smooth muscle bds 2010-2011

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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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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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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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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and Skeletal Muscle

ElectricalSingle-unit



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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and Skeletal Muscle

ContractionSingle-unit



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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smooth muscle bds 2010-2011

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