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The Synapse • Nervous system works because information flows from neuron to neuron • Neurons functionally connected by synapses – Junctions that mediate information transfer • From one neuron to another neuron • Or from one neuron to an effector cell © 2013 Pearson Education, Inc. Synapse Classification • Axodendritic—between axon terminals of one neuron and dendrites of others • Axosomatic—between axon terminals of one neuron and soma of others © 2013 Pearson Education, Inc. Important Terminology • Presynaptic neuron – Neuron conducting impulses toward synapse – Sends the information • Postsynaptic neuron (in Pns may be a neuron, muscle cell, or gland cell) – Neuron transmitting electrical signal away from synapse – Receives the information • Most function as both PLAY Animation: Synapses © 2013 Pearson Education, Inc. Figure 11.16 Synapses. Axodendritic synapses Dendrites Axosomatic synapses Cell body Axoaxonal synapses Axon Axon Axosomatic synapses Cell body (soma) of postsynaptic neuron © 2013 Pearson Education, Inc. Chemical Synapses • Specialized for release and reception of chemical neurotransmitters • Typically composed of two parts – Axon terminal of presynaptic neuron • Contains synaptic vesicles filled with neurotransmitter – Neurotransmitter receptor region on postsynaptic neuron's membrane • Usually on dendrite or cell body • Two parts separated by synaptic cleft – Fluid-filled space • Electrical impulse changed to chemical across synapse, then back into electrical © 2013 Pearson Education, Inc. Synaptic Cleft • 30 – 50 nm wide (~1/1,000,000th of an inch) • Prevents nerve impulses from directly passing from one neuron to next © 2013 Pearson Education, Inc. Synaptic Cleft • Transmission across synaptic cleft – Chemical event (as opposed to an electrical one) – Depends on release, diffusion, and receptor binding of neurotransmitters – Ensures unidirectional communication between neurons PLAY Animation: Neurotransmitters © 2013 Pearson Education, Inc. Information Transfer Across Chemical Synapses • AP arrives at axon terminal of presynaptic neuron • Causes voltage-gated Ca2+ channels to open – Ca2+ floods into cell • protein binds Ca2+ and promotes fusion of synaptic vesicles with axon membrane • Exocytosis of neurotransmitter into synaptic cleft occurs – Higher impulse frequency more released © 2013 Pearson Education, Inc. Information Transfer Across Chemical Synapses • Neurotransmitter diffuses across synapse • Binds to receptors on postsynaptic neuron – Often chemically-gated ion channels • Ion channels are opened • Causes an excitatory or inhibitory event (graded potential) • Neurotransmitter effects terminated © 2013 Pearson Education, Inc. Termination of Neurotransmitter Effects • Within a few milliseconds neurotransmitter effect terminated in one of three ways – Reuptake • By astrocytes or axon terminal – Degradation • By enzymes – Diffusion • Away from synaptic cleft © 2013 Pearson Education, Inc. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. Mitochondrion Synaptic cleft Axon terminal Synaptic vesicles Postsynaptic neuron © 2013 Pearson Education, Inc. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. Mitochondrion Synaptic cleft Axon terminal Synaptic vesicles Postsynaptic neuron © 2013 Pearson Education, Inc. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 3 Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis Mitochondrion Synaptic cleft Axon terminal Synaptic vesicles Postsynaptic neuron © 2013 Pearson Education, Inc. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 3 Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis 4 Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. © 2013 Pearson Education, Inc. Mitochondrion Synaptic cleft Axon terminal Synaptic vesicles Postsynaptic neuron Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters. Ion movement Graded potential 5 Binding of neurotransmitter opens ion channels, resulting in graded potentials. © 2013 Pearson Education, Inc. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters. Enzymatic degradation Reuptake Diffusion away from synapse 6 Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. © 2013 Pearson Education, Inc. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Presynaptic neuron Postsynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 3 Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis Mitochondrion Synaptic cleft Axon terminal Synaptic vesicles 4 Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. Postsynaptic neuron Ion movement Enzymatic degradation Graded potential Reuptake Diffusion away from synapse 5 Binding of neurotransmitter opens ion channels, resulting in graded potentials. 6 Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. © 2013 Pearson Education, Inc. Synaptic Delay • Time needed for neurotransmitter to be released, diffuse across synapse, and bind to receptors – 0.3–5.0 ms • Synaptic delay is rate-limiting step of neural transmission © 2013 Pearson Education, Inc. Postsynaptic Potentials • Neurotransmitter receptors cause graded potentials that vary in strength with – Amount of neurotransmitter released and – Time neurotransmitter stays in area © 2013 Pearson Education, Inc. Postsynaptic Potentials • Types of postsynaptic potentials – EPSP—excitatory postsynaptic potentials – IPSP—inhibitory postsynaptic potentials © 2013 Pearson Education, Inc. Excitatory Synapses and EPSPs • Neurotransmitter binding opens chemically gated channels • Allows simultaneous flow of Na+ and K+ in opposite directions • Na+ influx greater than K+ efflux net depolarization called EPSP (not AP) • EPSP help trigger AP if EPSP is of threshold strength – Can spread to axon hillock, trigger opening of voltage-gated channels, and cause AP to be generated © 2013 Pearson Education, Inc. Inhibitory Synapses and IPSPs • Reduces postsynaptic neuron's ability to produce an action potential – Makes membrane more permeable to K+ or Cl– • If K+ channels open, it moves out of cell • If Cl- channels open, it moves into cell – Therefore neurotransmitter hyperpolarizes cell • Inner surface of membrane becomes more negative • AP less likely to be generated © 2013 Pearson Education, Inc. Synaptic Integration: Summation • A single EPSP cannot induce an AP • EPSPs can summate to influence postsynaptic neuron • IPSPs can also summate • Most neurons receive both excitatory and inhibitory inputs from thousands of other neurons – Only if EPSP's predominate and bring to threshold AP © 2013 Pearson Education, Inc. Neurotransmitters • Language of nervous system • 50 or more neurotransmitters have been identified • Most neurons make two or more neurotransmitters – Neurons can exert several influences • Usually released at different stimulation frequencies • Classified by chemical structure and by function © 2013 Pearson Education, Inc. Classification of Neurotransmitters: Chemical Structure • Acetylcholine (ACh) – First identified; best understood – Released at neuromuscular junctions ,by some ANS neurons, by some CNS neurons – Synthesized from acetic and choline by enzyme choline acetyltransferase – Degraded by enzyme acetylcholinesterase (AChE) © 2013 Pearson Education, Inc. Classification of Neurotransmitters: Chemical Structure • Biogenic amines • Catecholamines – Dopamine, norepinephrine (NE), and epinephrine – Synthesized from amino acid tyrosine • Indolamines – Serotonin and histamine – Serotonin synthesized from amino acid tryptophan; histamine synthesized from amino acid histidine • Broadly distributed in brain – Play roles in emotional behaviors and biological clock • Some ANS motor neurons (especially NE) • Imbalances associated with mental illness © 2013 Pearson Education, Inc. Classification of Neurotransmitters: Chemical Structure • Peptides (neuropeptides) • Substance P – Mediator of pain signals • Endorphins – Beta endorphin, dynorphin and enkephalins – Act as natural opiates; reduce pain perception • Gut-brain peptides – Somatostatin and cholecystokinin © 2013 Pearson Education, Inc. Classification of Neurotransmitters: Function • Great diversity of functions • Can classify by – Effects – excitatory versus inhibitory – Actions – direct versus indirect © 2013 Pearson Education, Inc. Classification of Neurotransmitters: Function • Effects - excitatory versus inhibitory – Neurotransmitter effects can be excitatory (depolarizing) and/or inhibitory (hyperpolarizing) – Effect determined by receptor to which it binds • GABA and glycine usually inhibitory • Glutamate usually excitatory • Acetylcholine and NE bind to at least two receptor types with opposite effects © 2013 Pearson Education, Inc. – ACh excitatory at neuromuscular junctions in skeletal muscle – ACh inhibitory in cardiac muscle Basic Concepts of Neural Integration • Neurons function in groups • Groups contribute to broader neural functions • There are billions of neurons in CNS – Must be integration so the individual parts fuse to make a smoothly operating whole © 2013 Pearson Education, Inc. Figure 11.22 Simple neuronal pool. Presynaptic (input) fiber Facilitated zone © 2013 Pearson Education, Inc. Discharge zone Facilitated zone Patterns of Neural Processing: Serial Processing • Input travels along one pathway to a specific destination • System works in all-or-none manner to produce specific, anticipated response • Example – spinal reflexes – Rapid, automatic responses to stimuli – Particular stimulus always causes same response – Occur over pathways called reflex arcs • Five components: receptor, sensory neuron, CNS integration center, motor neuron, effector © 2013 Pearson Education, Inc. Figure 11.24 A simple reflex arc. Stimulus 1 Receptor Interneuron 2 Sensory neuron 3 Integration center 4 Motor neuron 5 Effector Spinal cord (CNS) Response © 2013 Pearson Education, Inc. Patterns of Neural Processing: Parallel Processing • Input travels along several pathways • Different parts of circuitry deal simultaneously with the information – One stimulus promotes numerous responses • Important for higher-level mental functioning • Example: a sensed smell may remind one of an odor and any associated experiences © 2013 Pearson Education, Inc.