smooth muscle Flashcards
all smooth muscles lack
the striated banding pattern found in cardiac and skeletal fibers
neurons to all smooth muscles are derived from the
autonomic nervous system (ANS)
characteristics of each smooth muscle
spindle-shaped
has a single nucleus
lacks striations
much smaller (especially shorter) than a skeletal muscle cell
often interconnected to form sheet-like layers of cells
thick (myosin) and thin (actin) filaments are not organized into
myofibrils or aligned into sarcomeres
dense bodies
anchor the thin (actin) filaments to the cell’s plasma membrane or to structures in the cytoplasm, and serve the same role as the Z discs of skeletal muscle
single-unit (unitary; visceral) smooth muscle
an entire group of muscle fibers responds to stimulation as a single unit (electrical and contractile activity is synchronous in all cells)
- occurs because each cell is connected to adjacent cells by gap junctions
- some of the cells have pacemaker activity and spontaneously generate action potentials, which spread to other cells via the gap junctions
- nerves (not primarily), hormones and local factors (primarily) can alter the contractile activity of the cells
- the activity of the entire muscle can be regulated by controlling the frequency with which the pacemaker fires
- a contraction can often be induced by stretching the muscle
examples of single-unit (unitary; visceral) smooth muscles
smooth muscles of the stomach and intestines
the bile ducts
the ureters
the uterus
muscle in many blood vessels
multi-unit smooth muscle
contain few or no gap junctions, and each cell thus responds independently
- richly innervated by the ANS
- the contractile response of the entire muscle depends on the number of fibers that are activated and the frequency of nerve stimulation
- although stimulation by axons/transmitter leads to some degree of depolarization and a contractile response, the muscle cells do not typically make action potentials
- may respond to hormones, but not usually stretch
examples of multi-unit smooth muscles
smooth muscles in the eyes
large airways
large arteries
attached to the hairs in the skin
in smooth muscle, a myosin molecule is formed from
6 polypeptide chains: 2 heavy and 4 light
a myosin molecule’s tail is formed by
most of the two heavy chains wrapped into a double helix
a myosin molecule’s 2 heads are each formed by
the folded end of one heavy chain plus 2 of the light chains
smooth muscles cells lack _____, meaning ______ is never in a position where it blocks myosin’s access to actin
troponin
tropomyosin
during contraction, inputs/signals to the cell increase Ca2+ concentration in the cell’s cytoplasm by
opening (maybe not directly) membrane Ca2+ channels that allow influx of extracellular Ca2+
OR
by stimulating the cell’s sarcoplasmic reticulum to release stored Ca2+
during contraction, Ca2+ binds reversibly
within the cell with the protein called calmodulin
during contraction, the calcium-calmodulin complex binds to and activates
myosin light chain kinase (MLCK)
during contraction, MLCK uses
ATPs to phosphorylate one of the light chains of each myosin head
(2 of 4 light chains are phosphorylated)
during contraction, the phosphorylated myosin head can now bind
repetitively with the actin (thin) filament and go through the entire cycling process of intermittent “pulls” (as in skeletal muscle), thus causing contraction
during relaxation, myosin must be dephosphorylated so that it is unable to bind to actin, this happens by:
a slowly-acting ATP-requiring pump transports calcium back into the sarcoplasmic reticulum or extracellular fluid, so that MLCK is no longer being activated
the activity level of the dephosphorylating enzyme, myosin light chain phosphatase, which is continuously active in smooth muscle cells, now outpaces that of MCLK and the amount of phosphorylated myosin declines, producing relaxation
the rates of ATP splitting and cross bridge cycling during contraction are much _____ in smooth muscle than in skeletal muscle
slower
the velocity of smooth muscle shortening is _____ in smooth muscle than in skeletal muscle
lower
smooth muscle does not undergo _____ during prolonged periods of activity
fatigue
latch state
once smooth muscle has developed full contraction, the tension it generates can often be maintained near the full level despite fairly dramatic declines in the level of energy consumption
the controlling inputs to smooth muscle cells that increase the cell’s cytosolic Ca2+ concentration include:
transmitters released by autonomic neurons
hormones
local factors such as: acidity, osmolarity, O2/CO2 concentration, and paracrines from nearby cells
stretch of the fiber
spontaneous electrical activity in the fiber’s plasma membrane
in some smooth muscles, action potentials (membrane depolarizations) are not necessary to release Ca2+ from the sarcoplasmic reticulum; instead,
internal second messenger molecules generated in response to the binding of external chemical messengers trigger the release of sarcoplasmic reticulum Ca2+, meaning the cell can contract in the absence of a membrane potential change
in smooth muscles that contract in response to the production of action potentials, what ions is responsbile for carrying positive charge into the cell during the rising phase of the action potential
Ca2+ (not Na+)
(a graded, threshold-reaching membrane depolarization in response to an input opens voltage-gated Ca2+ channels, and Ca2+ entry produces the rising phase)
some smooth muscle cells have a resting membrane potential that depolarizes slowly to threshold (that is that produces ______ ); thus these cells generate action potentials (and contract) spontaneously and rhythmically in the absence of
pacemaker potentials
neural, hormonal, or local inputs
some smooth muscle pacemaker cells have membrane potentials that drift up and down (due possibly to regular variation in ion flux across the membrane) to produce
slow waves
if any given slow wave is large enough to reach threshold, one action potential or often a
short burst of action potentials is produced
because slow waves are usually found in single-unit smooth muscle, when an action potential is produces, the action potential
spreads over the entire syncytial group of cells, and the group contracts as a unit
neuronal and hormonal regulation of contraction:
unlike in skeletal muscle, smooth muscle fibers do not have a
motor end-plate region
neuronal and hormonal regulation of contraction:
as the axon of a postganglionic autonomic neuron approaches, it divides into many branches, with each branch containing many swollen regions called
varicosities
neuronal and hormonal regulation of contraction:
transmitter-filled vesicles are within each varicosity, and some vesicles are discharged as
an action potential passes the varicosity
neuronal and hormonal regulation of contraction:
varicosities from a single axon may occur along (and thus influence)
several muscle fibers
neuronal and hormonal regulation of contraction:
a single muscle fiber may be near (and influenced by) varicosities of
postganglionic fibers of both sympathetic and parasympathetic neurons
neuronal and hormonal regulation of contraction:
some transmitters _______ contractile activity
enhance and others inhibit
neuronal and hormonal regulation of contraction:
a given transmitter can increase tension in one smooth muscle and
decrease tension in another
e.g. norepinephrine (binding to alpha-adrenergic receptors) enhances contraction of most blood vessel smooth muscle, but it relaxes bronchiole smooth muscle (by binding to beta2-adrenergic receptors)
neuronal and hormonal regulation of contraction:
smooth muscle plasma membranes contain receptors for a variety of hormones including:
epinephrine
cholecystokinin
angiotensin II
vasopressin
