Smooth Muscle Contraction Flashcards
(25 cards)
What is smooth muscle and Where are theyfound?
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
Structure of smooth muscle
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
Important Functional Proteins in smooth muscle
Contractile
Regulatory
Structural
Contractile
○Myosin – Molecular motor of Thick Filament
○Actin – Thin Filament protein, binds to myosin head
Regulatory
○Tropomyosin – Blocks site on actin for myosin binding
○Calmodulin – Signaling protein for contraction initiation
Structural (all associated with force transduction
○Desmin
○Filamin
○Vimentin
Myosin (Thick Filament)
○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).
Actin (Thin Filament)
○Longer filaments
○More plentiful than in skeletal muscle
Myosin & actin
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
Smooth muscle contraction occurs
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
Thick filament composed of
of myosin Myosin heads contain: ATPase function Ability to “pivot” Lights chains
Thin Filament:
Multiple g-actins form a long chain known as f-actin
F-actin is surrounded by
surrounded by tropomyosin which partially blocks the site on actin for myosin binding
Caldesemon is bound to..
Caldesemon is bound to the tropomyosin and regulates tropomyosin’s position on the thin filament
Tropomyosin
associated with actin, partially blocks the interaction between myosin and actin
Caldesmon complex
Ca2+ binding protein complex associated with actin, plays a role in “activation of the thin filament” (Controversial)
Calponin
associated with actin (link contractile proteins to cytoskeletal proteins)
Calmodulin
in cytoplasm
Binds Ca2+
Ca2+ binding is the first step in a series of events that ends with contraction
Structural proteins
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.
Types of smooth muscle
Multiunit muscle
Single-unit (or visceral)
Multiunit muscle
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
Single-unit (or visceral)
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
Smooth Muscle Contraction
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
Smooth muscle contraction Example:
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
