Ganong's Review of Medical Physiology, 23rd Edition

84
SECTION II
Physiology of Nerve & Muscle Cells

observed, with the inside negative relative to the outside of the
cell at rest. A
membrane potential results from separation of
positive and negative charges across the cell membrane (Fig-
ure 4–5). In neurons, the
resting membrane potential
is usu-
ally about –70 mV, which is close to the equilibrium potential
for K

• (Figure 4–6).
  In order for a potential difference to be present across a
  membrane lipid bilayer, two conditions must be met. First,
  there must be an unequal distribution of ions of one or more
  species across the membrane (ie, a concentration gradient).
  Two, the membrane must be permeable to one or more of
  these ion species. The permeability is provided by the exis-
  tence of channels or pores in the bilayer; these channels are
  usually permeable to a single species of ions. The resting
  membrane potential represents an equilibrium situation at
  which the driving force for the membrane-permeant ions
  down their concentration gradients across the membrane is
  equal and opposite to the driving force for these ions down
  their electrical gradients.

FIGURE 4–5

This membrane potential results from

separation of positive and negative charges across the cell

membrane.

The excess of positive charges (red

circles) outside the cell

and negative charges (blue circles) inside the cell at rest represents a

small fraction of the total number of ions present.

(From Kandel ER,

Schwartz JH, Jessell TM [editors]:

Principles of Neural Science,

4th ed. McGraw-Hill, 2000.).

• 
  

  –

  
  

   -
  
   -

  
  


• 
  


• 
  


• –


• 
  


• – –

+

+

+

+

+ +

+ + +

+

+ + +

+ + +

+

+

+

+

+

+

+ +

+

+

+

+

+

+ + + + + +

• 
  

  – – – – – – – –

  
  
  
  
  • 
    
  
  
  
  


• 
  


• 
  


• 
  


• 
  


• – –


• 
  


• 
  


• 
  


• 
  

Equal

+,–

Equal

+,–

Extracellular

side

Cytoplasmic

side

FIGURE 4–6
The changes in
(a)
membrane potential (mV) and
(b)
relative membrane permeability (P) to Na+ and K+ during an action po-
tential.
(From Widmaier EP, Raff H, Strang KT:
Vander’s Human Physiology.
McGraw-Hill, 2008.)

PK

PNa

Na+

K+

Membrane potential (mV)

Time (ms)

Relative membrane permeability

(a)

(b)

+30

0

–70

600

300

50

1

0 1 234

3

2

4

5

6

7

1

K+

Gated Na+

channel

Gated K+

channel