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Biology 103 - Main points/Questions

Biology 103 - Main points/Questions. What does a neuron look like? Why do membranes have charges? How can these charges change?. Functions of the Nervous System. Process and coordinate: sensory input: from inside and outside body motor commands:

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Biology 103 - Main points/Questions

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  1. Biology 103 - Main points/Questions What does a neuron look like? Why do membranes have charges? How can these charges change?

  2. Functions of the Nervous System • Process and coordinate: • sensory input: • from inside and outside body • motor commands: • control activities of peripheral organs (e.g., skeletal muscles) • Integration – occurs in the central nervous system • higher functions of brain: • intelligence, memory, learning, emotion

  3. Coordinating all the different body systems and interacting with the external world are the job of the body’s control systems – the nervous system and the endocrine (hormone) system.

  4. Aplysia (sea slug) neurons

  5. Neurons are nerve cells that transfer information within the body • Neurons use two types of signals to communicate: • electrical signals (long-distance) and • chemical signals (one cell to the next - short) • Nervous systems process information in three stages: sensory input, integration, and motor output

  6. Sensory input Integration Sensor Motor output Central nervous system (CNS) Effector Peripheral nervous system (PNS)

  7. Integration Sensory information White matter Response Spinal cord Sensory neuron Motor neuron Interneuron

  8. Three types of neurons • These stages use three basic types of neurons – • sensory • association and • motor

  9. Neuron Structure and Function • Most of a neuron’s organelles are in the cell body • Most neurons have dendrites, that receive signals from other neurons • The axon is typically a longer extension that transmits signals to other cells • Many axons are wrapped by other cells (glial cells) to speed signaling

  10. Glial Cells A typical neuron & formation of the myelin sheath

  11. Big idea: Neuron membranes have a charge. • Every cell has a voltage (difference in electrical charge) across its plasma membrane called a membrane potential • Messages are transmitted as changes in membrane potential • The resting potential is the membrane potential of a neuron not sending signals

  12. The Resting Potential • Why do neurons have a resting potential? • Lets look at one ion - potassium (K+) – that is found in your neurons • Cells have large amounts of potassium inside them and small amounts outside. • Neurons have channels that let potassium cross the membrane – what does this do?

  13. Electrochemical Gradients Figure 12–9c, d

  14. Electrochemical Gradients Figure 12–9a, b

  15. –90 mV Outer chamber Inner chamber 140 mM 5 mM KCI KCI • Potassium stops moving when charge is -90mV – Why? K+ Cl– Potassium channel (a) Membrane selectively permeable to K+

  16. The Resting Potential • Of course there are more charged ions and molecules inside a neuron • Sodium (Na+) is a key player in neuron signaling. • There is lots of sodium outside the cell!

  17. Two key ions for neurons • Other molecules and ions add negative charge to the inside of a neuron. OUTSIDE CELL [Na+] 150 mM [K+] 5 mM [A–] 100 mM [K+] 140 mM INSIDE CELL [Na+] 15 mM (a)

  18. The Resting Potential • In your neuron the concentration of K+ is greater inside the cell, while the concentration of Na+ is greater outside • How do your neurons maintain this difference?

  19. Active resting in neurons • Neurons are constantly working to maintain “resting” conditions • This is because the membrane leaks ions • A neuron at rest contains many open K+ channels and few open Na+ channels; so lots of K+ diffuses out of the cell

  20. Active transport allows cells to maintain concentration gradients that differ from their surroundings • The sodium-potassium pump is one type of active transport system

  21. EXTRACELLULAR FLUID [Na+] high [K+] low Na+ Na+ [Na+] low Na+ CYTOPLASM [K+] high 1 Cytoplasmic Na+ binds to the sodium-potassium pump.

  22. Na+ Na+ Na+ ATP P ADP Na+ binding stimulates phosphorylation by ATP. 2

  23. Na+ Na+ Na+ P Phosphorylation causes the protein to change its shape. Na+ is expelled to the outside. 3

  24. K+ K+ P P K+ binds on the extracellular side and triggers release of the phosphate group. 4

  25. K+ K+ Loss of the phosphate restores the protein’s original shape. 5

  26. K+ K+ K+ is released, and the cycle repeats. 6

  27. K+ constantly leaks out of the neuron • The flow of K+ ions out of the cell helps to maintain the resting potential • A neuron at rest has a potential about -70 mV

  28. Big idea: Action potentials are the signals conducted by axons • Signals are passed down an axon as spikes in membrane potential • These spikes, that briefly reverse membrane polarity, are called action potentials • These action potentials are the basic form of communication for neurons

  29. (a) Gentle touch 1 fires slowly 2 silent 1 2

  30. Changing membrane potential • Neurons contain gated ion channels that open or close in response to stimuli • Membrane potential changes in response to opening or closing of these channels • What would happen if K+ permeability increased?

  31. 3 Conditions of Gated Channels • Closed, but capable of opening • Open (activated) • Closed, not capable of opening (inactivated)

  32. Stimuli +50 • When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative • This is called hyperpolarization • What if Na+ gates open? 0 Membrane potential (mV) –50 Threshold Resting potential Hyperpolarizations –100 1 5 2 3 4 0 Time (msec) (a) hyperpolarizations

  33. Stimuli +50 • If gated Na+ channels open and Na+ diffuses into the cell • This causes a depolarization, a reduction in the membrane potential 0 Membrane potential (mV) Threshold –50 Resting potential Depolarizations –100 0 1 5 2 3 4 Time (msec) (b) depolarizations

  34. Stimuli +50 • If enough open the membrane in this region reaches threshold • At this point a large number of Na+ channels open and sodium pours in • What would this do to membrane potential? 0 Membrane potential (mV) Threshold –50 Resting potential Depolarizations –100 0 1 5 2 3 4 Time (msec) (b) depolarizations

  35. Strong depolarizing stimulus +50 Action potential • Membrane polarity flips! • Then these channels shut & K+ open • Potential drops back as K+ ions flow out • This spike in charge is an action potential! 0 Membrane potential (mV) –50 Threshold Resting potential –100 0 2 4 5 6 1 3 Time (msec) (c) Action potential

  36. This flipping and returning of the membrane potential is passed along a neuron down it’s axon • The action potential flows down the axon as depolarization is pushed ahead of the action potential (propagation)

  37. Big idea: Action potentials depolarization • starts with a slight of membrane (closer to 0mv) • often no action potential is fired if • at ~ -50mv channels open – allowing to pour (in/out) Threshold isn’t hit gated Na+

  38. Big idea: Action potentials • at ~ -50mv gated channels open – allowing Na+ to pour (out!) • this causes • They after a very short time (~1msec.) • channels also respond to voltage – but they are • pours (in/out) – reversing the charge again • They shut after driving charge membrane potential to flip slam shut K+ much slower K+ below resting

  39. Axon Plasma membrane Action potential Cytosol Na+

  40. Axon Plasma membrane Action potential Cytosol Na+ Action potential K+ Na+ K+

  41. Axon Plasma membrane Action potential Cytosol Na+ Action potential K+ Na+ K+ Action potential K+ Na+ K+

  42. Because the sodium gates lock shut an action potential cannot move “backwards” • During the refractory period after an action potential, a second action potential cannot be initiated • The refractory period is a result of a temporary inactivation of the Na+ channels

  43. Figure 34.5 How an action potential is generated

  44. Generation of Action Potentials

  45. What happens at the end of the axon? • Axons end at a synapse • This is a small gap between one neuron and another (or sometimes another cell) • Chemicals called neurotransmitters carry information across the gap

  46. Dendrites Stimulus Presynaptic cell Nucleus Axon hillock Cell body Axon Synapse Synaptic terminals Postsynaptic cell Neurotransmitter

  47. A synapse between two neurons

  48. 5 Na+ K+ Synaptic vesicles containing neurotransmitter Presynaptic membrane Voltage-gated Ca2+ channel Postsynaptic membrane Ca2+ 1 4 6 2 3 Synaptic cleft Ligand-gated ion channels

  49. The presynaptic neuron synthesizes and packages neurotransmitter in synaptic vesicles located in the synaptic terminal • The action potential causes the release of the neurotransmitter • The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell

  50. Postsynaptic neuron Synaptic terminals of pre- synaptic neurons 5 µm

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