Physics 414: Introduction to Biophysics. Professor Henry Greenside August 30, 2018

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1 Physics 414: Introduction to Biophysics Professor Henry Greenside August 30,

2 Start first homework before next Monday 2

3 Divide up into discussion groups 3

4 Continue and complete discussion about why do animals have heads 4

5 Some insights from the last class 1) Moving neurons closer together to form a brain can make the brain faster by reducing the transmission time for information to be sent between neurons. But: How fast does a brain need to be?! Moving some neurons closer means moving other neurons further away, how to optimize? Is signal delay dominant? 2) Moving neurons closer together to form a brain might reduce the power consumed by a brain, if the power needed to run a brain increases with the length of the axons (wire-like connections between neurons), which is actually the case But how expensive are brains to maintain metabolically? 3) Reason suggested in end-of-class question: moving neurons closer together may allow an increased number of connections between neurons so greater computational ability. (Calculation at board: cannot have all-to-all connections) 4) Any other reasons for animals to have brains? 5

6 Our arguments might also explain why larger brains are highly modular Felleman and Van Essen (1991) Macaque monkey visual modules 6

7 Minimizing delay time puzzle: why do some animals have eyes on stalks? Hammerhead shark 7

8 Movie Pan s Labyrinth (2006) 8

Minimum energy or minimum power arguments

9 Minimum energy or minimum power arguments tricky with evolution because of sexual selection Indian peacock Pavo cristatus Irish Elk or Giant Deer Megaloceros giganteus Male zebra finch singing to impress female: loudness, precision, complexity selected. Large human brains result of sexual selection that favors creativity, music, art, humor? 9

10 What is the response time of a mammalian brain? Volunteer from audience to estimate visual reflex time 10

11 Simple way to measure visual reflex time 11

12 Class discussion: Is a reflex time of 300 ms for mammals from evolutionary viewpoint? What does animal have to react to quickly in order to survive? 12

13 Can we understand reflex time of ~300 ms based on signal speeds of m/s? 13

14 Can we understand the speed range of m/s? What are possible mechanisms and speeds that organisms could use to transmit information between internal components like neurons? 14

15 Class discussion: why do multicellular creatures not use metallic wires to transmit signals? 15

$A few small creatures do use metal or conducting wires Coherent X-ray diffraction microscopy image of unstained magnetotactic prokaryote Magnetospirillum magnetotacticum.$

16 A few small creatures do use metal or conducting wires Coherent X-ray diffraction microscopy image of unstained magnetotactic prokaryote Magnetospirillum magnetotacticum. The reddish dots are ferromagnetic nanocrystals. Geobacter metallireducens a bacterium that creates tiny nanowires (although not metallic) to feed of electricity in its environment

17 Signal propagation not dominant delay in most brains: chemical synapses take 5-15 ms to respond Can we estimate how many synapses involved when someone grabs the ruler? 17

18 Biophysics: speed of pulses along axon increases with axon radius while speed of signals in wire independent of radius Speed of pulses (action potentials) for unmyelinated or myelinated axons: Radial dependence has profound impact on brain architecture: to be faster, need thicker wires which take up more volume and consume more energy. :( So why not shrink everything to bring neurons closer together, then don t need to increase v? 18

19 What determines size of a cell, of a neuron? Determines how closely one can shrink a brain to make it fast You will do a calculation related to this later in the course. If a cell needs oxygen O2 for its metabolism (true for all eukaryotes, many bacteria), then rate at which a cell can absorb oxygen by diffusion (molecular collisions) is proportional to its surface area, goes as R2. But metabolic power need of cell is proportional to its volume, R3, so there must be a maximum cell size, unless evolution develops an active (energy-consuming) transport mechanism. Many mammalian cells have diameters of order microns, many bacteria have diameters of order microns, with E. coli being about 2 microns long, 1 micron wide. 19

20 One-minute End-of-class Question 20

Primitively there is a pair of ganglia per body segment but there has been progressive fusion of ganglia both within and between segments.

Primitively there is a pair of ganglia per body segment but there has been progressive fusion of ganglia both within and between segments. Multicellular organisms contain systems of organs that carry out specialised functions that enable them to survive and reproduce examining the specialised cells and tissues involved in structure and function