Human Physiology, 14th edition (2016)

Interactions Between Cells and the Extracellular Environment 153

6.5 Cell Signaling

Cells communicate by signaling each other chemically.

These chemical signals are regulatory molecules released

by neurons and endocrine glands, and by different cells

within an organ.

will be described in chapter 7. An increased membrane permeabil-

ity to Na^1 drives the membrane potential toward E (^) Na  ( 1 66  mV)
for a short time. This is the reason that the term resting is used
to describe the membrane potential when it is not producing
impulses.
Role of the Na^1 /K^1 Pumps
Since the resting membrane potential is less negative than
E (^) K , some K^1 leaks out of the cell ( fig. 6.27 ). The cell is not
at equilibrium with respect to K^1 and Na^1 concentrations.
Nonetheless, the concentrations of K^1 and Na^1 are main-
tained constant because of the constant expenditure of energy
in active transport by the Na^1 /K^1 pumps. The Na^1 /K^1 pumps
act to counter the leaks and thus maintain the membrane
potential.
Actually, the Na^1 /K^1 pump does more than simply work
against the ion leaks; because it transports 3 Na^1 out of the cell
for every 2 K^1 that it moves in, it has the net effect of contrib-
uting to the negative intracellular charge (see fig.  6.19 ). This
electrogenic effect of the pumps adds approximately 3 mV to
the membrane potential. As a result of all of these activities, a
real cell has (1) a relatively constant intracellular concentra-
tion of Na^1 and K^1 and (2) a constant membrane potential (in
the absence of stimulation) in nerves and muscles of 2 65 mV
to 2 85 mV. The processes influencing the resting membrane
potential are summarized in figure 6.28.

Clinical Investigation CLUES

Jessica had an abnormal electrocardiogram (ECG) and

mild hypokalemia.

• What is hypokalemia, and how might it have been
  produced?


• How would hypokalemia affect the resting
  membrane potential of Jessica’s heart and her
  ECG?

| CHECKPOINTS

9a. Describe the potassium and sodium equilibrium

potentials.

9b. Define membrane potential and explain how it is

measured.

10a. Explain the relationship of the resting membrane

potential to the two equilibrium potentials.

10b. What role do the Na^1 /K^1 pumps play in establishing

the resting membrane potential?

LEARNING OUTCOMES

After studying this section, you should be able to:

11. Distinguish between synaptic, endocrine, and
    paracrine regulation.


12. Identify where receptor proteins are located within
    target cells.

Figure 6.28 The processes that influence the resting

membrane potential. As shown in this figure, the Na^1 /K^1 pumps

produce concentration gradients for Na^1 and K^1 , and the presence

of fixed anions and the different permeabilities of the plasma

membrane to diffusible ions results in their unequal distribution

across the plasma membrane. The greater permeability of the

membrane to K^1 causes the membrane potential to be closer to

the equilibrium potential for K^1  ( E (^) K ) than to Na^1  ( E (^) Na ). The resting
membrane potential is different for different cells; a value of 2 70 mV
is typical for mammalian neurons.
Fixed Anions

• Unequal permeabilities
  of plasma membrane
  to diffusible ions
  Uneven distribution of ions
  across the plasma membrane
  More permeable
  to K+
  K+ higher on inside
  Na+/K+ pump Na+/K+ pump
  Na+ higher on outside
  EK = –90 mV ENa = +66 mV
  Less permeable
  to Na+
  Resting membrane potential
  (RMP) = –70 mV
  The membrane potential and the permeability of the plasma
  membrane to ions discussed in the previous section set the
  stage for the discussion of nerve impulses in chapter 7. Nerve
  impulses are a type of signal that is conducted along the axon
  of a neuron. When the impulses reach the end of the axon, how-
  ever, the signal must somehow be transmitted to the next cell.
  Cell signaling refers to how cells communicate with each
  other. In certain specialized cases, the signal can travel directly