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CHAPTER 1 / Cardiac Anatomy and Physiology 17

typical sodium ion concentrations as in Table 1-2, the equilibrium membrane potential, the less is the current required to initiate ex-
potential for sodium ion is approximately 60 mV. This means citation but the speed and amplitude of depolarization are re-
that if the membrane were permeable to sodium ion, then the duced. If the resting potential is substantially depolarized, the cell
membrane potential would have to be 60 mV to halt net inward can be impossible to activate.
sodium current. At typical resting potentials of 90 mV, a large The resting membrane potential is altered by changes in the
electromotive force favors inward sodium current. The sodium ionic milieu on either side of the membrane and by hormones or
concentration is markedly higher in the extracellular space than it drugs that alter the relative permeabilities of potassium or sodium
is in the intracellular space. Thus, diffusion forces also favor in- ion. Factors that alter the action of the sodium–potassium pump
ward sodium current. At rest, however, there is minimal net move- alter the resting membrane potential. These include insulin and
ment of sodium ion because the sodium channels are closed. epinephrine (hyperpolarizing influences) and digoxin-like drugs
When the channels open during activation, the diffusional and (depolarizing influence).
electrical forces combine to produce a large, but transient, inward
current carried by sodium ion. The result is rapid depolarization. Ionic Activity. Although electrochemical gradients are most
The chloride ion concentration is higher in the extracellular frequently explained in terms of chemical concentration gradi-
space than in the intracellular space. Thus, diffusional force favors ents, it is actually each ion’s chemical activity that affects most cel-
inward movement of chloride ion. However, the resting membrane lular functions. Ionic activity reflects interactions between ions as
potential is at approximately the chloride ion equilibrium poten- well as the ion concentration. An ion’s activity is equal to its con-
tial. Thus, the negative potential opposes the net inward move- centration times its activity coefficient. It is possible to make rea-
ment of chloride ion. The resting muscle membrane is permeant to sonably accurate measurements of ionic activities within cells.
chloride ion, but there is scant net chloride ion movement. However, most descriptions of ion movements are based on ion
The sarcoplasmic calcium ion concentration is extremely low. concentration.
Calcium ions are actively removed from the sarcoplasm. Calcium
ions are taken up into the SR and pumped outward to the extra- Ion Movement Across the Myocardial
cellular space. The extracellular calcium ion concentration is in Cell Membrane
the millimolar range, approximately 10,000 times higher than the Passive Ion Movement. Ions traverse the sarcolemma pas-
intracellular concentration. Thus, a powerful concentration gradi- sively through membrane-bound, water-filled pores called chan-
ent would move calcium ions inward if a path were available. A nels. When a channel is open, any ions that are able to pass
powerful electrical force also favors inward movement. The cal- through the channel move according to the concentration and
cium ion equilibrium potential calculated from the Nernst equa- electrical gradient, as constrained by the channel dimensions.
tion is more positive than 100 mV. However, the resting mem- When the channel is closed, ions do not penetrate. The opening
brane is not permeant to calcium ion. As with the sodium ion, the and closing properties of an ion channel are referred to as its gat-
opening of a calcium ion channel evokes a large inward current. ing characteristics. The signal to open may be a change in the elec-
This inward current happens during activation. An increase in in- trical field (voltage-gated channel) or a change in the chemical mi-
tracellular calcium ion signals metabolic and contractile changes. lieu (receptor-gated channel). Changes in the internal or external
milieu may modify channel gating. Also, there can be time-
Calculating Membrane Resting Potential. At high extra- dependent effects. For example, a small depolarization opens the
cellular potassium ion concentrations, the Nernst equation for sodium channel; it closes after a few milliseconds.
potassium ion predicts resting membrane potential with good ac- An important channel characteristic is its ability to allow pas-
curacy. In and below the physiological range of external potassium sage of some ions while excluding others. This is called selective
ion concentrations, the membrane potential is slightly less nega- permeability. A theoretical model of an ionic channel is given in
tive than would be predicted based on potassium ion concentra- Figure 1-16.
tions. This state occurs because at very low external potassium ion The sodium channel is common in excitable cells and has been
concentrations, the membrane is slightly permeable to sodium well characterized. In Nobel prize–winning work, Hodgkin and
ion. Because concentration and electrical gradients for sodium ion Huxley 38 described the sodium current of the squid giant axon.
both favor inward sodium ion movement, an increase in sodium According to them, at rest, the membrane potential is negative,
ion permeability allows an inward trickling of sodium ions (an in- perhaps 90 mV, extracellular sodium ion concentration is high,
ward current). The membrane depolarizes, becoming several mil- and intracellular concentration low. Electrical and diffusion gra-
livolts more positive than the potassium ion equilibrium poten- dients favor inward sodium ion movement. Because the sodium
tial. The ratio of potassium and sodium permeabilities determines channel is closed, there is no path for the ions to travel. With a
the extent to which the resting membrane potential deviates from small depolarization the sodium channel opens. This opening of
the potassium ion equilibrium potential. Equations have been de- the sodium channel in response to a small depolarizing current is
veloped to predict resting membrane potential based on the rela- sometimes described as opening the activation (or m) gate. When
tive permeabilities and concentrations of various ions. These com- the activation gate opens, the sodium channel is then open; an in-
putations assume that the membrane is in a steady state and that ward depolarizing of sodium ion flows. Because both the electri-
there are no active ion pumps producing current. cal and concentration gradients are significant and favor inward
Typically, cardiac muscle cell resting membrane potential is ap- movement, this inward current is intense. After a few millisec-
proximately 90 mV. Excitation and propagation of excitation onds, however, another gate (sometimes called the inactivation or
depend on the resting membrane potential. The more negative the h gate) closes, halting the current. The h gate remains closed until
resting membrane potential, the more current is required to initi- the membrane is restored to a sufficiently negative voltage. At that
ate excitation, but the speed and amplitude of the subsequent de- time, the inactivation gate opens but no current flows because the
polarizing excitation are greater. The less negative is the resting activation gate has closed. With the closing of either gate, current