Transcript Chapter 3

Chapter 12
Nervous Tissue
Lecture Outline
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INTRODUCTION
• The nervous system, along with
the endocrine system, helps to
keep controlled conditions within
limits that maintain health and
helps to maintain homeostasis.
• The nervous system is
responsible for all our behaviors,
memories, and movements.
• The branch of medical science
that deals with the normal
functioning and disorders of the
nervous system is called
neurology.
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Chapter 12
Nervous Tissue
• Controls and integrates all body activities within limits
that maintain life
• Three basic functions
– sensing changes with sensory receptors
• fullness of stomach or sun on your face
– interpreting and remembering those changes
– reacting to those changes with effectors
• muscular contractions
• glandular secretions
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Major Structures of the Nervous System
• Brain, cranial nerves, spinal cord, spinal nerves, ganglia, enteric
plexuses and sensory receptors
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Structures of the Nervous System - Overview
• Twelve pairs of cranial nerves emerge from the base of the brain
through foramina of the skull.
– A nerve is a bundle of hundreds or thousands of axons, each of
which courses along a defined path and serves a specific region of
the body.
• The spinal cord connects to the brain through the foramen magnum of
the skull and is encircled by the bones of the vertebral column.
– Thirty-one pairs of spinal nerves emerge from the spinal cord, each
serving a specific region of the body.
• Ganglia, located outside the brain and spinal cord, are small masses of
nervous tissue, containing primarily cell bodies of neurons.
• Enteric plexuses help regulate the digestive system.
• Sensory receptors are either parts of neurons or specialized cells that
monitor changes in the internal or external environment.
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Functions of the Nervous Systems
• The sensory function of the nervous system is to sense
changes in the internal and external environment through
sensory receptors.
– Sensory (afferent) neurons serve this function.
• The integrative function is to analyze the sensory
information, store some aspects, and make decisions
regarding appropriate behaviors.
– Association or interneurons serve this function.
• The motor function is to respond to stimuli by initiating
action.
– Motor(efferent) neurons serve this function.
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Nervous System Divisions
• Central nervous system (CNS)
– consists of the brain and spinal cord
• Peripheral nervous system (PNS)
– consists of cranial and spinal nerves that contain both
sensory and motor fibers
– connects CNS to muscles, glands & all sensory receptors
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Subdivisions of the PNS
• Somatic (voluntary) nervous system (SNS)
– neurons from cutaneous and special sensory receptors to
the CNS
– motor neurons to skeletal muscle tissue
• Autonomic (involuntary) nervous systems
– sensory neurons from visceral organs to CNS
– motor neurons to smooth & cardiac muscle and glands
• sympathetic division (speeds up heart rate)
• parasympathetic division (slow down heart rate)
• Enteric nervous system (ENS)
– involuntary sensory & motor neurons control GI tract
– neurons function independently of ANS & CNS
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Organization of the Nervous System
• CNS is brain and spinal cord
• PNS is everything else
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Enteric NS
• The enteric nervous system (ENS) consists of neurons in
enteric plexuses that extend the length of the GI tract.
– Many neurons of the enteric plexuses function
independently of the ANS and CNS.
– Sensory neurons of the ENS monitor chemical changes
within the GI tract and stretching of its walls, whereas
enteric motor neurons govern contraction of GI tract
organs, and activity of the GI tract endocrine cells.
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HISTOLOGY OF THE NERVOUS SYSTEM
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Neuronal Structure & Function
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Neurons
• Functional unit of nervous system
• Have capacity to produce action potentials
– electrical excitability
• Cell body
– single nucleus with prominent nucleolus
– Nissl bodies (chromatophilic substance)
• rough ER & free ribosomes for protein
synthesis
– neurofilaments give cell shape and support
– microtubules move material inside cell
– lipofuscin pigment clumps (harmless aging)
• Cell processes = dendrites & axons
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Parts of a Neuron
Neuroglial cells
Nucleus with
Nucleolus
Axons or
Dendrites
Cell body
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Cell membrane
• The dendrites are the receiving or input portions of a
neuron.
• The axon conducts nerve impulses from the neuron to the
dendrites or cell body of another neuron or to an effector
organ of the body (muscle or gland).
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Dendrites
• Conducts impulses
towards the cell body
• Typically short, highly
branched & unmyelinated
• Surfaces specialized for
contact with other
neurons
• Contains neurofibrils &
Nissl bodies
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Axons
• Conduct impulses away
from cell body
• Long, thin cylindrical
process of cell
• Arises at axon hillock
• Impulses arise from initial
segment (trigger zone)
• Side branches
(collaterals) end in fine
processes called axon
terminals
• Swollen tips called
synaptic end bulbs
contain vesicles filled with
neurotransmitters
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Axonal Transport
• Cell body is location for most protein synthesis
– neurotransmitters & repair proteins
• Axonal transport system moves substances
– slow axonal flow
• movement in one direction only -- away from cell body
• movement at 1-5 mm per day
– fast axonal flow
• moves organelles & materials along surface of
microtubules
• at 200-400 mm per day
• transports in either direction
• for use or for recycling in cell body
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Axonal Transport & Disease
• Fast axonal transport route by which toxins or
pathogens reach neuron cell bodies
– tetanus (Clostridium tetani bacteria)
– disrupts motor neurons causing painful muscle
spasms
• Bacteria enter the body through a laceration or
puncture injury
– more serious if wound is in head or neck because
of shorter transit time
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Diversity in Neurons
• Both structural and functional features are used to classify
the various neurons in the body.
• On the basis of the number of processes extending from the
cell body (structure), neurons are classified as multipolar,
biopolar, and unipolar (Figure 12.4).
• Most neurons in the body are interneurons and are often
named for the histologist who first described them or for an
aspect of their shape or appearance. Examples are Purkinje
cells (Figure 12.5a) or Renshaw cells (Figure 12.5b).
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Structural Classification of Neurons
• Based on number of processes found on cell body
– multipolar = several dendrites & one axon
• most common cell type
– bipolar neurons = one main dendrite & one axon
• found in retina, inner ear & olfactory
– unipolar neurons = one process only(develops from a bipolar)
• are always sensory neurons
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Functional Classification of Neurons
• Sensory (afferent) neurons
– transport sensory information from skin, muscles,
joints, sense organs & viscera to CNS
• Motor (efferent) neurons
– send motor nerve impulses to muscles & glands
• Interneurons (association) neurons
– connect sensory to motor neurons
– 90% of neurons in the body
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Association or Interneurons
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Neuroglial Cells
•
•
•
•
Half of the volume of the CNS
Smaller cells than neurons
50X more numerous
Cells can divide
– rapid mitosis in tumor formation (gliomas)
• 4 cell types in CNS
– astrocytes, oligodendrocytes, microglia & ependymal
• 2 cell types in PNS
– schwann and satellite cells
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Astrocytes
• Star-shaped cells
• Form blood-brain
barrier by covering
blood capillaries
• Metabolize
neurotransmitters
• Regulate K+ balance
• Provide structural
support
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Microglia
• Small cells found near
blood vessels
• Phagocytic role -- clear
away dead cells
• Derived from cells that
also gave rise to
macrophages &
monocytes
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Ependymal cells
• Form epithelial
membrane lining
cerebral cavities &
central canal
• Produce cerebrospinal
fluid (CSF)
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Satellite Cells
• Flat cells surrounding
neuronal cell bodies in
peripheral ganglia
• Support neurons in
the PNS ganglia
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Oligodendrocytes
• Most common glial cell
type
• Each forms myelin
sheath around more
than one axons in CNS
• Analogous to Schwann
cells of PNS
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Myelination
• A multilayered lipid and protein covering called the myelin
sheath and produced by Schwann cells and
oligodendrocytes surrounds the axons of most neurons
(Figure 12.8a).
• The sheath electrically insulates the axon and increases the
speed of nerve impulse conduction.
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Schwann Cell
• Cells encircling PNS axons
• Each cell produces part of the myelin sheath
surrounding an axon in the PNS
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Axon Coverings in PNS
• All axons surrounded by a lipid & protein
covering (myelin sheath) produced by
Schwann cells
• Neurilemma is cytoplasm & nucleus
of Schwann cell
– gaps called nodes of Ranvier
• Myelinated fibers appear white
– jelly-roll like wrappings made of
lipoprotein = myelin
– acts as electrical insulator
– speeds conduction of nerve impulses
• Unmyelinated fibers
– slow, small diameter fibers
– only surrounded by neurilemma but no
myelin sheath wrapping
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Myelination in PNS
• Schwann cells myelinate (wrap around) axons in the PNS
during fetal development
• Schwann cell cytoplasm & nucleus forms outermost layer of
neurolemma with inner portion being the myelin sheath
• Tube guides growing axons that are repairing themselves
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Myelination in the CNS
• Oligodendrocytes myelinate axons in the CNS
• Broad, flat cell processes wrap about CNS axons, but the
cell bodies do not surround the axons
• No neurilemma is formed
• Little regrowth after injury is possible due to the lack of a
distinct tube or neurilemma
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Gray and
White Matter
• White matter = myelinated processes (white in color)
• Gray matter = nerve cell bodies, dendrites, axon terminals,
bundles of unmyelinated axons and neuroglia (gray color)
– In the spinal cord = gray matter forms an H-shaped inner
core surrounded by white matter
– In the brain = a thin outer shell of gray matter covers the
surface & is found in clusters called nuclei inside the CNS
• A nucleus is a mass of nerve cell bodies and dendrites inside
the CNS.
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Electrical Signals in Neurons
• Neurons are electrically excitable due to the voltage
difference across their membrane
• Communicate with 2 types of electric signals
– action potentials that can travel long distances
– graded potentials that are local membrane
changes only
• In living cells, a flow of ions occurs through ion
channels in the cell membrane
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Two Types of Ion Channels
• Leakage (nongated) channels are always open
– nerve cells have more K+ than Na+ leakage channels
– as a result, membrane permeability to K+ is higher
– explains resting membrane potential of -70mV in nerve
tissue
• Gated channels open and close in response to a stimulus
– results in neuron excitability
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Ion Channels
• Gated ion channels respond to voltage changes, ligands
(chemicals), and mechanical pressure.
– Voltage-gated channels respond to a direct change in the
membrane potential (Figure 12.10a).
– Ligand-gated channels respond to a specific chemical
stimulus (Figure 12.10b).
– Mechanically gated ion channels respond to mechanical
vibration or pressure.
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Gated Ion Channels
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Resting Membrane Potential
• Negative ions along inside of cell membrane & positive ions along
outside
– potential energy difference at rest is -70 mV
– cell is “polarized”
• Resting potential exists because
– concentration of ions different inside & outside
• extracellular fluid rich in Na+ and Cl
• cytosol full of K+, organic phosphate & amino acids
– membrane permeability differs for Na+ and K+
• 50-100 greater permeability for K+
• inward flow of Na+ can’t keep up with outward flow of K+
• Na+/K+ pump removes Na+ as fast as it leaks in
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Graded Potentials
• Small deviations from resting
potential of -70mV
– hyperpolarization = membrane
has become more negative
– depolarization = membrane has
become more positive
• The signals are graded, meaning
they vary in amplitude (size),
depending on the strength of the
stimulus and localized.
• Graded potentials occur most often
in the dendrites and cell body of a
neuron.
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How do Graded Potentials Arise?
• Source of stimuli
– mechanical stimulation of membranes with mechanical
gated ion channels (pressure)
– chemical stimulation of membranes with ligand gated ion
channels (neurotransmitter)
• Graded/postsynaptic/receptor or generator potential
– ions flow through ion channels and change membrane
potential locally
– amount of change varies with strength of stimuli
• Flow of current (ions) is local change only
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Generation of an Action Potential
• An action potential (AP) or impulse is a sequence of rapidly
occurring events that decrease and eventually reverse the
membrane potential (depolarization) and then restore it to
the resting state (repolarization).
– During an action potential, voltage-gated Na+ and K+
channels open in sequence (Figure 12.13).
• According to the all-or-none principle, if a stimulus reaches
threshold, the action potential is always the same.
– A stronger stimulus will not cause a larger impulse.
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Action Potential
• Series of rapidly occurring events that change and then restore the
membrane potential of a cell to its resting state
• Ion channels open, Na+ rushes in (depolarization), K+ rushes out
(repolarization)
• All-or-none principal = with stimulation, either happens one specific way
or not at all (lasts 1/1000 of a second)
• Travels (spreads) over surface of cell without dying out
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Depolarizing Phase of
Action Potential
• Chemical or mechanical stimulus
caused a graded potential to reach
at least (-55mV or threshold)
• Voltage-gated Na+ channels open
& Na+ rushes into cell
– in resting membrane, inactivation gate of sodium channel is open &
activation gate is closed (Na+ can not get in)
– when threshold (-55mV) is reached, both open & Na+ enters
– inactivation gate closes again in few ten-thousandths of second
– only a total of 20,000 Na+ actually enter the cell, but they change
the membrane potential considerably(up to +30mV)
• Positive feedback process
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Repolarizing Phase of
Action Potential
• When threshold potential of
-55mV is reached, voltage-gated
K+ channels open
• K+ channel opening is much
slower than Na+ channel
opening which caused depolarization
• When K+ channels finally do open, the Na+ channels have already
closed (Na+ inflow stops)
• K+ outflow returns membrane potential to -70mV
• If enough K+ leaves the cell, it will reach a -90mV membrane
potential and enter the after-hyperpolarizing phase
• K+ channels close and the membrane potential returns to the resting
potential of -70mV
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Refractory Period of
Action Potential
• Period of time during which
neuron can not generate
another action potential
• Absolute refractory period
– even very strong stimulus will
not begin another AP
– inactivated Na+ channels must return to the resting state before
they can be reopened
– large fibers have absolute refractory period of 0.4 msec and up
to 1000 impulses per second are possible
• Relative refractory period
– a suprathreshold stimulus will be able to start an AP
– K+ channels are still open, but Na+ channels have closed
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The Action Potential:
Summarized
• Resting membrane
potential is -70mV
• Depolarization is the
change from -70mV to
+30 mV
• Repolarization is the
reversal from +30 mV
back to -70 mV)
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Local Anesthetics
• Local anesthetics and certain neurotoxins
– Prevent opening of voltage-gated Na+ channels
– Nerve impulses cannot pass the anesthetized region
Examples:
– Novocaine and lidocaine
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Propagation of Action Potential
• An action potential spreads (propagates) over the surface of
the axon membrane
– as Na+ flows into the cell during depolarization, the
voltage of adjacent areas is effected and their voltagegated Na+ channels open
– self-propagating along the membrane
• The traveling action potential is called a nerve impulse
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Continuous versus Saltatory Conduction
• Continuous conduction (unmyelinated fibers)
– step-by-step depolarization of each portion of the
length of the axolemma
• Saltatory conduction
– depolarization only at nodes of Ranvier where there is
a high density of voltage-gated ion channels
– current carried by ions flows through extracellular fluid
from node to node
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Saltatory Conduction
• Nerve impulse conduction in which the impulse jumps from node
to node
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Speed of Impulse Propagation
• The propagation speed of a nerve impulse is not related to
stimulus strength.
– larger, myelinated fibers conduct impulses faster due to
size & saltatory conduction
• Fiber types
– A fibers largest (5-20 microns & 130 m/sec)
• myelinated somatic sensory & motor to skeletal muscle
– B fibers medium (2-3 microns & 15 m/sec)
• myelinated visceral sensory & autonomic preganglionic
– C fibers smallest (.5-1.5 microns & 2 m/sec)
• unmyelinated sensory & autonomic motor
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Encoding of Stimulus Intensity
• How do we differentiate a light touch from a firmer
touch?
– frequency of impulses
• firm pressure generates impulses at a higher
frequency
– number of sensory neurons activated
• firm pressure stimulates more neurons than does a
light touch
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Action Potentials in Nerve and Muscle
• Entire muscle cell membrane versus only the axon of the
neuron is involved
• Resting membrane potential
– nerve is -70mV
– skeletal & cardiac muscle is closer to -90mV
• Duration
– nerve impulse is 1/2 to 2 msec
– muscle action potential lasts 1-5 msec for skeletal & 10300msec for cardiac & smooth
• Fastest nerve conduction velocity is 18 times faster than
velocity over skeletal muscle fiber
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SIGNAL TRANSMISSION AT SYNAPSES
• A synapse is the functional junction between one neuron
and another or between a neuron and an effector such as a
muscle or gland.
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Signal Transmission at Synapses
• 2 Types of synapses
– electrical
• ionic current spreads to next cell through gap junctions
• faster, two-way transmission & capable of
synchronizing groups of neurons
– chemical
• one-way information transfer from a presynaptic neuron
to a postsynaptic neuron
– axodendritic -- from axon to dendrite
– axosomatic -- from axon to cell body
– axoaxonic -- from axon to axon
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Chemical Synapses
• Action potential reaches end bulb
and voltage-gated Ca+ 2 channels
open
• Ca+2 flows inward triggering
release of neurotransmitter
• Neurotransmitter crosses synaptic
cleft & binding to ligand-gated
receptors
– the more neurotransmitter
released the greater the change
in potential of the postsynaptic
cell
• Synaptic delay is 0.5 msec
• One-way information transfer
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Excitatory & Inhibitory Potentials
• The effect of a neurotransmitter can be either excitatory or
inhibitory
– a depolarizing postsynaptic potential is called an EPSP
• it results from the opening of ligand-gated Na+
channels
• the postsynaptic cell is more likely to reach threshold
– an inhibitory postsynaptic potential is called an IPSP
• it results from the opening of ligand-gated Cl- or K+
channels
• it causes the postsynaptic cell to become more
negative or hyperpolarized
• the postsynaptic cell is less likely to reach threshold
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Removal of Neurotransmitter
• Diffusion
– move down concentration
gradient
• Enzymatic degradation
– acetylcholinesterase
• Uptake by neurons or glia cells
– neurotransmitter
transporters
– Prozac = serotonin reuptake
inhibitor
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Three Possible Responses
• Small EPSP occurs
– potential reaches -56 mV only
• An impulse is generated
– threshold was reached
– membrane potential of at least -55 mV
• IPSP occurs
– membrane hyperpolarized
– potential drops below -70 mV
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Comparison of Graded & Action Potentials
• Origin
– GPs arise on dendrites and cell bodies
– APs arise only at trigger zone on axon hillock
• Types of Channels
– AP is produced by voltage-gated ion channels
– GP is produced by ligand or mechanically-gated
channels
• Conduction
– GPs are localized (not propagated)
– APs conduct over the surface of the axon
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Comparison of Graded & Action Potentials
• Amplitude
– amplitude of the AP is constant (all-or-none)
– graded potentials vary depending upon stimulus
• Duration
– The duration of the GP is as long as the stimulus
lasts
• Refractory period
– The AP has a refractory period due to the nature
of the voltage-gated channels, and the GP has
none.
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Summation
• If several presynaptic end bulbs release their
neurotransmitter at about the same time, the combined
effect may generate a nerve impulse due to summation
• Summation may be spatial or temporal.
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Spatial Summation
• Summation of effects of
neurotransmitters released from
several end bulbs onto one
neuron
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Temporal Summation
• Summation of effect of
neurotransmitters released from 2
or more firings of the same end
bulb in rapid succession onto a
second neuron
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Summation
• The postsynaptic neuron is an integrator, receiving and
integrating signals, then responding.
• If the excitatory effect is greater than the inhibitory effect but
less that the threshold level of stimulation, the result is a
subthreshold EPSP, making it easier to generate a nerve
impulse.
• If the excitatory effect is greater than the inhibitory effect and
reaches or surpasses the threshold level of stimulation, the
result is a threshold or suprathreshold EPSP and a nerve
impulse.
• If the inhibitory effect is greater than the excitatory effect, the
membrane hyperpolarizes (IPSP) with failure to produce a
nerve impulse.
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Neurotransmitters
• Both excitatory and inhibitory neurotransmitters are present
in the CNS and PNS; the same neurotransmitter may be
excitatory in some locations and inhibitory in others.
• Important neurotransmitters include acetylcholine,
glutamate, aspartate, gamma aminobutyric acid, glycine,
norepinephrine, epinephrine, and dopamine.
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Neurotransmitter Effects
• Neurotransmitter effects can be modified
– synthesis can be stimulated or inhibited
– release can be blocked or enhanced
– removal can be stimulated or blocked
– receptor site can be blocked or activated
• Agonist
– anything that enhances a transmitters effects
• Antagonist
– anything that blocks the action of a neurotranmitter
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Small-Molecule Neurotransmitters
• Acetylcholine (ACh)
– released by many PNS neurons & some CNS
– excitatory on NMJ but inhibitory at others
– inactivated by acetylcholinesterase
• Amino Acids
– glutamate released by nearly all excitatory neurons in
the brain ---- inactivated by glutamate specific
transporters
– GABA is inhibitory neurotransmitter for 1/3 of all brain
synapses (Valium is a GABA agonist -- enhancing its
inhibitory effect)
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Small-Molecule Neurotransmitters
• Biogenic Amines
– modified amino acids (tyrosine)
• norepinephrine -- regulates mood, dreaming,
awakening from deep sleep
• dopamine -- regulating skeletal muscle tone
• serotonin -- control of mood, temperature
regulation, & induction of sleep
– removed from synapse & recycled or destroyed by
enzymes (monoamine oxidase or catechol-0methyltransferase)
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Small-Molecule Neurotransmitters
• ATP and other purines (ADP, AMP & adenosine)
– excitatory in both CNS & PNS
– released with other neurotransmitters (ACh & NE)
• Gases (nitric oxide or NO)
– formed from amino acid arginine by an enzyme
– formed on demand and acts immediately
• diffuses out of cell that produced it to affect
neighboring cells
• may play a role in memory & learning
– first recognized as vasodilator that helps lower blood
pressure
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Neuropeptides
• 3-40 amino acids linked by peptide bonds
• Substance P -- enhances our perception of pain
• Pain relief
– enkephalins -- pain-relieving effect by blocking the
release of substance P
– acupuncture may produce loss of pain sensation
because of release of opioids-like substances such as
endorphins or dynorphins
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Strychnine Poisoning
• In spinal cord, Renshaw cells normally release an inhibitory
neurotransmitter (glycine) onto motor neurons preventing
excessive muscle contraction
• Strychnine binds to and blocks glycine receptors in the
spinal cord
• Massive tetanic contractions of all skeletal muscles are
produced
– when the diaphragm contracts & remains contracted,
breathing can not occur
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Neuronal Circuits
• Neuronal pools are organized into circuits (neural
networks.) These include simple series, diverging,
converging, reverberating, and parallel afterdischarge circuits (Figure 12.18 a-d).
• A neuronal network may contain thousands or even
millions of neurons.
• Neuronal circuits are involved in many important
activities
– breathing
– short-term memory
– waking up
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Neuronal Circuits
• Diverging -- single cell stimulates many others
• Converging -- one cell stimulated by many others
• Reverberating -- impulses from later cells repeatedly stimulate
early cells in the circuit (short-term memory)
• Parallel-after-discharge -- single cell stimulates a group of cells
that all stimulate a common postsynaptic cell (math problems)
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Regeneration & Repair
• Plasticity maintained throughout life
– sprouting of new dendrites
– synthesis of new proteins
– changes in synaptic contacts with other neurons
• Limited ability for regeneration (repair)
– PNS can repair damaged dendrites or axons
– CNS no repairs are possible
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Damage and Repair in the Peripheral Nervous
System (Figure 19.a)
• When there is damage to an axon, usually there are
changes, called chromatolysis, which occur in the cell body
of the affected cell; this causes swelling of the cell body and
peaks between 10 and 20 days after injury.
• By the third to fifth day, degeneration of the distal portion of
the neuronal process and myelin sheath (Wallerian
degeneration) occurs; afterward, macrophages phagocytize
the remains.
• Retrograde degeneration of the proximal portion of the fiber
extends only to the first neurofibral node.
• Regeneration follows chromatolysis; synthesis of RNA and
protein accelerates, favoring rebuilding of the axon and
often taking several months.
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Repair within the PNS
• Axons & dendrites may be repaired if
– neuron cell body remains intact
– schwann cells remain active and
form a tube
– scar tissue does not form too rapidly
•
Chromatolysis
– 24-48 hours after injury, Nissl bodies
break up into fine granular masses
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Repair within the PNS
• By 3-5 days,
– wallerian degeneration occurs
(breakdown of axon & myelin
sheath distal to injury)
– retrograde degeneration occurs
back one node
• Within several months, regeneration
occurs
– neurolemma on each side of injury
repairs tube (schwann cell mitosis)
– axonal buds grow down the tube to
reconnect (1.5 mm per day)
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Neurogenesis in the CNS
• Formation of new neurons from stem cells was not
thought to occur in humans
– 1992 a growth factor was found that stimulates adult
mice brain cells to multiply
– 1998 new neurons found to form within adult human
hippocampus (area important for learning)
• There is a lack of neurogenesis in other regions of the
brain and spinal cord.
• Factors preventing neurogenesis in CNS
– inhibition by neuroglial cells, absence of growth
stimulating factors, lack of neurolemmas, and rapid
formation of scar tissue
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Multiple Sclerosis (MS)
• Autoimmune disorder causing destruction of myelin
sheaths in CNS
– sheaths becomes scars or plaques
– 1/2 million people in the United States
– appears between ages 20 and 40
– females twice as often as males
• Symptoms include muscular weakness, abnormal
sensations or double vision
• Remissions & relapses result in progressive,
cumulative loss of function
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Epilepsy
• The second most common neurological disorder
– affects 1% of population
• Characterized by short, recurrent attacks initiated by
electrical discharges in the brain
– lights, noise, or smells may be sensed
– skeletal muscles may contract involuntarily
– loss of consciousness
• Epilepsy has many causes, including;
– brain damage at birth, metabolic disturbances,
infections, toxins, vascular disturbances, head
injuries, and tumors
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end
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