Smooth Muscle

Question 1

Where is smooth muscle?

Answer 1

Hollow organs + tubes

Helps regulate flow in:

• vascular system
• airways
• GI tract
• urogenital tract
• eye

Question 2

Functional roles of smooth muscle

Answer 2

Regulate flow by varying tube diameter - blood vessels during exercise

Control flow by occluding tube - sphincters

Walls of storage organ - expand + expel

Movement of large bulk - peristalsis (oesophagus)

Question 3

Describe the structural organisation of smooth muscle

Answer 3

Supported by and contains connective tissue

SM does not act on structures like bone (has no tendons)

Small cells (do not extend full length of muscle)

Group of cells arranged in sheets

Single sheet e.g. arterioles + airways

• circularly oriented
• maintains vessel diameter
• controls blood flow

Multiple sheets e.g. ileum

• 2 sheets perpendicular to each other
• longitudinal + circular layers
• vary diameter + length: peristalsis

Question 4

Describe smooth muscle cell structure

Answer 4

Small spindle-shaped cells

• Uni-nucleate
• No striations, not banded i.e. “smooth”
• No “z-bands” but “dense bodies”

Actin & myosin filaments present

• actin filaments anchored to “dense bodies”

Intracellular cytoskeleton harnesses pull

• Contracts inward, bulges
• Intercellular connections harness pull between cells

Question 5

What are Gap Junctions?

Answer 5

Join smooth muscle cells to one another

Non-selective channels allow intracellular communication

Signalling propagate between cells

Fibres act in unison: synchronised contraction of uterus during labour

Question 6

What is needed to start a SM contraction?

Answer 6

Requires Ca²⁺ from either:

• outside cell across plasma membrane
  
  • using voltage gated Ca²⁺ channels
• Ca²⁺ mobilised from intracellular stores
  
  • through activation of 2nd messenger

Increasing Ca²⁺ increases the level of tone by increasing the contraction of the SM cells

Varying the level of intracellular Ca²⁺ enables SM cells to vary level of contraction

Question 7

Example of structure made up of multiple sheets of smooth muscle

Answer 7

Ileum (final part of SI)

Question 8

Example of structure made up of a single sheet of smooth muscle

Answer 8

Arterioles + airways

Question 9

Describe the contractile mechanisms of smooth muscle

Answer 9

Contraction by sliding filament theory

(Interaction of actin w/ myosin to form X-bridges)

Ca2+ binds to calmodulin (biochemical effect)

Ca2+-calmodulin activates myosin light chain kinase (MLCK)

MLCK phosphorylates myosin light chain (MLC)

MLC must first be phosphorylated for actin to bind

This causes a contraction

Question 10

How can MLC bind to Actin

Answer 10

MLC must be phosphorylated to MLC-Phos to be able to bind to Actin to cause a contraction

Question 11

Relaxation of smooth muscle

Answer 11

Ca2+ in cytosol decreases

Ca2+ unbinds from calmodulin

MLCK decreases

MLCP (myosin phosphatase) removes phosphate group from myosin light chains which decreases myosin ATPase (MLCP from cGMP or cAMP)

Less myosin ATPase causes decreased muscle tension

Question 12

How does Rho-Kinase play a part in smooth muscle contraction?

Answer 12

RhoA activates Rho kinase, which phosphorylates MLCP, thereby inhibiting MLCP activity and increasing contraction and vascular tone.

Question 13

What receptors increase cAMP in smooth muscle?

Answer 13

Beta-adrenoceptors

Increasing cAMP leads to relaxation (airways)

No relaxation of vascular smooth muscle (nitrates used in angina)

Question 14

What are the other ways which smooth muscle tone can be regulated?

Answer 14

By contractile + relaxatory agents released from neurones, endothelium + blood-borne