may be differentially affected during the development of hypertrophy, thereby
increasing the risk of reentrant or nonsustained ventricular tachycardia seen
in African-American males.
In the freeze-fracture micrograph, the connexons are seen in circular
arrangements on the P face of the membrane. When the connexons of adja-
cent cells are in alignment, a pore of about 1.5 nm is open, and there is
continuity between the interior of the two cells. Gap junctions maintain
electrical or chemical coupling, between cells. Rapid nerve conduction in
some systems uses gap junctions to avoid the chemical synapse, which
requires the release of neurotransmitter. Mutations in connexins would
slow down normal nerve impulse conduction (answer a).Normal peri-
stalsis(answer b)requires normal gap junctions between smooth muscle
in the small intestine and would be slowed down in the absence of normal
gap junctions. Not all hepatocytes are innervated. Innervated and nonin-
nervated hepatocytes are connected by gap junctions, which allows for a
more coordinated effect of norepinephrine on hepatocytes to facilitate
release of blood glucose from stored glycogen in hepatocytes (answer d).
Adherence of epithelial cells to the basement membrane (answer e)is
dependent on integrins and hemidesmosomes, not gap junctions.
76.The answer is a.(Alberts, pp 1106–1108. Junqueira, pp 67–68.)Epithe-
lial cells require a basement membrane as a structural support. In most
epithelia, the basement membrane prevents penetration from the underly-
ing lamina propria into the epithelium. Basement membranes are a path-
way for migrating cells during development and repair processes (e.g.,
healing of skin wounds). In the kidney, the basement membrane of the
renal glomerulus forms a selective barrier for the filtration of the plasma.
Contractility and excitability (answers b and c)are characteristics that are
associated with muscle and nerve, respectively, not with the basement
membrane. Active ion transport and modification of secretory proteins
(answers d and e)are characteristics of the epithelia that are positioned on
the basement membrane, not of the basement membrane itself.
77.The answer is a.(Alberts, pp 1106–1108. Junqueira, pp 67–68.)The
regions labeled in the electron micrograph are a(lamina rara, also known as the
lamina lucida), b(lamina densa), c(reticular lamina), d(basal lamina), and e
(basal cell membrane). Junctional epidermolysis bullosa (JEB) represents a
disruption between laminin, specifically the beta and gamma chains, with
164 Anatomy, Histology, and Cell Biology