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L6 NPB 101 - Lecture 6 •  SmartSite: –  Lec 6...

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Unformatted text preview: Lecture 6 •  SmartSite: –  Lec 6 Notes •  Announcements: –  None •  Review •  Synapse Physiology •  Reading (Required): –  Neurotransmi1ers (pp. 107) & Neuropep:des (pp. 110 ­111) •  Reading (Recommended): –  Chapter 4 (pp 104 ­113) 1 REV: AcGon PotenGal 1.  Rapid stereotyped change in Em, which occurs in response to a sGmulus. 2.  There are 6 primary characterisGcs of an AP: a)  Large regeneraGve increase in PNa via PosiGve Feedback b)  Rapid repolarizaGon of Em by slow increase in PK c)  Every AP followed by 2 refractory periods d)  Every AP is All ­or ­None e)  AP conducted over the axon f)  AP are conducted rapidly 2 Synapses •  JuncGon between two neurons •  Primary means by which one neuron directly interacts with another neuron •  General Anatomy of a synapse –  Presynap:c neuron – conducts acGon potenGal toward synapse •  SynapGc knob – contains synapGc vesicles •  SynapGc vesicles – stores neurotransmiVer (carries signal across a synapse) –  Synap:c cleA – space between the presynapGc and postsynapGc neurons –  Postsynap:c neuron – neuron whose acGon potenGals are propagated away from the synapse 3 General Synapse Anatomy Fig. 4 ­15, pg. 105 4 Synapse Physiology •  SynapGc transmission –  Process of conversion of electrical AP informaGon into chemical Em change in postsynapGc cell. –  Occurs in 8 steps 1)  Axon AP depolarizes Axon terminal. 2)  Ca+2 permeability increased by opening voltage ­gated Ca+2 channels (ECa is PosiGve Em). 3)  Chemical NeurotransmiVer released as Ca+2 in cell leads to fusion of synapGc vesicles to presynapGc membrane and exocytosis of vesicles. 5 Synapse Physiology (cont’d) 4)  Diffusion of NT across synapGc cle^.   SynapGc delay: release/diffusion take Gme (~0.5 mSec). 5)  NT binds to receptors in postsynapGc membrane. 6)  PostsynapGc response:             Open/close chemically ­gated ion channels. Increase/decrease ion permeability. Increase/decrease ion flow through postsynapGc membrane. Quantal/graded response – not all ­or ­none. Excitatory Inhibitory 7)  2 basic types of response: 8)  Removal of NT from synapGc cle^ – specialized mechanisms limit amount of Gme NT is present. 6 The Chemical Synapse Fig. 4 ­15, pg. 105 7 PresynapGc & PostsynapGc Membranes Vesicular fusion and NT release NT diffusion Binding a postsynapGc receptor Ion flow Fig. 4 ­15, pg. 105 8 NeurotransmiVers •  •  •  •  Vary from synapse to synapse Same neurotransmiVer is always released at a parGcular synapse Quickly removed from the synapGc cle^ Some common neurotransmiVers –  –  –  –  –  –  –  –  –  –  Acetylcholine Dopamine Norepinephrine Epinephrine Serotonin Histamine Glycine Glutamate Aspartate Gamma ­aminobutyric acid (GABA) 9 NeurotransmiVers •  •  Acetylcholine – Used at the neuromuscular juncGon. In the brain it is involved in learning and memory. Biogenic amines: found in small numbers of neurons, but these neurons generally synapse in many areas of the brain. Dopamine– Involved in motor control. Loss causes Parkinson’s disease Norepinephrine–Involved in aVenGon, circadian rhythms. Epinephrine– Involved in aVenGon. (As a hormone, causes “fight or flight”) Serotonin– Involved in sleep/wake cycle, aggressive behavior, social hierarchies. Pharmacology suggests roles in percepGon (LSD) and mood (Prozac). –  Histamine– Modulates sleep. –  –  –  –  –  Glycine– inhibitory, important in the spinal cord. –  Glutamate– main excitatory neurotransmiVer in the brain –  Aspartate– excitatory, important in the spinal cord –  Loss of acetylcholine neurons may be involved in Alzheimer’s disease. •  Amino acids: •  •  Gamma ­aminobutyric acid (GABA)– main inhibitory neurotransmiVer in the brain Many other neurotransmiVers (also neuropepGdes). 10 SynapGc PotenGals •  Signal at synapse either excites (+) or inhibits ( ­) the postsynapGc neuron •  Sign depends upon direcGon of Em change –  Excitatory is Em depolarizaGon closer to Threshold –  Inhibitory is Em hyperpolarizaGon farther from Threshold –  Excitatory synapses –  Inhibitory synapses 11 •  Two types of synapses Excitatory PostsynapGc PotenGals •  EPSP: cell more excitable •  NT opens a chemically ­ gated nonspecific caGon channel. –  Increase PNa and PK –  Greater tendency for Na+ to enter cell (remember Ei) •  Results in depolarizaGon –  Small change in Em –  Graded potenGal Fig. 4 ­16a, pg. 106 12 Inhibitory PostsynapGc PotenGals •  IPSP: cell less excitable •  NT opens a chemically ­ gated K+ or Cl ­ channel. –  Increase PK or PCl –  Greater tendency for K+ to leave cell or Cl ­ to enter cell (remember Ei) •  Results in hyperpolarizaGon –  Small change in Em –  Graded potenGal Fig. 4 ­16b, pg. 106 13 SummaGon of SynapGc Input Fig. 4 ­17, pg. 109 14 SummaGon of SynapGc Input 15 PresynapGc InhibiGon (& FacilitaGon) Fig. 4 ­18, pg. 111 16 Morphology of an Electrical Synapse •  Less common •  Direct cell ­cell communicaGon •  BidirecGonal •  Electrical and/or Metabolic coupling –  EssenGally sharing cytoplasm •  Fast – only limited by ConducGon Velocity Fig. 3 ­6, pg. 60 17 ...
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