Electrophysiology

 Electrophysiology 

channels

• ionotropic; e.g., Na, K channels
• metabotropic; e.g., beta receptors

electrochemical equilibria = membrane potential that would keep any ion at its observed concentration inside and outside

-example; if K channels open the membrane will become hyperpolarized to its equilibrium potential which is - 95 mv.  this is how most inhibitory neurotransmitters work.

Ohm's Law  I = V/R  where I = current; V=voltage; R=resistance

Rectifying currents deviate from Ohm's law; i.e., current is not linear with voltage.

• inward
• outward

Conduction velocity = f(rate of change of voltage) - substances that block Na channels will slow conduction; e.g., high potassium; cocaine

Action potentials

all of heart action potentials summated to ECG

slow cells - pacemaker cells 

T type Ca channels activation by hyperpolarization

L type by depolarization

slope of phase 4, threshold, and resting potential determine heart rate

Another good figure below and explanation (from Lilly)

The maximum negative voltage of pace-maker cells is approximately -60 mV, substantially less negative than the resting potential of ventricular muscle cells (-90 mV). The persistently less negative membrane voltage of pacemaker cells causes the fast sodium channels within these cells to remain inactivated.

fast cells (myocytes; purkinje fibres)

this is a very helpful figure to understand fast action potentials:

Here is another way to understand ion currents in the fast action potential:

From Lilly; Pathophysiology of Heart Disease. Schematic representation of a myocyte action potential (AP) and relative net ion currents for Na+, Ca++, and K+. The resting potential is represented by phase 4 of the AP. Following de-polarization, Na+ influx results in the rapid upstroke of phase 0; a transient outward potassium current is responsible for partial repolarization during phase 1; slow Ca++ influx (and relatively low K+ efflux) results in the plateau of phase 2; and final rapid repolarization largely results from K+ efflux during phase 3.

Conduction velocity (dromotropy) determined by dV/dT

myocardium = .3-1 m/sec
av node = .02 - .1 m/sec  (important to allow time for filling of ventricles)