Nervous System (Ch. 48)
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Transcript of Nervous System (Ch. 48)
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Nervous System(Ch. 48)
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• Every time you move a muscle & every time you think a thought, your nerve cells are hard at work. They are processing information: receiving signals, deciding what to do with them, & dispatching new messages off to their neighbors. Some nerve cells communicate directly with muscle cells, sending them the signal to contract. Other nerve cells are involved solely in the bureaucracy of information, spending their lives communicating only with other nerve cells. But unlike our human bureaucracies, this processing of information must be fast in order to keep up with the ever-changing demands of life.
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Why do animals need a nervous system?
• What characteristics do animals need in a nervous system?
– fast – accurate– reset quickly
Remember…think aboutthe bunny… Poor bunny!
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Overview of information processing by nervous systems
Sensor
Effector
Motor output
Integration
Sensory input
Peripheral nervoussystem (PNS)
Central nervoussystem (CNS)
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Nervous system cells
dendrites
cell body
axon
synaptic terminal
Neuron a nerve cell
Structure fits function many entry points for
signal one path out transmits signal
signal direction
signaldirection
dendrite cell body axon synapse
myelin sheath
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Fun facts about neurons• Most specialized cell in
animals• Longest cell
– blue whale neuron• 10-30 meters
– giraffe axon• 5 meters
– human neuron• 1-2 meters
Nervous system allows for 1 millisecond response time
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• Think dominoes! – start the signal
• knock down line of dominoes by tipping 1st one trigger the signal
– propagate the signal• do dominoes move down the line?
no, just a wave through them! – re-set the system
• before you can do it again, have to set up dominoes again reset the axon
Transmission of a signal
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Transmission of a nerve signal• Neuron has similar system
– protein channels are set up – once first one is opened, the rest open in
succession• all or nothing response
– a “wave” action travels along neuron – have to re-set channels so neuron can react
again
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Cells: surrounded by charged ions
• Cells live in a sea of charged ions– anions (negative)
• more concentrated within the cell• Cl-, charged amino acids (aa-)
– cations (positive)• more concentrated in the extracellular fluid• Na+
Na+ Na+ Na+ Na+ Na+ Na+ Na+Na+ Na+ K+ Na+ Na+
Cl-
K+ Cl- Cl- Cl-K+
aa-K+ Cl- Cl-
aa- aa-aa-
aa- aa-K+
K+channel leaks K+ +
–
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Cells have voltage!• Opposite charges on opposite sides of cell membrane• This is an imbalanced condition.• The positively + charged ions repel each other as do the negatively -
charged ions. They “want” to flow down their electrical gradient and mix together evenly.
• This means that there is energy stored here, like a dammed up river.• Voltage is a measurement of stored electrical energy. Like “Danger High
Voltage” = lots of energy (lethal).– membrane is polarized
• negative inside; positive outside• charge gradient• stored energy (like a battery)
+ + + + + + + ++ + + + + + +
+ + + + + + + ++ + + + + + +
– – – – – – – ––– – – – –
– – – – – – – ––– – – – –
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Measuring cell voltageVoltage = measures the difference in concentration of charges.The positives are the “hole” you leave behind when you move an electron.Original experiments on giant squid neurons!
unstimulated neuron = resting potential of -70mV
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How does a nerve impulse travel?• Stimulus: nerve is stimulated
– reaches threshold potential • open Na+ channels in cell membrane• Na+ ions diffuse into cell
– charges reverse at that point on neuron• positive inside; negative outside • cell becomes depolarized
– + + + + + + ++ + + + + + +
– + + + + + + ++ + + + + + +
+ – – – – – – –– – – – – – –
+ – – – – – – –– – – – – – –Na+
The 1stdomino
goesdown!
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Gate
+ –
+
+
channel closed
channel open
How does a nerve impulse travel?• Wave: nerve impulse travels down neuron
– change in charge opens next Na+ gates down the line • “voltage-gated” channels
– Na+ ions continue to diffuse into cell– “wave” moves down neuron = action potential
– – + + + + + +– + + + + + +
– – + + + + + +– + + + + + +
+ + – – – – – –+ – – – – – –
+ + – – – – – –+ – – – – – –Na+
wave
The restof the
dominoes fall!
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How does a nerve impulse travel?• Re-set: 2nd wave travels down neuron
– K+ channels open• K+ channels open up more slowly than Na+ channels
– K+ ions diffuse out of cell– charges reverse back at that point
• negative inside; positive outside
+ – – + + + + +– – + + + + +
+ – – + + + + +– – + + + + +
– + + – – – – –+ + – – – – –
– + + – – – – –+ + – – – – –Na+
K+
wave
Setdominoesback upquickly!
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How does a nerve impulse travel?• Combined waves travel down neuron
– wave of opening ion channels moves down neuron– signal moves in one direction
• flow of K+ out of cell stops activation of Na+ channels in wrong direction
+ + – – + + + ++ – – + + + +
+ + – – + + + ++ – – + + + +
– – + + – – – –– + + – – – –
– – + + – – – –– + + – – – –Na+
wave
K+Readyfor
next time!
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How does a nerve impulse travel?• Action potential propagates
– wave = nerve impulse, or action potential– brain finger tips in milliseconds!
• K+ gates open more slowly than Na+ gates
+ + + + – – + ++ + + – – + +
+ + + + – – + ++ + + – – + +
– – – – + + – –– – – + + – –
– – – – + + – –– – – + + – –Na+
K+
wave
In theblink ofan eye!
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Voltage-gated channels• Ion channels open & close in response to changes in charge across
membrane – Na+ channels open quickly in response to depolarization & close
slowly– K+ channels open slowly in response to depolarization & close
slowly• Na+ channel closed when nerve isn’t doing anything.
+ + + + + – + ++ + + + – – +
+ + + + + – + ++ + + + – – +
– – – – – + – –– – – – + + –
– – – – – + – –– – – – + + –Na+
K+
wave
Structure& function!
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How does the nerve re-set itself?• After firing a neuron has to re-set itself
– Na+ needs to move back out– K+ needs to move back in– both are moving against concentration gradients
• need a pump!!
+ + + + + – – ++ + + + + – –
+ + + + + – – ++ + + + + – –
– – – – – + + –– – – – – + +
– – – – – + + –– – – – – + +Na+
Na+Na+
Na+ Na+Na+
K+K+K+K+ Na+ Na+
Na+Na+Na+
Na+Na+
Na+Na+
Na+
Na+
K+K+K+K+
K+K+
K+ K+
wave
K+
Na+
A lot ofwork todo here!
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How does the nerve re-set itself?• Sodium-Potassium pump
– active transport protein in membrane• requires ATP
– 3 Na+ pumped out– 2 K+ pumped in– re-sets charge
across membrane
ATP
That’s a lot of ATP !
Feed me somesugar quick!
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• Dominoes set back up again.• Na/K pumps are one of the main drains on ATP
production in your body. Your brain is a very expensive organ to run!
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Neuron is ready to fire again
Na+ Na+ Na+ Na+ Na+ Na+ Na+Na+ Na+ Na+ Na+ Na+ Na+
Na+ Na+ Na+ Na+ Na+ Na+ Na+
Na+ Na+ Na+ Na+ Na+ Na+
K+
K+ K+ K+ K+
K+
aa-K+ K+ K+
aa- aa-aa-
aa- aa-
+ + + + + + + ++ + + + + + +
+ + + + + + + ++ + + + + + +
– – – – – – – –– – – – – – –
– – – – – – – –– – – – – – –
resting potential
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1. Resting potential2. Stimulus reaches threshold
potential3. Depolarization
Na+ channels open; K+ channels closed
4. Na+ channels close; K+ channels open
5. Repolarizationreset charge gradient
6. UndershootK+ channels close slowly
Action potential graph
–70 mV
–60 mV
–80 mV
–50 mV
–40 mV
–30 mV
–20 mV
–10 mV
0 mV
10 mV DepolarizationNa+ flows in
20 mV
30 mV
40 mV
RepolarizationK+ flows out
ThresholdHyperpolarization(undershoot)
Resting potential Resting1
2
3
4
5
6
Mem
bra
ne
po
ten
tial
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Myelin sheath
signaldirection
Axon coated with Schwann cells insulates axon speeds signal
signal hops from node to node saltatory conduction
150 m/sec vs. 5 m/sec(330 mph vs. 11 mph)
myelin sheath
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myelin
axon
Na+
Na+
++ + + + –
–
action potential
saltatoryconduction
Multiple Sclerosis immune system (T cells)
attack myelin sheath loss of signal
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Synapse
Impulse has to jump the synapse!– junction between neurons– has to jump quickly from one
cell to next
What happens at the end of the axon?
How does the wave
jump the gap?
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axon terminal
synaptic vesicles
muscle cell (fiber)
neurotransmitteracetylcholine (ACh)receptor protein
Ca++
synapse
action potential
Chemical synapse Events at synapse
action potential depolarizes membrane
opens Ca++ channels neurotransmitter vesicles
fuse with membrane release neurotransmitter to
synapse diffusion neurotransmitter binds with
protein receptor ion-gated channels open
neurotransmitter degraded or reabsorbed
We switched…from an electrical signal
to a chemical signal
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• Calcium is a very important ion throughout your body. It will come up again and again involved in many processes.
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Synaptic terminals on the cell body of a postsynaptic neuron (colorized SEM)
Postsynapticneuron
Synapticterminal
of presynapticneurons
5 µ
m
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Nerve impulse in next neuron • Post-synaptic neuron
– triggers nerve impulse in next nerve cell• chemical signal opens ion-gated channels • Na+ diffuses into cell• K+ diffuses out of cell
– switch back to voltage-gated channel
– + + + + + + ++ + + + + + +
– + + + + + + ++ + + + + + +
+ – – – – – – –– – – – – – –
+ – – – – – – –– – – – – – –Na+
K+
K+K+
Na+ Na+
Na+
ion channel
binding site ACh
Here wego again!
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Summation of postsynaptic potentials
E1 E1 E1 E1E1E1 + E2 E1 + II
ActionpotentialAction
potentialRestingpotential
Threshold of axon ofpostsynaptic neuron
(a) Subthreshold, nosummation
(b) Temporal summation (c) Spatial summation (d) Spatial summationof EPSP and IPSP
Terminal branch of presynaptic neuron
Postsynaptic neuron E1
E1E1
E2
E1
IAxonhillock
0
–70
Mem
bra
ne p
oten
tial (
mV
)
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Neurotransmitters• Acetylcholine
– transmit signal to skeletal muscle• Epinephrine (adrenaline) & norepinephrine
– fight-or-flight response • Dopamine
– widespread in brain– affects sleep, mood, attention & learning– lack of dopamine in brain associated with Parkinson’s
disease– excessive dopamine linked to schizophrenia
• Serotonin– widespread in brain– affects sleep, mood, attention & learning
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• Nerves communicate with one another and with muscle cells by using neurotransmitters. These are small molecules that are released from the nerve cell and rapidly diffuse to neighboring cells, stimulating a response once they arrive. Many different neurotransmitters are used for different jobs: – glutamate excites nerves into action; – GABA inhibits the passing of information; – dopamine and serotonin are involved in the subtle messages of thought and
cognition. – The main job of the neurotransmitter acetylcholine is to carry the signal from
nerve cells to muscle cells. When a motor nerve cell gets the proper signal from the nervous system, it releases acetylcholine into its synapses with muscle cells. There, acetylcholine opens receptors on the muscle cells, triggering the process of contraction. Of course, once the message is passed, the neurotransmitter must be destroyed, otherwise later signals would get mixed up in a jumble of obsolete neurotransmitter molecules. The cleanup of old acetylcholine is the job of the enzyme acetylcholinesterase.
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Neurotransmitters • Weak point of nervous system
– any substance that affects neurotransmitters or mimics them affects nerve function
• gases: nitrous oxide, carbon monoxide• mood altering drugs:
– stimulants» amphetamines, caffeine, nicotine
– depressants» quaaludes, barbiturates
• hallucinogenic drugs: LSD, peyote• SSRIs: Prozac, Zoloft, Paxil• Poisons
• Selective serotonin reuptake inhibitor
Pity the Test Mice
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snake toxin blockingacetylcholinesterase active site
Acetylcholinesterase
acetylcholinesterase
neurotoxin in green
• Enzyme which breaks downacetylcholine neurotransmitter – acetylcholinesterase inhibitors =
neurotoxins• snake venom, sarin, insecticides
active site in red
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Since acetylcholinesterase has an essential function, it is a potential weak point in our nervous system. Poisons and toxins that attack the enzyme cause acetylcholine to accumulate in the nerve synapse, paralyzing the muscle. Over the years, acetylcholinesterase has been attacked in many ways by natural enemies. For instance, some snake toxins attack acetylcholinesterase.Acetylcholinesterase is found in the synapse between nerve cells and muscle cells. It waits patiently and springs into action soon after a signal is passed, breaking down the acetylcholine into its two component parts, acetic acid and choline. This effectively stops the signal, allowing the pieces to be recycled and rebuilt into new neurotransmitters for the next message. Acetylcholinesterase has one of the fastest reaction rates of any of our enzymes, breaking up each molecule in about 80 microseconds.
Is the acetylcholinesterase toxin a competitive or non-competitive inhibitor?
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Questions to ponder…
• Why are axons so long?– Transmit signal quickly. The synapse is the choke point. Reduce the
number of synapses & reduce the time for transmission
• Why have synapses at all?Decision points (intersections of multiple neurons) & control
points
• How do “mind altering drugs” work?– caffeine, alcohol, nicotine, marijuana…– Affect neurotransmitter release, uptake & breakdown. React with or
block receptors & also serve as neurotransmitter mimics
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• Why are axons so long?– Transmit signal quickly. The synapse is the choke point.
Reduce the number of synapses & reduce the time for transmission
• Do plants have — or need — nervous systems?– They react to stimuli — is that a nervous system?
Depends on how you define nervous system.– But if you can’t move quickly, there is very little adaptive
advantage of a nervous system running at the speed of electrical transmission.
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Organization of some nervous systems
Nerve netNervering
Radialnerve
Eyespot
BrainNerve cord
Transversenerve
Brain
Segmentalganglion
Ventral nervecord
Brain
Ventral nervecord
Segmentalganglia
Anteriornerve ring
Longitudinalnerve cords
Ganglia
Brain
Ganglia
Sensoryganglion
Spinalcord
(dorsalnervecord)
Brain
(d) Leech (annelid)(c) Planarian (flatworm)(b) Sea star (echinoderm)(a) Hydra (cnidarian)
(e) Insect (arthropod) (f) Chiton (mollusc) (g) Squid (mollusc) (h) Salamander (chordate)
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The vertebrate nervous systemCentral nervous
system (CNS)Peripheral nervous
system (PNS)
Brain
Spinal cord
Cranialnerves
Gangliaoutside
CNS
Spinalnerves
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Functional hierarchy of the vertebrate peripheral nervous system
Peripheralnervous system
Somaticnervoussystem
Autonomicnervoussystem
Sympatheticdivision
Parasympatheticdivision
Entericdivision
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The parasympathetic and sympathetic divisions of the autonomic nervous system
Parasympathetic division Sympathetic division
Action on target organs: Action on target organs:
Location ofpreganglionic neurons:
brainstem and sacralsegments of spinal cord
Neurotransmitterreleased by
preganglionic neurons:acetylcholine
Location ofpostganglionic neurons:
in ganglia close to orwithin target organs
Neurotransmitterreleased by
postganglionic neurons:acetylcholine
Constricts pupilof eye
Stimulates salivarygland secretion
Constrictsbronchi in lungs
Slows heart
Stimulates activityof stomach and
intestines
Stimulates activityof pancreas
Stimulatesgallbladder
Promotes emptyingof bladder
Promotes erectionof genitalia
Cervical
Thoracic
Lumbar
Synapse
Sympatheticganglia
Dilates pupilof eye
Inhibits salivary gland secretion
Relaxes bronchiin lungs
Accelerates heart
Inhibits activity of stomach and intestines
Inhibits activityof pancreas
Stimulates glucoserelease from liver;inhibits gallbladder
Stimulatesadrenal medulla
Inhibits emptyingof bladder
Promotes ejaculation and vaginal contractionsSacral
Location ofpreganglionic neurons:
thoracic and lumbarsegments of spinal cord
Neurotransmitterreleased by
preganglionic neurons:acetylcholine
Location ofpostganglionic neurons:some in ganglia close totarget organs; others ina chain of ganglia near
spinal cord
Neurotransmitterreleased by
postganglionic neurons:norepinephrine
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Development of the human brainEmbryonic brain regions Brain structures present in adult
Forebrain
Telencephalon
Midbrain
Hindbrain
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
Cerebrum (cerebral hemispheres; includes cerebralcortex, white matter, basal nuclei)
Diencephalon (thalamus, hypothalamus, epithalamus)
Midbrain (part of brainstem)
Pons (part of brainstem), cerebellum
Medulla oblongata (part of brainstem)
Midbrain Hindbrain
Forebrain
(a) Embryo at one month (b) Embryo at five weeks (c) Adult
MesencephalonMetencephalon
Myelencephalon
Spinal cord
Diencephalon
Telencephalon
Cerebral hemisphereDiencephalon:
Hypothalamus
ThalamusPineal gland
(part of epithalamus)
Brainstem:
Midbrain
Pons
Medullaoblongata
Cerebellum
Central canal
Spinal cord
Pituitarygland
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Ventricles, gray matter, and white matter
Gray matter
Whitematter
Ventricles
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Medulla, Pons and Midbrain
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The Cerebellum
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The Diencephalon
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The Cerebrum
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The human cerebrum viewed from the rear
Left cerebralhemisphere
Corpuscallosum
Neocortex
Right cerebralhemisphere
Basalnuclei
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The human cerebral cortexFrontal lobe
Temporal lobe Occipital lobe
Parietal lobe
Frontalassociation
area
Speech
Smell
Hearing
Auditoryassociation
areaVision
Visualassociation
area
Somatosensoryassociation
area
Reading
Speech
Taste
Som
atos
enso
ry c
orte
x
Mot
or c
orte
x
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Body representations in the primary motor and primary somatosensory cortices
Tongue
JawLips
Face
Eye
Brow
Neck
Thumb
Fingers
HandW
ristForearmE
lbow
ShoulderT
runk
Hip
Knee
Primarymotor cortex Abdominal
organs
Pharynx
Tongue
TeethGumsJaw
Lips
Face
Nose
Eye
FingersHand
ForearmE
lbowU
pper arm
Trunk H
ip
Leg
Thumb
Neck
Head
Genitalia
Primarysomatosensory cortex
Toes
Parietal lobeFrontal lobe
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Mapping language areas in the cerebral cortex
Hearingwords
Seeingwords
Speakingwords
Generatingwords
Max
Min
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Mechanism of long-term potentiation in the vertebrate brain
PRESYNAPTIC NEURON
NO
Glutamate
NMDAreceptor
Signal transduction pathways
NO
Ca2+
AMPA receptor
POSTSYNAPTIC NEURON
Ca2+ initiates the phos-phorylation of AMPA receptors,
making them more responsive. Ca2+ also causes more AMPA
receptors to appear in the postsynaptic membrane.
5
Ca2+ stimulates thepostsynaptic neuron to
produce nitric oxide (NO).
6
The presynapticneuron releases glutamate.1
Glutamate binds to AMPAreceptors, opening the AMPA-
receptor channel and depolarizingthe postsynaptic membrane.
2
Glutamate also binds to NMDAreceptors. If the postsynapticmembrane is simultaneously
depolarized, the NMDA-receptorchannel opens.
3
Ca2+ diffuses into thepostsynaptic neuron.
4
NO diffuses into thepresynaptic neuron, causing it to release more glutamate.
7
P
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Microscopic signs of Alzheimer’s diseaseSenile plaque Neurofibrillary tangle
20 m
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Ponder this…Any Questions??
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Make sure you can do the following:1. Compare and contrast the regulatory structures and
functions of the nervous and endocrine systems2. Diagram the processes by which nervous signals are
transmitted by and between neurons.3. Label all parts of a neuron4. Explain the relationships between the major divisions
of the mammalian nervous system.5. Explain the relationships between the major divisions
of the human brain6. Explain the causes of nervous system disruptions and
how disruptions of the nervous system can lead to disruptions of homeostasis.