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Chapter 12
*Lecture PowerPoint
Nervous Tissue
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Introduction
• The nervous system is one of great complexity
• Nervous system is the foundation of our
conscious experience, personality, and behavior
• Neurobiology combines the behavioral and life
sciences
11-2
Overview of the Nervous System
• Expected Learning Outcomes
– Describe the overall function of the nervous system.
– Describe its major anatomical and functional
subdivisions.
11-3
Overview of the Nervous System
• Endocrine and nervous systems maintain
internal coordination
– Endocrine system: communicates by means of
chemical messengers (hormones) secreted into to the
blood
– Nervous system: employs electrical and chemical
means to send messages from cell to cell
12-4
Overview of the Nervous System
• Nervous system carries out its task in three
basic steps
• Sense organs receive information about changes in
the body and the external environment, and transmit
coded messages to the spinal cord and the brain
• Brain and spinal cord process this information,
relate it to past experiences, and determine what
response is appropriate to the circumstances
• Brain and spinal cord issue commands to muscles
and gland cells to carry out such a response
12-5
Overview of the Nervous System
• Nervous system has two major anatomical
subdivisions
– Central nervous system (CNS)
• Brain and spinal cord enclosed in bony coverings
• Enclosed by cranium and vertebral column
– Peripheral nervous system (PNS)
• All the nervous system except the brain and spinal cord;
composed of nerves and ganglia
• Nerve—a bundle of nerve fibers (axons) wrapped in fibrous
connective tissue
• Ganglion—a knotlike swelling in a nerve where neuron cell
bodies are concentrated
12-6
Overview of the Nervous System
• Peripheral nervous system has two major
functional subdivisions
– Sensory (afferent) division: carries sensory signals
from various receptors to the CNS
• Informs the CNS of stimuli within or around the body
– Somatic sensory division: carries signals from
receptors in the skin, muscles, bones, and joints
– Visceral sensory division: carries signals from the
viscera of the thoracic and abdominal cavities
• Heart, lungs, stomach, and urinary bladder
12-7
Overview of the Nervous System
• Motor (efferent) division—carries signals from
the CNS to gland and muscle cells that carry out
the body’s response
– Effectors: cells and organs that respond to
commands from the CNS
– Somatic motor division: carries signals to skeletal
muscles
• Output produces muscular contraction as well as
somatic reflexes—involuntary muscle contractions
12-8
Overview of the Nervous System
• Visceral motor division (autonomic nervous system)
– Carries signals to glands, cardiac muscle, and smooth
muscle
– Involuntary, and responses of this system and its
receptors are visceral reflexes
– Sympathetic division
• Tends to arouse body for action
• Accelerating heart beat and respiration, while inhibiting
digestive and urinary systems
– Parasympathetic division
• Tends to have calming effect
• Slows heart rate and breathing
• Stimulates digestive and urinary systems
12-9
Subdivisions of the Nervous System
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Central nervous
system (CNS)
Brain
Peripheral nervous
system (PNS)
Spinal
cord
Nerves
Ganglia
Figure 12.1
12-10
Subdivisions of the Nervous System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Central nervous system
Brain
Peripheral nervous system
Spinal
cord
Visceral
sensory
division
Figure 12.2
Sensory
division
Somatic
sensory
division
Motor
division
Visceral
motor
division
Sympathetic
division
Somatic
motor
division
Parasympathetic
division
12-11
Properties of Neurons
• Expected Learning Outcomes
– Describe three functional properties found in all
neurons.
– Define the three most basic functional categories of
neurons.
– Identify the parts of a neuron.
– Explain how neurons transport materials between the
cell body and tips of the axon.
12-12
Universal Properties
• Excitability (irritability)
– Respond to environmental changes called stimuli
• Conductivity
– Neurons respond to stimuli by producing electrical
signals that are quickly conducted to other cells at
distant locations
• Secretion
– When electrical signal reaches end of nerve fiber, a
chemical neurotransmitter is secreted that crosses
the gap and stimulates the next cell
12-13
Functional Classes
• Three general classes of neurons (sensory,
interneuron, motor) based on function
• Sensory (afferent) neurons
– Specialized to detect stimuli
– Transmit information about them to the CNS
• Begin in almost every organ in the body and end in
CNS
• Afferent—conducting signals toward CNS
12-14
Functional Classes
• Three general classes of neurons (cont.)
• Interneurons (association neurons)
– Lie entirely within the CNS
– Receive signals from many neurons and carry out the
integrative function
• Process, store, and retrieve information and “make
decisions” that determine how the body will respond to
stimuli
– 90% of all neurons are interneurons
– Lie between and interconnect the incoming sensory
pathways and the outgoing motor pathways of the CNS
12-15
Functional Classes
• Three general classes of neurons (cont.)
• Motor (efferent) neuron
– Send signals out to muscles and gland cells (the
effectors)
• Motor because most of them lead to muscles
• Efferent neurons conduct signals away from the CNS
12-16
Classes of Neurons
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Peripheral nervous system
Central nervous system
1 Sensory (afferent)
neurons conduct
signals from receptors
to the CNS.
3 Motor (efferent)
neurons conduct
signals from the CNS
to effectors such as
muscles and glands.
2 Inter neurons
(association
neurons) are
confined to
the CNS.
Figure 12.3
12-17
Structure of a Neuron
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Soma—the control center of the
neuron
– Also called neurosoma, cell
body, or perikaryon
– Has a single, centrally located
nucleus with large nucleolus
– Cytoplasm contains
mitochondria, lysosomes, a Golgi
complex, numerous inclusions,
and extensive rough endoplasmic
reticulum and cytoskeleton
– Cytoskeleton consists of dense
mesh of microtubules and
neurofibrils (bundles of actin
filaments)
• Compartmentalizes rough ER
into dark-staining Nissl bodies
Dendrites
Soma
Nucleus
Nucleolus
Trigger zone:
Axon hillock
Initial segment
Axon collateral
Axon
Direction of
signal transmission
Internodes
Node of Ranvier
Myelin sheath
Schwann cell
Terminal
arborization
Synaptic knobs
(a)
Figure 12.4a
12-18
Structure of a Neuron
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Soma—the control center of the
neuron (cont.)
– No centrioles: no further cell
division
– Inclusions: glycogen granules,
lipid droplets, melanin, and
lipofuscin (golden brown pigment
produced when lysosomes digest
worn-out organelles)
• Lipofuscin accumulates with age
• Wear-and-tear granules
• Most abundant in old neurons
Dendrites
Soma
Nucleus
Nucleolus
Trigger zone:
Axon hillock
Initial segment
Axon collateral
Axon
Direction of
signal transmission
Internodes
Node of Ranvier
Myelin sheath
Schwann cell
Terminal
arborization
Synaptic knobs
(a)
Figure 12.4a
12-19
Structure of a Neuron
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dendrites
• Dendrites—vast number of
branches coming from a few
thick branches from the soma
– Resemble bare branches of a
tree in winter
– Primary site for receiving
signals from other neurons
– The more dendrites the
neuron has, the more
information it can receive and
incorporate into decision
making
– Provide precise pathway for
the reception and processing
of neural information
Soma
Nucleus
Nucleolus
Trigger zone:
Axon hillock
Initial segment
Axon collateral
Axon
Direction of
signal transmission
Internodes
Node of Ranvier
Myelin sheath
Schwann cell
Terminal
arborization
Synaptic knobs
(a)
Figure 12.4a
12-20
Structure of a Neuron
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dendrites
• Axon (nerve fiber)—
originates from a mound on
one side of the soma called
the axon hillock
Soma
Nucleus
Nucleolus
Trigger zone:
Axon hillock
Initial segment
– Cylindrical, relatively
unbranched for most of its
length
• Axon collaterals—branches
of axon
– Branch extensively on distal
end
– Specialized for rapid
conduction of nerve signals to
points remote to the soma
Axon collateral
Axon
Direction of
signal transmission
Internodes
Node of Ranvier
Myelin sheath
Schwann cell
Terminal
arborization
Synaptic knobs
(a)
Figure 12.4a
12-21
Structure of a Neuron
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Axon (nerve fiber) (cont.)
– Axoplasm: cytoplasm of axon
– Axolemma: plasma membrane
of axon
– Only one axon per neuron
– Schwann cells and myelin
sheath enclose axon
– Distal end, axon has terminal
arborization: extensive complex
of fine branches
Dendrites
Soma
Nucleus
Nucleolus
Trigger zone:
Axon hillock
Initial segment
Axon collateral
Axon
Direction of
signal transmission
Internodes
Node of Ranvier
• Synaptic knob (terminal
button)—little swelling that forms
a junction (synapse) with the
next cell
• Contains synaptic vesicles full
of neurotransmitter
Myelin sheath
Schwann cell
Terminal
arborization
Synaptic knobs
(a)
Figure 12.4a
12-22
Structure of a Neuron
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Multipolar neuron
– One axon and multiple dendrites
– Most common
– Most neurons in the brain and spinal cord
Dendrites
Axon
Multipolar neurons
• Bipolar neuron
– One axon and one dendrite
– Olfactory cells, retina, inner ear
Dendrites
• Unipolar neuron
– Single process leading away from the
soma
– Sensory from skin and organs to spinal
cord
Axon
Bipolar neurons
Dendrites
Axon
Unipolar neuron
• Anaxonic neuron
– Many dendrites but no axon
– Help in visual processes
Dendrites
Anaxonic neuron
Figure 12.5
12-23
Axonal Transport
• Many proteins made in soma must be transported to axon
and axon terminal
– To repair axolemma, serve as gated ion channel proteins, as
enzymes or neurotransmitters
• Axonal transport—two-way passage of proteins,
organelles, and other material along an axon
– Anterograde transport: movement down the axon away from
soma
– Retrograde transport: movement up the axon toward the soma
• Microtubules guide materials along axon
– Motor proteins (kinesin and dynein) carry materials “on their
backs” while they “crawl” along microtubules
• Kinesin—motor proteins in anterograde transport
• Dynein—motor proteins in retrograde transport
12-24
Axonal Transport
• Fast axonal transport—occurs at a rate of 20 to 400
mm/day
– Fast anterograde transport (up to 400 mm/day)
• Organelles, enzymes, synaptic vesicles, and small molecules
– Fast retrograde transport
• For recycled materials and pathogens—rabies, herpes
simplex, tetanus, polio viruses
– Delay between infection and symptoms is time needed for
transport up the axon
• Slow axonal transport or axoplasmic flow—0.5 to 10
mm/day
– Always anterograde
– Moves enzymes, cytoskeletal components, and new axoplasm
down the axon during repair and regeneration of damaged axons
– Damaged nerve fibers regenerate at a speed governed by slow
axonal transport
12-25
Supportive Cells (Neuroglia)
• Expected Learning Outcomes
– Name the six types of cells that aid neurons and state
their respective functions.
– Describe the myelin sheath that is found around
certain nerve fibers and explain its importance.
– Describe the relationship of unmyelinated nerve fibers
to their supportive cells.
– Explain how damaged nerve fibers regenerate.
12-26
Supportive Cells (Neuroglia)
• About 1 trillion (1012) neurons in the nervous
system
• Neuroglia outnumber the neurons by as much
as 50 to 1
• Neuroglia or glial cells
– Support and protect the neurons
– Bind neurons together and form framework for
nervous tissue
– In fetus, guide migrating neurons to their destination
– If mature neuron is not in synaptic contact with
another neuron it is covered by glial cells
• Prevents neurons from touching each other
• Gives precision to conduction pathways
12-27
Types of Neuroglia
• Four types occur only in CNS
– Oligodendrocytes
• Form myelin sheaths in CNS
• Each armlike process wraps around a nerve fiber forming
an insulating layer that speeds up signal conduction
– Ependymal cells
• Line internal cavities of the brain
• Cuboidal epithelium with cilia on apical surface
• Secretes and circulates cerebrospinal fluid (CSF)
– Clear liquid that bathes the CNS
12-28
Types of Neuroglia
• Four types occur only in CNS (cont.)
– Microglia
• Small, wandering macrophages formed white blood cell
called monocytes
• Thought to perform a complete checkup on the brain
tissue several times a day
• Wander in search of cellular debris to phagocytize
12-29
Types of Neuroglia
• Four types occur only in CNS (cont.)
– Astrocytes
• Most abundant glial cell in CNS
• Cover entire brain surface and most nonsynaptic
regions of the neurons in the gray matter of the CNS
• Diverse functions
– Form a supportive framework of nervous tissue
– Have extensions (perivascular feet) that contact blood
capillaries that stimulate them to form a tight seal called
the blood–brain barrier
– Convert blood glucose to lactate and supply this to the
neurons for nourishment
12-30
Types of Neuroglia
Cont.
• Nerve growth factors secreted by astrocytes promote
neuron growth and synapse formation
• Communicate electrically with neurons and may
influence synaptic signaling
• Regulate chemical composition of tissue fluid by
absorbing excess neurotransmitters and ions
• Astrocytosis or sclerosis—when neuron is damaged,
astrocytes form hardened scar tissue and fill space
formerly occupied by the neuron
12-31
Types of Neuroglia
• Two types occur only in PNS
– Schwann cells
• Envelope nerve fibers in PNS
• Wind repeatedly around a nerve fiber
• Produce a myelin sheath similar to the ones produced
by oligodendrocytes in CNS
• Assist in the regeneration of damaged fibers
– Satellite cells
• Surround the neurosomas in ganglia of the PNS
• Provide electrical insulation around the soma
• Regulate the chemical environment of the neurons
12-32
Neuroglial Cells of CNS
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Capillary
Neurons
Astrocyte
Oligodendrocyte
Perivascular feet
Myelinated axon
Ependymal cell
Myelin (cut)
Cerebrospinal fluid
Microglia
Figure 12.6
12-33
Glial Cells and Brain Tumors
• Tumors—masses of rapidly dividing cells
– Mature neurons have little or no capacity for mitosis and
seldom form tumors
• Brain tumors arise from:
– Meninges (protective membranes of CNS)
– Metastasis from nonneuronal tumors in other organs
– Often glial cells that are mitotically active throughout life
• Gliomas grow rapidly and are highly malignant
– Blood–brain barrier decreases effectiveness of
chemotherapy
– Treatment consists of radiation or surgery
12-34
Myelin
• Myelin sheath—an insulating layer around a
nerve fiber
– Formed by oligodendrocytes in CNS and Schwann
cells in PNS
– Consists of the plasma membrane of glial cells
• 20% protein and 80% lipid
• Myelination—production of the myelin sheath
–
–
–
–
Begins at week 14 of fetal development
Proceeds rapidly during infancy
Completed in late adolescence
Dietary fat is important to CNS development
12-35
Myelin
• In PNS, Schwann cell spirals repeatedly around
a single nerve fiber
– Lays down as many as a hundred layers of its own
membrane
– No cytoplasm between the membranes
– Neurilemma: thick, outermost coil of myelin sheath
• Contains nucleus and most of its cytoplasm
• External to neurilemma is basal lamina and a thin layer
of fibrous connective tissue—endoneurium
12-36
Myelin Sheath in PNS
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Axoplasm
Schwann cell
nucleus
Axolemma
Neurilemma
Figure 12.4c
(c)
Myelin sheath
Nodes of Ranvier and internodes
12-37
Myelin
• In CNS—oligodendrocytes reach out to
myelinate several nerve fibers in its immediate
vicinity
– Anchored to multiple nerve fibers
– Cannot migrate around any one of them like Schwann
cells
– Must push newer layers of myelin under the older
ones; so myelination spirals inward toward nerve fiber
– Nerve fibers in CNS have no neurilemma or
endoneurium
12-38
Myelin
• Many Schwann cells or oligodendrocytes are
needed to cover one nerve fiber
• Myelin sheath is segmented
– Nodes of Ranvier: gap between segments
– Internodes: myelin-covered segments from one gap
to the next
– Initial segment: short section of nerve fiber between
the axon hillock and the first glial cell
– Trigger zone: the axon hillock and the initial segment
• Play an important role in initiating a nerve signal
12-39
Myelination in CNS
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Oligodendrocyte
Myelin
Nerve fiber
Figure 12.7b
(b)
12-40
Myelination in PNS
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Schwann cell
Axon
Basal lamina
Endoneurium
Nucleus
(a)
Neurilemma
Myelin sheath
Figure 12.7a
12-41
Diseases of the Myelin Sheath
• Degenerative disorders of the myelin sheath
– Multiple sclerosis
• Oligodendrocytes and myelin sheaths in the CNS deteriorate
• Myelin replaced by hardened scar tissue
• Nerve conduction disrupted (double vision, tremors, numbness,
speech defects)
• Onset between 20 and 40 and fatal from 25 to 30 years after
diagnosis
• Cause may be autoimmune triggered by virus
12-42
Diseases of the Myelin Sheath
• Degenerative disorders of the myelin sheath (cont.)
– Tay–Sachs disease: a hereditary disorder of infants of
Eastern European Jewish ancestry
• Abnormal accumulation of glycolipid called GM2 in the
myelin sheath
– Normally decomposed by lysosomal enzyme
– Enzyme missing in individuals homozygous for Tay–Sachs
allele
– Accumulation of ganglioside (GM2) disrupts conduction of
nerve signals
– Blindness, loss of coordination, and dementia
• Fatal before age 4
12-43
Unmyelinated Nerve Fibers
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Neurilemma
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Myelin sheath
Unmyelinated
nerve fibers
Myelinated
axon
Schwann
cell cytoplasm
Basal
lamina
Neurilemma
Unmyelinated
axon
Schwann cell
(c)
3 µm
Basal lamina
c: © The McGraw-Hill Companies, Inc./Dr. Dennis Emery, Dept. of Zoology and Genetics, Iowa State University,
photographer
Figure 12.7c
Figure 12.8
• Schwann cells hold 1 to 12 small nerve fibers in grooves on the surface
• Membrane folds once around each fiber overlapping itself along the edges
• Mesaxon—neurilemma wrapping of unmyelinated nerve fibers
12-44
Conduction Speed of Nerve Fibers
• Speed at which a nerve signal travels along a
nerve fiber depends on two factors
– Diameter of fiber
– Presence or absence of myelin
• Signal conduction occurs along the surface of a
fiber
– Larger fibers have more surface area and conduct
signals more rapidly
– Myelin further speeds signal conduction
12-45
Conduction Speed of Nerve Fibers
• Conduction speed
–
–
–
–
Small, unmyelinated fibers: 0.5 to 2.0 m/s
Small, myelinated fibers: 3 to 15.0 m/s
Large, myelinated fibers: up to 120 m/s
Slow signals supply the stomach and dilate pupil where
speed is less of an issue
– Fast signals supply skeletal muscles and transport
sensory
– Signals for vision and balance
12-46
Regeneration of Nerve Fibers
• Regeneration of a damaged peripheral nerve fiber can
occur if:
– Its soma is intact
– At least some neurilemma remains
• Fiber distal to the injury cannot survive and degenerates
– Macrophages clean up tissue debris at the point of injury
and beyond
• Soma swells, ER breaks up, and nucleus moves off center
– Due to loss of nerve growth factor from neuron’s target cell
• Axon stump sprouts multiple growth processes
– Severed distal end continues to degenerate
12-47
Regeneration of Nerve Fibers
• Regeneration tube—formed by Schwann cells,
basal lamina, and the neurilemma near the injury
– Regeneration tube guides the growing sprout back to
the original target cells and reestablishes synaptic
contact
• Nucleus returns to normal shape
• Regeneration of damaged nerve fibers in the CNS
cannot occur at all
12-48
Regeneration of Nerve Fiber
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Endoneurium
Neuromuscular
junction
Myelin sheath
Muscle fiber
1
Normal nerve fiber
Local trauma
Macrophages
Degenerating
terminal
2 Injured fiber
Degenerating
Schwann cells
Degenerating axon
3
Degeneration of severed fiber
Schwann cells
Growth processes
4
Early regeneration
Regeneration
tube
Atrophy of
muscle fibers
• Denervation
atrophy of muscle
due to loss of nerve
contact by damaged
nerve
Retraction of
growth processes
Growth processes
5
Late regeneration
6 Regenerated fiber
Regrowth of
muscle fibers
Figure 12.9
12-49
Nerve Growth Factor
• Nerve growth factor (NGF)—
a protein secreted by a gland,
muscle, and glial cells and
picked up by the axon
terminals of the neurons
– Prevents apoptosis
(programmed cell death) in
growing neurons
– Enables growing neurons to
make contact with their target
cells
• Isolated by Rita Levi-Montalcini
in 1950s
• Won Nobel prize in 1986 with
Stanley Cohen
• Use of growth factors is now a
vibrant field of research
Figure 12.10
12-50
Electrophysiology of Neurons
• Expected Learning Outcomes
– Explain why a cell has an electrical charge difference
(voltage) across its membrane.
– Explain how stimulation of a neuron causes a local
electrical response in its membrane.
– Explain how local responses generate a nerve signal.
– Explain how the nerve signal is conducted down an
axon.
12-51
Electrophysiology of Neurons
• Galen thought that the brain pumped a vapor called
psychic pneuma through hollow nerves and squirted it
into the muscles to make them contract
• René Descartes in the seventeenth century supported
this theory
• Luigi Galvani discovered the role of electricity in muscle
contraction in the eighteenth century
• Camillo Golgi developed an important method for
staining neurons with silver in the nineteenth century
12-52
Electrophysiology of Neurons
• Santiago Ramón y Cajal set forth the neuron doctrine:
nervous pathway is not a continuous “wire” or tube, but a
series of cells separated by gaps called synapses
• Neuron doctrine brought up two key questions
– How does a neuron generate an electrical signal?
– How does it transmit a meaningful message to the
next cell?
12-53
Electrical Potentials and Currents
• Electrophysiology—cellular mechanisms for producing
electrical potentials and currents
– Basis for neural communication and muscle contraction
• Electrical potential—a difference in the concentration of
charged particles between one point and another
• Electrical current—a flow of charged particles from one
point to another
– In the body, currents are movements of ions, such as Na+
or K+, through gated channels in the plasma membrane
– Gated channels are opened or closed by various stimuli
– Enables cell to turn electrical currents on and off
12-54
Electrical Potentials and Currents
• Living cells are polarized
• Resting membrane potential (RMP)—charge
difference across the plasma membrane
– About −70 mV in a resting, unstimulated neuron
– Negative value means there are more negatively
charged particles on the inside of the membrane
than on the outside
12-55
The Resting Membrane Potential
• Resting membrane potential (RMP) exists
because of unequal electrolyte distribution
between extracellular fluid (ECF) and intracellular
fluid (ICF)
• RMP results from the combined effect of three
factors
– Ions diffuse down their concentration gradient through
the membrane
– Plasma membrane is selectively permeable and allows
some ions to pass easier than others
– Electrical attraction of cations and anions to each other
12-56
The Resting Membrane Potential
• Potassium ions (K+) have the greatest influence
on RMP
– Plasma membrane is more permeable to K+ than any
other ion
– Leaks out until electrical charge of cytoplasmic anions
attracts it back in and equilibrium is reached and net
diffusion of K+ stops
– K+ is about 40 times as concentrated in the ICF as in
the ECF
• Cytoplasmic anions cannot escape due to size
or charge (phosphates, sulfates, small organic
acids, proteins, ATP, and RNA)
12-57
The Resting Membrane Potential
• Membrane much less permeable to high
concentration of sodium (Na+) found outside
the cell
– Some leaks and diffuses into the cell down its
concentration gradient
– Na+ is about 12 times as concentrated in the ECF as
in the ICF
– Resting membrane is much less permeable to Na+
than to K+
12-58
The Resting Membrane Potential
• Na+/K+ pumps out 3 Na+ for every 2 K+ it brings in
– Works continuously to compensate for Na+ and K+
leakage, and requires great deal of ATP
• 70% of the energy requirement of the nervous system
– Necessitates glucose and oxygen be supplied to nerve
tissue (energy needed to create the resting potential)
– Pump contributes about −3 mV to the cell’s resting
membrane potential of −70 mV
12-59
The Resting Membrane Potential
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ECF
Figure 12.11
Na+ 145 m Eq/L
K+
Na+
channel
4 m Eq/L
K+
channel
Na+ 12 m Eq/L
K+ 150 m Eq/L
ICF
• Na+ concentrated outside of cell (ECF)
• K+ concentrated inside cell (ICF)
Large anions
that cannot
escape cell
12-60
Local Potentials
• Local potentials—disturbances in membrane
potential when a neuron is stimulated
• Neuron response begins at the dendrite,
spreads through the soma, travels down the
axon, and ends at the synaptic knobs
12-61
Local Potentials
• When neuron is stimulated by chemicals, light,
heat, or mechanical disturbance
– Opens the Na+ gates and allows Na+ to rush into the
cell
– Na+ inflow neutralizes some of the internal negative
charge
– Voltage measured across the membrane drifts toward
zero
– Depolarization: case in which membrane voltage
shifts to a less negative value
– Na+ diffuses for short distance on the inside of the
plasma membrane producing a current that travels
toward the cell’s trigger zone; this short-range change
in voltage is called a local potential
12-62
Local Potentials
• Differences of local potentials from action
potentials
– Graded: vary in magnitude with stimulus strength
• Stronger stimuli open more Na+ gates
– Decremental: get weaker the farther they spread from
the point of stimulation
• Voltage shift caused by Na+ inflow diminishes rapidly with
distance
– Reversible: when stimulation ceases, K+ diffusion out of
cell returns the cell to its normal resting potential
– Either excitatory or inhibitory: some neurotransmitters
(glycine) make the membrane potential more negative—
hyperpolarize it—so it becomes less sensitive and less
likely to produce an action potential
12-63
Local Potentials
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Dendrites Soma Trigger
zone
Axon
Current
ECF
Ligand
Receptor
Plasma
membrane
of dendrite
Na+
ICF
Figure 12.12
12-64
Action Potentials
• Action potential—more dramatic change
produced by voltage-regulated ion gates in the
plasma membrane
– Only occur where there is a high enough density of
voltage-regulated gates
– Soma (50 to 75 gates per m2 ); cannot generate
an action potential
– Trigger zone (350 to 500 gates per m2 ); where
action potential is generated
• If excitatory local potential spreads all the way to the
trigger zone and is still strong enough when it arrives,
it can open these gates and generate an action
potential
12-65
Action Potentials
• Action potential is a rapid up-and-down shift in the
membrane voltage
– Sodium ions arrive at the axon hillock
– Depolarize the membrane at that point
– Threshold: critical voltage to which local potentials must
rise to open the voltage-regulated gates
• −55 mV
• When threshold is reached, neuron “fires” and
produces an action potential
• More and more Na+ channels open in the trigger zone
in a positive feedback cycle creating a rapid rise in
membrane voltage, called spike
12-66
Action Potentials
• When rising membrane potential passes 0 mV, Na+
gates are inactivated
– Begin closing; when all closed, the voltage peaks at +35
mV
– Membrane now positive on the inside and negative on the
outside
– Polarity reversed from RMP—depolarization
• By the time the voltage peaks, the slow K+ gates are
fully open
– K+ repelled by the positive intracellular fluid now exit the
cell
– Their outflow repolarizes the membrane; shifts the
voltage back to negative numbers returning toward RMP
12-67
Action Potentials
– K+ gates stay open longer than the Na+ gates
• Slightly more K+ leaves the cell than Na+ entering
• Drops the membrane voltage 1 or 2 mV more negative
than the original RMP—negative overshoot—
hyperpolarization or afterpotential
– Na+ and K+ switch places across the membrane
during an action potential
12-68
Action Potentials
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Only a thin layer of the
cytoplasm next to the
cell membrane is
affected
3
5
0
Depolarization
Threshold
2
–55
Local
potential
• Action potential is often
called a spike, as it
happens so fast
Repolarization
Action
potential
mV
– In reality, very few ions
are involved
4
+35
1
7
–70
Resting membrane
potential
(a)
6
Hyperpolarization
Time
Figure 12.13a
12-69
Action Potentials
• Characteristics of action
potential versus a local
potential
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4
+35
– Follows an all-or-none law
Depolarization
– Irreversible: once started
goes to completion and
cannot be stopped
Repolarization
Action
potential
Threshold
• If threshold is not reached, –55
it does not fire
– Nondecremental: do not
get weaker with distance
5
0
mV
• If threshold is reached,
neuron fires at its
maximum voltage
3
2
Local
potential
1
7
–70
Resting membrane
potential
(a)
6
Hyperpolarization
Time
Figure 12.13a
12-70
Action Potential vs. Local Potential
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4
+35
3
+35
Spike
5
0
0
Repolarization
Action
potential
Threshold
mV
mV
Depolarization
2
–55
Local
potential
1
7
Hyperpolarization
–70
Resting membrane
potential
6
Hyperpolarization
–70
0
Time
(a)
(b)
10
20
30
40
50
ms
Figure 12.13a,b
12-71
Sodium and Potassium Channels
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
K+
Na+
K+
channel
Na+
channel
35
0
mV
mV
0
–70
2 Na+ channels open, Na+
enters cell, K+ channels
beginning to open
Resting membrane
potential
Depolarization begins
35
35
0
0
mV
3 Na+ channels closed, K+
channels fully open, K+
leaves cell
–70
–70
Depolarization ends,
repolarization begins
4 Na+ channels closed,
K+ channels closing
Figure 12.14
mV
1 Na+ and K+ channels closed
–70
Repolarization complete
12-72
The Refractory Period
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Relative
refractory
period
+35
0
mV
• During an action
potential and for a few
milliseconds after, it is
difficult or impossible to
stimulate that region of
a neuron to fire again
Absolute
refractory
period
• Refractory period—
the period of resistance
to stimulation
Threshold
–55
Resting membrane
potential
–70
Time
Figure 12.15
12-73
The Refractory Period
• Two phases of the refractory
period
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Relative
refractory
period
mV
– Absolute refractory period +35
• No stimulus of any strength
will trigger AP
0
• As long as Na+ gates are
open
• From action potential to
RMP
– Relative refractory period
–55
• Only especially strong
stimulus will trigger new AP–70
Absolute
refractory
period
– K+ gates are still open
and any effect of
incoming Na+ is opposed
by the outgoing K+
Threshold
Resting membrane
potential
Time
Figure 12.15
12-74
The Refractory Period
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Relative
refractory
period
+35
0
mV
• The refractory period
refers only to a small
patch of the neuron’s
membrane at one time
Absolute
refractory
period
• Other parts of the
neuron can be
stimulated while the
small part is in
refractory period
Threshold
–55
Resting membrane
potential
–70
Time
Figure 12.15
12-75
Signal Conduction in Nerve Fibers
• For communication to occur, the nerve signal
must travel to the end of the axon
• Unmyelinated fiber has voltage-regulated ion
gates along its entire length
• Action potential from the trigger zone causes
Na+ to enter the axon and diffuse into adjacent
regions beneath the membrane
12-76
Signal Conduction in Nerve Fibers
• The depolarization excites voltage-regulated
gates immediately distal to the action potential
• Na+ and K+ gates open and close producing a
new action potential
• By repetition the membrane distal to that is
excited
• Chain reaction continues to the end of the
axon
12-77
Signal Conduction in Nerve Fibers
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dendrites
Cell body
Axon
Signal
Action potential
in progress
Refractory
membrane
Excitable
membrane
++++–––++ ++++ +++++
––––+++–––––– –––– –
––––+++–––––– –––– –
++++–––++ ++++ +++++
+++++++++ –––+ +++ ++
–––––––––+++– –––– –
–––––––––+++– –––– –
+++++++++ –––+ +++ ++
+++++++++ ++++ ––– ++
––––––––––––– +++– –
––––––––––––– +++– –
+++++++++ ++++ ––– ++
Figure 12.16
12-78
Signal Conduction in Nerve Fibers
• Voltage-gated channels needed for APs
– Fewer than 25 per m2 in myelin-covered regions (internodes)
– Up to 12,000 per m2 in nodes of Ranvier
• Fast Na+ diffusion occurs between nodes
– Signal weakens under myelin sheath, but still strong enough to
stimulate an action potential at next node
• Saltatory conduction—nerve signal seems to jump from node to node
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 12.17a
(a)
Na+ inflow at node
generates action potential
(slow but non decremental)
Na+ diffuses along inside
of axolemma to next node
(fast but decremental)
Excitation of voltageregulated gates will
generate next action
potential here
12-79
Signal Conduction in Nerve Fibers
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
+
–
–
+
+
–
–
+
–
+
+
–
–
+
+
–
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
++
––
––
++
++
––
––
++
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
–
+
+
–
–
+
+
–
+
–
–
+
+
–
–
+
++
––
––
++
++
––
––
++
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
+
–
–
+
–
+
+
–
–
+
+
–
++
––
––
++
++
––
––
++
(b)
Action potential
in progress
Refractory
membrane
Excitable
membrane
• Much faster than conduction in unmyelinated fibers
12-80
Synapses
• Expected Learning Outcomes
– Explain how messages are transmitted from one
neuron to another.
– Give examples of neurotransmitters and
neuromodulators and describe their actions.
– Explain how stimulation of a postsynaptic cell is
stopped.
12-81
Synapses
• A nerve signal can go no further when it reaches the
end of the axon
– Triggers the release of a neurotransmitter
– Stimulates a new wave of electrical activity in the next
cell across the synapse
• Synapse between two neurons
– First neuron in the signal path is the presynaptic
neuron
• Releases neurotransmitter
– Second neuron is postsynaptic neuron
• Responds to neurotransmitter
12-82
Synapses
• Presynaptic neuron may synapse with a dendrite,
soma, or axon of postsynaptic neuron to form
axodendritic, axosomatic, or axoaxonic synapses
• A neuron can have an enormous number of synapses
– Spinal motor neuron covered by about 10,000 synaptic
knobs from other neurons
• 8,000 ending on its dendrites
• 2,000 ending on its soma
• In the cerebellum of brain, one neuron can have as
many as 100,000 synapses
12-83
The Discovery of Neurotransmitters
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Soma
Synapse
Axon
Presynaptic
neuron
Directionof
signal
transmission
Postsynaptic
neuron
(a)
Figure 12.18
Axodendritic synapse
Axosomatic
synapse
Axoaxonic synapse
(b)
12-84
The Discovery of Neurotransmitters
• Synaptic cleft—gap between neurons was discovered
by Ramón y Cajal through histological observations
• Otto Loewi, in 1921, demonstrated that neurons
communicate by releasing chemicals—chemical
synapses
– He flooded exposed hearts of two frogs with saline
– Stimulated vagus nerve of the first frog and the heart
slowed
– Removed saline from that frog and found it slowed heart of
second frog
– Named it Vagusstoffe (“vagus substance”)
• Later renamed acetylcholine, the first known neurotransmitter
12-85
The Discovery of Neurotransmitters
• Electrical synapses do exist
– Some neurons, neuroglia, and cardiac and single-unit
smooth muscle
– Gap junctions join adjacent cells
• Ions diffuse through the gap junctions from one cell to the
next
– Advantage of quick transmission
• No delay for release and binding of neurotransmitter
• Cardiac and smooth muscle and some neurons
– Disadvantage is they cannot integrate information and
make decisions
• Ability reserved for chemical synapses in which neurons
communicate by releasing neurotransmitters
12-86
Synaptic Knobs
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Axon of
presynaptic
neuron
Synaptic
knob
Soma of
postsynaptic
neuron
© Omikron/Science Source/PhotoResearchers, Inc.
Figure 12.19
12-87
Structure of a Chemical Synapse
• Synaptic knob of presynaptic neuron contains
synaptic vesicles containing neurotransmitter
– Many docked on release sites on plasma membrane
• Ready to release neurotransmitter on demand
– A reserve pool of synaptic vesicles located further away
from membrane
• Postsynaptic neuron membrane contains
proteins that function as receptors and ligandregulated ion gates
12-88
Structure of a Chemical Synapse
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Microtubules
ofcytoskeleton
Axon of presynaptic neuron
Mitochondria
Postsynaptic neuron
Synaptic knob
Synaptic vesicles
containing neurotransmitter
Synaptic cleft
Postsynaptic neuron
Neurotransmitter
receptor
Neurotransmitter
release
• Presynaptic neurons have synaptic vesicles with neurotransmitter
and postsynaptic have receptors and ligand-regulated ion channels
12-89
Neurotransmitters and Related
Messengers
• More than 100 neurotransmitters have been
identified
• Fall into four major categories according to chemical
composition
– Acetylcholine
• In a class by itself
• Formed from acetic acid and choline
– Amino acid neurotransmitters
• Include glycine, glutamate, aspartate, and -aminobutyric
acid (GABA)
12-90
Neurotransmitters and Related
Messengers
Cont.
– Monoamines
• Synthesized from amino acids by removal of the –COOH
group
• Retaining the –NH2 (amino) group
• Major monoamines
– Epinephrine, norepinephrine, dopamine (catecholamines)
– Histamine and serotonin
– Neuropeptides
12-91
Neurotransmitters and Related
Messengers
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Acetylcholine
CH3
O
+
H3C N CH2 CH2 O C CH3
Catecholamines
HO
CH3
OH
CH CH2 NH CH2
O
HO
C CH2 CH2 CH2 NH
Gly Gly
Try
Enkephalin
Pro
Ary Try Lys
OH
CH2 CH2 NH2
Norepinephrine
HO
C
CH
HO
NH
HO
Glycine
O
CH2 CH2 NH2
HO
Dopamine
O
C CH CH
NH2
C
CH2 CH2 NH2
OH
N
Asparticacid
O
O
C CH CH2 CH2
HO
NH2
C
OH
Glutamic acid
Thr Met Phe
Ser
Glu
Gly
Gly
SO4
Cholecystokinin
Try
Lys
HO
Leu Met
Phe Gly
Phe
Glu
Glu
Substance P
Phe
Asp Tyr Met Gly Trp Met Asp
GABA
O
HO
Met Phe
Epinephrine
HO
Amino acids
HO
Neuropeptides
Monoamines
ß-endorphin
Ser
Serotonin
Glu
N
N
CH2 CH2 NH2
Histamine
Thr
Pro
Leu
Val
Leu
Thr
Ala
Asn
Lys
Phe
Ile
Ile
Lys Asn Ala Tyr
Lys
Lys
Gly
Glu
Figure 12.21
12-92
Neurotransmitters and Related
Messengers
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Neuropeptides are chains of 2 to
40 amino acids
– Beta-endorphin and substance P
• Act at lower concentrations than
other neurotransmitters
• Longer lasting effects
• Stored in axon terminal as larger
secretory granules (called densecore vesicles)
• Some function as hormones or
neuromodulators
• Some also released from digestive
tract
– Gut–brain peptides cause food
cravings
Neuropeptides
Met Phe
Gly Gly
Try
Enkephalin
Pro
Ary Try Lys
Leu Met
Phe Gly
Phe
Glu
Glu
Substance P
Phe
Asp Tyr Met Gly Trp Met Asp
Thr Met Phe
Ser
Glu
Gly
Gly
SO4
Cholecystokinin
Try
Lys
ß-endorphin
Ser
Glu
Thr
Pro
Leu
Val
Leu
Thr
Ala
Asn
Lys
Phe
Ile
Ile
Lys Asn Ala Tyr
Lys
Lys
Gly
Glu
Figure 12.21
12-93
Synaptic Transmission
• Neurotransmitters
–
–
–
–
Synthesized by the presynaptic neuron
Released in response to stimulation
Bind to specific receptors on the postsynaptic cell
Alter the physiology of that cell
12-94
Synaptic Transmission
• A given neurotransmitter does not have the
same effect everywhere in the body
• Multiple receptor types exist for a particular
neurotransmitter
– 14 receptor types for serotonin
• Receptor governs the effect the neurotransmitter
has on the target cell
12-95
Synaptic Transmission
• Neurotransmitters are diverse in their action
– Some excitatory
– Some inhibitory
– Some the effect depends on what kind of receptor the
postsynaptic cell has
– Some open ligand-regulated ion gates
– Some act through second-messenger systems
12-96
Synaptic Transmission
• Three kinds of synapses with different modes of
action
– Excitatory cholinergic synapse
– Inhibitory GABA-ergic synapse
– Excitatory adrenergic synapse
• Synaptic delay—time from the arrival of a signal
at the axon terminal of a presynaptic cell to the
beginning of an action potential in the postsynaptic
cell
– 0.5 ms for all the complex sequence of events to occur
12-97
An Excitatory Cholinergic Synapse
• Cholinergic synapse—employs acetylcholine
(ACh) as its neurotransmitter
– ACh excites some postsynaptic cells
• Skeletal muscle
– Inhibits others
12-98
An Excitatory Cholinergic Synapse
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Presynaptic neuron
Figure 12.22
Presynaptic neuron
3
Ca2+
1
2
ACh
Na+
4
K+
5
Postsynaptic neuron
12-99
An Inhibitory GABA-ergic Synapse
• GABA-ergic synapse employs -aminobutyric acid as its
neurotransmitter
• Nerve signal triggers release of GABA into synaptic cleft
• GABA receptors are chloride channels
• Cl− enters cell and makes the inside more negative than
the resting membrane potential
• Postsynaptic neuron is inhibited, and less likely to fire
12-100
An Excitatory Adrenergic Synapse
• Adrenergic synapse employs the neurotransmitter
norepinephrine (NE), also called noradrenaline
• NE and other monoamines, and neuropeptides, act
through second-messenger systems such as cyclic
AMP (cAMP)
• Receptor is not an ion gate, but a transmembrane
protein associated with a G protein
• Slower to respond than cholinergic and GABA-ergic
synapses
• Has advantage of enzyme amplification—single
molecule of NE can produce vast numbers of product
molecules in the cell
12-101
An Excitatory Adrenergic Synapse
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Presynaptic neuron
Postsynaptic neuron
Neurotransmitter
receptor
Norepinephrine
Adenylate cyclase
G protein
–
– –
+
+ +
1
2
Ligandgated
channels
opened
3
5
Na+
cAMP
4
Enzyme activation
6
Metabolic
changes
Multiple
possible
effects
7
Postsynaptic
potential
Figure 12.23
Genetic transcription
Enzyme synthesis
12-102
Cessation of the Signal
• Mechanisms to turn off stimulation to keep
postsynaptic neuron from firing indefinitely
– Neurotransmitter molecule binds to its receptor for only
1 ms or so
• Then dissociates from it
– If presynaptic cell continues to release
neurotransmitter
• One molecule is quickly replaced by another and the neuron is
restimulated
12-103
Cessation of the Signal
• Stop adding neurotransmitter and get rid of that
which is already diffusion
– Neurotransmitter escapes the synapse into the nearby
ECF
– Astrocytes in CNS absorb it and return it to neurons
• Reuptake
– Synaptic knob reabsorbs amino acids and monoamines
by endocytosis
– Break neurotransmitters down with monoamine oxidase
(MAO) enzyme
– Some antidepressant drugs work by inhibiting MAO
• Degradation in the synaptic cleft
– Enzyme acetylcholinesterase (AChE) in synaptic cleft
degrades ACh into acetate and choline
– Choline reabsorbed by synaptic knob
12-104
Neuromodulators
• Neuromodulators—hormones, neuropeptides,
and other messengers that modify synaptic
transmission
– May stimulate a neuron to install more receptors in the
postsynaptic membrane adjusting its sensitivity to the
neurotransmitter
– May alter the rate of neurotransmitter synthesis,
release, reuptake, or breakdown
• Enkephalins—a neuromodulator family
– Small peptides that inhibit spinal interneurons from
transmitting pain signals to the brain
12-105
Neuromodulators
• Nitric oxide (NO)—simpler neuromodulator
– A lightweight gas released by the postsynaptic
neurons in some areas of the brain concerned with
learning and memory
– Diffuses into the presynaptic neuron
– Stimulates it to release more neurotransmitter
– One neuron’s way of telling the other to “give me
more”
– Some chemical communication that goes backward
across the synapse
12-106
Neural Integration
• Expected Learning Outcomes
– Explain how a neurons “decides” whether or not to
generate action potentials.
– Explain how the nervous system translates complex
information into a simple code.
– Explain how neurons work together in groups to
process information and produce effective output.
– Describe how memory works at cellular and
molecular levels.
12-107
Neural Integration
• Synaptic delay slows the transmission of nerve
signals
• More synapses in a neural pathway, the longer
it takes for information to get from its origin to its
destination
– Synapses are not due to limitation of nerve fiber
length
– Gap junctions allow some cells to communicate more
rapidly than chemical synapses
12-108
Neural Integration
• Then why do we have synapses?
– To process information, store it, and make decisions
– Chemical synapses are the decision-making devices of the
nervous system
– The more synapses a neuron has, the greater its
information-processing capabilities
– Pyramidal cells in cerebral cortex have about 40,000
synaptic contacts with other neurons
– Cerebral cortex (main information-processing tissue of your
brain) has an estimated 100 trillion (1014) synapses
• Neural integration—the ability of your neurons to
process information, store and recall it, and make
decisions
12-109
Postsynaptic Potentials
• Neural integration is based on the postsynaptic
potentials produced by neurotransmitters
• Typical neuron has a resting membrane potential
of −70 mV and threshold of about −55 mV
12-110
Postsynaptic Potentials
• Excitatory postsynaptic potential (EPSP)
– Any voltage change in the direction of threshold that
makes a neuron more likely to fire
• Usually results from Na+ flowing into the cell cancelling
some of the negative charge on the inside of the
membrane
– Glutamate and aspartate are excitatory brain
neurotransmitters that produce EPSPs
12-111
Postsynaptic Potentials
• Inhibitory postsynaptic potential (IPSP)
– Any voltage change away from threshold that makes a
neuron less likely to fire
• Neurotransmitter hyperpolarizes the postsynaptic cell and
makes it more negative than the RMP making it less likely to
fire
• Produced by neurotransmitters that open ligand-regulated
chloride gates
– Causing inflow of Cl− making the cytosol more negative
12-112
Postsynaptic Potentials
Cont.
– Glycine and GABA produce IPSPs and are inhibitory
– Acetylcholine (ACh) and norepinephrine are
excitatory to some cells and inhibitory to others
• Depending on the type of receptors on the target cell
• ACh excites skeletal muscle, but inhibits cardiac muscle
due to the different type of receptors
12-113
Postsynaptic Potentials
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
0
mV
–20
–40
Threshold
–60
Repolarization
Depolarization
–80
Stimulus
(a)
Resting membrane
potential
EPSP
Time
0
mV
–20
–40
Threshold
Resting membrane
potential
–60
IPSP
–80
Hyperpolarization
(b)
Stimulus
Time
Figure 12.24
12-114
Summation, Facilitation, and Inhibition
• One neuron can receive input from thousands of
other neurons
• Some incoming nerve fibers may produce EPSPs
while others produce IPSPs
• Neuron’s response depends on whether the net
input is excitatory or inhibitory
• Summation—the process of adding up
postsynaptic potentials and responding to their net
effect
– Occurs in the trigger zone
12-115
Summation, Facilitation, and Inhibition
• The balance between EPSPs and IPSPs enables the
nervous system to make decisions
• Temporal summation—occurs when a single synapse
generates EPSPs so quickly that each is generated before
the previous one fades
– Allows EPSPs to add up over time to a threshold voltage
that triggers an action potential
• Spatial summation—occurs when EPSPs from several
different synapses add up to threshold at an axon hillock
– Several synapses admit enough Na+ to reach threshold
– Presynaptic neurons cooperate to induce the
postsynaptic neuron to fire
12-116
Temporal and Spatial Summation
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
3 Postsynaptic
neuron fires
1 Intense stimulation
by one presynaptic
neuron
2 EPSPs spread
from one synapse
to trigger zone
(a) Temporal summation
3 Postsynaptic
neuron fires
1 Simultaneous stimulation
by several presynaptic
neurons
(b) Spatial summation
2 EPSPs spread from
several synapses
to trigger zone
Figure 12.25
12-117
Summation of EPSPs
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
+40
+20
mV
0
Action potential
–20
–40
–60
–80
Threshold
EPSPs
Resting
membrane
potential
Stimuli
Figure 12.26
Time
• Does this represent spatial or temporal summation?
12-118
Summation, Facilitation, and Inhibition
• Neurons routinely work in groups to modify each other’s action
• Facilitation—a process in which one neuron enhances the
effect of another one
– Combined effort of several neurons facilitates firing of
postsynaptic neuron
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Signal in presynaptic neuron
Signal in presynaptic neuron
Signal in inhibitory neuron
Figure 12.27
No activity in inhibitory
neuron
Neurotransmitter
No neurotransmitter
release here
Neurotransmitter
Excitation of postsynaptic
neuron
(a)
Inhibition of presynaptic
neuron
S
+
EPSP
No neurotransmitter
release here
R
No response in postsynaptic
neuron
IPSP
S
R
(b)
12-119
Summation, Facilitation, and Inhibition
• Presynaptic inhibition—process in which one presynaptic neuron
suppresses another one
– Opposite of facilitation
– Reduces or halts unwanted synaptic transmission
– Neuron I releases inhibitory GABA
• Prevents voltage-gated calcium channels from opening in synaptic
knob and presynaptic neuron releases less or no neurotransmitter
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Signal in presynaptic neuron
Signal in presynaptic neuron
Figure 12.27
Signal in inhibitory neuron
No activity in inhibitory
neuron
Neurotransmitter
No neurotransmitter
release here
Neurotransmitter
Excitation of postsynaptic
neuron
(a)
Inhibition of presynaptic
neuron
S
+
EPSP
No neurotransmitter
release here
R
No response in postsynaptic
neuron
(b)
IPSP
S
R
12-120
Neural Coding
• Neural coding—the way in which the nervous
system converts information to a meaningful
pattern of action potentials
• Qualitative information depends upon which
neurons fire
– Labeled line code: each nerve fiber to the brain
leads from a receptor that specifically recognizes a
particular stimulus type
12-121
Neural Coding
• Quantitative information—information about the
intensity of a stimulus is encoded in two ways
– One depends on the fact that different neurons
have different thresholds of excitation
• Stronger stimuli causes a more rapid firing rate
• Excitement of sensitive, low-threshold fibers gives
way to excitement of less sensitive, high-threshold
fibers as intensity of stimuli increases
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Neural Coding
Cont.
– Other way depends on the fact that the more
strongly a neuron is stimulated, the more frequently
it fires
• CNS can judge stimulus strength from the firing
frequency of afferent neurons
12-123
Neural Coding
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Action potentials
2g
5g
10 g
20 g
Time
Figure 12.28
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Neural Pools and Circuits
• Neural pools—neurons function in large groups,
each of which consists of millions of interneurons
concerned with a particular body function
– Control rhythm of breathing
– Moving limbs rhythmically when walking
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Input neuron
Figure 12.29
Facilitated zone
Discharge zone
Facilitated zone
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Neural Pools and Circuits
• Information arrives at a neural pool through one or
more input neurons
– Branch repeatedly and synapse with numerous
interneurons in the pool
– Some input neurons form multiple synapses with a
single postsynaptic cell
• Can produce EPSPs in all points of contact with that cell
• Through spatial summation, make it fire more easily than if they
synapsed with it at only one point
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Neural Pools and Circuits
Cont.
– Within the discharge zone of an input neuron
• That neuron acting alone can make the postsynaptic cells
fire
– In a broader facilitated zone, it synapses with still other
neurons in the pool
• Fewer synapses on each of them
• Can only stimulate those neurons to fire with the
assistance of other input neurons
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Neural Pools and Circuits
• Diverging circuit
– One nerve fiber branches and synapses with several
postsynaptic cells
– One neuron may produce output through hundreds of
neurons
• Converging circuit
– Input from many different nerve fibers can be funneled
to one neuron or neural pool
– Opposite of diverging circuit
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Neural Pools and Circuits
Cont.
• Reverberating circuits
– Neurons stimulate each other in linear sequence but
one cell restimulates the first cell to start the process all
over
– Diaphragm and intercostal muscles
• Parallel after-discharge circuits
– Input neuron diverges to stimulate several chains of
neurons
• Each chain has a different number of synapses
• Eventually they all reconverge on a single output neuron
• After-discharge—continued firing after the stimulus stops
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Neural Pools and Circuits
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Diverging
Converging
Input
Output
Output
Input
Reverberating
Parallel after-discharge
Figure 12.30
Input
Input
Output
Output
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Memory and Synaptic Plasticity
• Physical basis of memory is a pathway through
the brain called a memory trace or engram
– Along this pathway, new synapses were created or
existing synapses modified to make transmission easier
– Synaptic plasticity: the ability of synapses to change
– Synaptic potentiation: the process of making
transmission easier
• Kinds of memory
– Immediate, short- and long-term memory
– Correlate with different modes of synaptic potentiation
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Immediate Memory
• Immediate memory—the ability to hold
something in your thoughts for just a few
seconds
– Essential for reading ability
• Feel for the flow of events (sense of the present)
• Our memory of what just happened “echoes” in
our minds for a few seconds
– Reverberating circuits
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Short-Term Memory
• Short-term memory (STM)—lasts from a few
seconds to several hours
– Quickly forgotten if distracted
– Calling a phone number we just looked up
– Reverberating circuits
• Facilitation causes memory to last longer
– Tetanic stimulation: rapid arrival of repetitive signals at
a synapse
• Causes Ca2+ accumulation and postsynaptic cell more
likely to fire
– Posttetanic potentiation: to jog a memory
• Ca2+ level in synaptic knob stays elevated
• Little stimulation needed to recover memory
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Long-Term Memory
• Types of long-term memory
– Declarative: retention of events that you can put into
words
– Procedural: retention of motor skills
• Physical remodeling of synapses
– New branching of axons or dendrites
• Molecular changes—long-term potentiation
– Changes in receptors and other features increase
transmission across “experienced” synapses
– Effect is longer-lasting
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Long-Term Memory
• Molecular changes are called long-term
potentiation
• Method described
– Receptors on synaptic knobs are usually blocked by
Mg2+ ions
– When they bind glutamate and receive tetanic stimuli,
they repel Mg2+ and admit Ca2+ into the dendrite;
Ca2+ acts as second messenger
• More synaptic knob receptors are produced
• Synthesizes proteins involved in synapse remodeling
• Releases nitric oxide that triggers more neurotransmitter
release at presynaptic neuron
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Alzheimer Disease
• 100,000 deaths/year
– 11% of population over 65; 47% by age 85
• Memory loss for recent events, moody, combative, lose ability to
talk, walk, and eat
• Show deficiencies of acetylcholine (ACh) and nerve growth factor
(NGF)
• Diagnosis confirmed at autopsy
– Atrophy of gyri (folds) in cerebral cortex
– Neurofibrillary tangles and senile plaques
– Formation of β-amyloid protein from breakdown product of plasma
membranes
• Treatment—halt β-amyloid production
– Research halted due to serious side effects
– Give NGF or cholinesterase inhibitors
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Alzheimer Disease
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Neurons with
neurofibrillary
tangles
Shrunken
gyri
Wide sulci
Senile plaque
(b)
b: © Simon Fraser/Photo Researchers, Inc.
(a)
a: Custom Medical Stock Photo
Figure 12.31a
Figure 12.31b
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Parkinson Disease
• Progressive loss of motor function beginning in 50s or 60s—
no recovery
– Degeneration of dopamine-releasing neurons
• Dopamine normally prevents excessive activity in motor
centers (basal nuclei)
• Involuntary muscle contractions
– Pill-rolling motion, facial rigidity, slurred speech
– Illegible handwriting, slow gait
• Treatment—drugs and physical therapy
– Dopamine precursor (L-dopa) crosses brain barrier; bad side
effects on heart and liver
– MAO inhibitor slows neural degeneration
– Surgical technique to relieve tremors
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