Smooth muscle is one of three types of muscle tissue, alongside cardiac and skeletal muscle. It is a non-striated muscle tissue, lacking the characteristic markings of the other muscle types.
It is found in numerous bodily systems, including the ophthalmic, reproductive, respiratory and gastrointestinal systems, where it functions to contract and cause movements under involuntary control.
This article will discuss the ultrastructure of smooth muscle, its mode of contraction and function in the human body.
Structure of Smooth Muscle
Smooth muscle fibres are elongated and spindle-shaped cells which taper at both ends. This distinctive shape, along with the presence of a single central nucleus, helps to identify it histologically. These fibres are thousands of times shorter than skeletal muscle fibres.
Fig 1 – Smooth muscle fibres
Smooth muscle differs from striated muscle in many ways. T-tubules, myofibrils and sarcomeres are all absent, in contrast to striated muscle. Actin and myosin contractile proteins are present, as are thick and thin filaments. However, these are arranged differently – thin filaments are attached to a dense body (analogous to the Z-disc of skeletal and cardiac muscle).
Cross-Bridge Formation and Smooth Muscle Contraction
As smooth muscle cells lack troponin, cross-bridge formation is not regulated by the troponin-tropomyosin complex as it is in skeletal muscle. Instead, cross-bridge formation is regulated by the action of calcium-modulated protein, more commonly known as calmodulin. The process is as follows:
- Membrane depolarisation causes L-type voltage-gated calcium channels to open and extracellular calcium ions enter the cell down their concentration gradient.
- Intracellular calcium ions bind to calmodulin, forming the calcium-calmodulin complex which in turn activates myosin light chain (MLC) kinase.
- MLC kinase activates the myosin heads by phosphorylating them through ATP hydrolysis.
- Following phosphorylation, the myosin heads are able to attach to actin-binding sites and pull on the thin filaments.
- These filaments are anchored to the dense bodies, which are networked throughout the sarcoplasm. When thin filaments slide past the thick filaments, they pull on the dense bodies, causing the entire muscle fibre to contract.
Fig 2 – Diagram of smooth muscle fibre contraction
Relaxation of smooth muscle requires the myosin head to become dephosphorylated, which is mediated by myosin light chain phosphatase.
Muscle contraction can continue until ATP-dependent calcium pumps actively transport calcium ions back into the sarcoplasmic reticulum and extracellular fluid. A low concentration of calcium is always present within the sarcoplasm to maintain muscle tone and keep the muscle slightly contracted.
Furthermore, a specialised subtype of cross-bridge (latch-bridges) remains between myosin heads and actin, keeping the thick filaments linked together independently of ATP and calcium removal. Maintenance of muscle tone is especially important in the smooth muscle lining of the arterioles.
Control of Contraction
Contraction of smooth muscle is not under conscious control, hence it is referred to as an involuntary muscle. Triggers for smooth muscle contraction include hormones, neuronal stimulation by the autonomic nervous system and other local factors.
Neurons of the autonomic nervous system do not form an organised neuromuscular junction with smooth muscle. Instead, swellings of axons filled with neurotransmitter known as varicosities lie in close contact with the sarcolemma.
Smooth muscle is often spontaneously active, and can trigger action potentials without stimuli via the action of pacemaker cells in the walls of hollow organs. Its nerve supply acts to either stimulate or depress the activity of the fibre, depending on the neurotransmitter released. Examples of these pacemaker cells include the interstitial cells of Cajal in the GI tract.
In certain locations, muscle stretching can trigger its own contraction via the stress-relaxation response. In the stress-relaxation response, as the muscle stretches, the mechanical stress initiates muscle contraction, immediately followed by relaxation.
This is especially important in hollow organs such as the stomach or urinary bladder, which continuously expand when they fill. This response allows smooth muscle surrounding these organs to maintain muscle tone when the organ empties and shrinks, preventing premature emptying and ‘flabbiness’ in the empty organ.
Smooth Muscle Organisation
Smooth muscle is organised into two different ways. These two types of arrangement are found in different locations and have different characteristics.
| Single unit (visceral) smooth muscle | Multiunit smooth muscle | |
| Frequency | More common | Less common |
| Fibre coupling | Fibres joined by gap junctions, therefore are electrically coupled (if one fibre is stimulated to contract, they will all contract as a single unit) | Do not possess gap junctions, therefore fibres are not electrically coupled. Contraction doesn’t spread between cells and is confined to the cell originally stimulated |
| Stimulation of muscle fibres |
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| Location | Walls of all visceral organs except the heart | Large blood vessels, eyes and respiratory airways |
| Contraction | Produces slow, steady contractions to allow substances to move through the body, e.g. food in the GI tract | Produces asynchronous contractions |
Function of Smooth Muscle
Smooth muscle is found in numerous body systems and has a variety of functions depending on its location.
- Cardiovascular system – vascular smooth muscle cells are present in all vascular segments in the tunica media layer, excluding capillaries. Smooth muscle contraction and dilation (vasoconstriction and vasodilation respectively) controls blood vessel diameter, thereby controlling the distribution of blood and determining blood pressure.
- Respiratory system – smooth muscle layers are present in the walls of the bronchi and bronchioles, helping to regulate air flow into the lungs.
- Gastrointestinal tract – extensive layers of smooth muscle are present to help move food down the GI tract via peristalsis, and eject bile into the digestive tract from the gallbladder.
- Urinary system – layers of smooth muscle are found in the ureter walls and bladder to aid movement of urine out of the body.
- Male reproductive tract – layers of smooth muscle are present in the vas deferens to help move sperm through the system, and also functions to cause ejection of glandular secretions from the prostate, seminal vesicle and bulbourethral glands.
- Female reproductive tract – the myometrium of the uterus mostly consists of smooth muscle, and is stimulated by oxytocin to contract during labour to aid in birthing of the foetus.
- Ophthalmic system – ciliary muscle contracts and dilates to change the size and shape of the lens, changing the amount of light entering the eye.
- Integumentary system – hair follicles in the skin are associated with erector pili muscles, which contact to elevate hairs in response to changes in temperature.
Clinical Relevance – Leiomyomas
Leiomyomas are benign tumours developing from smooth muscle fibres. The most common location for development of leiomyomas is in the wall of the uterus, where they are known as fibroids. If large enough, fibroids can cause symptoms such as heavy periods and pain secondary to pressure on other nearby structures.
