PS231: Neurobiology III

Where neuron communication begins

A neuron is stimulated and/or inhibited by many other neurons at the dendrites

Excitatory inputs

cause the cell to fire (may cause AP, depolarized), generates excitatory postsynaptic potentials

releases neurotransmitters that depolarize the postsynaptic membrane

Inhibition

makes neurons less likely to fire an AP, generates inhibitory postsynaptic potentials (IPSPs).

releases neurotransmitters that hyperpolarize the postsynaptic membrane

Integration of excitation and inhibition

The neuron simultaneously “adds up” all of the excitatory inputs and “subtracts” all the

inhibitory inputs; if there’s enough excitation to reach “threshold,” (~ -55 mV), the

neuron generates an AP

The AP starts at the axon hillock and travels down the axon to cause neurotransmitter release at the synaptic terminal

Spatial summation

Excitatory potentials from multiple presynaptic inputs arrive at the same time to stimulate a neuron, causing an action potential in the post-synaptic neuron if the stimulation is sufficient for the neuron to reach “threshold” (VGNa+ channels open)

Temporal summation

Excitatory potentials from the same presynaptic input arrive close in time to one another (“train” of APs), and build on one another to potentially cause an action potential in the post-synaptic neuron

Threshold

Critical level of depolarization that must be reached in order to trigger an AP (-55 mV)

Rising phase

Rapid depolarization of the membrane (membrane becomes more positive), Na+ influx

Overshoot

The inside of the neuron is positively charged with respect to the outside (height never changes!)

Falling phase

Rapid repolarization of the membrane (membrane becomes more negative), K+ efflux

Undershoot

The inside of the cell is briefly more negative than the resting potential (RMP)

Absolute refractory period

A period of time (1 ms) where another AP cannot be generated

Generating multiple action potentials

• If a large enough electrical current is injected into a neuron, the membrane is

  depolarized sufficiently to fire multiple (“a train”) action potentials (right)

• In the nervous system, the intensity/strength of a signal is often encoded by firing frequency

• You don’t get bigger action potentials, you get more of them!

Ionic basis of the action potential (1)

At rest, the membrane is only permeable to K+ (potassium “leak” channels) Vm = EK
(g = conductance = ions flowing through channels)

Ionic basis of the action potential (2)

When the membrane becomes sufficiently depolarized (to “threshold”), voltage-gated Na+ channels in the axon hillock open, allowing Na+ to rush in, down its concentration gradient

Vm approaches ENa

Ionic basis of the action potential (3)

Soon thereafter, Na+ channels inactivate and because the membrane is highly depolarized, there is a strong driving force for K+ efflux via voltage-gated K+ channels (K+ flows out)

Depolarization

A shift in membrane potential toward a more positive value.

Occurs when Na⁺ ions enter the neuron, reducing the internal negativity.

If depolarization reaches the threshold (~−55 mV), it triggers an action potential.

Hyperpolarization

A shift toward a more negative membrane potential.

Happens when K⁺ ions exit or Cl⁻ ions enter, making the inside more negative than resting potential.

Makes the neuron less likely to fire an action potential.

Are There “Big” and “Little” Action Potentials?

No. Action potentials are all-or-none events.

Once threshold is reached, the AP is uniform in size and duration.

Frequency coding

Stronger stimuli cause more frequent action potentials.

Like turning up the volume—not by making each spike louder, but by sending more spikes per second.

Role of voltage-gated Na+ channels

Crucial for initiating the action potential.

Open when membrane reaches threshold (~−55 mV).

Allow rapid Na⁺ influx, causing depolarization.

Threshold concept

The minimum depolarization needed to open voltage-gated Na⁺ channels.

Typically around −55 mV.

Below this, no AP occurs; above it, AP fires fully.