The Neuron
Basic Constitution
• The major features: soma (the cell body),
dendrites (the receiving end) and axon (the
outgoing end)
• Neurons differ in size, shape and
physiological properties
Bipolar neuron
e.g., retina, olfactory bulb
Unipolar neuron
Invertebrates
Multipolar neuron
pyramidal cell
Neuronal State: Membrane Potential
• Membrane potential: the difference between electric
potentials within the cell body and its surrounding.
Speciality of Membrane
• Permeability of the membrane
• Ions types: Potassium, Sodium, Calcium and Chloride
• Ion channel types:
–
–
–
–
–
–
–
Leakage channel: always open
Voltage-gated channel: sensitive to the size of membrane potential
Neurotransmitter -gated channel: sensitive to the binding of
neurotransmitters
Ion bump : generating concentration difference for ions within and
outside the cell
And others
Resting potential
•Ion pump generates concentration difference of ions;
Under natural force, ions diffuse from high to low concentrations
•Positive charged ions (cations) leak through leakage channels,
but the membrane is not permeable to the anions binding to cations;
A electrical potential is generated.
•A balance is reached when the electrical force matches diffusion force
Neuronal Interaction
• Chemical synapses and neurotransmitters
• Synapses: the contact sites between
neurons
•Pre- and post- synaptic neurons
•Action potential triggers the release of
neurotransmitter
•Neurotransmitters drift across the synaptic
cleft
•Neurotransmitter-gated channels open,
generating electrical current (or postsynaptic
potential (PSP))
•Dependent on the sign of PSP, synapses are
clarified as excitatory and inhibitory ones.
Action potential:
•Membrane potential increases due to external input.
•When its value is larger than a threshold,
voltage-gated channels are open, ions influx and efflux the cell.
•Voltage-gated channels are closed very quick, leading to
an electrical pulse.
Summary of Neural Signaling Mechanism
•Resting potential
-Special permeability of the membrane for ions
-Ion bump account for the concentration gradient of ions
-Leakage channel only for cations
-A balance between electric and diffusion force
•Basic synaptic mechanism
-Action potential triggers the release of neurotransmitter at
the axon terminal of pre-synaptic neuron
-Neurotransmitter drift across the synaptic cleft
-Neurotransmitter opens the neurotransmitter-gated channels, generating
PSP at the post-synaptic neuron
•The mechanism for generating action potential?
-The increase of membrane potential triggers the opening of
voltage-gated channels
The Profile of Action Potential
•
Rising phase (Depolarization):
A sharp increase of the membrane potential to a positive value
•
Falling phase (de-polarization):
Followed the rising phase quickly, a sharp decrease in the
membrane potential, which even overshoot the resting potential
•
Returning phase:
The membrane potential gradually returns to the resting value
The minimal mechanism
• Rising phase (depolarization):
– The membrane voltage change triggers the opening of voltagegated sodium channels
– The influx of sodium depolarizes the neuron
(up to V=+35mV)
• Falling phase (de-polarization):
– The sodium channels become inactive at around 1ms after their
opening
– Meanwhile, the potassium channels open—the efflux of
potassium ions overshoots the resting potential (down to V=80mV).
• Returning phase
– The hyper-polarization causes the close of potassium and sodium
channels
– Ion bump may be called to retain the concentration gradients of
potassium and sodium ions
The status of channels
The opening of
Na+ channel
Inactive of Na+ channel;
Opening K+ channel
Closing of Na+ &
K+ channels
The opening of
leakage channel
Review Questions:
1. Name 3 types of neurons.
2. What is a membrane potential?
3. What ion channels/pumps are responsible for keeping
the resting membrane potential?
4. What ion channels/pumps are responsible for generating
the action potential?
5. By which ways are the neurons repolarized?
6. What is meant by the absolute refractory period?
Name 1,2,3,4,5,6 and 7
What is “A” & “B” ?