– Smooth muscle are voluntary and non-striated.
– Present in almost all the organs as bundles or sheaths.
– This forma a major contractile tissues of various organs .
* Smooth muscle fibers are present in following structures ;
– Wall of esophagus , stomach and intestine.
– Duct of digestive gland.
– Trachea , bronchial tube and alveolar ducts.
– Ureter , urinary bladder and urethra .
– Wall of blood vessels .
– In skin, arrector pilorum.
– Mammary gland , uterus , genital ducts, prostate gland and scrotum.
– Iris and ciliary body .
• Functions of smooth muscle :
(1) In CVS :
– The muscle fiber around blood vessel regulate blood pressure.
(2) In Respiratory system :
– In air passages , contraction and relaxation of muscle fiber alters the diameter of air passage and regulate the inflow and outflow of air.
(3) In Digestive system :
– Muscle fiber helps in movement of food substance and mixing of food substance .
(4) In Renal system :
– In renal blood flow : Regulate renal blood flow and glomerular filteration.
– In Urinary bladder : help in voiding urine to the exterior.
(5) In Reproductive system :
– In males , facilitates movement of sperms.
– In females , helps in movement of sperm through genital tract after sexual act , ovum movement through fallopian tube , expulsion of menstrual fluid and delivery of baby.
• Structure of smooth muscle :
– Muscle fibers are generally very small and are fusiform or elongated cells.
– Nucleus is centrally placed and is single and elongated.
– Normally nucleus consists of two or more cells.
* Contractile proteins and Myofilaments :
√ There are 3 contractile proteins can be seen in smooth muscle :
-Thick and thin filaments.
– Filaments are not arranged in orderly fashion.
– Thick filament includes myosin molecule and are scattered in sarcoplasm.
– Thin filament includes actin and Tropomyosin molecules.
* Dense bodies:
– These are present all over the sarcoplasm in network of intermediate Filaments, which is formed by protein Desmin.
– The Actin and Tropomyosin molecules of thin Filaments are attached to the dense bodies and some of the them are attaches firmly to sarcolemma.
– Muscle fibers twist- like corkscrew during contraction because dense bodies are not arranged in straight line.
– These muscle fibers bound together at dense bodies and helps to transmit the contraction from one cell to another .
* Covering and Tendons :
– The tendons and aponeurosis are absent but the muscle fibers are covered by connective tissue.
* Sarcotubular system :
– Muscle fibers are arranged in form of network .
– T- tubules are absent and L- tubules are poorly developed.
• Types of smooth muscle :
– There are two types ;
(1) Single – unit or visceral smooth muscle.
(2) Multiunit smooth muscle.
* Single – unit smooth muscle :
– More common
– Arranged like sheets or bundles of tissues.
– Present in small blood vessels and walls of organs ( organs of GIT , Urinary system , reproductive system , respiratory tract etc)
– It has a gap junctions , which allows rapid passage of action potential.
– Spontaneous rhythmical contractions occur because of presence of self- excitable pace maker.
– Control of action is myogenic.
– Due to rhythmic modulations in sodium – potassium pump , there is a unstable resting membrane potential with slow spike potentials .
– Action potential can be generated spontaneously and it can be elicited by electrical or hormonal stimulations.
– Action potential spreads rapidly throughout the sheets of cells and they make the cells to contract as a single unit .
This occurs with a plateau due to long depolarization and slow repolarization.
* Multiunit smooth muscle :
– They are less common
– Discrete individual muscle fibers .
– Present in ciliary muscle of eye, iris of eye , nicititating membrane ( in cat ), arrector pilli and larger blood vessels.
– Each muscle fiber is innervated by single nerve ending and there is no gap junction.
– No spontaneous contractions because of absence of pacemaker cells.
– Control of action is neurogenic.
– Resting membrane potential is stable .
– Action potential is cannot be generated spontaneously and can be elicited by neural and hormonal stimulation.
– Selective activation of each muscle fiber can contract independent of each other and there is no plateau.
• Control of smooth muscle :
– Muscle fiber is controlled by ;
(1) Nervous factors
(2) Humoral factors
* Nervous factors :
– Both the type of smooth muscles are innervated by nerves of both the division of autonomic nervous system.
– These nerve initiate contraction of only Multiunit smooth muscles and it do not initiate contraction of single unit smooth muscles.
– These nerves can regulate rate and force of contraction.
* Humoral factors :
– Humoral factors includes hormones, neurotransmitters and other humoral factors.
✓ Hormones and neurotransmitters :
– Action of these factors depends on the type of receptors present in membrane of smooth muscle fiber in particular area.
– These receptors are of two types ;
(1) Excitatory receptors : Contract muscle by producing Depolarization.
(2) Inhibitory receptors : Relax the muscle by producing hyperpolarization.
* Hormones and neurotransmitters acting on smooth muscles are :
2) Antidiuretic hormone
4) Angiotensin 2, 3, 4.
✓ Other humoral factors cause relaxation of smooth muscle are;
1) Lack of oxygen
2) Excess of carbon dioxide
3) Increase in hydrogen ion concentration.
5) Lactic acid
6) Excess of potassium ion
7) Decrease in calcium ion
8) Nitric oxide , endothelium- derived relaxing factor.