Hockenbury Discovering Psychology 5th txtbk

The Neuron51Neurotransmitter molecules that are not reabsorbed or that remain attached tothe receptor site are broken down or destroyed by enzymes. As you’ll see in thenext section, certain drugs can interfere with both of these processes, prolongingthe presence of the neurotransmitter in the synaptic gap.The number of neurotransmitters that a neuron can manufacture varies. Someneurons produce only one type of neurotransmitter, whereas others manufacturethree or more. Although estimates vary, scientists have identified more than100 different compounds that function as neurotransmitters in the brain.Each type of neurotransmitter has a chemically distinct shape. Like a key ina lock, a neurotransmitter’s shape must precisely match that of a receptor siteon the postsynaptic neuron for the neurotransmitter to affect that neuron. Keepin mind that the postsynaptic neuron can have many differently shaped receptorsites on its dendrites and other surfaces. Thus, a given neuron may be ableto receive several different neurotransmitters. The distinctive shapes of neurotransmittersand their receptor sites are shown schematically in Figure 2.5.NEAChExcitatory and Inhibitory MessagesA neurotransmitter communicates either an excitatory or an inhibitory message toa postsynaptic neuron. An excitatory message increases the likelihood that thepostsynaptic neuron will activate and generate an action potential. Conversely, aninhibitory message decreases the likelihood that the postsynaptic neuron will activate.If a postsynaptic neuron receives an excitatory and an inhibitory message simultaneously,the two messages cancel each other out.It’s important to note that the effect of any particular neurotransmitter dependson the particular receptor to which it binds. So, the same neurotransmitter can havean inhibitory effect on one neuron and an excitatory effect on another.Depending on the number and kind of neurotransmitter chemicals that arebound to the receptor sites on the adjoining neurons, the postsynaptic neurons aremore or less likely to activate. If the net result is a sufficient number of excitatorymessages, the postsynaptic neuron depolarizes, generates an action potential, andreleases its own neurotransmitters.When released by a presynaptic neuron, neurotransmitter chemicals cross hundreds,even thousands, of synaptic gaps and affect the intertwined dendrites ofadjacent neurons. Because the receiving neuron can have thousands of dendritesthat intertwine with the axon terminals of many presynaptic neurons, the numberof potential synaptic interconnections between neurons is truly mind-boggling.Each neuron in the brain communicates directly with an average of 1,000 otherneurons (Hyman, 2005). However, some specialized neurons have as many as100,000 connections with other neurons. Thus, in your brain alone, there are upto 100 trillion synaptic interconnections.Figure 2.5 Neurotransmitter and ReceptorSite Shapes Each neurotransmitter hasa chemically distinct shape. Like a key in alock, a neurotransmitter must perfectly fitthe receptor site on the receiving neuronfor its message to be communicated. Inthis figure, NE is the abbreviation for theneurotransmitter norepinephrine and AChis the abbreviation for acetylcholine.Neurotransmitters and Their EffectsYour ability to perceive, feel, think, move, act, and react depends on the delicate balanceof neurotransmitters in your nervous system. Too much or too little of a givenneurotransmitter can have devastating effects. Yet neurotransmitters are present inonly minuscule amounts in the human body. If you imagine trying to detect a pinchof salt dissolved in an Olympic-sized swimming pool, you will have some idea of theinfinitesimal amounts of neurotransmitters present in brain tissue.In this section, you’ll see that researchers have linked abnormal levels of specificneurotransmitters to various physical and behavioral problems (see Table 2.1 on thenext page). Never theless, it’s important to remember that any connection betweena particular neurotransmitter and a particular effect is not a simple one-to-one relationship.Many behaviors are the result of the complex interaction of different neurotransmitters.Further, neurotransmitters sometimes have different effects in differentareas of the brain.