Course Introduction: The Brain, chemistry, neural signaling

1. Course Introduction: The Brain, chemistry, neural signaling Jerome Feldman Srini Narayan CS182/Ling109/CogSci110 Spring 2008 feldman@icsi.berkeley.edu

2. http://inst.eecs.berkeley.edu/~cs182/sp08/Lecture Overview • Course introduction • Neural Processing: Basic Issues • Neural Communication: Basics

3. Instructor Contact • Instructor : Srini Narayanan • Office Hours : • Email : snarayan@icsi.berkeley.edu • Instructor : Jerome Feldman • Office Hours : Monday 1 – 2, Thur. 2:30-3:30 • Email : jfeldman@cs.berkeley.edu • TA: Leon Barrett • Office Hours : • Email : barrett@icsi.berkeley.edu

4. The Neural Theory of Language and Thought • This is a course on the current status of interdisciplinary studies that seek to answer the following questions: • How is it possible for the human brain, which is a highly structured network of neurons, to think and to learn, use, and understand language? • How are language and thought related to perception, motor control, and our other neural systems, including social cognition? • How do the computational properties of neural systems and the specific neural structures of the human brain shape the nature of thought and language? • What are the applications of neural computing and embodied language?

5. Learning • I hear and I forget • I see and I remember • I do and I understand • attributed to Confucius 551-479 B.C.

6. Tinbergen’s Four Questions How does it work? How does it improve fitness? How does it develop and adapt? How did it evolve?

7. Single Cell (Protozoan) Behaviors • No Nervous System • Foraging Behavior (move toward food) • Positive chemotaxis • Defensive/Avoidance Behavior • Negative chemotaxis • Reproduction • Asexual and Sexual reproduction using chemical messenger proteins (pheromones)

9. Earliest Nervous Systems • Earliest neurons dispensed hormones • Hydra, jellyfish, corals, sea anemones • Basic neural cell (Neuron) • Early differentiation into 3 types of neurons STIMULUS Effector Sensory Neuron Inter- Neuron Motor Neuron

10. Neural Processing

11. Neurons • cell body • dendrites (input structure) • receive inputs from other neurons • perform spatio-temporal integration of inputs • relay them to the cell body • axon (output structure) • a fiber that carries messages (spikes) from the cell to dendrites of other neurons

12. postsynaptic neuron science-education.nih.gov

13. Synapse • site of communication between two cells • formed when an axon of a presynaptic cell “connects” with the dendrites of a postsynaptic cell

14. Synapse axon of presynaptic neuron dendrite of postsynaptic neuron bipolar.about.com/library

15. Synapse • a synapse can be excitatory or inhibitory • arrival of activity at an excitatory synapse depolarizes the local membrane potential of the postsynaptic cell and makes the cell more prone to firing • arrival of activity at an inhibitory synapse hyperpolarizes the local membrane potential of the postsynaptic cell and makes it less prone to firing • the greater the synaptic strength, the greater the depolarization or hyperpolarization

17. UNIPOLAR BIPOLAR MULTIPOLAR CELLS

18. 1000 operations/sec 100,000,000,000 units 10,000 connections/ graded, stochastic embodied fault tolerant evolves, learns 1,000,000,000 ops/sec 1-100 processors ~ 4 connections binary, deterministic abstract crashes designed, programmed Brains ~ Computers

19. Motor cortex Somatosensory cortex Sensory associative cortex Pars opercularis Visual associative cortex Broca’s area Visual cortex Primary Auditory cortex Wernicke’s area

20. PET scan of blood flow for 4 word tasks

21. Somatotopy of Action Observation Foot Action Hand Action Mouth Action Buccino et al. Eur J Neurosci 2001

25. Neural Communication: 1 Processing within the cell

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

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

28. Artist’s rendition of a typical cell membrane

29. 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

30. 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+

31. 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+

33. 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.

34. 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.

35. Depolarization

36. 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 the membrane.

37. Repolarization

38. 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.

40. Action Potential

41. 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 enables saltatory conduction.

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

43. 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

44. 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.

45. 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

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

49. 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

50. Ionotropic Receptors