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5. Communication Between Neurons.docx

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Department
Psychology
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PSYC 3530
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Guy Proulx

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5. Communication Between Neurons Friday, September 20, 2013 12:42 PM Neurotransmitter Discovery  Otto Loewi identified the chemical that communicates a message to inhibit, or slow, a frog's heart rate as acetylcholine (ACh) and the chemical that carries an excitatory message to speed up frog heart rate as epinephrine (EP, or adrenaline) o Discovered neurotransmitters, the class of chemicals that carry messages from one neuron to the next  Cholinergic neurons-- neurons that release Ach  Norepinephrine (NE)-- also called noradrenaline (NA)-- replaces EP in mammals as the excitatory neurotransmitter in noradrenergic neurons  Each neurotransmitter can be either excitatory or inhibitory: its action is determined by the receptor with which it interacts   The Structure of Synapses   3 main parts of the synapse: 1. An axon terminal (the presynaptic membrane)  Contains synaptic vesicles that contain the chemical neurotransmitter  In some axon terminals, larger storage granules hold a number of synaptic vesicles 2. The membrane encasing the tip of an adjacent dendritic spine (the postsynaptic membrane) 3. The very small space separating these two structures (the synaptic cleft) o o Steps in Neurotransmission 1. During synthesis, either the transmitter is created by the cell's DNA or its building blocks are imported and stored in the axon terminal where they are synthesized o This divides neurotransmitters into 2 large classes: 1. A quicker-acting class derived from nutrient building blocks 2. A slower-acting class derived from DNA o Depending on the type of neurotransmitter they house, synaptic vesicles are stored in 3 ways: 1. Some are collected in storage granules 2. Others are attached to the microfilaments in the terminal button 3. Others are attached to the presynaptic membrane, ready to release a neurotransmitter into the synaptic cleft  When a vesicle is emptied from the presynaptic membrane, other vesicles move to take its place so that they, too, are ready to release their contents when needed  2. During release, the transmitter is transported to the presynaptic membrane and released in response to an action potential o The arrival of the action potential opens voltage-sensitive calcium channels on the presynaptic membrane, allowing an influx of calcium into the axon terminal o The incoming calcium ions bind to a chemical called calmodulin, forming a molecular complex that participates in two chemical reactions: one of them releases vesicles bound to the presynaptic membrane, and the other releases vesicles bound to filaments in the axon terminal 3. During receptor action, the transmitter traverses the synaptic cleft and interacts with receptors on the membrane of the target cell o Receptors-- receive the neurotransmitter substance o The type of neurotransmitter and the kind of receptors on the postsynaptic membrane determine whether the neurotransmitter  depolarizes the postsynaptic membrane and so has an excitatory action;  hyperpolarizes the postsynaptic membrane and so has an inhibitory action;  initiates other chemical reaction sequences that can modulate either the excitatory or the inhibitory effect or influence other functions of the postsynaptic neuron;  creates new synapses; or  brings about other changes in the cell. o Autoreceptors-- receptors on the presynaptic membrane that neurotransmitters may act upon to influence the cell that just released the transmitters o Synapses correspond to behavioral experience in that synapses that are used more often (e.g., a synapse that contracts an exercised muscle) develop more calcium channels and synaptic vesicles than do synapses that receive little use 4. During inactivation, the transmitter either is drawn back into the axon of the presynaptic cell or breaks down in the synaptic cleft. Otherwise, it would continue to work indefinitely. o Deactivation of a neurotransmitter takes place in at least 4 ways: It may…  Diffuse away from the synapse  Be degraded by enzymes in the synaptic cleft  Be brought back into the axon terminal in a process called reuptake  Be taken up by neighboring glial cells Types of Synapses  Axomuscular synapse-- axon terminal meets muscle  Capillary-- tiny blood vessel  Despite the versatility of synapses, they convey only 2 types of messages: excitatory (Type I synapse) or inhibitory (Type II synapse) o i.e., it either increases or decreases the probability that the cell in which it comes into contact will produce an action potential o Varieties of Neurotransmitters  There may be 100 or more neurotransmitters  4 experimental criteria used to identify neurotransmitters: 1. The chemical must be synthesized in the neuron or otherwise be present in it 2. When the neuron is active, the chemical must be released and produce a response in some target cell 3. The same response (receptor action) must be obtained when the chemical is experimentally placed on the target 4. A mechanism must exist for deactivating or removing the chemical from its site of action after its work is done   A chemical that is suspected of being a neurotransmitter but has not yet met all the criteria for proof is called a putative
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