Download Nerve Fiber Classification Nerve fibers are classified according to:

Nerve Fiber Classification
Nerve fibers are classified according to:
Diameter
Degree of myelination
Speed of conduction
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Synapses
A junction that mediates information transfer from
one neuron:
To another neuron
To an effector cell
Presynaptic neuron – conducts impulses toward the
synapse
Postsynaptic neuron – transmits impulses away
from the synapse
Most neurons will function as both
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Synapses
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Figure 11.17
Types of Synapses
Axodendritic – synapses between the axon of one
neuron and the dendrite of another
Axosomatic – synapses between the axon of one
neuron and the soma of another
Other types of synapses include:
Axoaxonic (axon to axon)
Dendrodendritic (dendrite to dendrite)
Dendrosomatic (dendrites to soma)
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Nervous System II: Anatomy Review, page 5
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Electrical Synapses
Contain protein channels made of connexin subunits that
connect cytoplasm of adjacent neurons
Electrically coupled
Fast transmission across synapse
Electrical synapses:
Are less common than chemical synapses
Correspond to gap junctions found in other cell types
Are important in the CNS in:
PLAY
Arousal from sleep, Mental attention, Emotions and
memory
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Nervous System II: Anatomy Review, page 6
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Chemical Synapses
Specialized for the release and reception of
chemical neurotransmitters
Typically composed of two parts:
1) Axonal terminal of the presynaptic neuron,
which contains synaptic vesicles
2) Receptor region on the dendrite(s) or soma of
the postsynaptic neuron
1 & 2 are separated by a synaptic cleft
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Synaptic Cleft
Fluid-filled space separating the presynaptic and
postsynaptic neurons
Prevents nerve impulses from directly passing
from one neuron to the next
Transmission across the synaptic cleft:
Is a chemical event (as opposed to an electrical
one)
Ensures unidirectional communication between
neurons
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Synaptic Cleft: Information Transfer
Nerve impulses reach the axonal terminal of the presynaptic
neuron and open Na+ channels as well as Ca2+ channels
1) Ca++ floods into the terminal from the extracellular matrix
2) Ca++ acts as an intracellular messenger directing synaptic
vessicles to fuse with the axon membrane and empty
neurotransmitter the synaptic cleft via exocytosis
3) Neurotransmitter crosses the synaptic cleft and binds to
receptors on the postsynaptic neuron
4) Postsynaptic membrane permeability changes, causing an
excitatory or inhibitory effect
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Synaptic Cleft: Information Transfer
n
tio ial
Ac tent
po
Ca2+
1
Neurotransmitter
Axon terminal of
presynaptic neuron
Postsynaptic
membrane
Mitochondrion
Axon of
presynaptic
neuron
Na+
Receptor
Postsynaptic
membrane
Ion channel open
Synaptic vesicles
containing
neurotransmitter
molecules
5
Degraded
neurotransmitter
2
Synaptic
cleft
Ion channel
(closed)
3
4
Ion channel closed
Ion channel (open)
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Figure 11.18
Step 5: Termination of Neurotransmitter Effects
Neurotransmitter bound to a postsynaptic neuron:
Produces a continuous postsynaptic effect
Blocks reception of additional “messages”
Must be removed from its receptor
Removal of neurotransmitters occurs when they:
Are degraded by enzymes
Are reabsorbed by astrocytes or the presynaptic
terminals
Diffuse away from the synaptic cleft
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Synaptic Delay
Time taken for neurotransmitter release, diffusion
across the synapse, and binding to receptors
Synaptic delay – 0.3-5.0 ms
Synaptic delay is the rate-limiting step of neural
transmission (e.g. the slowest step)
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Postsynaptic Potentials
Postsynaptic membrane receptors are chemically gated and
not voltage gated.
Thus, they can not be self-amplifying nor self-generating
Neurotransmitter receptors mediate changes in membrane
potential according to:
The amount of neurotransmitter released
The amount of time the neurotransmitter is bound to
receptors
The two types of postsynaptic potentials are:
EPSP – excitatory postsynaptic potentials
IPSP – inhibitory postsynaptic potentials
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Nervous System II: Synaptic Transmission, pages 7–12
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Excitatory Postsynaptic Potentials
Here, neurotransmitter binding causes
depolarization of the postsynaptic membrane
At the synapse, a single type of chemically gated
ion channel opens on postsynaptic membranes
This channel allows Na+ & K+ to diffuse
simultaneously through the membrane
Na+ has steeper electrochemical gradient
Thus, Na+ influx is greater than K+ efflux and net
depolarization occurs
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Excitatory Postsynaptic Potentials
If enough neurotransmitter binds, depolarization of
the post synaptic membrane can reach 0mV
However, post synaptic membranes DO NOT
generate APs.
Instead, Excitatory Postsynaptic Postentials
(EPSPs) occur.
EPSPs trigger an AP distally at the axon hillock
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EPSPs
EPSPs are brief, relatively weak graded
depolarization events
Currents created by EPSPs spread all the way to
the axon hillock where they:
Depolarize the axon to threshold
Axonal VGICs open
AP is generated
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Inhibitory Synapses and IPSPs
Neurotransmitter binding to a receptor at inhibitory
synapses:
Causes the membrane to become more permeable to
potassium and chloride ions
Leaves the charge on the inner surface negative
(hyperpolarization)
Reduces the postsynaptic neuron’s ability to produce an
action potential. How?
This will now require an even larger depolarizing
event to reach threshold and induce an AP
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Summation by the Postsynaptic Neuron
A single EPSP cannot induce an action potential in
the postsynaptic neuron
But 1000 of them can!
EPSPs must summate (add together) temporally or
spatially to induce an action potential
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Temporal summation
Occurs when one or more presynaptic neurons
transmit impulses in high frequency.
Successive EPSPs add on to one another
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Spatial Summation
Spatial summation – postsynaptic neuron is
stimulated by a large number of terminals at the
same time (usually from different neurons)
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Summation
Both EPSPs and IPSPs can summate
Most neurons receive both EPSPs and IPSPs as well as
both chemical and electrical synapses
The axon hillock responds to the most prominent
summation, be it excitatory or inhibitory
The axon hillock acts as a neural integrator
It’s voltage potential reflects the sum of all incoming
neural information
The most effective synapses are those close to the axon
hillock
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Summation
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Figure 11.20
Neurotransmitters
Chemicals used for neuronal communication with
the body and the brain
50 different neurotransmitters have been identified
Most neurons make and release more than one type
Classified chemically and functionally
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Chemical Neurotransmitters
Chemical classes are based on structure
Acetylcholine (ACh)
Biogenic amines
Amino acids
Peptides
Novel messengers: ATP and dissolved gases NO
and CO
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Neurotransmitters: Acetylcholine
First neurotransmitter identified, and best
understood
Released at the neuromuscular junction
Synthesized and enclosed in synaptic vesicles
Acetyl CoA + choline
Ach + CoA
(Acetyl CoA is acetic acid + Coenzyme A)
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Neurotransmitters: Acetylcholine
Degraded by the enzyme acetylcholinesterase
(AChE) into acetic acid and choline
Released by:
All neurons that stimulate skeletal muscle
Some neurons in the autonomic nervous system
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Neurotransmitters: Biogenic Amines
Include:
Catecholamines – dopamine, norepinephrine (NE),
and epinephrine
Indolamines – serotonin and histamine
Broadly distributed in the brain
Play roles in emotional behaviors and our
biological clock
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Synthesis of Catecholamines
Enzymes present in the cell
determine length of
biosynthetic pathway
Norepinephrine and
dopamine are synthesized in
axonal terminals
Epinephrine is released by
the adrenal medulla
Neurons possess only the
enzymes needed to make
their own neurotransmitter
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Figure 11.21
Neurotransmitters: Amino Acids
Include:
GABA – Gamma (γ)-aminobutyric acid
Glycine
Aspartate
Glutamate
Found only in the CNS
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Neurotransmitters: Peptides
Include:
Substance P – mediator of pain signals
Beta endorphin, dynorphin, and enkephalins
Act as natural opiates; reduce pain perception
Bind to the same receptors as opiates and
morphine
E.g. increase in enkephalis during labor
E.g. increase in endorphin release in athletes
(extra boost of strength)
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Neurotransmitters: Novel Messengers
ATP
Is found in both the CNS and PNS
Produces excitatory or inhibitory responses depending on
receptor type
Second messenger response depending on which type of
receptor it binds to.
Adenosine receptors:
Adenosine is an inhibitor in the brain
Caffeine blocks adenosine receptors
Coffee drinkers get their “rush”
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Neurotransmitters: Novel Messengers
Nitric oxide (NO)
Synthesized on demand and diffuses out of the cell
that made it
Binds to iron in guanydyl cyclase, the enzyme that
makes cyclic GMP, and activates it
Is involved in learning and memory
Carbon monoxide (CO) is a main regulator of
cGMP in the brain
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Functional Classification of Neurotransmitters
Two classifications: excitatory and inhibitory
Excitatory neurotransmitters cause depolarizations
(e.g., glutamate)
Inhibitory neurotransmitters cause
hyperpolarizations (e.g., GABA and glycine)
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Functional Classification of Neurotransmitters
Some neurotransmitters have both excitatory and
inhibitory effects
Determined by the receptor type of the postsynaptic neuron
Example: acetylcholine
Excitatory at neuromuscular junctions with skeletal
muscle
Inhibitory in cardiac muscle
(norepinephren also has both excitatory and inhibitory
effects, but are opposite of those of acetylcholine)
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Neurotransmitter Receptor Mechanisms
Direct: neurotransmitters that open ion channels
Promote rapid responses
Examples: ACh and amino acids
Indirect: neurotransmitters that act through second
messengers
Promote long-lasting effects
Examples: G-proteins, biogenic amines, peptides,
and dissolved gases
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Neurotransmitter Receptors: Channel-Linked Receptors
Composed of integral membrane protein
Ligand gated ion channels that mediate direct transmitter
action (ionotropic receptors)
Action is immediate, brief, simple, and highly localized
Ligand binds the receptor, and ions enter the cells
Excitatory receptors depolarize membranes (e.g. Ach,
glutamate, aspartate, ATP are ligands for cation channels
for Na+, K+, and Ca++)
Inhibitory receptors hyperpolarize membranes (e.g. GABA
and glycine are ligands for Cl- channels)
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Channel-Linked Receptors
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Figure 11.22a
Neurotransmitter Receptors: G Protein-Linked Receptors
Metabotropic receptors
Responses are indirect, slow, complex, prolonged, and
often diffuse
These receptors are transmembrane protein complexes
Examples: muscarinic ACh receptors, neuropeptides, and
those that bind biogenic amines
G-protein activation works by controlling production of
second messengers such as cyclic AMP, cyclic GMP,
diacylglycerol, or Ca++ which open or close ion channels
or activate kinase enzymes that initiate an enzymatic
cascade
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Neurotransmitter Receptor Mechanism
Ions flow
Blocked ion flow
(a)
Channel closed
Ion channel
Adenylate
cyclase
Channel open
Neurotransmitter (ligand)
released from axon terminal
of presynaptic neuron
3
1
PPi
4
GTP
5
cAMP
ATP
5
3
Changes in
membrane
permeability
and potential
GTP
2
GDP
Protein
synthesis
Enzyme
activation
GTP
Receptor
G protein
(b)
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Nucleus
Activation of
specific genes
Figure 11.22b
Neural Integration: Neuronal Pools
Functional groups of neurons that:
Integrate incoming information
Forward the processed information to its
appropriate destination
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Neural Integration: Neuronal Pools
Simple neuronal pool
Input fiber – presynaptic fiber
Discharge zone – neurons most closely associated
with the incoming fiber
Facilitated zone – neurons farther away from
incoming fiber
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Simple Neuronal Pool
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Figure 11.23
Types of Circuits in Neuronal Pools
Divergent Circuits
Converging Circuits
Oscillating Circuits
Parallel After-Discharge Circuits
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Types of Circuits in Neuronal Pools
Divergent – one incoming fiber stimulates ever
increasing number of fibers, often amplifying
circuits
Common in sensory and motor systems
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Figure 11.24a, b
Types of Circuits in Neuronal Pools
Convergent Circuits– opposite of divergent circuits,
resulting in either strong stimulation or inhibition
Pool receives inputs from several presynaptic neurons
and the circuit has a concentrating effect
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Figure 11.24c, d
Types of Circuits in Neuronal Pools
Reverberating or Oscillating Circuits – chain of
neurons containing collateral synapses with
previous neurons in the chain
Positive feedback, impulses are sent again and
again
Control rhythmic activities, e.g. breathing
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Figure 11.24e
Types of Circuits in Neuronal Pools
Parallel after-discharge circuit– parallel arrays of
post-synaptic neurons that stimulate a common
output cell
Signals reach the output cell at different times
creating a burst of impulses (after-discharge)
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Figure 11.24f
Patterns of Neural Processing
Serial Processing
One neuron stimulates the next, that neuron
stimulates the next, and so on…
Input travels along one pathway to a specific
destination
Works in an all-or-none manner
Example: spinal reflexes (see sidenote)
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Reflexes SideNote
Reflexes are rapid, automatic responses to stimuli. The stimulus always
causes the same response (stereotyped and dependable)
E.g. jerking hand away upon touching something hot.
E.g. eye blink when an object is too close
Reflexes occur over reflex arcs that have 5 essential components:
1) receptor
2) sensory neuron
3) CNS integration center
4) motor neuron
5) effector
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Patterns of Neural Processing
Parallel Processing
Input travels along several pathways
Information delivered by each pathway is dealt
with simultaneously by different parts of the neural
circuitry
Thus, one stimulus promotes numerous responses
E.g. a smell may remind one of the odor and
associated experiences
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Development of Neurons
The nervous system originates from the neural tube
and neural crest
The neural tube becomes the CNS
There is a three-phase process of differentiation:
Proliferation of cells needed for development
Migration – cells become amitotic and move
externally
Differentiation into neuroblasts
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Axonal Growth: How do you get “wired” up?
How do neurons find their correct target?
The growing tip of the axon interacts with its environment
Guided by:
Extracellular and cell surface adhesion proteins direct growth cone into
safe areas to grow
Scaffold laid down by older neurons
Orienting glial fibers
Release of nerve growth factor by astrocytes
Neurotropins released by other neurons “lure” the growing axon to
approach or retreat
Repulsion guiding molecules
Attractants released by target cells
Neurons that fail to make synapses die
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