Smooth Muscle
 Spindle-shaped cells 2-10
 m across & ~100
 m long

Have a thin endomysium

Organized into longitudinal and circular layers

Found in walls of contractile organs (except the heart)

Innervation of Smooth Muscle

Lacks neuromuscular junctions

Innervating nerves form varicosities

Varicosities release neurotransmitters into synaptic clefts called diffuse junctions

Microscopic Anatomy of Smooth Muscle

SR less developed than in sk mus & lacks specific pattern

No T tubules

Plasma membranes have caveoli

Ca 2+ sequestered in extracellular space near caveoli

Thin and thick filaments present, but no visible striations

Organization of Myofilaments in Smooth Muscle
 Intermediate filaments attached at “dense bodies”

More thick filaments per thin filament

Thick filaments have heads along their entire length

No troponin complex

Myofilaments arranged diagonally

Contraction of Smooth Muscle

Synchronized contraction
 sheets of smooth muscle
 Gap junctions
 electrically couple cells
 action potentials transmitted cell to cell

Some smooth muscle cells:

Act as pacemakers and set the contractile pace for whole sheets of muscle

Are self-excitatory and depolarize without external stimuli

Contraction Mechanism

Actin and myosin interact via to the sliding filament mechanism
 Resting State

Actin bound by caldesmon

Myosin light chain (MLC) dephosphorylated

Intracellular [Ca 2+ ] low

Rise in intracellular Ca 2+ triggers contraction
 Ca 2+ enters from the extracellular space (some from SR)

Increase in [Ca 2+ ] caused by:

Excitation by neurotransmitters (extracellular Ca)

Hormonal signaling via receptors & 2 nd messengers (SR
Ca)
 Ca flux can be inhibited by various inhibitory stimuli – hormonal & neuronal

Regulation of SM Contraction
Actin

Role of Ca 2+ for Contraction

Ca 2+ binds to calmodulin (Cam)

Ca-Cam binds to & activates myosin light chain kinase (MLCK)

CaCam binds to & inactivates caldesmon

Active caldesmon binds actin

CaCam-inactivated caldesmon can’t bind actin

The free actin is available for myosin to bind

Activated MLCK phosphorylates myosin head region allowing cross bridges with actin to form

Special Features of Smooth Muscle Contraction

Unique characteristics of smooth muscle include:

Smooth muscle tone

Slow, prolonged contractile activity

Low energy requirements

Response to stretch

Response to Stretch

Smooth muscle exhibits a phenomenon called stress-relaxation response in which:

Smooth muscle responds to stretch only briefly, and then adapts to its new length

The new length, however, retains its ability to contract

This enables organs such as the stomach and bladder to temporarily store contents

Hyperplasia

Certain smooth muscles can divide and increase their numbers by undergoing hyperplasia
 estrogen’s effect on the uterus

During pregnancy, estrogen stimulates uterine growth to accommodate growing fetus

Types of Smooth Muscle: Single Unit

The cells of single-unit smooth muscle, commonly called visceral muscle:

Contract rhythmically as a unit
 electrically coupled via gap junctions
 exhibit spontaneous action potentials
 arranged in opposing sheets
 exhibit stress-relaxation response

Types of Smooth Muscle: Multiunit

Multiunit smooth muscles are found:
 airways of lungs
 arteries
 arrector pili muscles
 internal eye muscles
 characteristics include:

Rare gap junctions

Infrequent spontaneous depolarizations

Structurally independent muscle fibers
 rich nerve supply, 1/motor units

Graded contractions in response to neural stimuli

Muscular Dystrophies

Group of inherited muscle degeneration diseases

Muscle fibers atrophy

Muscle tissue replaced by connective tissues (scar tissue)

Duchenne Muscular Dystrophy (DMD)
 sex-linked recessive inheritance (gene on X chromosome)

1/3500 live male births
 caused by a defect in the gene encoding the protein dystrophin
 dystrophin helps maintain muscle cell integrity during use