Neurons, Synapses, and Signaling

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6

Most synapses are chemical synapses.

Slide 38

The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal.

The action potential causes the release of the neurotransmitter.

The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell.

Slide 39

Chemical synapse

Voltage-gated

Ca2+ channel

Ca2+

1

2

3

4

Synaptic

cleft

Ligand-gated

ion channels

Postsynaptic

membrane

Presynaptic

membrane

Synaptic vesicles

containing

neurotransmitter

5

6

K+

Na+

Slide 40

Generation of Postsynaptic Potentials

Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell.

Neurotransmitter binding causes ion channels to open, generating a postsynaptic potential.

Slide 41

Postsynaptic potentials fall into two categories:

Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the membrane potential toward threshold.

Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold.

Slide 42

After release, the neurotransmitter

May diffuse out of the synaptic cleft

May be taken up by surrounding cells

May be degraded by enzymes

Slide 43

Summation of Postsynaptic Potentials

Unlike action potentials, postsynaptic potentials are graded and do not regenerate.

Most neurons have many synapses on their dendrites and cell body.

A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron.

If two EPSPs are produced in rapid succession, an effect called temporal summation occurs.

Slide 44

Summation of postsynaptic potentials

Terminal branch

of presynaptic

neuron

E1

E2

I

Postsynaptic

neuron

Threshold of axon of

postsynaptic neuron

Resting

potential

E1

E1

0

–70

Membrane potential (mV)

(a) Subthreshold, no

summation

(b) Temporal summation

E1

E1

Action

potential

I

Axon

hillock

E1

E2

E2

E1

I

Action

potential

E1 + E2

(c) Spatial summation

I

E1

E1 + I

(d) Spatial summation