Transcript Chapter 12 - Marion ISD

Nervous system cells
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Ch. 12
Introduction to the nervous
system video
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Nervous system Introduction
Functioncommunication
 Components
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 Brain,
spinal
cord, nerves
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Organization of the nervous
system
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Subdivisions
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Central Nervous system (CNS)
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Peripheral Nervous system (PNS)
Nerves
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Structural/functional center
Brain,spinal cord
Integrate sensory information
Evaluate sensory information
Initiate outgoing response
Cranial-from brain
Spinal-from spinal cord
Afferent and efferent divisions
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Afferent - information from environment
Efferent-from brain to muscles/glands
Innervation
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Somatic - carries info to muscles
 Autonomic-carries info to smooth
and cardiac muscle
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Efferent division
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Sympathetic - (fight or flight)
Parasympathetic-normal resting activities
(rest/repair)
Visceral sensory division-communicating between viscera and
brain - continual
Cells of the nervous system
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Glia - supportive function
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Types
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Astrocytes-star shaped
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Microglia-small
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If brain is inflamed, phagocytosis
Ependymal-thin sheets that line
cavities of cns
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Most numerous
Connect to neurons and capillaries
Transfer nutrients from blood to
neurons
Make up blood brain barrier
Produce and circulate fluid
Oligodendrocytes-hold nerve fibers
together and produce myelin sheath
Schwann-in pns
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Form myelin sheath
Gaps-nodes of ranvier
Essential for nerve regrowth
Microglia
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Cells of the nervous system
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Neurons-excitable cells that
conduct impulses
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Structure
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Cell body-ribosomes make
neurotransmitters which are
packaged into vessicles
Dendrites-one or more per
neuron
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Conduct nerve signals to cell
body
 Distal ends of sensory neurons
are receptors
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Axon-single process extending
from axon hillock
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Sometimes covered with myelin
- fatty layer
 Conducts nerve impulses away
from cell body
 Synaptic knob at end
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Cytoskeleton- neurofibrils - allow
rapid transport
Functional regions
Input - dendrites/cell body
 Summation-axon hillock
 Conduction-axon
 Output -synaptic knobs
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Classification of neurons
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Structural classification
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Multipolar-1axon, several dendrites
Bipolar-1axon,1dendrite
Unipolar-1axon which divides into 2
Functional classification
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Afferent-sensory neuron(conduct impulses to
spinal cord or brain)
Efferent-motor neuron(conduct impulses from
brain to muscle or gland)
Interneurons (bridge gap between sensory and
motor )
Reflex arc
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Signal conduction
route - from receptor
to and from CNS
 3 neuron arc-most
common - afferent
neuron, interneuron
and efferent neuron
 Two neuron arcsimplest form afferent and efferent
neuron
Synapse-Where nerve signals are
transmitted from one neuron to another
Types
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Electricalelectrical current
jumps gap
Chemical typical in adults
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Located at
junction of
synaptic knob of
one neuron and
dendrite/cell
body of another
Nerves-bundles of fibers held together with
connective tissue
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Connective tissue
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Endoneurium-surround
each fiber
Perineurium-hold together
bundles of nerves
(fascicles)
Epineurium-surround many
fascicles/blood
Tracts-bundles of nerve
fibers in CNS
White matter - myelinated
nerves
Gray matter-cell bodies,
dendrites
Nerves can be afferent,
efferent or Mixed
Repair of nerve fibers
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If the cell dies, the damage
is permanent.
 If the axon is damaged,
the axon can bypass the
damage and re-grow.
 Stages of repair
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Axon degeneration and
Removal of debris by
phagocytosis.
 Growth bypasses the
damaged axon with new
axon formation
 Schwann cells cover the
new growth
Nerve Impulses
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Living cells maintain a difference in the concentration across
membranes
Membrane potential - excess of positively charged ions
outside membrane, negatively charged inside
Polarized membrane-exhibits this difference
Magnitude measured in Volts or millivolts (mv).
Resting membrane potential is normally -70mv.
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Sodium potassium pump (active transport)-produces slight
excess of pos. ions outside. Transports sodium/potassium
inside/outside.
Local potential-slight shift away from resting potential
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Excitation occurs when -dditional sodium channels opened
(sodium is positively charged)allows potential to move towards
0 (depolarization)
 Action potential occurs when a certain number of sodium ions
diffuse inward.
 To resotre the neuron to resting potential, Inhibition occurs potassium channels open increasing potential (
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Action potential mechanism
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Stimulus triggers sodium gates
to open
Sodium moves into cell
Threshold potential point at
which impulse is triggered
All or none
Gates stay open for a short
time then close
Movement to resting potential
when potassium channels
open (repolarization)
Hyperpolarization precedes
achieving resting potential
again
Refractory period-membrane
resists repolarization (brief)
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Action potential conduction
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Reverse of polarity at peak of action
potential
Reversal causes electrical current to flow
between membrane regions and triggers
sodium channels to open in next segment.
This repeats
Action potential never moves backward
because of refractory period
In myelenated axons, action potentials only
occur at nodes of ranvier, jumping to next
node - called Saltatory conduction
Speed of conduction-depends on diameter
of fiber and presence or absence of myelin.
Types of synapses
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Electrical-action
potential continues to
postsynaptic membrane
Chemical-presynaptic
cells release chemical
messengers
(neurotransmitters)
across gap to
postsynaptic cell,
inducing action
potential.
Mechanism of synaptic
transmission
Action potential releases calcium
 Neurotransmitter release
 Neurotransmitter diffusion
 Post synaptic potential
 Neurotransmitter action termination by
re-uptake or taken up by glia
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Neurotransmitters
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Description-chemical
communicators
 Function - can be
excitatory or inhibibitory
 Classes of
neurotransmitters
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Acetylcholine
 Amines affect learning,
emotions, motor control
 Amino acids - most
common. Location PNS
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Neurotransmitter stimulation
of post-synaptic membrane
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Direct stimulation
second messenger
Anaesthetics
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Reduce pain sensation by the following
 Blocks
initiation or conduction of nerve
impulse
 Inhibit sodium channel opening
Antidepressants
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Depression - deficit of norepinephrine,
dopamine, serotonin
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Possible causes:
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Neurotransmitters are not present in the synaptic cleft in
enough quantity
Neurotransmitters are receycled too quickly
Neurotransmitters are not produced by the ribosomesin
enough quantitiy
Drugs inhibit the enzymes that inactivate
neurotransmitters
 Inhibit reuptake of neurotransmitters
Cycle of life
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Nerve tissue development
 Begins
in fetal ectoderm
 Learning - formation of new synapses
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Aging - degeneration