Understanding Neural Signaling in the Brain

1. Course Introduction: The Brain, chemistry, neural signaling Jerome Feldman CS182/Ling109/CogSci110 Spring 2007 feldman@icsi.berkeley.edu

2. Learning • I hear and I forget • I see and I remember • I do and I understand • attributed to Confucius 551-479 B.C. • There is no erasing in the brain

3. Instructor Access • Instructor : Jerry Feldman • Office Hours : Monday, Thursday 1 – 2 Soda 739 • Email : jfeldman@cs.berkeley.edu • TA: Leon Barrett • Office Hours :TBA, Soda 739 • 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?

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

8. 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

9. How does it all work?

12. Artist’s rendition of the Actin molecule structure

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

15. Artist’s rendition of a typical cell membrane

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

17. Neural Processing

18. Neurons • cell body – metabolism, protein synthesis • dendrites (input structure) • receive inputs from other neurons • perform spatio-temporal integration of inputs • relay information to the cell body • axon (output structure) • a branching fiber that carries the message (spikes) from the cell to other neurons

19. postsynaptic neuron science-education.nih.gov

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

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

22. 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 – usually connects on dendrite • arrival of activity at an inhibitory synapse hyperpolarizes the local membrane potential of the postsynaptic cell and makes it less prone to firing – usually connects on cell body • the greater the synaptic strength, the greater the depolarization or hyperpolarization

24. UNIPOLAR BIPOLAR MULTIPOLAR CELLS

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

26. PET scan of blood flow for 4 word tasks

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

28. Neural Communication: 1 Communication within the cell

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

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

31. Artist’s rendition of a typical cell membrane

32. 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

33. 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+

34. 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+

35. 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+

37. 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

38. 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.

39. Before Depolarization

40. 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.

41. Depolarization

42. 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.

43. Repolarization

44. 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.

46. 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.

47. Action Potential

48. 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.

49. 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

50. postsynaptic neuron science-education.nih.gov