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From the simple to the complex

Anatomy by Level of Organization

The “Coming Out” of the Electrical Synapse

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In a chemical synapse, a nerve impulse can travel in only one direction. In contrast, in an electrical synapse, the impulse travels in both directions.

Also, across a chemical synapse, the impulse is transmitted with a 0.5-millisecond delay, while across an electrical synapse, the delay is almost non-existent.


The term “synapse” designates the point where the axon of one neuron connects to a dendrite of another. This word comes from the Greek syn (together) and haptein (join). In the animal kingdom, neurons can be connected to each other in two very different ways:
  • by an electrical synapse, in which the two cells touch and are connected by tiny holes, which lets the nerve impulse pass directly from one neuron to the other; or

  • by a chemical synapse, where the two cells do not touch and the nerve impulse needs particular molecules to bridge the gap between them.

Chemical synapses are slower than electrical ones but are also far more flexible. This valuable flexibility is the foundation of all learning.

On either side of the synapse, the axon and dendrite have evolved specialized molecules to perform their respective tasks.


Neurotransmitters are chemical molecules that “ferry” nerve impulses across the synapse from one neuron to the next.

Each type of neurotransmitter has a molecular form that lets it bind to the right site on the second neuron to produce its particular effect.

The neurotransmitter thus acts somewhat like a key. If it is the right shape for the next neuron (shown here as a lock), it will produce an effect on that neuron.

Neurotransmitters are divided into two categories according to the effect that they have on the second neuron once they are released into the synaptic gap.

Neurotransmitters that help this neuron to propagate the nerve impulse are classified as excitatory neurotransmitters.

Neurotransmitters that reduce the likelihood of this neuron’s propagating the impulse are called inhibitory neurotransmitters.

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