Download The Neuron

The Nervous System
Cell Structure
Cells of the Central Nervous System
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Receiver (AFFECTOR) cells from sense
organs
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Sender cells (EFFERENT) to motor
organs/muscles
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Also: Specialized “helper” cells
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Total Neurons: About 100 Billion
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Specialized nerve cells in CNS and PNS
Neurons
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Neurons: Specialized nerve cells in CNS and PNS
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Convey sensory information into the brain
Carry out operations involved in thoughts, feeling,
behavior
Transmit commands to body to control muscles and
organs
Many different specializations
Many neurons:
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26-29 billion in higher brain areas
70 billion in cerebellum
1 billion in spinal cord
Parts of a Neuron
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Soma:
regulates life functions (metabolism)
Contains nucleus (and chromosomes), organelles, etc.
Dendrites:
branched projections from soma
receive transmissions from other neurons
Axon:
long projection extending from soma
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transmits information to next neuron
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axon hillock: beginning of axon, important in transmission
Parts of a Neuron
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Terminals or Terminal Buttons
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bulbs on end of branched portion of axon
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synaptic bulbs or terminals
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contain neurotransmitters
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Synapse or synaptic cleft
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space between neurons
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space between one neuron's synaptic bulb/other's dendrites
Myelin sheath:
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glue like structures that hold neurons in place
Myelin Sheath
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insulates neurons
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made of glial cells or “glue cells”
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In CNS: oligodendroglial cells
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In PNS: Schwann cells
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allows synaptic transmission to occur by jumping down axon
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exposed area between sheath = Nodes of Ranvier
Several kinds of neurons
and helper cells
 Affector or Receptor neurons:
– SENSORY neurons: embedded in sense organs
– specialized to receive stimulation from environment and
send to the brain
 May be UNIPOLAR or BIPOLAR
– Single axon
– Two axons: end of axon branches into two
 Axon and dendrites extend in several directions from
body
Several kinds of neurons
and helper cells
 Effector or Motor neurons
– specialized to contract muscles and
stimulate glandular secretions
– acted upon by nerves and neurons
 MULTIPOLAR: have multiple axons which
extend from soma in several directions
– Dendrites on one end; axon on other
Several kinds of neurons
and helper cells
 Interneurons
– Connect one neuron to another in same part or
region of brain/spinal cord
– Multipolar
– Seem to be missing the axon (it is very short or
nonexistent)
Several kinds of neurons
and helper cells
 Glial cells: form the myelin sheath
– Wrap around on outside of most human neurons:
 Insulate and wrap around neurons:
– Speeds up neural transmission
– Also help with storing of neurotransmitter
– Important part of waste system for neuron as well
 Multiple Sclerosis = allergy to own myelin
Neural Membrane
and Its potential:
HOW THE NEURON WORKS
The Cell Membrane
 Cell membrane =
– The cell wall or “skin” of the cell
– most critical factor in neural communication
– Only about 8 micrometers ( 8 millionths of a meter) thick
 Composed of lipid (fat) and protein
– Lipid molecules arranged head to tail
– Heads are water soluable (attract water)
– Tails are water insoluable (repel water)
 Heads point towards surrounding fluid, tails turn away
– This creates double layer membrane
Neuron lipid layer
Neuron Potentials
 Membrane holds cell together
– More importantly: CONTROLS environment within and
outside cell
 Semipermeable:
– allows some molecules in/out
– H20; O2, CO2 pass freely
 Selective permeability:
– Keeps out some substances
– Allows others in only under certain circumstances
– Protein channels: open and close to let molecules in when
neuron is active
Neuron Potential
 Polarization is result of this selective membrane
permeability:
– means that the cell has an electrical charge
 Potential: difference in electrical charge between the
inside and outside of a cell (or any two points)
 Neuron has three critical potentials:
– Resting potential: cell is at rest
– Action potential: cell is active and sending a signal
– Refractory potential: cell is recovering
Resting potential
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Neurons sit at base level: RESTING POTENTIAL
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Resting potential = approximately -70 mV
Why -70 mV?
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70 mV charge due to unequal distribution of electrical charges on
two sides of cell membrane:
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inside of axon is negatively charged:
more K+ ; more A-
outside of axon is positively charged:
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More Na+; some Cl-
Remember: axon has voltage of -70mV at
resting potential
How can neuron stabilize at resting potential?
 Diffusion: Molecules diffuse from area of high concentration to low
concentration until “evened out”:
 Concentration gradient: ions move to side of membrane where less
concentrated
 Electrical gradient: ions attracted to side that is of opposite charge
 Other factors upset this diffusion:
– Neuron wall semi permeable: anions too large to pass through membrane
– Negative charge of anions repels Cl- ions so they don’t move inside
 During an ACTION POTENTIAL, membrane undergoes changes:
– opens gates into/out of cell
– Results in changes to the t charge of the concentration
Na+K+ Pump or Ion Pump
 K+ and Na+ are special:
– K+ tends to move out of cell
 concentration gradient is stronger than electrical gradient
– Na+ tends to move into cell:
 both gradients pull Na+ inside
 K+ and Na+ pass through membrane via
special protein channels
– Most Na+ and K+ remain in place during resting
potential
– Those that do get through are returned via NA+K+
pump
Na+K+ Pump or Ion Pump
• Na+ K ion pump = large protein
molecules that move Na+ ions to
outside/K+ to inside
• Rate of exchange: 3 Na+/2 K+
• Maintains membrane as more
negative inside than out
• Metabolic process: uses energy
(about 40% of energy
expenditure of the cell!)
Neuron Potential
 Polarization is result of this selective membrane
permeability:
– means that the cell has an electrical charge
 Potential: difference in electrical charge between the
inside and outside of a cell (or any two points)
 Neuron has three critical potentials:
– Resting potential: cell is at rest
– Action potential: cell is active and sending a signal
– Refractory potential: cell is recovering
Action Potential
 Action potential = abrupt depolarization
of membrane that allows neuron to
communicate over long distances
 Neuron becomes excited and sends a signal
via neurotransmitter release
 if incoming message is sufficient in
strength: Causes an ACTION POTENTIAL
Action Potential
 Dendrites (usually) receive incoming neurotransmitter
– Chemical fits in “lock” on dendrite
– Alters the shape of the cell wall
 PARTIAL DEPOLARIZATION at dendrites:
– Allows changes in cell wall that will change voltage inside the
neuron
– THIS depolarization Is decremented: decreases with
time/distance
– Also called local potential because has only a local effect
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Action Potential
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All or None Law: Voltage change is either sufficient to
stimulate action potential, or not (no wimpy action potentials!)
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Voltage changes from -70 to +40 mV and back again
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This “depolarization” begins at the axon hillock
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inside of axon becomes negative due to movement of ions
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NaCl goes out of axon
outside of axon becomes positive: K+ goes in
result is voltage change as switching of ions occurs
depolarization moves down axon in wavelike form in
myelinated neurons
The Neuron Fires
 Voltage across cell membrane is stored energy; if this
stored energy is released, get tremendous changes
within cell
 During action potential: Na+ channels open
– Remember: thousands of Na+ ions held on outside- now they
rush in through these channels
– Approximately 500x greater than normal number of Na+ ions
– Small area inside membrane is depolarized, first to 0 and
then to +30 to +40mV
– This small area will then spread down axon: cell wall opens
and ion exchange occurs at Nodes of Ranvier
As reach peak of Action Potential
 K+ ions move out due to diffusion.
 K+ also moves out due to electrostatic pressure because
the inside of the cell is temporarily positive.
 Membrane returns to near resting potential or BELOW
 This entire event takes approximately 1 millisecond.
 Because nearby sodium channels open, a new action
potential is triggered at the adjacent patch of membrane.
Refractory Period
 Absolute refractory period: Neuron repolarizes
– cell environment moves back toward resting potential during
refractory period
– Resetting neuron back to resting potential
– Cell absolutely cannot fire during this period
 Then Relative Refractory period:
– Neuron can fire,
– but only with extra stimulation
Absolute Refractory Period
 Neuron cannot fire during this period
 Due to action of Ion pump:
– Ion pump kicks into action at end of action potential
– Pumps ions K+ in and Na+ out
– Over does it a bit: cell ends up just below resting potential
– Until returns to resting potential, very difficult, if not impossible, for
cell to fire
Important functions of
Absolute Refractory Period
 When the Na+ channels close during the
action potential, that part of the axon
cannot fire again.
 This limits how frequently the neuron can
fire.
 This also prevents backward spread of
depolarization, so action potentials move
only toward the terminals.
Important functions of
Relative Refractory Period
 Plays role in intensity coding in axon
 K+ channels open for just milliseconds longer
following absolute refractory period
 Makes inside of cell slightly more negative; harder to
fire
 Rate law: Stronger stimuli trigger new action
potentials earlier in recovery, so the axon encodes
intensity as rate of firing.
 Thus: only stronger stimulation can set off neuron
Remember:
 Movement of action potentials down the axon
is not a flow of ions but a chain of events.
 Because the action potential “jumps” from node
to node this is called saltatory conduction.
 When the action potential reaches the
terminals it passes the message on to the next
cell “in line”…..and it begins again
Nondecremental conduction
 Action potential different from local potential in
several important ways:
– Local potential = graded potential- it varies in magnitude
depending on strength of stimulus that produced it;
action potential is ungraded
– Action potential obeys all or none law: occurs at full
strength or not at all
 Action potential is nondecremental: does NOT lose
strength at each successive point (local potentials
do degrade)
Neurotoxins and Ion channels
 Neurotoxins affect ion channels involved in the action potential.
 The puffer fish produces tetrodotoxin: blocks sodium channels.
 Scorpion venom keeps sodium channels open, prolonging the
action potential.
 Beneficial drugs affect these ion channels as well.
– Local anesthetics block sodium channels.
– Some general anesthetics work by opening potassium channels.
 Ion channels can be modified to control neurons by light.
– This allows greater precision in stimulating neurons and identifying
pathways in the brain.
Again: Three Steps for firing
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Resting potential: voltage is about -70mV
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Action potential
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– Dendrites receive incoming signals
– If sufficient, cell goes into firing mode
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Voltage changes from -70mV to +40mV
Ions exchange places
Repeats itself rapidly down axon
Only in places where myelin sheath doesn’t cover: Nodes of Ranvier
Refractory Period:
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below resting or lower than -70mV
Cell recovers from firing
Absolute refractor period: Brief time period when cannot fire again
Relative refractory period: Brief time period when difficult for it to fire again.
 Take home lesson:
– Axon encodes stimulus intensity by controlling FIRING RATE not size
of action potential
Why so fast?
Thank your Glial cells
• Remember: Glial cells:
– Non neural cells
– Provide a supporting function to neurons
– Account for 90% of cells in adult human brain
• Function: Help hold neurons together, assist in
neurotransmission
– Provide supports for the nervous system
– In periphery are rather rigid:
• E.g., Schwann cells
– In CNS: are soft and squishy:
• E.g., Oligodenroglia cells
Glial Cells speed conduction
• Neurons conduct impulses from 1 to 120 meters/sec or 270 mph
– Still fairly slow
 Since reaction time critical for survival, body found ways to
increase conduction speed
• Vary thickness of axons to provide less resistance
• Motor neurons: diameter of 0.5 mm can attain conduction speed of
30m/sec
• But: conduction speed not increase in direct proportion to size: is
power function, thus must find alternative way
 Alternative way: use graded local potentials
Myelination = solution to increasing
speed of neural transmission
• Vertebrates are myelinated: allows
SALTATORY conduction
– Action potential jumps from node to node
– Myelin also helps increase speed via
capacitance: resists movement of ions during
graded potential
 Overall effect: 100x greater conduction
speed; reduced work for Ion pump
Other functions of Glial Cells
 Important in fetal development: scaffold that guides new
neurons to destination
 Provide energy to cell
 Serve as waste system for neurons
 Aid in development and maintenance of neural
connections
– Get 7x more connections when glial cells available
 Help in conducting action potential
Now: Why an action potential?
 Allows release of neurotransmitter
 Neurotransmitter is a chemical substance
– Remember: even chemical substances contain a
charge
 Several specific kinds- each act on certain
neurons
 Most neurons respond to and release one kind
of neurotransmitter
Now: Why an action potential?
 Neurotransmitter stored in synaptic vesicles
• Remember, these are at the END of the axon
• Next to the synapse
 Action potential opens channels that allow Ca+ ions to
enter terminals from extracellular fluid
– Ca+ ions cause vesicles nearest the membrane to fuse
with membrane
– Membrane then opens and transmitter is dumped into
synapse
 Neurotransmitter then diffuses across synapse to
postsynaptic neuron and attaches to chemical receptor
– And if enough NT moves across, it all starts again!