Smooth muscle

►Involuntary muscle

►Innervated by autonomic nerves

►Lack striated pattern (smooth)

Types

►Single unit (visceral smooth muscle)

►Multi unit smooth muscle

Single unit smooth muscle

►Fibers connected by gap junctions

►Function as a single unit (syncytium)

►Stimulated by

►Nervous stimuli

►Non-nervous stimuli (chemicals or physical)

►Present in walls of hollow viscera

►GIT, reproductive & urinary tracts and small blood vessels

Multi unit smooth muscle

►Each fiber functions independently

►Separate nerves to each fibers

►Stimulated by nervous signals only

►Present in

►Ciliary muscle, iris, piloerector muscle

Smooth muscle fiber

►Spindle shaped cell

►Single nucleus

►Smaller than skeletal muscle fiber

►No ‘T’ tubules -caveolae present

►Calmodulin in place of troponin

►Have capability to divide (unlike skeletal muscle)

Contractile unit

►Actin filaments -attached to dense bodies

►Lack troponin

►Myosin filaments

►Single filament within a ‘sarcomere’

Contraction of smooth muscle

►Ca++performs two functions

►Generation of action potential

►Initiation of contraction

Relaxation of smooth muscle

►Role of myosin phosphatase

►Dephosphorylates myosin head

►Removal of Ca++

►Ca++pump located at fiber membrane

►Dissociation of Ca++-Calmodulin complex

Contraction of smooth muscle

Source of Ca++

►ECF

►Some from sarcoplasmic reticulum (role of caveolae)

Characteristics of smooth muscle contraction

►Less ATPase activity of myosin head

►Slow cycling of myosin cross bridges

►Decreased energy requirement

►Slow onset of contraction & relaxation

►Increased force of muscle contraction

Stress relaxation

►When stretch increases

►Contractile filaments adjust to new position to undo effect of rise in stretch

►Rise in BP, filling of urinary bladder

►Reverse stress relaxation

►When stretch decreases

►Contractile filaments adjust to new position to undo effect of decrease in BP

►Fall in BP

Latch mechanism

►Sustained contraction with little excitatory signal (exactly opposite to skeletal muscle)

►Less use of energy

►Myosin heads remain attached (latched) with actin for much longer time

Electrical activity

►In single unit smooth muscle

►Recording from many muscle cells

►In multi unit smooth muscle fiber

►Action potential can not be recorded from a single cell

Action potential in smooth muscle

Resting membrane potential

►-50 to -60 mv (unstable)

►Depolarization

►Opening of voltage gated Ca++channels

►Repolarization

►Opening of voltage gated K+channels

►Spike potentials

►Action potentials with plateau

►Action potentials on slow waves

Spike potentials

►Typical action potential

►Due to Ca++influx and K+efflux

Action potential with plateau

►Due to Ca++influx and delayed K+efflux

►Leads to sustained contractions

►Occurs in ureters, uterus etc

Slow waves (pacemaker waves)

►Waxing and waning of Na+-K+pump

►At threshold action potential initiates

►Occurs in GIT

Stimuli

►Nervous

►Non-nervous

►Hormones

►Paracrine factors

►NO, prostacyclin, adenosine etc

►Physical

►Stretch

Smooth muscle cell membrane contains receptors

►Receptors either excite or inhibit smooth muscle

Nervous stimuli

►Autonomic nerves

►Make diffuse junctions over sheet of single unit smooth muscles

►Either contract or relax smooth muscle

►Contact junctions with multi unit smooth muscle fibers

Diffuse junctions (with single unit smooth muscle cells)

►No direct contact of autonomic nerves with muscle cells

►Nerves terminate on sheet of cells

►Nerves form varicosities

►Non-nervous stimuli

►Hormones and paracrine factors

►Action through ion channels

►Opening of Na+or Ca++channels (stimulation)

►Opening of K+channels (inhibition)

►Action through 2ndmessenger system

►Stimulation or inhibition of smooth muscle

2ndmessenger system Relaxation of vascular smooth muscle by NO

Non-nervous stimuli

►Physical stretch

►Contraction of smooth muscle

►Bulk laxatives (fiber diet)