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Chapter3 Power Point Lecture


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Chapter3 Power Point Lecture

  1. 1. Chapter 3 Synapses
  2. 2. The Concept of the Synapse <ul><li>Neurons communicate by transmitting chemicals at junctions called “synapses” </li></ul><ul><li>In 1906, Charles Scott Sherrington coined the term synapse to describe the specialized gap that existed between neurons. </li></ul><ul><li>Sherrington conducted his research investigating how neurons communicate with each other by studying reflexes (automatic muscular responses to stimuli). </li></ul>
  3. 3. Fig. 3-1, p. 52
  4. 4. The Concept of the Synapse <ul><li>Sherrington observed three important points about reflexes: </li></ul><ul><ul><li>Reflexes are slower than conduction along an axon. </li></ul></ul><ul><ul><li>Several weak stimuli presented at slightly different times or slightly different locations produces a stronger reflex than a single stimulus does. </li></ul></ul><ul><ul><li>As one set of muscles relaxes, another set becomes excited. </li></ul></ul>
  5. 5. The Concept of the Synapse <ul><li>Sherrington observed a difference in the speed of conduction of the reflex arc from that of the speed of an action potential. </li></ul><ul><li>He believed the difference must be accounted for by the time it took for communication between neurons to occur. </li></ul><ul><li>Evidence validated the idea of the synapse. </li></ul>
  6. 6. Fig. 3-2, p. 53
  7. 7. The Concept of the Synapse <ul><li>Sherrington observed that repeated stimuli over a short period of time produced a stronger response. </li></ul><ul><li>Led to the idea of temporal summation or that repeated stimuli can have a cumulative effect and can produce a nerve impulse when a single stimuli is too weak. </li></ul>
  8. 8. Fig. 3-3, p. 54
  9. 9. The Concept of the Synapse <ul><li>Sherrington also noticed that several small stimuli on a similar location produced a reflex when a single stimuli did not. </li></ul><ul><li>This led to the idea of spatial summation or that synaptic input from several locations can have a cumulative effect and trigger a nerve impulse. </li></ul>
  10. 10. Fig. 3-4, p. 54
  11. 11. The Concept of the Synapse <ul><li>Excitatory postsynaptic potential (EPSP) is a graded potential that decays over time and space. </li></ul><ul><li>The cumulative effect of EPSPs are the basis for temporal and spatial summation. </li></ul>
  12. 12. The Concept of the Synapse <ul><li>Sherrington also noticed that during the reflex that occurred, the foot of a dog that was pinched retracted while the other three feet were extended. </li></ul><ul><li>He suggested that an interneuron in the spinal cord sent an excitatory message to the flexor muscles of one leg and an inhibitory message was sent to the other three legs. </li></ul>
  13. 13. Fig. 3-5, p. 55
  14. 14. The Concept of the Synapse <ul><li>This led to the idea of inhibitory postsynaptic potential or the temporary hyperpolarization of a membrane. </li></ul><ul><li>An ISPS occurs when synaptic input selectively opens the gates for positively charged potassium ions to leave the cell or for negatively charged chloride ions to enter the cells. </li></ul><ul><li>Serves as an active “brake”, that suppresses excitation. </li></ul>
  15. 15. Fig. 3-6, p. 55
  16. 16. The Concept of the Synapse <ul><li>Neurons can have thousands of synapses. </li></ul><ul><li>Both temporal and spatial summation can occur within a neuron. </li></ul><ul><li>The likelihood of an action potential depends upon the ratio of IPSPs to EPSPs at a given moment. </li></ul>
  17. 17. The Concept of the Synapse <ul><li>The spontaneous firing rate refers to the periodic production of action potentials despite synaptic input. </li></ul><ul><li>EPSPs increase the number of action potentials above the spontaneous firing rate. </li></ul><ul><li>IPSPs decrease the number of action potentials below the spontaneous firing rate. </li></ul>
  18. 18. Chemical Events at the Synapse <ul><li>Transmission of a message across the synapse occurs by chemical means. </li></ul><ul><li>Neurotransmitters are chemicals that travel across the synapse and allow communication between neurons. </li></ul>
  19. 19. Chemical Events at the Synapse <ul><li>The major sequence of events that allow communication between neurons across the synapse are as follows: </li></ul><ul><ul><li>The neuron synthesizes chemicals that serve as neurotransmitters. </li></ul></ul><ul><ul><li>Neurons store neurotransmitters in axon terminals or transport them there. </li></ul></ul><ul><ul><li>An action potential triggers the release of neurotransmitters into the synaptic cleft. </li></ul></ul>
  20. 20. Chemical Events at the Synapse (cont.) <ul><li>The neurotransmitters travel across the cleft and attach to receptors on the postsynaptic neuron. </li></ul><ul><li>The neurotransmitters separate from the receptors. </li></ul><ul><li>The neurotransmitters are taken back into the presynaptic neuron, diffuse away, or are inactivated by chemicals. </li></ul><ul><li>The postsynaptic cell may send negative feedback to slow the release of further neurotransmitters. </li></ul>
  21. 21. Fig. 3-8, p. 59
  22. 22. Chemical Events at the Synapse <ul><li>Major categories of neurotransmitters include the following: </li></ul><ul><ul><li>Amino acids. </li></ul></ul><ul><ul><li>Peptides. </li></ul></ul><ul><ul><li>Acetylcholine. </li></ul></ul><ul><ul><li>Monoamines. </li></ul></ul><ul><ul><li>Purines. </li></ul></ul><ul><ul><li>Gases. </li></ul></ul>
  23. 23. Table 3-1, p. 60
  24. 24. Chemical Events at the Synapse <ul><li>Neurons synthesize neurotransmitters and other chemicals from substances provided by the diet. </li></ul><ul><ul><li>Catecholimines are synthesized and contain a catechol group and an amine group. </li></ul></ul><ul><ul><li>Acetylcholine is synthesized from choline found in milk, eggs, and nuts. </li></ul></ul><ul><ul><li>Tryptophan serves as a precursor for serotonin. </li></ul></ul>
  25. 25. Fig. 3-9, p. 61
  26. 26. Chemical Events at the Synapse <ul><li>Smaller neurotransmitters are synthesized in the presynaptic terminal and held there for release. </li></ul><ul><ul><li>Example: acetylcholine </li></ul></ul><ul><li>Larger neurotransmitters are synthesized in the cell body and transported down the axon. </li></ul><ul><ul><li>Example: peptides </li></ul></ul>
  27. 27. Chemical Events at the Synapse <ul><li>Vesicles are tiny spherical packets located in the presynaptic terminal where neurotransmitters are held for release. </li></ul><ul><li>Exocytosis refers to the excretion of the neurotransmitter from the presynaptic terminal into the synaptic cleft. </li></ul><ul><ul><li>Triggered by an action potential arriving fro the axon. </li></ul></ul>
  28. 28. Fig. 3-10, p. 61
  29. 29. Chemical Events at the Synapse <ul><li>Transmission across the synaptic cleft by a neurotransmitter takes fewer than 10 microseconds. </li></ul><ul><li>Most individual neurons release at least two or more different kinds of neurotransmitters. </li></ul><ul><li>A neuron may respond to more types of neurotransmitters than it releases. </li></ul>
  30. 30. Chemical Events at the Synapse <ul><li>An ionotropic effect refers to when a neurotransmitter attaches to receptors and immediately opens ion channels. </li></ul><ul><li>Ionotropic effects occur very quickly and are very short lasting. </li></ul><ul><li>Most of the brain’s excitatory ionotropic synapses use glutamate or acetylcholine as a neurotransmitter. </li></ul>
  31. 31. Fig. 3-11, p. 63
  32. 32. Chemical Events at the Synapse <ul><li>Metabotropic effects refer to when a neurotransmitter attaches to a receptor and initiates a sequence of metabolic reactions that are slower and longer lasting. </li></ul><ul><li>Metabotropic events include such behaviors as hunger, fear, thirst, or anger. </li></ul><ul><li>When neurotransmitters attach to a metabotropic receptor, it bends the rest of the protein . </li></ul><ul><li>Bending allows a portion of the protein inside the neuron to react with other molecules. </li></ul>
  33. 33. Chemical Events at the Synapse <ul><li>The portion inside the neuron activates a G-protein –one that is coupled to guanosine triphosphate (GTP), an energy storing molecule. </li></ul><ul><li>G-protein increases the concentration of a “second-messenger”. </li></ul><ul><li>The second messenger communicates to areas within the cell. </li></ul><ul><ul><li>May open or close ion channels, alter production of activating proteins, or activate chromosomes. </li></ul></ul>
  34. 34. Fig. 3-12, p. 63
  35. 35. Chemical Events at the Synapse <ul><li>Metabotropic effects utilize a number of different neurotransmitters and are often called neuromodulators because they do not directly excite or inhibit the postsynaptic cell. </li></ul><ul><li>Instead, neuromodulators: </li></ul><ul><ul><li>increase or decrease the release of other neurotransmitters </li></ul></ul><ul><ul><li>alter the response of postsynaptic cells to various inputs. </li></ul></ul>
  36. 36. Chemical Events at the Synapse <ul><li>A hormone is a chemical secreted by a gland or other cells that is transported to other organs by the blood where it alters activity. </li></ul><ul><li>Endocrine glands are responsible for the production of hormones. </li></ul><ul><li>Hormones are important for triggering long-lasting changes in multiple parts of the body. </li></ul>
  37. 37. Table 3-2a, p. 64
  38. 38. Table 3-2b, p. 64
  39. 39. Chemical Events at the Synapse <ul><li>Protein hormones and peptide hormones are composed of chains of amino acids and attach to membrane receptors where they activate second messenger systems. </li></ul><ul><li>Hormones secreted by the brain can control the release of other hormones. </li></ul>
  40. 40. Fig. 3-13, p. 65
  41. 41. Chemical Events at the Synapse <ul><li>The pituitary gland is attached to the hypothalamus and consists of two distinct glands that each release a different set of hormones: </li></ul><ul><li>Anterior pituitary- composed of glandular tissue and synthesizes six hormones. </li></ul><ul><li>Posterior pituitary- composed of neural tissue and can be considered an extension of the hypothalamus </li></ul>
  42. 42. Fig. 3-14, p. 65
  43. 43. Chemical Events at the Synapse <ul><li>Neurons in the hypothalamus synthesize the hormones oxytocin and vasopressin which migrate down axons to the posterior pituitary. </li></ul><ul><ul><li>Also known as antidiuretic hormones </li></ul></ul><ul><li>The hypothalamus secretes releasing hormones which flow through the blood and stimulate the anterior pituitary to release a number of other hormones. </li></ul>
  44. 44. Fig. 3-15, p. 66
  45. 45. Chemical Events at the Synapse <ul><li>The hypothalamus maintains a fairly constant circulating level of hormones through a negative-feedback system. </li></ul><ul><ul><li>Example : TSH- releasing hormone </li></ul></ul>
  46. 46. Fig. 3-16, p. 66
  47. 47. Chemical Events at the Synapse <ul><li>Neurotransmitters released into the synapse do not remain and are subject to either inactivation or reuptake . </li></ul><ul><li>Reuptake refers to when the presynaptic neuron take sup most of the neurotransmitter molecules intact and reuses the. </li></ul><ul><li>Transporters are special membrane proteins that facilitate reuptake. </li></ul><ul><ul><li>Example: Serotonin is taken back up into the presynaptic terminal. </li></ul></ul>
  48. 48. Chemical Events at the Synapse <ul><li>Examples of inactivation and reuptake include: </li></ul><ul><ul><li>Acetylcholine is broken down by acetylcholinesterase into acetate and choline. </li></ul></ul><ul><li>Some serotonin and catecholamine molecules are converted into inactive chemicals: </li></ul><ul><ul><li>COMT and MAO are enzymes that convert catecholamine transmitters into inactive chemicals. </li></ul></ul>
  49. 49. Chemical Events at the Synapse <ul><li>Research has begun to investigate the role of events at the synapse and their effects on personality. </li></ul><ul><li>Research suggests that some dopamine receptors may be related to “pleasure-seeking” and “thrill-seeking” behaviors. </li></ul>
  50. 50. Drugs and the Synapse <ul><li>The study of the influence of various kinds of drugs has provided us with knowledge about many aspects of neural communication at the synaptic level. </li></ul><ul><li>Drugs work by mimicking our own neurochemistry. </li></ul><ul><ul><li>Example: receptors in the brain respond to LSD and cocaine </li></ul></ul><ul><li>Drugs alter various stages of synaptic processing. </li></ul>
  51. 51. Fig. 3-1, p. 52
  52. 52. Drugs and the Synapse <ul><li>Drugs either facilitate or inhibit activity at the synapse. </li></ul><ul><ul><li>Antagonistic drugs block the effects of neurotransmitters. </li></ul></ul><ul><ul><li>Agonist drugs mimic or increase the effects of neurotransmitters. </li></ul></ul>
  53. 53. Drugs and the Synapse <ul><li>Drugs work by doing one or more of the following to neurotransmitters: </li></ul><ul><ul><li>Increasing the synthesis. </li></ul></ul><ul><ul><li>Causing vesicles to leak. </li></ul></ul><ul><ul><li>Increasing release. </li></ul></ul><ul><ul><li>Decreasing reuptake. </li></ul></ul><ul><ul><li>Blocking the breakdown into inactive chemical. </li></ul></ul><ul><ul><li>Directly stimulating or blocking postsynaptic receptors. </li></ul></ul>
  54. 54. Drugs and the Synapse <ul><li>A drug has an affinity for a particular type of receptor if it binds to that receptor. </li></ul><ul><ul><li>Can vary from strong to weak. </li></ul></ul><ul><li>The efficacy of the drug is its tendency to activate the receptor. </li></ul><ul><li>Drugs can have a high affinity but low efficacy. </li></ul>
  55. 55. Drugs and the Synapse <ul><li>Almost all abused drugs stimulate dopamine release in the nucleus accumbens, </li></ul><ul><ul><li>small subcortical area rich in dopamine receptors </li></ul></ul><ul><ul><li>an area responsible for feelings of pleasure </li></ul></ul><ul><li>Sustained bursts of dopamine in the nucleus accumbens inhibit cells that release the inhibitory neurotransmitter GABA </li></ul>
  56. 56. Fig. 3-18, p. 72
  57. 57. Drugs and the Synapse <ul><li>Drugs are categorized according to their predominant action or effect upon behavior </li></ul><ul><li>Stimulant drugs increase excitement, alertness, motor activity and elevate mood. </li></ul><ul><li>Examples: amphetamines, cocaine, methylphenidate (Ritalin), MDMA (Ecstasy), nicotine </li></ul><ul><li>Stimulant drugs directly stimulate dopamine receptor types D 2 , D 3 , and D 4 . </li></ul>
  58. 58. Fig. 3-19, p. 72
  59. 59. Drugs and the Synapse <ul><li>Amphetamine stimulate dopamine synapses by increaseing the release of dopamine from the presynaptic terminal. </li></ul><ul><li>Cocaine blocks the reuptake of dopamine, norepinephrine, and serotonin. </li></ul><ul><li>Methylphenidate (Ritalin) also blocks the reuptake of dopamine but in a more gradual and more controlled rate. </li></ul><ul><ul><li>Often prescribed for people with ADD </li></ul></ul>
  60. 60. Drugs and the Synapse <ul><li>Ecstasy increases the release of dopamine at low doses that account for its stimulant properties. </li></ul><ul><li>Ecstasy increases the release of serotonin at higher doses accounting for its hallucinogenic properties. </li></ul><ul><li>Research indicates ecstasy use may contribute to higher incidences of anxiety and depression as well as memory loss and other cognitive deficits. </li></ul>
  61. 61. Fig. 3-20, p. 73
  62. 62. Drugs and the Synapse <ul><li>Nicotine is the active ingredient in tobacco. </li></ul><ul><li>Nicotine stimulates one type of acetylcholine receptor known as the nicotinic receptor . </li></ul><ul><li>Nicotinic receptors are found in the central nervous system, the nerve-muscle junction of skeletal muscles and in the nucleus accumbens. </li></ul><ul><li>Nicotinic receptors are also abundant in the nucleus accumbens and facilitate dopamine release. </li></ul>
  63. 63. Drugs and the Synapse <ul><li>Opiate drugs are those that are derived from (or similar to those derived from) the opium poppy. </li></ul><ul><li>Opiates decrease sensitivity to pain and increase relaxation. </li></ul><ul><li>Examples: morphine, heroin, methadone. </li></ul>
  64. 64. Drugs and the Synapse <ul><li>The brain produces peptides called endorphins. </li></ul><ul><li>Endorphin synapses may contribute to certain kinds of reinforcement by inhibiting the release of GABA indirectly. </li></ul><ul><li>Inhibiting GABA indirectly releases dopamine. </li></ul><ul><li>Endorphins attach to the same receptors to which opiates attach. </li></ul>
  65. 65. Drugs and the Synapse <ul><li>Opiates also block the locus coeruleus. </li></ul><ul><ul><li>involved in our response to arousing stimuli by release of norepinephrine </li></ul></ul><ul><ul><li>also involved in memory storage. </li></ul></ul>
  66. 66. Drugs and the Synapse <ul><li>Tetrahydocannabinol (THC) is the active ingredient in marijuana. </li></ul><ul><li>THC attaches to cannabinoid receptors throughout the brain but especially the cerebral cortex, cerebellum, basal ganglia, and hippocampus. </li></ul><ul><li>Anandamide and 2-AG are the endogenous chemicals that attach to these receptors. </li></ul>
  67. 67. Drugs and the Synapse <ul><li>The location of the receptors in the brain may account for the subjective effects of loss of time, an intensification of sensory experience, and also memory impairment. </li></ul><ul><li>The cannabinoid receptors are located on the presynaptic neuron and inhibit the release of glutamate and GABA. </li></ul>
  68. 68. Drugs and the Synapse <ul><li>Hallucinogenic drugs cause distorted perception. </li></ul><ul><li>Many hallucinogenic drugs resemble serotonin in their molecular shape. </li></ul><ul><li>Hallucinogenic drugs stimulate serotonin type 2A receptors (5-HT 2A ) at inappropriate times or for longer duration than usual thus causing their subjective effect. </li></ul>
  69. 69. Fig. 3-21, p. 75
  70. 70. Table 3-3, p. 76