2005B14 Describe the processes of excitation and contraction within smooth muscle cells.

 

List:

·       Structure compared with striated

·       Function compared with striated

·       Excitation and contraction process

 

Structure (cf. striated)

Membrane structures

·  Can be single unit (syncytial with gap junctions) or multi unit (stand-alone cells)

·  No single end-plate, instead multiple nerve branches with varicosities

·  Poorly developed T tubules = caveolae

Contractile structures

·  Smooth cf. striated (chaotic actin-myosin distribution)

·  Dense bodies cf. Z line

Other intracellular

·  Single nucleus

·  Poorly developed sarcoplasmic reticulum (SR); most Ca2+ derived from ECF

 

Function (cf. striated)

Electrical

·  Wandering membrane potential -40 to -60mV, threshold potential -35mV

·  Automaticity occurs

Mechanical

·  No fixed length-tension relationship

·  Greater force of contraction

·  Lower energy consumption

·  Tendency for sustained contraction via latch bridge mechanism

 

Excitation and contraction process

Stimulus

·  Spontaneous depolarisation

·  Mechanical stretch -> reflex contraction

·  Autonomic nervous system (acetylcholine)

·  Hormones – autocrine, paracrine, endocrine

Excitation and contraction

·  Action potential

·  Opening of L-Ca2+ channel

·  Ca2+ influx from ECF >> SR

·  Ca2+-calmodulin -> ↑MLCK activity, ↓MLCP activity

·  MLCK phosphorylates myosin ATPase

·  Myosin ATPase phosphorylates myosin

·  Actin-myosin cross-bridge cycling occurs

Relaxation

·  ↓ICF [Ca2+]

·  ↑MLCP activity, ↓MLCK activity

·  Myosin detaches from actin

Latch bridge mechanism

·  Actin and myosin adopt a fixed, contracted conformation

·  Sustained contraction despite low ICF [Ca2+] and inactivated myosin-ATPase