Hockenbury Discovering Psychology 5th txtbk

50 CHAPTER 2 Neuroscience and Behavioraxon terminalsBranches at the end of the axon that containtiny pouches, or sacs, called synapticvesicles.synaptic vesicles(sin-AP-tick VESS-ick-ulls) Tiny pouches orsacs in the axon terminals that containchemicals called neurotransmitters.neurotransmittersChemical messengers manufactured by aneuron.synaptic transmission(sin-AP-tick) The process through whichneurotransmitters are released by oneneuron, cross the synaptic gap, and affectadjoining neurons.reuptakeThe process by which neurotransmitter moleculesdetach from a postsynaptic neuronand are reabsorbed by a presynaptic neuronso they can be recycled and used again.In general terms, chemical communication occurs when the presynaptic neuroncreates a chemical substance that diffuses across the synaptic gap and is detected bythe postsynaptic neuron. This one-way communication process between one neuronand another has many important implications for human behavior.More specifically, here’s how chemical communication takes place between neurons.As you’ve seen, when the presynaptic neuron is activated, it generates an actionpotential that travels to the end of the axon. At the end of the axon are several smallbranches called axon terminals. Floating in the interior fluid of the axon terminalsare tiny sacs called synaptic vesicles (see Figure 2.4). The synaptic vesicles hold specialchemical messengers manufactured by the neuron, called neurotransmitters.When the action potential reaches the axon terminals, some of the synaptic vesicles“dock” on the axon terminal membrane, then release their neurotransmitters into thesynaptic gap. These chemical messengers cross the synaptic gap and attach to receptorsites on the dendrites and other surfaces of the surrounding neurons. This journeyacross the synaptic gap is slower than electrical transmission but is still extremely rapid;it takes just a few millionths of a second. The entire process of transmitting informationat the synapse is called synaptic transmission.What happens to the neurotransmitter molecules after they’ve attached to the receptorsites of the postsynaptic neuron? Most often, they detach from the receptorand are reabsorbed by the presynaptic neuron so they can be recycled and used again.This process is called reuptake. Reuptake also occurs with many of the neurotransmittersthat failed to attach to a receptor and are left floating in the synaptic gap.(a) Path of Neural Impulse1. Action potential travels alongaxon of sending neuron.(b) Enlarged View of a SynapseAxon terminal ofpresynaptic neuronDendrite ofreceiving neuron2. Action potential triggersrelease of neurotransmittersby synaptic vesicles.(c) Synaptic TransmissionSynaptic gap betweenpresynaptic andpostsynaptic neuronFigure 2.4 Communication BetweenNeurons: The Process of Synaptic TransmissionAs you follow the steps in thisprogressive graphic, you can trace the sequenceof synaptic transmission in whichneurotransmitters are released by thesending, or presynaptic, neuron, cross thetiny fluid-filled space called the synapticgap, and attach to receptor sites on thereceiving, or postsynaptic, neuron.Na+(sodium ions)Synaptic vesiclesNeurotransmittersReceptor sites on the dendrite ofpostsynaptic neuron3. Neurotransmitters crosssynaptic gap and bind to thecorrectly shaped receptor sites onthe postsynaptic neuron.