Nervous System Chapter 12 PDF

Overview of the Nervous System 1 Endocrine and nervous systems maintain internal coordination – Endocrine system: communicates by means of chemical messengers (hormones) secreted into to the blood – Ner...

Overview of the Nervous System 1 Endocrine and nervous systems maintain internal coordination – Endocrine system: communicates by means of chemical messengers (hormones) secreted into to the blood – Nervous system: employs electrical and chemical means to send messages from cell to cell ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Overview of the Nervous System 2 Nervous system carries out its task in three basic steps Sense organs receive information about changes in the body and external environment, and transmit coded messages to the brain and spinal cord (CNS: central nervous system) CNS processes this information, relates it to past experiences, and determines appropriate response CNS issues commands to muscles and gland cells to carry out such a response ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Overview of the Nervous System 3 Two major subdivisions of nervous system – Central nervous system (CNS) Brain and spinal cord enclosed by cranium and vertebral column – Peripheral nervous system (PNS) All the nervous system except the brain and spinal cord; composed of nerves and ganglia Nerve—a bundle of nerve fibers (axons) wrapped in fibrous connective tissue Ganglion—a knot-like swelling in a nerve where neuron cell bodies are concentrated ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Overview of the Nervous System 4 Peripheral nervous system contains sensory and motor divisions each with somatic and visceral subdivisions – Sensory (afferent) division: carries signals from receptors to CNS Somatic sensory division: carries signals from receptors in the skin, muscles, bones, and joints Visceral sensory division: carries signals from the viscera (heart, lungs, stomach, and urinary bladder) ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Overview of the Nervous System 5 Motor (efferent) division—carries signals from CNS to effectors (glands and muscles that carry out the body’s response) – Somatic motor division: carries signals to skeletal muscles Output produces muscular contraction as well as somatic reflexes—involuntary muscle contractions – Visceral motor division (autonomic nervous system)—carries signals to glands, cardiac and smooth muscle Its involuntary responses are visceral reflexes ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Overview of the Nervous System 6 Visceral motor division (autonomic nervous system) – Sympathetic division Tends to arouse body for action Accelerating heart beat and respiration, while inhibiting digestive and urinary systems – Parasympathetic division Tends to have calming effect Slows heart rate and breathing Stimulates digestive and urinary systems They work opposite to each other ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Subdivisions of the Nervous System 1 Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.1 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Subdivisions of the Nervous System 2 Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.2 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Figure 11.2 Levels of organization in the nervous system. Central nervous system (CNS) Peripheral nervous system (PNS) Brain and spinal cord Cranial nerves and spinal nerves Integrative and control centers Communication lines between the CNS and the rest of the body Sensory (afferent) division Motor (efferent) division Somatic and visceral sensory Motor nerve fibers nerve fibers Conducts impulses from the CNS Conducts impulses from to effectors (muscles and glands) receptors to the CNS Somatic nervous Autonomic nervous Somatic sensory fiber Skin system system (ANS) Somatic motor Visceral motor (voluntary) (involuntary) Conducts impulses Conducts impulses from the CNS to from the CNS to skeletal muscles cardiac muscles, smooth muscles, Visceral sensory fiber and glands Stomach Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Parasympathetic Mobilizes body systems division during activity Conserves energy Promotes house- keeping functions during rest Sympathetic motor fiber of ANS Heart Structure Function Sensory (afferent) division of PNS Parasympathetic motor fiber of ANS Bladder Motor (efferent) division of PNS © 2013 Pearson Education, Inc. ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Universal Properties of Neurons Excitability (irritability) – Respond to environmental changes called stimuli Conductivity – Respond to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations Secretion – When an electrical signal reaches the end of nerve fiber, the cell secretes a chemical neurotransmitter that influences the next cell ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Functional Classes of Neurons Sensory (afferent) neurons – Detect stimuli and transmit information about them toward the CNS Interneurons – Lie entirely within CNS connecting motor and sensory pathways (about 90% of all neurons) – Receive signals from many neurons and carry out integrative functions (make decisions on responses) Motor (efferent) neuron – Send signals out to muscles and gland cells (the effectors) ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Three Functional Classes of Neurons Copyright © McGraw-Hill Education. Permission required for reproduction or display. Peripheral nervous Central nervous system system 1)Sensory (afferent) neurons conduct signals from receptors to the CNS. 3)Motor (efferent) 2)Interneuron neurons conduct s are signals from the confined to CNS to effectors the CNS. such as muscles and glands. Figure 12.3 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Structure of a Neuron 1 Neurosoma—control center of neuron – Also called neurosoma or cell body – Has a single, centrally located nucleus with large nucleolus – Cytoplasm contains mitochondria, lysosomes, Golgi complex, inclusions, extensive rough ER and cytoskeleton Inclusions: glycogen, lipid droplets, melanin, and lipofuscin pigment (produced when lysosomes digest old organelles) Cytoskeleton has dense mesh of microtubules and neurofibrils (bundles of actin filaments) that compartmentalizes rough ER into dark-staining chromatophilic substance ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Structure of a Neuron 2 Dendrites—branches that come off the neurosoma – Primary site for receiving signals from other neurons – The more dendrites the neuron has, the more information it can receive – Provide precise pathways for the reception and processing of information Axon (nerve fiber)—originates from a mound on the neurosoma called the axon hillock Axon is cylindrical, relatively unbranched for most of its length – Axon collaterals—branches of axon – Branch extensively on distal end – Specialized for rapid conduction of signals to distant points – Axoplasm: cytoplasm of axon – Axolemma: plasma membrane of axon – Only one axon per neuron (some neurons have none) – Myelin sheath may enclose axon- missing in MS ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Structure of a Neuron 3 Distal end of axon has terminal arborization: extensive complex of fine branches Axon terminal—little swelling that forms a junction (synapse) with the next cell – Contains synaptic vesicles full of neurotransmitter ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  General Structure of a Neuron Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.4a ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Click to edit Master title style Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Nucleus Axon (impulse- Myelin sheath gap Nucleolus generating Impulse (node of Ranvier) and -conducting direction Chromatophilic substance (rough region) Axon endoplasmic terminals reticulum) Schwann cell (secretory region) Axon hillock Terminal branches © 2013 Pearson Education, Inc. ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Axonal Transport 1 Many proteins made in neurosoma must be transported to axon and axon terminal – To repair axolemma, serve as gated ion channels, enzymes or neurotransmitters Axonal transport—two-way passage of proteins, organelles, and other material along an axon – Anterograde transport: movement down the axon away from neurosoma – Retrograde transport: movement up the axon toward the neurosoma Microtubules guide materials along axon – Motor proteins (kinesin and dynein) carry materials “on their backs” while they “crawl” along microtubules Kinesin—motor proteins in anterograde transport Dynein—motor proteins in retrograde transport ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Axonal Transport 2 Fast axonal transport—rate of 20 to 400 mm/day – Fast anterograde transport Organelles, enzymes, synaptic vesicles, and small molecules – Fast retrograde transport For recycled materials and pathogens—rabies, herpes simplex, tetanus, polio viruses – Delay between infection and symptoms is time needed for transport up the axon Slow axonal transport—0.5 to 10 mm/day – Always anterograde – Moves enzymes, cytoskeletal components, and new axoplasm down the axon during repair and regeneration of damaged axons – Damaged nerve fibers regenerate at a speed governed ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Supportive Cells (Neuroglia) About 1 trillion neurons in the nervous system Neuroglia outnumber neurons by at least 10 to 1 Neuroglia or glial cells – Protect neurons and help them function – Bind neurons together and form framework for nervous tissue – In fetus, guide migrating neurons to their destination – If mature neuron is not in synaptic contact with another neuron, it is covered by glial cells ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Types of Neuroglia 1 Four types of glia occur in CNS: oligodendrocytes, ependymal cells, microglia, and astrocytes – Oligodendrocytes Form myelin sheaths in CNS that speed signal conduction – Arm-like processes wrap around nerve fibers – Ependymal cells Line internal cavities of the brain; secrete and circulate cerebrospinal fluid (CSF) – Cuboidal epithelium with cilia on apical surface – Microglia Wander through CNS looking for debris and damage – Develop from white blood cells (monocytes) and become concentrated in areas of damage ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Types of Neuroglia 2 Astrocytes – Most abundant glial cell in CNS, covering brain surface and most nonsynaptic regions of neurons in the gray matter – Diverse functions: – Form supportive framework – Have extensions (perivascular feet) that contact blood capillaries and stimulate them to form a seal called the blood–brain barrier – Monitor neuron activity and regulate blood flow to match metabolic need – Convert glucose to lactate and supply this to neurons – Secrete nerve growth factors – Communicate electrically with neurons – Regulate chemical composition of tissue fluid by absorbing excess neurotransmitters and ions – Astrocytosis or sclerosis—when neuron is damaged, astrocytes form hardened scar tissue and fill in space ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Neuroglial Cells of CNS Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.6 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Types of Neuroglia 3 Two types occur only in PNS – Schwann cells Envelope nerve fibers in PNS Wind repeatedly around a nerve fiber Produce a myelin sheath similar to the ones produced by oligodendrocytes in CNS Assist in regeneration of damaged fibers – Satellite cells Surround the neurosomas in ganglia of the PNS, act like astrocytes in the PNS Provide electrical insulation around the neurosoma Regulate the chemical environment of the neurons ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Myelin 1 Myelin sheath—insulation around a nerve fiber – Formed by oligodendrocytes in CNS and Schwann cells in PNS – Consists of the plasma membrane of glial cells 20% protein and 80% lipid Myelination—production of the myelin sheath – Begins at week 14 of fetal development – Proceeds rapidly during infancy – Completed in late adolescence ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Myelin 2 In PNS, Schwann cell spirals repeatedly around a single nerve fiber – Lays down as many as one hundred layers of membrane – No cytoplasm between the membranes – Neurilemma: thick, outermost coil of myelin sheath Contains nucleus and most of its cytoplasm External to neurilemma is basal lamina and a thin layer of fibrous connective tissue— endoneurium ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Myelin Sheath in PNS Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.4c ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Myelination in PNS Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.7a ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Myelin 3 In CNS—an oligodendrocyte myelinates several nerve fibers in its immediate vicinity – Anchored to multiple nerve fibers – Cannot migrate around any one of them like Schwann cells – Must push newer layers of myelin under the older ones; so myelination spirals inward toward nerve fiber – Nerve fibers in CNS have no neurilemma or endoneurium ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Myelination in CNS Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.7b ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Myelin 4 Many Schwann cells or oligodendrocytes are needed to cover one nerve fiber Myelin sheath is segmented – Nodes of Ranvier: gap between segments – Internodes: myelin-covered segments from one gap to the next – Initial segment: short section of nerve fiber between the axon hillock and the first glial cell – Trigger zone: the axon hillock and the initial segment Play an important role in initiating a nerve signal ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Glial Cells and Brain Tumors Tumors—masses of rapidly dividing cells – Mature neurons have little or no capacity for mitosis and seldom form tumors Brain tumors arise from: – Meninges (protective membranes of CNS) – Metastasis from nonneuronal tumors in other organs – Often glial cells that are mitotically active throughout life Gliomas grow rapidly and are highly malignant – Blood–brain barrier decreases effectiveness of chemotherapy – Treatment consists of radiation or surgery ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Diseases of the Myelin Sheath 1 Degenerative disorders of the myelin sheath – Multiple sclerosis Oligodendrocytes and myelin sheaths in the CNS deteriorate Myelin replaced by hardened scar tissue Nerve conduction disrupted (double vision, tremors, numbness, speech defects) Onset between 20 and 40 and fatal from 25 to 30 years after diagnosis Cause may be autoimmune triggered by virus ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Unmyelinated Nerve Fibers 1 Many CNS and PNS fibers are unmyelinated ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Unmyelinated Nerve Fibers 2 Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.8 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Conduction Speed of Nerve Fibers 1 Speed at which a nerve signal travels along surface of nerve fiber depends on two factors – Diameter of fiber Larger fibers have more surface area and conduct signals more rapidly – Presence or absence of myelin Myelin further speeds signal conduction ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Conduction Speed of Nerve Fibers 2 Conduction speed – Small, unmyelinated fibers: 0.5 to 2.0 m/s – Small, myelinated fibers: 3 to 15.0 m/s – Large, myelinated fibers: up to 120 m/s – Slow signals sent to the gastrointestinal tract where speed is less of an issue – Fast signals sent to skeletal muscles where speed improves balance and coordinated body movement ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Electrical Potentials and Currents Electrophysiology—study of cellular mechanisms for producing electrical potentials and currents – Basis for neural communication and muscle contraction Electrical potential—a difference in concentration of charged particles between one point and another – Living cells are polarized and have a resting membrane potential – Cells have more negative particles on inside of membrane than outside – Neurons have about −70 mV resting membrane potential Electrical current—a flow of charged particles from one point to another – In the body, currents are movements of ions, such as or , through channels in the plasma membrane – Gated channels are opened or closed by various stimuli – Enables cell to turn electrical currents on and off ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  The Resting Membrane Potential 1 Resting membrane potential (RMP) exists because of unequal electrolyte distribution between extracellular fluid (ECF) and intracellular fluid (ICF) RMP results from the combined effect of three factors – Ions diffuse down their concentration gradient through the membrane – Plasma membrane is selectively permeable and allows some ions to pass easier than others – Electrical attraction of cations and anions to each other ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  The Resting Membrane Potential 2 Potassium has greatest influence on RMP – Plasma membrane is more permeable to than any other ion – Leaks out until electrical charge of cytoplasmic anions attracts it back in and equilibrium is reached (no more net movement of ) – is about 40 times as concentrated in the ICF as in the ECF Cytoplasmic anions cannot escape due to size or charge (phosphates, sulfates, small organic acids, proteins, ATP, and RNA) ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  The Resting Membrane Potential 3 Membrane is not very permeable to sodium but RMP is slightly influenced by it – is about 12 times as concentrated in the ECF as in the ICF – Some leaks into the cell, diffusing down its concentration and electrical gradients – This leakage makes RMP slightly less negative than it would be if RMP were determined solely by ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  The Resting Membrane Potential 4 / pump moves 3 out for every 2 it brings in – Works continuously to compensate for and leakage, and requires great deal of ATP (1 ATP per exchange) 70% of the energy requirement of the nervous system – Necessitates glucose and oxygen be supplied to nerve tissue (energy needed to create the resting potential) – The exchange of 3 positive charges for only 2 positive charges contributes about −3 mV to the cell’s resting membrane potential of −70 mV ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Ionic Basis of the Resting Membrane Potential Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.11 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Local Potentials 1 Local potentials—changes in membrane potential of a neuron occurring at and nearby the part of the cell that is stimulated Different neurons can be stimulated by chemicals, light, heat, or mechanical disturbance A chemical stimulant binds to a receptor on the neuron – Opens gates and allows to enter cell – Entry of a positive ion makes the cell less negative; this is a depolarization: a change in membrane potential toward zero mV – entry results in a current that travels toward the cell’s trigger zone; this short-range change in voltage is called a local potential ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Local Potentials 2 Properties of local potentials (unlike action potentials) – Graded: vary in magnitude with stimulus strength Stronger stimuli open more gates – Decremental: get weaker the farther they spread from the point of stimulation Voltage shift caused by inflow diminishes with distance – Reversible: if stimulation ceases, the cell quickly returns to its normal resting potential – Either excitatory or inhibitory: some neurotransmitters make the membrane potential more negative— hyperpolarize it—so it becomes less likely to produce an action potential ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Excitation of a Neuron by a Chemical Stimulus Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.12 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Action Potentials 1 Action potential—dramatic change in membrane polarity produced by voltage- gated ion channels – Only occurs where there is a high enough density of voltage-regulated gates – Neurosoma (50 to 75 gates per ); cannot generate an action potential – Trigger zone (350 to 500 gates per ); where action potential is generated If excitatory local potential reaches trigger zone and is still strong enough, it can open these gates and generate an action potential ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Action Potentials 2 Action potential is a rapid up-and-down shift in the membrane voltage involving a sequence of steps: – Arrival of current at axon hillock depolarizes membrane – Depolarization must reach threshold: critical voltage (about -55 mV) required to open voltage-regulated gates – Voltage-gated channels open, enters and depolarizes cell, which opens more channels resulting in a rapid positive feedback cycle as voltage rises ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Action Potentials 3 (Steps in action potential shift in membrane voltage, Continued) – As membrane potential rises above 0 mV, channels are inactivated and close; voltage peaks at about +35 mV – Slow channels open and outflow of repolarizes the cell – channels remain open for a time so that membrane is briefly hyperpolarized (more negative than RMP) – RMP is restored as leaks in and extracellular is removed by astrocytes ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  An Action Potential Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.13a ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written Actions of the Sodium and Potassium Channels During an Action Potential Copyright © McGraw-Hill Education. Permission required for reproduction or display. 1) and channels closed 2) channels open, enters cell, channels beginning to open 3) channels closed, 4) channels closed, channels fully open, channels closing leaves cell Figure 12.14 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Action Potentials 4 Only a thin layer of the cytoplasm next to the cell membrane is affected – In reality, very few ions are involved Action potential is often called a spike, as it happens so fast Characteristics of action potential (unlike local potential) – Follows an all-or-none law If threshold is reached, neuron fires at its maximum voltage If threshold is not reached, it does not fire – Nondecremental: do not get weaker with distance – Irreversible: once started, goes to completion and cannot be stopped ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Action Potential in Real-time Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.13 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  The Refractory Period During an action potential and for a few milliseconds after, it is difficult or impossible to stimulate that region of a neuron to fire again Refractory period—the period of resistance to stimulation Two phases – Absolute refractory period No stimulus of any strength will trigger AP Lasts as long as gates are open, then inactivated – Relative refractory period Only especially strong stimulus will trigger new AP gates are still open and any effect of incoming is opposed by the outgoing Generally lasts until hyperpolarization ends Only a small patch of neuron’s membrane is refractory at one time (other parts of the cell can be stimulated) ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  The Absolute and Relative Refractory Periods in Relation to the Action Potential Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.15 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Signal Conduction in Nerve Fibers 1 Unmyelinated fibers have voltage-gated channels along their entire length Action potential at trigger zone causes to enter the axon and diffuse into adjacent regions; this depolarization excites voltage-gated channels Opening of voltage-gated ion channels results in a new action potential which then allows diffusion to excite the membrane immediately distal to that Chain reaction continues until the nerve signal reaches the end of the axon – The nerve signal is like a wave of falling dominoes – Called continuous conduction ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Continuous Conduction of a Nerve Signal in an Unmyelinated Fiber Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.16 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Signal Conduction in Nerve Fibers 2 Myelinated fibers conduct signals with saltatory conduction—signal seems to jump from node to node Nodes of Ranvier contain many voltage-gated ion channels, while myelin-covered internodes contain few When enters the cell at a node, its electrical field repels positive ions inside the cell As these positive ions move away, their positive charge repels their positive neighbors, transferring energy down the axon rapidly (conducting the signal) Myelin speeds up this conduction by minimizing leakage of out of the cell and further separating the inner positive ions from attraction of negative ions outside cell – But the signal strength does start to fade in the internode When signal reaches the next node of Ranvier it is strong enough to open the voltage gated ion channels, and a new, full-strength action potential occurs ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Saltatory Conduction of a Nerve Signal in a Myelinated Fiber (a) Copyright © McGraw-Hill Education. Permission required for reproduction or display. inflow at node Positive charge flows Depolarization of generates action rapidly along axon and membrane at next node potential. depolarizes membrane; opens channels, triggering signal grows weaker with new action potential. distance. Figure 12.17a ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Saltatory Conduction of a Nerve Signal in a Myelinated Fiber (b) Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.17b ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Synapses 1 A nerve signal can go no further when it reaches the end of the axon – Triggers the release of a neurotransmitter – Stimulates a new wave of electrical activity in the next cell across the synapse Synapse between two neurons – First neuron in the signal path is the presynaptic neuron Releases neurotransmitter – Second neuron is postsynaptic neuron Responds to neurotransmitter ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Synapses 2 Presynaptic neuron may synapse with a dendrite, neurosoma, or axon of postsynaptic neuron to form axodendritic, axosomatic, or axoaxonic synapses A neuron can have an enormous number of synapses – Spinal motor neuron covered by about 10,000 axon terminals from other neurons 8,000 ending on its dendrites 2,000 ending on its neurosoma In the cerebellum of brain, one neuron can have as many as 100,000 synapses ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Synaptic Relationships Between Neurons Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.18 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Structure of a Chemical Synapse 1 Axon terminal of presynaptic neuron contains synaptic vesicles containing neurotransmitter – Many vesicles are docked on release sites on plasma membrane ready to release neurotransmitter – A reserve pool of synaptic vesicles is located further away from membrane Postsynaptic neuron membrane contains proteins that function as receptors and ligand-regulated ion gates ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Structure of a Chemical Synapse 3 Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.20 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Neurotransmitters and Related Messengers Neurotransmitters are molecules1that are released when a signal reaches a synaptic nob that binds to a receptor on another cell and alter that cell’s physiology More than 100 neurotransmitters have been identified but most fall into four major chemical categories: acetylcholine, amino acids, monoamines, and neuropeptides – Acetylcholine In a class by itself Formed from acetic acid and choline – Amino acid neurotransmitters Include glycine, glutamate, aspartate, and - ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Neurotransmitters and Related Messengers 2 – Monoamines Synthesized from amino acids by removing the –COOH group while retaining the (amino) group Include the catecholamines: – Epinephrine, norepinephrine, dopamine Also include histamine, ATP, and serotonin – Purines Include adenosine and ATP (adenosine triphosphate) ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Neurotransmitters and Related – Gases Messengers 3 Nitric oxide (NO) and carbon monoxide (CO) – Inorganic exceptions to the usual definition of neurotransmitters. They are synthesized as needed rather than stored in synaptic vesicles Diffuse out of the axon terminal rather than being released by exocytosis Diffuse into the postsynaptic neuron rather than bind to a surface receptor – Neuropeptides Chains of 2 to 40 amino acids Stored in secretory granules Include: cholecystokinin and substance P Some function as hormones or neuromodulators Some also released from digestive tract – Gut–brain peptides cause food cravings ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Synaptic Transmission Synapses vary – Some neurotransmitters are excitatory, others are inhibitory, and sometimes a transmitter’s effect differs depending on the type of receptor on the postsynaptic cell – Some receptors are ligand-gated ion channels and others act through second messengers Next we consider three kinds of synapses: – Excitatory cholinergic synapse – Inhibitory GABA-ergic synapse – Excitatory adrenergic synapse ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  An Excitatory Cholinergic Synapse 1 Cholinergic synapse—uses acetylcholine (ACh) – Nerve signal arrives at axon terminal and opens voltage-gated channels – enters knob and triggers exocytosis of Ach – Ach diffuses across cleft and binds to postsynaptic receptors – The receptors are ion channels that open and allow and to diffuse – Entry of causes a depolarizing postsynaptic potential – If depolarization is strong enough, it will cause an action potential at the trigger zone ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  An Excitatory Cholinergic Synapse 2 Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.22 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  An Inhibitory GABA-ergic Synapse GABA-ergic synapse employs -aminobutyric acid as its neurotransmitter Nerve signal triggers release of GABA into synaptic cleft GABA receptors are chloride channels enters cell and makes the inside more negative than the resting membrane potential Postsynaptic neuron is inhibited, and less likely to fire ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  An Excitatory Adrenergic Synapse Adrenergic synapse employs the neurotransmitter norepinephrine (NE), also called noradrenaline NE and other monoamines, and neuropeptides, act through second-messenger systems such as cyclic AMP (cAMP) Receptor is not an ion gate, but a transmembrane protein associated with a G protein Slower to respond than cholinergic and GABA- ergic synapses Has advantage of enzyme amplification—single molecule of NE can produce vast numbers of product molecules in the cell ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Transmission at an Adrenergic Synapse Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.23 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Cessation of the Signal Synapses must turn off stimulation to keep postsynaptic neuron from firing indefinitely Presynaptic cell stops releasing neurotransmitter Neurotransmitter only stays bound to its receptor for about 1 ms and then is cleared 6 - Acetylcholine is broken down by acetylcholinesterase (AchE) in the synaptic cleft After degradation, the presynaptic cell reabsorbs the fragments of the molecule for recycling 7 - Axon terminal reabsorbs neurotransmitter by endocytosis Monoamine transmitters are broken down after reabsorption by monoamine oxidase 8 - Neurotransmitter diffuses into nearby ECF Astrocytes in CNS absorb it and return it to neurons ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Cessation of the Signal: Steps 6-8 Copyright © McGraw-Hill Education. Permission required for reproduction or display. Figure 12.22 ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written  Neural Integration Neural integration—the ability to process, store, and recall information and use it to make decisions Chemical synapses allow for decision making – Brain cells are incredibly well connected allowing for complex integration Pyramidal cells of cerebral cortex have about 40,000 contacts with other neurons – Trade off: chemical transmission involves a synaptic delay that makes information travel slower than it would be if there was no synapse ©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written