Smooth muscle

Question 1

Where is smooth muscle present?(and roles?)

Answer 1

• Vascular system-(e.g-capilliaries, arteries and veins)(Hypertension)
• Airways-(Trachea, bronchioles)(Asthma)
• GI tract-(Stomach,sphincters)(Gastric spasticity)
• Urinogenital tract-(bladder,uterus,vas deferens)(Infertility & incontinence)
• Eye-(Iris & cilary body)

Question 2

Why do you target smooth mucle for hypertension?

Answer 2

• Reduce vasoconstriction
• Current targets are receptors and target mech of contraction

Question 3

Why do you target smooth mucle for angina?

Answer 3

• Relaxation of coronary artery
• Current targets inc rec and signalling
• e.g K+ channels, NO

Question 4

Why do you target smooth mucle for asthma?

Answer 4

• Stimulation of B-adrenoreceptors-relax smooth muscle
• Inhibit msucarinic contraction

Question 5

Why do you target smooth mucle for Constipation?

Answer 5

• Stimulation of GI smooth muscle

Question 6

Why do you target smooth mucle for Diarrhoea?

Answer 6

• Inhibition of GI smooth muscle contraction

Question 7

What conditions involve treatment of Smooth muscle?

Answer 7

• Hypertension
• Angina
• Asthma
• Constipation
• Diarrhoea

Question 8

What are the functional roles of Smooth muscle?

Answer 8

• Regulate flow by varying tube diameter (e.g blood vessels during excercise)(Constriction & Dilation)
• Control flow by occluding tube(e.g Sphincters)
• Walls of storage organ (e.g bladder)(expand and expel)
• Movt of large bulk (e.g. Oesophagus,intestine) (swallowing-peristalsis)

Question 9

What is the structure of smooth muscle?

Answer 9

• Supported by and contains connective tissue. -SM does not act on structures e.g. bone(No tendons)
• Groups of small cells arranged in sheets
• Single sheet e.g. arterioles&airways (circularly orientated, controls blood flow)
• Multiple sheets e.g ileum
  1.2 sheets perpendicular to each other
  2. Longitudinal & Circular layers
  3. Vary diameter & length: peristalsis

(See slides for diagram)

Question 10

What is peristalsis?

Answer 10

• Cyclical contraction & relaxation of sheets of smooth muscle that line tubular tissue
  1.Lumen cavity constricts & dilates alternately
  2.Synch and wavelike
  3.Unidirectional movt of lumen contents
• Nerve plexus btw 2 layers of muscle
  1.E.g. myenteric plexus
  2.Nerve network
  3.Co-ordinates contr-relaxation cycle at local level

Question 11

How does smooth muscle contract?

Answer 11

• Actin & Myosin filament present
  1.Actin filaments anchored to “dense bodies”
  2.No troponin or troponin C
  3.Sliding filament theory of contraction
• Intracellular cytoskeleton harnesses pull
  1.Contracts inward, bulges
  2.Intracellular conncetions harness pull betw cels

Question 12

Explain sliding filament theory.

Answer 12

Question 13

What are varicosities?

Answer 13

• no specific junction between neurone and smooth muscle – more general release of neurotransmitters onto receptors over the surface of the muscle

Question 14

What is a single unit muscle?

Answer 14

• Electrically coupled to one another by gap junctions
  1.Non-selective
  2.e.g. calcium
  3.fibres act in unison:synch contract & relaxation
• 4.e.g. uniform,co-ord contraction of uterus during labour

Question 15

What are the contractile properties of a single-unit SM?

Answer 15

• Stretch-relaxation response
  1.When SM is suddenly stretched it initially inc tension, then relaxes, but at a new longer length.
  2.Allows hollow organs to accommodate variable amounts of contents without inc in pressure. Prev uncontrolled expulsion
• 3.e.g.bladder, uterus

Question 16

What is the role of Ca2+ in the excitation-contraction coupling?

Answer 16

• Contraction req inc intracellular Ca2+ from either influx across the mem (depo signals activate Voltage-gated CA2+ channels) or from intracellular stores (through activation of 2nd messenger e.g. G-protein coupled receptor)
• Many single-unit SM cells have enough cytosolic Ca2+ to maintain a low level of tone
• Inc Ca2+ inc tone by inc contraction os SM cells
• Varying the level of Ca2+ enables variation in level of contr

Question 17

What is the contractile mechanism of SM?

(Look at pp for diag)

Answer 17

• Contraction by sliding-filament-theory
  1.Interaction of actin with myosin to form X-bridges BUT
  2.SkM- Ca2+ binds to troponin C - causes conformational change
  3.SM-Ca2+ binds Calmodulin -Biochem effect
  4.Ca2+ calmodulin activates myosin light chin kinase (MCLK)
  5.MCLK phosphorylates myosin light chain (MLC)
• 6.MLC must first be Phos for actin to bind