Neurons - Cognitive Neuroscience Historical background Are...

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Unformatted text preview: Cognitive Neuroscience Historical background Are cognitive functions localized? What is the role of neurons? Progress in science? Data accumulation Technological advances Conceptual/intellectual progress " is only through historical study that one can really appreciate new ideas, see the faults in existing theories and determine the best path to follow for innovative new research" (Finger, 1994) Localization of the mind to the heart or to the brain? Aristotle (384322 B.C.) Cardiocentric thought heart was the seat of emotional and intellectual functions Heart active, central, blood + chicken's body remains active after decapitation! Brain: cools the seething heart. Because heat rises and because the brain is cool, the network of blood vessels blanketing the cool brain seemed well suited to act like a radiator... The large size relative to other animals is because humans are warmer than other animals.. The large cool human brain.... Allowed for the most efficient type of cooling and thus assured the most rational of hearts (Finger, pg 14) Earliest views regarding localization of mental function Ventricular localization Perception/imagination: lateral ventricles Cognition: third ventricle Memory: fourth ventricle st Held from the 1 centuries through the Middle Ages and Renaissance Why? Ventricular Localization Based on the view that animal spirits (CSF) are the instrument of the soul. The soul acts on these spirits by controlling their flow thru the nerves and thus directing the actions of the body. Influenced by the technology of the time: mechanical dolls operating with hydraulic mechanisms. (e.g., gardens of Versailles) Descartes: "Why do you withdraw your hand from the fire? The force of the external object is such to provoke a greater dilation of the nervous tubes that open up in the surface of the interior cavity of the brain, thus allowing them to conduct a greater flow of the spirits that issue from the pineal gland. These spirits then modify the disposition of the cerebral tubules and enter the nerves of the limb where they cause the hand to withdraw". th th 17 and 18 centuries Ventricles are unlikely seat of higher mental functions since the ventricles of humans are not dissimilar from those of animals with clearly inferior mental abilities Cerebral cortex is the site of higher mental activities, lower structures are responsible for motor and perceptual functions However, no specific localization of these functions within these general areas th 19 Century Localization of respiration to the medulla Le Gallois Localization of sensory and motor functions to the dorsal and ventral roots of the spinal cord respectively Bell, Magendie If functions can be localized within these structures, perhaps there is also localization of higher functions in the cortex? Competing Views Functional localization: certain parts of the brain perform specific higher functions there may be interaction between them but the interaction is sufficiently limited that we can usefully understand the brain by describing components, their function, their localization Holism: the cortex is a dynamic whole which is more than the sum of its parts just as we cannot appreciate music by analyzing individual notes or squareness by looking at each line separately, cannot understand the brain by breaking it down into component functions Franz Joseph Gall (17581828) Physician and outstanding anatomist (materialist and atheist) Gall Cortical localization: The brain is composed of as many organs as there are mental faculties Phrenology: These mental organs vary in size and their size affects the shape of the skull One can study a man's character and faculties by studying the external configuration of the skull Evidence: skulls of individuals with exceptional faculties: poets, statesmen, lunatics, mothers who committed infanticide, etc. Broca (18241880) Physician, scientist highly respected (head of the leftwing, anticlerical Paris Society of Anthropology) Broca (18241880) Put to the test the hypothesis that language is a faculty of the anterior frontal lobes: Tan (Leborgne) had extreme difficulty in speaking but no paralysis of lips or tongue, nor language comprehension difficulties rd On autopsy, damage localized to the 3 convolution of the frontal lobe of the left hemisphere John Hughlings Jackson (1831 1911) Observed epileptic seizures, did autopsies Concluded seizures due to excessive activity in cortex Saw that spasms originate and move across body in predictable ways. Concluded that motor function of different body parts is localized. Motor localization confirmed by others in 1870 by electrical stimulation to cortex of dogs. Tan's brain: What would you conclude? Neurons: Continuous or contiguous? Fused neural networks or independent neurons? If fused--How can there be localization of function as suggested by Broca's findings? Golgi stain Silver nitrate stain: stains random neurons discovered by Golgi (1873) in a kitchen working by candlelight Convinced most (but not Golgi himself) Purkinje cell (cerebellum) Hippocampus impregnated by the Golgi stain (from an original preparation from Golgi's laboratory kept in the Institute of Pathology of the University of Pavia). Purkinje cells of the cerebellum. The Golgi stain reveals their extensive dendritic branches (from an original preparation from Golgi's laboratory kept in the Institute of Pathology of the University of Pavia). The Neuron Doctrine Neurons are discrete cells Unidirectional flow (dendrites to axons) Basic computational unit Prototypical neuron input (dendrites) summation/processing (cell body) output (axon) Different morphologies Glial Cells (10 times the number of neurons) Astrocytes: At synapses, axons Now known to communicate with neurons, other astrocytes. Microglia: Repair tissue damage Oligodendrocytes: myelin (CNS) Schwann cells: myelin (PNS) Myelin Preliminaries Ion: atom that has lost or gained an electron has charge + K : Potassium + Na : Sodium Cl : Chloride Charged particles create electric fields Voltage Electrical "pressure" on charged particle in an electric field. Membrane potential: Voltage next to cell membrane. Due to separation of positive and negative ions. Measured in millivolts (Mv) Membrane Proteins + + + + Active pumps: Na /K pump: 3 Na out / 2 K in + 20 times more K inside + 10 times more Na outside + + Ion channels: passageways for ions Na , K , Cl nongated: always open to specific ions + most are K channels + neural membrane is selectively permeable to K gated: opening/closing is triggered by certain conditions Two Forces (1) Thermal energy gives random motion. + + More K on inside, so more K moves intoout than outtoin. Net flow outward (along concentration gradient). (2) Electrical gradient + As K moves outward, inside of cell becomes negative. Electrical field pulls + ions inward. Electrochemical equilibrium: Balance between the 2 flows: + + K out = K in Net flow = 0 Equilibrium potential depends on concentration ratio, as given by Nernst equation. + For K , 20:1 E = 75 mV K Resting membrane potential: Net current flow = 0 70 mV (Not exactly equal to E because K membrane somewhat permeable to other ions.) Conduction If electrical current (charge + or ) introduced into the neuron, the charge travels (conduction) Passive conduction Due to conductivity of cytoplasm, membrane, extracellular fluid. Does not involve gated ion channels. Active conduction Change of membrane conductivity due to opening of gated ion channels. Passive Conduction The axon is a "leaky hose" (due to ion channels) The distance current will passively travel is determined, among other things by: The strength of the original current (graded response) The diameter of the axon (resistance) Generally no further than 1mm In some places (e.g., retina) 1mm is sufficient to signal to a neighboring neuron, but typically it is not Passive conduction Action Potential 1 passive conduction of + charge depolarization of membrane (moves toward 0) 2 if depolarization reaches threshold (50 mV) + + gated Na channels open, Na rushes in rapid reversal of polarization (+40 mV) the action potential + gated Na channels close 3 triggers opening of gated K + channels, K + rushes out hyperpolarization (75 mV) (1 ms) + + 4 Na /K pump returns cell to the resting potential (1 2 ms) Action potential: Summary At rest, the inside of the neuron is slightly negative. When stimulated past threshold, sodium channels open and sodium rushes into the axon, causing a region of positive charge within the axon. The region of positive charge causes nearby sodium channels to open. Just after the sodium channels close, the potassium channels open wide, and potassium exits the axon. This process continues as a chainreaction along the axon. The influx of sodium depolarizes the axon, and the outflow potassium repolarizes the axon. The sodium/potassium pump restores the resting concentrations of sodium and potassium ions This page and its contents Copyright 1997, Carlos Finlay and Michael R. Markham. The action potential Historically considered all or none: once the threshold is reached, a depolarization of fixed size takes place (not affected by the size/intensity of the original current) New evidence with naturalistic stimulation of pyramidal cells shows height and width systematically vary as a function of membrane conductivity over past 50 ms. (Polavieja et al., 2005, Joural of Neuroscience) Spikes of Different Shapes From Figure 1 of (Polavieja et al., 2005) The action potential Refractory period: a brief period of time during which it is more difficult to generate and action potential + absolute refractory period: Na channels cannot reopen for a few milliseconds action potentials travel in one direction down axon. relative refractory period: Period of membrane hyperpolarization. Long distance communication Passive currents > action potential > passive currents > action potential >...... Slow....... And still a leaky hose..... For rapid and efficient communication need to insulate the leaky hose. myelin: insulation allowing passive currents travel further, action potentials can generated less often Nodes of Ranvier: gaps in myelin where action potentials are generated > saltatory conduction (~120 m/sec) Neural signaling at the synapse Synapse (~100 trillion/brain) presynaptic membrane axon synaptic cleft (20 nanometers, 20 billionths of a meter) postsynaptic membrane dendrite Chemical Signaling: Transmitter release Presynaptic membrane: 1 Action potential arrives at the axon terminals depolarization 2+ 2 Voltagegated Ca channels open 3vesicles with neurotransmitter fuse to presynaptic membrane 4transmitter released into synaptic cleft (vesicles are later refilled) Chemical signaling: Transmitter uptake Postsynaptic membrane: 1 neurotransmitter binds with receptors 2 triggers opening and/or closing of chemically gated channels + + (e.g., Na , K ,Cl ) Excitatory synapses: ion influx/efflux depolarization of the postsynaptic cell (EPSP) Inhibitory synapses: ionic influx/efflux hyperpolarization of the postsynaptic cell (IPSP) Synaptic Strength Variation in magnitude of postsynaptic potential Presynaptic: vesicle size, number probability of vesicle fusing to membrane 2+ Ca concentration Postsynaptic: density of receptors Neurons as computational devices: Summation and Convergence Convergence: neurons receive signals from many other neurons Spatial summation: all currents received by the postsynaptic neuron are combined (EPSP and IPSP) and the sum will determine whether or not an action potential is generated (typically at the axon hillock) and a signal transmitted to other neurons Temporal summation: potentials linger and sum up over time, increasing the probability that an action potential will be generated Spike rate: larger summed signal faster spike rate ~10 billion neurons typical neuron has 1,00010,000 synapses ~60100 trillion synapses A LOT of computation! Some Exceptions to the Neuron Doctrine Electrical synapses Information processing by astrocytes Electrical Synapses Gap junctions: precisely aligned, paired channels forming a pore, allowing ionic current to flow passively from one neuron to another Only recently confirmed in the mammalian brain Probably throughout, but confirmed in: Olivary nucleus, neocortex, hippocampus, thalamus, retina Electrical synapses Extremely rapid information transmission (fraction of a ms) Bidirectional flow synchronization of activity between neurons Graded transmission Tripartite Synapse Astrocyte surrounds synapse. Astrocyte has receptors for neurotransmitters. 2+ Neurotransmitters can increase Ca level in astrocyte. 2+ Ca wave can spread to other astrocytes. 2+ Ca level affects gliotransmitter release from astrocytes. Gliotransmitters can affect presynaptic and postsynaptic neural function. Back to Neurons What does it mean when a neuron fires? Neurons are pattern detectors. Input connections determine preferred pattern. Firing means that input is similar to preferred pattern. Characterization Easy at Periphery Vision light a certain point of space Auditory sound at certain frequency Motor output contract certain portion of muscle Higher Levels Meaning determined by connections between levels and within levels. Highly complex may be difficult to categorize in terms of words / concepts. ...
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This note was uploaded on 07/29/2008 for the course NEUROSCIEN 70 taught by Professor Whitney during the Spring '08 term at Johns Hopkins.

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