Overview

In the media, the smooth muscle cells themselves make the extracellular matrix, because the media has no fibroblasts. They produce elastin, type III collagen and proteoglycans.

In elastic arteries, the media consists of dozens of fenestrated elastic lamellae alternating with layers of smooth muscle cells. In muscular arteries it consists of about 3 to 40 layers of smooth muscle. An external elastic lamina marks its outer boundary. In veins the media is thin, with only a few layers of muscle.

Control of contraction. Vascular smooth muscle contracts when Ca²⁺ binds calmodulin, which activates myosin light-chain kinase. It relaxes when myosin light-chain phosphatase is active. Nitric oxide relaxes it through soluble guanylate cyclase, cGMP and protein kinase G. Muscle tone is set by sympathetic nerves (noradrenaline on α1 receptors), circulating hormones (angiotensin II, vasopressin, adrenaline), local metabolites, endothelial mediators and the myogenic response to stretch.

Anatomy

Histology: Concentric layers of spindle-shaped smooth muscle cells with interposed elastic lamellae and collagen. An external elastic lamina is present in muscular arteries.

Function

Changes vessel diameter (vasoconstriction and vasodilation), which controls resistance and flow.
Stores and releases elastic energy (elastic arteries).
Gives the wall its strength against arterial pressure.

Clinical significance

Medial degeneration, aneurysm and dissection

In medial degeneration, elastic fibres are lost and mucoid material accumulates, weakening the aortic wall. It occurs in Marfan syndrome (fibrillin-1), Loeys–Dietz syndrome (TGF-β signalling), vascular Ehlers–Danlos syndrome (type III collagen) and long-standing hypertension. It predisposes to aneurysm and dissection.

Drugs acting on vascular smooth muscle

Several drug classes act on vascular smooth muscle. Dihydropyridine calcium channel blockers such as amlodipine block its Ca²⁺ entry. Nitrates release nitric oxide. α1-blockers block sympathetic constriction. Hydralazine and minoxidil are direct vasodilators. Pressor agents such as noradrenaline and vasopressin constrict it.

Test yourself

Which papillary muscle is most likely to rupture after an inferior myocardial infarction?

Anterolateral papillary muscle of the LV
Posteromedial papillary muscle of the LV
Anterior papillary muscle of the RV
Septal papillary muscle of the RV

Answer: Posteromedial papillary muscle of the LV

The posteromedial papillary muscle usually has a single blood supply from the posterior interventricular artery. The anterolateral muscle has a dual supply from the LAD and circumflex. Rupture causes acute severe mitral regurgitation.

In the femoral triangle, which structure lies immediately medial to the femoral artery?

Femoral nerve
Femoral vein
Great saphenous vein
Inguinal ligament

Answer: Femoral vein

From lateral to medial the order is NAVY: Nerve, Artery, Vein, Y-fronts (the femoral canal). This is why femoral venous access is about 1 cm medial to the femoral pulse.

Where is the great saphenous vein most reliably found at the ankle?

Anterior to the medial malleolus
Posterior to the lateral malleolus
Posterior to the medial malleolus
Anterior to the lateral malleolus

Answer: Anterior to the medial malleolus

The great saphenous vein lies in front of the medial malleolus, with the saphenous nerve beside it. This is the classic site for an emergency venous cut-down. The small saphenous vein passes behind the lateral malleolus with the sural nerve.