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http://upload.wikimedia.org Nervous Tissue Note Much of the text material is from, “Principles of Anatomy and Physiology” by Gerald J. Tortora and Bryan Derrickson (2009, 2011, and 2014). I don’t claim authorship. Other sources are noted when they are used. The lecture slides are mapped to the three editions of the textbook based on the color-coded key below. 14th edition 13th edition 12th edition Same figure or table reference in all three editions 2 Outline • • • • • • • Microscopic level Electrical activity in neurons Signal transmission at synapses Neurotransmitters Neural circuits Plasticity, regeneration, and repair Two neurological disorders 3 Microscopic Level 4 Types of Cells in the Nervous System • Neurons mediate most of the information processing functions of the nervous system. • Neuroglia support, nourish, and protect the neurons and their functions. Page 402 Page 450 Page 417 5 Neurons • Neurons and muscle fibers are electrical excitable—they respond to certain types of stimuli to transduce energy to action potentials. • An action potential is an electrical signal that propagates (travels) along the membrane of the axon of a neuron. • This is due to movement of sodium ions into and potassium ions out of the axon. • Some axons are very short to propagate action potentials over distances of 1 mm or less, while others can be very long to propagate over much longer distances, such as between the brain and spinal cord. Transduce = convert energy from one form to another. Page 402 Page 450 Page 417 6 A Neuron 7 Parts of a Neuron • The cell body, or soma, is similar to the generalized cell discussed in the biology review. • Dendrites (Latin for “little trees”) are input areas to a neuron that are typically organized as branched, tree-like structures that extend from the cell body. • The axon is a thin, cylindrical projection often extending from the cell body—it propagates action potentials toward another neuron, a muscle fiber, or gland. Page 402 Page 450 Page 417 Figure 12.2 8 http://upload.wikimedia.org Cortical Neurons Pyramidal cells in the cerebral cortex (primary motor area). 9 Synapses and Neurotransmitters • The synapse is the site of communication between a neuron and another neuron, a neuron and a muscle fiber, or a neuron and a gland cell. • Axons typically branch at their distal ends, and swell into end bulbs that have vesicles which store neurotransmitters. • Some neurons release 2 or 3 neurotransmitters; however, somatic motor neurons that innervate skeletal muscles have only acetylcholine (ACh). Page 404 Page 452 Page 419 10 Synapse http://theora.com 11 Synapses and Neurotransmitters (continued) • When released in response to an action potential, neurotransmitter diffuses across the synaptic cleft to excite or inhibit another neuron. • ACh at the neuromuscular junction in skeletal muscle is always excitatory. Page 404 Page 452 Page 419 12 Santiago Ramón y Cajal Structure of the mammalian retina (1900) 1852-1934 All images on this page and the next page are from: http://upload.wikimedia.org 13 Santiago Ramón y Cajal (continued) Optic tectum of a sparrow Purkinjie cells and granular cells in the pigeon cerebellum. Hippocampus of a rodent 14 Structural Classification • A neuron can be classified based on the processes that extend from its cell body. • Multipolar neurons usually have several dendrites and one axon— most neurons in the brain and spinal cord are of this type. • Bipolar neurons have one main dendrite and one axon—they are found in some sensory systems including the retina, inner ear, and olfactory area of the brain. • Unipolar neurons have dendrites and an axon fused together to form a continuous process—they are found in certain sensory receptors of the skin. Neuronal processes = dendrites and axons. Page 404 Page 452 Page 419 Figure 12.3 15 Structural Classification (continued) Multipolar Bipolar Unipolar http://webanatomy.net 16 Naming Conventions • Neurons are sometimes named for who discovered them or their shape or appearance. • Examples include: Purkinjie cells in the cerebellum—named for their discoverer, the Czech anatomist, Jan Evangelista Purkinje. - Pyramidal cells in the motor cortex of the cerebral hemispheres— named for their pyramid-like shape. - Page 405 Page 453 Page 420 Figure 12.5 17 Functional Classification • Sensory, or afferent, neurons have sensory receptors or neurons at one of their ends. • When a stimulus activates a sensory receptor, an action potential is propagated into the CNS. • Motor, or efferent, neurons propagate action potentials from the CNS to effectors (muscle or glands) via the cranial nerves or spinal nerves. Page 406 Page 454 Page 420 Figure 12.10 Figure 12.11 Figure 12.11 18 Functional Classification (continued) • Interneurons are positioned in the CNS between some sensory and motor neurons. • Within the cerebral cortex, interneurons are known as association neurons. • These neurons are organized in complex networks to integrate and process sensory information. Page 406 Page 454 Page 421 19 Axonal Transport • Substances are synthesized or recycled in the cell bodies of neurons and transported to the axons and their end bulbs. • Slow axonal transport enables the one-way movement of axoplasm (cytoplasm) toward the end bulbs. • Fast axonal transport uses proteins as “motors” powered by ATP to move materials in both directions along microtubules in the axons. • The materials include organelles, and complex molecules that form the axolemma (plasma membrane), end bulbs, and synaptic vesicles. Page 404 Page 454 Page 419 20 Neuroglia • Neuroglia or glial cells make-up about 50 percent of the total volume of the central nervous system. • They are much smaller than neurons, but 5 to 50 times more numerous. • They were originally thought to be the “glue” (in Latin) that holds nervous tissue together. • Neuroglia are now known to be involved in functioning of the nervous system.••• Page 406 Page 454 Page 421 Figure 12.6 21 http://www.anatomybox.com Neuroglia (continued) Neuroglia interspersed among neurons. 22 Neuroglia (continued) • Unlike neurons, neuroglia can multiply and divide (through mitosis) in mature mammalian nervous systems (including humans). • Neuroglia can multiply to fill-in the space formerly occupied by neurons in injury or disease. • Brain tumors (gliomas) from neuroglia can be highly-malignant, grow rapidly, and metastasize. Page 406 Page 454 Page 421 23 CNS Neuroglia • Astrocytes have many processes, and are the largest and most numerous of the neuroglia in the central nervous system. • The primary function is to form the blood-brain barrier by wrapping their processes around capillaries in the brain. • Oligodendrocytes resemble astrocytes, but they are smaller and have fewer processes. • Their processes form the myelin sheath that encircles axons in the CNS. Central nervous system (CNS) = brain and spinal cord. Page 406 Page 454 Page 421 Figure 12.6 24 CNS Neuroglia (continued) • Microglia are small cells with slender processes and spine-like projections. • They function as phagocytes to remove cellular debris in the CNS. • Ependymal cells are cuboidal (cube-shaped) with microvilli and cilia. • These cells line the ventricles of the brain and central canal of the spinal cord, and monitor and support the circulation of cerebrospinal fluid. Phagocyte = a cell that engulfs and digests debris and invading microorganisms. (http://wordnetweb.princeton.edu) Page 406 Page 455 Page 422 Figure 12.6 25 PNS Neuroglia • Schwann cells encircle the axons in the peripheral nervous system to form the myelin sheath. • Unlike oligodendrocytes, a Schwann cell will encircle only one axon. • Satellite cells surround the cell bodies of neurons in the ganglia of the PNS. • The cells provide structural support and regulate exchange of materials between neurons and interstitial fluid. Peripheral nervous system (PNS) = all nervous tissues outside of the CNS. Page 408 Page 455 Page 421 Figure 12.7 26 Myelin Sheath • The myelin sheath that surrounds many types of axons consists of a multi-layered lipid and protein covering. • The sheath electrically insulates the axon and increases the velocity (speed) of action potentials for reasons that will be discussed when we cover the electrical activity of neurons. • Axons without a myelin sheath (such as slow pain fibers) are said to be unmyelinated. Page 408 Page 456 Page 423 Figure 12.8 27 http://upload.wikimedia.org Myelin Sheath (continued) Electron micrograph. 28 Myelin Sheath (continued) • The amount of myelin sheath progressively increases from birth to maturity in humans. • An infant’s response to stimuli is neither rapid nor coordinated due in part to the lack of much myelination. • Myelination (progressive development of the myelin sheath) continues through adolescence. Page 409 Page 456 Page 423 29 Demyelination • Demyelination is the loss or destruction of myelin sheath from axons. • It occurs in certain neurological disorders including multiple sclerosis (MS) and Tay-Sachs disease. • Demyelination degrades or slows the conduction of action potentials, and enables “cross-talk” among axons. • Demyelinating diseases will be discussed in more detail later in this learning module. Page 409 Page 457 Page 423 30 Definitions • Nucleus is a cluster of cell bodies of neurons in the CNS (not to be confused with cell nucleus). • Ganglion is a cluster of neuronal cell bodies, usually in the PNS. • Fiber tract is a bundle of axons connecting neurons in the brain or spinal cord (CNS). • Nerve is a bundle of axons in the PNS. Nucleus = singular for nuclei. Ganglion = singular for ganglia. Page 410 Page 457 Page 424 31 Gray and White Matter • Some areas of a freshly-dissected brain or spinal cord are gray and other areas are white in appearance. • Gray matter includes neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. • White matter is primarily composed of myelinated axons—their lipid coverings are white. • Blood vessels in gray and white matter provide oxygen and nourishment to neurons and neuroglia and remove waste products from cellular respiration. Page 410 Page 458 Page 425 Figure 12.9 32 http://upload.wikimedia.org Gray and White Matter (continued) Human brain, mid-sagittal section, right lateral view. 33 Electrical Activity in Neurons 34 Electrical Excitability • Neurons and muscle fibers have a unique characteristic of being electrically-excitable. • Neurons generate graded potentials and action potentials, which are electrical signals. • Graded potentials integrate information from other neurons and convey it over short distances. • Action potentials enable information to be conveyed over a wide range of distances. Transient = temporary or brief. Page 410 Page 458 Page 427 35 Electrical Excitability (continued) • The generation of electrical signals depends on two basic features of a neuron or muscle fiber: A resting membrane potential, measured as voltage, that changes in response to electrical or chemical stimuli. - Ion channels in the axolemma for Na+ and K+ (sodium and potassium ions). - Axolemma = plasma membrane of an axon. Page 412 Page 460 Page 428 36 Ion Channels • Ion channels in the axolemma allow some certain ions to follow their chemical or electrical gradients. • Ions follow their chemical gradients from higher to lower concentration. • Cations (positively charged ions) move toward a negatively charged area—an electrical gradient. • Anions (negatively charged ions) move toward a positively charged area—also an electrical gradient. • Molecular gates in the axolemma guard the ion channels—they must be moved to open the channels. Page 412 Page 460 Page 436 Figure 12.11 Figure 12.11 Figure 12.12 37 Types of Ion Channels • Leakage channels randomly alternate between open and closed states. • Voltage-gated channels open and close in response to membrane potentials—they are involved in the generation and propagation of action potentials. • Ligand-gated channels open and close in response to chemical stimuli including neurotransmitters. • Mechanically-gated channels open and close in response to mechanical stimulation (such as touch, pressure, and sound) and tissue stretching. Page 412 Page 460 Page 428 Figure 12.11 Figure 12.11 Figure 12.12 38 Ligand-Gated Channel http://www.hcc.uce.ac.uk Ligand = an ion, a molecule, or a molecular group that binds to another chemical entity to form a larger complex. (www.thefreedictionary.com) 39 Resting Membrane Potential • A resting membrane potential results from the buildup of anions (-) in the cytoplasm, and a buildup of cations (+) in the extracellular space. • This buildup of anions and cations occurs in close proximity to the axolemma.• • The separation of negative and positive electrical charges across the axolemma is a form of potential energy. • This potential energy can be measured using tiny electrodes and a voltmeter. Page 414 Page 462 Page 430 Figure 12.12 Figure 12.12 Figure 12.13 40 http://faculty.irsc.edu Anion and Cation Distribution 41 Resting Membrane Potential (continued) • The resting membrane potential is measured in millivolts (mV), where 1.0 mV equals 0.001 volt—this is very small compared to a AA battery of 1.5 volts. • The greater the difference in electrical charge across the plasma membrane, the larger the resting membrane potential. Page 414 Page 462 Page 430 Figure 12.12 Figure 12.12 Figure 12.13 42 http://upload.wikimedia.org Squid Axon Loligo vulgaris, European (common) squid. 43 Microelectrodes • A microelectrode is inserted into the cell to measure the resting membrane potential. • A reference electrode is positioned outside the cell in the extracellular space. • The electrodes are connected to a voltmeter to record the membrane potential. Glass microelectrode http://www.medicine.mcgill.ca/physio/vlab/rmp/images Page 414 Page 463 Page 430 Figure 12.12 Figure 12.12 Figure 12.13 44 http://ncbi.nlm.nih.gov Voltage Measurement 45 Voltages • A cells that has a resting membrane potential is said to be polarized. • Most somatic cells are polarized, but only neurons and muscle fibers are electrically-excitable. • The resting membrane potentials of neurons ranges between -40 and -90 mV, with a typical value of -70mV. • A minus sign indicates the inside of the cell is negative compared to its outside. Page 415 Page 463 Page 431 Figure 12.12 Figure 12.12 Figure 12.13 46 Major Factors • The resting membrane potential of neurons is determined by three major factors: Unequal distribution of cations and anions across the axolemma. - Sodium-potassium transport pump. - Inability of most anions (especially proteins) to exit the cell because they are too large to fit through the ion channels. - Page 415 Page 463 Page 431 Figure 12.13 Figure 12.12 Figure 12.14 47 Graded Potentials • A graded potential is a voltage change from the resting membrane potential that makes the neuron either more polarized or less polarized. • Graded potentials are hyperpolarizing (more polarized) or depolarizing (less polarized). Page 416 Page 464 Page 432 Figure 12.14 Figure 12.14 Figure 12.15 48 Graded Potentials (continued) • A graded potential occurs when a stimulus causes ion channels to open or close in the plasma membrane. • Graded potentials vary in their amplitude (voltage) depending on the intensity of the stimulus. • The opening of ion channels produces current spread in the immediate area. • Graded potentials decay as they spread across the plasma membrane (this is known as decremental spread). Page 416 Page 464 Page 432 Figure 12.14 Figure 12.14 Figure 12.15 49 Summation • Graded potentials can combine with other graded potentials in a process known as summation. • Two or more depolarizing potentials can produce a greater depolarizing potential. • Two or more hyperpolarizing potentials can produce a greater hyperpolarizing potential. • Two equal graded potentials of opposite polarization will negate each other. Page 416 Page 466 Page 434 Figure 12.16 Figure 12.16 Figure 12.17 50 What’s In a Name? • Graded potentials also have different names depending on the stimulus that initiates them and where they occur in nervous tissue. • At neuron-neuron and neuron-muscle fiber synapses, graded potentials from the release of a neurotransmitter are known as postsynaptic potentials. • Graded potentials that occur in sensory receptors and sensory neurons are known as receptor potentials and generator potentials, respectively. 51 Threshold • An action potential is generated in the axolemma when a depolarization reaches the threshold value of the neuron or muscle fiber. • The threshold is about -55 mV for a resting membrane potential of -70mV. • An action potential is not generated when a weak depolarization (known as a subthreshold stimulus) does not reach the threshold value. • Although the threshold value of a neuron does not change, different neurons may have slightly different thresholds. Page 419 Page 466 Page 434 Figure 12.19 Figure 12.19 Figure 12.20 52 Action Potential • The action potential is a sequence of rapidly-occurring events that reverses the membrane potential and then restores it to its resting state. • An action potential spikes and becomes positive (to about +30 mV) during the depolarizing phase. • The spike represents about a 100 mV change, or about one-tenth of a volt. • It declines during the repolarizing phase and returns to the resting state (-70mV in our example). Page 417 Page 466 Page 434 Figure 12.18 Figure 12.18 Figure 12.19 53 Action Potential (continued) http://encefalus.com 54 Action Potential (continued) • Two different types of voltage-gated ion channels open and close during an action potential. • The first to open are the Na+ channels that enable sodium ions to rush into the cell along their electrochemical gradient to initiate the depolarizing phase. • K+ channels then open, allowing potassium ions to flow-out of the cell along their chemical gradient to initiate the repolarizing phase. • An after-hyperpolarizing phase occurs because K+ channels remain open for a short period of time after the resting membrane potential is reached. Page 419 Page 466 Page 436 Figure 12.20 Figure 12.20 Figure 12.21 55 Sodium-Potassium Transport Pump • The ion or sodium-potassium transport pump, made-up of proteins in the axolemma, is powered by ATP. • The transport pump transports Na+ out of the cell and transports K+ into the cell. • It is instrumental in restoring the resting membrane potential to -70 mV after generation of an action potential. Page 420 Page 466 Page 436 Figure 12.20 Figure 12.20 Figure 12.21 56 All-or-None Principle • An action potential either occurs or does not occur—it will be generated if a depolarizing graded potential reaches or exceeds the threshold value of the neuron. • There are no in-between states; only all-or-none in the generation of action potentials Page 420 Page 468 Page 436 57 Refractory Periods http://encefalus.com 58 Absolute Refractory Period • Another action potential cannot be generated when the Na+ gates are open—this state is the absolute refractory period. • Large-diameter axons have shorter absolute refractory periods than small-diameter axons; therefore, more action potentials can be generated in a given period of time. • Maximum frequencies range between 10 to 1,000 action potentials per second depending on the time duration of the absolute refractory period. Page 420 Page 468 Page 436 Figure 12.18 Figure 12.18 Figure 12.19 59 Relative Refractory Period • The relative refractory period occurs after the Na+ gates close and the K+ gates open. • A second action potential may be triggered by a superthreshold depolarization during this period. Page 420 Page 468 Page 436 Figure 12.18 Figure 12.18 Figure 12.19 60 Propagation • Action potentials usually propagate from the trigger zone—the axon hillock where a cell body transitions to an axon—to the end bulbs of the axon where neurotransmitters are released. • Action potentials are of constant amplitude as they travel along the axolemma. • In comparison, graded potentials decay with time and over distance. Page 420 Page 470 Page 438 Figure 12.21 Figure 12.21 Figure 12.22 61 Propagation (continued) • An action potential is continually regenerated along the axolemma since it is preceded by a depolarizing current that reaches or exceeds the threshold value. • Action potentials, depending on the location of the trigger zone, can propagate in ether direction along an axon—this is not consistent with the textbook, and we will discuss why.• Axons can be two-way streets while chemical synapses serve as one-way doors. (metaphorically speaking) Page 420 Page 470 Page 438 Figure 12.21 Figure 12.21 Figure 12.22 62 http://tainano.com Propagation (continued) 63 Continuous Conduction • Continuous conduction involves the depolarization and repolarization of each infinitesimal segment of the axolemma during propagation of an action potential. • Continuous conduction occurs in unmyelinated axons and in muscle fibers. Page 422 Page 470 Page 438 Figure 12.21 Figure 12.21 Figure 12.22 64 Saltatory Conduction • In saltatory conduction, action potentials are generated only at the nodes of Ranvier (areas of axolemma not covered by myelin sheath). • The Na+ and K+ gates in the axolemma are exposed since there is no myelin sheath at these nodes. • An action potential seems to jump from one node to the next as it propagates along the axolemma. • Due to saltatory conduction, action potentials propagate much more rapidly along myelinated than unmyelinated axons. Saltare = to jump (in Latin). Page 422 Page 470 Page 438 Figure 12.21 Figure 12.21 Figure 12.22 65 Saltatory Conduction (continued) http://qwickstep.com 66 Propagation Speed • The factors that affect the propagation (conduction) speed of an action potential include: Amount of myelination - Axon diameter - Temperature - • All are direct relationships—increases in any or all of the three factors increase propagation speed. Page 422 Page 470 Page 438 67 Classification of Nerve Fibers • A fibers—largest-diameter, myelinated axons. • B fibers—smaller-diameter, myelinated axons. • C fibers—smallest-diameter, unmyelinated axons. Page 422 Page 471 Page 439 68 Frequency Coding • The amplitude of action potentials generated by a neuron does not change based on the all-of-none principle. • The intensity of a stimulus is instead encoded in the frequency of the action potentials. • The greater the intensity of a stimulus to a limit, the higher rate of action potential generation (“firing rate”). • A second factor in encoding the intensity of the stimulus is the number of axons in a bundle or nerve that are recruited (activated) by the stimulus. Page 423 Page 472 Page 440 69 Frequency Coding (continued) http://www1.lf1.cuni.cz 70 Signal Transmission at Synapses 71 Synapse • The junction between neuron and neuron, neuron and muscle fiber, or neuron and gland is known as a synapse. • The neuron sending the signal is known as the presynaptic neuron, and the neuron receiving the message is the postsynaptic neuron. • The three configurations of synapses are axodendritic, axosomatic, and axoaxonic. • The two basic types of synapses—electrical and chemical—differ in their structures and functions. Page 424 Page 473 Page 441 72 Electrical Synapses • Action potentials can cross between adjacent cells via very narrow gap junctions in electrical synapses. • Ions flow from one cell to the next through tubular connexions in the gap junctions to enable action potentials to rapidly spread from cell to cell. • Gap junctions are found in cardiac (heart) muscle, smooth muscle, and the nervous system of developing mammalian embryos. • Electrical synapses allow electrical synchronization within groups of muscle fibers to enable coordinated contractions. Page 424 Page 473 Page 441 73 Chemical Synapses • The presynaptic and postsynaptic neurons in a chemical synapse are in close proximity, but they do not physically touch. • The two neurons are separated by a synaptic cleft, a gap of 20 to 50 nm filled with interstitial fluid. • Action potentials are not conducted across the synaptic cleft in chemical synapses. • A chemical called a neurotransmitter is released from the end bulbs of the presynaptic neuron and diffuses across the cleft. Page 425 Page 473 Page 441 Figure 12.23 74 Chemical Synapses (continued) • The neurotransmitter binds to receptors of the plasma membrane of the postsynaptic neuron. • Most synapses in the nervous systems of mammals are chemical in nature. Page 425 Page 473 Page 441 Figure 12.23 75 http://biologyclass.neurobio.arizona.edu Chemical Synapse 76 Synaptic Events • In response to an action potential, voltage-gated Ca2++ channels open in the plasma membrane of the end bulbs in the presynaptic neuron. • The inflow of Ca2++ triggers the process of exocytosis—the vesicles merge with the plasma membrane to release their contents (neurotransmitter) into the synaptic cleft. • The neurotransmitter passively diffuses through the interstitial fluid in the synaptic cleft, and binds to protein receptors of the postsynaptic membrane. Page 425 Page 473 Page 441 Figure 12.23 Figure 12.22 Figure 12.23 77 Neurotransmitter Receptors • The receptor sites on the postsynaptic membrane usually bind only one type of neurotransmitter. • When the neurotransmitter binds to the postsynaptic receptor, an ion channel opens in the plasma membrane, and a postsynaptic (graded) potential is generated. • Neurotransmitter receptors are either ionotropic or metabotropic based on their protein structures. Ionotopic = a hormone activates or deactivates ionotropic receptors (ligandgated ion channels). The effect can be either positive or negative, whether the effect is a depolarization or a hyperpolarization respectively. Metabotropic = responds on activation with glutamate binding by initiating a number of intracellular biochemical events which modulate synaptic and neuronal activity. They are not directly linked to any specific ion channels (Both definitions from http://www.encyclo.co.uk) Page 426 Page 475 Page 443 78 Postsynaptic Potentials • A depolarizing or hyperpolarizing graded postsynaptic potential is generated depending on the neurotransmitter and location of the synapse in the post-synaptic neuron. • The graded potential is either an excitatory postsynaptic potential (EPSP) or a inhibitory postsynaptic potentials (IPSP). • EPSPs and IPSPs correspond to depolarizations and hyerpolarizations as we discussed. Page 427 Page 475 Page 443 79 EPSP and IPSP EPSP—depolarizing IPSP—hyperpolarizing http://www.igi.tugraz.at 80 Postsynaptic Potentials (continued) • The time required to generate a postsynaptic potential, known as synaptic delay, is about 0.5 msec. • Chemical synapses respond more slowly than electrical synapses due to this time delay. • Information transfer is in only one direction at chemical synapses, from presynaptic neuron to postsynaptic neuron • A chemical synapse, in essence, serves as a one-way door while an axon can be a two-way street. Page 427 Page 475 Page 443 Figure 12.23 Figure 12.22 Figure 12.23 81 Neurotransmitter Removal • Rapid removal of neurotransmitter from the synaptic cleft is essential for continued synaptic function. • If the neurotransmitter were to remain in the synaptic cleft, it could continue to stimulate the postsynaptic neuron, muscle fiber, or gland for as long as it lingered. • The neurotransmitter is removed by diffusion out of the synaptic cleft, enzymes, and re-uptake by cells, or a combination of these processes. Page 427 Page 475 Page 443 82 Summation • Neurons in the CNS have input from up to 1,000 to 10,000 synapses with other neurons. • Excitatory and inhibitory inputs are summated to produce a postsynaptic potential. • Spatial summation involves the summation of postsynaptic potentials on different but nearby locations on the plasma membrane of the postsynaptic neuron. Page 429 Page 477 Page 445 Figure 12.25 Figure 12.24 Figure 12.25 83 Summation (continued) • Temporal summation involves the summation of postsynaptic potentials at the same location on the neuron but at slightly different times. • Spatial summation and temporal summation act together determine if an action potential will be generated based on the processes we discussed. Page 429 Page 477 Page 445 Figure 12.26 Figure 12.25 Figure 12.26 84 http://biologyclass.neurobio.arizona.edu Summation (continued) 85 Neurotransmitters 86 Overview • Over 100 chemical substances are known or thought to be neurotransmitters. • Some neurotransmitters bind to postsynaptic receptors and function rapidly to open ion channels in the plasma membrane (ionotropic). • Other neurotransmitters function via second-messenger systems in the post-synaptic membrane to influence chemical reactions inside the cell (metabotropic). • Both processes involve the excitation or inhibition of postsynaptic neurons. Second-messenger system = a chemical substance inside a cell that carries information farther along the signal pathway from the internal part of a membrane-spanning receptor embedded in the cell membrane. It may be in the form of an enzyme's product or ion fluxes. (http://medical-dictionary.thefreedictionary.com) Page 432 Page 480 Page 448 Figure 12.27 Figure 12.26 Figure 12.27 87 Small-Molecule Neurotransmitters • Acetylcholine (ACh) is the most widely-studied neurotransmitter in the CNS and especially the PNS. • ACh is excitatory at certain synapses, including neuromuscular junctions, but it is inhibitory at some types of synapses in the brain. • The amino acids, glutumate and aspartate, are excitatory at some synapses in the brain. Page 432 Page 480 Page 448 Figure 12.27 Figure 12.26 Figure 12.27 88 GABA and Glycine • Gamma amino butyric acid (GABA) and glycine, an amino acid, are inhibitory. • As many as one-third of the synapses in the brain involve GABA as a neurotransmitter. • Anti-anxiety drugs such as Valium® enhance the effects of GABA, including at synapses in the limbic system, the collection of structures deep in the telencepahalon that can mediate emotional states. Page 432 Page 480 Page 448 Figure 12.27 Figure 12.26 Figure 12.27 89 Biogenic Amines • The biogenic amines include norepinephrine, dopamine, and serotonin. • Norepinephrine functions in sleep, dreaming, and emotional responses. • Dopamine functions in emotional responses, pleasurable experiences, and addictive behaviors. • Dopamine also helps to regulate skeletal muscle tone and aspects of body movements in deep structures of the brain (for example, the basal ganglia). • Serotonin functions in sensory perception, sleep induction, temperature regulation, food appetite, and control of emotional moods (by regulating deep structures). Page 432 Page 480 Page 448 Figure 12.27 90 Other Small-Molecule Neurotransmitters • Small-molecule neurotransmitters also include: - Adenosine triphosphate (ATP) Purines (a type of nucleotide) Nitric oxide (NO) Page 432 Page 482 Page 450 Figure 12.27 Figure 12.26 Figure 12.27 91 Neuropeptides • Neuropeptides are neurotransmitters of 3 to 40 amino acids linked by peptide bonds through dehydration synthesis. • Neuropeptides known as enkephalins have analgesic effects that are about 200 times stronger than morphine. • Opioid forms known as endorphins have strong analgesic effects, and are thought to be involved in a number of emotional states and mental illnesses. Analgesic = pain-relieving. Page 434 Page 491 Page 450 Figure 12.27 Figure 12.26 Figure 12.27 92 Neural Circuits 93 Overview • The CNS has many billions of neurons—most are organized in complex networks known as neural circuits. • Different types of circuits are thought to process specific types of information. • In a simple-series network, one presynaptic neuron stimulates one postsynaptic neuron. • Most neural networks are more complex than a simple-series network. Page 435 Page 483 Page 451 94 Types of Neural Circuits • Diverging networks relay sensory information to different areas of the brain. • Converging networks integrate information from different areas of the brain to stimulate somatic motor neurons that activate skeletal muscle contractions. • Reverberating networks may regulate complex muscular activities and short-term memory. • Parallel after-discharge circuits may mediate mental activities such as mathematical calculations. Page 435 Page 482 Page 451 Figure 12.28 Figure 12.27 Figure 12.28 95 Neural Circuit in a Flat Worm http://www.wormbook.org 96 Plasticity, Regeneration, and Repair 97 Plasticity • The nervous system exhibits plasticity, that is the capacity to change based on external stimuli (that is, experience). • Changes include new dendritic growth, synthesis of new proteins, and modifications to synaptic connections. • Despite plasticity, neurons in mammals have a limited ability to regenerate when they are damaged or destroyed (especially in the CNS). Regeneration = the ability for replication or self-repair. Page 436 Page 484 Page 452 98 Neurogenesis • The growth of new neurons from undifferentiated stem cells and progenitor cells is known as neurogenesis. • Neurons appear and disappear in the brains of some migrating songbirds each year. • New neurons typically do not form in the adult brains of humans and other primates. • The human brain develops new synaptic connections in response to learning. Page 436 Page 485 Page 452 99 Neurogenesis (continued) http://mindsparke.com 100 Neurogenesis (continued) • In humans, new neurons have been discovered in the hippocampus, a structure located deep in the telencephalon. • The hippocampus is involved in learning, and in particular in the transfer of short-term memory traces to long-term memory. • More research needs to be conducted to determine the scope (if any) of neurogenesis in humans. Page 436 Page 485 Page 453 101 PNS Damage and Repair • Axons and dendrites in the peripheral nervous system may undergo repair in humans, but only if the cell body is intact, Schwann cells remain functional, and scar tissue has not yet formed. • A person who has injured the axons in an upper limb, for example, has a good chance of recovering some or all of the nerve function. Page 436 Page 485 Page 453 Figure 12.29 Figure 12.28 Figure 12.29 102 Two Neurological Disorders 103 Multiple Sclerosis • Multiple sclerosis (MS) is a disease caused by the progressive deterioration of the myelin sheath of neurons in the CNS. • The myelin sheath deteriorates into scleroses of hardened scars and plaques. • The damage slows and eventually short-circuits the propagation of action potentials. Page 437 Page 486 Page 454 104 Multiple Sclerosis (continued) http://icimmedics.com 105 Multiple Sclerosis (continued) • MS is an autoimmune disease involving the body’s immune system. • The disease effects about 350,000 people in the United States, and about two million people worldwide. • Its onset usually occurs between ages 20 and 40—it occurs in females about twice as often as males. Page 437 Page 486 Page 454 106 Multiple Sclerosis (continued) • The most common form of the disease is relapsing-remitting MS. • The earliest symptoms can include a feeling of heaviness or weakness in the skeletal muscles, abnormal sensations, and doublevision. • An episode may be followed by a period of remission lasting up to 1 to 2 years. • Then, over time, the progressive loss of neural function steadily continues. Page 437 Page 486 Page 454 107 Epilepsy • Epilepsy involves recurrent seizures of motor, sensory, and association systems of the brain. • Epileptic seizures affect about 1 to 3 percent of the world’s population. • Seizures are triggered by abnormal electrical discharges from neurons in different structures of the brain. Page 437 Page 486 Page 454 108 Epilepsy (continued) • The abnormal electrical discharges send action potentials over their conduction pathways to other neurons, which can be recruited for the seizure. • The patterns of electrical discharges during a seizure can be chaotic and random. Page 437 Page 486 Page 454 109 Epilepsy (continued) • Immediately before (pre-ictal period) or during a generalized seizure, lights, noises, or smells may be sensed, although the sensory organs have not been stimulated • The effects are because the sensory areas of the cerebral cortex are activated. • Skeletal muscles may contract involuntarily due to involvement of the motor cortex in what is known as grand mal, clonic-tonic, or generalized seizure. Page 437 Page 486 Page 454 110 Epilepsy—EEG Patterns EEG of a generalized seizure http://www.thebarrow.org 111 Epilepsy—Seizure Types • Partial seizures, which occur in a localized area on one side of the brain, typically produce mild symptoms. • Generalized seizures involve large areas on both sides of the brain, and can result in unconsciousness. • Temporal lobe seizures involve areas of the cerebral cortex and the limbic system. • The limbic system mediates emotional states, which can be affected by seizure activity. Page 437 Page 486 Page 454 112 Epilepsy—Causes • Epilepsy can result from many causes, although in many cases it is idiopathic. • Known causes include: - - Brain damage at birth Head injuries Tumors and abscesses of the brain Metabolic disturbances Infections Toxins Vascular problems Idiopathic = of unknown cause. Page 437 Page 486 Page 454 113 Epilepsy—Treatment • Epilepsy can often (but not always) be alleviated or controlled by antiepileptic drugs. • Surgical intervention to remove or contain the epileptic focus may be needed in severe cases. • Support groups are available for persons who have epilepsy and their families and friends. Page 437 Page 486 Page 454 114