Nervous system works because information flows from neuron to neuron 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 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.

30 – 50 nm wide (~1/1,000,000th of an inch) 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.

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 Action potential arrives at axon terminal. 1 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 Action potential arrives at axon terminal. 1 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 2 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 Action potential arrives at axon terminal. 1 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 2 Mitochondrion Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis 3 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 Action potential arrives at axon terminal. 1 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 2 Mitochondrion Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis 3 Synaptic cleft Axon terminal Synaptic vesicles Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. 4 Postsynaptic neuron © 2013 Pearson Education, Inc.

ion channels, resulting in graded potentials. 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.

terminated by reuptake through transport proteins, enzymatic 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 Action potential arrives at axon terminal. 1 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 2 Mitochondrion Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis 3 Synaptic cleft Axon terminal Synaptic vesicles Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. 4 Postsynaptic neuron Ion movement Enzymatic degradation Graded potential Reuptake Diffusion away from synapse Binding of neurotransmitter opens ion channels, resulting in graded potentials. 5 Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. 6 © 2013 Pearson Education, Inc.

Synaptic delay is rate-limiting step of neural transmission 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.

Language of nervous system 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 ACh excitatory at neuromuscular junctions in skeletal muscle ACh inhibitory in cardiac muscle © 2013 Pearson Education, Inc.

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 Discharge zone Facilitated zone © 2013 Pearson Education, Inc.

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 Interneuron 1 Receptor 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.