Smooth Muscle Contraction

Question 1

What is smooth muscle and Where are theyfound?

Answer 1

Digestive tract
provides the forces for movement of food through the tract (peristalsis)
Blood vessels
Smooth muscle is found generally in sheets or layers of various organs
The individual cells are “spindle shaped”.
May be interconnected via gap junctions (connexons)
Contain actin and myosin that interact to allow for contraction
regulates BV diameter and blood pressure
Respiratory tract
regulates the diameter of airways
Urinary Bladder
relps to expel urine from the bladder
Uterus
provides the forces for parturition (child birth)
Skin

Question 2

Structure of smooth muscle

Answer 2

Spindle-shaped: diameter 2-10 µm, length 50-200 µm
Few mitochondria, poorly developed sarcoplasmic reticulum
Sarcolemma (cell membrane)
○Numerous voltage-dependent Ca++ channels
○Receptor proteins
○No T-tubules

Question 3

Important Functional Proteins in smooth muscle

Answer 3

Contractile
Regulatory
Structural

Question 4

Contractile

Answer 4

○Myosin – Molecular motor of Thick Filament

○Actin – Thin Filament protein, binds to myosin head

Question 5

Regulatory

Answer 5

○Tropomyosin – Blocks site on actin for myosin binding

○Calmodulin – Signaling protein for contraction initiation

Question 6

Structural (all associated with force transduction

Answer 6

○Desmin
○Filamin
○Vimentin

Question 7

Myosin (Thick Filament)

Answer 7

○Class II (two myosin heavy chains), but different isoforms from skeletal muscle
○ATPase activity is slower
○The myosin light chain in the head controls contraction and relaxation
○Myosin filaments lie between actin fibers so that the entire length of the actin filament is covered by globular heads (actin slides along myosin for longer distances).

Question 8

Actin (Thin Filament)

Answer 8

○Longer filaments

○More plentiful than in skeletal muscle

Question 9

Myosin & actin

Answer 9

Not arranged in sarcomeres, but in long bundles that extend diagonally within the cell , forming a lattice around the nucleus (No cross striations)
Actin is attached to dense bodies within the cytoplasm and terminate to dense bodies within the membrane
The dense bodies function like Z-lines in the skeletal muscle and anchor the thin filaments
High # of actin filaments and smaller # of myosin filaments
Actin filaments leaving one dense body overlap one myosin filament, placed at half distance between 2 dense bodies
Oblique arrangement causes smooth muscle fibers to become globular when they contract
Lack of striations= smooth

Question 10

Smooth muscle contraction occurs

Answer 10

Smooth muscle contraction occurs when the myosin filament (green) pulls the actin filament along its length

This occurs due to the myosin head groups (which protrude form the myosin filament interact with actin

Question 11

Thick filament composed of

Answer 11

of myosin
Myosin heads contain:
ATPase function
Ability to “pivot”
Lights chains

Question 12

Thin Filament:

Answer 12

Multiple g-actins form a long chain known as f-actin

Question 13

F-actin is surrounded by

Answer 13

surrounded by tropomyosin which partially blocks the site on actin for myosin binding

Question 14

Caldesemon is bound to..

Answer 14

Caldesemon is bound to the tropomyosin and regulates tropomyosin’s position on the thin filament

Question 15

Tropomyosin

Answer 15

associated with actin, partially blocks the interaction between myosin and actin

Question 16

Caldesmon complex

Answer 16

Ca2+ binding protein complex associated with actin, plays a role in “activation of the thin filament” (Controversial)

Question 17

Calponin

Answer 17

associated with actin (link contractile proteins to cytoskeletal proteins)

Question 18

Calmodulin

Answer 18

in cytoplasm
Binds Ca2+
Ca2+ binding is the first step in a series of events that ends with contraction

Question 19

Structural proteins

Answer 19

Make up the cytoskeleton
They form a network of intermediate filaments that:
attach to “attachment dense bodies” in membrane
cross and contact the “cytoplasmic dense bodies”
Attachment dense bodies in one cell are in contact to attachment dense bodies in neighboring cells
Dense Bodies & the intermediate filaments serve for transmitting the mechanical forces from one cell to the other.

Question 20

Types of smooth muscle

Answer 20

Multiunit muscle

Single-unit (or visceral)

Question 21

Multiunit muscle

Answer 21

consists of discrete muscle fibers, that contract independently of the others

   - in the intrinsic muscles of the eye (iris) – change the shape of the lens to focus light on the retina
   - in the piloerector muscles of the skin – when these muscle cells contract the hairs become erect

Question 22

Single-unit (or visceral)

Answer 22

composed of hundreds to hundred thousands cells that contract simultaneously, as a single unit (functional syncitium).
-internal organs
Cells are electrically connected through gap junctions – action potential in one cell spreads rapidly to make the entire sheet of tissue contract

Question 23

Smooth Muscle Contraction

Answer 23

Contraction is initiated by elevations in intracellular Ca2+ caused by:

influx of Ca2+ through membrane channels
○Induced by action potentials via the autonomic NS
○Induced by chemical signals

release of calcium from sarcoplasmic reticulum (SR) following activation of some membrane receptors which, through second messengers, open Ca2+ channels in the SR membrane

Question 24

Smooth muscle contraction Example:

Answer 24

Vasculature of (arteries & arterioles) skin, GI, kidney, brain

○Have a1-adrenergic receptors (Gq coupled)
Epinephrine binds to a1-adrenergic receptor → activates Gq → ↑PLC activity, → ↑IP3, → ↑ Ca2+ release from ER/SR → ↑ cytosolic [Ca2+] → smooth muscle contraction → vessel constriction → decreased blood flow.
1.Ca2+ binds to calmodulin
2.Ca2+-CaM activates an enzyme myosin light chain kinase (MLCK)
3.MLCK phosphorylates the regulatory myosin light chains
4.Ca2+ also binds to a caldesmon complex on the thin filament which appears to shift tropomyosin, allowing the myosin head to bind actin more tightly or allow more cross bridges (controversial, modulatory role)
5.When myosin is phosphorylated, its globular heads can interact with actin (main initiation step for contraction)
6.Myosin becomes active and cross-bridges pull actin and create muscle tension
Thus smooth muscle contraction is controlled by a myosin-linked regulatory process

Question 25

Relaxation

Answer 25

Free Ca is removed from cytosol by:
1.Ca2+ pump in sarcoplasmic reticulum membrane
2.Ca2+ pump in plasma membrane
3.Na+-Ca2+ pump (antiport system) in plasma membrane
Decreased free cytosolic Ca2+ causes Ca2+ to unbind from calmodulin
In the absence of Ca2+-CaM, the myosin light chain kinase inactivates
Myosin light chain is dephosphorylated by myosin phosphatase
Contraction ceases