1. DISTRIBUTION OF SMOOTH MUSCLE

2. STRUCTURES HAVING SMOOTH MUSCLE

1. Wall of organs such as:
   • Esophagus
   • Stomach
   • Intestine in gastrointestinal tract
2. Ducts of digestive glands.
3. Trachea, bronchial tube and alveolar ducts of respiratory tract.
4. Ureter, urinary bladder and urethra in excretory system.
5. Wall of blood vessels in circulatory system.
6. Arrector pili of skin.
7. Mammary glands, uterus, genital ducts, prostate gland and scrotum in reproductive system.
8. Iris and ciliary body of the eye.

3. FUNCTIONS OF SMOOTH MUSCLE

Smooth muscles are concerned with very important functions in different parts of the body.

1. In Cardiovascular System

• Smooth muscle fibers around the blood vessels regulate:
  • Blood pressure
  • Blood flow through different organs and regions of the body

2. In Respiratory System

• Contraction and relaxation of smooth muscle fibers of air passages:
• Alter the diameter of air passage.
• Regulate the inflow and outflow of air.

3. In Digestive System

• Smooth muscle fibers in the digestive tract help in:
  1. Movement of food substances.
  2. Mixing of food substance with digestive juices.
  3. Absorption of digested material.
  4. Elimination of unwanted substances.
• Sphincters along the digestive tract regulate flow of food materials.

4. In Renal System

• Smooth muscle fibers in renal blood vessels regulate:
  • Renal blood flow
  • Glomerular filtration
• Smooth muscles in ureters propel urine from kidneys to urinary bladder through ureters.
• Smooth muscles present in urinary bladder help in voiding urine to the exterior.

5. In Reproductive System

In Males

• Smooth muscle fibers facilitate the movement of:
  • Sperms
  • Secretions from accessory glands

In Females

• Smooth muscles:
  • Accelerate the movement of sperms through genital canal after sexual act.
  • Move ovum into uterus through fallopian tube.
  • Help in expulsion of menstrual fluid.
  • Help in delivery of the fetus.

4. STRUCTURE OF SMOOTH MUSCLE

Myofibrils and Sarcomere

• Well-defined myofibrils and sarcomere are absent in smooth muscles.
• Therefore, alternating dark and light bands are absent.
• Absence of dark and light bands gives the nonstriated appearance to smooth muscle.

Myofilaments and Contractile Proteins

• Contractile proteins in smooth muscle fiber are:
  • Actin
  • Myosin
  • Tropomyosin
• Troponin or troponin-like substance is absent.
• Thick and thin filaments are present in smooth muscle fibers.
• Filaments are not arranged in an orderly fashion as in skeletal muscle.
• Thick filaments formed by myosin molecules:
  • Have many numbers of cross bridges than in skeletal muscle.
• Thin filaments are formed by:
  • Actin
  • Tropomyosin molecules

Dense Bodies

• Dense bodies are special structures of smooth muscle fibers to which:
  • Actin
  • Tropomyosin
  of thin filaments are attached.

Sarcotubular System

• Sarcotubular system in smooth muscle fibers is present in the form of a network.
• T-tubules are absent.
• L-tubules are poorly developed.

5. TYPES OF SMOOTH MUSCLE FIBERS

6. SINGLE-UNIT OR VISCERAL SMOOTH MUSCLE FIBERS

Features of Single-Unit Smooth Muscle Fibers

1. Muscle fibers are arranged in:
   • Sheets
   • Bundles
2. Cell membrane of adjacent fibers fuses at many points to form gap junctions.
3. Through gap junctions, impulses move freely from one cell to another.
4. Thus, a functional syncytium is developed.
5. Syncytium contracts as a single unit.
6. In this way, visceral smooth muscle resembles cardiac muscle more than skeletal muscle.
7. Single-unit smooth muscle fibers are present in walls of organs such as:
   • Gastrointestinal organs
   • Uterus
   • Ureters
   • Respiratory tract
   • Etc.

7. MULTIUNIT SMOOTH MUSCLE FIBERS

Features of Multiunit Smooth Muscle Fibers

1. Muscle fibers are individual fibers.
2. Each muscle fiber is innervated by a single nerve ending.
3. Each muscle fiber has an outer membrane made up of myofilament, which helps to insulate and separate the muscle fibers from one another.
4. Control of muscle fibers is mainly by nerve signals.
5. Smooth muscle fibers do not exhibit spontaneous contractions.

Multiunit Smooth Muscles are Present in

• Ciliary muscles of the eye
• Iris of the eye
• Nictitating membrane (in cat)
• Arrector pili
• Smooth muscles of blood vessels
• Urinary bladder

8. DIFFERENCES BETWEEN SINGLE-UNIT AND MULTIUNIT SMOOTH MUSCLE

9. ELECTRICAL ACTIVITY IN SINGLE-UNIT SMOOTH MUSCLE

Resting Membrane Potential

• Resting membrane potential in single-unit smooth muscle fiber is very much unstable.
• It ranges between −50 mV and −75 mV.
• Sometimes, it reaches a low level of −25 mV.

Cause for Unstable Resting Membrane Potential

• Unstable resting membrane potential is caused by appearance of some wave-like fluctuations called slow waves.
• Slow waves occur in a rhythmic fashion at a frequency of 4 to 10 per minute with an amplitude of 10 to 15 mV.
• Slow-wave rhythm may be due to rhythmic modulations in the activity of sodium-potassium pump.
• Slow wave is not an action potential and cannot cause contraction of the muscle.
• It initiates the action potential.

10. ACTION POTENTIAL IN SINGLE-UNIT SMOOTH MUSCLE

Three types of action potential occur in single-unit smooth muscle:

1. Spike potential
2. Spike potential initiated by slow-wave rhythm
3. Action potential with plateau

1. Spike Potential

• Spike potential in single-unit smooth muscle is different from that in skeletal muscles.
• Average duration of spike potential varies between 30 and 50 milliseconds.
• Its amplitude is very low and it does not reach the isoelectric base.
• It is due to nervous and other stimuli.
• It leads to contraction of the muscle.

2. Spike Potential Initiated by Slow-Wave Rhythm

• Sometimes, slow-wave rhythm of resting membrane potential initiates spike potentials, which lead to contraction of the muscle.
• Spike potentials occur rhythmically at a rate of about one or two spikes at the peak of each slow wave.
• The slow waves cause rhythmic contractions of smooth muscles.
• This type of potential appears mostly in muscles which are self-excitatory and contract themselves without any external stimuli.
• Therefore, spike potentials initiated by slow-wave rhythm are also called pacemaker waves.
• Smooth muscles showing rhythmic contractions are present in some visceral organs such as intestine.

3. Action Potential With Plateau

• This type of action potential starts with rapid depolarization as in skeletal muscle.
• But repolarization does not occur immediately.
• Muscle remains depolarized for long periods of about 100 to 1,000 milliseconds.
• This forms the plateau (stable period) in action potential.
• This type of action potential is responsible for sustained contraction of smooth muscle fibers.
• After long depolarized state, slow repolarization occurs.

Tonic Contraction of Smooth Muscle Without Action Potential

• Smooth muscles of some visceral organs maintain a state of partial contraction called tonus or tone.
• Tonic contraction of the muscle occurs without any action potential or any stimulus.
• Sometimes, tonic contraction occurs due to action of some hormones.

11. IONIC BASIS OF ACTION POTENTIAL

12. ELECTRICAL ACTIVITY IN MULTIUNIT SMOOTH MUSCLE

13. CONTRACTILE PROCESS IN SMOOTH MUSCLE

Calcium-Calmodulin Complex

• Stimulation of ATPase activity of myosin in smooth muscle is different from that in skeletal muscle.
• In smooth muscle, myosin has to be phosphorylated for activation of myosin ATPase.

Phosphorylation of Myosin

1. Calcium, which enters the sarcoplasm from extracellular fluid, combines with a protein calmodulin and forms calcium-calmodulin complex.
2. This complex activates an enzyme called calmodulin-dependent myosin light chain kinase.
3. This enzyme in turn causes phosphorylation of myosin followed by activation of myosin ATPase.
4. Now, the sliding of actin filaments starts.
5. Phosphorylated myosin gets attached to the actin molecule for longer period.
6. This is called latch-bridge mechanism.
7. It is responsible for sustained contraction of the muscle with expenditure of little energy.
8. Relaxation of the muscle occurs due to dissociation of calcium-calmodulin complex.

14. LENGTH-TENSION RELATIONSHIP: PLASTICITY

Significance of Plasticity

• Tension produced by muscle fiber is not directly proportional to resting length of the muscle fiber.
• In other words, Starling’s law is not applicable to smooth muscle.
• In skeletal and cardiac muscles, Starling’s law is applicable and the force of contraction is directly proportional to the initial length of the muscle fibers.

15. NEUROMUSCULAR JUNCTION IN SMOOTH MUSCLE

Three Types of Nerve Endings on Smooth Muscle Fibers

1. In some smooth muscles, nerve fibers diffuse on the sheet of smooth muscle fibers without making any direct contact with the muscle.
   • The diffused nerve fibers form diffused junctions which contain neurotransmitters.
2. In some smooth muscle fibers, axon terminal ends in the form of many varicosities which contain the neurotransmitter.
3. In some of the multiunit smooth muscle fibers, a synaptic cleft is present between the varicosities and the membrane of smooth muscle fibers, which resembles the contact junction in skeletal muscle.
   • This gap is a junction and it functions as a neuromuscular junction of skeletal muscle.

16. CONTROL OF SMOOTH MUSCLE ACTIVITIES

Nervous Factors

• Single-unit smooth muscle and multiunit smooth muscle are innervated by nerves of both divisions of the autonomic nervous system.
• All these nerves initiate contraction of multiunit smooth muscles only.
• Nerves supplying single-unit smooth muscles regulate the rate and force of contraction.

Humoral Factors

• Activity of smooth muscle is also controlled by humoral factors which include:
  • Hormones
  • Neurotransmitters
  • Other humoral factors
• Action of hormones and neurotransmitters depends upon the type of receptors present in the membrane of smooth muscle fibers in particular area.
• There are two types of receptors:
  • Excitatory receptors
  • Inhibitory receptors
• If excitatory receptors are present, hormones or neurotransmitters contract the muscle by producing depolarization.
• If inhibitory receptors are present, hormones or neurotransmitters relax the muscle by producing hyperpolarization.

Humoral Factors Which Cause Contraction of Smooth Muscles

1. Acetylcholine
2. Antidiuretic hormone (ADH)
3. Adrenaline
4. Angiotensin I, II and IV
5. Endothelin
6. Histamine
7. Oxytocin
8. Serotonin

Humoral Factors Which Cause Relaxation of Smooth Muscles

1. Lack of oxygen
2. Excess carbon dioxide
3. Increase in hydrogen ion concentration
4. Adenosine
5. Lactic acid
6. Excess potassium ion
7. Decrease in calcium
8. Nitric oxide (NO), the endothelium-derived relaxing factor (EDRF)