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Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
Lect 4-synapse-8-
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Lect 4-synapse-8-

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  1. The Synapses SMS1084 Dr. Mohanad R. Alwan
  2. SYNAPSES <ul><li>A. Introduction </li></ul><ul><li>1) A synapse is a junction between 2 nerve cells or a nerve cell and a muscle cell. </li></ul><ul><ul><li>Usually these are formed between axon terminals and cell dendrites or body. </li></ul></ul>
  3. <ul><li>B) Anatomy of A Synapse </li></ul><ul><li>1) Synaptic knob with transmitter vesicles and presynaptic membrane. </li></ul><ul><li>2) Synaptic cleft. </li></ul><ul><li>3) Postsynaptic membrane with receptors for transmitters. </li></ul>
  4. <ul><li>C) Function of The Synapse </li></ul><ul><li>1) Electrical impulse in knob causes influx of Ca ++ , vesicles rupture, release transmitters. </li></ul><ul><li>2) Transmitters diffuse across,cleft attach to receptors. </li></ul>
  5. <ul><li>3) Receptors open gates for ions. </li></ul><ul><li>a) Can be direct </li></ul><ul><li>b) May be indirect via G-proteins (which diffuse from receptor to ion channel.) </li></ul><ul><li>4) Transmitters can be broken down, taken up by presynaptic membrane or diffuse away. </li></ul>
  6. Electrical Synapses <ul><li>Electrical synapses are formed when two neurons are connected by gap junctions ( syncitia ). </li></ul><ul><li>These are very rare in mammalian adult nervous systems. </li></ul><ul><ul><li>Primarily found in the control of certain eye movements </li></ul></ul><ul><li>These are more common in the developing fetal nervous system . </li></ul>
  7. Chemical Synapses <ul><li>Chemical synapses convert the electrical signal (AP) into a chemical signal (neurotransmitter) that is transmitted to the next cell. </li></ul><ul><li>Chemical synapses are unidirectional. </li></ul><ul><ul><li>Neurotransmitter released by one neuron </li></ul></ul><ul><ul><li>Neurotransmitter signal interpreted by other neuron </li></ul></ul><ul><li>The connection made at a chemical synapses contains a small gap (20-50 nm) between the connected neurons called a synaptic cleft . </li></ul>
  8. View of The Chemical Synapse & Function
  9. Neurotransmitter <ul><li>Neurotransmitter is made by the pre-synaptic neurone and is stored in synaptic vessels at the end of the axon. </li></ul><ul><li>The membrane of the post-synaptic neurone has chemical-gated ion channels called neuroreceptors . These have specific binding sites for neurotransmitters. </li></ul>
  10. <ul><li>1) Acetylcholine was first discovered, there are many others. </li></ul><ul><li>2) Monoamines </li></ul><ul><li>a) The catacholamines: dopamine, norepinephrine, epinephrine? </li></ul><ul><li>b) Serotonin </li></ul>D) Transmitters
  11. <ul><li>3) Amino acids </li></ul><ul><li>a) glutamate </li></ul><ul><li>b) GABA and glycine - inhibitory </li></ul><ul><li>4) Amines such as acetylcholine and histamine. </li></ul><ul><li>5) Polypeptides such as enkephalins and endorphins. </li></ul><ul><li>6) Gases such as nitric oxide and CO. </li></ul>
  12. Neurotransmitters
  13. <ul><li>E) One-way Conduction </li></ul><ul><li>1) Synapses make nervous system a one- way system - dendrites do not have neurotransmitter vesicles. </li></ul><ul><li>2) Synaptic delay </li></ul>
  14. POSTSYNAPTIC POTENTIALS <ul><li>The interaction of neurotransmitters with their receptors can cause changes in the membrane potential. </li></ul><ul><li>The response to a given neurotransmitter depends on the type of receptor present on the postsynaptic site. </li></ul><ul><li>Excitatory postsynaptic potentials cause a depolarization of the membrane (EPSP). </li></ul><ul><li>Inhibitory postsynaptic potentials cause a hyperpolarization of the membrane (IPSP). </li></ul>
  15. Types of Postsynaptic Potentials
  16. <ul><li>G) Synaptic Integration </li></ul><ul><li>1) Most synapses involve many neurons. </li></ul><ul><li>2) Whether or not postsynaptic membrane has an action potential is dependent upon algebraic sum of EPSPs and IPSPs. </li></ul>
  17. <ul><li>3) Spatial summation occurs when numerous synaptic knobs release their transmitters. </li></ul><ul><li>4) Temporal summation can occur if neuron releases transmitter rapidly. </li></ul>
  18. Neuromuscular Junctions <ul><li>Same stages as cholinergic synapses, but in this case the postsynaptic membrane is the muscle fibre membrane, (Sarcolemma). Depolarisation of the sarcolemma leads to contraction of muscle fibre. </li></ul>
  19. Cholinergic Synapses <ul><li>Acetylcholine is a common transmitter. </li></ul><ul><li>Synapses that have acetylcholine transmitter are called cholinergic synapses. </li></ul><ul><li>Some neurones form more than 1 synapse. </li></ul><ul><li>This is an electron micrograph of synapses between nerve fibres and a neurone cell body. </li></ul>
  20. What happens at a cholinergic synapse? Stage 1 <ul><li>An action potential arrives at presynaptic membrane. Voltage gated calcium channels in the presynaptic membrane open, calcium ions enter the presynaptic neurone. </li></ul>
  21. What happens at a cholinergic synapse? Stage 2 <ul><li>Calcium ions cause synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft. </li></ul>
  22. What happens at a cholinergic synapse? Stage 3 <ul><li>Acetylcholine diffuses cross the synaptic cleft and binds to specific neuroreceptor sites in the post synaptic membrane. </li></ul>
  23. What happens at a cholinergic synapse? Stage 4 <ul><li>Sodium channels open. Sodium ions diffuse into the postsynaptic membrane causing depolarisation, which may initiate an action potential. </li></ul>
  24. What happens at a cholinergic synapse? Stage 5 <ul><li>Acetylcholinesterase breaks down acetylcholine. The products diffuse back into the presynaptic neurone where acetycholine is resynthesised using ATP from the mitochondria. </li></ul>
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  29. <ul><li>H) Chemicals Which Affect The Synapse </li></ul><ul><li>1) Many chemicals that affect nervous system do so by affecting synapse. </li></ul><ul><li>a) Clostridium botulinum toxin. </li></ul><ul><li>b) Clostridium tetanii toxin. </li></ul><ul><li>c) Caffeine </li></ul><ul><li>d) Anesthetics </li></ul><ul><li>e) Strychnine </li></ul><ul><li>f) Psychotropic drugs </li></ul>
  30. The Role Of Membrane Ion Channels <ul><li>Plasma membrane of neurons is filled with channels that allow specific ions to cross. </li></ul><ul><li>Ion channels fall into 1 of 2 categories: </li></ul><ul><ul><li>Passive or leakage channels – usually open and allow specific ions to pass (i.e., K + ). </li></ul></ul><ul><ul><li>Gated channels – only open when appropriate signal received. </li></ul></ul><ul><ul><ul><li>Chemically (ligand) gated channels – only open when the appropriate chemical or neurotransmitter present. </li></ul></ul></ul><ul><ul><ul><li>Voltage gated channels – only open when the membrane voltage is at an appropriate level. </li></ul></ul></ul>
  31. Types of Ion Channels
  32. The Resting Membrane Potential <ul><li>If we measure voltage between the inside of a neuron and the outside we find that the neuron is more negative inside than outside with a potential of about –70 mV. </li></ul>
  33. Source of The Resting Membrane Potential <ul><li>The resting membrane potential results from the concentrations of ions that are in & out of the cell and the permeability to those ions. The resting membrane potential is about –70 mV. </li></ul>
  34. Membrane Potentials <ul><li>Neurons use electrical signals to receive, integrate and send information. </li></ul><ul><li>Electrical potential of the membrane can change </li></ul><ul><li>There are two types of electrical changes to the membrane: </li></ul><ul><ul><li>Graded potentials - short distance </li></ul></ul><ul><ul><li>Action potentials - long distance (through the axon). </li></ul></ul>
  35. HABISSSSS SSSSSSS SSSSSSSS SSSSSSS

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