Excitation and contraction of smooth muscles Flashcards
2 types of smooth muscles
multi unit and unitary smooth muscles
multi unit smooth muscles
are separate distinct smooth muscle fibers, each one of the function independently and each innervated by a single nerve ending. A basement membrane covers which is made of thin collagen and glycoprotein
e.g of multi unit smooth muscles
cilliary and iris muscles of eye, piloerector muscles of hair
unitary smooth muscles
called as visceral or syncytial smooth muscles are hundreds and thousands of smooth muscle fiber that contracts together
they are arranged in sheets and bundles and their cell membranes are adherent to one another so the contraction force can travel in the neighboring fibers
they also have gap junctions for movement of ions for action potential or without action potential
why unitary SM is called syncytial and visceral
it syncytial interconnections between fibers and is present in the walls of most viscera of the body, GIT, bile duct, uterus, ureters and many blood vessels
physical basis for smooth muscle contraction
in SM the actin filaments are attached to dense bodies, the dense bodies are either attached to cell membrane or dispersed inside the cell, membrane dense bodies are attached to adjacent cell by protein bridges and that’s how the the force of contraction is transmitted from one cell to another. interspread between the actin filaments is the myosin filaments
describe contractile unit of SM
many actin filaments radiating from two dense bodies , their ends overlap with myosin filamets midway between the dense bodies
what acts as Z disk in SM
the dense bodies
side polar
side polar cross bridges(present on myosin filament) means the crossbridge on one side hinge towards in one direction meanwhile the other bridge hinge towards the opposite so when the myosin filament pulls an actin filament it pulls one in one direction towards one side and other in the other direction on opposite side
regulation of contraction by calcium ions in SM
- the concentration of calcium ions increase in the cytosolic SM because of influx of CA from calcium channels or sarcoplasmic reticulum
- calcium ions reversibly join with calmodulin
- CA-calmodulin complex then attaches and activates the MYOSIN LIGHT CHAIN KINASE (phosphorylated enzyme)
- one of the myosin light chains of myosin head (regulatory chain) becomes phosphorylated cause of myosin kinase
- the non phosphorylated cannot attach/ detach with actin meanwhile the phosphorylated ones can
source of calcium ions in SM that causes contraction are
the calcium ions are diffused in the muscles from the extracellular fluid at the time of action potential. CA concentration in ECF is 10-3 molar and in muscles is 10-7 which allows rapid diffusion of CA ions inside
latent period
the time required for the diffusion of CA ions inside is 200-300 milliseconds before the contraction occurs which is 50 times greater for SM than in skeletal muscles
ROLE of smooth sarcoplasmic reticulum in Smooth muscles
the sarcoplasmic reticulum has caveolae which is invagination of cell membrane and is a rudimentary analog of transverse tubules
when action potential reaches the caveolae, calcium ions are released from sarco reticulum, works in same was as t transverse tubule cause release of CA from longitudinal sarc reticulum
the more extensive the sarc reticulum is the more rapidly SE contracts
smooth muscles contraction is dependent on ECF CA concentration
if the CA concentration falls in ECF to 1/3 to 1/10, the smooth muscle contraction ceases
what causes smooth muscles relaxation
in order to relax smooth muscles, ca ions must be removed from the muscles , the Ca ions are removed by the Calcium pumps back to ECF or Sarc reticulum
the calcium pumps of Smooth muscles aren’t as rapid as sarcoplasmic reticular pump of skeletal muscles so a muscle contraction continues for seconds as compare to hundredth or tenth of a second in skeletal
myosin phosphatase
when the calcium levels drop below a critical level, the process mention of calcium pumping reverse, myosin phosphatase phosphorylates the regulatory light chain, and then the cycle stops and contraction ceases
possible mechanism of regulation latching phenomena
as myosin kinase and phosphatase is highly activated, the cycling frequency of myosin head and contraction is high
so the myosin head remains attach for longer period of time with actin
even when then kinase and phosphatase decreases so does the frequency of cycling but deactivation of these enzymes allows the myosin heads to remain attach and the static force of contraction is maintained.
diffuse junction
autonomic nerve fibers that innervate the smooth muscles branches diffusely on the sheet of the muscle fiber
these nerve fibers are not in direct contact with the smooth muscle fiber but form diffused junction, release the transmitter on the matrix coat of the smooth muscle
how excitations travel in the inner layer of smooth muscles from outer layer
they travel through action potentials are additional diffusion
varicosities
the axon terminal of smooth muscle cells have varicosities. Schwann cells envelop the exons are interrupted so that transmitter is release from the varicosities
how trasnmitter is released from varicosities
it is released through vesicles from the varicosities and contains norepinephrine in place of acetylcholine sometimes
contact junctions
in multi unit smooth muscles the varicosities are distant from the muscle cell membrane by only 20-30 nanometers which is as synaptic cleft so called contact junction
excitation of visceral smooth muscles by muscle stretch
when visceral smooth muscles are stretched a spontenous action potential is generated due to
- normal slow wave potentials
- a decrease in overall negativity of membrane potential cause of stretching
example of exciation of SM by stretching
persistalasis in intestine cause the inetstinal contents to move towards the anus
depolarization of multi unit smooth muscles without action potential
multi unit smooth muscles contratcs due to an extrinsic stimuli. acetylcholine or norepinephrine released into the muscle fiber cause depolarization and this elicit contractions
smooth muscle contraction in response to local tissue chemical factor
smooth muscles are contractile and respond rapidly to the change in environment surrounding interstitial fluid and by changes in BP
the blood vessels are contracted but they relax as the blood flow increase
1decrease in O2 LEVEL causes vasodilation
2..increase co2 level causes vasodilation
3.increase H2 ion concentration cause vasodilation
chemicals causing vasodilation
adenosine, increase in phosphate ions, nitric oxide and lactic acid
hormones that effect smooth muscles and thorgh what
they affect through HORMONE GATED EXCITORY RECEPTORS or NOT WORK BY INHIBTORY RECEPTORS. EG: norepinephrine, epinephrine, neostigmine II, oxytocin, serotonin, endothelin, vasopressin
what does smooth muscles does not contain?
troponin complex
vale of side polar cross bridges
it allows the the smooth muscle to contract the whole lenght of muscle which is 80% of the length and not 30% like the skeletal muscles
slow cycling of myosin cross bridges
skeletal muscle myosin cross bridges their attachment to actin, then release
from the actin, and reattachment for the next cycle—is
much slower than in skeletal muscle
major factor the determines the force of contraction is
the amount of time that the cross bridges are attached to the actin filaments is what determines the force of contraction and it is greatly increased in smooth muscles.
why there is slow cycling of myosin cross bridges in smooth muscles?
less amount of ATPase in the cross bridges head so low amount of atp degradation and low energy for attachment and reattachment
Low Energy Requirement to Sustain Smooth Muscle
Contraction.
Only 1/10 to 1/300 as much energy is
required to sustain the same tension of contraction in
smooth muscle as in skeletal muscle.
latch mechanism
once the muscle has contracted, the amount of more excitation can be reduced to half of its initial level and so can be the energy required for the excitation which is 1/300 the energy required for sustained skeletal muscle contraction.
stress-relaxation and
reverse stress-relaxation Their importance
they allow a hollow organ to maintain
about the same amount of pressure inside its lumen
despite sustained, large changes in volume.
relaxation of smooth muscle fibers
the calcium needs to be pumped back into the ecf though the calcium pumps. which is slower than the calcium pump in skeletal muscles.
what happens when ca level falls below a critical level?
the calcium calmodulin process occurs but the myosin heads are not phosphorylated
myosin phosphatase splits the phosphate from the regulatory chain
muscle excitation travels how from outer to inner layer by diffusion junction
by action potential or additional diffusion
why acytelcholine and norepinephrine have different affect on different smooth muscle fibers
they are first attached to receptor proteins which are either excitatory receptors or inhibitory receptors
resting potential of smooth muscles
-50 to -60
Action Potentials in Unitary Smooth Muscle.
(1) spike potentials or (2) action
potentials with plateaus
duration of spike action potential
10-50 milliseconds
spike action potentials can be elicited by
electric stimulation
action potential by hormones
stretch in the muscles
spontaneous generation in the muscle itself
Action Potentials with Plateaus
it initiates in the same way as spike potential but it takes time in repolarization to several hundred to 1000 millisecond.
Action Potentials with Plateaus takes place where
ureters, uterus in some conditions and cardiac muscles.
two task that calcium performs in smooth muscles
prolonged contraction that causes the plateaus action potential and it acts directly on the muscles to cause contraction.
