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Synapses Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.17 Synaptic Cleft Fluid-filled space separating the presynaptic and postsynaptic neurons Prevents nerve impulses from directly passing from one neuron to the next Transmission across the synaptic cleft: Is a chemical event (as opposed to an electrical one) Ensures unidirectional communication between neurons Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Synaptic Cleft: Information Transfer Ca2+ 1 Neurotransmitter Axon terminal of presynaptic neuron Postsynaptic membrane Mitochondrion Axon of presynaptic neuron Na+ Receptor Postsynaptic membrane Ion channel open Synaptic vesicles containing neurotransmitter molecules 5 Degraded neurotransmitter 2 Synaptic cleft Ion channel (closed) 3 4 Ion channel closed Ion channel (open) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.18 Synaptic Cleft: Information Transfer Ca2+ 1 Axon terminal of presynaptic neuron Axon of presynaptic neuron Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.18 Synaptic Cleft: Information Transfer Ca2+ 1 Axon terminal of presynaptic neuron Mitochondrion Axon of presynaptic neuron Synaptic vesicles containing neurotransmitter molecules 2 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.18 Synaptic Cleft: Information Transfer Ca2+ 1 Axon terminal of presynaptic neuron Mitochondrion Postsynaptic membrane Axon of presynaptic neuron Synaptic vesicles containing neurotransmitter molecules 2 Synaptic cleft Ion channel (closed) 3 Ion channel (open) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.18 Synaptic Cleft: Information Transfer Ca2+ 1 Neurotransmitter Axon terminal of presynaptic neuron Postsynaptic membrane Mitochondrion Axon of presynaptic neuron Na+ Receptor Postsynaptic membrane Ion channel open Synaptic vesicles containing neurotransmitter molecules 2 Synaptic cleft Ion channel (closed) 3 4 Ion channel (open) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.18 Synaptic Cleft: Information Transfer Ca2+ 1 Neurotransmitter Axon terminal of presynaptic neuron Postsynaptic membrane Mitochondrion Axon of presynaptic neuron Na+ Receptor Postsynaptic membrane Ion channel open Synaptic vesicles containing neurotransmitter molecules 5 Degraded neurotransmitter 2 Synaptic cleft Ion channel (closed) 3 4 Ion channel closed Ion channel (open) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.18 Synaptic Delay Neurotransmitter must be released, diffuse across the synapse, and bind to receptors Synaptic delay – time needed to do this (0.3-5.0 ms) Synaptic delay is the rate-limiting step of neural transmission Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Postsynaptic Potentials The two types of postsynaptic potentials are: EPSP – excitatory postsynaptic potentials IPSP – inhibitory postsynaptic potentials Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Excitatory Postsynaptic Potential (EPSP) Open Na+ channels Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.19a Inhibitory Postsynaptic (IPSP) Open K+ and Cl- channels Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.19b Summation A single EPSP cannot induce an action potential Temporal summation Spatial summation Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.20 Chemical Neurotransmitters 50 different neurotransmitters Acetylcholine (ACh) Biogenic amines Amino acids Peptides Novel messengers: ATP and dissolved gases NO and CO Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters: Acetylcholine Degraded by the enzyme acetylcholinesterase (AChE) Released by: All neurons that stimulate skeletal muscle Some neurons in the autonomic nervous system Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Tetanus – bacterial toxin Neurotransmitters: Biogenic Amines Catecholamines – dopamine, norepinephrine (NE), and epinephrine Indolamines – serotonin and histamine emotional behaviors Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters: Amino Acids Include: GABA – Gamma ()-aminobutyric acid Glycine Aspartate Glutamate Found only in the CNS Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters: Peptides Include: Substance P – mediator of pain signals Beta endorphin, dynorphin, and enkephalins Act as natural opiates; reduce pain perception Bind to the same receptors as opiates and morphine Gut-brain peptides – somatostatin, and cholecystokinin Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters: Novel Messengers ATP Is found in both the CNS and PNS Produces excitatory or inhibitory responses depending on receptor type Induces Ca2+ wave propagation in astrocytes Provokes pain sensation Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters: Novel Messengers Nitric oxide (NO) Activates the intracellular receptor guanylyl cyclase Is involved in learning and memory Carbon monoxide (CO) is a main regulator of cGMP in the brain Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Functional Classification of Neurotransmitters Two classifications: excitatory and inhibitory Excitatory neurotransmitters cause depolarizations (e.g., glutamate) Inhibitory neurotransmitters cause hyperpolarizations (e.g., GABA and glycine) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Functional Classification of Neurotransmitters Some neurotransmitters have both excitatory and inhibitory effects Determined by the receptor type of the postsynaptic neuron Example: acetylcholine Excitatory at neuromuscular junctions with skeletal muscle Inhibitory in cardiac muscle Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitter Receptor Mechanisms Direct: neurotransmitters that open ion channels Promote rapid responses Examples: ACh Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings