Biol 219 Lec 12 Fall 2016

Transcription

1 Cell-to-Cell: Neurons Communicate at Synapses Electrical synapses pass electrical signals through gap junctions Signal can be bi-directional Synchronizes the activity of a network of cells Primarily in the CNS Chemical synapses use neurotransmitters that cross synaptic clefts Synapse region where neuron meets its target cell Parts of synapse: 1. Axon terminal of presynaptic cell 2. Membrane of postsynaptic cell 3. Synaptic cleft 1

2 Biol 219 Lec 12 Fall 2016 Electrical Synapses Chemical Synapses 2

3 Terminology Local and long-distance chemical communication Paracrine acting on cells in the immediate vicinity Autocrine acting on itself Neurocrine a chemical secreted by a neuron Neurotransmitter: rapid onset Neuromodulator: acts more slowly Neurohormone: released into the bloodstream Neurotransmitters 1. Acetylcholine 2. Amines 3. Amino acids 4. Purines 5. Gases 6. Peptides 7. Lipids Slide 2 A Chemical Synapse Figure 8.19a Synaptic Communication Neurotransmitter Release An ac tion pote ntial depolariz e s the axon terminal. Action potential arrives at axon terminal. Synaptic vesicle with neurotransmitter molecules Synaptic cleft Postsynaptic cell Re c e ptor 3

4 Slide 3 Slide 4 Figure 8.19a Synaptic Communication Figure 8.19a Synaptic Communication Neurotransmitter Release Neurotransmitter Release An ac tion pote ntial depolariz e s the axon terminal. An ac tion pote ntial depolariz e s the axon terminal. Action potential arrives at axon terminal. Synaptic vesicle with neurotransmitter molecules The depolarization opens voltagegated Ca 2+ channels, and Ca 2+ enters the cell. Action potential arrives at axon terminal. Synaptic vesicle with neurotransmitter molecules The depolarization opens voltagegated Ca 2+ channels, and Ca 2+ enters the cell. Ca lcium entry triggers ex ocy tosis of synaptic vesicle contents. Ca 2+ Docking protein Ca 2+ Synaptic cleft Synaptic cleft Postsynaptic cell Voltage-gated Ca 2+ channel Re c e ptor Postsynaptic cell Voltage-gated Ca 2+ channel Re c e ptor Slide 5 Slide 6 Figure 8.19a Synaptic Communication Figure 8.19a Synaptic Communication Neurotransmitter Release Neurotransmitter Release An ac tion pote ntial depolariz e s the axon terminal. An ac tion pote ntial depolariz e s the axon terminal. Action potential arrives at axon terminal. Synaptic vesicle with neurotransmitter molecules The depolarization opens voltagegated Ca 2+ channels, and Ca 2+ enters the cell. Action potential arrives at axon terminal. Synaptic vesicle with neurotransmitter molecules The depolarization opens voltagegated Ca 2+ channels, and Ca 2+ enters the cell. Ca lcium entry triggers ex ocy tosis of synaptic vesicle contents. Ca lcium entry triggers ex ocy tosis of synaptic vesicle contents. Docking protein Ca 2+ Synaptic cleft Neurotransmitter diffuses across the synaptic cleft and binds with receptors on the postsynaptic cell. Docking protein Ca 2+ Synaptic cleft Neurotransmitter diffuses across the synaptic cleft and binds with receptors on the postsynaptic cell. Neurotransmitter binding initiates Postsynaptic cell Voltage-gated Ca 2+ channel Re c e ptor Postsynaptic cell Voltage-gated Ca 2+ channel Ce ll re s ponse Re c e ptor a response in the postsynaptic cell. 4

5 Figure 8.18 A chemical synapse Schwann cell Axon terminal Removal of neurotransmitter from synaptic cleft Mitochondr io n Vesicles with neur otr ansm itter 1. Diffusion 2. Enzymatic degradation 3. Reuptake and uptake Synaptic cleft Muscle fiber Fig. 8.19b Fig

6 Receptor subtypes Ionotropic alter ion channel function Metabotropic work through second messenger systems Fig. 6.8a Copy right 2010 Pears on Educ ation, Inc. Sy napse between a motor neuron and a skeletal muscle fiber Ac ety lc holine (ACh) is the neurotransmitter at the NMJ ACh binds to nicotinic AChreceptors on the postsynaptic membrane (= motor end plate) Nicotinic receptors are chemically-gated ion channels that are permeable to Na+ Na enters the postsynaptic cell, resulting in a graded depolarization of the muscle fiber The Neuromuscular Junction Na + Ion Channels and Membrane Potentials A. Ungated Channels Example: K + leak channel Function: Resting Membrane Potential B. Gated Channels 1. Voltage-gated Channels Example: Voltage-gated Na + channel Function: Action Potential 2. Chemically-gated Channels (Ligand-gated channels) Example: ACh receptor-channel (Nicotinic AChreceptor) Function: Graded Potentials: synaptic transmission 3. Mechanically-gated Channels Example: Sensory receptors (several types) Function: Graded Potentials: sensory transduction 6

7 Postsynaptic Potential Graded potential in the postsynaptic cell membrane that results from binding of neurotransmitter to receptors. Excitatory postsynaptic potential (EPSP) Postsynaptic Potentials Excitatory postsynaptic potential (EPSP) - increases the likelihood of a response (AP) by the postsynaptic cell Inhibitory postsynaptic potential (IPSP) - decreases the likelihood of a response (AP) by the postsynaptic cell Neural Integration Each neuron receives multiple inputs from other neurons. Spatial Summation EPSPs from different synapses can add together to exceed threshold. Excitatory postsynaptic potentials (EPSPs) Inhibitory postsynaptic potentials (IPSPs) 7

8 Spatial Summation with Inhibition IPSPs tend to cancel out EPSPs to prevent reaching threshold. Temporal Summation EPSPs that occur close together in time can add together to exceed threshold. Neural Networks Divergent and Convergent Pathways Responses of Postsynaptic Cells 8

9 Acetylcholine (Cholinergic) Receptors Nicotinic Cholinergic Receptor - Ion channel-receptor - Fast response - Always excitatory Locations: Neuromuscular junction Ganglia of Autonomic NS Acetylcholine (Cholinergic) Receptors Muscarinic Cholinergic Receptor - G protein-coupled receptor - Slower response - mediated by a G protein which activates a second messenger signaling system - Can be excitatory or inhibitory Locations: Target cells of the parasympathetic division of the autonomic NS 9

10 Norepinephrine and Epinephrine (Adrenergic) Receptors Locations: Target cells of the sympathetic division of the autonomic NS Alpha (α) Adrenergic Receptors - G protein-coupled receptors - activate a second messenger system - excitatory Location: vascular smooth muscle (vasoconstriction) β receptor Norepinephrine and Epinephrine (Adrenergic) Receptors Locations: Target cells of the sympathetic division of the autonomic NS Beta (β) Adrenergic Receptors - G protein-coupled receptors - activate a second messenger system - excitatory or inhibitory Locations: cardiac muscle (excitatory); bronchial smooth muscle (inhibitory) 10

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