Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 2Overview of Cellular Physiology in Medical Physiology 39

intracellularly to intermediate filaments. However, they con-
tain integrins rather than cadherins. Focal adhesions also
attach cells to their basal laminas. As noted previously, they
are labile structures associated with actin filaments inside the
cell, and they play an important role in cell movement.

GAP JUNCTIONS

At gap junctions, the intercellular space narrows from 25 nm to
3 nm, and units called connexons in the membrane of each cell

are lined up with one another (Figure 2–9). Each connexon is

made up of six protein subunits called connexins. They sur-

round a channel that, when lined up with the channel in the cor-

responding connexon in the adjacent cell, permits substances to

pass between the cells without entering the ECF. The diameter

of the channel is normally about 2 nm, which permits the pas-

sage of ions, sugars, amino acids, and other solutes with molec-

ular weights up to about 1000. Gap junctions thus permit the

rapid propagation of electrical activity from cell to cell, as well

as the exchange of various chemical messengers. However, the

FIGURE 2–9 Gap junction connecting the cytoplasm of two cells. A) A gap junction plaque, or collection of individual gap junctions, is
shown to form multiple pores between cells that allow for the transfer of small molecules. Inset is electron micrograph from rat liver (N. Gilula).
B) Topographical depiction of individual connexon and corresponding 6 connexin proteins that traverse the membrane. Note that each connexin
traverses the membrane four times. (Reproduced with permission from Kandel ER, Schwartz JH, Jessell TM [editors]: Principles of Neural Science, 4th ed. McGraw-Hill, 2000.)

6 connexin subunits =

1 connexon (hemichannel)

Presynaptic

cytoplasm

Extracellular loops for

homophilic interactions

Each of the 6 connexins

has 4 membrane-spanning

regions

Cytoplasmic loops

for regulation

Extracellular

space

Channel formed

by pores in

each membrane

B

Postsynaptic

cytoplasm

Presynaptic

cytoplasm

A

Normal

extracellular

space

3.5 nm 20 nm