Excitation and contraction of smooth muscles

Question 1

2 types of smooth muscles

Answer 1

multi unit and unitary smooth muscles

Question 2

multi unit smooth muscles

Answer 2

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

Question 3

e.g of multi unit smooth muscles

Answer 3

cilliary and iris muscles of eye, piloerector muscles of hair

Question 4

unitary smooth muscles

Answer 4

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

Question 5

why unitary SM is called syncytial and visceral

Answer 5

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

Question 6

physical basis for smooth muscle contraction

Answer 6

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

Question 7

describe contractile unit of SM

Answer 7

many actin filaments radiating from two dense bodies , their ends overlap with myosin filamets midway between the dense bodies

Question 8

what acts as Z disk in SM

Answer 8

the dense bodies

Question 9

side polar

Answer 9

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

Question 10

regulation of contraction by calcium ions in SM

Answer 10

1. the concentration of calcium ions increase in the cytosolic SM because of influx of CA from calcium channels or sarcoplasmic reticulum
2. calcium ions reversibly join with calmodulin
3. CA-calmodulin complex then attaches and activates the MYOSIN LIGHT CHAIN KINASE (phosphorylated enzyme)
4. one of the myosin light chains of myosin head (regulatory chain) becomes phosphorylated cause of myosin kinase
5. the non phosphorylated cannot attach/ detach with actin meanwhile the phosphorylated ones can

Question 11

source of calcium ions in SM that causes contraction are

Answer 11

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

Question 12

latent period

Answer 12

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

Question 13

ROLE of smooth sarcoplasmic reticulum in Smooth muscles

Answer 13

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

Question 14

smooth muscles contraction is dependent on ECF CA concentration

Answer 14

if the CA concentration falls in ECF to 1/3 to 1/10, the smooth muscle contraction ceases

Question 15

what causes smooth muscles relaxation

Answer 15

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

Question 16

myosin phosphatase

Answer 16

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

Question 17

possible mechanism of regulation latching phenomena

Answer 17

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.

Question 18

diffuse junction

Answer 18

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

Question 19

how excitations travel in the inner layer of smooth muscles from outer layer

Answer 19

they travel through action potentials are additional diffusion

Question 20

varicosities

Answer 20

the axon terminal of smooth muscle cells have varicosities. Schwann cells envelop the exons are interrupted so that transmitter is release from the varicosities

Question 21

how trasnmitter is released from varicosities

Answer 21

it is released through vesicles from the varicosities and contains norepinephrine in place of acetylcholine sometimes

Question 22

contact junctions

Answer 22

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

Question 23

excitation of visceral smooth muscles by muscle stretch

Answer 23

when visceral smooth muscles are stretched a spontenous action potential is generated due to

1. normal slow wave potentials
2. a decrease in overall negativity of membrane potential cause of stretching

Question 24

example of exciation of SM by stretching

Answer 24

persistalasis in intestine cause the inetstinal contents to move towards the anus

Question 25

depolarization of multi unit smooth muscles without action potential

Answer 25

multi unit smooth muscles contratcs due to an extrinsic stimuli. acetylcholine or norepinephrine released into the muscle fiber cause depolarization and this elicit contractions

Question 26

smooth muscle contraction in response to local tissue chemical factor

Answer 26

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

Question 27

chemicals causing vasodilation

Answer 27

adenosine, increase in phosphate ions, nitric oxide and lactic acid

Question 28

hormones that effect smooth muscles and thorgh what

Answer 28

they affect through HORMONE GATED EXCITORY RECEPTORS or NOT WORK BY INHIBTORY RECEPTORS. EG: norepinephrine, epinephrine, neostigmine II, oxytocin, serotonin, endothelin, vasopressin

Question 29

what does smooth muscles does not contain?

Answer 29

troponin complex

Question 30

vale of side polar cross bridges

Answer 30

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

Question 31

slow cycling of myosin cross bridges

Answer 31

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

Question 32

major factor the determines the force of contraction is

Answer 32

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.

Question 33

why there is slow cycling of myosin cross bridges in smooth muscles?

Answer 33

less amount of ATPase in the cross bridges head so low amount of atp degradation and low energy for attachment and reattachment

Question 34

Low Energy Requirement to Sustain Smooth Muscle

Contraction.

Answer 34

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.

Question 35

latch mechanism

Answer 35

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.

Question 36

stress-relaxation and

reverse stress-relaxation Their importance

Answer 36

they allow a hollow organ to maintain
about the same amount of pressure inside its lumen
despite sustained, large changes in volume.

Question 37

relaxation of smooth muscle fibers

Answer 37

the calcium needs to be pumped back into the ecf though the calcium pumps. which is slower than the calcium pump in skeletal muscles.

Question 38

what happens when ca level falls below a critical level?

Answer 38

the calcium calmodulin process occurs but the myosin heads are not phosphorylated
myosin phosphatase splits the phosphate from the regulatory chain

Question 39

muscle excitation travels how from outer to inner layer by diffusion junction

Answer 39

by action potential or additional diffusion

Question 40

why acytelcholine and norepinephrine have different affect on different smooth muscle fibers

Answer 40

they are first attached to receptor proteins which are either excitatory receptors or inhibitory receptors

Question 41

resting potential of smooth muscles

Answer 41

-50 to -60

Question 42

Action Potentials in Unitary Smooth Muscle.

Answer 42

(1) spike potentials or (2) action

potentials with plateaus

Question 43

duration of spike action potential

Answer 43

10-50 milliseconds

Question 44

spike action potentials can be elicited by

Answer 44

electric stimulation
action potential by hormones
stretch in the muscles
spontaneous generation in the muscle itself

Question 45

Action Potentials with Plateaus

Answer 45

it initiates in the same way as spike potential but it takes time in repolarization to several hundred to 1000 millisecond.

Question 46

Action Potentials with Plateaus takes place where

Answer 46

ureters, uterus in some conditions and cardiac muscles.

Question 47

two task that calcium performs in smooth muscles

Answer 47

prolonged contraction that causes the plateaus action potential and it acts directly on the muscles to cause contraction.