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The Nervous System
Cell Structure
Cells of the Central Nervous System
Receiver (AFFECTOR) cells from sense
Sender cells (EFFERENT) to motor
Also: Specialized “helper” cells
Total Neurons: About 100 Billion
Specialized nerve cells in CNS and PNS
Neurons: Specialized nerve cells in CNS and PNS
Convey sensory information into the brain
Carry out operations involved in thoughts, feeling,
Transmit commands to body to control muscles and
Many different specializations
Many neurons:
26-29 billion in higher brain areas
70 billion in cerebellum
1 billion in spinal cord
Parts of a Neuron
regulates life functions (metabolism)
Contains nucleus (and chromosomes), organelles, etc.
branched projections from soma
receive transmissions from other neurons
long projection extending from soma
transmits information to next neuron
axon hillock: beginning of axon, important in transmission
Parts of a Neuron
Terminals or Terminal Buttons
bulbs on end of branched portion of axon
synaptic bulbs or terminals
contain neurotransmitters
Synapse or synaptic cleft
space between neurons
space between one neuron's synaptic bulb/other's dendrites
Myelin sheath:
glue like structures that hold neurons in place
Myelin Sheath
insulates neurons
made of glial cells or “glue cells”
In CNS: oligodendroglial cells
In PNS: Schwann cells
allows synaptic transmission to occur by jumping down axon
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
– Single axon
– Two axons: end of axon branches into two
 Axon and dendrites extend in several directions from
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
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:
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
– 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
Neurons sit at base level: RESTING POTENTIAL
Resting potential = approximately -70 mV
Why -70 mV?
70 mV charge due to unequal distribution of electrical charges on
two sides of cell membrane:
inside of axon is negatively charged:
more K+ ; more A-
outside of axon is positively charged:
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
 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
– Those that do get through are returned via NA+K+
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
– 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
– Allows changes in cell wall that will change voltage inside the
– THIS depolarization Is decremented: decreases with
– Also called local potential because has only a local effect
Action Potential
All or None Law: Voltage change is either sufficient to
stimulate action potential, or not (no wimpy action potentials!)
Voltage changes from -70 to +40 mV and back again
This “depolarization” begins at the axon hillock
inside of axon becomes negative due to movement of ions
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
 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
 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
 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
Resting potential: voltage is about -70mV
Action potential
– Dendrites receive incoming signals
– If sufficient, cell goes into firing mode
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:
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
– 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
• 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
– 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
 Several specific kinds- each act on certain
 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
 Neurotransmitter then diffuses across synapse to
postsynaptic neuron and attaches to chemical receptor
– And if enough NT moves across, it all starts again!