Human Anatomy and Physiology Eleventh Edition (Chapter 11 Part A): PDF

Human Anatomy and Physiology Eleventh Edition Chapter 11 Part A Fundamentals of the Nervous System and...

Human Anatomy and Physiology Eleventh Edition Chapter 11 Part A Fundamentals of the Nervous System and Nervous Tissue PowerPoint® Lecture Slides prepared by Karen Dunbar Kareiva, Ivy Tech Community College Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.1 Functions of Nervous System Nervous system is master controlling and communicating system of body Cells communicate via electrical and chemical signals – Rapid and specific – Usually cause almost immediate responses Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.1 Functions of Nervous System Nervous system has three overlapping functions 1. Sensory input ▪ Information gathered by sensory receptors about internal and external changes 2. Integration ▪ Processing and interpretation of sensory input 3. Motor output ▪ Activation of effector organs (muscles and glands) produces a response Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Nervous System’s Functions Sensory input Integration Motor output Figure 11.1 The nervous system’s functions. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.1 Functions of Nervous System Nervous system is divided into two principal parts: – Central nervous system (CNS) ▪ Brain and spinal cord of dorsal body cavity ▪ Integration and control center – Interprets sensory input and dictates motor output – Peripheral nervous system (PNS) ▪ The portion of nervous system outside CNS ▪ Consists mainly of nerves that extend from brain and spinal cord – Spinal nerves to and from spinal cord – Cranial nerves to and from brain Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Nervous System Central nervous Peripheral nervous system (CNS) system (PNS) Brain Cranial nerves Spinal cord Spinal nerves Ganglia Figure 11.2 The nervous system. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.1 Functions of Nervous System Peripheral nervous system (PNS) has two functional divisions – Sensory (afferent) division ▪ Somatic sensory fibers: convey impulses from skin, skeletal muscles, and joints to CNS ▪ Visceral sensory fibers: convey impulses from visceral organs to CNS – Motor (efferent) division ▪ Transmits impulses from CNS to effector organs – Muscles and glands ▪ Two divisions – Somatic nervous system – Autonomic nervous system Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.1 Functions of Nervous System Somatic nervous system – Somatic motor nerve fibers conduct impulses from CNS to skeletal muscle – Voluntary nervous system ▪ Conscious control of skeletal muscles Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.1 Functions of Nervous System Autonomic nervous system – Consists of visceral motor nerve fibers – Regulates smooth muscle, cardiac muscle, and glands – Involuntary nervous system – Two functional subdivisions ▪ Sympathetic ▪ Parasympathetic ▪ Work in opposition to each other Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Organization of 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 system system (ANS) Somatic (voluntary) Visceral (involuntary) motor nerve fibers motor nerve fibers Conducts impulses Conducts impulses from the CNS to from the CNS to skeletal muscles cardiac muscle, smooth muscle, and glands Sympathetic division Parasympathetic Mobilizes body systems division during activity Conserves energy Promotes house- keeping functions Structure during rest Function Figure 11.3 Organization of the nervous system. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.2 Neuroglia Nervous tissue histology Nervous tissue consists of two principal cell types – Neuroglia (glial cells): small cells that surround and wrap delicate neurons – Neurons (nerve cells): excitable cells that transmit electrical signals Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuroglia of the CNS Four main neuroglia support CNS neurons – Astrocytes – Microglial cells – Ependymal cells – Oligodendrocytes Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuroglia of the CNS Astrocytes – Most abundant, versatile, and highly branched of glial cells – Cling to neurons, synaptic endings, and capillaries Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuroglia of the CNS Astrocytes (cont.) – Functions include: ▪ Support and brace neurons ▪ Play role in exchanges between capillaries and neurons ▪ Guide migration of young neurons ▪ Control chemical environment around neurons ▪ Respond to nerve impulses and neurotransmitters ▪ Influence neuronal functioning ▪ Participate in information processing in brain Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuroglia (1 of 5) Capillary Neuron Astrocyte (a) Astrocytes are the most abundant CNS neuroglia. Figure 11.4a Neuroglia. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuroglia of the CNS Microglial cells – Small, ovoid cells with thorny processes that touch and monitor neurons – Migrate toward injured neurons – Can transform to phagocytize microorganisms and neuronal debris Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuroglia Neuron Microglial cell (b) Microglial cells are defensive cells in the CNS. Figure 11.4b Neuroglia. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuroglia of the CNS Ependymal cells – Range in shape from squamous to columnar – May be ciliated ▪ Cilia beat to circulate CSF – Line the central cavities of the brain and spinal column – Form permeable barrier between cerebrospinal fluid (CSF) in cavities and tissue fluid bathing CNS cells Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved  Neuroglia Fluid-filled cavity Cilia Ependymal cells Brain or spinal cord tissue (c) Ependymal cells line cerebrospinal fluid–filled CNS cavities. Figure 11.4c Neuroglia. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuroglia of the CNS Oligodendrocytes – Branched cells – Processes wrap CNS nerve fibers, forming insulating myelin sheaths in thicker nerve fibers Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuroglia Axons Oligodendrocytes Myelin sheath Myelin sheath gap (d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers. Figure 11.4d Neuroglia. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuroglia of PNS Two major neuroglia seen in PNS Satellite cells – Surround neuron cell bodies in PNS – Function similar to astrocytes of CNS Schwann cells (neurolemmocytes) – Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers ▪ Similar function as oligodendrocytes – Vital to regeneration of damaged peripheral nerve fibers Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuroglia Satellite cells Cell body of neuron Schwann cells (forming myelin sheath) Nerve fiber (e) Satellite cells and Schwann cells (which form myelin sheaths) surround neurons in the PNS. Figure 11.4e Neuroglia. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.3 Neurons Neurons (nerve cells) are structural units of nervous system Large, highly specialized cells that conduct impulses Special characteristics – Extreme longevity (lasts a person’s lifetime) – Amitotic, with few exceptions – High metabolic rate: requires continuous supply of oxygen and glucose All have cell body and one or more processes Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Cell Body Also called the perikaryon or soma Biosynthetic center of neuron – Synthesizes proteins, membranes, chemicals – Rough ER (chromatophilic substance, or Nissl bodies) Contains spherical nucleus with nucleolus Some contain pigments In most, plasma membrane is part of receptive region that receives input info from other neurons Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Structure of a Motor Neuron Cell body (biosynthetic center Dendrites(receptive and receptive region) regions) Nucleus Axon (impulse-generating and conducting Initial Nucleolus region) segment Myelin sheath gap Chromatophilic of axon substance(rough endoplasmic Axon Axon hillock Schwann cell Terminals reticulum) Impulse (a) Terminal branches (secretory direction region) Figure 11.5a Structure of a motor neuron. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Cell Body Most neuron cell bodies are located in CNS – Nuclei: clusters of neuron cell bodies in CNS – Ganglia: clusters of neuron cell bodies in PNS Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes Armlike processes that extend from cell body – CNS contains both neuron cell bodies and their processes – PNS contains chiefly neuron processes (whose cell bodies are in CNS) Tracts – Bundles of neuron processes in CNS Nerves – Bundles of neuron processes in PNS Two types of processes – Dendrites – Axon Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved IP Anatomy Review Animation: Nervous I Click here to view ADA compliant IP Anatomy Review Animation: Nervous I https://mediaplayer.pearsoncmg.com/assets/secs-ipweb-nervous1-anat-rev Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Structure of a Motor Neuron Cell body (biosynthetic center Dendrites(receptive and receptive region) regions) Nucleus Axon (impulse-generating and conducting Initial Nucleolus region) segment Myelin sheath gap Chromatophilic of axon substance(rough endoplasmic Axon Axon hillock Schwann cell Terminals reticulum) Impulse (a) Terminal branches (secretory direction region) Figure 11.5a Structure of a motor neuron. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes Dendrites – Motor neurons can contain 100s of these short, tapering, diffusely branched processes ▪ Contain same organelles as in cell body – Receptive (input) region of neuron – Convey incoming messages toward cell body as graded potentials (short distance signals) – In many brain areas, finer dendrites are highly specialized to collect information ▪ Contain dendritic spines, appendages with bulbous or spiky ends Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved  Structure of a Motor Neuron Neuron cell body Dendritic spine (b) Figure 11.5b Structure of a motor neuron. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes The axon: structure – Each neuron has one axon that starts at cone-shaped area called axon hillock – In some neurons, axons are short or absent; in others, axon comprises almost entire length of cell ▪ Some axons can be over 1 meter long – Long axons are called nerve fibers – Axons have occasional branches called axon collaterals – Axons branch profusely at their end (terminus) ▪ Can number as many as 10,000 terminal branches – Distal endings are called axon terminals or terminal boutons Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes The axon: functional characteristics – Axon is the conducting region of neuron – Generates nerve impulses and transmits them along axolemma (neuron cell membrane) to axon terminal ▪ Terminal: region that secretes neurotransmitters, which are released into extracellular space ▪ Can excite or inhibit neurons it contacts – Carries on many conversations with different neurons at same time Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes Myelin sheath – Composed of myelin, a whitish, protein-lipid substance – Function of myelin ▪ Protect and electrically insulate axon ▪ Increase speed of nerve impulse transmission – Myelinated fibers: segmented sheath surrounds most long or large-diameter axons – Nonmyelinated fibers: do not contain sheath ▪ Conduct impulses more slowly Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes Myelination in the PNS – Formed by Schwann cells ▪ Wraps around axon in jelly roll fashion ▪ One cell forms one segment of myelin sheath – Plasma membranes have less protein ▪ No channels or carriers, so good electrical insulators ▪ Interlocking proteins bind adjacent myelin membranes Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved  Schwann cell plasma membrane Schwann cell cytoplasm 1 A Schwann cell envelops an axon. Axon Schwann cell nucleus PNS Nerve Fiber Myelination 2 The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers. 3 The Schwann cell cytoplasm is forced Myelin sheath from between the membranes. The tight membrane wrappings surrounding the axon Figure 11.6a PNS nerve form the myelin fiber myelination. Schwann cell cytoplasm sheath. (a) Myelination of a nerve fiber (axon) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes Myelination in the PNS (cont.) – Myelin sheath gaps ▪ Gaps between adjacent Schwann cells ▪ Sites where axon collaterals can emerge ▪ Formerly called nodes of Ranvier – Nonmyelinated fibers ▪ Thin fibers not wrapped in myelin; surrounded by Schwann cells but no coiling; one cell may surround 15 different fibers Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Structure of a Motor Neuron Cell body (biosynthetic center Dendrites(receptive and receptive region) regions) Nucleus Axon (impulse-generating and conducting Initial Nucleolus region) segment Myelin sheath gap Chromatophilic of axon substance(rough endoplasmic Axon Axon hillock Schwann cell Terminals reticulum) Impulse (a) Terminal branches (secretory direction region) Figure 11.5a Structure of a motor neuron. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes Myelin sheaths in the CNS – Formed by processes of oligodendrocytes, not whole cells – Each cell can wrap up to 60 axons at once – Myelin sheath gap is present Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes Myelin sheaths in the CNS (cont.) – White matter: regions of brain and spinal cord with dense collections of myelinated fibers ▪ Usually fiber tracts – Gray matter: mostly neuron cell bodies and nonmyelinated fibers Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuroglia Axons Oligodendrocytes Myelin sheath Myelin sheath gap (d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers. Figure 11.4d Neuroglia. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurons Functional classification of neurons – Three types of neurons grouped by direction in which nerve impulse travels relative to CNS 1. Sensory – Transmit impulses from sensory receptors toward CNS – Almost all are unipolar – Cell bodies are located in ganglia in PNS 2. Motor – Carry impulses from CNS to effectors – Multipolar – Most cell bodies are located in CNS (except some autonomic neurons) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurons Functional classification of neurons (cont.) – Three types (cont.) 3. Interneurons – Also called association neurons – Lie between motor and sensory neurons – Shuttle signals through CNS pathways – Most are entirely within CNS – 99% of body’s neurons are interneurons Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Human Anatomy and Physiology Eleventh Edition Chapter 11 Part B Fundamentals of the Nervous System and Nervous Tissue PowerPoint® Lecture Slides prepared by Karen Dunbar Kareiva, Ivy Tech Community College Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.4 Membrane Potentials Like all cells, neurons have a resting membrane potential Unlike most other cells, neurons can rapidly change resting membrane potential Neurons are highly excitable Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Basic Principles of Electricity Role of membrane ion channels – Large proteins serve as selective membrane ion channels ▪ K+ ion channel allows only K+ to pass through – Two main types of ion channels ▪ Leakage (nongated) channels, which are always open ▪ Gated channels, in which part of the protein changes shape to open/close the channel – Three main gated channels: chemically gated, voltage−gated, or mechanically gated Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Basic Principles of Electricity Chemically gated (ligand-gated) channels – Open only with binding of a specific chemical (example: neurotransmitter) Voltage-gated channels – Open and close in response to changes in membrane potential Mechanically gated channels – Open and close in response to physical deformation of receptors, as in sensory receptors Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Basic Principles of Electricity When gated channels are open, ions diffuse quickly: – Along chemical concentration gradients from higher concentration to lower concentration – Along electrical gradients toward opposite electrical charge Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved  Generating the Resting Membrane Potential A voltmeter can measure potential (charge) difference across membrane of resting cell Resting membrane potential of a resting neuron is approximately −70 mV – The cytoplasmic side of membrane is negatively charged relative to the outside – The actual voltage difference varies from −40 mV to −90 mV – The membrane is said to be polarized Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating the Resting Membrane Potential Potential generated by: – Differences in ionic composition of ICF and ECF – Differences in plasma membrane permeability Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating the Resting Membrane Potential Differences in ionic composition – ECF has higher concentration of Na+ than ICF ▪ Balanced chiefly by chloride ions (Cl−) – ICF has higher concentration of K+ than ECF ▪ Balanced by negatively charged proteins – K+ plays most important role in membrane potential Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Resting Membrane Potential Generating a resting membrane potential depends on (1) differences in K+ and Na+ concentrations inside and outside cells, and (2) differences in permeability of the plasma membrane to these ions. Focus Figure 11.1-1 Resting Membrane Potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating the Resting Membrane Potential Differences in plasma membrane permeability – Impermeable to large anionic proteins – Slightly permeable to Na+ (through leakage channels) ▪ Sodium diffuses into cell down concentration gradient – 25 times more permeable to K+ than sodium (more leakage channels) ▪ Potassium diffuses out of cell down concentration gradient – Quite permeable to Cl– Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating the Resting Membrane Potential Differences in plasma membrane permeability (cont.) – More potassium diffuses out than sodium diffuses in ▪ As a result, the inside of the cell is more negative ▪ Establishes resting membrane potential – Sodium-potassium pump (Na+/K+ ATPase) stabilizes resting membrane potential ▪ Maintains concentration gradients for Na+ and K+ ▪ Three Na+ are pumped out of cell while two K+ are pumped back in Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating a Resting Membrane Potential Depends on (1) Differences in and Na+ Concentrations Inside and Outside Cells, and (2) Differences in Permeability of the Plasma Membrane to these Ions Equilibrium potential Focus Figure 11.1 Resting Membrane Potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Changing the Resting Membrane Potential Membrane potential changes when: – Concentrations of ions across membrane change – Membrane permeability to ions changes Changes produce two types of signals – Graded potentials ▪ Incoming signals operating over short distances – Action potentials ▪ Long-distance signals of axons Changes in membrane potential are used as signals to receive, integrate, and send information Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Changing the Resting Membrane Potential Terms describing membrane potential changes relative to resting membrane potential – Depolarization: decrease in membrane potential (moves toward zero and above) ▪ Inside of membrane becomes less negative than resting membrane potential ▪ Probability of producing impulse increases – Repolarization: return in membrane potential towards resting membrane potential – Hyperpolarization: increase in membrane potential (away from zero) ▪ Inside of membrane becomes more negative than resting membrane potential ▪ Probability of producing impulse decreases Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Depolarization and Hyperpolarization of the Membrane Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.5 Graded Potentials Short-lived, localized changes in membrane potential – The stronger the stimulus, the more voltage changes and the farther current flows Triggered by stimulus that opens gated ion channels – Results in depolarization or sometimes hyperpolarization Named according to location and function – Receptor potential (generator potential): graded potentials in receptors of sensory neurons – Postsynaptic potential: neuron graded potential Current flows but dissipates quickly and decays – Graded potentials are signals only over short distances Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Spread and Decay of a Graded Potential Stimulus Depolarized region + + + + + + + + − + + + + + + + − − − − − − − − + − − − − − − − Plasma membrane − − − − − − − − + − − − − − − − + + + + + + + + − + + + + + + + (a) Depolarization: A small patch of the membrane (red area) depolarizes. Figure 11.10a The spread and decay of a graded potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Spread and Decay of a Graded Potential Once gated ion channel opens, depolarization spreads from one area of membrane to next + + + + + + + − − − + + + + + + − − − − − − − + + + − − − − − − − − − − − − − + + + − − − − − − + + + + + + + − − − + + + + + + (b) Depolarization spreads: Opposite charges attract each other. This creates local currents (black arrows) that depolarize adjacent membrane areas, spreading the wave of depolarization. Figure 11.10b The spread and decay of a graded potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.5 Graded Potentials In order for an action potential to be generated at the axon hillock, stimuli must be depolarizing and above threshold Graded potentials can summate to reach threshold Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.6 Action Potentials Principal way neurons send signals – Means of long-distance neural communication Occur only in muscle cells and axons of neurons Brief reversal of membrane potential with a change in voltage of ~100 mV Action potentials (APs) do not decay over distance as graded potentials do In neurons, also referred to as a nerve impulse Involves opening of specific voltage-gated channels Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Action Potential Focus Figure 11.2 Action Potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating an Action Potential Four main steps 1. Resting state: All gated Na+ and K+ channels are closed ▪ Only leakage channels for Na+ and K+ are open – Maintains the resting membrane potential ▪ Each Na+ channel has two voltage-sensitive gates – Activation gates: closed at rest; open with depolarization, allowing Na+ to enter cell – Inactivation gates: open at rest; block channel once it is open to prevent more Na+ from entering cell ▪ Each K+ channel has one voltage-sensitive gate – Closed at rest – Opens slowly with depolarization Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Action Potential Focus Figure 11.2 Action Potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating an Action Potential 2. Depolarization: Na+ channels open ▪ Depolarizing local currents open voltage-gated Na+ channels, and Na+ rushes into cell ▪ Na+ activation and inactivation gates open ▪ Na+ influx causes more depolarization, which opens more Na+ channels – As a result, ICF becomes less negative ▪ At threshold (–55 to –50 mV), positive feedback causes opening of all Na+ channels – Results in large action potential spike – Membrane polarity jumps to +30 mV Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Action Potential Focus Figure 11.2 Action Potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating an Action Potential 3. Repolarization: Na+ channels are inactivating, and K+ channels open ▪ Na+ channel inactivation gates close – Membrane permeability to Na+ declines to resting state – AP spike stops rising ▪ Voltage-gated K+ channels open – K+ exits cell down its electrochemical gradient ▪ Repolarization: membrane returns to resting membrane potential Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Action Potential Focus Figure 11.2 Action Potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating an Action Potential 4. Hyperpolarization: Some K+ channels remain open, and Na+ channels reset ▪ Some K+ channels remain open, allowing excessive K+ efflux – Inside of membrane becomes more negative than in resting state ▪ This causes hyperpolarization of the membrane (slight dip below resting voltage) ▪ Na+ channels also begin to reset Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Action Potential Focus Figure 11.2 Action Potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Action Potential The events Each step corresponds to one part of the AP graph. Na+ 1 Sodium Potassium channel channel Activation Inactivation gates K+ gate 1 Resting state: All gated Na+ and K+ channels are closed. 2 4 Na+ Na+ K+ K+ 4 Hyperpolarization: Some K+ channels remain 2 Depolarization: Na+ channels open, allowing open, and Na+ channels reset. Na+ entry. 3 Focus Figure 11.2 Action Potential. 3 Repolarization: Na+ channels are inactivating. K+ channels open, allowing K+ to exit. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Action Potential Focus Figure 11.2 Action Potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved A&P Flix: Generation of an Action Potential Click here to view ADA compliant A&P Flix: Generation of an Action Potential https://mediaplayer.pearsoncmg.com/assets/apf-generation-action-potential Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating an Action Potential Repolarization resets electrical conditions, not ionic conditions After repolarization, Na+/K+ pumps (thousands of them in an axon) restore ionic conditions Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Threshold and the All-or-None Phenomenon Not all depolarization events produce APs For an axon to “fire,” depolarization must reach threshold voltage to trigger AP At threshold: – Membrane is depolarized by 15 to 20 mV (from approx. -70mV to 50-55mV) – Na+ permeability increases – Na+ influx exceeds K+ efflux – The positive feedback cycle begins All-or-None: An AP either happens completely, or does not happen at all Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Propagation of an Action Potential Propagation allows AP to be transmitted from origin down entire axon length toward terminals Na+ influx through voltage gates in one membrane area cause local currents that cause opening of Na+ voltage gates in adjacent membrane areas – Leads to depolarization of that area, which in turn causes depolarization in next area Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Propagation of an Action Potential Once initiated, an AP is self-propagating – In nonmyelinated axons, each successive segment of membrane depolarizes, then repolarizes – Propagation in myelinated axons differs Since Na+ channels closer to the AP origin are still inactivated, no new AP is generated there – AP occurs only in a forward direction Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Propagation of an Action Potential Membrane potential (mV) (AP) +30 Voltage at 0 ms -70 Recording electrode − −+ + + ++ + + + ++ + + + + + +− − − −− − − − −− − − − − + +− − − − − − − − − − − − − − − −+ + + ++ + + + ++ + + + + (a) Time = 0 ms. Action potential has not yet reached the recording electrode. Resting potential Figure 11.11a Propagation of an action Peak of action potential potential (AP). Hyperpolarization Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved  Membrane potential (mV) Propagation of an Action Potential Voltage (AP) +30 at 2 ms -70 + + + + + + − − − + + + + + + + − − − − − − + + + − − − − − − − − − − − − − + + + − − − − − − − + + + + + + − − − + + + + + + + (b) Time = 2 ms. Action potential peak reaches the recording electrode. Resting potential Peak of action potential Figure 11.11b Propagation of an action potential (AP). Hyperpolarization Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Propagation of an Action Potential (AP) Membrane potential (mV) Voltage at 2 ms +30 Voltage Voltage at 0 ms at 4 ms -70 Recording electrode −−++ + + + + + + + + + + + + + + ++ + + − − − + + + + + + + + + ++ + + + + + + + − − − + + ++−− − − − − − − − − − − − − − − −− − − + + + − − − − − − − − − −− − − − − − − − + + + − − ++−− − − − − − − − − − − − − − − −− − − + + + − − − − − − − − − −− − − − − − − − + + + − − −−++ + + + + + + + + + + + + + + ++ + + − − − + + + + + + + + + ++ + + + + + + + − − − + + (a) Time = 0 ms. Action potential has (b) Time = 2 ms. Action potential (c) Time = 4 ms. Action potential not yet reached the recording peak reaches the recording peak has passed the recording electrode. electrode. electrode. Membrane at the Resting potential recording electrode is still hyperpolarized. Peak of action potential Hyperpolarization Figure 11.11 Propagation of an action potential (AP). Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved A&P Flix: Propagation of an Action Potential Click here to view ADA compliant A&P Flix: Propagation of an Action Potential https://mediaplayer.pearsoncmg.com/assets/apf-propagation-action-potential Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Coding for Stimulus Intensity All action potentials are alike and are independent of stimulus intensity CNS tells difference between a weak stimulus and a strong one by frequency of impulses – Frequency is number of impulses (APs) received per second – Higher frequencies mean stronger stimulus Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Relationship Between Stimulus Strength and Action Potential Frequency Membrane potential (mV) Action potentials +30 Amplitude of AP remains constant -70 Stimulus Stimulus Threshold voltage 0 Time (ms) A subthreshold The stronger the stimulus, Figure 11.12 Relationship stimulus does not the more frequently APs between stimulus strength and generate an AP. are generated. action potential frequency. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Refractory Periods Refractory period: time in which neuron cannot trigger another AP – Voltage-gated Na+ channels are open, so neuron cannot respond to another stimulus Two types – Absolute refractory period ▪ Time from opening of Na+ channels until resetting of the channels ▪ Ensures that each AP is an all-or-none event ▪ Enforces one-way transmission of nerve impulses Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Refractory Periods – Relative refractory period ▪ Follows absolute refractory period – Most Na+ channels have returned to their resting state – Some K+ channels still open – Repolarization is occurring ▪ Threshold for AP generation is elevated ▪ Only exceptionally strong stimulus could stimulate an AP Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Absolute and Relative Refractory Periods in an AP Absolute refractory Relative refractory period period Depolarization (Na+ enters) Membrane potential (mV) +30 0 Repolarization (K+ leaves) Hyperpolarization Figure 11.13 -70 Absolute and relative Stimulus refractory periods in an 0 1 2 3 4 5 AP. Time (ms) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Conduction Velocity APs occur only in axons, not other cell areas AP conduction velocities in axons vary widely Rate of AP propagation depends on two factors: 1. Axon diameter ▪ Larger-diameter fibers have less resistance to local current flow, so have faster impulse conduction ▪ Think of moving through a narrow vs. wide hallway 2. Degree of myelination ▪ Two types of conduction depending on presence or absence of myelin – Continuous conduction – Saltatory conduction Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Conduction Velocity – Continuous conduction: slow conduction that occurs in nonmyelinated axons – Saltatory conduction: occurs only in myelinated axons and is about 30 times faster ▪ Myelin sheaths insulate and prevent leakage of charge ▪ Voltage-gated Na+ channels are located at myelin sheath gaps ▪ APs generated only at gaps ▪ Electrical signal appears to jump rapidly from gap to gap Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Action Potential Propagation in Nonmyelinated and Myelinated Axons Stimulus Size of voltage (a) In bare plasma membranes, voltage decays. Without voltage-gated channels, as on a dendrite, voltage decays because current leaks across the membrane. Figure 11.14a Action potential propagation in nonmyelinated and myelinated axons. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Action Potential Propagation in Nonmyelinated and Myelinated Axons Stimulus Voltage-gated ion channel (b) In nonmyelinated axons, conduction is slow (continuous conduction). Voltage-gated Na+ and K+ channels regenerate the action potential at each point along the axon, so voltage does not decay. Conduction is slow because it takes time for ions and for gates of channel proteins to move, and this must occur before voltage can be regenerated. Figure 11.14b Action potential propagation in nonmyelinated and myelinated axons. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Action Potential Propagation in Nonmyelinated and Myelinated Axons Myelin Myelin Stimulus sheath sheath gap Myelin 1 mm sheath (c) In myelinated axons, conduction is fast (saltatory conduction). Myelin keeps current in axons (voltage doesn’t decay much). Action potentials are generated only in the myelin sheath gaps and appear to jump rapidly from gap to gap. Figure 11.14c Action potential propagation in nonmyelinated and myelinated axons. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 11.2 Multiple sclerosis (MS) is an autoimmune disease that affects primarily young adults Myelin sheaths in CNS are destroyed when immune system attacks myelin – Turns myelin into hardened lesions called scleroses – Impulse conduction slows and eventually ceases – Demyelinated axons increase Na+ channels, causing cycles of relapse and remission Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 11.2 Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, incontinence Treatment: drugs that modify immune system activity Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Human Anatomy and Physiology Eleventh Edition Chapter 11 Part C Fundamentals of the Nervous System and Nervous Tissue PowerPoint® Lectures Slides prepared by Karen Dunbar Kareiva, Ivy Tech Community College Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.7 The Synapse Nervous system works because information flows from neuron to neuron Neurons are functionally connected by synapses, junctions that mediate information transfer – From one neuron to another neuron – Or from one neuron to an effector cell Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.7 The Synapse Presynaptic neuron: neuron conducting impulses toward synapse (sends information) Postsynaptic neuron: neuron transmitting electrical signal away from synapse (receives information) – In PNS may be a neuron, muscle cell, or gland cell Most function as both Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Synapses Figure 11.15a Synapses. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Synapses Synaptic connections – Axodendritic: between axon terminals of one neuron and dendrites of others – Axosomatic: between axon terminals of one neuron and soma (cell body) of others – Less common connections: ▪ Axoaxonal (axon to axon) ▪ Dendrodendritic (dendrite to dendrite) ▪ Somatodendritic (dendrite to soma) – Two main types of synapses: ▪ Chemical synapse ▪ Electrical synapse Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses Most common type of synapse Specialized for release and reception of chemical neurotransmitters Typically composed of two parts – Axon terminal of presynaptic neuron: contains synaptic vesicles filled with neurotransmitter – Receptor region on postsynaptic neuron’s membrane: receives neurotransmitter ▪ Usually on dendrite or cell body – Two parts separated by fluid-filled synaptic cleft Electrical impulse changed to chemical across synapse, then back into electrical Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses Transmission across synaptic cleft – Synaptic cleft prevents nerve impulses from directly passing from one neuron to next – Chemical event (as opposed to an electrical one) – Depends on release, diffusion, and receptor binding of neurotransmitters – Ensures unidirectional communication between neurons Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Animation: Neurotransmitters Click here to view ADA compliant Animation: Neurotransmitters https://mediaplayer.pearsoncmg.com/assets/neurotransmitters Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses Information transfer across chemical synapses – Six steps are involved: 1. AP arrives at axon terminal of presynaptic neuron 2. Voltage-gated Ca2+ channels open, and Ca2+ enters axon terminal – Ca2+ flows down electrochemical gradient from ECF to inside of axon terminal Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses Transmit Signals from One Neuron to Another Using Neurotransmitters Focus Figure 11.3 Chemical Synapse. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses Transmit Signals from One Neuron to Another Using Neurotransmitters Focus Figure 11.3 Chemical Synapse. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses 3. Ca2+ entry causes synaptic vesicles to release neurotransmitter – Ca2+ causes synaptotagmin protein to react with SNARE proteins that control fusion of synaptic vesicles with axon membrane – Fusion results in exocytosis of neurotransmitter into synaptic cleft – The higher the impulse frequency, the more vesicles exocytose, leading to a greater effect on the postsynaptic cell Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses Transmit Signals from One Neuron to Another Using Neurotransmitters Focus Figure 11.3 Chemical Synapse. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses 4. Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane – Often chemically gated ion channels 5. Binding of neurotransmitter opens ion channels, creating graded potentials – Binding causes receptor protein to change shape, which causes ion channels to open Causes a graded potential in postsynaptic cell Can be an excitatory or inhibitory event Some receptor proteins are also ion channels Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses Transmit Signals from One Neuron to Another Using Neurotransmitters Focus Figure 11.3 Chemical Synapse. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses Transmit Signals from One Neuron to Another Using Neurotransmitters Focus Figure 11.3 Chemical Synapse. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses 6. Neurotransmitter effects are terminated – As long as neurotransmitter is binding to receptor, graded potentials will continue, so process needs to be regulated – Within a few milliseconds, neurotransmitter effect is terminated in one of three ways Reuptake by astrocytes or axon terminal Degradation by enzymes Diffusion away from synaptic cleft Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses Transmit Signals from One Neuron to Another Using Neurotransmitters Focus Figure 11.3 Chemical Synapse. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Electrical Synapses Less common than chemical synapses Neurons are electrically coupled – Joined by gap junctions that connect cytoplasm of adjacent neurons – Communication is very rapid and may be unidirectional or bidirectional – Found in some brain regions responsible for eye movements or hippocampus in areas involved in emotions and memory – Most abundant in embryonic nervous tissue Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.8 Postsynaptic Potentials Neurotransmitter receptors cause graded potentials that vary in strength based on: – Amount of neurotransmitter released – Time neurotransmitter stays in cleft Depending on effect of chemical synapse, there are two types of postsynaptic potentials – EPSP: excitatory postsynaptic potentials – IPSP: inhibitory postsynaptic potentials Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Postsynaptic Potentials and Their Summation Focus Figure 11.4 Postsynaptic Potentials and Their Summation. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 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, resulting in local net graded potential depolarization called excitatory postsynaptic potential (EPSP) EPSPs trigger AP if EPSP is of threshold strength – Can spread to axon hillock and trigger opening of voltage-gated channels, causing AP to be generated Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Postsynaptic Potentials and Their Summation Focus Figure 11.4 Postsynaptic Potentials and Their Summation. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Inhibitory Synapses and IPSPs Neurotransmitter binding to receptor opens chemically gated channels that allow entrance/exit of ions that cause hyperpolarization – Makes postsynaptic membrane more permeable to K+ or Cl– ▪ If K+ channels open, it moves out of cell ▪ If Cl– channels open, it moves into cell – Reduces postsynaptic neuron’s ability to produce an action potential ▪ Moves neuron farther away from threshold (makes it more negative) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Postsynaptic Potentials and Their Summation Focus Figure 11.4 Postsynaptic Potentials and Their Summation. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Integration and Modification of Synaptic Events Summation by the postsynaptic neuron – A single EPSP cannot induce an AP, but EPSPs can summate (add together) to influence postsynaptic neuron ▪ IPSPs can also summate – Most neurons receive both excitatory and inhibitory inputs from thousands of other neurons ▪ Only if EPSPs predominate and bring to threshold will an AP be generated – Two types of summations: temporal and spatial Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Spatial summation of graded potentials – Spatial summation ▪ Postsynaptic neuron is stimulated by large number of terminals simultaneously – Many receptors are activated, each producing EPSPs, which can then add together Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Temporal summation of graded potentials – Temporal summation ▪ One or more presynaptic neurons transmit impulses in rapid-fire order – First impulse produces EPSP, and before it can dissipate another EPSP is triggered, adding on top of first impulse Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure Animation: Comparison of Graded Potentials and Action Potentials Click here to view ADA compliant Animation: Comparison of Graded Potentials and Action Potentials https://mediaplayer.pearsoncmg.com/assets/sci-ap-comparison-of-graded-potentials-and-action- potentials Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Copyright Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved