Psychobiology Chapter 2
Psychobiology Chapter 2 PSYC2070
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This 7 page Class Notes was uploaded by Karly Lord on Thursday January 21, 2016. The Class Notes belongs to PSYC2070 at University of Cincinnati taught by Dr. Kenneth King in Fall 2016. Since its upload, it has received 69 views. For similar materials see Psychobiology in Psychlogy at University of Cincinnati.
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Date Created: 01/21/16
Chapter 2 Thursday, January 21, 26:29 PM The Concept of the Synapse Synapse -‐ term was introduced by Charles Scott Sherrington in 1906. He used it to the describe a specialized gap between neurons that is used as a point of communication. Properties of Synapses • Reflexes -‐ automatic muscular responses to stimuli ○ Reflex arc -‐the circuit from sensory neuron to muscle response -‐ a sensory neuron excites a second neuron which excites a motor neuron which excites a muscle ○ Requires communication between neurons • Sherrington's observed properties of reflexes that suggest special processes at the gap between neurons ○ Reflexes are slower than conduction along an axon ○ Several weak stimuli presented at nearby places or times produce a stronger reflex than one stimulus alone does ○ When one set of muscles becomes excited, a different set becomes relaxed Speed of a reflex and delayed transmission at the synapse • There is a delay between being pinched and the leg flexing ○ An impulse has to travel up an axon from the skin receptor to the spinal cord and back down to the leg muscle ○ The delay occurs when one neuron communications with another Temporal summation • A cumulative effect of repeated stimuli within a brief time ○ A light pinch does not evoke a reflex, but a few rapid repeated pinches did -‐ a single pinch didn’t reach the threshold for excitation for the next neuron • Presynaptic neuron -‐ the neuron that delivers transmission • Postsynaptic neuron -‐ the neuron that receives it ○ The excitation from in the postsynaptic neuron decays over time but it can combine with a second excitation that follows it quickly ○ Each excitation adds together until the combination exceeds the threshold of the neuron, producing an action potential • A graded depolarization-‐ excitatory postsynaptic potential (EPSP) ○ Results from a flow of sodium ions into the neuron ○ Graded potentials, unlike action potentials, can be depolarizations or can combine with a second excitation that follows it quickly ○ Each excitation adds together until the combination exceeds the threshold of the neuron, producing an action potential • A graded depolarization-‐ excitatory postsynaptic potential (EPSP) ○ Results from a flow of sodium ions into the neuron ○ Graded potentials, unlike action potentials, can be depolarizations or hyperpolarizations § Depolarization -‐ excites another neuron § Hyperpolarization -‐ inhibits another neuron ○ Excites Spatial summation • Combination of effects of activity from two or more synapses onto a single neuron ○ Synaptic inputs from separate locations combine their effects on a neuron -‐pinching in two different places at the same time can produce a reflex while either of them on their own would not have • Temporal and spatial summation usually occu- neuron might receive input from several axons in succession Inhibitory synapses • Sherrington pinched one leg of a dog and that leg's flexor muscles contracted (drew extremity toward the body) while the other 3 legs' extensor muscles contracted (move extremity away from the body) ○ An interneuron sends messages to inhibit the extensor muscles in the leg that was pinched and the flexor muscles in the other three legs • Hyperpolarization of a membrane-‐ inhibitory postsynaptic potential (IPSP) ○ Resembles an ESPS ○ Opens gates for potassium ions or chloride ions to leave the cell ○ Inhibits Relationship among EPSP, IPSP, and Action Potentials • EPSPs and IPSPs compete with one another ○ Balance increases or decreases the neuron's frequency of action potentials ○ IPSP inhibits action potentials while EPSP excites them Chemical Events at the Synapse The Discovery of Chemical Transmission at Synapses • T.R. Elliot in 1905 reported that applying adrenaline directly to the surface of the heart, stomach, or pupils produces the same effects as those of the sympathetic nervous system ○ Suggested that the sympathetic nerves stimulate muscles by using adrenaline or a similar chemical • Otto Loewi in 19 ecreased a frog's heart rate and collected fluid from the heart and transferred it to a second frog's heart. The second heart decreased the heart, stomach, or pupils produces the same effects as those of the sympathetic nervous system ○ Suggested that the sympathetic nerves stimulate muscles by using adrenaline or a similar chemical • Otto Loewi in 19 ecreased a frog's heart rate and collected fluid from the heart and transferred it to a second frog's heart. The second heart decreased heart rate as well ○ Repeated by increasing heart rate and transferring fluid again. The second heart showed increased heart rate ○ Concluded nerves send messaged by releasing chemicals The Sequence of Chemical Events at a Synapse • Major chemical events in synapses (shown in figure 2-i)n 2.2 sect ○ The neuron synthesizes chemicals that serve as neurors ransmitte-‐ smaller neurotransmitters in the axon terminals and neuropeptides in the cell body ○ Action potentials travel down onn ables calcium to enter the cell which releases neurotransmitters from the presynaptic terminals and into the synaptic cleft, the space between presynaptic and postsynaptic neurons ○ The released molecules diffuse across the cleft, attach to receptors, and alter the activity of the postsynaptic neuron ○ The neurotransmitter molecules separate from their receptors ○ The neurotransmitter molecules may be taken back into the presynaptic neuron for recycling or they may diffuse away ○ Some postsynaptic cells send reverse messages to control the further release of neurotransmitter by presynaptic cells Types of neurotransmitters • Chemicals that are released from a neuron that affect-‐neuron neurotransmitters • Amino acids- containing an amine group ○ Glutamate, GABA, glycine, aspartate • Acetylcholine -‐a modified amino acid • Monoamine -‐ chemicals formed by a change in certain amino acids ○ Serotonin, dopamine, norepinephrine, epinephrine • Neuropeptides -‐ chains of amino acids ○ Endorphins, substance P, neuropeptide Y, many others • Purines -‐ including adenosine and its derivatives ○ ATP, adenosine, maybe others • Gases ○ Nitric oxide, maybe others § Nitric oxide is released by many small local neurons § Poisonous in large quantities and difficult to make in a lab § Many neurons release nitric oxide when they are stimulated, dilating the blood vessels in the area and increasing blood flow • Gases ○ Nitric oxide, maybe others § Nitric oxide is released by many small local neurons § Poisonous in large quantities and difficult to make in a lab § Many neurons release nitric oxide when they are stimulated, dilating the blood vessels in the area and increasing blood flow Synthesis of transmitters • Nearly all neurotransmitters are synthesized from amino acids, obtained from proteins in the diet • Catecholamines -‐ compounds that contain catechol and an amine group Epinephrine, norepinephrine, and dopamine ○ Storage of transmitters • Vesicles -‐tiny nearly spherical packets with high concentrations of neurotransmitter molecules ○ Vesicles are located in presynaptic terminals • MAO -‐ monoamine oxidase ○ Enzyme in neurons that release serotonin, dopamine, or norepinephrine ○ Breaks down transmitters into inactive chemicals ○ Blocking MAO with antidepressant drugs can increase the brain's supply of serotonin, dopamine, and norepinephrine Release and diffusion of transmitters • Depolarization opens calcium gates which cause e-‐ bursts of release of neurotransmitter from the presynaptic neuron • Neurotransmitters take no more than .01 milliseconds to diffuse across the cleft Activating receptors of the postsynaptic cell • When a neurotransmitter attaches to its receptor, the receptor may open a channel or it may produce a slower but longer effect ○ Open a chanl ionotropic effect ○ Slower but longer ctm etabotropic effect Ionotropic effect • A neurotransmitter binds to an ionotropic receptor and twists it enough to open its central channel • Sodium and potassium channels on an axon -latgde -‐ channels controlled by neurotransmitters are called -gated or ligand-‐gated channels • Begin quickly -‐ sometimes within less than a millisecond after the transmitter attaches ○ Decays with a-‐life of about 5 ms • Most of the excitatory ionotropic synapses use the neurotransmitter glutamate -‐ most abundant neurotransmitter in the nervous system ○ Acetylcholine is excitory in most cases • Most of the inhibitory ionotropic synapses -opeA chloride gates, enable chloride ions to cross the membrane into the cell more rapidly than usual • Most of the excitatory ionotropic synapses use the neurotransmitter glutamate -‐ most abundant neurotransmitter in the nervous system ○ Acetylcholine is excitory in most cases • Most of the inhibitory ionotropic synapses -opeA chloride gates, enable chloride ions to cross the membrane into the cell more rapidly than usual ○ Glycine is common inhibitory transmitter as well Metabotropic effects and second messenger systems • Transmitters exert metabotropic effects by initiating a sequence of metabolic reactions that are slower and longer lasting than ionotropic effects ○ 30 ms or more after the release of the transmitter ○ Typically last up to a few seconds but sometimes longer • Uses many neurotransmitter-‐s dopamine, norepinephrine, seratonin, sometimes glutamate and GABA • Neurotransmitter attaches to the metabotropic receptor, bends the receptor protein that goes through the membrane ○ Other side of the receptor is attached -ptoit in coupled to guanosine triphosphate (GTP), energy-‐storing molecule ○ Bending the receptor detaches the G protein so it takes its energy elsewhere in the cell § Results in increased concentration of a second -‐ er chemical that initiates communication to many areas within the neuron • An ionotropic synapse has effects localized to one point on the membrane; metabotropic synapse influences activity in much or all of the cell and over a long time by way of its second messenger Neuropeptides • Often referred to as neuromodulators • Many differences between neuropeptides and other neurotransmitters ○ Place synthesized: neuropeptid-c ell body; neurotransmi s-‐ presynaptic terminal ○ Place released: neuropeptid-d endrites or cell body and sides of axon; neurotransmitters -‐ axon terminal ○ Released by: neuropeptides -‐ repeated depolarization; neurotransmitters -‐ single action potential ○ Effect on neighboring cells: neuro-‐ they release neuropeptide too; neurotransmitters -‐ no effect ○ Spread of effects: neuropeedsi ffuse to wide area; neurotransmitters -‐ effect mostly on receptors of the adjacent postsynaptic cell ○ Duration of effects: neuropeptides -‐many minutes; neurotr-‐rs less than a few seconds to a few seconds Variation in receptors • At least 26 types of GABA receptors and at least 7 families of serotonin receptors postsynaptic cell ○ Duration of effects: neuropeptides -‐many minutes; neurotr-‐rs less than a few seconds to a few seconds Variation in receptors • At least 26 types of GABA receptors and at least 7 families of serotonin receptors • Receptors differ in chemical properties, responses to drugs, and roles in behavior • A given receptor can have different effects on different people or even different parts of one brain Drugs that act by binding to receptors • Hallucinogenic drugs- drugs that distort perception Chemically resemble serotonin ○ • Nicotine ○ Stimulates a family of acetylcholine receptors (known as nicotinic receptors) ○ Abundant on neurons that release dopamine • Opiate drugs -‐ morphine, heroin, and methadone ○ Opiates attach to the same receptors as endorphins Inactivation and reuptake of neurotransmitters • Neurotransmitters are inactivated; neuropeptides are not and instead diffuse away • Acetylcholine is broken down by the enzyme acetylcholinesterase into two fragments -‐ acetate and choline ○ Choline diffuses back to the presynaptic neuron and reconnects with acetate already in the cell to form acetylcholine again • Serotonin and the catecholamines do not break down into in-‐gments a detach from the receptor instead • Reuptake -‐ presynaptic neuron takes up the released neurotransmitter molecules intact and reuses them ○ Occurs through special membrane proteins called transporters ○ Any transmitter molecules not taken up by transporters are broken down by the enzyme COMT • Stimulant drugs (amphetamine, cocaine) inhibit d-deicer eases reuptake and prolongs dopamine's effects ○ Most antidepressants also block the dopamine transporter • Methylphenidate (ritalin) blocks dopamine like cocaine but has a gradual increase in the drug's concentration followed by a slow decline Negative feedback from the postsynaptic cell • Autoreceptors -‐ receptors that respond to the released transmitter by inhibiting further synthesis and release ○ Provide negative feedback • Nitric oxide, anandamine, G -‐ chemicals that travel back to the presynaptic terminal to inhibit further release of transmitter Negative feedback from the postsynaptic cell • Autoreceptors -‐ receptors that respond to the released transmitter by inhibiting further synthesis and release ○ Provide negative feedback • Nitric oxide, anandamine, 2G -‐ chemicals that travel back to the presynaptic terminal to inhibit further release of transmitter • Cannabinoids-‐ the active chemicals in marijuana ○ Bind to anandamine -‐AG receptors so the presynaptic cell, unaware that it didn't send any message, stops sending ○ Decrease both excitatory and inhibitory messages from many neurons *Table 2.3 in sect-2l outlines some drugs and their effects Electrical Synapses • at an electrical synapse -‐ the membrane of one neuron comes into direct contact with the membrane of another ○ Contact is called a gap junction ○ Fairly large pores of the membranes line up so the two neurons act almost as if they were a single neuron Hormones • A chemical secreted by cells in one part of the body and conveyed by the blood to influence other cells • Endocrine (hormone -‐producing glands-‐) hypothalamus, pineal gland, pituitary gland, parathyroid glands, thyroid glands, thymus, liver, adrenal gland, kidney, pancreas, ovary (in female), placenta (in female during pregnancy), testis (in male) • Two types of hormones are protein and peptide ○ Protein hormones-‐ composed of long chains of amino acids ○ Peptide hormones-‐ composed of short chains of amino acids • Pituitary gland ○ Anterior pituitary ○ Posterior pituitary § Composed of neural tissue § Can be considered an extension of the hypothalamus • The hypothalamus ○ Synthesize the hormones oxytocin and vasopressin ○ Secrets releasing hormo-fl ow through the blood to the anterior pituitary
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