY
© 2015 Pearson Education, Inc.
+
+
= Sodium ion
= Potassium ion
Figure 12-14 Generation of an Action Potential (Part 7 of 9).
4
Closing of Potassium
Channels
–90 mV
+
+
+
– – –
+
+ + +
+
+
– –
+
+
+
+
–
+
The voltage-gated sodium channels
remain inactivated until the membrane
has repolarized to near threshold levels.
At this time, they regain their normal
status: closed but capable of opening.
The voltage-gated potassium channels
begin closing as the membrane reaches
the normal resting potential (about
–70 mV). Until all these potassium
channels have closed, potassium ions
continue to leave the cell. This produces
a brief hyperpolarization.
KEY
© 2015 Pearson Education, Inc.
+
+
= Sodium ion
= Potassium ion
12-5 Action Potential
•  The Refractory Period
•  The time period:
•  From beginning of action potential
•  To return to resting state
•  During which membrane will not respond normally
to additional stimuli
© 2015 Pearson Education, Inc.
A&P FLIX Generation of an Action Potential
© 2015 Pearson Education, Inc.
12-5 Action Potential
•  Absolute Refractory Period
•  Sodium channels open or inactivated
•  No action potential possible
•  Relative Refractory Period
•  Membrane potential almost normal
•  Very large stimulus can initiate action potential
© 2015 Pearson Education, Inc.
12-5 Action Potential
•  Powering the Sodium–Potassium Exchange
Pump
•  To maintain concentration gradients of Na+ and K+
over time
•  Requires energy (1 ATP for each 2 K+/3 Na+
exchange)
•  Without ATP
•  Neurons stop functioning
© 2015 Pearson Education, Inc.
Figure 12-14 Generation of an Action Potential (Part 9 of 9).
ABSOLUTE REFRACTORY PERIOD
3
+30
Membrane potential (mV)
0
D E P O L A R I Z AT I O N
R E P O L A R I Z AT I O N
Threshold
1
4
A graded depolarization
brings an area of excitable
membrane to threshold
(–60 mV).
During the relative refractory
period, the membrane can
respond only to a larger-thannormal stimulus.
During the absolute refractory
period, the membrane cannot
respond to further stimulation.
0
1
Time (msec)
© 2015 Pearson Education, Inc.
Potassium channels
close, and both sodium
and potassium channels
return to their
normal states.
Voltage-gated sodium
channels open and
sodium ions move into
the cell. The membrane
potential rises to +30 mV.
–40
–70
Sodium channels close,
voltage-gated potassium
channels open, and potassium
ions move out of the cell.
Repolarization begins.
2
Resting
potential
–60
RELATIVE REFRACTORY PERIOD
2
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)
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
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)
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
A&P FLIX Propagation of an Action Potential
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
12-6 Axon Diameter and Speed
•  Three Groups of Axons
1.  Type A fibers
2.  Type B fibers
3.  Type C fibers
•  These groups are classified by:
•  Diameter
•  Myelination
•  Speed of action potentials
© 2015 Pearson Education, Inc.
12-6 Axon Diameter and Speed
•  Type A Fibers
•  Myelinated
•  Large diameter
•  High speed (140 m/sec)
•  Carry rapid information to/from CNS
•  For example, position, balance, touch, and motor
impulses
© 2015 Pearson Education, Inc.
12-6 Axon Diameter and Speed
•  Type B Fibers
•  Myelinated
•  Medium diameter
•  Medium speed (18 m/sec)
•  Carry intermediate signals
•  For example, sensory information, peripheral
effectors
© 2015 Pearson Education, Inc.
12-6 Axon Diameter and Speed
•  Type C Fibers
•  Unmyelinated
•  Small diameter
•  Slow speed (1 m/sec)
•  Carry slower information
•  For example, involuntary muscle, gland controls
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
12-7 Synapses
•  Synaptic Activity
•  Action potentials (nerve impulses)
•  Are transmitted from presynaptic neuron
•  To postsynaptic neuron (or other postsynaptic
cell)
•  Across a synapse
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
Figure 12-16 Events in the Functioning of a Cholinergic Synapse (Part 1 of 4).
1
An arriving action potential depolarizes the axon terminal
of a presynaptic neuron.
Mitochondrion
1
1
A
Ch
A
Ch
A
Ch
Synaptic
vesicle
A
Ch
Axon terminal
Synaptic cleft
Postsynaptic neuron
© 2015 Pearson Education, Inc.
ACh receptor
(chemically
gated Na+
channel)
Figure 12-16 Events in the Functioning of a Cholinergic Synapse (Part 2 of 4).
2
Calcium ions (Ca2+) enter the cytosol of the axon terminal.
This results in ACh release from the synaptic vesicles by
exocytosis.
Mitochondrion
Voltage-gated
Ca2+ channel
A
Ch
2
2
Ca2+
Ca2+
A
Ch
2
A
Ch
© 2015 Pearson Education, Inc.
A
Ch
2
A
Ch
Figure 12-16 Events in the Functioning of a Cholinergic Synapse (Part 3 of 4).
3
ACh diffuses across the synaptic cleft and binds to
receptors on the postsynaptic membrane. Sodium channels
open, producing a graded depolarization.
A
Ch
A
Ch
Na+
© 2015 Pearson Education, Inc.
A
Ch
3
Chemically
gated Na+
channels
3
Initiation of a
graded potential,
or action potential, if
threshold is reached at
the initial segment
Na+
Figure 12-16 Events in the Functioning of a Cholinergic Synapse (Part 4 of 4).
4
Depolarization ends as ACh is broken down into acetate and
choline by AChE. The axon terminal reabsorbs choline from
the synaptic cleft and uses it to resynthesize ACh.
Mitochondrion
Acetyl-CoA
4
CoA
A
Ch
Acetylcholine
Ch
A
Ch
Axon terminal
Choline Ch
4
Acetate A
Acetylcholinesterase
(AChE)
Postsynaptic neuron
© 2015 Pearson Education, Inc.
A
Ch
Propagation
of action
potential
(if generated)
12-7 Synapses
•  Synaptic Fatigue
•  Occurs when neurotransmitter cannot recycle fast
enough to meet demands of intense stimuli
•  Synapse inactive until ACh is replenished
© 2015 Pearson Education, Inc.
12-8 Neurotransmitters and Neuromodulators
•  Other Neurotransmitters
•  At least 50 neurotransmitters other than ACh,
including:
•  Biogenic amines
•  Amino acids
•  Neuropeptides
•  Dissolved gases
© 2015 Pearson Education, Inc.
12-8 Neurotransmitters and Neuromodulators
•  Important Neurotransmitters
•  Other than acetylcholine
•  Norepinephrine (NE)
•  Dopamine
•  Serotonin
•  Gamma aminobutyric acid (GABA)
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
12-8 Neurotransmitters and Neuromodulators
•  Four Classes of Opioids
1. 
2. 
3. 
4. 
Endorphins
Enkephalins
Endomorphins
Dynorphins
© 2015 Pearson Education, Inc.
12-8 Neurotransmitters and Neuromodulators
•  How Neurotransmitters and Neuromodulators
Work
•  Direct effects on membrane channels
•  For example, ACh, glycine, aspartate
•  Indirect effects via G proteins
•  For example, E, NE, dopamine, histamine, GABA
•  Indirect effects via intracellular enzymes
•  For example, lipid-soluble gases (NO, CO)
© 2015 Pearson Education, Inc.
12-8 Neurotransmitters and Neuromodulators
•  Direct Effects
•  Ionotropic effects
•  Open/close gated ion channels
© 2015 Pearson Education, Inc.
Figure 12-17a Mechanisms of Neurotransmitter Function.
Examples: ACh, glutamate,
aspartate
a Direct effects
ACh
+
Binding
site
Chemically gated channel
© 2015 Pearson Education, Inc.
+
+
+
+
+
12-8 Neurotransmitters and
Neuromodulators
•  Indirect Effects – G Proteins
•  Work through second messengers
•  Enzyme complex that binds GTP
•  Link between neurotransmitter (first messenger)
and second messenger
•  Activate enzyme adenylyl cyclase
•  Which produces second messenger cyclic-AMP
(cAMP)
© 2015 Pearson Education, Inc.
Figure 12-17b Mechanisms of Neurotransmitter Function.
b Indirect effects
by G proteins
Examples: E, NE, dopamine,
histamine, GABA
Neurotransmitter
Receptor
G protein
(inactive)
G protein
(active)
ATP
Opens ion
channels
Adenylate
cyclase
cAMP
Activates enzymes that
change cell metabolism
and activity
© 2015 Pearson Education, Inc.
12-8 Neurotransmitters and Neuromodulators
•  Indirect Effects – Intracellular Receptors
•  Lipid-soluble gases (NO, CO)
•  Bind to enzymes in brain cells
© 2015 Pearson Education, Inc.
Figure 12-17c Mechanisms of Neurotransmitter Function.
c
Indirect effects by
intracellular enzymes
Examples: Nitric oxide,
carbon monoxide
Nitric oxide
Opens ion
channels
Production
of secondary
messengers
Changes in cell
metabolism and activity
© 2015 Pearson Education, Inc.
Activation
of enzymes
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
© 2015 Pearson Education, Inc.
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
© 2015 Pearson Education, Inc.
12-9 Information Processing
•  Temporal Summation
•  Multiple times
•  Rapid, repeated stimuli at one synapse
•  Spatial Summation
•  Multiple locations
•  Many stimuli, arrive at multiple synapses
© 2015 Pearson Education, Inc.
Figure 12-18a Temporal and Spatial Summation.
1
First stimulus arrives
FIRST
STIMULUS
Initial
segment
2
Second stimulus arrives and is
added to the first stimulus
3
Action potential is generated
SECOND
STIMULUS
Threshold
reached
a 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
on the first.
© 2015 Pearson Education, Inc.
ACTION
POTENTIAL
PROPAGATION
Figure 12-18b Temporal and Spatial Summation.
1
Two stimuli arrive simultaneously
TWO
SIMULTANEOUS
STIMULI
2
Action potential is generated
ACTION
POTENTIAL
PROPAGATION
Threshold
reached
b 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.
© 2015 Pearson Education, Inc.
12-9 Information Processing
•  Facilitation
•  A neuron becomes facilitated
•  As EPSPs accumulate
•  Raising membrane potential closer to threshold
•  Until a small stimulus can trigger action potential
© 2015 Pearson Education, Inc.
12-9 Information Processing
•  Summation of EPSPs and IPSPs
•  Neuromodulators and hormones
•  Can change membrane sensitivity to
neurotransmitters
•  Shifting balance between EPSPs and IPSPs
© 2015 Pearson Education, Inc.
Figure 12-20a 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
a Presynaptic inhibition
© 2015 Pearson Education, Inc.
4. Reduced
effect on
postsynaptic
membrane
Figure 12-20b 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
b Presynaptic facilitation
© 2015 Pearson Education, Inc.
4. Increased
effect on
postsynaptic
membrane