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Chapter 5

PSYB65 Chapter 5.doc

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Zachariah Campbell

PSYB65 – Chapter 5 Neurotransmitter Discovery: - Otto Loewi – identified the chemical that communicates a message to inhibit or slow a frog’s heart as acetylcholine (ACh) o The chemical that carries an excitatory message to speed up frog heart rate is epinephrine (EP) - Groups of neurons that release a chemical neurotransmitter of a certain type are named after that neurotransmitter o Ex. neurons that release ACh are called acetylcholine neurons or cholinergic neurons  Neurons that release EP are called epinephrine neurons. - In mammals, a closely related neurotransmitter, norepinephrine (NE) also called noradrenaline, replaces EP as the excitatory neurotransmitter in noradrenergic neurons - Walter Cannon – demonstrated that cholinergic neurons and noradrenergic neurons play complementary roles in controlling many bodily functions in the ANS o They constitute chemical systems that produce widespread and coordinated influences on behaviour o Coined the phrases:  Rest and digest – summarizing the collective inhibitory actions of acetylcholine neurons in the parasympathetic ANS  Fight or flight – summarizing the collective excitatory actions of noreppinephrine neurons in the sympathetic ANS - Each neurotransmitter can be either excitatory or inhibitory, its action is determined by the receptor with which it interacts o Ex. Acetylcholine is inhibitory by means of a receptor on organs of the ANS, but excitatory on body muscles connected to the SNS The Structure of Synapses: - The 3 main parts of a synapse o Axon terminal o Synaptic cleft – small space separating these two structures o Postsynaptic membrane – membrane encasing the tip of an adjacent dendritic spine  Presynaptic membrane – membrane of the axon terminal, consist largely of protein molecules • The protein molecules mostly serve as channels, pumps and receptor sites - Within the axon terminal are many other specialized structures, including: o Mitochondria o Synaptic vesicles – round granules that contain the chemical neurotransmitter o Tubules – that give the terminal button its shape - Storage granules – in some axon terminals, hold a number of synaptic vesicles Steps in Neurotransmission: - Information is transmitted across a synapse in 4 basic steps, each step requires a different chemical reaction: o During synthesis – either the transmitter is created by the cell’s DNA or its building blocks are imported and stored in the axon terminal o During release – the transmitter is transported to the presynaptic membrane and released in response to an action potential o During receptor action – the transmitter traverses the synaptic cleft and interacts with receptors on the membrane of the target cell o During inactivation – the transmitter either is drawn back into the axon of the presynaptic cell or breaks down in the synaptic clef Step 1: Transmitter Synthesis and Storage: - Neurotransmitters are derived in 2 basic ways o Some are synthesized as proteins in the cell body according to instructions contained in the neuron’s DNA  The neurotransmitters are packaged in membranes on the Golgi bodies and transported on microtubules to the axon terminal  Messenger RNA may be transported to the synapse, where it directs the synthesis of a transmitter within the axon terminal o Other neurotransmitters are synthesized in the axon terminal from building blocks derived from food  Transporter proteins in the cell membrane absorb these precursor chemicals from the blood supply  Mitochondria in the axon terminal provide the energy for synthesizing neurotransmitters from their precursor chemicals - These 2 basic modes of synthesis divided most neurotransmitter substances into 2 large classes: o A quicker-acting class derived from nutrient building blocks o A slower-acting class of proteins derived from DNA - Synaptic vesicles are stored in 3 ways: o Some are collected in storage granules o Others are attached to the microfilaments in the terminal button o 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 they can release their contents when needed Step 2: Neurotransmitter Release: - The action potential triggers the release of a neurotransmitter from a presynaptic membrane rich in voltage-sensitive calcium channels o The surrounding extracellular fluid is rich in calcium ions (Ca2+) - The arrival of the action potential opens the voltage-sensitive calcium channels, 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 2 chemical reactions: • One of them releases vesicles bound to the presynaptic membrane • Other releases vesicles bound to filaments in the axon terminal - The vesicles released from the presynaptic membrane empty their contents into the synaptic cleft through the process of exocytosis o The membrane surrounding the transmitter substances fuses with the cell membrane o The vesicles that were bound to the filaments are then transported to the membrane to replace the vesicles that were just released there Step 3: Activation of Receptor Sites: - A neurotransmitter released from the presynaptic membrane diffuses across the synaptic cleft and binds to specialized protein molecules in the postsynaptic membrane o These transmitter-activated protein molecules are called receptors because the sites that they occupy on the membrane receive the transmitter substance - The type of neurotransmitter and the kind of receptors on the postsynaptic membrane determine whether the neurotransmitter: o Depolarizes the postsynaptic membrane and so has an excitatory action o Hyperpolarizes the postsynaptic membrane and so has an inhibitory action o Initiates other chemical reaction sequences that can modulate either the excitatory or the inhibitory effect  Or influence other functions of the postsynaptic neuron o Creates new synapses o Brings about other changes in the cell - A neurotransmitter may interact with autoreceptors on its own presynaptic membrane o It may influence the cell that just released it o Autoreceptors receive messages from their own axon terminal - The amount of neurotransmitter released from the presynaptic membrane in response to a single ac
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