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Chapter 12
Lecture
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Nervous Tissue
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• overview of the nervous
system
• properties of neurons
• supportive cells
(neuroglia)
• electrophysiology of
neurons
• synapses
Neurofibrils
• neural integration
(d)
Figure 12.4d
Axon
12-2
Overview of Nervous System
• Maintenance of internal coordination
– endocrine system - communicates by means of chemicals
– nervous system - employs electrical and chemical means
• nervous system carries out its task in three basic steps:
• sense organs receive information and transmit coded
messages to the spinal cord and the brain
• brain and spinal cord process this information and
determine what response is appropriate
• brain and spinal cord issue commands to muscles and
gland cells to carry out such a response
12-3
Two Major Anatomical
Subdivisions of Nervous System
• central nervous system (CNS)
– brain and spinal cord enclosed in bony coverings
• 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 knot-like swelling in a nerve where neuron
cell bodies are concentrated
12-4
Subdivisions of Nervous System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Central
nervous
system
(CNS) Brain
Peripheral
nervous
system (PNS)
Spinal
cord
Nerves
Ganglia
Figure 12.1
12-5
Sensory Divisions of PNS
• 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
12-6
Motor Divisions of PNS
• motor (efferent) division – carries signals from the CNS to
gland and muscle cells for body’s response
– somatic motor division – carries signals to skeletal muscles
• voluntary muscular contraction as well as somatic
reflexes – involuntary muscle contractions
– visceral motor division (autonomic nervous system) carries signals to glands, cardiac muscle, and smooth muscle
• involuntary - visceral reflexes
• sympathetic division - arouse body for action
– fight and flight
• parasympathetic division - calming effect
– rest and digest
12-7
Subdivisions of 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
12-8
Universal Properties of Neurons
• excitability (irritability)
– respond to environmental changes called stimuli
• conductivity
– stimuli produce electrical signals that are quickly
conducted to other cells at distant locations
• secretion
– signal reaches end of nerve fiber, a chemical
neurotransmitter is secreted that crosses the gap
and stimulates the next cell
12-9
Functional Types of Neurons
• sensory (afferent) neurons
– specialized to detect stimuli
– transmit information about them to the CNS
• interneurons (association) neurons
– lie entirely within the CNS
– receive signals from many neurons and carry out the
integrative function
• process, store, & retrieve info and ‘make decisions’
– 90% of all neurons are interneurons
• motor (efferent) neuron
– send signals out to muscles and gland cells (the effectors)
• motor because most of them lead to muscles
12-10
Functional Classes of Neurons
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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 Interneurons
(association
neurons) are
confined to
the CNS.
Figure 12.3
12-11
Structure of a Neuron
• Soma (body) – the control center
of the neuron
Dendrites
Soma
– one centrally located nucleus
– inclusions – glycogen granules,
lipid droplets, melanin, and
lipofuscin (golden brown
pigment produced when
lysosomes digest worn-out
organelles)
• accumulates with age
• wear-and-tear granules
• most abundant in old neurons
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
Figure 12.4a
Synaptic
knobs
(a)
12-12
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
Soma
Nucleus
Nucleolus
Trigger zone:
Axon hillock
Initial segment
Axon collateral
Axon
– 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
Direction of
signal transmission
Internodes
Node of Ranvier
Myelin sheath
Schwann cell
Terminal
arborization
Figure 12.4a
Figure 12.4a
Synaptic knobs
(a)
12-13
Structure of a Neuron
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• axon (nerve fiber) –
– starts at a mound on soma called
the axon hillock
– cylindrical, relatively unbranched
– branch extensively on distal end
– rapid conduction of nerve signals
to points remote to the soma
– only one axon per neuron
– Schwann cells and myelin
sheath enclose axon
– distal end, axon has terminal
arborization – extensive
complex of fine branches
• synaptic knob - swelling at tip
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
Figure 12.4a
Figure 12.4a
Synaptic knobs
(a)
12-14
Neuron Structure
• multipolar neuron
– one axon and multiple dendrites
– most common
– most neurons in the brain and spinal
cord
Dendrites
Axon
Multipolar neurons
Dendrites
• bipolar neuron
– one axon and one dendrite
– olfactory cells, retina, inner ear
Axon
• unipolar neuron
– single process leading away from the
soma
– sensory from skin and organs to
spinal cord
• anaxonic neuron
– many dendrites but no axon
– help in visual processes
Bipolar neurons
Dendrites
Axon
Unipolar neuron
Dendrites
12-15
Anaxonic neuron
Neuroglial Cells
• about a trillion (1012) neurons in body
• 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
in fetus, guide migrating neurons to their destination
if mature neuron is not in synaptic contact with
another neuron, it’s covered by glial cells
• prevents neurons from touching each other
• gives precision to conduction pathways
12-16
Six Types of Neuroglial Cells
• four types occur only in CNS
– oligodendrocytes
• form myelin sheaths in CNS
• each arm-like process wraps around a nerve fiber - forms
insulating layer that speeds up signal conduction
– ependymal cells
• lines internal cavities of the brain
• secretes and circulates cerebrospinal fluid (CSF)
– microglia
• small, wandering macrophages
• thought to perform a complete checkup on the brain
tissue several times a day
• wander in search of cellular debris to phagocytize
12-17
Six Types of Neuroglial Cells
• four types occur only in CNS
– astrocytes
• most abundant glial cell in CNS - esp. brain
• diverse functions
– form a supportive framework of nervous tissue
– have extensions (perivascular feet) that contact
blood capillaries and form a tight seal called the
blood-brain barrier
– convert blood glucose to lactate - food for neurons
– secrete nerve growth factors
– regulate chemical composition of tissue fluid by
absorbing excess neurotransmitters and ions
12-18
Six Types of Neuroglial Cells
• two types occur only in PNS
– Schwann cells
• envelope nerve fibers in PNS
• wind repeatedly around a nerve fiber
• produces 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
12-19
Neuroglial Cells of CNS
Capillary
Neurons
Astrocyte
Oligodendrocyte
Perivascular feet
Myelinated axon
Ependymal cell
Myelin (cut)
Microglia
Cerebrospinal fluid
Figure 12.6
12-20
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)
– by metastasis from non-neuronal tumors in other
organs
– most come from 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
12-21
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
• myelination – production of the myelin sheath
–
–
–
–
begins the 14th week of fetal development
proceeds rapidly during infancy
completed in late adolescence
dietary fat is important to nervous system
development
12-22
Myelin
• in PNS, Schwann cell spirals around a single nerve fiber
– as many as a hundred layers of its own membrane
– neurilemma – thick outermost coil of myelin sheath
• contains nucleus and most of its cytoplasm
• in CNS – oligodendrocytes myelinate several 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
12-23
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 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
12-24
Myelin Sheath in PNS
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Axoplasm
Axolemma
Schwann cell
nucleus
Neurilemma
Figure 12.4c
(c)
Myelin sheath
nodes of Ranvier and internodes
12-25
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
12-26
Myelination in CNS
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Oligodendrocyte
Myelin
Nerve fiber
Figure 12.7b
(b)
12-27
Diseases of Myelin Sheath
– multiple sclerosis
• oligodendrocytes & myelin sheaths in the CNS deteriorate
• myelin replaced by hardened scar tissue
• nerve conduction disrupted (double vision, tremors,
numbness)
• cause may be autoimmune, triggered by virus
– Tay-Sachs disease - a hereditary disorder of infants of
Eastern European Jewish ancestry
• abnormal accumulation of glycolipid (GM2) in myelin sheath
– enzyme missing in homozygous individuals
– accumulation of GM2 disrupts conduction of nerve
signals
12-28
– blindness, loss of coordination, and dementia
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
• conduction speed
–
–
–
–
–
small, unmyelinated fibers - 0.5 - 2.0 m/sec
small, myelinated fibers 3 - 15.0 m/sec
large, myelinated fibers - up to 120 m/sec
slow signals supply the stomach and dilate pupil
fast signals supply skeletal muscles and transport sensory
signals for vision and balance
12-29
Electrical Potentials and Currents
• electrical potential – a difference in the concentration of charged
particles between one point and another
• electrical current – flow of charged particles from one point to
another
– in the body, currents are movement of ions, such as Na+ or K+
through gated channels in the plasma membrane
• living cells are polarized
• resting membrane potential (RMP) – charge difference across the
plasma membrane
– -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
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
12-31
Creation of Resting Membrane Potential
• potassium ions (K+) have the greatest influence on RMP
– plasma membrane more permeable to K+ than any other ion
– leaks out until electrical charge of cytoplasmic anions attracts
it back in and equilibrium is reached
– 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)
• membrane much less permeable to high concentration of sodium
(Na+) found outside the cell
– Na+ is about 12X as concentrated in the ECF as in the ICF
12-32
Creation of Resting Membrane Potential
• Na+/K+ pumps pump out 3 Na+ for every 2 K+ they bring
in
– work continuously to compensate for Na+ and K+
leakage, and requires great deal of ATP
• 70% of the energy requirement of the nervous
system
– requires 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
12-33
Ionic Basis of Resting Membrane
Potential
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ECF
Na+ 145 mEq/L
K+
Na+
channel
K+
channel
4 mEq/L
Figure 12.11
Na+ 12 mEq/L
ICF
K+ 150 mEq/L
Large anions
that cannot
escape cell
• Na+ concentrated outside of cell (ECF)
• K+ concentrated inside cell (ICF)
12-34
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
• when neuron is stimulated by chemicals, light, heat or
mechanical disturbance
12-35
Local Potentials
• How it works:
– stimulus opens the Na+ gates - Na+ rushes into cell
– Na+ inflow neutralizes some of the internal negative charge
– voltage measured across the membrane drifts toward zero
– depolarization - when membrane voltage shifts to a less
negative value
– Na+ diffuses for short distance on the inside of the plasma
membrane – this short-range change in voltage is called a
local potential
12-36
Characteristics of Local Potentials
• local potentials
– are graded - vary in magnitude with stimulus strength
• stronger stimuli open more Na+ gates
– are decremental - get weaker the farther they spread
• voltage shift caused by Na+ inflow diminishes rapidly
with distance
– are reversible - when stimulation ceases, K+ diffusion
out of cell returns the cell to its normal RP
– can be either excitatory or inhibitory - make the cell
more or less likely to fire
– this is different from action potentials, which we will
cover shortly
12-37
Excitation of a Neuron by a
Chemical Stimulus
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dendrites Soma Trigger
zone
Axon
Current
ECF
Ligand
Receptor
Plasma
membrane
of dendrite
Na+
ICF
Figure 12.12
12-38
Action Potentials
• action potential – more dramatic change than local potential
– only occur where there is a high density of voltage-regulated
gates
– trigger zone (350–500 gates/m2 ) – where AP 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
• action potential is a rapid shift in the membrane voltage
– sodium ions arrive at the axon hillock (trigger zone)
– depolarize the membrane at that point
– threshold – critical voltage to which local potentials must rise
to open the voltage-regulated gates
12-39
Action Potentials
1.) when threshold is reached, neuron ‘fires’ and produces
an action potential
2.) more and more Na+ channels open in in the trigger zone
in a positive feedback cycle creating a rapid rise in
membrane voltage – spike
3.) when rising membrane potential passes 0 mV, Na+
gates are inactivated
• begin closing
• membrane now positive on the inside and negative on
the outside (depolarization)
12-40
Action Potentials(cont.)
4.) by the time the voltage peaks, slow K+ gates are fully
open
• K+ repelled by the positive intracellular fluid now exit
the cell
• their outflow repolarizes the membrane
5.) 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
12-41
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Action Potentials
mV
• action potential is often called
+35
a spike – happens so fast
• action potential vs. local
potential
– follows an all-or-none law 0
• if threshold is reached,
neuron fires at its
maximum voltage
–55
• if threshold is not
reached it does not fire
–70
– nondecremental - do not
get weaker with distance
– irreversible - once started
(a)
goes to completion and
cannot be stopped
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4
3
5
Depolarization
Repolarization
Action
potential
Threshold
2
Local
potential
1
7
Resting membrane
potential
6
Hyperpolarization
Time
Figure 12.13a
12-43
Sodium and Potassium Gates
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
K+
Na+
K+
gate
Na+
gate
35
0
mV
mV
35
0
2 Na+ gates open,
Na+enters cell, K+ –70
Depolarization begins
gatesbeginning
to open
35
Na+ gates closed,
0
+
K gates fully
3
open, K+ leaves
–70
cell
Depolarization ends,
35
repolarization begins
Figure 12.14
mV
mV
1 Na+ and K+ gates
–70
closed
Resting membrane
potential
0
4 Na+ gates closed,
K+ gates closing –70
Repolarization complete
12-44
The Refractory Period
• during an action potential and for a
few milliseconds after, that region of
a neuron cannot fire again.
Copyright © The McGraw-Hill Companies, Inc.
Permission required
for reproduction
Absolute
Relativeor display.
+35
refractory
period
mV
• refractory period – the period of
resistance to stimulation
0
• two phases of the refractory period
– absolute refractory period
• cannot trigger AP
• as long as Na+ gates are open
–
55
– relative refractory period
–70
• only especially strong
stimulus will trigger new AP
– K+ gates are still open
• occurring only at a small patch of the
neuron’s membrane at one time
refractory
period
Threshold
Resting membran
potential
Time
Figure 12.15
12-45
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Nerve Signal Conduction
Unmyelinated Fibers
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Dendrites
Cell body
Axon
Signal
Action potential + + + + – – – + + + + + + + + + + +
––––+++–––––––––––
in progress
Refractory
membrane
Excitable
membrane
–––– +++–––––––––––
++++–––+++++++++++
+++++++++–––++++++
–––––––––+++––––––
–––––––––+++––––––
+++++++++–––++++++
+++++++++++++–––++
–––––––––––––+++––
–––––––––––––+++––
+++++++++++++–––++
Figure 12.16
12-47
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Saltatory Conduction
Myelinated Fibers
• voltage-gated channels
– most are packed in nodes of Ranvier
• fast Na+ diffusion occurs between nodes
– signal weakens, but can still stimulate an AP at next node
• saltatory conduction – nerve signal seems to jump from
node to node
Figure 12.17a
Na+ inflow at node
generates action potential
(slow but nondecremental)
Na+ diffuses along inside
of axolemma to next node
(fast but decremental)
Excitation of voltage-regulated
gates will generate next action
potential here
12-49
Saltatory Conduction
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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(b)
Action potential
in progress
Refractory
membrane
Excitable
membrane
Figure 12.17b
• much faster than conduction in unmyelinated
fibers
12-50
Synapses
• a nerve signal can go no further when it reaches the end
of the axon
– triggers the release of a neurotransmitter (chemical)
– stimulates a new wave of electrical activity in the cell
across the synapse
• synapse between two neurons
– 1st neuron in the signal path is the presynaptic
neuron
• releases neurotransmitter
– 2nd neuron is postsynaptic neuron
• responds to neurotransmitter
12-51
Synapses
• presynaptic neuron may synapse with a dendrite,
soma, or axon of postsynaptic neuron
• neuron can have an enormous number of synapses
– spinal motor neuron covered by about 10,000
synaptic knobs from other neurons
• 8000 ending on its dendrites
• 2000 ending on its soma
• in cerebellum of brain, one neuron can have as many as
100,000 synapses
12-52
Synaptic Relationships Between
Neurons
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Soma
Synapse
Axon
Presynaptic
neuron
Direction of
signal
transmission
Postsynaptic
neuron
(a)
Axodendritic synapse
Axosomatic
synapse
(b)
Figure 12.18
Axoaxonic synapse
12-53
Synaptic Knobs
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Axon of
presynaptic
neuron
Synaptic
knob
Soma of
postsynaptic
neuron
© Omikron/Science Source/Photo Researchers, Inc
Figure 12.19
12-54
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 ligand-regulated ion gates
12-55
Structure of a Chemical Synapse
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Axon of
presynaptic
neuron
Postsynaptic neuron
Microtubules
of cytoskeleton
Mitochondria
Synaptic knob
Synaptic vesicles
containing neurotransmitter
Synaptic cleft
Neurotransmitter receptor
Postsynaptic neuron
Neurotransmitter
release
• presynaptic neurons have synaptic vesicles with
neurotransmitter and postsynaptic have receptors and
ligand-regulated ion channels
12-56
Neurotransmitters and Related
Messengers
• more than 100 neurotransmitters have been identified
• four major categories according to chemical composition
– acetylcholine
• in a class by itself
– amino acid neurotransmitters
• include glycine, glutamate, aspartate, and -aminobutyric
acid (GABA)
– monoamines
• synthesized from amino acids
• major monoamines are:
– epinephrine, norepinephrine, dopamine
– histamine and serotonin
– neuropeptides
12-57
Function of Neurotransmitters
at Synapse
• synthesized by the presynaptic neuron
• released in response to stimulation
• bind to specific receptors on the
postsynaptic cell
• alter the physiology of that cell
12-58
Effects of Neurotransmitters
• 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
12-59
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
12-60
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Cessation of the Signal
• mechanisms to turn off stimulation
– neurotransmitter molecule binds to its receptor for only 1 msec 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
• stop adding neurotransmitter and get rid of what’s there
– stop signals in presynaptic fiber
– getting rid of neurotransmitter by:
• diffusion - moves off into ECF
• reuptake
– synaptic knob reabsorbs by endocytosis
– break neurotransmitters down with monoamine oxidase
(MAO) enzyme
• degradation in the synaptic cleft
12-62
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
• 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 informationprocessing capabilities.
– cerebral cortex (main information-processing tissue of your
brain) has an estimated 100 trillion (1014) synapses
12-63
Postsynaptic Potentials - EPSP
• 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
• excitatory postsynaptic potentials (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
12-64
Postsynaptic Potentials - IPSP
• inhibitory postsynaptic potentials (IPSP)
– any voltage change away from threshold that makes a
neuron less likely to fire
• neurotransmitter hyperpolarizes the postsynaptic
cell, makes it more negative than the RMP, making it
less likely to fire
12-65
Postsynaptic Potentials
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0
mV
–20
–40
Threshold
–60
Repolarization
Depolarization
–80
(a)
Resting membrane
potential
EPSP
Stimulus
Time
0
mV
–20
–40
Threshold
–60
IPSP
Resting membrane
potential
Figure 12.24
–80
Hyperpolarization
(b)
Stimulus
Time
12-66
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
• the balance between EPSPs and IPSPs enables the nervous
system to make decisions
12-67
Summation, Facilitation, and Inhibition
• temporal summation – 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 – 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
12-68
Temporal and Spatial Summation
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3 Postsynaptic
neuron fires
1 Intense stimulation
by one presynaptic
neuron
2 EPSPs spread
from one synapse
to trigger zone
(a) Temporal summation
2
1 Simultaneous
stimulation by several
presynaptic neurons
(b) Spatial summation
3 Postsynaptic
neuron fires
EPSPs spread from
several synapses
to trigger zone
Figure 12.25
12-69
Summation of EPSPs
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
+40
+20
0
mV
Action potential
–20
–40
–60
–80
Threshold
EPSPs
Resting
membrane
potential
Stimuli
Figure 12.26
Time
• does this represent spatial or temporal summation?
12-70
Summation, Facilitation, and Inhibition
• facilitation – one neuron enhances the effect of another one
– often combined effort of several neurons
• presynaptic inhibition – one neuron suppresses another
– the opposite of facilitation
– reduces or halts unwanted synaptic transmission
Signal in presynaptic neuron
Signal in presynaptic neuron
Signal in inhibitory neuron
No activity in
inhibitory
neuron
I
I
Neurotransmitter
No neurotransmitter
release here
Neurotransmitter
(a)
Excitation of
postsynaptic
neuron
Inhibition of presynaptic
neuron
S
EPSP
No neurotransmitter
release here
R No response in postsynaptic
neuron
(b)
Figure 12.27
IPSP
S
R
12-71
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
• quantitative information – intensity of a stimulus is encoded in
two ways:
– one depends on the fact that different neurons have different
thresholds of excitation
– other way depends on the fact that the more strongly a neuron
is stimulated, the more frequently it fires
12-72
Neural Coding
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Action potentials
2g
5g
10 g
20 g
Time
Figure 12.28
12-73
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
• deficiencies of acetylcholine (ACh) & nerve growth factor
(NGF)
• diagnosis confirmed at autopsy
– neurofibrillary tangles and senile plaques
– formation of beta-amyloid protein from breakdown
product of plasma membrane
12-74
Alzheimer Disease Effects
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Shrunken
gyri
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Neurons with
neurofibrillary
tangles
Wide sulci
Senile plaque
(b)
(a)
© Simon Fraser/Photo Researchers, Inc.
Custom Medical Stock Photo, Inc.
Figure 12.31a
Figure 12.31b
12-75
Parkinson Disease
• progressive loss of motor function beginning in 50’s or 60’s no recovery
– degeneration of dopamine-releasing neurons
• dopamine normally prevents excessive activity in motor
centers
• 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 & liver
– MAO inhibitor slows neural degeneration
12-76
– surgical technique to relieve tremors