288
Chapter 9
cells, it divides into many branches, each branch containing a
series of swollen regions known as
varicosities
(
Figure 9–37
).
Each varicosity contains many vesicles fi
lled with neurotrans-
mitter, some of which are released when an action potential
passes the varicosity. Varicosities from a single axon may be
located along several muscle cells, and a single muscle cell may
be located near varicosities belonging to postganglionic fi bers
of both sympathetic and parasympathetic neurons. Therefore,
a number of smooth muscle cells are infl uenced by the neu-
rotransmitters released by a single nerve fi ber, and a single
smooth muscle cell may be infl uenced by neurotransmitters
from more than one neuron.
Whereas some neurotransmitters enhance contractile
activity, others decrease contractile activity. Thus, in contrast
to skeletal muscle, which receives only excitatory input from its
motor neurons, smooth muscle tension can be either increased
or decreased by neural activity.
Moreover, a given neurotransmitter may produce oppo-
site effects in different smooth muscle tissues. For example,
norepinephrine, the neurotransmitter released from most
postganglionic sympathetic neurons, enhances contraction of
most vascular smooth muscle by acting on alpha-adrenergic
receptors. By contrast, the same neurotransmitter produces
relaxation of airway (bronchiolar) smooth muscle by acting on
beta-2 adrenergic receptors. Thus, the type of response (excit-
atory or inhibitory) depends not on the chemical messenger
per se,
but on the receptors the chemical messenger binds to in
the membrane and on the intracellular signaling mechanisms
those receptors activate.
In addition to receptors for neurotransmitters, smooth
muscle plasma membranes contain receptors for a variety of
hormones. Binding of a hormone to its receptor may lead to
either increased or decreased contractile activity.
Varicosity
Synaptic
vesicles
Mitochondrion
Autonomic
nerve fiber
Varicosities
Sheet of
cells
Figure 9–37
Innervation of smooth muscle by a postganglionic autonomic neuron. Neurotransmitter, released from varicosities along the branched axon,
diffuses to receptors on muscle cell plasma membranes. Both sympathetic and parasympathetic neurons follow this pattern, often overlapping in
their distribution.
Figure 9–36
Generation of action potentials in smooth muscle fi bers.
(a) Some smooth muscle cells have pacemaker potentials that drift to
threshold at regular intervals. (b) Pacemaker cells with a slow wave
pattern drift periodically toward threshold; excitatory stimuli can
depolarize the cell to reach threshold and fi re action potentials.
Action
potential
Pacemaker
potential
Threshold
potential
Time (min)
0
+30
–60
Membrane potential (mV)
Action
potentials
Slow waves
Time (min)
0
+30
–60
Membrane potential (mV)
(b)
(a)
Threshold
potential
Excitatory stimulus applied
previous page 316 Vander's Human Physiology The Mechanisms of Body Function read online next page 318 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off