Download PHYSIOLOGY OF THE NERVOUS SYSTEM

PHYSIOLOGY OF THE
NERVOUS SYSTEM
Neurons are IRRITABLE
• Ability to respond
to a stimulus!
• (What’s a
stimulus?)
How do neurons respond to
stimuli?
• By generating an ACTION POTENTIAL!
• Action Potential = “Nerve Impulse”
• Reminder: Which part of the neuron
generates impulses?
• NOTE:
– A stimulus must be strong enough (above the
threshold level) to generate an Action Potential
– Examples…
So how does an Action Potential
get generated???
• Involves:
1. Ions
2. Ion Channels in Neuron Membranes
3. Electricity! (Currents, Voltage)
1. Ions Involved:
•
•
•
•
•
K+
Na+
ClCa++
Various Anions (A- )
2. Ion Channels
• Transport Proteins embedded in
Phospholipid Membrane of Axon!
• 2 Major Types:
– Passive (Leakage) Channels
– Active (Gated) Channels:
• Voltage-Gated Channels
• Chemically-Gated Channels
• Mechanically-Gated Channels
The Big Idea
AN ELECTRICAL IMPULSE
IS GENERATED WHEN
IONS MOVE ACROSS THE
AXON MEMBRANE
THROUGH OPEN
CHANNELS!!
3. ELECTRICITY
• Overall,
– The body is Electrically Neutral!
• ( # + charges = # - charges)
Voltage
Measures the Potential Energy due to
separation of charges across a membrane!
Current
Flow of electrical charge from one point to
another. Can be used to do WORK!!
(Kinetic Energy)
When Charges are Separated
Across the Neuron Membrane…
• Neuron is “AT REST”
–(
Potential Energy,
Kinetic Energy)
• RESTING MEMBRANE POTENTIAL:
– Separation of Charge across membrane…
– More (-) charges inside than (+) outside
– Voltage Reading = –70 millivolts
– Membrane is POLARIZED !
Resting Membrane Potential
The Polarized State
What Maintains the
Resting Membrane Potential?
Maintaining the differences
in [ion] between the
intracellular and
extracellular fluids!!
Distribution of Ions during
Resting Potential State
•
•
•
•
K+ :
inside membrane
Na+ :
outside membrane
Cl- :
outside membrane
Various Anions (A- ) :
inside membrane
Permeability Across the
Membrane
• K+
• Na+
• High Permeability
(Leakage Channels)
• Low Permeability
• Cl-
• High Permeability
• Anions-
• IMPERMEABLE!!!
The Sodium Potassium Pump
• K+ constantly leaks across membrane to
outside! ( Why?)
• K+ pumped back into cell!
• Na+ pumped out of cell!
• Ratio : 3 Na+ OUT / 2 K+ IN
• ACTIVE TRANSPORT MECHANISM!!
(What does that mean??)
Let the Fun Begin…
Generation of a Nerve Impulse
1. Signal is detected/received and relayed to
axon membrane.
2. RESTING MEMBRANE POTENTIAL
ACTION POTENTIAL!!
(aka: Nerve Impulse!)
Action Potentials
• Caused by a shift in membrane potentials
along the axon!
• Detection of STIMULUS
Shift in Membrane
Permeabilities!
So what’s the deal with Action
Potentials?
• An Action Potential (AP) is:
a wave of positive charges
that travel undiminished in strength
along the axon
The Chain of Events
1.
2.
3.
4.
Stimulus is detected
Gated Ion Channels open
Na+ rushes into the cell
Charge on cell membrane becomes
DEPOLARIZED
( -70 mv  0 mv)
5. If stimulus is strong enough (above
THRESHOLD LEVEL),
MORE Na+ GATES OPEN!!
6. Complete DEPOLARIZATION occurs!
(-70mv  0mv  +30mv)
7. Na+ channels open further down the
neuron
8. AP travels down length of neuron!
Action Potentials are
ALL OR NOTHING !!!
• IF A STIMULUS DOES NOT REACH
THRESHOLD LEVEL…. NO AP!
• IF A STIMULUS REACHES THRESHOLD
LEVEL, AP IS
GENERATED
AND TRAVELS DOWN
LENGTH OF THE AXON!!!
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