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8/8/2019 Smooth Muscle BDS 2010-2011
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Dr Hoe See ZiauDepartment of Physiology
Faculty of MedicineUniversity of Malaya
020810 BDS 2010/2011 Smooth Muscle Lecture
SMOOTH MUSCLE
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Types of Muscle
Skeletal Muscle
striated
voluntary
Cardiac Muscle
striated
involuntary
Smooth Muscle
non-striated
involuntary
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Smooth Muscle
Found in the walls of hollow organs
Main functions:
To provide motility, e.g.:
Propulsion of chyme along thegastrointestinal tract
Propulsion of urine along the ureter
To maintain tension
In the wall of blood vessel
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Smooth Muscle Fibre
A spindle-shaped cell
Small
2 - 10m diameter
50 - 400m long (does not extend the full length of a muscle) Arranged in sheets
Single nucleated
No striations,,lacking sarcomeres
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Smooth Muscle Fibre
No Z line, but has dense bodies
No T-tubules
Present of caveolae invaginationof surface membrane, act like T-
tubules
Contains actin, myosin II &tropomyosin
Does not contain troponin, but
calmodulin instead
Actin anchored to dense bodies
Innervation: Autonomic: Sympathetic& parasympathetic
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Types of Smooth Muscle
1. Single-unit / unitary smooth muscle
2. Multi-unit smooth muscle
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Single-Unit Smooth Muscle
Fibres arranged in bundles of sheets
Presence of gap junction
Allow flow of ions between cells
Allow muscle to function as asyncytium
Has an inherent rhythm
Found in walls of many visceralorgans of body visceral smooth
muscle
Present in:
Gastrointestinal tract, bile duct
Urogenital system
Airways of respiratory system
Many blood vessels
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Multi-unit Smooth Muscle
Discrete, separate fibresfunctioning independently
No interconnecting bridges
Ensure individual musclecontraction and more precise
control No inherent rhythm relatively
stable membrane potential
Regulation mainly by nervesignals, similar to skeletal muscle
Present in: Ciliary muscle,
Iris muscle
Piloerector muscle
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Electrical Properties of Single-Unit
Smooth Muscle Myogenic
Self-excitable, does not require nervous
input to contract Spontaneous electrical activity
Unstable resting membrane potential: -50
mV to -60 mV Two types of spontaneous depolarisations:
Pacemaker potential
Slow-wave potential
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Pacemaker Potential
Can develop from pacemaker cells that can arise anywhere in the smoothmuscle
Passive ionic fluxes due to automatic changes in channel permeability
They gradually depolarise threshold potential (
-35 mV) action potential produced (due to influx of Ca2+)
which would spread to other cells through gap junctions
contraction
Following repolarisation, the membrane again begins to depolarise
sequence of action potentials occurs tonic state of contractile activity
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Pacemaker Potential
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Slow-wave Potential
Gradually alternating hyperpolarising and depolarising swings inpotential
Caused by automatic cyclical changes in the rate at which Na+ areactively transported outward across the membrane
A burst of action potential occurs if a depolarising swing brings themembrane to threshold
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Contraction of Smooth Muscle
Action potential fire whenslow wave potentials reachthreshold
The force and duration ofmuscle contraction aredirectly related to thefrequency of actionpotentials
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Importance of Ca2+
Smooth muscle membrane
Plenty of voltage-gated Ca2+ channels
Very few voltage-gated Na+ channels
Importance of Ca2+
Influx of Ca2+ (major source of Ca2+)
Generates action potential (depolarisation phase)
Participates in the contraction process
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Contraction of Smooth Muscle
Initiation of contraction
Delayed: contraction begins 200 ms after the actionpotential
Peak contraction 500 ms after the action potential
Excitation-contraction coupling very slow process
Contraction
Syncytium
Continuous and irregular
Does not require nerve supply
Partial contraction always present tonus (constantstate of partial contraction)
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Contraction of Smooth Muscle
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Contraction of Smooth Muscle
Changes in cytosolic [Ca2+]control the contractile activityin smooth muscle fibres
Ca2+ binds to calmodulin, nottroponin C
Smooth muscle contractionoccurs by a sliding-filament
mechanism Myosin can only interact with
actin when phosphorylated
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Cross-Bridge Activation
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Contraction and Relaxation:
Sequence of Events1. Binding of neurotransmitter to receptor on surface membrane
2. influx of Ca2+ into cell and release of Ca2+ from sarcoplasmic reticulum(SR)
3. Ca2+ binds with calmodulin to form Ca2+-calmodulin complex (CaM)
4. Activation of myosin light chain kinase (MLCK) by CaM
5. Phosphorylation of myosin active myosin-Pi (ATP ADP)
6. Active myosin binds with actin
7. Actin slides along myosin contraction8. Ca2+ returned to SR and ECF
9. Dephosphorylation of myosin by myosin light chain phosphatase
10. May still get sustained contraction due to latch bridges
11. Finally relaxation occurs when CaM dissociates
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Contraction of Smooth Muscle
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Relaxation of Smooth Muscle
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Contraction and Relaxation
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Control of Contraction Neural:
Dual innervated: sympathetic and parasympathetic
Except on wall of blood vessel: sympathetic only
Effect depends on the type of receptor
Does not initiate contraction
Modulates or modifies the strength and rate of contraction
Hormonal
Adrenaline
Noradrenaline
Oestrogen
Progesterone
Gastrin
Others
ACh
Hypoxia
Hypercapnia
pH
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Nerve Supply ANS: Sympathetic & parasympathetic
Not discrete junction but each nerve terminal can travel acrosssurface of one or more smooth muscle
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Nerve Supply
Fewer nerves More nerves
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Plasticity of Smooth Muscle
Response to stretch
When muscle is stretched response is contraction
When stretched further response is relaxation
Stress-relaxation property of smooth muscle
To allow smooth muscle to adjust to stretch without increasingpressure on contents of organ, e.g.: receptive relaxation of stomachwall
Length-tension relationship
For any given tension developed variable
stretch tension
stretch tension
No correlation
No resting length
Referred to as plasticity of smooth muscle
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Plasticity of Smooth Muscle
Stretch leads to contraction and increased tension On sustained stretch, tension reduces
Constant Stretch
Tensio
n
(g)
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Comparisons between Smooth Muscles
and Skeletal Muscle
StructureSingle-unit
smooth muscleMulti-unit
smooth muscleSkeletal muscle
Structure Unstriated Unstriated Striated
StructureNo sarcomeres;
dense bodies
No sarcomeres;
dense bodies
Sarcomeres; Z-
lines
Nucleus Single-nucleated Single-nucleated Multi-nucleated
Cell shapeSmall; spindleshaped
Small; spindleshape
Long; shaped likecylinder
Gap junctions Many Few None
InnervationANS; excitation orinhibition
ANS; excitation orinhibition
Somatic NS;excitation only
Nerve:muscle NMJ Varicosities Varicosities
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Comparisons between Smooth Muscles
and Skeletal Muscle
ElectricalSingle-unit
smooth muscleMulti-unit
smooth muscleSkeletal muscle
RMP Unstable Stable Stable
Spontaneousfiring
Yes No No
Pacemaker cells Yes No No
Action potential
DepolarisationRepolarisation
Influx of Ca2+Efflux of K+
Influx of Ca2+Efflux of K+
Influx of Na+Efflux of K+
Nerve initiation No; modify Yes Yes
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Comparisons between Smooth Muscles
and Skeletal Muscle
ContractionSingle-unit
smooth muscleMulti-unit
smooth muscleSkeletal muscle
Sliding filamentmechanism
Yes Yes Yes
Source of Ca2+ ECF; SR ECF; SR SR
Site for Ca2+ Calmodulin Calmodulin Troponin C
Rate Slow Slow Fast
Duration Longer; sustained Longer Short
Syncytium Yes No No
Plasticity Yes No No
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