Smooth muscle contraction

Question 1

What does smooth muscle not contain that striated do?

Answer 1

-No tropomyosin/troponin system. This is where dense bodies of actin/myosin interact

Question 2

Where are smooth muscles found?

Answer 2

Found in ‘walls’ of tubular organs like
-Blood vessels
-GI tract
-airways
-Uterus
-Bladder
-Eye

Question 3

What does contraction of smooth vessels in blood vessels lead to?

Answer 3

-Contraction or relaxation of circular smooth muscle cells
-This leads to change in diameter which change blood flow

Question 4

What does contraction of smooth muscle in GI tract lead to?

Answer 4

-Contraction or relaxation of circular and longitudinal smooth muscle
-this leads to change in movement of the gut(Peristalsis)

Question 5

Why is regulation of smooth muscle important in disease?

Answer 5

-Regulation of smooth muscle is an important goal in treating disease like hypertension, COPD, IBS etc.

Question 6

What is the contraction like in smooth muscles and due to what reason?

Answer 6

Slow, Sustained, graded contraction due to time taken to switch on an off Ca2+ stimulated contractile events

Question 7

What are smooth muscles innervated by ?

Answer 7

Innervated by autonomic nervous system(ANS)

Question 8

What is myogenic contraction and can it occur in smooth muscle?

Answer 8

Can have spontaneous contractions(myogenic)

Question 9

What transmitter is released from parasympathetic nerves for smooth muscles?

Answer 9

Ach

Question 10

What transmitter is released from sympathetic nerves for smooth muscles?

Answer 10

Noradrenaline

Question 11

What transmitter is released from NANC(non-NA-non-ACH) nerves for smooth muscles?

Answer 11

Nitric oxide

Question 12

What transmitter is released from adrenal medulla for smooth muscles?

Answer 12

Adrenaline

Question 13

How is cytosolic Ca2+ increased?

Answer 13

-Increase in Ca2+ influx from extracellular medium
-Release of Ca2+ from internal Ca2+ stores(Sarcoplasmic reticulum)

Question 14

What are the steps involved in neurotransmission to smooth muscle via autonomic nerves?

Answer 14

1. Release of excitatory transmitter(Ach,NA) binds to Gq GPCR which causes depolarisation and leads to junctional depolarising potential
2. This meets the threshold for VGCCs an opens them allowing a Ca2+ influx
   i. VGCCs cause upstroke of action potenital
   ii. This leads to muscle contraction
3. When K+ channels open, this results in repolarisation and close VGCCs
   i. This leads to a decrease in Ca2+ leading to muscle relaxation

Question 15

Steps involved in smooth muscle contraction

Answer 15

1. Increase in ca2+ influx
   a. Stimulation of GPCR(Gq family)
   i. Ach on Mus(M3) in airways
   ii. Noradrenaline on alpha1 in blood vessels
   b. Action on ion channels produce depolarisation
   i. From resting membrane potential of –60mV to –30mV
   c. Junctional depolarising potential generated and results in activation of VGCCs with a threshold of –40mv
   i. Produces Ca2+
   d. Stimulation of GPCR(Gq family) generates IP3 which binds to IP3 receptors on the sarcoplasmic reticulum
   i. This results in a release of Ca2+ from the sarcoplasmic reticulum, increasing cytosolic concentration of Ca2+
   e. This results in an increase in Ca2+ concentration within the cytosol
   f. Smooth muscle contraction uses IP3 receptors, not ryanodine receptors(RyR in cardiac/skeletal muscle contraction)
   
   2. Smooth muscle uses Ca2+-modulin-Myosin light chain kinase system produce contraction
      a. Intracellular Ca2+ binds to calmodulin(CaM)
      b. Ca2+-modulin activates myosin light chain kinase(MLCK)
      c. MLCK phosphorylates light chains in myosin heads and increases myosin ATPase activity
      Active myosin crossbridge slide along actin and create muscle tension