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Overview

Overview. Course introduction Neural Processing: Basic Issues Neural Communication: Basics Vision, Motor Control: Models. Neural Communication: 1 Communication within and between cells. Transmission of information. Information must be transmitted within each neuron and between neurons.

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Overview

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  1. Overview • Course introduction • Neural Processing: Basic Issues • Neural Communication: Basics • Vision, Motor Control: Models

  2. Neural Communication: 1 Communication within and between cells

  3. Transmission of information Information must be transmitted • within each neuron • and between neurons

  4. The Membrane • The membrane surrounds the neuron. • It is composed of lipid and protein.

  5. The Resting Potential • There is an electrical charge across the membrane. • This is the membrane potential. • The resting potential (when the cell is not firing) is a 70mV difference between the inside and the outside. outside + + + + + - inside - - - - Resting potential of neuron = -70mV

  6. Artist’s rendition of a typical cell membrane

  7. Ions and the Resting Potential • Ions are electrically-charged molecules e.g. sodium (Na+), potassium (K+), chloride (Cl-). • The resting potential exists because ions are concentrated on different sides of the membrane. • Na+ and Cl- outside the cell. • K+ and organic anions inside the cell. Cl- Na+ Cl- Na+ Na+ Na+ outside inside Organic anions (-) K+ Organic anions (-) Organic anions (-) K+

  8. Ions and the Resting Potential • Ions are electrically-charged molecules e.g. sodium (Na+), potassium (K+), chloride (Cl-). • The resting potential exists because ions are concentrated on different sides of the membrane. • Na+ and Cl- outside the cell. • K+ and organic anions inside the cell. Cl- Na+ Cl- Na+ Na+ Na+ outside inside Organic anions (-) K+ Organic anions (-) Organic anions (-) K+

  9. Maintaining the Resting Potential • Na+ ions are actively transported (this uses energy) to maintain the resting potential. • The sodium-potassium pump (a membrane protein) exchanges three Na+ ions for two K+ ions. Na+ Na+ Na+ outside inside K+ K+

  10. Neuronal firing: the action potential • The action potential is a rapid depolarization of the membrane. • It starts at the axon hillock and passes quickly along the axon. • The membrane is quickly repolarized to allow subsequent firing.

  11. Before Depolarization

  12. Na+ + - - + Na+ Na+ Action potentials: Rapid depolarization • When partial depolarization reaches the activation threshold,voltage-gated sodium ion channels open. • Sodium ions rush in. • The membrane potential changes from -70mV to +40mV.

  13. Depolarization

  14. Na+ K+ + K+ - Na+ Na+ K+ Action potentials: Repolarization • Sodium ion channels close and become refractory. • Depolarization triggers opening of voltage-gated potassium ion channels. • K+ ions rush out of the cell, repolarizing and then hyperpolarizing the membrane.

  15. Repolarization

  16. The Action Potential • The action potential is “all-or-none”. • It is always the same size. • Either it is not triggered at all - e.g. too little depolarization, or the membrane is “refractory”; • Or it is triggered completely.

  17. +40 Membrane potential (mV) [C] [B] 0 [A] [D] excitation threshold -70 Time (msec) 0 1 2 3 Course of the Action Potential • The action potential begins with a partial depolarization (e.g. from firing of another neuron ) [A]. • When the excitation threshold is reached there is a sudden large depolarization [B]. • This is followed rapidly by repolarization [C] and a brief hyperpolarization [D]. • There is a refractory period immediately after the action potential where no depolarization can occur [E] [E]

  18. Action Potential Local Currents depolarize adjacent channels causing depolarization and opening of adjacent Na channels Question: Why doesn’t the action potential travel backward?

  19. Conduction of the action potential. • Passive conduction will ensure that adjacent membrane depolarizes, so the action potential “travels” down the axon. • But transmission by continuous action potentials is relatively slow and energy-consuming (Na+/K+ pump). • A faster, more efficient mechanism has evolved: saltatory conduction. • Myelination provides saltatory conduction.

  20. Myelination • Most mammalian axons are myelinated. • The myelin sheath is provided by oligodendrocytes and Schwann cells. • Myelin is insulating, preventing passage of ions over the membrane.

  21. Saltatory Conduction • Myelinated regions of axon are electrically insulated. • Electrical charge moves along the axon rather than across the membrane. • Action potentials occur only at unmyelinated regions: nodes of Ranvier. Myelin sheath Node of Ranvier

  22. Synaptic transmission • Information is transmitted from the presynaptic neuron to the postsynaptic cell. • Chemical neurotransmitters cross the synapse, from the terminal to the dendrite or soma. • The synapse is very narrow, so transmission is fast.

  23. Structure of the synapse • An action potential causes neurotransmitter release from the presynaptic membrane. • Neurotransmitters diffuse across the synaptic cleft. • They bind to receptors within the postsynaptic membrane, altering the membrane potential. terminal extracellular fluid synaptic cleft presynaptic membrane dendritic spine postsynaptic membrane

  24. Neurotransmitter release • Ca2+ causes vesicle membrane to fuse with presynaptic membrane. • Vesicle contents empty into cleft: exocytosis. • Neurotransmitter diffuses across synaptic cleft. Ca2+

  25. Ionotropic receptors (ligand gated) • Synaptic activity at ionotropic receptors is fast and brief (milliseconds). • Acetylcholine (Ach) works in this way at nicotinic receptors. • Neurotransmitter binding changes the receptor’s shape to open an ion channel directly. ACh ACh

  26. Ionotropic Receptors

  27. Metabotropic Receptors (G-Protein)

  28. Excitatory postsynaptic potentials (EPSPs) • Opening of ion channels which leads to depolarization makes an action potential more likely, hence “excitatory PSPs”: EPSPs. • Inside of post-synaptic cell becomes less negative. • Na+ channels (NB remember the action potential) • Ca2+ . (Also activates structural intracellular changes -> learning.) Ca2+ Na+ + outside inside -

  29. Inhibitory postsynaptic potentials (IPSPs) • Opening of ion channels which leads to hyperpolarization makes an action potential less likely, hence “inhibitory PSPs”: IPSPs. • Inside of post-synaptic cell becomes more negative. • K+(NB remember termination of the action potential) • Cl-(if already depolarized) Cl- + outside inside - K+

  30. Postsynaptic Ion motion

  31. Requirements at the synapse For the synapse to work properly, six basic events need to happen: • Production of the Neurotransmitters • Synaptic vesicles (SV) • Storage of Neurotransmitters • SV • Release of Neurotransmitters • Binding of Neurotransmitters • Lock and key • Generation of a New Action Potential • Removal of Neurotransmitters from the Synapse • reuptake

  32. Integration of information • PSPs are small. An individual EPSP will not produce enough depolarization to trigger an action potential. • IPSPs will counteract the effect of EPSPs at the same neuron. • Summation means the effect of many coincident IPSPs and EPSPs at one neuron. • If there is sufficient depolarization at the axon hillock, an action potential will be triggered. axon hillock

  33. Three Nobel Prize Winners on Synaptic Transmission Arvid Carlsson discovered dopamine is a neurotransmitter. Carlsson also found lack of dopamine in the brain of Parkinson patients. Paul Greengard studied in detail how neurotransmitters carry out their work in the neurons. Dopamine activated a certain protein (DARPP-32), which could change the function of many other proteins. Eric Kandel proved that learning and memory processes involve a change of form and function of the synapse, increasing its efficiency. This research was on a certain kind of snail, the Sea Slug (Aplysia). With its relatively low number of 20,000 neurons, this snail is suitable for neuron research.

  34. Neuronal firing: the action potential • The action potential is a rapid depolarization of the membrane. • It starts at the axon hillock and passes quickly along the axon. • The membrane is quickly repolarized to allow subsequent firing.

  35. Overview • Course introduction • Neural Processing: Basic Issues • Neural Communication: Basics • Vision, Motor Control: Models

  36. Motor Control: Basics

  37. Hierarchical Organization of Motor System • Primary Motor Cortex and Premotor Areas

  38. Primary motor cortex (M1) Hip Trunk Arm Hand Foot Face Tongue Larynx

  39. Motor Control Basics • Reflex Circuits • Usually Brain-stem, spinal cord based • Interneurons control reflex behavior • Central Pattern Generators • Cortical Control

  40. postsynaptic neuron science-education.nih.gov

  41. Flexor- Crossed Extensor Reflex (Sheridan 1900) Reflex Circuits With Inter-neurons Painful Stimulus

  42. Gaits of the cat: an informal computational model

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