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Invertebrate nervous system organization -Nervous system structure reflects the body plan organization Invertebrate nervous systems: cnidarians (anemones, medusoid jellyfish) flatworms/roundworms (planaria, nematodes) segmented worms (polychaetes, oligochaetes, hirudinae) arthropods (crustacea and insects) mollusks (snails, octopods/squid) pre-vertebrates: hemichordates, cephalochordates agnathans (lamprey) 1 Cnidarians Flatworms nerve net - little or no collecting of neurons into ganglia, no anterior “brain” With the appearance of bilateral symmetry: came cephalization = concentation of neurons into ganglia at the anterior end http://cas.bellarmine.edu/tietjen/ Research/RobotAnimalModels.htm Do have neural “plexi” (sing. plexus), loose collections of neurons analogous to the enteric nervous system in vertebrates With the appearance of segmentation: segmental “ganglia” Leech serotonin immunoreactivity single cell dye filling 2 nerves to periphery intersegmental connectives segmental ganglion Arthropods -- fusion and specialization of segments accompanied By f & s of ganglia supraesophogeal subesophogeal thoracic abdominal www.st-andrews.ac.uk/ ~wjh/jumping/motorsys.htm thoracic abdominal With the elaboration of sophisticated sense organs at the anterior (head) end of the animal there is an elaboration and expansion of the circumesophegeal ganglia. Supraesoph. dominated by sensory processing circuits. With the specialization of thoracic segments for locomotion there is specialization of thoracic ganglia. Abdominal ganglia serve motor, sensory and reproductive functions of the abdominal segments. Successively more fusion of segmental ganglia from crustacea to insects 3 Gastropod mollusks Tritonia diomedea Tritonia festiva supra and subesophogeal ganglia and pleural gangia 1 mm Buccal ganglia 4 Vertical lobe retina optic nerves optic lobe Lateral view of octopus brain …. octopi live 1-2 yrs….. what a waste? 5 Spontaneous activity Procerebral lobe of Limax maximus – 100,000 small neurons odor Leech Locust AP P Crayfish 6 Giant glial cells in leech ganglion Processes of packet glia surrounding neural somata http://www.uni-kl.de/FB-Biologie/AG-Deitmer/Confocal/confocal_gallery.htm 7 Isolation of individual neurons from the Aplysia CNS. Lovell P , Moroz L L Integr. Comp. Biol. 2006;46:847-870 © The Author 2006. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: [email protected]. http://www.youtube.com/watch?v=V6H01cUSpfQ 8 Two ways to construct central pattern generators persistent excitation = driver + _ + The half-centre oscillator _ The neurogenic leech heart (a rhythm generator) All inhibitory synapses– where does the excitatory drive come from? Neurogenic leech heart Hyperpolarize 1 HN Hyperpolarize an HN…. note the re-setting of the phase This capacity to “reset” rhythm is a property of neurons that are central elements in central pattern generators. 9 Box 16C The autonomy of central pattern generators: evidence from the lobster stomatogastric ganglion (Part 1) (photo Marie Suver MIT) Pyloric Dilator -- C. Goldsmith, C. Staedele Illinois State U 10 Box 16C Autonomy of central pattern generators: evidence from the lobster stomatogastric ganglion PD neuron activity patterns Concept: Polymorphic networks One set of physical connections can generate different outputs depending on neuromodulation Diffuse networks of neurons mixed in with body tissues Formation of ganglia Bilateral symmetry and concentration of sensory structures at one end leads to cephalization Body segmentation is accompanied by segmention of nervous system = segmental ganglia Specialization and fusion of segmental ganglia reflecting specialization of segments Molluscs – continue process of fusing ganglia into larger “brains” Limitations to the invertebrate plan…. Diffusion of O2 and nutrients into a thick tissue… Terms to know and be able to explain: nerve net cephalization ventral nerve cord ganglion/ganglia commissure connective nerve trunk/nerve segmental ganglion/ganglia central pattern generator 11 BIRD ucumberland.edu LAMPREY FISH HUMAN FROG Very early in its evolution, the vertebrate brain underwent modifications that set the stage for later evolutionary trends. Briefly, the modifications were these:1.The hindbrain became divided into a ventral portion, called the medulla oblongata, a dorsal portion, the cerebellum, and the anterior pons. The medulla became specialized as a control center for some autonomic and somatic pathways concerned with vital functions (such as breathing, blood pressure, and heartbeat) and as a connecting tract between the spinal cord and the more anterior parts of the brain. The pons is above the medulla and also acts as a connecting tract. The cerebellum enlarged and became a structure concerned with balance, equilibrium, and muscular coordination. Fish Frog Lamprey (agnathan) 12 2.The midbrain became specialized as the optic lobes, visual centers associated with the optic nerves. 3.The forebrain became divided into an anterior portion consisting of the cerebrum, with its prominent olfactory bulbs, and a posterior portion consisting of the thalamus and hypothalamus. During the course of vertebrate evolution, there have been few changes in the hindbrain, though the cerebellum has become larger and more complex in many animals. The major evolutionary change has been the steady increase in size and importance of the cerebrum, with a corresponding decrease in relative size and importance of the midbrain (see Figure below and next slide). Bird You 13 Increase in size of forebrain Decrease in relative size of midbrain Increased function for forebrain (cerebrum), e.g. more visual processing, more somatosensory processing, more associational processes. Increase in the number of layers of neurons in the cerebrum (cortex) addition of 6-layered neo-cortex to pre-existing 3layered archi-cortex (hippocampus and pyriform, == olfactory cortex) Increase in size of cerebellum (a complex motor-learning/ sensory integration area). Large expansion of “associational” higher processing areas in neocortex Folding of neocortex (upon itself and infolding) to accommodate increased area within the cranial cavity Expansion of area devoted to specialized functions in some species, e.g. audition in dolphins and owls, somatosensory areas in blind, burrowing moles etc. Cool brain facts A cubic mm of neocortex contains 90,000 neurons 400 meters of dendrites 4 km of axons About 7x108 synapses The most cell dense part of the brain is the granule layer of cerebellum (not neocortex) with about 7x106 neurons per cubic mm, each of which is about 7-8µm diameter. Human surface area of cortex – 2500 cm2 Surface area of 3/4 of a sphere with 7 cm radius = 460 cm2 Folding of cortex = 5 to 6-fold increase in area. (brain volume is 140cm3 which is equivalent to a sphere with 7 cm radius, 75% of this is neocortex so roughly assume area is 75% of the area of a 7cm radius sphere) 14 Brain regions expand to meet computational demands: e.g. in weakly electric fish the cerebellum devoted to generating and interpreting electric field signals = biggest part of weakly electric fish brain. Sensory inputs are usually organized so that information from adjacent areas is processed by neurons located near each other in brain regions all along the processing chain. Somatotopy in the 15 The amount of brain tissue devoted to processing information varies depending upon the amount of information that is needed to be processed. peripheral afferents somatosensory subdivisions 16 17 Expansion of forebrain is not equally distributed – expansion occurred mainly in so-called “association” areas Frog Neocortex Hippocampus Mouse 18 Frog Frog Cerebrum Neocortex = 3 layered “Archicortex” Hippocampus Mouse neocortex Hippocampus Projection neurons - long axons “Principal” or Projection Neurons Mouse 6 “layers” appeared early in mammalian evolution (Triassic Jurassic) Front Neurosci. 2015; 9: 162. doi: 10.3389/fnins. 2015.00162 E.g. Cortical pyramidal neurons Action potential output 19 INTERNEURONS short axons -- mainly local connections Spiny stellate Basket neuron Chandelier Axon Axon 100µm Mouse Axon Inhibitory (GABA) Excitatory (glutamate) Neural communication….exciting! Next section of course: Chapters 2-5. Where does neural ‘electricity’ come from? active transport and passive diffusion…. Nernst, Goldman-Hodgkin-Katz Active vs passive properties of neurons Signal transmission by axons, Signal transfer by synapses 20 The simple myotatic reflex circuit illustrates several points about the functional organization of neural circuits (Fig. 1.7) • PN01050.JPG Sherrington Relative frequency and timing of action potentials in different components of the myotatic reflex (Fig.1.8) • PN01060.JPG Extracellular electrical recordings + + 21 Intracellularly Recorded Reponses Underlying the Myotatic Reflex (Fig. 1.9) • PN01072.JPG Intracellular recordings To prepare for exams: Be able to describe the processes involved in signaling in a simple reflex circuit like this. How are electrical signals converted to chemical and then back to electrical? What causes delays in the transfer of signals? What would slow down or speed up signaling? What determines the sign (inhibition vs excitation) of the synaptic potentials. Why are “inhibitory” potentials “inhibitory” Neuro-electricity is due to ionic currents that are generated by the diffusion of 4 essential ions…. Na+, K+, Ca2+, Cl- 22 Figure 2.2 Recording passive and active electrical signals in a nerve cell Figure 2.1 Types of neuronal electrical signals Receptor potential “passive” Synaptic potential “passive” Action potential “regenerative ” = “active” 23 Where does the non-zero voltage difference, which we call the membrane potential come from? It is produced by maintaining different concentrations of charged molecules (mainly ions) inside and outside a selectively permeable cell surface membrane (the plasmalemma = cell membrane). TOTAL 490-600 mM TOTAL 185 mM TOTAL 1030 mM TOTAL 262 mM 24 TOTAL 490-600 mM TOTAL 185 mM TOTAL 1030 mM TOTAL 262 mM Figure 2.3 Transporters and channels move ions across neuronal membranes What is the role of pumps in setting membrane potentials? 25 Permeable only to K+ ….. at 18 °C If we have more of one ion on one side of a membrane then we have a concentration gradient and a charge gradient = possible energy sources – BUT there is only an actual energy source if ion is out of its ‘equilibrium condition’. 26 Permeability is needed to “harvest” the energy stored in the concentration gradient. No permeability no “voltage” will be generated Seems like we could have energy stored in in the form of voltage differences (batteries) or concentration differences…. In fact cells have both kinds of energy storage mechanisms = electrochemical gradients How much--- what “direction”. ?? etc. For our questions about how neurons will generate time variant electrical signals we first need to know how much potential electrical energy is stored due to the concentration gradient for EACH ionic species. That way we know what kind of voltage changes (large/ small, positive/negative) we could produce if we allowed a particular ion to move (flow as electrical current) down its energy gradient…. 27 for unequal distribution of charged molecules across a membrane: A general equation to relate stored electrical energy to stored chemical energy derives from equating chemical to electrical energy Electrical “work” = zFV and Chemical “work” = RT*ln[X]1/[X]2 at equilibrium, the chemical work that can be done by a concentration difference of ions is equal to the negative of the electrical work that goes into separating the charges. F= Faraday’s constant (96,000 Coulombs/mole), R = universal gas constant (8.314 VCK-1 mol) z = valence of charge (e.g. +1) At equilibrium chemical and electrical forces are equal and opposite When the energy stored in the electrical gradient (V) = the negative of the energy stored in the chemical gradient then the forces cancel and net diffusion is zero: zFV = - RT*ln ([X]1/[X]2) Thus: V = -(RT/zF)*ln ([X]1/[X]2) or V = +(RT/zF)*ln ([X]2/[X]1) :: {ln a/b = -ln (b/a)} This particular voltage difference where the forces balance is called the Equilibrium potential for ion x Also written as Veq(x) or Vx or Ex 28 At equilibrium chemical and electrical forces are equal and opposite When the energy stored in the electrical gradient (V) = the negative of the energy stored in the chemical gradient then the forces cancel and net diffusion is zero: zFV = - RT*ln ([X]1/[X]2) Thus: V = -(RT/zF)*ln ([X]1/[X]2) or V = +(RT/zF)*ln ([X]2/[X]1) :: {ln a/b = -ln (b/a)} This particular voltage difference where the forces balance is called the Equilibrium potential for ion x Also written as Veq(x) or Vx or Ex Ex == Equilibrium potential of ion x (ALSO known as the reversal potential) It can be calculated as the “Nernst” potential for ion X. By convention we use outside over inside which gives us V of the inside relative to the outside 29 Ex == Equilibrium potential of ion x (ALSO known as the reversal potential) It can be calculated as the “Nernst” potential for ion X. By convention we use outside over inside which gives us V of the inside relative to the outside Nernst: Ex = (RT/zF) *ln ( [X]outside / [X]inside) Note -- we can use log10 or ln. Since ln(a) = 2.303log10(a): Ex = ((2.303*RT)/zF) *log10 ( [X]outside / [X]inside) Ex is measured “inside” with respect to “outside”. Therefore “[X]outside is the numerator in our equation -----------------------------------------------------------------------z is ion’s valence (+1, +2, -1, -2 etc) T = temperature in °Kelvin (273 + °K = °C) 2.303 RT/F = .058 Volts (58 mV) @ 18 °C; = .062 Volts (62 mV) @ 37°C = .056 Volts (56 mV) @ 10°C Notice that the voltage change is not accompanied by a measureable change in concentration….. Only a small fraction of the total ions in the compartments have to re-distribute to create the gradient across the thin membrane 30 Figure 2.7 Resting membrane potential is determined PRIMARILY by the K+ concentration gradient – WHY??? at a T° of 18C of course! Why does the real data tail off at low [K]out? Dead man walking…… Practical applications of Nernst….. Sodium pental or (barbituate) pentobarbital Pancurium bromide (paralytic) Potassium chloride (heart stopper) 31 Recall Nernst equation (for one ionic species) Veq = Eion = +(2.303RT/zF)* log ([ion1]o/ +[ion1]i) Recall Nernst equation (for one ionic species) Veq = Eion = +(2.303RT/zF)* log ([ion1]o/ +[ion1]i) REAL NEURONS have more than just one permeant ion species Goldman-Hodgkin Katz (GHK) equation Vm = +(2.303RT/zF)* log ( ( Pion1[ion1]o +Pion2[ion2]o) ( Pion1[ion1]i +Pion2[ion2]i) ) ( “P’ refers to the relative permeabilty of the ion species ) 32 Where does ionic permeability come from? What is conductance and how does it relate to permeability in the GHK equation? The tricky thing for many is understanding the difference between conductance and current… 33 Figure 2.5 Membrane potential influences ion fluxes The membrane potential of a neuron can change from the “resting” potential to a new potential whenever: a) permeability of the membrane changes for one or more of the ionic species that is not at equilibrium inside/outside the neuron. -- permeability can be changed quickly -- thus electrical changes can occur quickly b) the concentration of an ionic species to which the membrane has permeability is changed. If this occurs it is usually much slower than (a c) The activity of an electrogenic pump increases or decreases. V will change slowly 34