Lecture 3

Neural connectome

mapping provides a framework for understanding it, reveals potential avenues for information flow

C. Elegans

nematode worm with only 302 neurons, had its connectome fully mapped over 30 years

Chemical information

neurotransmitters at synapses, neuromodulators

Electrical information

graded potentials, action potentials, electrical synapses

Luigi Galvani

credited with the discovery of bioelectricity (1700s); applied electric current to dissected frog legs which caused them to twitch

Giovanni Aldini

used body of executed prisoner; minutes after execution, Aldini performed galvanization on the body

Electrophysiology

the use of equipment to study bioelectricity, as pioneered by Galvani and others

Action potential

Hodgkin, Huxley, Cole and the voltage-clamp technique

Ion channel function

Erwin Neher, Bert Sakmann and the patch-clamp technique

Extracellular recording

record voltage/ion fluxes along the outside surface of a cell

Intracellular sharp electrodes

record voltage/ion fluxes across the cell membrane

Whole-cell patch electrodes

record voltage/ion fluxes across the cell membrane; large access into cell let's you change the intracellular saline

Protein engineers combined

1. GFP 2. The Ca2+ sensor protein calmodulin (CaM) 3. The M13 alpha helix of the muscle protein myosin light chain kinase

GCaMP

increases fluorescence when Ca2+ levels rise

Aps

cause Ca2+ influx which activates CaM which binds to M13 helix and this pulls on the GFP protein to change its structure and make it more fluorescent

Optical technologies:

• Voltage-sensitive fluorescent dyes

• Genetically encoded fluorescent voltage indicators

View many neurons without damaging

why might optical techniques be advantageous over classical electrophysiological approaches

CAT scan

changes cytoplasm of cell to determine which ion channels are active at which point in time

Electricity

movement of charged particles through a conductor/resistor; referred to as current, units are amperes

Electrons

changed particles that move through metal wires

Cations and anions

charged particles that move through aqueous solutions

Coulombs

used to count/measure charges

Current

number of coulombs moving through a conductor per second

Ohm's law

I = V * G or I = V/R or V = IR

I

current in amperes

V

electrical potential energy in volts

G

conductance of the conductor in siemens

R

resistance in ohms

Amplifier

used to record electrical events across all membranes

Excitable cells

membrane voltage originates from ion concentration gradients across the cell membrane

Applying voltage across conductor

will produce a current

Ion channels

conductors in cells across the cell membrane

Membrane voltage

will produce current through open ion channels ; some conduct cations and other anions

Cytoplasm

conductor from dendrites to soma to axons

Capacitor

cell membrane; consists of two conductive plates (ex. Cytoplasmic and extracellular salines) separated by an insulator (cell membrane)

Insulator

must be thin so that charges can sense each other across the plates

Opposite charges

accumulate along opposite plates when a voltage is applied and ions do not travel directly through the membrane; still a current results from the repulsion/attraction of ions across the membrane

Applied charges

accumulate until the energy in the capacitor matches the applied voltage; capacitor can hold applied voltage

Hydrophobic environments

not preferred by ions

Batteries

provide energy and capacitors can borrow energy from these

Ability of a capacitor to hold charges

C=q/V

C

capacitance (in coulombs/volts or farads); determined by physical properties of the insulator, the conductive plates, and the surface area of the capacitor

q

charge in coulombs

V

electrical potential energy in volts

Large capacitance

can store a lot of charge per unit volt

Bigger membrane

bigger capacitance

Ion gradients in cells

batteries (V) that move charges onto the cell membrane ©, leading to charging on the membrane voltage

Batteries conduct

charges through ion channels that provide pathways for charging Vm

Nernst equation

various applied voltages or batteries each made up of a different ion gradient are defined by this