Lecture 3 - Cellular Neurophysiology

Membrane voltage

Ionic concentration gradients and membrane permeability determine

GHK Equation

Used to calculate the resting membrane potential

Generates signals

Voltage across the neuronal membranes

Passive voltage

• Changes the membrane time constant and length constant.

• The magnitude of change is proportional to current injected

• Injected current changes (changes Q —> changes V) charge density on membrane by adding/taing change —> changes difference of + and -

Types of neuronal electrical signals

Passive and active

Membrane time constant

Passive voltage changes

Depolarization

Changes in membrane voltage towards a positive value

Hyperpolarization

Membrane voltage change that moves in the negative direction

Membrane capacitance causes…

membrane voltage to take time to change

• W/o conductance & only capacitance, Vm would keep going up in response to current injection as more and more change gets deposited on membrane

• W/o capactitance and only conductance Vm would change instantly in response to current injection (no caapcitor to charge Vm is across conductor only)

Membrane time constant

Defines the rate of voltage change

• Time it takes for Vm to change to 63% of max voltage change (Vmax)

• Determined by membrane capacitance and resistance

• τ = RC

Ic

Current that charges the membrane voltage (once charged, Ic stops flowing)

Ii

current through the membrane’s ion channels (is at max once the membrane is maximally charged)

Larger Vmax tells us about … and means…

• resistance (conductance) of the membrane

• larger membrane resistance

Passive voltage changes

Membrane length constant

Membrane ptl (Vm)

decays w/distance from site of current injection

Membrane length constant

Dependent on membrane resistance and axial resistance

Membbrane resistance decreases by (membrane condtance increased)—>

Having more ion channels, current will flow easier out thru membrane more easily —> decreasing λ (distance from site of current injection where Vm is 37% of Vmax)

Action potentials

• active voltage signals that are caused by passive voltage depolarization that reaches the AP threshold

• Caused by a positive feedback loop

• Regenerative signals —> don’t decay w/distance like passive voltage signals

AP propoagation

Each adjacent segment of an axon regenerates AP in the same way

Axon insulation

• Speeds up passive voltage change and makes AP conduction velocity along the axon more reliable

• From oligodendrocytes

• Made of myelin

Nodes of ranvier

Exposed axon segments in between segments of myelin seath

Lowering capacitance speeds up

Changing voltage across a capacitor

Insulating axon does …

• Decreases membrane capacitance

• Increases membrane resistance by increasing membrane thickness

Increased Rm (define and effect)

• Increased length constant in myelinated segments

• Less charge lost across the membrane due to increased Rm—> allows voltage to remain above AP threshold for greater distances —> increases reliability

λ (length constant) equation

sqrt(rm/ra)

• Rm = Membrane Resistance

• Ra = Internal Resistance

Hodgkin and Huxley

• discovered ionic currents and ion channels involved in generating AP

• recorded action potentials and currents w/2-electrode voltage clamp method

• found channel voltage dependence arose from charges in membrane that move under influence of electric field across membrane (gating currents)

Ion channels

• Conductance of an ionic species across the membrane is same as membrane permeability

• Ions pass through membranes via channels

• Channels closed —> ions can’t pass through them —> conductance = 0

• Some ion channels are gated by voltage

Voltage clamping squid giant axon used to…

• Measure ionic currents

• Current injected to maintain voltage at command voltage = & opp. to current flowing across membrane > can be used to measure ionic currents at diff. voltages

• Allows you to set membrane voltage at whatever value>measure resulting flowing current

Drugs help block…

• Specific ion channels to identify ionic currents responsible for action potential

• Leaves just K+ current

Na+ and K+ currents determined by…

1. Magnitude of Na+ and K+ conductance gna and gx (measure of # open Na+) and K+ channels in membrane

2. Electrochemical driving force on Na+ ions and K+ ions

• Difference between membrane voltage & equilibrium ptl for that ion (Vm - Ena and Vm - Ek)

Ionic currents for each ion (at every membrane voltage) formula

I_k = gk(Vm - Ek)

• gk = conductance

• (Vm - Ek) = driving force