Physiology of Smooth Muscle

Question 1

What are the 3 types of muscle?

Answer 1

• Skeletal- attached to skeleton
• Cardiac- heart muscle only
• Smooth- located in tubes and walls of hollow organs

Question 2

Describe skeletal muscle

Answer 2

• Voluntary control

- Fibrous- long multi-nucleated cells acting as one giant cell, and striated

Question 3

Describe cardiac muscle

Answer 3

• Only heart, myogenically active
• Spontaneous polarisation leads to contraction
• Y-shaped, single nucleated (mostly), striated, distant connected cells, intercalated discs

Question 4

Describe striations

Answer 4

Arrangement of contractile filaments in muscle, thick and thin filaments

Question 5

Describe the structure of a sarcomere

Answer 5

• Shrinks during contraction as filaments overlap
• I band- light thin filaments
• A band- thick and thin
• H zone- centre of A band, only thick
• M-line divides H zone, connects thick filaments
• Cross-bridge between myosin head and actin filament

Question 6

Describe the power stroke

Answer 6

• Involves energy, ATP required (not used to drive contraction, dissociates myosin head from actin)
• ATP binds to and is hydrolysed by myosin head, phosphate binds to myosin head- ‘cocked’
• Myosin binds to actin, phosphate released, reverts to original structure
• Repeated- more cross bridges- more muscle shrinkage

Question 7

Describe the thin filament

Answer 7

• Tropomyosin wrapped around actin
• Troponin bound to tropomyosin
• 3 troponins- T, C and I
• Calcium ions bind to troponin

Question 8

What is the significance of calcium ions?

Answer 8

• Muscle stimulation- calcium ions release from sarcoplasmic reticulum storage
• Binds to troponin C- moves troponin T and exposes binding site on actin
• Troponin T pulls tropomyosin away- cross bridge can now form

Question 9

Describe rigor mortis after death

Answer 9

• No ATP produced to break cross-bridges
• Muscles are tense
• Fades as muscles begin to break down

Question 10

Describe smooth muscle

Answer 10

Structurally similar throughout body: visceral/single unit or multi-unit

Question 11

Describe visceral smooth muscle

Answer 11

• Single unit
• Common and widely distributed
• Many cells- single unit, small levels of innervation (gap junctions)
• Continuous irregular tone

Question 12

Describe multi-unit smooth muscle

Answer 12

• Rarer than single unit

- Discrete nerve supply, independent behaviour

Question 13

What are the key features of smooth muscle?

Answer 13

• Involuntary control (ANS, beyond conscious control)
• Spindle-shaped, single-nucleated, smaller than skeletal, no striations- ‘smooth’
• Same contractile apparatus (actin, myosin and tropomyosin)- arranged differently
• No troponin complex

Question 14

Describe the arrangement of smooth muscle

Answer 14

• Not striated
• Different arrangement of actin and myosin (mesh like)
• Troponin replaced by calmodulin
• Calmodulin- ubiquitous calcium binding protein found in a number of cells

Question 15

Describe smooth muscle contraction

Answer 15

• Power stroke and cross-bridge formation
• Loose lattice effect- shrinkage in smooth muscle size with contraction
• Calcium- calmodulin complex- activates myosin light chain kinase- phosphorylates myosin light chain
• Myosin interaction with actin- contraction

Question 16

What are the calcium sources for contraction?

Answer 16

• Sarcoplasmic reticulum for skeletal muscle

- Poorly developed in smooth muscle- interstitial fluid and some intracellular stores

Question 17

How is smooth muscle stimulated?

Answer 17

• Depolarisation of membrane- opens voltage gated calcium channels
• Calcium-induced calcium release- positive feedback
• Activation of MLCK- contraction
• Receptor activation- agonist induced increases in calcium channel or IP3 through ligand-gated calcium channel or IP3 pathway
• No changes in membrane potential

Question 18

Describe smooth muscle relaxation

Answer 18

• Dephosphorylation of myosin and removal of calcium
• Myosin dephosphorylated by enzyme, calcium actively pumped out
• > Calcium ATPase (primary)
• > Na+/Calcium ion exchanger (secondary)
• Or actively pumped into intracellular stores- calcium ATPase

Question 19

What are the contractile characteristics

Answer 19

• Can shrink up to 25% in length in contraction
• Slow contraction/relaxation rate
• Generated equivalent tension to skeletal muscle
• Low energy expenditure, does not fatigue, few mitochondria
• Generates similar force as skeletal muscle, per unit of cross-sectional are, despite 1/3 actin
• > low rate of cross bridge cycling- tension linked to number of cross bridges
• Anaerobic ATP generator