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Chapter 5

PSY295 - Chapter 5 Notes (Communication Between Neurons).docx

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University of Toronto Mississauga
Katherine Krpan

PSY 295 CHAPTER 5: COMMUNICATION BETWEEN NEURONS Acetylcholine neurotransmitter that communicates a message to inhibit or slow Epinephrine chemical that carries an excitatory message to speed up a process Cholinergic neurons class of neurons that release Ach Norepinephrine replaces epinephrine as the excitatory neurotransmitter in mammals Noradrenergic class of neurons that release norepinephrine neurons Synaptic Cleft small space separating axon terminal and the tip of dendritic spine Postsynaptic membrane membrane on the tip of the dendritic spine Presynaptic membrane the membrane of the axon terminal Synaptic Vesicles round granules that contain the chemical neurotransmitter Storage Granules larger storage granules found in some axon terminals that hold a number of synaptic vesicles Receptors transmitter activated protein molecules that occupy the membrane that receive the transmitter substance Reuptake neurotransmitters that are brought back into the axon terminal by this process Axodendrtic synapse axon terminal of a neuron meets a dendrite Axomuscular synapse axon terminal of a neuron meets a muscle cell Axosomatic synapse axon terminal ends on a cell body Axoaxonic synapse axon terminal ends on another axon Axosynaptic synapse axon terminal ends at another terminal Axoextracellular synapses axon terminals with no specific target (release to extracellular fluid) Axosecretory synapse axon terminal synapses with a tiny blood vessel called a capillary and secrets its transmitter directly into the blood (more general effect) Dendodendrtic synapses dendrites may send messages to other dendrites Renshaw loop - a feedback circuit where an interneuron synapses back on the motor neurons cell body to inhibit it from rapid firing after a previously fired synapse Glycine neurotransmitter that plays the inhibitory role in the Renshaw loop Small-molecule transmitters synthesized and packaged for use in axon terminal; act quickly at the synapse Dopamine plays a role in coordinating movement, in attention and learning and in behaviours that are reinforcing Rate-limiting factor a factor that limits the synthesis of a chemical reaction, ex. Tyrosine hydroxylase for dopamine synthesis Serotonin amine transmitter involved in regulating mood and aggression, appetite and arousal, the perception of pain and respiration Glutamate amino acid transmitter involved in excitatory transmission in the forebrain and cerebellum Gamma-aminobutyric acid amino acid transmitter involved in inhibitory transmission in the forebrain and cerebellum Histamine amino acid transmitter involved in arousal and waking; also can cause constriction of smooth muscles, which may lead to allergies Neuropeptides multifunctional chains of amino acids that act as neurotransmitters are made through the translation of mRNA from instructions contained in the neurons DNA Met-enkephalin; Leu- enkephalin; Beta-endorphin opium and a group of related synthetic chemicals mimic the actions of these three peptide transmitters Nitric Oxide; Carbon Monoxide activate metabolic processes in cells, including those modulating the production of other neurotransmitters Ionotropic Receptors allows for the movement of charged atoms across a cell membrane when the membranes charge fluctuates Metabotropic Receptors a single protein that spans the cell membrane, does not possess a pore of its own through which ions can flow, so it must act indirectly Guanyl-nucleotide-binding proteins (G proteins) family of proteins that translates the transmitters message into biochemical activity within the cell Second Messenger carries a message to other structures within the cell Activating system coordinates wide areas of the brain to act in concert Alzheimers disease disease that takes away your memory shows losses of cholinergic neurons at autopsy Schizophrenia a behavioural disorder characterized by delusions, hallucinations, disorganized speech, blunted emotion and other symptoms that is linked to excessive DA activity PSY 295 Major Depression characterized by prolonged feelings of worthlessness and guilt, the disruption of normal eating habits, sleep disturbances and a slowing of behaviour, and frequent thoughts of suicide related to decreases in the activity of noradrenergic neurons Mania opposite of depression (excessive excitability); may be related to in activity of noradrenergic neurons Obsessive Compulsive Disorder a condition where the affected person compulsively repeats acts and has repetitive and often unpleasant thoughts related to increases in serotonergic activity Neurotransmitter Discovery: (1921) Otto Loewi identified two important chemicals that communicate inhibitory and excitatory messages respectfully, acetylcholine and epinephrine Discovered new class of chemicals that carry messages from one neuron to the next, neurotransmitters At about the same time, Walter Cannon demonstrated that cholinergic neurons and noradrenergic neurons play complementary roles in controlling many bodily functions in the autonomic nervous system Each neurotransmitter can be either excitatory or inhibitory: action is determined by the receptor with which it interacts Class of receptors for neurotransmitters are excitatory and other class is inhibitory Ex. Ach is inhibitory on organs of the ANS but excitatory on body muscles connected to the somatic nervous system Receptor subtypes can further expand the dimension of excitatory or inhibitory influence of a neurotransmitter, for example a short lasting action at one site vs. a long lasting action at another The Structure of Synapses: Figure 5.2 Chemical Synapse = shows a vivid picture of a synapse, real and pictured Three main parts of a synapse are an axon terminal, the membrane encasing the tip of an adjacent dendritic spine and the very small space separating these two structures, known as the synaptic cleft Parts of the postsynaptic membrane often consists largely of protein molecules specialized for receiving chemical messages Parts of the presynaptic membrane consists largely of protein molecules that serve as channels and pumps and receptor sites Mitochondria are also found in the axon terminal along with other specialized structures including, synaptic vesicles, tubules that give the terminal button its shape and sometimes storage granules ..Smile Steps in Neurotransmission: 4 basic steps that involve different chemical reactions 1. Synthesis the transmitter is created by the cells DNA or its building blocks are imported and stored in the axon terminal 2. Release the transmitter is transported to the presynaptic membrane and released in response to an action potential 3. Receptor Action the transmitter traverses the synaptic cleft and interacts with receptors on the membrane of the target cell 4. Inactivation the transmitter is either drawn back into the axon of the presynaptic cell or breaks down in the synaptic cleft. Otherwise, it would continue to work indefinitely Step 1 Transmitter Synthesis and Storage: Derived in two basic ways: Synthesized as proteins in the cell body according to DNA instructions; packaged on the Golgi bodies and transported to the axon terminal Slower actingPSY 295 Synthesized in the axon terminal from building blocks derived from food; transporter proteins absorb these precursor chemicals from the blood supply Quicker Synaptic vesicles are stored in three ways: Collected in storage granules Attached to the microfilaments in the terminal button Attached to the presynaptic membrane; replaced quickly by other vesicles after release Step 2 Neurotransmitter Release: Action potential triggers the release of a neurotransmitter from a presynaptic membrane rich in voltage sensitive calcium channels (surrounding extracellular fluid is rich in Ca+ ions; check out figure 5.4 Pg 115) Incoming calcium ions bind to the chemical calmodulin that participates in 2 chemical reactions: Releases vesicles bound to the presynaptic membrane through exocytosis Releases vesicles bound to filaments in the axon terminal replace the vesicles that were formally bound to the presynaptic membrane Step 3 Activation of Receptor Sites: Neurotransmitters bind to specialized, transmitter-activated protein molecules, known as receptors which 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 inhibitory effect or influence other functions of the postsynaptic neuron Creates new syna
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