Download Lecture 048 - Neurons and Nervous Systems

Colonie High AP Biology
DeMarco/Goldberg
Nervous System Cells
 Neuron

a nerve cell
dendrites
signal
direction
Chapter 45
cell body
Neurons and Nervous Systems
 Structure fits function
many entry points
for signal
 one path out
 transmits signal

axon
signal direction
synapse
dendrite  cell body  axon  terminal branches 
Types of Neurons
Fun Facts About Neurons
 Most specialized cell in
cell body
sensory neuron
“afferent”
cell body
axon
dendrites
interneuron
“associative”
dendrites
cell
body
animals
 Longest cell

blue whale neuron

giraffe axon

human neuron
 10-30 meters
 5 meters
 1-2 meters
motor neuron
“efferent”
Transmission of a Nerve Signal
 Think dominoes!

start the signal
 knock down line of dominoes by tipping 1st one
 trigger the signal

propagate the signal
Nervous system allows for
~ millisecond response times
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
 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
a “wave” action travels along neuron
 have to re-set channels so neuron can
react again

Colonie High AP Biology
DeMarco/Goldberg
Cells: Surrounded by Charged Ions
 Cells live in a sea of charged ions

Cells have voltage!
 Opposite charges on opposite sides of
anions
cell membrane
 more concentrated within the cell
 Cl-, charged amino acids (aa-)
 charge gradient
 more concentrated in the extracellular fluid
 K+, Na+
K+
Na+
Na+
K+
aa-
K+
 stored energy (like a battery)
+ + + + + + + + + + + + + + +
Na+
aaCl-
Na+
ClK+
Na+
aa-
Na+
K+
aa-
K+
Na+
ClCl-
Na+
Na+
Na+
Na+
Cl-
aa-
– – – – – – – – – – – – – –
– – – – – – – – – – – – – –
–
channel
leaks K+
membrane is polarized
 negative inside; positive outside
cations
+


+ + + + + + + + + + + + + + +
aa- Cl-
Measuring Cell Voltage
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+
+ – – – – – – – – – – – – – –
– + + + + + + + + + + + + + +
unstimulated neuron = resting potential of ~60 mV
How does a nerve impulse travel?
 Wave: nerve impulse travels down neuron



How does a nerve impulse travel?
 Re-set: 2nd wave travels down neuron
Gate
+
change in charge opens
+ –
+
next Na+ gates down the line
 “voltage-gated” channels
channel
Na+ ions continue to diffuse into cell channel
closed
open
“wave” moves down neuron = action potential

K+ channels open

K+ ions diffuse out of cell
charges reverse back at that point
 K+ channels up more slowly than Na+ channels

 negative inside; positive outside
K+
– – – + + + + + + + + + + + +
+ + + – – – – – – – – – – – –
+ – – – – + + + + + + + + + +
– + + + + – – – – – – – – – –
Na+
Na+
+ + + – – – – – – – – – – – –
– – – + + + + + + + + + + + +
wave 
– + + + + – – – – – – – – – –
+ – – – – + + + + + + + + + +
wave 
Colonie High AP Biology
DeMarco/Goldberg
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
How does a nerve impulse travel?
 Action potential propagates


wave = nerve impulse, or action potential
brain  finger tips in milliseconds!
K+
K+
+ + + – – – – + + + + + + + +
– – – + + + + – – – – – – – –
+ + + + + + + – – – – + + + +
– – – – – – – + + + + – – – –
Na+
Na+
– – – + + + + – – – – – – – –
+ + + – – – – + + + + + + + +
– – – – – – – + + + + – – – –
+ + + + + + + – – – – + + + +
wave 
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
wave 
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!!
K+
Na+
K+
– – – – – – – – – + + + – – –
+ + + + + + + + + – – – + + +
Na+
Na+

active transport protein in membrane
 requires ATP
Na+
3
pumped out
+
 2 K pumped in
 re-sets charge
across
membrane

ATP
Na+
Na+
Na+ +
K
K+
Na+
+
+Na
Na+ Na
Na+
K+
K+
Na+ K+
K+
+
Na+ K+
Na+ K+ K
Na+
K+
Na+
– – – – – – – – – – + + + + –
+ + + + + + + + + + – – – – +
wave 
wave 
How does the nerve re-set itself?
 Na+ / K+ pump
K+
K+ + + + + + + + – – – – +
+ + +
– – – – – – – – – – + + + + –
+ + + + + + + + + – – – + + +
– – – – – – – – – + + + – – –
Na+
Na+
Neuron is ready to fire again…
Na+
Na+
Na+
K+
aa-
aaNa+
Na+
Na+
K+
Na+
Na+
K+
Na+
aa-
K+
Na+
Na+
Na+
Na+
K+
aaNa+
Na+
Na+
K+
Na+
Na+
Na+
Na+
K+
aa-
aa- K+
K+
Na+
Na+
Na+
Na+
Na+
Na+
resting potential
+ + + + + + + + + + + + + + +
– – – – – – – – – – – – – – –
– – – – – – – – – – – – – – –
+ + + + + + + + + + + + + + +
Na+
Colonie High AP Biology
DeMarco/Goldberg
Action Potential Graph
Myelin Sheath
 Axon coated by Schwann cells
1. Resting potential
2. Stimulus reaches
40 mV
4
10 mV Depolarization
Na+ flows in
0 mV
–10 mV
3
–20 mV
 saltatory conduction
Repolarization
K+ flows out

5
–30 mV
–40 mV
–50 mV
Threshold
–60 mV
2
–70 mV
–80 mV
1
insulate axon
speeds signal
 signal hops from node to node
20 mV
Hyperpolarization
(undershoot)
Resting potential
150 m/sec vs. 5 m/sec
(330 mph vs. 11 mph)
6 Resting
myelin sheath
saltatory
conduction
What happens at the end of the axon?
 Impulse has to jump the synapse!
Na+
myelin
+
+

30 mV
action potential
axon

signal
direction
Membrane potential
threshold potential
3. Depolarization
Na+ channels open;
K+ channels closed
4. K+ channels open;
Na+ channels close;
5. Repolarization
reset charge gradient
6. Undershoot: K+
channels close slowly
+
+
+

–
–

junction between neurons
has to jump quickly from one cell to next
Na+
Multiple Sclerosis
 immune system (T cells)
attack myelin sheath
 loss of signal
The Synapse
 Events at synapse
axon terminal
action potential
synaptic vesicles
synapse
Ca++
receptor protein
neurotransmitter
acetylcholine
(ACh)
muscle cell (fiber)
action potential
depolarizes membrane
 opens Ca++ channels
 neurotransmitter
vesicles fuse with
membrane
 release neurotransmitter
to synaptic cleft
 neurotransmitter binds
with protein receptor

 ion-gated channels open

neurotransmitter
degraded or reabsorbed
synapse
Acetylcholinesterase
 Enzyme which breaks
down acetylcholine
neurotransmitter

neurotoxins = inhibitors
 snake venom, sarin, insecticides
neurotoxin
in green
active site
in red
acetylcholinesterase
snake toxin blocking
acetylcholinesterase active site
Colonie High AP Biology
DeMarco/Goldberg
Nerve Impulse in Next Neuron
 Post-synaptic neuron

Neurotransmitters
 Acetylcholine

triggers nerve impulse in next nerve cell
 chemical signal opens ion-gated channels
 Na+ diffuses into cell

K+
binding site
Na+

Na+
ACh

diffuses out of cell


K+
K+
Na+
+ – – – – – – – – – – – – – –
– + + + + + + + + + + + + + +
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

Na+
– + + + + + + + + + + + + + +
+ – – – – – – – – – – – – – –

widespread in brain
lack of dopamine in brain associated with Parkinson’s disease
excessive dopamine linked to schizophrenia
pleasure & reward pathways
 Serotonin
ion channel

transmit signal to skeletal muscle
 Dopamine

widespread in brain
affects
 Glutamate

excites neurons into action
 GABA

inhibits passing of information
Questions to ponder…
 Why are axons so long?
 Why have synapses at all?
 How do “mind altering drugs” work?

caffeine, alcohol, nicotine, marijuana…
 Do plants have a nervous system?

amphetamines, caffeine, nicotine
Do they need one?
 depressants
 hallucinogenic drugs
 Prozac
 poisons
Muscle Contraction
 Nerve signal
stimulates muscle
cell’s sarcoplasmic
reticulum (SR) to
release stored Ca+2
Ca+2 Triggers Muscle Action
 At rest, tropomyosin
blocks myosin-binding
sites on actin
 Ca+2 binds to
troponin complex
shape change
causes movement
of tropomyosintroponin complex
 exposes myosinbinding sites
on actin

Colonie High AP Biology
DeMarco/Goldberg
How it all works…
How Ca+2 Controls Muscle
 Action potential causes Ca+2 release from SR
 Sliding filament

model
 Troponin moves tropomyosin uncovering
exposed actin binds
to myosin
 fibers slide past
each other
ATP

myosin binding site on actin
 Myosin binds actin
ATP
muscle doesn’t relax
until Ca+2 is pumped
back into SR
 requires ATP
1
uses ATP to "ratchet" each time
releases, "unratchets" & binds to next actin
 Myosin pulls actin chain along
 Sarcomere shortens
shorten muscle cell
 muscle contraction



 ratchet system

Ca+2 binds to troponin

Z lines move closer together
 Whole fiber shortens  contraction!
 Ca+2 pumps restore Ca+2 to SR  relaxation!
ATP
ATP
Put it all together…

pumps use ATP
Cephalization = Brain Evolution
Cephalization = clustering of neurons in “brain” at front (anterior)
end of bilaterally symmetrical animals  where sense organs are
2
associative
neurons
3
ATP
nerve cords
nerve
ribs
7
nerve
net
4
Cnidarian
6
5
Cephalization = Brain Evolution
Flatworm
Platyhelminthes
Echinoderm
Simplest nervous
system
no control of
complex actions
ATP
radial
nerve
More organization
but still based on
nerve nets;
supports more
complex movement
Simplest, defined
central nervous
system
more complex muscle
control
Evolution of Vertebrate Brain
 increase in interneurons in brain region
central nervous system
peripheral
nerves
forebrain
giant
axon
brain
brain
ventral
nerve
cords
forebrain
dominant cerebrum
Shark
hindbrain
Frog
Crocodile
Earthworm
Cat
Mollusk
Arthropod
More complex
brains
connected to all
other parts of body
by peripheral nerves
More complex
brains in predators
most sophisticated
invertebrate
nervous system
Further brain
development
ganglia = neuron
clusters along
CNS
Human
Spinal cord
Hind: Medulla oblongata
Hind: Cerebellum
Optic tectum
Midbrain
Fore: Cerebrum
Olfactory tract
forebrain
Bird