Neurons, Synapses, and Signaling

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During the refractory period after an action potential, a second action potential cannot be initiated. This ensures that an impulse moves along the axon in one direction only.

The refractory period is a result of a temporary inactivation of the Na+ channels.

The refractory period is a period of “normal” repolarization when the Na+ K+ pump restores the membrane to its original polarized condition.

Slide 31

Conduction of Action Potentials

An action potential can travel long distances by regenerating itself along the axon.

At the site where the action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membrane.

Inactivated Na+ channels behind the zone of depolarization prevent the action potential from traveling backwards. Action potentials travel in only one direction: toward the synaptic terminals.

Slide 32

Conduction of an

Action Potential

Signal

Transmission

Axon

Plasma

membrane

Cytosol

Action

potential

Na+

Action

potential

Na+

K+

K+

Action

potential

K+

K+

Na+

Slide 33

Conduction Speed

The speed of an action potential increases with the axon’s diameter.

In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase.

Myelin sheaths are made by glia— oligodendrocytes in the CNS and Schwann cells in the PNS.

Slide 34

Schwann cells and the myelin sheath

Axon

Myelin sheath

Schwann

cell

Nodes of

Ranvier

Schwann

cell

Nucleus of

Schwann cell

Node of Ranvier

Layers of myelin

Axon

Slide 35

Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na+ channels are found.

Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction.

Slide 36

Saltatory conduction

Cell body

Schwann cell

Depolarized region

(node of Ranvier)

Myelin

sheath

Axon

Slide 37

Neurons communicate with other cells at synapses

At electrical synapses, the electrical current flows from one neuron to another.

At chemical synapses, a chemical neurotransmitter carries information across the gap junction = synapse.