Human Anatomy and Physiology Chapter 11 PowerPoint PDF

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

Human Anatomy and Physiology Eleventh Edition Chapter 11 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 Why This Matters Understanding neurotransmitter function will help you be aware of how drugs affect a patient’s nervous system Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Video: Why This Matters (Career Connection) Click here to view ADA compliant video: Why This Matters (Career Connection) https://mediaplayer.pearsoncmg.com/assets/secs_wtm_ch_11_dea_v3 Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.1 Functions of Nervous System (1 of 6) 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 (2 of 6) 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 Figure 11.1 The nervous system’s functions. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.1 Functions of Nervous System (3 of 6) 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 Figure 11.2 The nervous system. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.1 Functions of Nervous System (4 of 6) 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 (5 of 6) 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 (6 of 6) 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 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 (1 of 6) 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 (2 of 6) 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 (3 of 6) 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) Figure 11.4a Neuroglia. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuroglia of the CNS (4 of 6) 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 (2 of 5) Figure 11.4b Neuroglia. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuroglia of the CNS (5 of 6) 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 (3 of 5) Figure 11.4c Neuroglia. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuroglia of the CNS (6 of 6) 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 (4 of 5) 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 (5 of 5) 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 (1 of 2) 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 Neuron Cell Body (2 of 2) 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 (1 of 10) Armlike processes that extend from cell body – CNS contains both neuron cell bodies and their processes – PNS contains chiefly neuron processes 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 Animation: 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 (1 of 3) Figure 11.5a Structure of a motor neuron. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes (2 of 10) 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 (2 of 3) Figure 11.5b Structure of a motor neuron. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes (3 of 10) 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 (4 of 10) 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 – Axons rely on cell bodies to renew proteins and membranes – Quickly decay if cut or damaged Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes (5 of 10) The axon: functional characteristics (cont.) – Axons have efficient internal transport mechanisms  Molecules and organelles are moved along axons by motor proteins and cytoskeletal elements – Movement occurs in both directions  Anterograde: away from cell body – Examples: mitochondria, cytoskeletal elements, membrane components, enzymes  Retrograde: toward cell body – Examples: organelles to be degraded, signal molecules, viruses, and bacterial toxins Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 11.1 Certain viruses and bacterial toxins damage neural tissues by using retrograde axonal transport – Example: polio, rabies, and herpes simplex viruses, and tetanus toxin Research is under way to investigate using retrograde transport to treat genetic diseases – Viruses containing “corrected” genes or microRNA to suppress defective genes can enter cell through retrograde transport Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes (6 of 10) 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 (7 of 10) Myelination in the PNS – Formed by Schwann cells  Wraps around axon in jelly roll fashion  One cell forms one segment of myelin sheath – Outer collar of perinuclear cytoplasm (formerly called neurilemma): peripheral bulge containing nucleus and most of cytoplasm – 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 PNS Nerve Fiber Myelination (1 of 2) Figure 11.6a PNS nerve fiber myelination. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved PNS Nerve Fiber Myelination (2 of 2) Figure 11.6b PNS nerve fiber myelination. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes (8 of 10) 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 (3 of 3) Figure 11.5a Structure of a motor neuron. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes (9 of 10) 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 – No outer collar of perinuclear cytoplasm – Thinnest fibers are unmyelinated, but covered by long extensions of adjacent neuroglia Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neuron Processes (10 of 10) 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 Figure 11.4d Neuroglia. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurons (1 of 3) Structural classification – Three types grouped by number of processes 1. Multipolar: three or more processes (1 axon, others dendrites) – Most common and major neuron type in CNS 2. Bipolar: two processes (one axon, 1one dendrite) – Rare (ex: retina and olfactory mucosa) 3. Unipolar: one T-like process (two axons) – Also called pseudounipolar – Peripheral (distal) process: associated with sensory receptor – Proximal (central) process: enters CNS Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 11.2-1 Comparison of Structural Classes of Neurons Table 11.2-1 Comparison of Structural Classes of Neurons Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 11.2-2 Comparison of Structural Classes of Neurons Table 11.2-2 Comparison of Structural Classes of Neurons Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurons (2 of 3) 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 (3 of 3) 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 Table 11.2-3 Comparison of Structural Classes of Neurons Table 11.2-3 Comparison of Structural Classes of Neurons 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 (1 of 8) Opposite charges are attracted to each other Energy is required to keep opposite charges separated across a membrane Energy is liberated when the charges move toward one another When opposite charges are separated, the system has potential energy Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Basic Principles of Electricity (2 of 8) Definitions – Voltage: a measure of potential energy generated by separated charge  Measured between two points in volts (V) or millivolts (mV)  Called potential difference or potential – Charge difference across plasma membrane results in potential  Greater charge difference between points = higher voltage Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Basic Principles of Electricity (3 of 8) Definitions (cont.) – Current: flow of electrical charge (ions) between two points  Can be used to do work  Flow is dependent on voltage and resistance – Resistance: hindrance to charge flow  Insulator: substance with high electrical resistance  Conductor: substance with low electrical resistance Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Basic Principles of Electricity (4 of 8) Definitions (cont.) – Ohm’s law: gives relationship of voltage, current, resistance Current (I)  voltage (V)/resistance (R) – Current is directly proportional to voltage  Greater the voltage (potential difference), greater the current  No net current flow between points with same potential – Current is inversely proportional to resistance  The greater the resistance, the smaller the current Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Basic Principles of Electricity (5 of 8) 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 (6 of 8) 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 Operation of Gated Channels Figure 11.7 Operation of gated channels. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Basic Principles of Electricity (7 of 8) 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 Basic Principles of Electricity (8 of 8) Electrochemical gradient: electrical and chemical gradients combined Ion flow creates an electrical current, and voltage changes across membrane – Expressed by rearranged Ohm’s law equation: V = IR Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating the Resting Membrane Potential (1 of 7) 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 (2 of 7) 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 Measuring Membrane Potential in Neurons Figure 11.8 Measuring membrane potential in neurons. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating the Resting Membrane Potential (3 of 7) 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 A&P Flix: Resting Membrane Potential Click here to view ADA compliant Animation: A&P Flix: Resting Membrane Potential https://mediaplayer.pearsoncmg.com/assets/apf-resting-membrane-potential Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Resting Membrane Potential (1 of 2) 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 (4 of 7) 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 (5 of 7) 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 K+ and Na+ Concentrations Inside and Outside Cells, and (2) Differences in Permeability of the Plasma Membrane to these Ions Focus Figure 11.1 Resting Membrane Potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Changing the Resting Membrane Potential (6 of 7) 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 (7of 7) 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 – 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 Figure 11.9 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 Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Spread and Decay of a Graded Potential (1 of 3) Figure 11.10a The spread and decay of a graded potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Graded Potentials (1 of 2) Once gated ion channel opens, depolarization spreads from one area of membrane to next Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Spread and Decay of a Graded Potential (2 of 3) Figure 11.10b The spread and decay of a graded potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Graded Potentials (2 of 2) 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 (3 of 3) Figure 11.10c The spread and decay of a graded potential. 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 (1 of 7) Focus Figure 11.2 Action Potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating an Action Potential (1 of 5) 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 (2 of 7) Focus Figure 11.2 Action Potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating an Action Potential (2 of 5) 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 (3 of 7) Focus Figure 11.2 Action Potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating an Action Potential (3 of 5) 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 (4 of 7) Focus Figure 11.2 Action Potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generating an Action Potential (4 of 5) 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 (5 of 7) Focus Figure 11.2 Action Potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Action Potential (6 of 7) Focus Figure 11.2 Action Potential. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Action Potential (7 of 7) 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 Animation: 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 (5 of 5) 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 Resting Membrane Potential (2 of 2) Focus Figure 11.1-1 Resting Membrane Potential. 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 – 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 (1 of 2) 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 (2 of 2) 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 (AP) (1 of 3) Figure 11.11a Propagation of an action potential (AP). Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Propagation of an Action Potential (AP) (2 of 3) Figure 11.11b Propagation of an action potential (AP). Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Propagation of an Action Potential (AP) (3 of 3) 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 Animation: 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 Figure 11.12 Relationship between stimulus strength and action potential frequency. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Refractory Periods (1 of 2) 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 (2 of 2) – 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 – Think of a disobedient (refractory) dog – if he is absolutely refractory he will never come when called, but if he is relatively refractory, he may come but only if you call loud enough Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Absolute and Relative Refractory Periods in an AP Figure 11.13 Absolute and relative refractory periods in an AP. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Conduction Velocity (1 of 4) 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 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 (2 of 4) – 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 (1 of 3) 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 (2 of 3) 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 (3 of 3) 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 (1 of 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 (2 of 2) Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, incontinence Treatment: drugs that modify immune system activity May not be able to prevent, but maintaining high blood levels of vitamin D may reduce risk of development Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Conduction Velocity (3 of 4) Nerve fibers are classified according to diameter, degree of myelination, and speed of conduction Fall into three groups: – Group A fibers  Largest diameter  Myelinated somatic sensory and motor fibers of skin, skeletal muscles, and joints  Transmit at 150 m/s (~300 mph) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Conduction Velocity (4 of 4) – Group B fibers  Intermediate diameter  Lightly myelinated fibers  Transmit at 15 m/s (~30 mph) – Group C fibers  Smallest diameter  Unmyelinated  Transmit at 1 m/s (~2 mph) – B and C groups include ANS visceral motor and sensory fibers that serve visceral organs Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical–Homeostatic Imbalance 11.3 Impaired AP impulse propagation can be caused by a number of chemical and physical factors. Local anesthetics act by blocking voltage-gated Na+channels. Cold temperatures or continuous pressure interrupt blood circulation and delivery of oxygen to neurons – Cold fingers get numb, or foot “goes to sleep” Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.7 The Synapse (1 of 2) 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 (2 of 2) 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 (1 of 3) Figure 11.15a Synapses. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Synapses (2 of 3) Figure 11.15b Synapses. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Synapses (3 of 3) 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 (1 of 7) 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 (2 of 7) 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 (3 of 7) 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 (1 of 6) 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 (2 of 6) Focus Figure 11.3 Chemical Synapse. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses (4 of 7) 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 (3 of 6) Focus Figure 11.3 Chemical Synapse. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses (5 of 7) 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 (4 of 6) 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 (5 of 6) Focus Figure 11.3 Chemical Synapse. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses (6 of 7) 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 (6 of 6) Focus Figure 11.3 Chemical Synapse. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Chemical Synapses (7 of 7) Synaptic delay – Time needed for neurotransmitter to be released, diffuse across synapse, and bind to receptors  Can take anywhere from 0.3 to 5.0 ms – Synaptic delay is rate-limiting step of neural transmission  Transmission of AP down axon can be very quick, but synapse slows transmission to postsynaptic neuron down significantly  Not noticeable, because these are still very fast 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 (1 of 7) 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 (2 of 7) 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 (3 of 7) 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 (1 of 4) 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 Integration and Modification of Synaptic Events (2 of 4) – 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 – 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 Postsynaptic Potentials and Their Summation (4 of 7) Focus Figure 11.4 Postsynaptic Potentials and Their Summation. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Postsynaptic Potentials and Their Summation (5 of 7) Focus Figure 11.4 Postsynaptic Potentials and Their Summation. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Postsynaptic Potentials and Their Summation (6 of 7) Focus Figure 11.4 Postsynaptic Potentials and Their Summation. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Postsynaptic Potentials and Their Summation (7 of 7) Focus Figure 11.4 Postsynaptic Potentials and Their Summation. 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-pot entials Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Integration and Modification of Synaptic Events (3 of 4) Synaptic potentiation – Repeated use of synapse increases ability of presynaptic cell to excite postsynaptic neuron  Ca2+ concentration increases in presynaptic terminal, causing release of more neurotransmitter  Leads to more EPSPs in postsynaptic neuron – Potentiation can cause Ca2+ voltage gates to open on postsynaptic neuron  Ca2+ activates kinase enzymes, leading to more effective response to subsequent stimuli – Long-term potentiation: learning and memory Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Integration and Modification of Synaptic Events (4 of 4) Presynaptic inhibition – Release of excitatory neurotransmitter by one neuron is inhibited by another neuron via an axoaxonal synapse – Less neurotransmitter is released, leading to smaller EPSPs Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Endomembrane System (1 of 4) Table 11.3-1 Comparison of Graded Potentials and Action Potentials. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Endomembrane System (2 of 4) Table 11.3-2 Comparison of Graded Potentials and Action Potentials. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Endomembrane System (3 of 4) Table 11.3-3 Comparison of Graded Potentials and Action Potentials. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Endomembrane System (4 of 4) Table 11.3-4 Comparison of Graded Potentials and Action Potentials. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.9 Neurotransmitters Language of nervous system 50 or more neurotransmitters have been identified Most neurons make two or more neurotransmitters – Neurons can exert several influences Usually released at different stimulation frequencies Classified by: – Chemical structure – Function Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurotransmitters by Chemical Structure (1 of 12) Acetylcholine (ACh) – First identified and best understood – Released at neuromuscular junctions  Also used by many ANS neurons and some CNS neurons – Synthesized from acetic acid and choline by enzyme choline acetyltransferase – Degraded by enzyme acetylcholinesterase (AChE) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurotransmitters by Chemical Structure (2 of 12) Biogenic amines – Catecholamines  Dopamine, norepinephrine (NE), and epinephrine: made from the amino acid tyrosine – Indolamines  Serotonin: made from the amino acid tryptophan  Histamine: made from the amino acid histidine – All widely used in brain: play roles in emotional behaviors and biological clock – Used by some ANS motor neurons  Especially NE – Imbalances are associated with mental illness Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurotransmitters by Chemical Structure (3 of 12) Amino acids – Amino acids make up all proteins: therefore, it is difficult to prove which are neurotransmitters – Amino acids that are proven neurotransmitters  Glutamate  Aspartate  Glycine  GABA: gamma ()-aminobutyric acid Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurotransmitters by Chemical Structure (4 of 12) Peptides (neuropeptides) – Strings of amino acids that have diverse functions  Substance P – Mediator of pain signals  Endorphins – Beta endorphin, dynorphin, and enkephalins: act as natural opiates; reduce pain perception  Gut-brain peptides – Somatostatin and cholecystokinin play a role in regulating digestion Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurotransmitters by Chemical Structure (5 of 12) Purines – Monomers of nucleic acids that have an effect in both CNS and PNS  ATP, the energy molecule, is now considered a neurotransmitter  Adenosine is a potent inhibitor in brain – Caffeine blocks adenosine receptors  Can induce Ca2+ influx in astrocytes Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurotransmitters by Chemical Structure (6 of 12) Gases and lipids – Gasotransmitters  Nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide gases (H2S)  Bind with G protein–coupled receptors in brain  Lipid soluble and are synthesized on demand  NO involved in learning and formation of new memories, as well as brain damage in stroke patients, and smooth muscle relaxation in intestine  H2S acts directly on ion channels to alter function Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurotransmitters by Chemical Structure (7 of 12) – Endocannabinoids  Act at same receptors as THC (active ingredient in marijuana)  Most common G protein–linked receptors in brain  Lipid soluble  Synthesized on demand  Believed to be involved in learning and memory  May be involved in neuronal development, controlling appetite, and suppressing nausea Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurotransmitters by Chemical Structure (8 of 12) Neurotransmitters exhibit a great diversity of functions Functions can be grouped into two classifications: – Effects – Actions Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurotransmitters by Chemical Structure (9 of 12) Effects: excitatory versus inhibitory – Neurotransmitter effects can be excitatory (depolarizing) and/or inhibitory (hyperpolarizing) – Effect determined by receptor to which it binds  GABA and glycine are usually inhibitory  Glutamate is usually excitatory  Acetylcholine and NE bind to at least two receptor types with opposite effects – ACh is excitatory at neuromuscular junctions in skeletal muscle – ACh is inhibitory in cardiac muscle Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurotransmitters by Chemical Structure (10 of 12) Actions: direct versus indirect – Direct action: neurotransmitter binds directly to and opens ion channels  Promotes rapid responses by altering membrane potential  Examples: ACh and amino acids – Indirect action: neurotransmitter acts through intracellular second messengers, usually G protein pathways  Broader, longer-lasting effects similar to hormones  Biogenic amines, neuropeptides, and dissolved gases Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurotransmitters by Chemical Structure (11 of 12) Actions: direct versus indirect (cont.) – Neuromodulator: chemical messenger released by neuron that does not directly cause EPSPs or IPSPs but instead affects the strength of synaptic transmission  May influence synthesis, release, degradation, or reuptake of neurotransmitter  May alter sensitivity of the postsynaptic membrane to neurotransmitter.  May be released as a paracrine – Effect is only local Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification of Neurotransmitters by Chemical Structure (12 of 12) Channel-linked receptors – Ligand-gated ion channels – Action is immediate and brief – Excitatory receptors are channels for small cations  Na+ influx contributes most to depolarization – Inhibitory receptors allow Cl– influx that causes hyperpolarization Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Channel-Linked Receptors Cause Rapid Synaptic Transmission Figure 11.16 Channel-linked receptors cause rapid synaptic transmission. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neurotransmitter Receptors (1 of 2) G protein–linked receptors – Responses are indirect, complex, slow, and often prolonged – Involves transmembrane protein complexes – Cause widespread metabolic changes – Examples:  Muscarinic ACh receptors  Receptors that bind biogenic amines  Receptors that bind neuropeptides Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Neurotransmitter Receptors (2 of 2) G protein–linked receptors (cont.) – Mechanism:  Neurotransmitter binds to G protein–linked receptor, activating G protein  Activated G protein controls production of second messengers, such as cyclic AMP, cyclic GMP, diacylglycerol, or Ca2+  Second messengers can then: – Open or close ion channels – Activate kinase enzymes – Phosphorylate channel proteins – Activate genes and induce protein synthesis Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved G Protein–Coupled Receptors Cause the Formation of Intracellular Second Messengers (1 of 7) Figure 11.17 G protein–coupled receptors cause the formation of intracellular second messengers. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved G Protein–Coupled Receptors Cause the Formation of Intracellular Second Messengers (2 of 7) Figure 11.17 G protein–coupled receptors cause the formation of intracellular second messengers. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved G Protein–Coupled Receptors Cause the Formation of Intracellular Second Messengers (3 of 7) Figure 11.17 G protein–coupled receptors cause the formation of intracellular second messengers. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved G Protein–Coupled Receptors Cause the Formation of Intracellular Second Messengers (4 of 7) Figure 11.17 G protein–coupled receptors cause the formation of intracellular second messengers. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved G Protein–Coupled Receptors Cause the Formation of Intracellular Second Messengers (5 of 7) Figure 11.17 G protein–coupled receptors cause the formation of intracellular second messengers. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved G Protein–Coupled Receptors Cause the Formation of Intracellular Second Messengers (6 of 7) Figure 11.17 G protein–coupled receptors cause the formation of intracellular second messengers. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved G Protein–Coupled Receptors Cause the Formation of Intracellular Second Messengers (7 of 7) Figure 11.17 G protein–coupled receptors cause the formation of intracellular second messengers. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 11.10 Neural Integration Neural integration: neurons functioning together in groups Groups contribute to broader neural functions There are billions of neurons in CNS – Must have integration so that the individual parts fuse to make a smoothly operating whole Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Organization of Neurons: Neuronal Pools Neuronal pool: functional groups of neurons – Integrate incoming information received from receptors or other neuronal pools – Forward processed information to other destinations Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Organization of Neurons: Neuronal Pools (cont.) Simple neuronal pool – Single presynaptic fiber branches and synapses with several neurons in pool – Discharge zone: neurons closer to incoming fiber are more likely to generate impulse – Facilitated zone: neurons on periphery of pool are farther away from incoming fiber; usually not excited to threshold unless stimulated by another source Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Simple Neuronal Pool Figure 11.18 Simple neuronal pool. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Patterns of Neural Processing (1 of 3) Serial processing – Input travels along one pathway to a specific destination  One neuron stimulates next one, which stimulates next one, etc. – System works in all-or-none manner to produce specific, anticipated response – Best example of serial processing is a spinal reflex Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Patterns of Neural Processing (2 of 3) Serial processing (cont.) – Reflexes  Rapid, automatic responses to stimuli  Particular stimulus always causes same response  Occur over pathways called reflex arcs that have five components: – Receptor – Sensory neuron – CNS integration center – Motor neuron – Effector Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved A Simple Reflex Arc Figure 11.19 A simple reflex arc. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Patterns of Neural Processing (3 of 3) Parallel processing – Input travels along several pathways – Different parts of circuitry deal simultaneously with the information  One stimulus promotes numerous responses – Important for higher-level mental functioning – Example: A sensed smell may remind one of an odor and any associated experiences Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Types of Circuits Circuits: patterns of synaptic connections in neuronal pools Four types of circuits – Diverging – Converging – Reverberating – Parallel after-discharge Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Types of Circuits in Neuronal Pools (1 of 4) Figure 11.20a Types of circuits in neuronal pools. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Types of Circuits in Neuronal Pools (2 of 4) Figure 11.20b Types of circuits in neuronal pools. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Types of Circuits in Neuronal Pools (3 of 4) Figure 11.20c Types of circuits in neuronal pools. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Types of Circuits in Neuronal Pools (4 of 4) Figure 11.20d Types of circuits in neuronal pools. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Developmental Aspects of Neurons (1 of 4) Nervous system originates from neural tube and neural crest formed from ectoderm The neural tube becomes CNS – Neuroepithelial cells of neural tube proliferate into number of cells needed for development – Neuroblasts become amitotic and migrate – Neuroblasts sprout axons to connect with targets and become neurons Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Developmental Aspects of Neurons (2 of 4) Growth cone: prickly structure at tip of axon that allows it to interact with its environment via: – Cell surface adhesion proteins (laminin, integrin, and nerve cell adhesion molecules, or N-CAMs), which provide anchor points – Neurotropins that attract or repel the growth cone – Nerve growth factor (NGF), which keeps neuroblast alive – Filopodia are growth cone processes that follow signals toward target Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Developmental Aspects of Neurons (3 of 4) Once axon finds its target, it then must find right place to form synapse – Astrocytes provide physical support and the cholesterol needed for construction of synapses About two-thirds of neurons die before birth – If axons do not form a synapse with their target, they are triggered to undergo apoptosis (programmed cell death) – Many other cells also undergo apoptosis during development Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Developmental Aspects of Neurons (4 of 4) During childhood and adolescence learning reinforces certain synapses and prunes away others – Recent evidence suggests genes that promote excessive synaptic pruning may predispose an individual to schizophrenia Neurons are amitotic after birth; however, there are a few special neuronal populations that continue to divide – Olfactory neurons and hippocampus Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved A neuronal growth cone Figure 11.21 A neuronal growth cone. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Copyright Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved