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Synaptic Transmission

Signals must travel from one neuron to another, but neurons are separated by a space called the synapse, or synaptic cleft. The nerve impulse cannot “jump” across the synapse, so a different mechanism must be used to propagate the signal from the axon of the presynaptic cell to a dendrite of the postsynaptic cell. This is done using neurotransmitters, chemicals that can diffuse across the synapse and stimulate the postsynaptic cells by interacting with specialized receptors located there. Neurotransmitters are synthesized and stored in tiny vesicles near the end of each axon; as the nerve impulse approaches this location, gated channels for calcium (Ca++) open, allowing calcium ions to diffuse into the cell. This event causes the storage vesicles to fuse with the plasma membrane, and the neurotransmitter molecules are released into the synaptic space. When they couple with their corresponding receptors, an action potential may be triggered in the postsynaptic cell, stimulating a new nerve impulse (see Figure 14.1). After neurotransmitter molecules exert their action, they must be quickly removed as the neuron gets ready to receive another signal. Sometimes they are enzymatically broken down, and in other cases they are reabsorbed by the presynaptic axon to be “recycled” and used again later. To sum up, the propagation of the nerve impulse and subsequent stimulation of adjacent neurons by neurotransmitters explains how electrical signals travel through nerves.

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