Transcript Nervous System

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Identify the principle parts of the nervous
system
Describe the cells that make up the nervous
system
Describe what starts and stops a nerve
impulse (action potential)
The role of neurotransmitters
Compare the functions of the CNS & PNS
Identify the principle parts of the brain
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20% of cells in nervous system are neurons
and the rest are neuroglial cells – support
cells
Support & protection
Maintenance of surrounding chemical
concentrations
No impulse generation
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Astrocytes – star shaped, largest & most
numerous, form tight sheaths around capillaries
of the brain
Microglia – small, usually stationary –but enlarge
and mobilize in degenerating brain tissue for
phagocytosis
Oligodendroglia – produce myelin sheath that
envelopes nerve fibers in the brain and spinal
cord
Schwann cells – found in nerves only (not in brain
or spinal cord); help to form myelin sheath
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In PNS these neuroglial cells produce myelin –
a fatty insulating material surrounding axon –
called the myelin sheath = myelinated
neurons
Nodes of Ranvier are short uninsulated gaps
where the surface of the axon is exposed
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Energy saving – prevents slow leakage of Na+
in / K+ out (recall: sodium-potassium pump =
3 Na+ out & 2 K+ in)
Speeds transmission of impulses – local
depolarizing at Nodes of Ranvier is much faster
than continuous propagation down an
unmyelinated axon = saltatory conduction –
action potentials appear to jump from node to
node (5 mph vs. 250 mph)
Helps damaged or severed axons of the PNS to
regenerate – severed end near soma can regrow
in sheath channel (weeks-year +)
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In CNS these protective sheaths of myelin do
not regenerate when the cell dies – this is why
spinal cord injuries or MS causes permanent
damage
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Identify the principle parts of the nervous
system
Describe the cells that make up the nervous
system
Describe what starts and stops a nerve
impulse (action potential)
The role of neurotransmitters
Compare the functions of the CNS & PNS
Identify the principle parts of the brain
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Junctions that allow transfer of information
from one neuron:
◦ to another neuron
◦ to an effector cell (muscle)
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Pre synaptic neuron – transmits signal to
synapse
Post synaptic neuron – transmits signal away
from synapse
Synaptic cleft is the fluid filled space
separating the pre/post synaptic neurons
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Convert an electrical signal (action potential)
into a chemical signal (neurotransmitter)
4 step sequence
1) Action potential arrives at axon bulb, Ca channels
open and Ca diffuses into axon bulb
2) Ca causes vesicles containing neurotransmitters
to fuse with presynaptic membrane and release
contents into synaptic cleft
3) Neurotransmitter molecules bind to receptors on
post synaptic membrane opening gated Na
channels
4) Na diffuses into postsynaptic membrane & starts
a new action potential (electrical signal)
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> 50 chemicals can function as
neurotransmitters!
Excitatory – encourage the generation of new
impulses in post synaptic neuron
Inhibitory – prevents generation of action
potentials in post synaptic neuron
Some neurotransmitters can do both
depending on the receptor that they bind to
Many small graded potentials are needed to
fire the impulse across the cleft
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Prompt removal of neurotransmitter causes
signals to stop – 3 ways to remove
1) Taken back up by presynaptic neuron &
repackaged
2) Destroyed by enzymes in synaptic cleft
3) Diffuse away from synaptic cleft  general
circulation and destroyed
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Convergence – one neuron receives
information from several neurons
Divergence – action potential will go to
several neurons
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Neurons may integrate and process
thousands of simultaneous incoming
stimulatory and inhibitory signals before
generating and transmitting their own action
potentials
Bottom line: individual neurons do not “see,
smell or hear” yet combined actions allow us
to experience these complex sensations
Filter of information – extraneous information
expelled during REM sleep
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Muscle cells are targets of presynaptic
neurons –
Skeletal muscle does not process information
Neuromuscular junction in large with many
points of contact between neuron and muscle
cell
Threshold is reached in skeletal muscle every
time a motor neuron sends a single action
potential
So the nervous system has absolute control
of skeletal muscle!