Exam 2 Content

Measuring voltage

Across two points. Red lead is the positive lead and black is for negative. So if red is on a more negative side than black, will read out a negative voltage.

Voltage

Potential energy or potential difference. Amount of work required to move a charged particle from one place to another.

DC

Direct current. Reasonably constant voltage instead of sinusoidal. What comes out of a battery. The kinds of voltage produced by the body.

AC

Alternating current. Current that comes out of wall outlet. Sinusoidal voltage/current source. Much more efficient for distributing power over a long distance. Can be scaled up or down using a transformer.

Passive sign convention

Place positive sign on side of resistor closest to positive terminal of power source or designate direction of current and from pos o neg.

Ohmic

A circuit element that obeys Ohm’s law, so a linear relationship between current and voltage, related by the slope R.

Diode

Useful non-ohmic element. Current can only flow through the component in one direction. Fluid equivalent is a valve.

KVL

Sum of voltages around any closed loop is zero

KCL

Sum of currents at any node is zero. Based on conservation of mass and volume.

Essential node

A circuit location where 2 or more branches meet.

Branch current method

1) assign a current to each branch without a current source

2) assign a voltage to each element that doesn’t have a specified voltage. Place ± according to passive sign convention

3) Apply KVL to each independent loop, and KCL to each essential node

4) solve the system of equations

Capacitor

Storage of charge

Capacitor capacitance

Defined by surface area of plates, permittivity constant, and distance between plates.

Windkessel Model

Blood vessel model. Vales as diodes, resistance of vessels as resistors, compliance of vessels represented by capacitor (storage of blood volume and potential energy to drive blood flow between heart beats)

Effect of valves

Prevent backflow of blood through body. Close when the pressure on the inside side is lower than on the output side.

Effect of resistive elements

Slow down current/flow rate, allowing for a slow enough flow at capillaries to have exchange of gas and nutrients.

Effect of compliance vessels

Smooth out flow (less pulsatile) and store potential energy to continue moving blood through body between heart beats. Ensures a smoother and more laminal flow is attained in the capillaries to allow for proper gas and nutrient exchange.

Biological membranes

Act as electrical capacitors. Based on selective permeability and charge gradients established by ion pumps, maintain charge difference across membrane.

Membrane resistance and capacitance form the time constant telling you how fast the membrane potential responds to ion channel currents.

Ion flow through membrane

1) passive channels

2) active channels

3) ion pumps

Nernst potential

For each ion in the cellular environment, there is an equilibrium potential where the net flux of the ion across the membrane is zero. Driven by concentration and charge gradients. Also known as equilibrium potential

Dominant ions in cells

Potassium, sodium, and chloride (K+, Na+, Cl-)

Equivalent circuit for the flow of an ion across a membrane

Assembled with a voltage source equivalent to the nernst potential and a resistor representing the resistance the ion faces to move across the cell membrane.

Squid gian axon

Can represent the movement of major ions across the cell membrane as a circuit with an equivalent capacitance. Can use this to solve for the resting membrane potential.

Solving for resting membrane potential

Orient nerns’t potential voltage sources with positive side toward inside of cells. Represent ground as being outside of the cell. Place individual ion circuits in parallel with Cm.

Changes during nerve excitation

Membrane potential increases as resistance for ions decreases. Leads to a smaller time constant and a faster ability for the cell membrane to respond to changes in current.

Action Potential

Upon stimulation, membrane potential rises above a threshold value, leading to full depolarization of the cell driven by increased conductance of Na+. Repolarization driven by increased conductance of K+. Closing of Na+ gates starts the decrease in voltage potential.

-threshold events

-all or none events

-temporarily change the membrane potential from eqm potential