Smooth stabilises thin filaments.
Stimulation - nervous, hormal and stretch.
ADH - vesopressin
Thin filaments suggest actin filaments as opposed to the thick ones, which are myosin filaments, both being proteins present in muscle fibres; the filaments slide along each other to shorten (= contract) muscle.
A signal from a nerve cell (neurone) or stimulation by a hormone or the stretching of special receptors (called proprioceptors) in muscle (these latter give the brain feedback regarding muscle length/state and posture thus helping with balance and controlled movement) are the 3 ways a smooth muscle can be "coaxed" to contract.
Oxytocin is one of the hormones released during labour and induces contraction of the smooth muscle of the uterus to generate the "contactions" that culminate in childbirth (NB: a woman cannot voluntarily decide to "eject" her baby - it is an involuntary process hence mediated by smooth muscle and not by striated [voluntary] muscle).
ADH (Anti-Diuretic Hormone) [secreted by the posterior pituitary gland] as the name suggests, tends to inhibit diuresis (=urine production). it achieves this by stimulating the reabsorption of water by the ascending limb of the loop of Henle [through an action on the juxta-glomerular complex] in the kidneys (the loop of Henle is the penultimate part of the nephron [functional unit of the kidney], of which there are 1.3 million in each kidney) the final part being the collecting duct. As more water is reabsorbed into the blood compartment, obviously less is left over in the urine compartment leadng to oliguria (less urine).
Angiotenin II is produced by the breakdown of Angiotensin I with the aid of ACE (Angiotensin Converting Enzyme). It tends to promote contraction of vascular smooth muscle leading to narrowing of arterioles - this increases peripheral resistance resulting in a rise in blood pressure (drugs known as ACE inhibitors e. g. captopril, enalapril, are useful in the treatment of hypertension (high blood pressure) as they reduce the vasoconstricting effect of angiotensin II.
Hope this helps.
Mukesh (A level Biology tutor)