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 voltage-gated 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 Inactive of Na+ channel; Opening K+ channel The opening of Na+ channel Closing of Na+ & K+ channels The opening of leakage channel

Review Questions: Name 3 types of neurons. What is a membrane potential? What ion channels/pumps are responsible for keeping the resting membrane potential? What ion channels/pumps are responsible for generating the action potential? By which ways are the neurons repolarized? What is meant by the absolute refractory period?

Name 1,2,3,4,5,6 and 7 What is “A” & “B” ?