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

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

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

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