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17.2 How neurons communicate (Page 3/20)

Art connection

The formation of an action potential can be divided into five steps: (1) A stimulus from a sensory cell or another neuron causes the target cell to depolarize toward the threshold potential. (2) If the threshold of excitation is reached, all Na ⁺ channels open and the membrane depolarizes. (3) At the peak action potential, K ⁺ channels open and K ⁺ begins to leave the cell. At the same time, Na ⁺ channels close. (4) The membrane becomes hyperpolarized as K ⁺ ions continue to leave the cell. The hyperpolarized membrane is in a refractory period and cannot fire. (5) The K ⁺ channels close and the Na ⁺ /K ⁺ transporter restores the resting potential.

The action potential is conducted down the axon as the axon membrane depolarizes, then repolarizes.

Myelin and the propagation of the action potential

For an action potential to communicate information to another neuron, it must travel along the axon and reach the axon terminals where it can initiate neurotransmitter release. The speed of conduction of an action potential along an axon is influenced by both the diameter of the axon and the axon’s resistance to ion leakage. Myelin acts as an insulator that prevents current from leaving the axon; this increases the speed of action potential conduction. In demyelinating diseases like multiple sclerosis, action potential conduction slows because ions leak from previously insulated axon areas. The nodes of Ranvier, illustrated in [link] are gaps in the myelin sheath along the axon. These unmyelinated spaces are about one micrometer long and contain voltage gated Na ⁺ and K ⁺ channels. Flow of ions through these channels, particularly the Na ⁺ channels, regenerates the action potential over and over again along the axon. This ‘jumping’ of the action potential from one node to the next is called saltatory conduction . If nodes of Ranvier were not present along an axon, the action potential would propagate very slowly since Na ⁺ and K ⁺ channels would have to continuously regenerate action potentials at every point along the axon instead of at specific points. Nodes of Ranvier also save energy for the neuron since the channels only need to be present at the nodes and not along the entire axon.

Nodes of Ranvier are gaps in myelin coverage along axons. Nodes contain voltage-gated K ⁺ and Na ⁺ channels. Action potentials travel down the axon by jumping from one node to the next.

Synaptic transmission

The synapse or “gap” is the place where information is transmitted from one neuron to another. Synapses usually form between axon terminals and dendritic spines, but this is not universally true. There are also axon-to-axon, dendrite-to-dendrite, and axon-to-cell body synapses. The neuron transmitting the signal is called the presynaptic neuron, and the neuron receiving the signal is called the postsynaptic neuron. Note that these designations are relative to a particular synapse—most neurons are both presynaptic and postsynaptic. There are two types of synapses: chemical and electrical.

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MCQ 2 FlashCards 2 Terms 12

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OpenStax, Human biology. OpenStax CNX. Dec 01, 2015 Download for free at http://legacy.cnx.org/content/col11903/1.3

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