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Chapter 7 The Nervous System: Neurons and Synapses Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-1 Nervous System (NS) Is divided into: Central nervous system (CNS) = brain and spinal cord, eye Peripheral nervous system (PNS) = cranial and spinal nerves Enteuic ns (gut) Autonomic- parasympathetic Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-4 Nervous System (NS) continued Consists of 2 kinds of cells: Neurons and supporting cells (= glial cells) Neurons are functional units of NS Glial cells maintain homeostasis Are 5X more common than neurons Are important for myelination Structural support neural survival-neurotrophin neurotoxicity protection Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-5 Neurons continued Have a cell body, dendrites and axon Cell body contains the nucleus Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-8 Neurons Gather and transmit information by: Responding to stimuli Producing and sending electrochemical impulses Releasing chemical messages Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-7 Neurons continued Dendrites receive information, convey it to cell body Axons conduct impulses away from cell body Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-10 Neurons continued Axonal transport moves large and insoluble compounds bidirectionally along microtubules; very fast Anterograde transport moves materials away from cell body Uses the molecular motor kinesin Retrograde transport moves materials toward cell body Uses the molecular motor dynein Viruses and toxins can enter CNS this way Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-12 Functional Classification of Neurons Sensory/Afferent neurons conduct impulses into CNS Motor/Efferent neurons carry impulses out of CNS Association/ Interneurons integrate NS activity Located entirely inside CNS Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-13 Structural Classification of Neurons Pseudounipolar: Cell body sits along side of single process e.g. sensory neurons Bipolar: Dendrite and axon arise from opposite ends of cell body e.g. retinal neurons Multipolar: Have many dendrites and one axon e.g. motor neurons Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-14 Supporting/Glial Cells PNS has Schwann and satellite cells Schwann cells myelinate PNS axons Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-16 Supporting/Glial Cells continued CNS has oligodendrocytes, microglia, astrocytes and ependymal cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-17 Supporting/Glial Cells continued Each oligodendrocyte myelinates several CNS axons Ependymal cells appear to be neural stem cells Other glial cells are involved in NS maintenance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-18 Myelination In PNS each Schwann cell myelinates 1mm of 1 axon by wrapping round and round axon Electrically insulates axon Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-19 Myelination continued Uninsulated gap between adjacent Schwann cells is called the node of Ranvier Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-20 Axon Regeneration Occurs much more readily in PNS than CNS Oligodendrocytes produce proteins that inhibit regrowth And form glial scar tissue that blocks regrowth Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-21 Nerve Regeneration continued When axon in PNS is severed: Distal part of axon degenerates Schwann cells survive; form regeneration tube Tube releases chemicals that attract growing axon Tube guides regrowing axon to synaptic site Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-22 Neurotrophins Promote fetal nerve growth Required for survival of many adult neurons Important in regeneration NGF-Nerve Growth Factor Read the abstract and intro to the paper on WEBCT… Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-23 Astrocytes Most common glial cell Involved in: Buffering K+ levels Recycling neurotransmitters Regulating adult neurogenesis Releasing transmitters that regulate neuronal activity Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-24 Blood-Brain Barrier Allows only certain compounds to enter brain Formed by capillary specializations in brain That appear to be induced by astrocytes Capillaries are not as leaky as those in body Gaps between adjacent cells are closed by tight junctions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-25 Resting Membrane Potential (RMP) At rest, all cells have a negative internal charge and unequal distribution of ions: Results from: Large cations being trapped inside cell Na+/K+ pump and limited permeability keep Na+ high outside cell K+ is very permeable and is high inside cell Attracted by negative charges inside Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-27 Excitability Excitable cells can discharge their RMP quickly By rapid changes in permeability to ions Neurons and muscles do this to generate and conduct impulses Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-28 Membrane Potential (MP) Changes Measured by placing 1 electrode inside cell and 1 outside Depolarization occurs when MP becomes more positive Hyperpolarization: MP becomes more negative than RMP Repolarization: MP returns to RMP Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-29 Membrane Ion Channels MP changes occur by ion flow through membrane channels Some channels are normally open; some closed K+ leakage channels are always open Closed channels have molecular gates that can be opened Voltage-gated (VG) channels are opened by depolarization VG K+ channels are closed in resting cells Na+ channels are VG; closed in resting cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-30 Model of a Voltage-gated Ion Channel ) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-31 The Action Potential (AP) Is a wave of MP change that sweeps along the axon from soma to synapse Wave is formed by rapid depolarization of the membrane by Na+ influx; followed by rapid repolarization by K+ efflux Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-33 Mechanism of Action Potential Depolarization: threshold, VG Na+ channels open Na+ driven inward by its electrochemical gradient This adds to depolarization, opens more channels Termed a positive feedback loop Causes a rapid change in MP from –70 to +30 mV At Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-34 Mechanism of Action Potential continued Repolarization: Na+ channels close; VG K+ channels open Electrochemical gradient drives K+ outward Repolarizes axon back to RMP VG Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-35 Mechanism of Action Potential continued Depolarization and repolarization occur via diffusion Do not require active transport After an AP, Na+/K+ pump extrudes Na+, recovers K+ Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-36 Mechanism of Action Potential continued Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-37 APs Are All-or-None When MP reaches threshold an AP is irreversibly fired Because positive feedback opens more and more Na+ channels Shortly after opening, Na+ channels close and become inactivated until repolarization Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-38 How Stimulus Intensity is Coded Increased stimulus intensity causes more APs to be fired Size of APs remains constant Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-39 Refractory Periods Absolute refractory period: Membrane cannot produce another AP because Na+ channels are inactivated Relative refractory period occurs when VG K+ channels are open, making it harder to depolarize to threshold Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-40 Conduction in an Unmyelinated Axon After axon hillock reaches threshold and fires AP, its Na+ influx depolarizes adjacent regions to threshold Generating a new AP Process repeats all along axon So AP amplitude is always same Conduction is slow Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-43 Conduction in Myelinated Axon Ions can't flow across myelinated membrane Thus no APs occur under myelin and no current leaks This increases current spread Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-44 Conduction in Myelinated Axon continued Gaps in myelin are called Nodes of Ranvier APs occur only at nodes VG Na+ channels are present only at nodes Current from AP at 1 node can depolarize next node to threshold Fast because APs skip from node to node Called saltatory conduction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-45 Synaptic Transmission 7-46 Synapse Is a functional connection between a neuron (presynaptic) and another cell (postsynaptic) There are chemical and electrical synapses Synaptic transmission at chemical synapses is via neurotransmitters (NT) Electrical synapses are rare in NS Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-47 Electrical Synapse Depolarization flows from presynaptic into postsynaptic cell through channels called gap junctions Formed by connexin proteins Found in smooth and cardiac muscles, brain, and glial cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-48 Chemical Synapse Synaptic cleft separates terminal bouton of presynaptic from postsynaptic cell NTs are in synaptic vesicles Vesicles fuse with bouton membrane; release NT by exocytosis Amount of NT released depends upon frequency of APs Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-49 Synaptic Transmission APs travel down axon to depolarize bouton Open VG Ca2+ channels in bouton Ca2+ is driven in by electrochemical gradient Triggers exocytosis of vesicles; release of NTs Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-50 Synaptic Transmission continued Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-54 Acetylcholine (ACh) Most widely used NT Used in brain and ANS; used at all neuromuscular junctions Has nicotinic and muscarinic receptor subtypes These can be excitatory or inhibitory Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-55 Nicotinic ACh Channel Formed by 5 polypeptide subunits 2 subunits contain ACh binding sites Opens when 2 AChs bind Permits diffusion of Na+ into and K+ out of postsynaptic cell Inward flow of Na+ dominates Produces EPSPs Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-57 Muscarinic ACh Channel Binding of 1 ACh activates G-protein cascade which affects gated K+ channels Opens some, causing hyperpolarization Closes others, causing depolarization Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-59 Acetylcholinesterase (AChE) Inactivates ACh, terminating its action; located in cleft Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-60 Neurotransmitters 7-61 Acetylcholine in the PNS Cholinergic neurons use acetylcholine as NT The large synapses on skeletal muscle are termed end plates or neuromuscular junctions (NMJ) Produce large EPSPs called end-plate potentials Open VG channels beneath end plate Cause muscle contraction Curare blocks ACh action at NMJ Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-62 Monoamine NTs Include serotonin, norepinephrine and dopamine Serotonin is derived from tryptophan Norepinephrine and dopamine are derived from tyrosine Called catecholamines Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-63 Monoamine NTs continued After release, are mostly inactivated by: Presynaptic reuptake And breakdown by monoamine oxidase (MAO) MAO inhibitors are antidepressants -TRANSPORTER Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-64 Serotonin Involved in regulation of mood, behavior, appetite and cerebral circulation LSD is structurally similar SSRIs (serotonin-specific reuptake inhibitors) are antidepressants e.g., Prozac, Zoloft, Paxil, Luvox Block reuptake of serotonin, prolonging its action Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-66 Dopamine There are 2 major dopamine systems in brain Nigrostriatal dopamine system originates in the substantia nigra and is involved in motor control Degeneration of this system causes Parkinson's disease Mesolimbic dopamine system is involved in behavior and emotional reward Most addictions activate this system Overactivity contributes to schizophrenia Which is treated by anti-dopamine drugs Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-67 Norepinephrine (NE) Used in PNS and CNS In PNS is a sympathetic NT In CNS affects general level of arousal Amphetamines stimulate NE pathways Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-68 Polypeptide NTs (neuropeptides) continued CCK promotes satiety following meals Substance P is a pain NT Endorphins, enkephalins and dynorphin are endogenous opioid NTs Promote analgesia and mediate many placebo effects Effects are blocked by naloxone, an opiate antagonist Neuropeptide Y is most common neuropeptide Inhibits glutamate in hippocampus Powerful stimulator of appetite Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-71 Polypeptide NTs (neuropeptides) continued Endocannabinoids - similar to THC in marijuana Are the only lipid NTs Not stored in vesicles; are produced from lipids of the plasma membrane Are retrograde NTs: they act on neuron that releases them And thereby may be involved in learning Like THC, they stimulate appetite Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 7-73
