SMOOTH MUSCLE CONTRACTION

• Smooth muscle is composed of far smaller fibers, typically measuring 1 to 5 micrometers in diameter and only 20 to 500 micrometers in length. In contrast, skeletal muscle fibers are significantly larger
• SMOOTH MUSCLE

  Composed of smaller fibers

  -Involuntary control

  -Contraction is slower and sustained

  -Found in internal organs
• SKELETAL MUSCLE

  Larger in diameter

  -Voluntary control

  -Contraction is rapid and powerfu

  -Attached to bones (responsible for body movement)

  -Prone to fatigue during sustained or repetitive contractions
• Multi-unit smooth muscle

  Composed of discrete, separate smooth muscle fibers.

  ➢ Fibers operate independently, often innervated by a single nerve ending.
• Multi-unit smooth muscle

  • Examples include ciliary muscle of the eye, iris muscle of the eye, and piloerector muscles.
• Unitary Smooth Muscle

  •  Also called syncytial smooth muscle or visceral smooth muscle.
  • ➢  Represents a mass of hundreds to thousands of smooth muscle fibers contracting together as a single unit.
• UNITARY SMOOTH MUSCLES:

  • Found in the walls of most viscera, including the gastrointestinal tract, bile ducts, ureters, uterus, and m
• 1. Smooth muscles contract less systematically than skeletal muscles, which are the striped muscles used for movement.
• 1. Actin and myosin filaments, calcium ions, and a special capacity to contract over a broad range (up to 80% of their length) in contrast to skeletal muscles are all involved in the action.
• Calcium ions and a special energy molecule called ATP jump in to make smooth muscle contraction happen.
• 2 BASIS OF CONTRACTILE MECHANISM IN SMOOTH MUSCLE

  CHEMICAL AND PHYSICAL
• Actin filaments are spread throughout the muscle, some attached to the outer cell membrane, others inside the cell.
• Most myosin filaments in smooth muscle have special "side polar" cross-bridges.
• SMOOTH MUSCLE ONLY NEEDS Low Energy Need to Sustain Muscle Contraction
• Similar to the skeletal muscle, smooth muscle also needs an ion that will initiate a spike that produces muscle contraction. It is called a calcium ion.
• smooth muscle doesn't have troponin, which are the regulatory protein that is activated by the calcium to cause skeletal muscle contraction
• the smooth muscle has CALMODULIN instead of TROPONIN in muscle contraction
• Calmodulin does muscle contraction by activating the myosin cross-bridges.
• 1. calmodulin-calcium complex then joins and activates the myosin light chain kinase, which is a phosphorylating enzyme
• Smooth muscle can be stimulated to contract by multiple types of signals, and in several other ways. :

  • nervous signals
  • hormonal stimulation
  • stretch of the muscle
• • Smooth muscles are innervated by autonomic nerve fibers.
• AUTONOMIC NEURONS HAVE

  • varicosities along the axon, which release neurotransmitters in the matrix around the smooth muscle fibers.
• • The transmitter diffuses through the matrix, by a form of so-called diffuse junctions and stimulates the outer layer of muscle fibers.
• • Schwann cells that envelop the axons are interrupted so that transmitter substance can be secreted through the walls of the varicosities.
• • In the varicosities are vesicles of the autonomic nerve fiber endings containing acetylcholine in some fibers and norepinephrine in others--and occasionally other substances as well.
• • In unitary smooth muscles, deeper fibers are stimulated by the conduction of action potential, through the gap junctions between the fibers.
• • In multi-unit smooth muscles, each muscle fiber is stimulated independently, by closely located varicosities these are called contact junctions.
• The most important transmitter substances secreted by the autonomic nerves:

  • Acetylcholine
  • Norepinephrine
• • Acetylcholine is an excitatory transmitter substance for smooth muscle fibers in some organs but an inhibitory transmitter for smooth muscle in other organs.
• • when acetylcholine inhibits a fiber, norepinephrine usually excites it.
• 1. In the normal resting state, many of these small blood vessels remain contracted. But when extra blood flow to the tissue is needed, multiple factors can relax the vessel wall, thus allowing for increased flow
• Lack of oxygen in the local tissues causes smooth muscle relaxation
• Excess carbon dioxide causes vasodilatation.
• Increased hydrogen ion concentration causes vasodilatation.
• • Adenosine is a naturally occurring substance that relaxes and dilates blood vessels. Adenosine also affects the electrical activity of the heart. Adenosine is used to help restore normal heartbeats in people with certain heart rhythm disorders. Adenosine is also used during a stress test of the heart
• • Lactic acid is a byproduct of anaerobic metabolism, in which the body produces energy without using oxygen.
• Since the discovery of lactic acid, the popular notion has been that it is responsible for muscle fatigue and tissue damage induced by the lactic acid following an intense workout (oxygen levels are low)
• 1. A hormone causes contraction of a smooth muscle when the muscle cell membrane contains hormone-gated excitatory receptors for the respective hormone.
• Conversely, the hormone causes inhibition if the membrane contains inhibitory receptors for the hormone rather than excitatory receptors.
• 1. Some hormone receptors in the smooth muscle membrane open sodium or calcium ion channels and depolarize the membrane, the same as after nerve stimulation.