smooth muscle quiz Flashcards
describe the shape of smooth muscle cells
short and fusiform in shape, with centrally located nucleus
how are smooth muscle cells different from skeletal and cardiac muscle cells?
- cells are not striated
- actin and myosin are arranged differently from the sarcomeres
where are dense bodies of smooth muscle found?
some scattered within the cell and some attached to the membrane
where are membrane dense bodies bonded?
some are bonded to membrane dense bodies of adjacent cells
what are attached to the dense bodies?
actin filaments
what does actin lack?
troponin
what does calcium bind to in smooth muscle?
binds to regulatory protein calmodulin which is associated with myosin filaments
where are myosin filaments located in smooth muscle?
they are interspersed in between actin filaments, halfway between dense bodies
when myosin heads are together and contract what happens to the actin?
the 2 actin filaments collapse together in the contracted state
what are the intermediate filaments responsible for?
structural proteins forming a framework for the actin and myosin filaments
side polar cross-bridges between actin and myosin do what?
myosin heads on one side of the myosin molecule hinge (bend) in one direction while heads on the opposite side hinge in the opposite direction
describe the sarcoplasmic reticulum in smooth muscle
sarcoplasmic reiticulum is sparse and there are no t-tubules
where does the calcium come from in smooth muscle?
little is stored in sarcoplasmic reticulum, most comes from extracellular fluid and enters through calcium channels in sarcolemma
what are other names for visceral cells?
unitary, synctial, single-unit
describe a visceral smooth muscle
numerous cells arranged into a single sheet of interconnected cells which all contract simultaneously
what connect visceral smooth muscle cells?
they are connected by numerous gap junctions which transmit action potentials (as well as ions) between cells
where is visceral smooth muscle usually found?
typically found in hollow organs i.e. stomach, intestines, uterus, bile duct, bladder and most blood vessels
how many layers of smooth muscle are there typically? how are they positioned? What is the exception
there are usually 2 layers, one longitudinally and one transversely; the stomach has a third that runs obliquely
what is a multi-unit smooth muscle composed of?
individual smooth muscle cells, unconnected to nearby cells
what is each multi-unit cell covered by?
a thin membrane composed of fine collagen fibers and glycoproteins which insulates it from nearby cells
each multi-unit smooth muscle cell receives its own ______?
innervation
where are multi-unit smooth muscle cells found?
in the constrictor and dilator muscles of the iris, arrector pili, pulmonary air passages and walls of the largest arteries
describe the 4 steps of smooth muscle contraction
1 calcium enters the cell and binds to calmodulin
2 calcium-calmodulin complex binds to and activates myosin light chain kinase (MLCK)
3 MLCK phosphorylates one of the light chains within a myosin head which allows the head to bind to actin
4 myosin-actin cross bridge repeats until available calcium is depleted
how is the cross bridge cycle different in smooth muscle from skeletal muscle?
it is slower than in skeletal muscle- the cross bridge is intact for longer period of time
how is the myosin light chain in the contraction cycle de-phosphorylated?
by the action of myosin phosphatase
how is the relaxation of contraction in smooth muscle different than in skeletal and cardiac muscle?
it is much slower
what are the steps in the relaxation of contraction in smooth muscle?
1 calcium channels open up much more slowly and slow the entry of calcium into the cell, the channels stay open longer as well
2 there is slow removal of calcium by calcium pumps
what does the lengthening of the cross bridge cycle and slowness of contraction and relaxation of smooth muscle produce?
prolonged muscle tone without excessive energy expenditure
what is the nervous input for smooth muscle contraction?
the predominant stimulus for multi-unit smooth muscle, less common for single-unit
what are the 4 regulations of smooth muscle contraction?
1 nervous input
2 hormonal input
3 muscle stretch
4 nearby chemical environment of the muscle cell (lack of oxygen, excessive CO2, increased H+ concentrations)
