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Nervous Tissue
Note
Much of the text material is from, “Principles of Anatomy and
Physiology” by Gerald J. Tortora and Bryan Derrickson (2009,
2011, and 2014). I don’t claim authorship. Other sources are
noted when they are used.
The lecture slides are mapped to the three editions of the
textbook based on the color-coded key below.
14th edition
13th edition
12th edition
Same figure or table reference in all three editions
2
Outline
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Microscopic level
Electrical activity in neurons
Signal transmission at synapses
Neurotransmitters
Neural circuits
Plasticity, regeneration, and repair
Two neurological disorders
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Microscopic Level
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Types of Cells in the Nervous System
•
Neurons mediate most of the information processing functions of the
nervous system.
•
Neuroglia support, nourish, and protect the neurons and their functions.
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Neurons
•
Neurons and muscle fibers are electrical excitable—they respond
to certain types of stimuli to transduce energy to action potentials.
•
An action potential is an electrical signal that propagates (travels)
along the membrane of the axon of a neuron.
•
This is due to movement of sodium ions into and potassium ions
out of the axon.
•
Some axons are very short to propagate action potentials over distances of 1 mm or less, while others can be very long to propagate
over much longer distances, such as between the brain and spinal
cord.
Transduce = convert energy from one form to another.
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A Neuron
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Parts of a Neuron
•
The cell body, or soma, is similar to the generalized cell discussed in
the biology review.
•
Dendrites (Latin for “little trees”) are input areas to a neuron that are
typically organized as branched, tree-like structures that extend from
the cell body.
•
The axon is a thin, cylindrical projection often extending from the cell
body—it propagates action potentials toward another neuron, a muscle
fiber, or gland.
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Figure 12.2
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http://upload.wikimedia.org
Cortical Neurons
Pyramidal cells in the cerebral cortex (primary motor area).
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Synapses and Neurotransmitters
•
The synapse is the site of communication between a neuron and
another neuron, a neuron and a muscle fiber, or a neuron and a
gland cell.
•
Axons typically branch at their distal ends, and swell into end bulbs
that have vesicles which store neurotransmitters.
•
Some neurons release 2 or 3 neurotransmitters; however, somatic
motor neurons that innervate skeletal muscles have only acetylcholine (ACh).
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Synapse
http://theora.com
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Synapses and Neurotransmitters (continued)
•
When released in response to an action potential, neurotransmitter
diffuses across the synaptic cleft to excite or inhibit another neuron.
•
ACh at the neuromuscular junction in skeletal muscle is always excitatory.
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Santiago Ramón y Cajal
Structure of the mammalian
retina (1900)
1852-1934
All images on this page and the next page are from:
http://upload.wikimedia.org
13
Santiago Ramón y Cajal (continued)
Optic tectum
of a sparrow
Purkinjie cells and
granular cells in the
pigeon cerebellum.
Hippocampus of
a rodent
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Structural Classification
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A neuron can be classified based on the processes that extend from
its cell body.
•
Multipolar neurons usually have several dendrites and one axon—
most neurons in the brain and spinal cord are of this type.
•
Bipolar neurons have one main dendrite and one axon—they are
found in some sensory systems including the retina, inner ear, and
olfactory area of the brain.
•
Unipolar neurons have dendrites and an axon fused together to form
a continuous process—they are found in certain sensory receptors of
the skin.
Neuronal processes = dendrites and axons.
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Figure 12.3
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Structural Classification (continued)
Multipolar
Bipolar
Unipolar
http://webanatomy.net
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Naming Conventions
•
Neurons are sometimes named for who discovered them or their shape
or appearance.
•
Examples include:
Purkinjie cells in the cerebellum—named for their discoverer, the
Czech anatomist, Jan Evangelista Purkinje.
- Pyramidal cells in the motor cortex of the cerebral hemispheres—
named for their pyramid-like shape.
-
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Figure 12.5
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Functional Classification
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Sensory, or afferent, neurons have sensory receptors or neurons at
one of their ends.
•
When a stimulus activates a sensory receptor, an action potential is
propagated into the CNS.
•
Motor, or efferent, neurons propagate action potentials from the CNS
to effectors (muscle or glands) via the cranial nerves or spinal nerves.
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Figure 12.10
Figure 12.11
Figure 12.11
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Functional Classification (continued)
•
Interneurons are positioned in the CNS between some sensory and
motor neurons.
•
Within the cerebral cortex, interneurons are known as association
neurons.
•
These neurons are organized in complex networks to integrate and
process sensory information.
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Axonal Transport
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Substances are synthesized or recycled in the cell bodies of neurons
and transported to the axons and their end bulbs.
•
Slow axonal transport enables the one-way movement of axoplasm
(cytoplasm) toward the end bulbs.
•
Fast axonal transport uses proteins as “motors” powered by ATP to
move materials in both directions along microtubules in the axons.
•
The materials include organelles, and complex molecules that form
the axolemma (plasma membrane), end bulbs, and synaptic vesicles.
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Neuroglia
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Neuroglia or glial cells make-up about 50 percent of the total volume
of the central nervous system.
•
They are much smaller than neurons, but 5 to 50 times more numerous.
•
They were originally thought to be the “glue” (in Latin) that holds nervous tissue together.
•
Neuroglia are now known to be involved in functioning of the nervous
system.•••
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Figure 12.6
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http://www.anatomybox.com
Neuroglia (continued)
Neuroglia interspersed among neurons.
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Neuroglia (continued)
•
Unlike neurons, neuroglia can multiply and divide (through mitosis) in
mature mammalian nervous systems (including humans).
•
Neuroglia can multiply to fill-in the space formerly occupied by neurons in injury or disease.
•
Brain tumors (gliomas) from neuroglia can be highly-malignant, grow
rapidly, and metastasize.
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CNS Neuroglia
•
Astrocytes have many processes, and are the largest and most numerous of the neuroglia in the central nervous system.
•
The primary function is to form the blood-brain barrier by wrapping their
processes around capillaries in the brain.
•
Oligodendrocytes resemble astrocytes, but they are smaller and have
fewer processes.
•
Their processes form the myelin sheath that encircles axons in the CNS.
Central nervous system (CNS) = brain and spinal cord.
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Figure 12.6
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CNS Neuroglia (continued)
•
Microglia are small cells with slender processes and spine-like projections.
•
They function as phagocytes to remove cellular debris in the CNS.
•
Ependymal cells are cuboidal (cube-shaped) with microvilli and
cilia.
•
These cells line the ventricles of the brain and central canal of the
spinal cord, and monitor and support the circulation of cerebrospinal
fluid.
Phagocyte = a cell that engulfs and digests debris and invading
microorganisms.
(http://wordnetweb.princeton.edu)
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Figure 12.6
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PNS Neuroglia
•
Schwann cells encircle the axons in the peripheral nervous system to
form the myelin sheath.
•
Unlike oligodendrocytes, a Schwann cell will encircle only one axon.
•
Satellite cells surround the cell bodies of neurons in the ganglia of the
PNS.
•
The cells provide structural support and regulate exchange of materials
between neurons and interstitial fluid.
Peripheral nervous system (PNS) = all nervous tissues outside of
the CNS.
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Figure 12.7
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Myelin Sheath
•
The myelin sheath that surrounds many types of axons consists of a
multi-layered lipid and protein covering.
•
The sheath electrically insulates the axon and increases the velocity
(speed) of action potentials for reasons that will be discussed when
we cover the electrical activity of neurons.
•
Axons without a myelin sheath (such as slow pain fibers) are said to
be unmyelinated.
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Figure 12.8
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http://upload.wikimedia.org
Myelin Sheath (continued)
Electron micrograph.
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Myelin Sheath (continued)
•
The amount of myelin sheath progressively increases from birth to
maturity in humans.
•
An infant’s response to stimuli is neither rapid nor coordinated due
in part to the lack of much myelination.
•
Myelination (progressive development of the myelin sheath) continues through adolescence.
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Demyelination
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Demyelination is the loss or destruction of myelin sheath from axons.
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It occurs in certain neurological disorders including multiple sclerosis
(MS) and Tay-Sachs disease.
•
Demyelination degrades or slows the conduction of action potentials,
and enables “cross-talk” among axons.
•
Demyelinating diseases will be discussed in more detail later in this
learning module.
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Definitions
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Nucleus is a cluster of cell bodies of neurons in the CNS (not to be
confused with cell nucleus).
•
Ganglion is a cluster of neuronal cell bodies, usually in the PNS.
•
Fiber tract is a bundle of axons connecting neurons in the brain or
spinal cord (CNS).
•
Nerve is a bundle of axons in the PNS.
Nucleus = singular for nuclei.
Ganglion = singular for ganglia.
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Gray and White Matter
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Some areas of a freshly-dissected brain or spinal cord are gray and
other areas are white in appearance.
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Gray matter includes neuronal cell bodies, dendrites, unmyelinated
axons, axon terminals, and neuroglia.
•
White matter is primarily composed of myelinated axons—their lipid
coverings are white.
•
Blood vessels in gray and white matter provide oxygen and nourishment to neurons and neuroglia and remove waste products from cellular respiration.
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Figure 12.9
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http://upload.wikimedia.org
Gray and White Matter (continued)
Human brain,
mid-sagittal section, right lateral view.
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Electrical Activity in Neurons
34
Electrical Excitability
•
Neurons and muscle fibers have a unique characteristic of being
electrically-excitable.
•
Neurons generate graded potentials and action potentials, which
are electrical signals.
•
Graded potentials integrate information from other neurons and
convey it over short distances.
•
Action potentials enable information to be conveyed over a wide
range of distances.
Transient = temporary or brief.
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Electrical Excitability (continued)
•
The generation of electrical signals depends on two basic features of a
neuron or muscle fiber:
A resting membrane potential, measured as voltage, that changes
in response to electrical or chemical stimuli.
- Ion channels in the axolemma for Na+ and K+ (sodium and potassium ions).
-
Axolemma = plasma membrane of an axon.
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Ion Channels
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Ion channels in the axolemma allow some certain ions to follow their
chemical or electrical gradients.
•
Ions follow their chemical gradients from higher to lower concentration.
•
Cations (positively charged ions) move toward a negatively charged
area—an electrical gradient.
•
Anions (negatively charged ions) move toward a positively charged
area—also an electrical gradient.
•
Molecular gates in the axolemma guard the ion channels—they must
be moved to open the channels.
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Figure 12.11
Figure 12.11
Figure 12.12
37
Types of Ion Channels
•
Leakage channels randomly alternate between open and closed
states.
•
Voltage-gated channels open and close in response to membrane
potentials—they are involved in the generation and propagation of
action potentials.
•
Ligand-gated channels open and close in response to chemical
stimuli including neurotransmitters.
•
Mechanically-gated channels open and close in response to mechanical stimulation (such as touch, pressure, and sound) and tissue
stretching.
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Figure 12.11
Figure 12.11
Figure 12.12
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Ligand-Gated Channel
http://www.hcc.uce.ac.uk
Ligand = an ion, a molecule, or a molecular group that
binds to another chemical entity to form a larger complex.
(www.thefreedictionary.com)
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Resting Membrane Potential
•
A resting membrane potential results from the buildup of anions (-) in
the cytoplasm, and a buildup of cations (+) in the extracellular
space.
•
This buildup of anions and cations occurs in close proximity to the
axolemma.•
•
The separation of negative and positive electrical charges across
the axolemma is a form of potential energy.
•
This potential energy can be measured using tiny electrodes and a
voltmeter.
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Figure 12.12
Figure 12.12
Figure 12.13
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http://faculty.irsc.edu
Anion and Cation Distribution
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Resting Membrane Potential (continued)
•
The resting membrane potential is measured in millivolts (mV), where
1.0 mV equals 0.001 volt—this is very small compared to a AA battery
of 1.5 volts.
•
The greater the difference in electrical charge across the plasma membrane, the larger the resting membrane potential.
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Figure 12.12
Figure 12.12
Figure 12.13
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http://upload.wikimedia.org
Squid Axon
Loligo vulgaris, European (common) squid.
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Microelectrodes
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A microelectrode is inserted into the cell to measure the resting membrane potential.
•
A reference electrode is positioned outside the cell in the extracellular
space.
•
The electrodes are connected to a voltmeter to record the membrane
potential.
Glass microelectrode
http://www.medicine.mcgill.ca/physio/vlab/rmp/images
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Figure 12.12
Figure 12.12
Figure 12.13
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http://ncbi.nlm.nih.gov
Voltage Measurement
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Voltages
•
A cells that has a resting membrane potential is said to be polarized.
•
Most somatic cells are polarized, but only neurons and muscle fibers
are electrically-excitable.
•
The resting membrane potentials of neurons ranges between -40
and -90 mV, with a typical value of -70mV.
•
A minus sign indicates the inside of the cell is negative compared to
its outside.
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Figure 12.12
Figure 12.12
Figure 12.13
46
Major Factors
•
The resting membrane potential of neurons is determined by three
major factors:
Unequal distribution of cations and anions across the axolemma.
- Sodium-potassium transport pump.
- Inability of most anions (especially proteins) to exit the cell
because they are too large to fit through the ion channels.
-
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Figure 12.13
Figure 12.12
Figure 12.14
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Graded Potentials
•
A graded potential is a voltage change from the resting membrane
potential that makes the neuron either more polarized or less polarized.
•
Graded potentials are hyperpolarizing (more polarized) or depolarizing (less polarized).
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Figure 12.14
Figure 12.14
Figure 12.15
48
Graded Potentials (continued)
•
A graded potential occurs when a stimulus causes ion channels to open
or close in the plasma membrane.
•
Graded potentials vary in their amplitude (voltage) depending on the
intensity of the stimulus.
•
The opening of ion channels produces current spread in the immediate
area.
•
Graded potentials decay as they spread across the plasma membrane
(this is known as decremental spread).
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Figure 12.14
Figure 12.14
Figure 12.15
49
Summation
•
Graded potentials can combine with other graded potentials in a process known as summation.
•
Two or more depolarizing potentials can produce a greater depolarizing potential.
•
Two or more hyperpolarizing potentials can produce a greater hyperpolarizing potential.
•
Two equal graded potentials of opposite polarization will negate each
other.
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Figure 12.16
Figure 12.16
Figure 12.17
50
What’s In a Name?
•
Graded potentials also have different names depending on the stimulus
that initiates them and where they occur in nervous tissue.
•
At neuron-neuron and neuron-muscle fiber synapses, graded potentials
from the release of a neurotransmitter are known as postsynaptic potentials.
•
Graded potentials that occur in sensory receptors and sensory neurons
are known as receptor potentials and generator potentials, respectively.
51
Threshold
•
An action potential is generated in the axolemma when a depolarization reaches the threshold value of the neuron or muscle fiber.
•
The threshold is about -55 mV for a resting membrane potential of
-70mV.
•
An action potential is not generated when a weak depolarization
(known as a subthreshold stimulus) does not reach the threshold
value.
•
Although the threshold value of a neuron does not change, different
neurons may have slightly different thresholds.
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Figure 12.19
Figure 12.19
Figure 12.20
52
Action Potential
•
The action potential is a sequence of rapidly-occurring events that
reverses the membrane potential and then restores it to its resting
state.
•
An action potential spikes and becomes positive (to about +30 mV)
during the depolarizing phase.
•
The spike represents about a 100 mV change, or about one-tenth
of a volt.
•
It declines during the repolarizing phase and returns to the resting
state (-70mV in our example).
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Figure 12.18
Figure 12.18
Figure 12.19
53
Action Potential (continued)
http://encefalus.com
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Action Potential (continued)
•
Two different types of voltage-gated ion channels open and close during an action potential.
•
The first to open are the Na+ channels that enable sodium ions to rush
into the cell along their electrochemical gradient to initiate the depolarizing phase.
•
K+ channels then open, allowing potassium ions to flow-out of the cell
along their chemical gradient to initiate the repolarizing phase.
•
An after-hyperpolarizing phase occurs because K+ channels remain
open for a short period of time after the resting membrane potential is
reached.
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Figure 12.20
Figure 12.20
Figure 12.21
55
Sodium-Potassium Transport Pump
•
The ion or sodium-potassium transport pump, made-up of proteins in
the axolemma, is powered by ATP.
•
The transport pump transports Na+ out of the cell and transports K+ into
the cell.
•
It is instrumental in restoring the resting membrane potential to -70 mV
after generation of an action potential.
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Figure 12.20
Figure 12.20
Figure 12.21
56
All-or-None Principle
•
An action potential either occurs or does not occur—it will be generated
if a depolarizing graded potential reaches or exceeds the threshold
value of the neuron.
•
There are no in-between states; only all-or-none in the generation of
action potentials
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Refractory Periods
http://encefalus.com
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Absolute Refractory Period
•
Another action potential cannot be generated when the Na+ gates
are open—this state is the absolute refractory period.
•
Large-diameter axons have shorter absolute refractory periods than
small-diameter axons; therefore, more action potentials can be generated in a given period of time.
•
Maximum frequencies range between 10 to 1,000 action potentials
per second depending on the time duration of the absolute refractory
period.
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Figure 12.18
Figure 12.18
Figure 12.19
59
Relative Refractory Period
•
The relative refractory period occurs after the Na+ gates close and the
K+ gates open.
•
A second action potential may be triggered by a superthreshold depolarization during this period.
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Figure 12.18
Figure 12.18
Figure 12.19
60
Propagation
•
Action potentials usually propagate from the trigger zone—the axon
hillock where a cell body transitions to an axon—to the end bulbs of
the axon where neurotransmitters are released.
•
Action potentials are of constant amplitude as they travel along the
axolemma.
•
In comparison, graded potentials decay with time and over distance.
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Figure 12.21
Figure 12.21
Figure 12.22
61
Propagation (continued)
•
An action potential is continually regenerated along the axolemma
since it is preceded by a depolarizing current that reaches or exceeds
the threshold value.
•
Action potentials, depending on the location of the trigger zone, can
propagate in ether direction along an axon—this is not consistent with
the textbook, and we will discuss why.•
Axons can be two-way streets while chemical synapses serve
as one-way doors.
(metaphorically speaking)
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Figure 12.21
Figure 12.21
Figure 12.22
62
http://tainano.com
Propagation (continued)
63
Continuous Conduction
•
Continuous conduction involves the depolarization and repolarization
of each infinitesimal segment of the axolemma during propagation of
an action potential.
•
Continuous conduction occurs in unmyelinated axons and in muscle
fibers.
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Figure 12.21
Figure 12.21
Figure 12.22
64
Saltatory Conduction
•
In saltatory conduction, action potentials are generated only at the
nodes of Ranvier (areas of axolemma not covered by myelin sheath).
•
The Na+ and K+ gates in the axolemma are exposed since there is
no myelin sheath at these nodes.
•
An action potential seems to jump from one node to the next as it
propagates along the axolemma.
•
Due to saltatory conduction, action potentials propagate much more
rapidly along myelinated than unmyelinated axons.
Saltare = to jump (in Latin).
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Figure 12.21
Figure 12.21
Figure 12.22
65
Saltatory Conduction (continued)
http://qwickstep.com
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Propagation Speed
•
The factors that affect the propagation (conduction) speed of an action
potential include:
Amount of myelination
- Axon diameter
- Temperature
-
•
All are direct relationships—increases in any or all of the three factors
increase propagation speed.
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Classification of Nerve Fibers
•
A fibers—largest-diameter, myelinated axons.
•
B fibers—smaller-diameter, myelinated axons.
•
C fibers—smallest-diameter, unmyelinated axons.
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Frequency Coding
•
The amplitude of action potentials generated by a neuron does not
change based on the all-of-none principle.
•
The intensity of a stimulus is instead encoded in the frequency of the
action potentials.
•
The greater the intensity of a stimulus to a limit, the higher rate of
action potential generation (“firing rate”).
•
A second factor in encoding the intensity of the stimulus is the number of axons in a bundle or nerve that are recruited (activated) by the
stimulus.
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Frequency Coding (continued)
http://www1.lf1.cuni.cz
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Signal Transmission at Synapses
71
Synapse
•
The junction between neuron and neuron, neuron and muscle fiber,
or neuron and gland is known as a synapse.
•
The neuron sending the signal is known as the presynaptic neuron,
and the neuron receiving the message is the postsynaptic neuron.
•
The three configurations of synapses are axodendritic, axosomatic,
and axoaxonic.
•
The two basic types of synapses—electrical and chemical—differ in
their structures and functions.
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Electrical Synapses
•
Action potentials can cross between adjacent cells via very narrow
gap junctions in electrical synapses.
•
Ions flow from one cell to the next through tubular connexions in the
gap junctions to enable action potentials to rapidly spread from cell
to cell.
•
Gap junctions are found in cardiac (heart) muscle, smooth muscle,
and the nervous system of developing mammalian embryos.
•
Electrical synapses allow electrical synchronization within groups of
muscle fibers to enable coordinated contractions.
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Chemical Synapses
•
The presynaptic and postsynaptic neurons in a chemical synapse are
in close proximity, but they do not physically touch.
•
The two neurons are separated by a synaptic cleft, a gap of 20 to 50
nm filled with interstitial fluid.
•
Action potentials are not conducted across the synaptic cleft in chemical synapses.
•
A chemical called a neurotransmitter is released from the end bulbs
of the presynaptic neuron and diffuses across the cleft.
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Figure 12.23
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Chemical Synapses (continued)
•
The neurotransmitter binds to receptors of the plasma membrane of
the postsynaptic neuron.
•
Most synapses in the nervous systems of mammals are chemical in
nature.
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Figure 12.23
75
http://biologyclass.neurobio.arizona.edu
Chemical Synapse
76
Synaptic Events
•
In response to an action potential, voltage-gated Ca2++ channels
open in the plasma membrane of the end bulbs in the presynaptic
neuron.
•
The inflow of Ca2++ triggers the process of exocytosis—the vesicles
merge with the plasma membrane to release their contents (neurotransmitter) into the synaptic cleft.
•
The neurotransmitter passively diffuses through the interstitial fluid in
the synaptic cleft, and binds to protein receptors of the postsynaptic
membrane.
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Figure 12.23
Figure 12.22
Figure 12.23
77
Neurotransmitter Receptors
•
The receptor sites on the postsynaptic membrane usually bind only
one type of neurotransmitter.
•
When the neurotransmitter binds to the postsynaptic receptor, an ion
channel opens in the plasma membrane, and a postsynaptic (graded)
potential is generated.
•
Neurotransmitter receptors are either ionotropic or metabotropic based
on their protein structures.
Ionotopic = a hormone activates or deactivates ionotropic receptors (ligandgated ion channels). The effect can be either positive or negative, whether
the effect is a depolarization or a hyperpolarization respectively.
Metabotropic = responds on activation with glutamate binding by initiating a
number of intracellular biochemical events which modulate synaptic and
neuronal activity. They are not directly linked to any specific ion channels
(Both definitions from http://www.encyclo.co.uk)
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Postsynaptic Potentials
•
A depolarizing or hyperpolarizing graded postsynaptic potential is
generated depending on the neurotransmitter and location of the
synapse in the post-synaptic neuron.
•
The graded potential is either an excitatory postsynaptic potential
(EPSP) or a inhibitory postsynaptic potentials (IPSP).
•
EPSPs and IPSPs correspond to depolarizations and hyerpolarizations as we discussed.
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EPSP and IPSP
EPSP—depolarizing
IPSP—hyperpolarizing
http://www.igi.tugraz.at
80
Postsynaptic Potentials (continued)
•
The time required to generate a postsynaptic potential, known as
synaptic delay, is about 0.5 msec.
•
Chemical synapses respond more slowly than electrical synapses
due to this time delay.
•
Information transfer is in only one direction at chemical synapses,
from presynaptic neuron to postsynaptic neuron
•
A chemical synapse, in essence, serves as a one-way door while
an axon can be a two-way street.
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Figure 12.23
Figure 12.22
Figure 12.23
81
Neurotransmitter Removal
•
Rapid removal of neurotransmitter from the synaptic cleft is essential
for continued synaptic function.
•
If the neurotransmitter were to remain in the synaptic cleft, it could
continue to stimulate the postsynaptic neuron, muscle fiber, or gland
for as long as it lingered.
•
The neurotransmitter is removed by diffusion out of the synaptic cleft,
enzymes, and re-uptake by cells, or a combination of these processes.
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Summation
•
Neurons in the CNS have input from up to 1,000 to 10,000 synapses
with other neurons.
•
Excitatory and inhibitory inputs are summated to produce a postsynaptic potential.
•
Spatial summation involves the summation of postsynaptic potentials
on different but nearby locations on the plasma membrane of the postsynaptic neuron.
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Figure 12.25
Figure 12.24
Figure 12.25
83
Summation (continued)
•
Temporal summation involves the summation of postsynaptic potentials at the same location on the neuron but at slightly different times.
•
Spatial summation and temporal summation act together determine
if an action potential will be generated based on the processes we
discussed.
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Figure 12.26
Figure 12.25
Figure 12.26
84
http://biologyclass.neurobio.arizona.edu
Summation (continued)
85
Neurotransmitters
86
Overview
•
Over 100 chemical substances are known or thought to be neurotransmitters.
•
Some neurotransmitters bind to postsynaptic receptors and function
rapidly to open ion channels in the plasma membrane (ionotropic).
•
Other neurotransmitters function via second-messenger systems in
the post-synaptic membrane to influence chemical reactions inside
the cell (metabotropic).
•
Both processes involve the excitation or inhibition of postsynaptic
neurons.
Second-messenger system = a chemical substance inside a cell that carries
information farther along the signal pathway from the internal part of a
membrane-spanning receptor embedded in the cell membrane. It may be in
the form of an enzyme's product or ion fluxes.
(http://medical-dictionary.thefreedictionary.com)
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Figure 12.27
Figure 12.26
Figure 12.27
87
Small-Molecule Neurotransmitters
•
Acetylcholine (ACh) is the most widely-studied neurotransmitter in
the CNS and especially the PNS.
•
ACh is excitatory at certain synapses, including neuromuscular
junctions, but it is inhibitory at some types of synapses in the brain.
•
The amino acids, glutumate and aspartate, are excitatory at some
synapses in the brain.
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Figure 12.27
Figure 12.26
Figure 12.27
88
GABA and Glycine
•
Gamma amino butyric acid (GABA) and glycine, an amino acid, are
inhibitory.
•
As many as one-third of the synapses in the brain involve GABA as
a neurotransmitter.
•
Anti-anxiety drugs such as Valium® enhance the effects of GABA,
including at synapses in the limbic system, the collection of structures deep in the telencepahalon that can mediate emotional states.
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Figure 12.27
Figure 12.26
Figure 12.27
89
Biogenic Amines
•
The biogenic amines include norepinephrine, dopamine, and serotonin.
•
Norepinephrine functions in sleep, dreaming, and emotional responses.
•
Dopamine functions in emotional responses, pleasurable experiences,
and addictive behaviors.
•
Dopamine also helps to regulate skeletal muscle tone and aspects of
body movements in deep structures of the brain (for example, the basal
ganglia).
•
Serotonin functions in sensory perception, sleep induction, temperature
regulation, food appetite, and control of emotional moods (by regulating
deep structures).
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Figure 12.27
90
Other Small-Molecule Neurotransmitters
•
Small-molecule neurotransmitters also include:
-
Adenosine triphosphate (ATP)
Purines (a type of nucleotide)
Nitric oxide (NO)
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Figure 12.27
Figure 12.26
Figure 12.27
91
Neuropeptides
•
Neuropeptides are neurotransmitters of 3 to 40 amino acids linked by
peptide bonds through dehydration synthesis.
•
Neuropeptides known as enkephalins have analgesic effects that are
about 200 times stronger than morphine.
•
Opioid forms known as endorphins have strong analgesic effects, and
are thought to be involved in a number of emotional states and mental
illnesses.
Analgesic = pain-relieving.
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Figure 12.27
Figure 12.26
Figure 12.27
92
Neural Circuits
93
Overview
•
The CNS has many billions of neurons—most are organized in complex networks known as neural circuits.
•
Different types of circuits are thought to process specific types of
information.
•
In a simple-series network, one presynaptic neuron stimulates one
postsynaptic neuron.
•
Most neural networks are more complex than a simple-series network.
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Types of Neural Circuits
•
Diverging networks relay sensory information to different areas of the
brain.
•
Converging networks integrate information from different areas of the
brain to stimulate somatic motor neurons that activate skeletal muscle
contractions.
•
Reverberating networks may regulate complex muscular activities and
short-term memory.
•
Parallel after-discharge circuits may mediate mental activities such as
mathematical calculations.
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Figure 12.28
Figure 12.27
Figure 12.28
95
Neural Circuit in a Flat Worm
http://www.wormbook.org
96
Plasticity, Regeneration, and Repair
97
Plasticity
•
The nervous system exhibits plasticity, that is the capacity to change
based on external stimuli (that is, experience).
•
Changes include new dendritic growth, synthesis of new proteins, and
modifications to synaptic connections.
•
Despite plasticity, neurons in mammals have a limited ability to regenerate when they are damaged or destroyed (especially in the CNS).
Regeneration = the ability for replication or self-repair.
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Neurogenesis
•
The growth of new neurons from undifferentiated stem cells and progenitor cells is known as neurogenesis.
•
Neurons appear and disappear in the brains of some migrating songbirds each year.
•
New neurons typically do not form in the adult brains of humans and
other primates.
•
The human brain develops new synaptic connections in response to
learning.
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Neurogenesis (continued)
http://mindsparke.com
100
Neurogenesis (continued)
•
In humans, new neurons have been discovered in the hippocampus, a
structure located deep in the telencephalon.
•
The hippocampus is involved in learning, and in particular in the transfer of short-term memory traces to long-term memory.
•
More research needs to be conducted to determine the scope (if any) of
neurogenesis in humans.
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PNS Damage and Repair
•
Axons and dendrites in the peripheral nervous system may undergo
repair in humans, but only if the cell body is intact, Schwann cells
remain functional, and scar tissue has not yet formed.
•
A person who has injured the axons in an upper limb, for example,
has a good chance of recovering some or all of the nerve function.
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Figure 12.29
Figure 12.28
Figure 12.29
102
Two Neurological Disorders
103
Multiple Sclerosis
•
Multiple sclerosis (MS) is a disease caused by the progressive deterioration of the myelin sheath of neurons in the CNS.
•
The myelin sheath deteriorates into scleroses of hardened scars and
plaques.
•
The damage slows and eventually short-circuits the propagation of action potentials.
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Multiple Sclerosis (continued)
http://icimmedics.com
105
Multiple Sclerosis (continued)
•
MS is an autoimmune disease involving the body’s immune system.
•
The disease effects about 350,000 people in the United States, and
about two million people worldwide.
•
Its onset usually occurs between ages 20 and 40—it occurs in females
about twice as often as males.
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Multiple Sclerosis (continued)
•
The most common form of the disease is relapsing-remitting MS.
•
The earliest symptoms can include a feeling of heaviness or weakness in the skeletal muscles, abnormal sensations, and doublevision.
•
An episode may be followed by a period of remission lasting up to 1
to 2 years.
•
Then, over time, the progressive loss of neural function steadily continues.
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Epilepsy
•
Epilepsy involves recurrent seizures of motor, sensory, and association
systems of the brain.
•
Epileptic seizures affect about 1 to 3 percent of the world’s population.
•
Seizures are triggered by abnormal electrical discharges from neurons
in different structures of the brain.
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Epilepsy (continued)
•
The abnormal electrical discharges send action potentials over their
conduction pathways to other neurons, which can be recruited for the
seizure.
•
The patterns of electrical discharges during a seizure can be chaotic
and random.
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Epilepsy (continued)
•
Immediately before (pre-ictal period) or during a generalized seizure,
lights, noises, or smells may be sensed, although the sensory organs
have not been stimulated
•
The effects are because the sensory areas of the cerebral cortex are
activated.
•
Skeletal muscles may contract involuntarily due to involvement of the
motor cortex in what is known as grand mal, clonic-tonic, or generalized seizure.
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Epilepsy—EEG Patterns
EEG of a generalized seizure
http://www.thebarrow.org
111
Epilepsy—Seizure Types
•
Partial seizures, which occur in a localized area on one side of the
brain, typically produce mild symptoms.
•
Generalized seizures involve large areas on both sides of the brain,
and can result in unconsciousness.
•
Temporal lobe seizures involve areas of the cerebral cortex and the
limbic system.
•
The limbic system mediates emotional states, which can be affected
by seizure activity.
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Epilepsy—Causes
•
Epilepsy can result from many causes, although in many cases it is
idiopathic.
•
Known causes include:
-
-
Brain damage at birth
Head injuries
Tumors and abscesses of the brain
Metabolic disturbances
Infections
Toxins
Vascular problems
Idiopathic = of unknown cause.
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Epilepsy—Treatment
•
Epilepsy can often (but not always) be alleviated or controlled by antiepileptic drugs.
•
Surgical intervention to remove or contain the epileptic focus may be
needed in severe cases.
•
Support groups are available for persons who have epilepsy and their
families and friends.
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