smooth muscle physiology Flashcards
Characteristic of smooth m.
NO STRIATION
ACTIN AND MYOSIN FILAMENT S ARE NOT ARRANGED IN FROM OF MYOFIBRILLS
long spindle shape
ONE CENTRALLY PLACED NUCLEUS
SMOOTH M. locations
the wall of hollow visceral organs (stomach, intestine, etc), blood vessels and respiratory passages
t/f Smooth muscle cells have a poor developed SR , have no T-tubules.
true
smooth m. is INVOLUNTARY and can be regulated by
autonomic nervous system
hormones
local physiological conditions
smooth muscle have extensive communication among cells by
GAP JUNCTIONS
corkscrew shape OCCURS WHen smmoth m. is contracted or relaxed
contracted
3 elements are absent in smooth m.
organization of myofilaments into myofibrills
no sarcomeres
no straitions
anchoring sites for the thin actin filaments
are electron dense structures in sarcoplasm located very close to sarcolemma
resemble the Z lines
dense bodies
Functional types of smooth muscle
single-unit and multi-unit
Functional type of smooth m. are determined based on
number of nerve fibers required to activate the smooth muscle tissue.
innervated by only one (or very few) nerve fibers per bundle
contract in unison due to gap junctions
Distribute on the walls of hollow visceral organs (such as gut, bile ducts, uterus, ureter, bladder) and blood vessels
cell membranes adhere by ADHERENS
Single-unit smooth m.
contains fewer (or no) gap junctions, so each cell requires its own electrical impulse
multi-unit smooth m.
t/f Only some fibers receive innervation from autonomic nerve fiber, the rest of other fibers receive stimulation through the gap junctions
true
display “intrinsic, or myogenic” electrical activity and contraction in response to stretch. the stretch induced by an increase in the volume of a ureter or a section of the digestive tract can stimulate myogenic contraction. Such contraction DOES NOT require stimulation by autonomic nerves
Single-unit smooth m.
unitary smooth m. contain what type of cells?
pacemaker cells
t/f unitary smooth m. has intrinsic and extrinsic rhythmic activity
true
Pacemaker in unitary smooth m. are in charge of
intrinsic or myogenic rythmicity
Autonomic motor nerves in unitary smooth m. cells are In charge of
extrinsic rhythmicity
what type of smooth m. is found in Ciliary muscles, iris, piloerector muscles, & airways?
multi unit
Composed of discrete, separate smooth muscle fibers
Each fiber operates (contracts) independently
Each fiber receives innervation from an autonomic nerve fiber
No “intrisic”rhythmic activity
Only excited by “extrinsic” autonomic motor nerves
Muscle fibers form motor unit
multi-unit smooth m.
t/f skeletal m. has no gap junctions and no adeherens junction between muscle fibers
true
t/f smooth m. cells have no troponin, instead they contain CALMODULIN a protein that, when bound to Ca2+, activates myosin light-chain kinase (MLCK)
true
Large amount of Ca2+ in cytosol is required for contraction.
- Ca2+ enters cytosol exclusively from internal SR release.
- Contraction responds only to “all-or-none ” action potentials (=spike potentials).
- Actional potential is mediated by voltage-gated Na+ channels on the plasma membrane.
skeletal m.
Minimal amount of Ca2+ in cytosol is enough for contraction. Any pharmaceutical or physical stimuli which can elevate the cytosolic Ca2+ concentration will also lead to a smooth muscle contraction.
- Ca2+ enters cytosol primarily from outside (extracellular) through voltage-gated Ca2+ channels on the plasma membrane.
- Contraction responds do not necessarily to the action potential. For examples, unitary smooth muscle in gut contract only to the actional potential, but that in blood vessels and multiunitary smooth muscle can contract to the graded depolarization.
- Actional potential is mediated by voltage-gated Ca2+ channels on the plasma membrane.
smooth m
Innervated by somatic motor neuron; all of fibers are innervated; one fiber receive only one nerve terminal input
skeletal m.
Innervated by autonomic motor neuron; in multi-unit smooth muscle, each fiber is innervated; while in unitary smooth muscle, only some fibers receive autonomic innervation. The rest of muscle fibers receive stimulations through gap junctions.
smooth m.
The post-synaptic membrane of neuromuscular junctions (NMJs) are in the form of motor end-plates;
ACh (acetylcholine) serves as the neurotransmitter; its nicotine (N)-type receptors (AChR) are concentrated at the motor end-plates.
skeletal m.
Absence of motor end-plates in NMJs, autonomic neurotransmitter receptors (M-type for ACh, α- or β- type for Norepinephrine) located at the entire surface of smooth muscle cell. The released neurotransmitters act diffusely
smooth m. fiber
