Fundamentals of the nervous system
General Organization
- CNS and PNS
- PNS subgroups
The basic units- the cells
- Neurons
- Glial cells
Neurophysiology
- Resting, graded and action potentials
Neural interactions
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Neurophysiology
inside
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Opposite electrical charges attract each other
In case negative and positive charges are
separated from each other, their coming
together liberates energy
Thus, separated opposing electrical charges
carry a potential energy
Neurophysiology
• Voltage (V)
measure of differences in electrical potential
energy generated by separated charges
• Current (I)
the flow of electrical charge between two points
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• Resistance (R)
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outside
hindrance to charge flow
inside
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Ohm’s law
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Current: ions
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outside
inside
Voltage: potential across the membrane
Resistance: membrane permeability
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Resistance: membrane permeability
outside
inside
How can ions move across the membrane?
Ion channels
1) Leak channels
2) Chemically (ligand)
– gated channels
- Can be ion-specific or
not (e.g. the
Acetylcholine receptor at
the neural-muscular
junctions is permeable to
all cations)
3) Voltage – gated channels
- Ion selective
- Gates can open (and close) at different speeds
4) Mechanically – gated channels
- Found in sensory receptors
The driving force:
the electrochemical gradient
outside
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inside
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The driving force:
the electrochemical gradient
K+
Na+
K+
Na+
In a resting state, Potassium is the key player
Potassium wants to
go out (chemical
force), but also
wants to go in
(electric force)
Potassium will diffuse via
leak channels until
equilibrium is reached
(higher concentrations
INSIDE)
Potassium wants to go out
Sodium wants to go in
K+
Na+
K+
Na+
- The neuronal membrane is much less permeable
to Na+ than to K+ . The result: Na+ stays out
- How do we keep this gradient?
The sodium/potassium pump acts to reserve
an electrical gradient
- Requires ATP
- Throwing 2 K+ in,
while throwing 3
Na+ out
The resting
membrane potential
is Negative
K+
Na+
K+
Na+
This is the resting membrane potential
But we can change it
The Membrane is Polarized
Depolarization
Making the cell less polarized
Hyperpolarization
Making the cell more polarized
This is the resting membrane potential
How can we change it?
Stimulus
How can we depolarize a cell?
Example
A chemical stimulus
Cell body
Axon
Dendrites
Sodium channels opening
leads to depolarization
-70 mV
- Generation of a graded potential (aka local)
A short-range change in a membrane potential
upon a stimulus
The graded potential is increased with a
stronger stimulus
Think about a membrane with 50 channels
Stimulating them with 4 ligand molecules or 40 will
make a difference
A graded potential can spread locally
- Cations will move towards
a negative charge
- The site next to the original
depolarization event will
also depolarize, creating
another graded potential
Membrane potential
- A Graded/local potentialA short-range change in a
membrane potential upon a stimulus
- Graded potentials spread locally but die out
Membrane potential
Who said you have to depolarize?
A stimulus can lead to hyperpolarization
How would that occur?