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Functions of smooth muscles and their electrical properties
Learning Objectives
By the end of the lecture, the student should be able to :
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Describe the characteristics of smooth muscle.
Explain the mechanism of smooth muscle contraction.
Give us the primary function of smooth muscle cells.
Give details of the electrical properties of smooth muscles.
Describe a few related disease conditions.
SMOOTH MUSCLES
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Smooth muscle is an involuntary non-striated muscle.
Innervated by autonomic nerve
It is divided into two sub-groups; the single-unit (unitary) and multiunit smooth muscle.
LOCATION
Smooth muscle is found in many places:
Tunica media layer of large (aorta) and small arteries,
Arterioles,veins&lymphatic vessels,
The urinary bladder, uterus, male and female reproductive tracts,
 Gastrointestinal tract,
Respiratory tract,
Arrector pili of skin,
The ciliary
 Glomeruli of
muscle, and iris of the eye,
the kidneys
STRUCTURE
General
Functions
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primary function of smooth muscle cells is to change, by contracting and relaxing, shape and stiffness of hollow
organs.
Forming lining of blood vessels
Specialized smooth muscle within afferent arteriole of juxtaglomerular apparatus secretes renin in response to
osmotic and pressure changes.
it is believed to secrete ATP in tubuloglomerular regulation of glomerular filtration rate.
General Functions
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Smooth muscle forms precapillary sphincters in metarterioles which regulates blood flow in capillary beds of various
organs and tissues
contractile function of vascular smooth muscle is critical to regulating the luminal diameter of small arteries-arterioles
called resistance vessels.
resistance arteries contribute significantly to setting level of blood pressure
Smooth muscle-containing tissue often must be stretched, so elasticity is an important
attribute of smooth muscle.
Smooth muscle cells may secrete a complex extracellular matrix containing collagen
(predominantly types I and III), elastin, glycoproteins, and proteoglycans.
These fibers with their extracellular matrices contribute to the viscoelasticity of these
tissues.
General Functions
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By relaxing or contracting, smooth muscle regulates blood flow through the vasculature, airflow in the lungs, movement of
food and waste through the gut, and uterine delivery of neonates.
It is also clear that smooth muscle cells synthesize extracellular matrix proteins and proteoglycans to fill the extracellular
space between muscle cells.
General Functions
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smooth muscle cells have significant capacity to synthesize, secrete, and respond to small soluble signaling molecules.
smooth muscle cells from many organs respond to pro-inflammatory stimuli, such as interleukin-(IL)-1β , IL-5, IL-13 and tissue
necrosis factor-(TNF)-α , by synthesizing a remarkably wide spectrum of signaling proteins including cytokines, chemokines, and
peptide growth factors.
Specific functions to GIT
SECRETARY FUNCTIONS
CONTRACTILE FUNCTIONS
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digestive tract smooth muscle contracts in a rhythmic peristaltic fashion, rhythmically forcing foodstuffs through digestive tract as result of phasic contraction.
There are differences in myosin heavy and light chains that also correlate with these differences in contractile patterns and kinetics of contraction between tonic and phasic
smooth muscle
Electrical properties
What is electricity in smooth muscles?
 Smooth muscle fibers are connected to their neighbors by gap junction. which are permeable to ions and thereby transmit electrical current from muscle fiber to muscle fiber.
 Ionic conncecivity, without cytoplasmic continuity from fiber to fiber, accounts for the electrical syncytial properties of smooth muscle, which confers electrical behavior
analogous to that of cardiac muscle.
Spread of Electric Activity
 Electrical activity and associated contractions spread from a point of initiation (e.g., the pacemaker region) in three dimensions throughout the bulk of the muscle.
 The distance and direction of spread of electrical activity in the electrical syncytium are controlled by the ENS.
Basic patterns of electrical activity
ELECTRICAL PACEMAKER
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The Interstitial cell of Cajal (ICC) is a type of interstitial cell found in the gastrointestinal tract that serves as a pacemaker which
creates the basal electrical rhythm leading to contraction of the smooth muscle (peristalsis)
Properties
Properties
 They are always present in the small intestine, where they decrease in frequency along a gradient from the duodenum to the ileum.
 In the gastric antrum, the terms slow wave and action potential are used interchangeably for the same electrical event.
 when action potentials are associated with electrical slow waves, they occur during the plateau phase of the slow wave.
Related diseases
References
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Lauralee Sherwood - Human Physiology From Cells to Systems.
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Publications.
– fifth Edition.