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Chapter 30

Chapter 30. General Principles of the Neuron Activities. Contents. Neurotransmission Neurotransmitter and Receptor Synaptic Plasticity Properties of the Synaptic Neurotransmission. Part I. NEUROTRANSMISSION. I Synapse. 1.Chemical synapse (Classical Synapse)

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Chapter 30

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  1. Chapter 30 General Principles of the Neuron Activities

  2. Contents Neurotransmission Neurotransmitter and Receptor Synaptic Plasticity Properties of the Synaptic Neurotransmission

  3. Part I NEUROTRANSMISSION

  4. I Synapse • 1.Chemical synapse (Classical Synapse) • Predominates in the vertebrate nervous system • 2.Non-synaptic chemical transmission • 3.Electrical synapse • Via specialized gap junctions • Does occur, but rare in vertebrate NS • Astrocytes can communicate via gap junctions

  5. 1. Chemical Synapse • Terminal bouton is separated from postsynaptic cell by synaptic cleft. • Vesicles fuse with axon membrane and NT released by exocytosis. • Amount of NTs released depends upon frequency of AP.

  6. 2. Non-synaptic chemical transmission • Sympathetic Nerve • The postganglionic neurons innervate the smooth muscles. • No recognizable endplates or other postsynaptic specializations; • The multiple branches are beaded with enlargements (varicosities) that are not covered by Schwann cells and contain synaptic vesicles Fig. : Ending of postganglionic autonomic neurons on smooth muscle

  7. Non-synaptic chemical transmission continued • In noradrenergic neurons, the varicosities are about 5m, with up to 20,000 varicosities per neuron • Transmitter is released at each varicosity • One neuron innervate many effector cells. Fig. : Ending of postganglionic autonomic neurons on smooth muscle

  8. Innervation of Adrenal Medulla

  9. 3. Electrical Synapse • Impulses can be regenerated without interruption in adjacent cells. • Gap junctions: • Adjacent cells electrically coupled through a channel. • Each gap junction is composed of 12 connexin proteins. • Examples: • Smooth and cardiac muscles, brain, and glial cells.

  10. Electrical Synapses Communication takes place by flow of electric current directly from one neuron to the other No synaptic cleft or vesicles cell membranes in direct contact Communication not polarized- electric current can flow between cells in either direction

  11. Chemical Synapse Electrical Synapse Purves, 2001

  12. II The Chemical Synapse and Signal Transmission

  13. II The Chemical Synapse and Signal Transmission • The chemical synapse • specialized junction that transfers nerve impulse information from a pre synaptic membrane to a postsynaptic membrane using neurotransmitters and enzymes

  14. Synaptic connections • ~100,000,000,000 neurons in human brain • Each neuron contacts ~1000 cells • How many synapses?

  15. Chemical Synapses • Neurotransmitter- chemical intermediary released from one neuron and influences another • Synaptic cleft- a small gap between the sending (presynaptic) and the receiving (postsynaptic) site

  16. Chemical Synapses • Synaptic vesicles- small spherical or oval organelles contain chemical transmitter used in transmission • Polarization- communication occurs in only one direction, from sending presynaptic site to receiving postsynaptic site

  17. 1. Synaptic Transmission Model • Precursor transport • NT synthesis • Storage • Release • Activation • Termination ~diffusion, degradation, uptake, autoreceptors

  18. Postsynaptic Membrane Presynaptic Axon Terminal Terminal Button Dendritic Spine

  19. (1) Precursor Transport

  20. _ _ _ NT (2) Synthesis enzymes/cofactors

  21. (3) Storage in vesicles

  22. NT Vesicles Terminal Button Dendritic Spine Synapse

  23. (4) Release Terminal Button Dendritic Spine Synapse Receptors

  24. Terminal Button Dendritic Spine AP Synapse

  25. Exocytosis Ca2+

  26. Each vesicle contains one quanta of neurotransmitter (approximately 5000 molecules) –quanta release

  27. (5) Activation

  28. (6) Termination

  29. (6.1) Termination by... Diffusion

  30. (6.2) Termination by... Enzymatic degradation

  31. (6.3) Termination by... Reuptake

  32. (6.4) Termination by... Autoreceptors A

  33. Autoreceptors • On presynaptic terminal • Binds NT • same as postsynaptic receptors • different receptor subtype • Decreases NT release & synthesis • Metabotropic receptors

  34. Synaptic Transmission • AP travels down axon to bouton. • Voltage Gated Ca2+ channels open. • Ca2+ enters bouton down concentration gradient. • Inward diffusion triggers rapid fusion of synaptic vesicles and release of NTs. • Ca2+ activates calmodulin, which activates protein kinase. • Protein kinase phosphorylates synapsins (突触蛋白）. • Synapsins aid in the fusion of synaptic vesicles.

  35. Synaptic Transmission (continued) • NTs are released and diffuse across synaptic cleft. • NT (ligand) binds to specific receptor proteins in postsynaptic cell membrane. • Chemically-regulated gated ion channels open. • EPSP: depolarization. • IPSP: hyperpolarization. • Neurotransmitter inactivated to end transmission.

  36. 2 EPSP and IPSP EPSP: Excitatory postsynaptic potential IPSP: Inhibitory postsynaptic potential

  37. EPSP • An AP arriving in the presynaptic terminal cause the release of neurotransmitter • The molecules bind and activate receptor on the postsynaptic membrane

  38. EPSP • Opening transmitter-gated ions channels ( Na+) in postsynaptic- membrane • Both an electrical and a concentration gradient driving Na+ into the cell • The postsynaptic membrane will become depolarized (EPSP).

  39. EPSP • No threshold. • Decreases resting membrane potential. • Closer to threshold. • Graded in magnitude. • Have no refractory period. • Can summate.

  40. IPSP A impulse arriving in the presynaptic terminal causes the release of neurotransmitter The molecular bind and active receptors on the postsynaptic membrane open CI- or, K+ channels CI- influx or K+ outflux produce a hyperpolarization in the postsynaptic membrane.

  41. IPSPs • No threshold. • Hyperpolarize postsynaptic membrane. • Increase membrane potential. • Can summate. • No refractory period.

  42. 3 Synaptic Inhibition A • Presynaptic inhibition • Postsynaptic inhibition B

  43. (1) Postsynaptic inhibition • Concept: • effect of inhibitory synapses on the postsynaptic membrane. • Mechanism: • IPSP • inhibitory interneuron • Types: Afferent collateral (reciprocal) inhibition) Recurrent inhibition.

  44. Postsynaptic inhibition Reciprocal inhibition

  45. Postsynaptic inhibition 2) Recurrent inhibition

  46. (2) Presynaptic inhibition • Concept: the inhibition occurs at the presynaptic terminals before the signal ever reaches the synapse. • The basic structure: an axon-axon synapse (presynaptic synapse) between A and B. • Neuron A has no direct effect on neuron C, but it exert a presynaptic effect on ability of B to Influence C. • decrease the amount of neuro- transmitter released from B (Presynaptic inhibition) B A A A B C C

  47. Mechanisms Presynaptic inhibition • Activation of the presynaptic receptors increases CI- conductance  EPSP to decrease the size of the AP reaching the excitatory ending  reduces Ca2+ entry and consequently the amount of excitatory transmitter decreased.

  48. A B Presynaptic Inhibition Excitatory Synapse + • A active • B more likely to fire • Add a 3d neuron ~

  49. A B + C Presynaptic Inhibition Excitatory Synapse + • Axon-axon synapse • C is excitatory ~

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