Physiology of Smooth Muscle Flashcards
What are the 3 types of muscle?
- Skeletal- attached to skeleton
- Cardiac- heart muscle only
- Smooth- located in tubes and walls of hollow organs
Describe skeletal muscle
- Voluntary control
- Fibrous- long multi-nucleated cells acting as one giant cell, and striated
Describe cardiac muscle
- Only heart, myogenically active
- Spontaneous polarisation leads to contraction
- Y-shaped, single nucleated (mostly), striated, distant connected cells, intercalated discs
Describe striations
Arrangement of contractile filaments in muscle, thick and thin filaments
Describe the structure of a sarcomere
- 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
Describe the power stroke
- 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
Describe the thin filament
- Tropomyosin wrapped around actin
- Troponin bound to tropomyosin
- 3 troponins- T, C and I
- Calcium ions bind to troponin
What is the significance of calcium ions?
- 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
Describe rigor mortis after death
- No ATP produced to break cross-bridges
- Muscles are tense
- Fades as muscles begin to break down
Describe smooth muscle
Structurally similar throughout body: visceral/single unit or multi-unit
Describe visceral smooth muscle
- Single unit
- Common and widely distributed
- Many cells- single unit, small levels of innervation (gap junctions)
- Continuous irregular tone
Describe multi-unit smooth muscle
- Rarer than single unit
- Discrete nerve supply, independent behaviour
What are the key features of smooth muscle?
- 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
Describe the arrangement of smooth muscle
- 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
Describe smooth muscle contraction
- 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
What are the calcium sources for contraction?
- Sarcoplasmic reticulum for skeletal muscle
- Poorly developed in smooth muscle- interstitial fluid and some intracellular stores
How is smooth muscle stimulated?
- 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
Describe smooth muscle relaxation
- 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
What are the contractile characteristics
- 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
