Monday April 7, 2014. Introduction to the nervous system and biological electricity

1. Monday April 7, 2014. Introduction to the nervous system and biological electricity 1. Pre-lecture quiz 2. A word about prelecture readings 3. Introduction to the nervous system 4. Neurons and nerves 5. Resting membrane potential

2. A word about the readings • Today’s readings were fine. (section 45.1 – Principles of electrical signaling is good background) • Wed: pp. 891-898 (section 45.2 – dissecting action potentials & 45.3 The synapse). • Fri: pp. 899-904 (section 45.4 The vertebrate nervous system)

3. The ability of animals to respond RAPIDLY to the environment and to move is due to the electrical properties of neurons and muscles.

4. Venus flytrap can send a signal to close that travels 1 to 3 cm/s. Action potentials along neurons travel up to 100 meters per second (or 10,000 cm/s).

5. Mary Shelley wrote Frankenstein in 1818 long before we knew about neurons. Why did she choose to use electricity to bring Frankenstein to life? She knew about the work by Galvani on frog legs.

6. Galvani & frogs legs Galvani showed the applying a current to a frog nerve could make the muscles twitch. Previously, folks thought that nerves were pipes or tubes. Galvani introduced the idea “biological electricity”. Many of his speculations were incorrect but he is credited with the important insight that animals use electricity in nerve and muscle cells.

7. Central vs. peripheral nervous system

8. The brain integrates sensory information and sends signals to effector cells. Sensory neuron CNS (brain  spinal cord) Sensory receptor Interneuron Motor neuron (part of PNS) Effector cells

9. Examplesof sensory receptors in vertebrates • Nocirecptors = pain stimuli • Thermorecptors = changes in temperature • Mechanoreceptors = changes in pressure • Chemoreceptors = detection of specific molecules • Photoreceptors = detection of light • Electroreceptors = detection of electric fields • Magnetoreceptors = detection of magnetic fields

10. Information flow through neurons Nucleus Dendrites Collect electrical signals Cell body Integrates incoming signals and generates outgoing signal to axon Axon Passes electrical signals to dendrites of another cell or to an effector cell

11. Nerves are bundles of neurons surrounded by connective tissue

12. Neurons form networks for information flow

14. An introduction to membrane potentials • A difference of electrical charge between any two points creates a difference in electrical potential, or a voltage. • Ions carry a charge, and in virtually all cells, the cytoplasm and extracellular fluid contain unequal distributions of ions. Therefore, there is a separation of charge across the membrane called a membrane potential. • Membrane potentials are a form of electrical potential and are measured in millivolts (mV). In neurons, membrane potentials are typically about 70–80 mV. • A flow of charged ions is an electric current.

15. Electrical Properties of Cells • All cells maintain a voltage difference across their membranes (Emembrane): • Two factors a required to establish a membrane potential • There must be a concentration gradient for an ion • The membrane must be somewhat permeable to that ion Outside of cell Microelectrode 0 mV K channel –65 mV Inside of cell

16. A quick lesson from physics . . . freely permeable membrane With a permeable membrane, it takes force to keep the distribution of ions. [Na+] high [Na+] low How much force (voltage) is required to maintain the imbalance? Answer: Nernst Equation

17. A quick lesson from physics . . . (see Box 45.1) freely permeable membrane Nernst Equation [Na+] high [Na+] low E=voltage R=gas constant T=temperature in Kelvin F=faraday’s constant (charge carried by mole of an ion) Z = valance (1 for Na+, -1 for Cl-) X1 and X2 are concentrations in the two sides.

18. A quick lesson from physics . . . (see Box 45.1) freely permeable membrane Nernst Equation [Na+] high [Na+] low E=voltage R=gas constant T=temperature in Kelvin F=faraday’s constant (charge carried by mole of an ion) Z = valance (1 for Na+, -1 for Cl-) X1 and X2 are concentrations in the two sides. Altered under physiological conditions Unaltered under physiological conditions

19. freely permeable membrane compartment 1 [Na+] high compartment 2 [Na+] low Assume the only thing changing are the concentrations of Na+ in the two compartments and consider the following scenarios. Scenario 1: [Na+] in compartment 1 = 500mM, [Na+] in compartment 2 = 50mM Scenario 2: [Na+] in compartment 1 = 700mM, [Na+] in compartment 2 = 50mM Which one of the two scenarios results in a larger value for E?

20. freely permeable membrane compartment 1 [Na+] high compartment 2 [Na+] low Scenario 1: [Na+] in compartment 1 = 500mM, [Na+] in compartment 2 = 50mM log (500 / 50) = 1 Scenario 2: [Na+] in compartment 1 = 700mM, [Na+] in compartment 2 = 1.46 log (700 / 50) = 1.146

21. Outside of cell Microelectrode 0 mV K channel –65 mV Inside of cell

23. Outside of cell Increasing [K+] outside the neuron Microelectrode 0 mV Equilibrium! K channel Increasingly negative charge inside the neuron –65 mV Inside of cell

24. Animation of resting potential • https://www.youtube.com/watch?v=YP_P6bYvEjE

25. Homework: Calculate the membrane potential for each of the three ions below. at 20° C

26. Calculating the total resting potential – the Goldman Equation The Goldman Equation extends the Nernst Equation to consider the relative permeabilities of the ions (P): Ions with higher P have a larger effect on Emembrane at 20° C Permeabilitieschange during an action potential and how this allows neurons to “fire”.