VPB 121: Veterinary Physiology - II (2+1)

SMOOTH MUSCLE TISSUE

• Contains action and myosin filaments with are not orderly arranged. So no striations.
• Two kinds
  • Visceral smooth muscles: About 99% of the smooth muscles are of the unitary type. The fibers are tightly bound together top form continuous network called ‘syncytium’ and wrap around sheets that form part of the walls of hollow visceral such as uterus and stomach, intestines, urinary bladder etc. When a neuron stimulation a fiber impulse travels over the other fibers via ‘gap junctions’ so that contraction occurs in wave over many adjacent fiber direct electrical conduction
  • Multi unit smooth muscle tissue (Direct electrical conduction): consist of individual fibers each with its own motor nerve endings, similar to that of skeletal muscles. About 1% of the smooth muscle is present as multiunit smooth muscle. Seen in the walls of blood vessels (sympathetic stimulation), erector pili muscles that attach to hair follicles, intrinsic muscles of eye such as iris, nictitating membrane.

• Both kinds of smooth muscles can contract and relax more slowly than skeletal muscles tissue (peculiar arrangement of thin and thick filaments). Smooth muscles can maintain forceful contraction for longer time (labour pain of uterus) than skeletal muscles. Smooth muscles can be stretched with out developing tension eg., Urinary Bladder, stomach wall etc.
• The actin filaments are attached to protein – dense bodies which in turn are attached to cell membrane. There are only 1 myosin filaments to every 12 – 15 actin but each myosin filament for more numbers of cross bridges – 3 kg/cm² pressure. Latent period 150 – 200 ms and total time 1 to 3 sec for contraction and relaxation with in about 30 times as long as the single twitch of skeletal muscle. Swiveling action of cross bridge is much slower ie., less ATPase activity. This economy of energy utilization by smooth muscle is important to overall function of the body, because organs such as intestines, UB, GB, must maintain moderate tone throughout life.
  • Contration duration is longer in smooth muscles.
  • Muscle proteins loosely organized so no myofibrils but force of contraction is almost equal to skeletal muscles.
  • Extra cellular materials like collagen, elastin are produced by smooth muscle.
  • Smooth muscles are seen as sheets (GIT tract) or bundles (Erector pili) and have no tendons except gizzard (Central tendon)
  • Actin molecules are held by dense bodies (similar to Z-lines)
  • Contain α- actin similar to Z line
  • No tropomyosin, instead Calmodulin
  • In smooth muscle, myosin and actin happens only when light chain is phosphorylated.
  • More of voltage gated Ca²⁺ channels - slow channels. These channels allow entry even in the absence of action potential.
  • ECF (Ca²⁺) has got strong influence on force contraction.
  • Similar to cardiac muscle.
  • Prolonged contraction and relaxation period (200-300 msec.).
  • No T-tubules
  • Stretching – contraction because of single unit smooth muscle depolarised and spread over large area (Gut).
  • Multi-unit smooth muscle found in ciliary body, iris, ductus deference and walls of large arteries. Here each muscle fiber in innervated separately & contracts only when it receives synaptic stimulation.

Action Potential in smooth muscle

• They are two types
  • Spike potentials and
  • AP with plateaus ie., prolonged repolarization causing sustained contraction as in uterus.