Excitation and contraction of smooth muscle Smooth muscle

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Excitation and contraction of smooth muscle

Excitation and contraction of smooth muscle

Smooth muscle Important component of many organ systems: q 1. GI 2. Ureters, uterus

Smooth muscle Important component of many organ systems: q 1. GI 2. Ureters, uterus 3. Respiratory system - bronchi 4. Blood vessels q Not under voluntary control. q Multiple ways of regulation of activity.

Types of smooth muscle q Multi-Unit q q ciliary muscle and iris muscle of

Types of smooth muscle q Multi-Unit q q ciliary muscle and iris muscle of the eye, piloerector muscle Unitary (syncytial or visceral)

Physical structure of smooth muscle Contractile unit similar as in skeletal muscle – but

Physical structure of smooth muscle Contractile unit similar as in skeletal muscle – but no such regularity. Sidepolar arrangement of myosin heads (in skeletal muscle bipolar) Dense bodies instead of Zdiscs.

Physical structure of smooth muscle Coupling of cells: q 1. dense bodies (physical) 2.

Physical structure of smooth muscle Coupling of cells: q 1. dense bodies (physical) 2. gap junctions (electrochemical)

Regulation of contraction by Ca 2+ ions

Regulation of contraction by Ca 2+ ions

Smooth muscle cross-bridge cycling q Cross-bridge cycle in smooth muscle is slower than in

Smooth muscle cross-bridge cycling q Cross-bridge cycle in smooth muscle is slower than in skeletal muscle. q Due to slower ATP-ase activity of myosin head.

Comparison of smooth muscle to skeletal muscle contraction Slow cycling of myosin bridges (1/10

Comparison of smooth muscle to skeletal muscle contraction Slow cycling of myosin bridges (1/10 to 1/100 of skeletal muscle). Fraction of time the cross-bridges remain attached to actin is long low energy requirement for contraction. Slowness of onset of contraction and relaxation (30 x longer than skeletal muscle). Maximum force of contraction greater than skeletal. “Latch” mechanism: sustained muscle contraction with little use of energy.

“Latch” mechanism

“Latch” mechanism

“Latch” mechanism

“Latch” mechanism

Control systems of smooth muscle

Control systems of smooth muscle

Regulation of contraction Contraction/relaxation can be elicited by: 1. Neural stimuli (autonomic nervous system)

Regulation of contraction Contraction/relaxation can be elicited by: 1. Neural stimuli (autonomic nervous system) 2. Hormones (adrenalin, vasopresin, ACh, angiotensin, oxitocin, histamin) i local factors (p. O 2, p. CO 2, H+) 3. Mechanical stimulus (strech stress relaxation) 4. Spontanous rhythmicity (pacemaker)

Membrane potential and contraction

Membrane potential and contraction

2+ Ca and contraction

2+ Ca and contraction

2+ Ca and contraction q Smooth muscle contraction is dependent on extracellular Ca 2+

2+ Ca and contraction q Smooth muscle contraction is dependent on extracellular Ca 2+ ion concentration q Cellular entry of extracellular Ca 2+ via: q 1. Opening of voltage-gated Ca 2+ channel (action potential) 2. Opening of ligand-gated Ca 2+ channel (no action potentials) Diffusion of Ca 2+ to all intracellular contractile proteins 50 x longer latent period than in skeletal muscle.

2+ Ca and contraction q Some smooth muscle cells have more developed SR. q

2+ Ca and contraction q Some smooth muscle cells have more developed SR. q Faster contraction after excitation.

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