SMOOTH MUSCLE

INTRODUCTION

 

– 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 :

(1) Actin 

(2) Myosin

(3) Tropomyosin.

(4)Myofilaments : 

-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 connetive 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 :

1) Acetylcholine

2) Antidiuretic hormone

3) Adrenaline

4) Angiotensin 2, 3, 4.

5) Endothelin

6) Histamine

7) Noradrenaline

8) Oxytocin

9) Serotonin.

 

✓ Other humoral factors cause relaxation of smooth muscle are; 

1) Lack of oxygen

2) Excess of carbon dioxide

3) Increase in hydrogen ion concentration.

4) Adenosine

5) Lactic acid

6) Excess of potassium ion

7) Decrease in calcium ion

8) Nitric oxide , endothelium- derived relaxing factor.

 

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