SMALL STEPS

Each input nudges the voltage

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EPSP

An excitatory postsynaptic potential is a small depolarization that moves the cell toward threshold

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IPSP

An inhibitory postsynaptic potential is a small shift that moves the cell away from threshold

One input is rarely enough to fire a neuron.

THE SUM

Inputs add up

Several excitatory inputs close together can reach threshold and trigger a spike. An inhibitory input pulls the voltage back.

The neuron is a tiny calculator.

TWO KINDS

Summation in time and space

Temporal Inputs that arrive close together in time add up before they fade

Spatial Inputs at different places on the cell add up at the cell body

Both Real neurons use a mix of both

Timing and location both matter.

THE DECISION

Where the spike starts

A typical neuron receives thousands of synapses. Their combined effect is read out at the start of the axon, called the axon initial segment, where voltage-gated sodium channels are packed densely. If the sum reaches threshold there, a spike starts.

The axon's first stretch is the trigger point.

CHANGING WITH USE

Synapses can strengthen

Synapses are not fixed. Repeated activity can strengthen them, a process called long-term potentiation, and glutamate receptors are central to it. This is one cellular basis of learning and memory, and a reason that signaling chemistry is worth measuring.

The strength of each connection is itself a kind of memory.

UNIT 13 STUDY COMPLETE

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Ready for the Fab Challenge?

You've seen how a neuron adds excitatory and inhibitory inputs and fires when they reach threshold.