cells, it divides into many branches, each branch containing a
series of swollen regions known as
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
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.
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
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.
Membrane potential (mV)
Membrane potential (mV)
Excitatory stimulus applied